CSDCOAbstracts.txt

14c07ec0-b582-394b-8248-f2dd04271ccf
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Abstract: Cores of organic-rich muds from the tropical meteorite crater Lake Bosumtwi, Ghana, contain laminae of authigenic calcite polyhedra and aragonite needles, as well as scattered diagenetic calcite, Mg-calcite spherulites, and aggregates of dolomite crystals. This example provides a model for the interpetation of ancient lacustrine carbonates from organic-rich environments.-from Authors

7cf29acb-e8c6-35f9-976c-9f206f97e5c0
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Abstract: Variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of Lake Bosumtwi, Ghana reflect climatically induced changes to the lake and the catchment flora. Cores spanning the last 27.5 kyr of sedimentation in the lake show large oscillations in δ13Corg PDB and δ15Norg Air. The late Pleistocene record is particularly detailed, showing changes of ca. 20‰ in carbon and ca. 10‰ in nitrogen isotopic compositions. These variations are of complex origin. Although different in magnitude, major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the Last Glacial Maximum. The Allerød-Younger Dryas period also seems to have been marked by generally dry conditions in this part of tropical West Africa. Nitrogen isotopic evidence suggests that during the period 9.2-3.2 kyr the lake had an extremely stable water column, probably due to the absence of a windy or cool, dry season, or both. Regular circulation of the water column recommenced during the late Holocene and has persisted until the present day. © 1992.

0b93375f-1108-356f-9ce5-ab94a476f2d4
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Abstract: Radiocarbon analyses of pollen, ostracodes, and total organic carbon (TOC) provide a reliable chronology for the sediments deposited in Bear Lake over the past 30,000 years. The differences in apparent age between TOC, pollen, and carbonate fractions are consistent and in accord with the origins of these fractions. Comparisons among different fractions indicate that pollen sample ages are the most reliable, at least for the past 15,000 years. The post-glacial radiocarbon data also agree with ages independently estimated from aspartic acid racemization in ostracodes. Ages in the red, siliclastic unit, inferred to be of last glacial age, appear to be several thousand years too old, probably because of a high proportion of reworked, refractory organic carbon in the pollen samples. Age-depth models for five piston cores and the Bear Lake drill core (BL00-1) were constructed by using two methods: quadratic equations and smooth cubic-splinefits. The two types of age models differ only in detail for individual cores, and each approach has its own advantages. Specific lithological horizons were dated in several cores and correlated among them, producing robust average ages for these horizons. The age of the correlated horizons in the red, siliclastic unit can be estimated from the age model for BL00-1, which is controlled by ages above and below the red, siliclastic unit. These ages were then transferred to the correlative horizons in the shorter piston cores, providing control for the sections of the age models in those cores in the red, siliclastic unit. These age models are the backbone for reconstructions of past environmental conditions in Bear Lake. In general, sedimentation rates in Bear Lake have been quite uniform, mostly between 0.3 and 0.8 mm yr-1 in the Holocene, and close to 0.5 mm yr-1 for the longer sedimentary record in the drill core from the deepest part of the lake. Copyright © 2009 The Geological Society of America.

705ea60b-322b-33a6-9258-14da6923ddc2
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Abstract: Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (δ13C and δ15N), and Hydrogen Index (HI). These data are combined with published biogenic silica and water content analyses to produce a detailed palaeolimnological history of the lake over the past ca. 17.5 ka. Late Pleistocene desiccation produced a lake-wide discontinuity marked by a vertisol. Sediments below the discontinuity are characterised by relatively low TOC and HI values, and high C/N, δ13C and δ15N, reflecting the combined influence of abundant terrestrial plant material and generally unfavourable conditions for organic matter preservation. A thin muddy interval with lower δ13C and higher HI and water content indicates that dry conditions were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly flooded. Abundant macrophytic plant debris and high TOC and δ13C values at the upper vertisol surface probably reflect a marginal swamp. δ13C values decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequate supply of nitrogen, but increasing δ15N values reflect intense utilisation of the lake's DIN reservoir, probably due to a dramatic rise in productivity as nutrients were released to the lake from the flooded land surface. An abrupt drop in δ13C and δ15N values around 13.8-13.6 ka reflects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and δ13C values fell as 12C-rich CO2 released by bacterial decomposition of the organic material was brought into the epilimnion. A weak drop in HI values suggests greater oxygen supply to the hypolimnion at this time. Better mixing was probably due to increased wind intensity and may mark the onset of the Younger Dryas in the region. After the period of deep mixing, the water column became more stable. TOC, C/N, δ13C and HI values were at a maximum during the period between 10 and 4 ka, when the lake probably had a stratified water column with anoxic bottom waters. A gradual decrease in values over the last 4000 yrs suggest a change to a more seasonal climate, with periodic mixing of the water column. Rising sediment accumulation rates and a trend to more uniform surface water conditions over the last 2000 yrs are probably a result of increased anthropogenic impact on the lake and its catchment. Following a maximum at the time of the rapid lake-level rise during the terminal Pleistocene, δ15N has remained relatively low and displays a gradual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of correlation between δ13C and δ15N, and the absence of any evidence for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the principal source of nitrogen throughout the Holocene. The importance of atmospheric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.

f592acb3-8e1f-3797-9cc2-f93563f0b01c
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Abstract: Lake Bosumtwi is one of the most widely studied palaeoclimate archives in West Africa. Results from numerous AMS 14C dates of samples from four piston cores from Lake Bosumtwi show that an abrupt sedimentary transition from a mid-Holocene sapropel to calcareous laminated muds occurred at about 3200 cal yr B.P. High-resolution analyses of the nitrogen isotopic composition of organic matter across this transition confirm its abrupt nature, and suggest that the change may signal a step toward increased aridity and intensified surface winds that affected western equatorial Africa from Ghana to the Congo basin. Northern and Eastern Africa experienced a similar abrupt shift toward aridity during the late Holocene, but at about 5000 cal yr B.P., a difference in timing that illustrates the regional nature of climate changes during the Holocene and the importance of feedback mechanisms in regulating Holocene climate variability. Furthermore, an abrupt change at about 3000 cal yr B.P. occurs at several sites adjacent to the tropical and subtropical Atlantic, which may hint at major changes in the surface temperatures of the tropical Atlantic and/or Pacific at this time. © 2003 University of Washington. Published by Elsevier Inc. All rights reserved.

75470f94-73f5-3f0d-964e-5c557a6cb359
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Abstract: Results are presented from the analysis of three sediment cores from Lake Bosumtwi using mineralogy, sedimentology, palynology and grass cuticle content. The longest core is 16.9 m long and spans approximately the last 27 500 yr of the lake's history. The oldest part of the sequence, from c.27 500 to 24 500 BP, is poorly understood, but seems to contain evidence of both high and low water levels. Between 23 000 and 19 500 BP the lake water was relatively dilute, but became more saline and alkaline between 19 000 and 16 000 BP when the lake level must have been low. Regressions of the lake are registered just before 13 500 BP, at c.3750 BP and after 2000 BP. Maximum lake level seems to have occurred between 9000 and 4000 BP. Grass cuticle fragments are prominent in sediments deposited before c.9000 BP, confirming the pollen evidence that grassland formerly existed around the Bosumtwi crater. -from Authors

4a2da9c7-fbff-3664-a07e-9ccb8d51e94a
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Abstract: Optical and geochemical techniques were applied to organic matter of Late Quaternary sediments from Lakes Victoria and Rukwa, East Africa. Variations in total organic carbon (TOC) and hydrogen content (expressed as Hydrogen Index, HI) can be related to known changes in lake level and used to identify periods of very low water or subaerial exposure in basinal sequences. TOC and HI both decline as an exposure surface is approached, mainly due to the selective removal of unstable components by bacterial respiration and inorganic oxidation. Inert organic material of very low HI is typically concentrated at the overlying transgression surface. The carbon isotopic composition of the particulate organic matter tends to be less negative in sediments associated with exposure surfaces. Although weathering may be partially responsible, isotopic variations are mainly due to changes in the relative importance of material from plants using C4 photosynthesis, such as grasses and sedges, growing around the shore and on the exposed lake muds, and of material from planktonic algae and terrestrial C3 plants. © 1989.

cad52eec-294c-3fc4-825f-bd58d2d6c1f4
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Abstract: We present preliminary results from the study of 23.50-m core from Lake Barombi Mbo, representing the last 25,000 years. The lake is in an explosion crater formed during Quaternary time. The very laminated sediment is composed mostly of clay containing 5-10% organic carbon. Each couplet is commonly composed of a basal lamina rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper lamina often with siderite. A perturbed section near the base of the core, before ca. 21,000 yr B.P., could be the result of a violent release of gas, such CO2, comparable to the recent Nyos gas eruption. The paleomagnetic studies exhibit high-frequency oscillations interpreted as paleosecular variations of the local geomagnetic field. This first record obtained on the African continent can be closely compared to the type record obtained in Western Europe. The pollen results demonstrate the presence of a forest refuge in West Cameroon during the last great arid period, ca. 18,000 yr B.P. When equatorial forest was broken up, elements of montane vegetation spread to the lowlands. These phenomena resulted from a drying and cooling climate. © 1990.

8c30e66b-4d67-3aae-8418-55683bf3d408
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Abstract: About 150 km of high-resolution, seismic reflection (Compressed High-Intensity Radar Pulse) profiles (approximately 20 m penetration) were collected in Mono Lake in order to define the uppermost sedimentary architecture of the basin, which has been heavily impacted by recent volcanic, tectonic, and climatic processes. The study also provides an important background for ongoing efforts to obtain paleoenvironmental records from sediment cores in the lake. The history of four seismic-stratigraphic units in the upper 20 m of section are inferred from the data, and the interpretations are generally consistent with previous interpretations of lake history for the past 2000 years, including a major lowstand at 1941 m. No shorelines below the previously documented major lowstand at 1941 m were found. A relatively steep slope segment, whose toe is at about 1918 m, and which occurs on the southern and western margins of the deep basin of the lake, is interpreted as the relict foreset slope of deposition from prograding western tributaries. This topography is unconformably overlain by a unit of interbedded tephra and lake sediments of variable lithology, which contains tephra of the North Mono (600–625 cal yr BP) eruption in its upper part. The tephra-rich unit is overlain by a mostly massive mudflow deposit that is locally more than 18 m thick and that is distributed in a radial pattern around Paoha Island. The evidence suggests that within the past few hundred years, rapid uplift of Paoha Island through thick, preexisting lake deposits led to widespread slope failures, which created a terrain of disrupted, intact blocks near the island, and a thick, fluid mudflow beyond. As is common in mudflows, the mudflow moved up the depositional slope of the lake floor, terminating against the preexisting slopes, likely in multiple surges. Since about 1700 Common Era, fine-grained, well-laminated sediments have accumulated in the deep parts of the lake at anomalously rapid rates, probably driven by continued rapid, small-scale shedding of sediment from Paoha Island.

2c3a3999-b087-3db8-9516-e0a86b108000
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Abstract: 

47d70a83-0fa4-3b04-a2c8-06f195f4520a
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Abstract: New biomarker analyses from Lake Albert, East Africa spanning ~15-9ka show the most extreme, abrupt, multi-stage climate and environmental shifts during the last deglacial transition of anywhere in Africa. Records of hydroclimate expressed in compound specific δD values from terrestrial leaf waxes and a TEX86 paleotemperature record support multiple stages of pronounced drying and cooling from 13.8 to 11.5ka and demonstrate the dynamic behavior of the low latitude tropics during the deglaciation. The vegetation response, illustrated by compound specific δ13C values and fossil pollen records, was an expansion of C4 grassland when the region was cool and arid. These results advance our understanding of a spatially and temporally complex regional response to global climate forcing, suggesting weakening of the Indian Ocean monsoon at the end of the Pleistocene that coincides with a minor decrease in the rate of the Atlantic Meridional Overturning Circulation (AMOC) and during a time of stepwise cooling in the northern high latitudes.

b28135e4-65bd-38e0-be44-a37d3d7ec068
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Abstract: Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416-16421]. This resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an approximately 125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (approximately 35-15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa.

6c42f1f7-095c-3e53-9a46-2d2defbb1f31
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Abstract: Analyses of carbon isotopic ratios in concomitant bulk sediment samples and fossil grass epidermal fragments from Lake Bosumtwi, Ghana, demonstrate that both records reflect paleohydrologic variability. However, the bulk sediment signal is dominated by within-lake processes, whereas the fossil grass epidermal record provides the terrestrial vegetation response to changes in available moisture. The direction of change is similar, but the magnitude and timing of response are different. During the terminal Pleistocene, the aquatic record shows a dramatic, flip-flop behavior (bulk sediment δ13C: -4 to -32‰), while the terrestrial grass epidermis record is much more muted (δ13C shifts from -11 to -15‰). Furthermore, during the transition to the relatively moister conditions of the Holocene, a dominance of C4 plants persisted for at least 80000 years after limnological changes began. On the other hand, the epidermal isotopic record shows a much more dramatic response than that provided by the bulk sediment to the onset of drier, more seasonally contrasted conditions during the late Holocene. These results emphasize the need to consider varying response times of the biogeochemical systems that control the production of proxies, especially when attempting to correlate widely separated records based upon fundamentally different proxies. Copyright 2003 by the American Geophysical Union.

30996725-2661-34c5-a0c6-c97e6122b9a2
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Abstract: We present preliminary results from the study of a 23.50 m core (BM-6) representing the last 25 000 years. The core was collected in Barombi Mbo, an explosion crater lake formed probably during the Quaternary. The finely laminated sediment are composed mostly of dark brown to green clay rich organic matter (5-10% organic carbon). Each couplet is commonly composed of a lower unit rich in quartz, plant debris, muscovite and sponge spicules, and of a more clayey upper unit often with siderite (FeCO3) crystals. The average periodicity for one couplet is between 6 and 20 radiocarbon years. The pollen results, which are compared with those of another forested site in Ghana, demonstrate the presence of a forest refuge in West Cameroon during the last major arid period, about 18 000 years BP. At the same time that equatorial forest was broken up, elements of montain vegetation descended to the lowlands. To provide an explanation for these phenomena marked by a drying and cooling of the climate, modern examples of extensions of montain biotopes to low altitude are described. These localized extensions are due to the persistence of cloud cover, often of stratiform type, generated over the relatively cold water of ocean upwellings. Such lowering of sea-surface temperature might be the primary regional cause of the changes of climate and vegetation that occurred in humid tropical Africa. The upwelling, presently synchronous throughout the Guinea Gulf, amplify the trade winds, which could account for the observed changes inland. © 1991.

a5ad8187-cb80-309c-a1e7-fd19b300f0cf
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Abstract: During the summer of 2011, the Bighorn Basin Coring Project (BBCP) recovered over 900m of overlapping core from 3 different sites in late Paleocene to early Eocene fluvial deposits of northwestern Wyoming. BBCP cores are being used to develop high-resolution proxy records of the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM2) hyperthermal events. These events are short-term, large magnitude global warming events associated with extreme perturbations to the earth's carbon cycle. Although the PETM and ETM2 occurred ~ 55-52 million years ago, they are analogous in many ways to modern anthropogenic changes to the carbon cycle. By applying various sedimentological, geochemical, and palynological methods to the cores, we hope to better understand what caused these events, study the biogeochemical and ecological feedbacks that operated during them, and reveal precisely how they impacted continental environments. Core recovery was > 98% in all holes and most drilling was carried out without fluid additives, showing that continuous coring of continental smectitic deposits like these can be achieved with minimal risk of contamination to molecular biomarkers. Cores were processed in the Bremen Core Repository where the science team convened for 17 days to carry out data collection and sampling protocols similar to IODP projects. Initial results show that the weathered horizon extends to as much as ~ 30m below the surface and variations in magnetic susceptibility within the cores record an interplay between grain size and pedogenesis. Previous investigations of outcrops near the BBCP drill sites allow detailed evaluation of the effects of weathering on common proxy methods. Studies of lithofacies, organic geochemistry, stable isotope geochemistry, calibrated XRF core scanning, paleomagnetics, and palynology are underway and will represent the highest resolution and most integrated proxy records of the PETM from a continental setting yet known. An extensive outreach program is in place to capitalize on the educational value associated with the Bighorn Basin's unusually complete record of Phanerozoic earth history. © Author(s) 2013.

ea8bdad4-6675-3a95-86a8-cfca4743686b
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Abstract: Biomarkers of aquatic algae and compound-specific carbon isotopes are used to examine past variations in algal community composition and primary productivity in Lake Malawi. We note major changes at the Pleistocene-Holocene boundary. From the Last Glacial Maximum until ∼11.8 calendar years before present (kyr B.P.), Lake Malawi was characterized by either low rates of primary productivity or algal productivity that was dominated by a group other than diatoms. At the onset of the Holocene, conditions similar to those of modern Lake Malawi were initiated, with diatoms and nitrogen-fixing cyanobacteria becoming more important contributors to primary productivity. The transition at ∼11.8 kyr B.P. is likely related to a shift in the dominant wind direction over Lake Malawi, resulting from a southward shift in the mean latitudinal position of the Inter-tropical Convergence Zone during the last glacial. Throughout the 23-kyr B.P. record, the effects of wind-induced upwelling are important and may be the main control on the carbon isotopic composition of algal lipids through delivery of 13C-depleted CO2, derived from organic matter decomposition in deeper waters, to the photic zone. Relationships are also suggested between thermal stratification and primary productivity, with cool conditions at ∼12 and 8 kyr B.P. resulting in weaker thermal stratification and increased primary productivity. The opposite situation occurs at 4.9 kyr B.P., when significantly warmer temperatures and decreased algal productivity are observed. Thus, in lentic systems such as Lake Malawi the major influence of climate on algal ecology appears to be through feedbacks in physical circulation mechanisms. © 2009, by the American Society of Limnology and Oceanography, Inc.

3690e727-51ba-3d07-801a-f961810cef26
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Abstract: Relatively few well-dated and high-resolution paleoclimate records of the past few centuries presently exist from tropical East Africa. Here, we examine the bulk and molecular geochemical records of two varved sediment cores from Lake Malawi, which together provide a continuous record of environmental variability in East Africa of the last 730 years. We observe a number of changes in the aquatic ecosystem of Lake Malawi, which are likely attributed to both natural climatic forcing and anthropogenic activities. Biomarkers of dinoflagellates (dinosterol) and bacterivorous ciliates (tetrahymanol) display increased accumulation rates from ~ 1900 AD to the present, while a simultaneous decrease in accumulation rates of diatom biomarkers (isololiolide/loliolide) is observed. Increased accumulation rates of retene, a compound derived from conifers, are also noted since ~ 1930 AD and likely reflect increased soil erosion due to deforestation of the Lake Malawi watershed. Spectral analysis of the high-resolution TOC record indicates a periodicity of 204 years, similar to the 206 year cycle noted in 14C and 10Be records, suggesting a link between East African climate and solar forcing.

2cfb0315-0a34-3667-83dc-c6a0f3b368d1
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Abstract: The tropics play a major role in global climate dynamics, and are vulnerable to future climate change. We present a record of East African climate since 55 ka, preserved in Lake Malawi sediments, that indicates rapid shifts between discrete climate modes related to abrupt warming (D-O) events observed in Greenland. Although the timing of the Malawi events cannot be determined exactly, our age model implies that they occur prior to their Greenland counterparts, consistent with southward excursions of the Intertropical Convergence Zone during Greenland stadials. The magnitude of each of the events recorded in Malawi sediments corresponds to the scale of the subsequent Greenland warming. This suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw. Copyright 2007 by the American Geophysical Union.

b6369e40-6ca5-3b96-a22b-6fe41c091b00
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Abstract: Plant leaf wax carbon isotopes provide a record of C3 versus C4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and an antiphase relationship prevailed. The abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the Intertropical Convergence Zone (ITCZ) during the last glacial. Southward displacements of the ITCZ are also linked to arid conditions in southeast Africa during the Younger Dryas and the Little Ice Age. © 2007 The Geological Society of America.

65f1d6d4-8ab9-3d1d-bc11-f53d2d6e8125
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Abstract: Oxygen and carbon isotopes from a continuous, 120-m-long, carbonate-rich core from Bear Lake, Utah-Idaho, document dramatic fluctuations in the hydrologic budget of the lake over the last 250,000 yr. Isotopic analyses of bulk sediment samples capture millennial-scale variability. Ostracode calcite was analyzed from 78 levels, mainly from the upper half of the core where valves are better preserved, to compare the isotopic value of purely endogenic carbonate with the bulk sediment, which comprises both endogenic and detrital components. The long core exhibits three relatively brief intervals with abundant endogenic aragonite (50±10%) and enriched δ18O and δ13C. These intervals are interpreted as warm/dry periods when the lake retracted into a topographically closed basin. We correlate these intervals with the interglacial periods of marine oxygen-isotope stages 1, 5e, and 7a, consistent with the presently available geochronological control. During most of the time represented by the core, the lake was fresher than the modern lake, as evidenced by depleted δ18O and δ13C in bulk-sediment carbonate. © 2006 Elsevier Ltd. All rights reserved.

636b84b8-08d8-3c09-ac5e-9881345c137e
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Abstract: Between 10,500 and 9000 cal yr BP, δ18O values of benthic ostracodes within glaciolacustrine varves from Lake Superior range from - 18 to - 22‰ PDB. In contrast, coeval ostracode and bivalve records from the Lake Huron and Lake Michigan basins are characterized by extreme δ18O variations, ranging from values that reflect a source that is primarily glacial (∼ - 20‰ PDB) to much higher values characteristic of a regional meteoric source (∼ - 5‰ PDB). Re-evaluated age models for the Huron and Michigan records yield a more consistent δ18O stratigraphy. The striking feature of these records is a sharp drop in δ18O values between 9400 and 9000 cal yr BP. In the Huron basin, this low δ18O excursion was ascribed to the late Stanley lowstand, and in the Lake Michigan basin to Lake Agassiz flooding. Catastrophic flooding from Lake Agassiz is likely, but a second possibility is that the low δ18O excursion records the switching of overflow from the Lake Superior basin from an undocumented northern outlet back into the Great Lakes basin. Quantifying freshwater fluxes for this system remains difficult because the benthic ostracodes in the glaciolacustrine varves of Lake Superior and Lake Agassiz may not record the average δ18O value of surface water. © 2009 University of Washington.

6c6f5aee-e62d-373d-8fa2-b84dff093e68
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ID: 6c6f5aee-e62d-373d-8fa2-b84dff093e68
Tag: []
Abstract: Understanding the evolution of Arctic polar climate from the protracted warmth of the middle Pliocene into the earliest glacial cycles in the Northern Hemisphere has been hindered by the lack of continuous, highly resolved Arctic time series. Evidence from Lake El'gygytgyn, in northeast (NE) Arctic Russia, shows that 3.6 to 3.4 million years ago, summer temperatures were ~8°C warmer than today, when the partial pressure of CO2 was ~400 parts per million. Multiproxy evidence suggests extreme warmth and polar amplification during the middle Pliocene, sudden stepped cooling events during the Pliocene-Pleistocene transition, and warmer than present Arctic summers until ~2.2 million years ago, after the onset of Northern Hemispheric glaciation. Our data are consistent with sea-level records and other proxies indicating that Arctic cooling was insufficient to support large-scale ice sheets until the early Pleistocene.

24eadb55-36e6-377b-bb30-2e7ad030c4d4
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ID: 24eadb55-36e6-377b-bb30-2e7ad030c4d4
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Abstract: The evolution of the early Great Lakes was driven by changing ice sheet geometry, meltwater influx, variable climate, and isostatic rebound. Unfortunately none of these factors are fully understood. Sediment cores from Fenton Lake and other sites in the Lake Superior basin have been used to document constantly falling water levels in glacial Lake Minong between 9,000 and 10,600 cal (8.1–9.5 ka) BP. Over three meters of previously unrecovered sediment from Fenton Lake detail a more complex lake level history than formerly realized, and consists of an early regression, transgression, and final regression. The initial regression is documented by a transition from gray, clayey silt to black sapropelic silt. The transgression is recorded by an abrupt return to gray sand and silt, and dates between 9,000 and 9,500 cal (8.1–8.6 ka) BP. The transgression could be the result of increased discharge from Lake Agassiz overflow or the Laurentide Ice Sheet, and hydraulic damming at the Lake Minong outlet. Alternatively ice advance in northern Ontario may have blocked an unrecognized low level northern outlet to glacial Lake Ojibway, which switched Lake Minong overflow back to the Lake Huron basin and raised lake levels. Multiple sites in the Lake Huron and Michigan basins suggest increased meltwater discharges occurred around the time of the transgression in Lake Minong, suggesting a possible linkage. The final regression in Fenton Lake is documented by a return to black sapropelic silt, which coincides with varve cessation in the Superior basin when Lake Agassiz overflow and glacial meltwater was diverted to glacial Lake Ojibway in northern Ontario.

618471d1-215a-3c07-af3b-48f302456efc
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ID: 618471d1-215a-3c07-af3b-48f302456efc
Tag: []
Abstract: We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand. The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035 +/- 170 cal. BP (calibrated years before 1950, ca 7950-8250 C-14 BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway. (C) 2004 Elsevier Ltd. All rights reserved.

6702a2c5-a599-3907-8d10-9ec00be9a52d
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ID: 6702a2c5-a599-3907-8d10-9ec00be9a52d
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Abstract: Decadal and centennial mean state changes in South American summer monsoon (SASM) precipitation during the last 2,300 years are detailed using an annually resolved authigenic calcite record of precipitation δ(18)O from a varved lake in the Central Peruvian Andes. This unique sediment record shows that δ(18)O peaked during the Medieval Climate Anomaly (MCA) from A.D. 900 to 1100, providing evidence that the SASM weakened considerably during this period. Minimum δ(18)O values occurred during the Little Ice Age (LIA) between A.D. 1400 and 1820, reflecting a prolonged intensification of the SASM that was regionally synchronous. After the LIA, δ(18)O increased rapidly, particularly during the current warm period (CWP; A.D. 1900 to present), indicating a return to reduced SASM precipitation that was more abrupt and sustained than the onset of the MCA. Diminished SASM precipitation during the MCA and CWP tracks reconstructed Northern Hemisphere and North Atlantic warming and a northward displacement of the Intertropical Convergence Zone (ITCZ) over the Atlantic, and likely the Pacific. Intensified SASM precipitation during the LIA follows reconstructed Northern Hemisphere and North Atlantic cooling, El Niño-like warming in the Pacific, and a southward displacement of the ITCZ over both oceans. These results suggest that SASM mean state changes are sensitive to ITCZ variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. Continued Northern Hemisphere and North Atlantic warming may therefore help perpetuate the recent reductions in SASM precipitation that characterize the last 100 years, which would negatively impact Andean water resources.

02aeed0f-c55c-3954-9d5b-9e7a0f1e2d82
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ID: 02aeed0f-c55c-3954-9d5b-9e7a0f1e2d82
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Abstract: The termination of the African Humid Period (AHP) about 5 thousand years ago (ka) was the most dramatic climate shift in northern and equatorial Africa since the end of the Pleistocene. Based on TEX86 paleotemperature data from Lake Turkana, Kenya, we show that a temperature shift of 2–4 °C occurred over the two millennia spanning the end of the AHP, with the warmest conditions occurring at ∼5 ka. We note a similar shift, though of a smaller magnitude, in other East African temperature records from Lakes Malawi and Tanganyika, as well as Mt. Kilimanjaro. Additionally, we document the temperature history for the last 220 years from Lake Turkana that indicates the thermal anomaly at 5 ka was warmer than the present day Lake Turkana temperatures and on par with modern temperatures of Lakes Tanganyika and Malawi. We suggest that the thermal response at the end of the AHP may be linked to local insolation during September–November, when local air temperature rises to an annual maximum over Lakes Malawi and Tanganyika and a secondary maximum over Lake Turkana and Mt. Kilimanjaro. September–November insolation peaked at ∼5 ka and likely caused air and water temperatures in the region to rise to maxima at that time.

779dd372-835a-35ee-9e3f-29c5f6a2139c
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ID: 779dd372-835a-35ee-9e3f-29c5f6a2139c
Tag: []
Abstract: The Holocene temperature history of Iceland is not well known, despite Iceland's climatically strategic location at the intersection of major surface currents in the high-latitude North Atlantic. Existing terrestrial records reveal spatially heterogeneous changes in Iceland's glacier extent, vegetation cover, and climate over the Holocene, but these records are temporally discontinuous and mostly qualitative. This paper presents the first quantitative estimates of temperatures throughout the entire Holocene on Iceland. Mean July temperatures are inferred based upon subfossil midge (Chironomidae) assemblages from three coastal lakes in northern Iceland. Midge data from each of the three lakes indicate broadly similar temperature trends, and suggest that the North Icelandic coast experienced relatively cool early Holocene summers and gradual warming throughout the Holocene until after 3 ka. This contrasts with many sites on Iceland and around the high-latitude Northern Hemisphere that experienced an early to mid-Holocene "thermal maximum" in response to enhanced summer insolation forcing. Our results suggest a heightened temperature gradient across Iceland in the early Holocene, with suppressed terrestrial temperatures along the northern coastal fringe, possibly as a result of sea surface conditions on the North Iceland shelf. © 2007 Elsevier Ltd. All rights reserved.

ae8ebb3a-0455-348a-aa64-f5bf6a65b76c
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Abstract: A sedimentary record from lake Stora Vioarvatn in northeast Iceland records environmental changes over the past 2000 years. Downcore data include chironomid (Diptera: Chironomidae) assemblage data and total organic carbon, nitrogen, and biogenic silica content. Sample scores from detrended correspondence analysis (DCA) of chironomid assemblage data are well correlated with measured temperatures at Stykkishólmur over the 170 year instrumental record, indicating that chironomid assemblages at Stora Vioarvatn have responded sensitively to past temperature changes. DCA scores appear to be useful for quantitatively inferring past temperatures at this site. In contrast, a quantitative chironomid-temperature transfer function developed for northwestern Iceland does a relatively poor job of reconstructing temperature shifts, possibly due to the lake's large size and depth relative to the calibration sites or to the limited resolution of the subfossil taxonomy. The pre-instrumental climate history inferred from chironomids and other paleolimnological proxies is supported by prior inferences from historical documents, glacier reconstructions, and paleoceanographic studies. Much of the first millennium AD was relatively warm, with temperatures comparable to warm decades of the twentieth century. Temperatures during parts of the tenth and eleventh centuries AD may have been comparably warm. Biogenic silica concentrations declined, carbon:nitrogen ratios increased, and some chironomid taxa disappeared from the lake between the thirteenth and nineteenth centuries, recording the decline of temperatures into the Little Ice Age, increasing soil erosion, and declining lake productivity. All the proxy reconstructions indicate that the most severe Little Ice Age conditions occurred during the eighteenth and nineteenth centuries, a period historically associated with maximum sea-ice and glacier extent around Iceland. © 2008 Springer Science+Business Media B.V.

0afca877-9267-38a3-8e0b-6fefefc7fdd3
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ID: 0afca877-9267-38a3-8e0b-6fefefc7fdd3
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Abstract: The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding today's at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

002764be-70ca-3ec4-a6f5-7e0bc89ac8e9
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ID: 002764be-70ca-3ec4-a6f5-7e0bc89ac8e9
Tag: []
Abstract: Palynological, sedimentological and geochemical analyses performed on the Villarquemado paleolake sequence (987 m a.s.l, 40°30′N; 1°18′W) reveal the vegetation dynamics and climate variability in continental Iberia over the last 13,500 cal yr BP. The Lateglacial and early Holocene periods are characterized by arid conditions with a stable landscape dominated by pinewoods and steppe until ca. 7780 cal yr BP, despite sedimentological evidence for large paleohydrological fluctuations in the paleolake. The most humid phase occurred between ca. 7780 and 5000 cal yr BP and was characterized by the maximum spread of mesophytes (e.g., Betula, Corylus, Quercus faginea type), the expansion of a mixed Mediterranean oak woodland with evergreen Quercus as dominant forest communities and more frequent higher lake level periods. The return of a dense pinewood synchronous with the depletion of mesophytes characterizes the mid-late Holocene transition (ca. 5000 cal yr BP) most likely as a consequence of an increasing aridity that coincides with the reappearance of a shallow, carbonate wetland environment. The paleohydrological and vegetation evolution shows similarities with other continental Mediterranean areas of Iberia and demonstrates a marked resilience of terrestrial vegetation and gradual responses to millennial-scale climate fluctuations. Human impact is negligible until the Ibero-Roman period (ca. 2500 cal yr BP) when a major deforestation occurred in the nearby pine forest. The last 1500 years are characterized by increasing landscape management, mainly associated with grazing practices shaping the current landscape.

2119b0bd-b4b4-3e13-b74b-c0ac7858b8c5
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ID: 2119b0bd-b4b4-3e13-b74b-c0ac7858b8c5
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Abstract: The 318m thick lacustrine sediment record from Lake El'gygytgyn, northeastern Russian Arctic cored by the international El'gygytgyn Drilling Project provides unique opportunities for the time-continuous reconstruction of the regional paleoenvironmental history for the past 3.6 Myr. Pollen studies of the lower 216m of the lacustrine sediments demonstrate their value as an excellent archive of vegetation and climate changes during the Late Pliocene and Early Pleistocene. About 3.5-3.35 Myr BP, the vegetation at Lake El'gygytgyn, now an area of tundra was dominated by spruce-larch-fir-hemlock forests. After ca. 3.35 Myr BP dark coniferous taxa gradually disappeared. A very pronounced environmental change took place ca. 3.31-3.28 Myr BP, corresponding to the Marine Isotope Stage (MIS) M2, when treeless tundra- and steppe-like habitats became dominant in the regional vegetation. Climate conditions were similar to those of Late Pleistocene cold intervals. Numerous coprophilous fungi spores identified in the pollen samples suggest the presence of grazing animals around the lake. Following the MIS M2 event, larch-pine forests with some spruce mostly dominated the area until ca. 2.6 Myr BP, interrupted by colder and drier intervals ca. 3.043-3.025, 2.935-2.912, and 2.719-2.698 Myr BP. At the beginning of the Pleistocene, ca. 2.6 Myr BP, noticeable climatic deterioration occurred. Forested habitats changed to predominantly treeless and shrubby environments, which reflect a relatively cold and dry climate. Peaks in observed green algae colonies (Botryococcus) around 2.53, 2.45, 2.32-2.305, 2.20 and 2.16-2.15 Myr BP suggest a spread of shallow water environments. A few intervals (i.e., 2.55-2.53, ca. 2.37, and 2.35-2.32 Myr BP) with a higher presence of coniferous taxa (mostly pine and larch) document some relatively shortterm climate ameliorations during Early Pleistocene glacial periods. © Author(s) 2014.

f7a46e85-a8cb-3ed0-a350-ac66594fbd92
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Abstract: Radiocarbon-based age models from two lakes on Adak Island, Alaska, were compared using correlated tephra to check the accuracy of the radiocarbon ages and to develop a regional tephrochronology. Andrew and Heart Lakes are located adjacent to and 5km south of Mt. Moffett on the northern part of Adak Island in the central Aleutian Arc. The downcore trends in radiocarbon ages of macrofossils, the 1963 spike in plutonium (239+240Pu) activity, and a lead (210Pb) profile from Andrew Lake were modeled with smooth spline fits. Tephras were correlated based on magnetic susceptibility, particle morphology, and internal stratigraphy including color and particle size. Of the 19 pure tephras in Heart Lake and 21 in Andrew Lake, ten could be confidently correlated between the two lakes. The composite tephrostratigraphy from the two lakes includes at least 30 unique tephras deposited over 9600 years. Radiocarbon ages obtained from mixed aquatic and terrestrial macrofossils with ages hundreds of years too old were inferred for Heart Lake by correlated tephra ties to the Andrew Lake age model. The eroding tephra-soil deposits that crop out along the banks of Heart Lake, and its large shallow platform, may facilitate remobilization of previously stored sediments. Previous studies on Adak Island of archeological sites and tephra-soil outcrops described five tephras that are correlated with a subset of tephras from the lakes based on stratigraphic succession, inferred age, and physical description. Age-depth models for Heart and Andrew Lakes as well as tephrochronology for Adak Island provide geochronological context for future studies. © 2013 Elsevier B.V.

78f2ebff-a5d5-3efd-bde8-4d3c095ebe8f
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ID: 78f2ebff-a5d5-3efd-bde8-4d3c095ebe8f
Tag: []
Abstract: Geochemical analysis of the varved (annually laminated) sediments of a small, deep urban lake in east-central Minnesota shows that some water-quality and watershed indicators are approaching prehistoric (pre-1850s) values after an excursion to anomalous levels during the 1920s through 1950s. This high-resolution paleolimnological information (annually resolved for most of the 20th century), including varve thickness measurements on a digital image, carbon stable isotopic composition of organic matter, and sedimentary component quantification, provides historical perspective to lake managers planning remedial measures with reference to the "natural" state of the lake, especially in regard to the perception that water quality has declined only in recent decades (i.e., in the late 20th century). The lake is at present eutrophic and oligomictic, with oxygen-depleted bottom waters enriched in dissolved solids relative to surface waters; an alum treatment has contributed to the problem of persistent stratification at the same time as it has reduced phosphorus and increased transparency in lake surface waters. The sedimentary record shows the lake's strong response to agricultural, recreational, and urban development in the watershed, gradual improvement beginning at the time of sanitary sewer installation in the 1960s, and only a minor additional response to a concerted remediation effort in the mid-1980s to 1990s. Indices of terrestrial inputs and algal productivity show evidence of a lake that was at its most heavily impacted during the mid-20th century, and which has improved in some respects since the 1960s. © 2008 Copyright by the North American Lake Management Society.

568f6856-728e-3219-86ab-00756e877961
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ID: 568f6856-728e-3219-86ab-00756e877961
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Abstract: We report a 150,000 year record of the abundance of biogenic silica (BSi) in drill cores from the northern and central basins of Lake Malawi. The periods of highest diatom burial at both sites occurred around 65-69 and 51-60 thousand years ago (ka) after the termination of prolonged, intense drought in the region. These peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin. The droughts that preceded these BSi depositional episodes occurred at precessional frequency, corresponding to times of minimum austral spring (or summer?) insolation in the region prior to 60 ka. Subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity. However, biogenic silica records do not show significant sensitivity to precessional forcing. The BSi burial flux in the north basin displays strong millennial-scale variability since 50 ka, with peak values occurring during cold times in the Northern Hemisphere, as reflected in the Greenland ice core records. We observe no relationship between BSi and deposition of volcanic tephras in this system. The central basin core also displays millennial-scale variability in BSi abundance during the past 50 ky, of comparable magnitude to that in the north basin, but without systematic correlation to the north basin BSi or Greenland ice core records. In general, we conclude that BSi profiles from the north and (probably) south basins are more readily interpreted in terms of regional climate dynamics than are BSi profiles from more central locations in long, narrow tropical rift lakes. Diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low-latitude lacustrine systems.

a4932169-f7fe-35af-b32d-caca3465322c
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ID: a4932169-f7fe-35af-b32d-caca3465322c
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Abstract: Many paleoclimate and landscape change studies in the American Midwest have focused on the Late Glacial and early Holocene time periods (~ 16–11 ka), but little work has addressed landscape change in this area between the Last Glacial Maximum and the Late Glacial (~ 22–16 ka). Sediment cores were collected from 29 new lake and bog sites in Ohio and Indiana to address this gap. The basal radiocarbon dates from these cores show that initial ice retreat from the maximal last-glacial ice extent occurred by 22 ka, and numerous sites that are ~ 100 km inside this limit were exposed by 18.9 ka. Post-glacial environmental changes were identified as stratigraphic or biologic changes in select cores. The strongest signal occurs between 18.5 and 14.6 ka. These Midwestern events correspond with evidence to the northeast, suggesting that initial deglaciation of the ice sheet, and ensuing environmental changes, were episodic and rapid. Significantly, these changes predate the onset of the Bølling postglacial warming (14.8 ka) as recorded by the Greenland ice cores. Thus, deglaciation and landscape change around the southern margins of the Laurentide Ice Sheet happened ~ 7 ka before postglacial changes were felt in central Greenland.

3185e5b4-606a-31a9-8486-c25c757b20cd
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Abstract: We present results of analysis of biological (diatoms and ostracodes) and non-biological (Ti, Ca / Ti, total inorganic carbon, magnetic susceptibility) variables from an 8.8 m long, high-resolution (∼ 20 yr sample-1) laminated sediment sequence from Lake Santa María del Oro (SMO), western Mexico. This lake lies at a sensitive location between the dry climates of northern Mexico, under the influence of the North Pacific subtropical high-pressure cell and the moister climates of central Mexico, under the influence of the seasonal migration of the intertropical convergence zone and the North American monsoon (NAM). The sequence covers the last 2000 years and provides evidence of two periods of human impact in the catchment, shown by increases in the diatom Achnanthidium minutissimum. The first from AD 100 to 400 (Early Classic) is related to the shaft and chamber tombs cultural tradition in western Mexico, and the second is related to Post-Classic occupation from AD 1100 to 1300. Both periods correspond to relatively wet conditions. Three dry intervals are identified from increased carbonate and the presence of ostracodes and aerophilous Eolimna minima. The first, from AD 500 to 1000 (most intense during the late Classic, from AD 600 to 800), correlates with the end of the shaft and chamber tradition in western Mexico after ca. AD 600. This late Classic dry period is the most important climatic signal in the Mesoamerican region during the last 2000 years, and has been recorded at several sites from Yucatan to the Pacific coast. In the Yucatan area, this dry interval has been related with the demise of the Maya culture at the end of the Classic (AD 850 to 950). The last two dry events (AD 1400 to 1550 and 1690 to 1770) correspond with the onset of, and the late, Little Ice Age, and follow largely the Spörer and Maunder minima in solar radiation. The first of these intervals (AD 1400 to 1550) shows the most intense signal over western Mexico; however this pattern is different at other sites. Dry/wet intervals in the SMO record are related with lower/higher intensity of the NAM over this region, respectively.

6020ce99-372a-31df-9c51-ef18df95b572
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ID: 6020ce99-372a-31df-9c51-ef18df95b572
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Abstract: Processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences, but this approach often requires records that contain the start of primary succession. Most lakes in the upper Midwestern U.S. were formed by glaciers at the end of the last Ice Age approximately 11,700 cal yr BP. Devils Lake, Wisconsin is a rare example of a lake from this region whose sediments extend into the Pleistocene and may include the Last Glacial Maximum. Sediment magnetic, geochemical, pollen, and charcoal records were generated from a 10 m core whose basal sediments may be 28,000 years old. Together with a previously published pollen record, these proxies combine to reveal a history of long-term climatic, vegetative and geologic change during the late Pleistocene to Holocene. We identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site. Productivity in the lake was low during the late Pleistocene and increased during the Holocene, as reflected by the sediment lithology, which shows a sudden shift from glacial vivianite-rich and organic-poor clastic-dominated sediments to Holocene diatomaceous sapropels. Several important processes initiated around 17,000 cal yr BP, including the onset of organic matter accumulation and fire in the terrestrial ecosystem. However, the post-glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation, likely a spruce tundra, survived near the site. A switch to a hardwood forest period during the Holocene also led to a change in the fire regime, with increased frequency of burning. Aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record. Nutrient cycling (as recorded by sedimentary δ15N) was variable but not directional, and appeared to be correlated with climate conditions early in the record, and terrestrial ecosystem processes later in the record. Throughout the Holocene magnetic mineral concentration decreased as productivity increased, and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50% derived from soil or biogenic sources. Magnetic grain size, Ambrosia pollen percentages, and charcoal concentration increased and δ15N decreased in the most recent part of the record, due to anthropogenic influence in the region including agricultural activities.

13c87aaf-0ece-30ea-a71e-ac9c58ef2f1e
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ID: 13c87aaf-0ece-30ea-a71e-ac9c58ef2f1e
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Abstract: This paper focuses on the characterization and genesis of mass movement deposits (MMDs) in the Quaternary and Pliocene sediments of Lake El'gygytgyn, Far East Russian Arctic. Three partly overlapping holes were drilled into the 320 m long sediment record at International Continental Scientific Drilling Program (ICDP) Site 5011-1 in the lake basin, recovering the Quaternary almost completely, and the Pliocene down to 3.6 Ma with 52% recovery. Mass movement deposits were investigated in all three cores, based on macroscopical core descriptions, radiographic images, high-resolution magnetic susceptibility and gamma-ray density. Five different types of MMDs were identified: turbidites, grain-flow deposits, debrites, slumps and slides. These are formed by transitional mass movement processes, and thus can be co-generic. An initial slope failure is thought to transform into a debris flow that deforms frontal sediments, partly disintegrates and dilutes into a turbidity flow. Turbidites are by far the most frequent MMD type in the lake center. They occur throughout the record in all pelagic sedimentary facies, but they are thinner in facies formed during cold climate conditions. All other MMDs, by contrast, incise exclusively the pelagic facies deposited during warm climates. In the 123 m thick Quaternary composite sediment record 230 mass movement events are identified, comprising 33% of the sediment length. Turbidites contribute 93% of the number of Quaternary MMDs, but only 35% of their thickness. In the Pliocene sediments between 123 and 320 m, 181 additional mass movement deposits are identified, which constitute ∼33% of the recovered sediments. The mean recurrence interval for MMDs is 11 and 5 ka in the Quaternary and Pliocene, respectively. © Author(s) 2013.

491f8ccf-1ba4-352d-aecd-2fa0d9847bc8
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ID: 491f8ccf-1ba4-352d-aecd-2fa0d9847bc8
Tag: []
Abstract: A 136-m-long drill core of sediments was recovered from tropical high-altitude Lake Titicaca, Bolivia-Peru, enabling a reconstruction of past climate that spans four cycles of regional glacial advance and retreat and that is estimated to extend continuously over the last 370,000 yr. Within the errors of the age model, the periods of regional glacial advance and retreat are concordant respectively with global glacial and interglacial stages. Periods of ice advance in the southern tropical Andes generally were periods of positive water balance, as evidenced by deeper and fresher conditions in Lake Titicaca. Conversely, reduced glaciation occurred during periods of negative water balance and shallow closed-basin conditions in the lake. The apparent coincidence of positive water balance of Lake Titicaca and glacial growth in the adjacent Andes with Northern Hemisphere ice sheet expansion implies that regional water balance and glacial mass balance are strongly influenced by global-scale temperature changes, as well as by precessional forcing of the South American summer monsoon. © 2007 University of Washington.

23f76c08-2f7d-3eb8-b936-cae696008c0d
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ID: 23f76c08-2f7d-3eb8-b936-cae696008c0d
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Abstract: Few records in the alpine landscape of western North America document the geomorphic and glaciologic response to climate change during the Pleistocene–Holocene transition. While moraines can provide snapshots of glacier extent, high-resolution records of environmental response to the end of the Last Glacial Maximum, Younger Dryas cooling, and subsequent warming into the stable Holocene are rare. We describe the transition from the late Pleistocene to the Holocene using a ~ 17,000-yr sediment record from Swiftcurrent Lake in eastern Glacier National Park, MT, with a focus on the period from ~ 17 to 11 ka. Total organic and inorganic carbon, grain size, and carbon/nitrogen data provide evidence for glacial retreat from the late Pleistocene into the Holocene, with the exception of a well-constrained advance during the Younger Dryas from 12.75 to 11.5 ka. Increased detrital carbonate concentration in Swiftcurrent Lake sediment reflects enhanced glacial erosion and sediment transport, likely a result of a more proximal ice terminus position and a reduction in the number of alpine lakes acting as sediment sinks in the valley.

3e5bef6e-31ef-31ff-8849-2e120ee21544
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ID: 3e5bef6e-31ef-31ff-8849-2e120ee21544
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Abstract: Iceberg Lake, a glacier-dammed proglacial lake in southern Alaska, contains a 1,500+ year varve record complicated by a history of episodic lake-level changes associated with fluctuations in ice-dam thickness and position. To better understand the basinwide glaciolacustrine response to late Holocene climate variability, we collected five cores from two areas in the lake, including a previously unexamined deepwater area distal from inlet streams. Based on eight AMS 14 C dates, and correlations among our cores and previously documented outcrops, we describe ~1,000 years of stratigraphy from each area. Deposition at both areas was dominated by fine-grained varves, but cores from the distal area uniquely contain coarser deposits, including rhythmites and graded sand beds, that we attribute to deposition of a subaqueous outwash fan-delta between ~1250 and 1650 AD. We attribute this event to thickening of the impounding glacier and consequent incursion of the glacier margin, and an associated lateral moraine, into the lake. This result suggests an early onset of the Little Ice Age (LIA) glacial advance in this region. Changes in basinwide circulation and sedimentation associated with this event probably caused minor thickening of varves used previously to reconstruct summer temperatures, reducing sensitivity of that record to early LIA cooling. The basinwide impact of this event illustrates the potentially significant spatial and temporal variability of lacustrine sedimentary processes in dynamic glacial landscapes.

76748791-5f4a-3454-862b-e9916448b8d2
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ID: 76748791-5f4a-3454-862b-e9916448b8d2
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Abstract: Forcing mechanisms of tropical climate in continental areas remain poorly understood, due in large part to a lack of continuous, long-term, high-fidelity records. Sediment core T97-52V from Lake Tanganyika provides new insight into the timing and mechanisms behind East African climate change over the past 90+ kyr. This record is particularly important, because, other than a recently recovered scientific drill core from Lake Malawi, there are no other continuous, well-dated records from East Africa prior to 60 ka. The high resolution age model presented here provides a large degree of age certainty for the past 45+ kyr, and our suite of proxies allows a thorough examination of Lake Tanganyika's dynamics. From core stratigraphy and chemical analyses, we present evidence of a lake level drop greater than 400 m sometime prior to ~ 90 ka, much greater than that inferred for the LGM, suggesting a period of intense aridity sometime around 100 ka. Additionally, core T97-52 V preserves evidence of worm burrows detected by X-radiographic imagery as indicated by burrow-shaped deposits of iron oxide, indicating a shallow lake at the time of deposition of that material. Intermittently high lake levels between ~ 78 ka and ~ 72 ka developed at the same time as a weakened Asian monsoon and a pluvial phase in Northeast Brazil, suggesting a global reorganization of climate, possibly forced by a reduction in orbital eccentricity. Over the past 60 ka this core preserves the same events recorded in a core collected ~ 100 km away in the southern basin of Lake Tanganyika, including an unexplained increase in biogenic silica at ~ 37 ka, suggesting that this vast lake is responding coherently across both major bathymetric basins to regional and global climate forcing.

a0cda8d8-3ec9-3824-816d-ed7d25854412
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Abstract: The global cycling of anthropogenic trace metals intensified during the twentieth century, impacting aquatic systems throughout the world. There are, however, few quantitative records showing the history of this contamination in large rivers. Here we present a well-dated sedimentary record of trace metal accumulation in Lake St. Croix, a natural riverine lake on the St. Croix River (Minnesota and Wisconsin, USA), revealing the history of heavy metal inputs to the river over the past 200 years. Concentrations of Hg, Pb, Ag, Cd, Cr and Zn and stable Pb isotopes were measured in eight 210Pb-dated sediment cores collected from profundal depositional areas throughout the lake. Time trends of trace metal concentrations and accumulation rates differed greatly between the upper lake (above Valley Creek) and the lower lake, reflecting contrasting sediment sources along the flow axis of the lake. For most of the study period (1800–2000 AD), sediment deposited throughout the lake derived almost exclusively from the suspended sediment load carried by the main-stem river into the lake. From 1910 through 1970, however, large inputs of eroded soils and stream channel sediments from side-valley tributaries resulted in greatly increased sediment and trace metal accumulation in the lower lake. Anthropogenic accumulation rates of Hg, Pb, Cd, Zn, and Ag in the upper lake correlate well with those from Square Lake, a small, relatively undisturbed nearby lake that has received trace metal inputs almost exclusively via atmospheric deposition. The similarity of these records suggests that atmospheric deposition was primarily responsible for trace metal accumulation trends in upper Lake St. Croix. Trace metal accumulation in the lower lake was also strongly influenced by atmospherically derived inputs, but metal contributions from native soils were important, as well, during the period of elevated sediment inputs from side-valley tributaries. Concentrations and accumulation rates of trace metals in both upper and lower lake sediments have decreased substantially since the 1970s due to decreased atmospheric inputs and sediment loadings, but accumulation rates remain well above pre-settlement values. Metal inputs to Lake St. Croix have been far lower than those to nearby Lake Pepin, located on the Mississippi River downstream of the Minneapolis-St. Paul metropolitan area, but there nevertheless remains a clear record of anthropogenic impact on the relatively pristine St. Croix River.

a010ef9d-7ce7-3e97-8f94-7e6b00bcbea9
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ID: a010ef9d-7ce7-3e97-8f94-7e6b00bcbea9
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Abstract: Fossil pollen analyses from northern Lake Malawi, southeast Africa, provide a high-resolution record of vegetation change during the Pleistocene/Holocene transition (~ 18–9 ka). Recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the Younger Dryas (YD). The Lake Malawi record tracks regional vegetation changes and allows comparison with other tropical African records identifying vegetation opening and local forest maintenance during the YD. Our record shows a gradual decline of afromontane vegetation at 18 ka. Around 14.5 ka, tropical seasonal forest and Zambezian miombo woodland became established. At ~ 13 ka, drier, more open formations gradually became prevalent. Although tropical seasonal forest taxa were still present in the watershed during the YD, this drought-intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways. The establishment of drought-tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after ~ 11.8 ka. The onset of the driest, most open vegetation type was coincident with a lake low stand at the beginning of the Holocene. This study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution.

74672ac9-b888-308d-9491-2637ce29b663
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ID: 74672ac9-b888-308d-9491-2637ce29b663
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Abstract: Paleoclimate proxy records reveal a pervasive cooling event with a Northern Hemispheric extent ~9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an ~45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly ~9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron–North Bay–Ottawa River–St. Lawrence River valleys.

ac0aacc4-94ed-3738-8f7c-ebdc297260b2
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ID: ac0aacc4-94ed-3738-8f7c-ebdc297260b2
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Abstract: The 318-m-thick sediment record from Lake El'gygytgyn provides unique opportunities for a detailed examination of environmental changes during the Réunion Subchron polarity reversal event (2.1384-2.1216. Myr. BP) in the northeastern Russian Arctic. The paper describes vegetation and climate fluctuations between ~. 2.15 and 2.10. Myr. BP as inferred from palynological data. Biome reconstructions indicate that throughout this interval the tundra (TUND) biome generally has higher affinity scores as compared to cold steppe (STEP) or cold deciduous forest (CLDE). An exception is the climatic optimum between ~. 2.139 and 2.131. Myr. BP, coinciding with Marine Isotope Stage 81 (approximately the Réunion Subchron), when the CLDE biome has the highest scores. Landscape-openness indices suggest that more closed vegetation characterized most of the interval between 2.146 and 2.127. Myr. BP, when deciduous forest and shrubs expanded in the regional vegetation and climate was relatively warm and wet. Peaks in green algal colonies (Botryococcus) and Zygnema-type spores ~. 2.150-2.146, ~. 2.131-2.123, and ~. 2.112-2.102. Myr. BP indicate expansions of shallow-water habitats and lowered lake levels. Comparisons with biome reconstructions from other interglacial intervals at Lake El'gygytgyn suggest that precession-related summer insolation intensity and obliquity-related duration of summer daylight are major controls on the onset of interglaciations, whereas obliquity probably plays a more significant role on vegetation succession at northern high latitudes during the Pleistocene.

2acbc0df-e9c6-38fa-b314-eb68170db875
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ID: 2acbc0df-e9c6-38fa-b314-eb68170db875
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Abstract: We present a TEX86-derived surface water temperature record for Lake Malawi that provides the first continuous continental record of temperature variability in the continental tropics spanning the past ~ 74 kyr with millennial-scale resolution. Average temperature during Marine Isotope Stage (MIS) 5A was 26.5 °C, with a range from 25.7 to 27.3 °C, comparable to Holocene temperatures. MIS 4 was a relatively cold period with temperatures generally decreasing from 25.5 °C at 68 ka to a minimum of 20 °C at ~ 60 ka, 1.5-2 °C colder than the Last Glacial Maximum (LGM). Termination of MIS 4 is characterized by a rapid increase of 3-4 °C in only ~ 0.5 kyr. Temperatures were relatively stable throughout MIS 3 at the resolution of this study, with an average of 23.8 °C and a range from 25.1 to 22.9 °C. The lack of millennial-scale temperature variability during MIS 3 suggests that Lake Malawi's documented response to the bipolar seesaw (Brown et al., 2007) is not reflected in its thermal history. Our temperature estimates for the LGM and Holocene are consistent with a previously published TEX86 record from Lake Malawi with a temperature of ~ 22.6 °C for the LGM, ~ 25-26 °C in the mid Holocene and ~ 25-28 for the late Holocene. In general the present extended TEX86 record indicates that temperature variability in tropical East Africa during late MIS 5 and MIS 4 was as great as that associated with the deglaciation and Holocene. A decrease in Southern Hemisphere insolation between 70 and 60 ka may have played an important role in forcing temperatures during MIS 4, but after 60 ka other factors, such as the extent of the polar ice sheets, or atmospheric CO2 may have forced temperature in tropical Africa to a greater extent than local summer insolation.

bf80d6d4-ff62-385a-8ab3-d0f13d805526
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ID: bf80d6d4-ff62-385a-8ab3-d0f13d805526
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Abstract: The Lake Kivu catchment in the East African Rift is subject to various geologic hazards, including frequent volcanic eruptions, earthquakes, and potential limnic overturns and degassing events. Integration of high-resolution seismic reflection data, 14C dated sediment cores, and lake-floor bathymetry reveals large axial and transverse turbidite systems in the eastern basin of the lake. The turbidites were sourced by hyperpycnal river flows during exceptional floods, and the temporal occurrence of the turbidites was climatically controlled. The turbidite record over the past ∼12 k.y. is correlated with the regional paleohydrologic records from tropical East Africa. Our study suggests that flood-introduced turbidites preserved in deep lakes are indicators of hydrological changes, and that extreme floods in Lake Kivu's recent history may have triggered deep mixing events. This study also has implications for the current degassing efforts in Lake Kivu; potential geologic hazards may be triggered by extraordinary turbidity currents, and need to be considered in the design and deployment of gas extraction facilities.

58592249-1c12-3617-a55d-9425973766ce
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ID: 58592249-1c12-3617-a55d-9425973766ce
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Abstract: Annually laminated (varved) Holocene sediments from Derby Lake, Michigan, display variations in endogenic calcite abundance reflecting a long-term (millennial-scale) decrease in burial punctuated with frequent short-term (decadal-scale) oscillations due to carbonate dissolution. Since 6000 cal yr B. P., sediment carbonate abundance has followed a decreasing trend while organic-carbon abundance has increased. The correlation between organic-carbon abundance and the sum of March-April-October-November insolation has an r(2) value of 0.58. We interpret these trends to represent a precession-driven lengthening of the Holocene growing season that has reduced calcite burial by enhancing net annual organic-matter production and associated calcite dissolution. Correlations with regional paleoclimate records suggest that changes in temperature and moisture balance have impacted the distribution of short-term oscillations in carbonate and organic-matter abundance superimposed on the precession-driven trends.

9ded25d9-ff07-3ed4-a726-f17036d8152f
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ID: 9ded25d9-ff07-3ed4-a726-f17036d8152f
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Abstract: The 3.6 Ma sediment record of Lake El'gygytgyn/NE Russia, Far East Russian Arctic, represents the longest continuous climate archive of the terrestrial Arctic. Its elemental composition as determined by X-ray fluorescence scanning exhibits significant changes since the mid-Pliocene caused by climate-driven variations in primary production, postdepositional diagenetic processes, and lake circulation as well as weathering processes in its catchment. During the mid-to late Pliocene, warmer and wetter climatic conditions are reflected by elevated Si/Ti ratios, indicating enhanced diatom production in the lake. Prior to 3.3 Ma, this signal is overprinted by intensified detrital input from the catchment, visible in maxima of clastic-related proxies, such as K. In addition, calcite formation in the early lake history points to enhanced Ca flux into the lake caused by intensified weathering in the catchment. A lack of calcite deposition after ca. 3.3 Ma is linked to the development of permafrost in the region triggered by cooling in the mid-Pliocene. After ca. 3.0 Ma the elemental data suggest a gradual transition to Pleistocene-style glacial-interglacial cyclicity. In the early Pleistocene, the cyclicity was first dominated by variations on the 41 kyr obliquity band but experienced a change to a 100 kyr eccentricity dominance during the middle Pleistocene transition (MPT) at ca. 1.2-0.6 Ma. This clearly demonstrates the sensitivity of the Lake El'gygytgyn record to orbital forcing. A successive decrease of the baseline levels of the redox-sensitive Mn/Fe ratio and magnetic susceptibility between 2.3 and 1.8 Ma reflects an overall change in the bottom-water oxygenation due to an intensified occurrence of pervasive glacial episodes in the early Pleistocene. The coincidence with major changes in the North Pacific and Bering Sea paleoceanography at ca. 1.8 Ma implies that the change in lake hydrology was caused by a regional cooling in the North Pacific and the western Beringian landmass and/or changes in the continentality. Further increases in total organic carbon and total nitrogen content after ca. 1.6 Ma are attributed to reduced organic matter decay in the sediment during prolonged anoxic periods. This points to more extensive periods of perennial ice coverage, and thus, to a progressive shifts towards more intense peak glacial periods. In the course of the Pleistocene glacial-interglacial sequence eight so-called "super-interglacials" occur. Their exceptionally warm conditions are reflected by extreme Si/Ti peaks accompanied by lows in Ti, K, and Fe, thus indicating extraordinary high lake productivity. © 2014 Author(s).

39606956-995a-3bd8-853d-c8d1f974d498
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ID: 39606956-995a-3bd8-853d-c8d1f974d498
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Abstract: Terrestrial plant communities have the potential to respond to climate change rapidly, if dominant species are killed by a series of extreme events, or slowly, if the cumulative effects of shorter-term climate fluctuations result in long-term compositional change. We used pollen and charcoal records from a lake and a testate amoebae-derived history of water-table depth in a nearby peatland to assess the response of the jack pine-dominated forests of northwestern Wisconsin to the climate variability of the last ~2000 years. The hydrology record and the charcoal record indicate that the climate near Warner Lake over the last ~2000 years was characterized by multidecadal variation in moisture availability with no apparent multicentennial-long trends in moisture balance or fire frequency. However, the pollen record suggests that there were multicentennial-scale changes in the vegetation composition around Warner Lake. Direct comparison of the three proxy records is challenging, because of their differing temporal resolutions and the complexity of potential ecological responses to climate variability. Therefore, we developed an interpretive model to compare multiple simulated proxies under two scenarios of environmental variability in order to determine under what conditions apparently contradictory records are likely to be found. The interpretive model reveals that a record of multicentennial-long change in vegetation is possible if multidecadal climate variability interacts with ecological processes influencing the direction and magnitude of succession. Compositional changes in the Warner Lake pollen record could reflect long-term variation in temperature, seasonality or other climate factors independent of moisture balance; however it is also possible that multidecadal moisture variability interacted with ecological processes affecting recruitment and mortality of species following fires of varying size and severity. Decadal-scale climatic variability can lead to altered successional pathways and to changes in forest composition that last for centuries.

e69c3be8-5e37-3fd4-b7a4-07aae40674e2
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ID: e69c3be8-5e37-3fd4-b7a4-07aae40674e2
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Abstract: The processes that control climate in the tropics are poorly understood. We applied compound-specific hydrogen isotopes (delta D) and the TEX86 (tetraether index of 86 carbon atoms) temperature proxy to sediment cores from Lake Tanganyika to independently reconstruct precipitation and temperature variations during the past 60,000 years. Tanganyika temperatures follow Northern Hemisphere insolation and indicate that warming in tropical southeast Africa during the last glacial termination began to increase similar to 3000 years before atmospheric carbon dioxide concentrations. delta D data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial-scale events recorded in Northern Hemisphere monsoonal climate records. This implies that precipitation in tropical southeast Africa is more strongly controlled by changes in Indian Ocean sea surface temperatures and the winter Indian monsoon than by migration of the Intertropical Convergence Zone.

e91c782a-1387-3ec3-b157-dc6c639965f3
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Abstract: Here we present a detailed multi-proxy record of the climate and environmental evolution at Lake El'gygytgyn, Far East Russian Arctic during the period 430-395 ka covering the marine isotope stage (MIS) 12/11 transition and the thermal maximum of super interglacial MIS 11c. The MIS 12/11 transition at Lake El'gygytgyn is characterized by initial warming followed by a cold reversal implying similarities to the last deglaciation. The thermal maximum of MIS 11c is characterized by full and remarkably stable interglacial conditions with mean temperatures of the warmest month (MTWM) ranging between ca. 10-15 C; annual precipitation (PANN) ranging between ca. 300-600 mm; strong in-lake productivity coinciding with dark coniferous forests in the catchment; annual disintegration of the lake ice cover; and full mixis of the water column. Such conditions persisted, according to our age model, for ca. 27 ± 8 kyr between ca. 425-398 ka. The Lake El'gygytgyn record closely resembles the climate pattern recorded in Lake Baikal (SE Siberia) sediments and Antarctic ice cores, implying interhemispheric climate connectivity during MIS 11c. © 2013. CC Attribution 3.0 License.

220281d5-99f6-3d9f-9017-e73d46b4b74e
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ID: 220281d5-99f6-3d9f-9017-e73d46b4b74e
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Abstract: We tested the use of visible to near-infrared (VNIR) reflectance spectroscopy to characterize the relative abundances of clay minerals in sediments from Lake Towuti, a large tectonic lake in Sulawesi, Indonesia. We measured VNIR spectra of lake and river sediments from Lake Towuti and its catchment to identify clay minerals, fit major VNIR absorption features with a modified Gaussian model to estimate relative abundances of these minerals, and compared these absorptions to the samples’ chemistry to test the utility of VNIR spectroscopy to characterize sediment compositional variations. We found that major absorptions are caused by vibrations of Al–OH in kaolinite (2.21 μm), Fe–OH in nontronite (2.29 μm), Mg–OH in saponite and serpentine (2.31 μm), and Mg–OH in serpentine (2.34 μm). This was confirmed with X-ray diffraction data. The correlations between absorption band areas for Fe–OH, Al–OH, and Mg–OH vibrations and Fe, Al and Mg concentrations, respectively, are statistically significant, varying between r = 0.51 and r = 0.90, and spatial variations in inferred clay mineralogy within the lake are consistent with variations in the geology of the catchment. We conclude that VNIR spectroscopy is an effective way to characterize the clay mineralogy of lake sediments, and can be used to investigate changes in mineral inputs to lake deposits.

4cafef06-af45-32a8-a1f3-631c47f2896e
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ID: 4cafef06-af45-32a8-a1f3-631c47f2896e
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Abstract: Lacustrine and tufa deposits from Laguna de Taravilla (Iberian Range, Guadalajara province, Spain, 40°39′ N, 1°59′ W, 1100 m a.s.l.) have been analyzed using sedimentological, mineralogical, geochemical and palynological techniques. A preliminary chronological framework is presented, based on U/Th, 14C AMS, 210Pb and 137Cs analyses. The lacustrine-tufa system developed in a hanging tributary valley of the Upper Tajo River and is composed of a large perched springline tufa build-up, and a barrage tufa dam that impounds Laguna de Taravilla. The Taravilla tufa stable-isotope compositions are similar to other examples in central and southern Spain and they plot in the same field as other lowland European stream tufas. These values are coherent with the range of isotopic compositions measured in the Taravilla spring and lake water. Several sediment cores retrieved from the Laguna de Taravilla have been dated with 14C AMS and analyzed using a multiproxy approach including magnetic properties, sedimentology, geochemistry, stable isotopes, palynology and ostracode assemblages. Sedimentary facies analyses show the dominance of clastic depositional processes in the lacustrine depositional system and suggest the potential of the sequences as palaeoflood records. Sands and coarse silts reflect periods of increased alluvial activity of the inlet. The dominance of clastic depositional processes and the input of detrital carbonate hinder the use of lake mud stable-isotope compositions as environmental indicators. Phases of increased tufa growth occurred during the Late Pleistocene (Last Glacial to Interglacial transition from oxygen isotopic stage 6 to 5) and during the Late Glacial and Early Holocene. Although the Taravilla chronology does not allow a detailed analysis of flood frequency, the reconstructed evolution is coherent with the palaeoflood history of the Tajo River for the last 2000 years, particularly with an increase during the last 500 years. The increase in flood frequency coincides with other evidence of wetter and colder climate and environmental change in Central Spain and in Europe during the Little Ice Age. © 2007 Elsevier B.V. All rights reserved.

0f5fcb20-3ab4-3ddf-9cc9-0514b203cbf8
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Abstract: Lake St. Croix is a natural impoundment of the lowermost 37 km of the St. Croix River in Minnesota and Wisconsin, making this one of a few large river systems in the world possessing a long-term depositional basin at its terminus. The river's relatively pristine condition led to its designation as a National Scenic Riverway in 1968, but increasing urbanization in its lower reaches has raised concerns about impacts on water quality. This study was initiated to reconstruct historical loadings of suspended sediment and phosphorus (P) from the sediment record in Lake St. Croix. Twenty-four piston cores, with an average length of 2 m, were collected along eight transects of the lake. Dated chronologies from Pb-210, Cs-137 and C-14 were used to calculate the rate of sediment accumulation in the lake over the past 100+ years. Diatom microfossil analysis was used to reconstruct historical lakewater P concentrations over the same time period, and sediment P analysis quantified the amount of P trapped in lake sediments. Using a whole-lake mass balance approach, the loading of sediment and P to Lake St. Croix over the last 100+ years was calculated. Beginning in 1850, sediment accumulation increased dramatically to a peak in 1950-1960 of eight times background rates prior to European settlement. The peak is driven largely by sediment contributions from small side-valley catchment tributaries to the downstream half of the lake. The total P load to the lake increased sharply after 1940 and remains high, at around four times the level of pre-European settlement conditions. The timing of peak sediment and P loading to the lake shows that early settlement activities, such as logging and the conversion of forest and prairie to agricultural land between 1850 and 1890, had only modest impacts on the lake. By contrast, the mid-1900s brought major increases in sediment and P loading to the lake, suggesting that relatively recent activities on the landscape and changes to nutrient balances in the watershed have caused the current eutrophic condition of this important recreational and natural resource.

54ae7919-a2c6-35e1-821d-d24e336102b6
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ID: 54ae7919-a2c6-35e1-821d-d24e336102b6
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Abstract: Lake Agassiz was ponded on the northward-sloping surface of the Hudson Bay and Arctic Ocean basins, as the Laurentide Ice Sheet retreated. The level of Lake Agassiz abruptly fell ~ 12.9 cal (11 14C) ka BP, exposing the lake floor over a large region for > 1000 yr. The routing of overflow during this (Moorhead low-water) period is uncertain, and there is evidence on the continent and in ocean basins for both an easterly route through the Great Lakes–St. Lawrence to the North Atlantic and for a northwesterly route through the Clearwater–Athabasca–Mackenzie system to the Arctic Ocean. The Moorhead low water phase coincides with the Younger Dryas cooling, and a cause–effect relationship has been proposed by attributing a change in ocean thermohaline circulation to the re-routing of Lake Agassiz freshwaters from the Gulf of Mexico to more northern oceans. Paleoclimatic interpretations from ecosystems in lake sediments in the region, and a simple calculation of the paleohydrological budget of Lake Agassiz, indicate that the climate remained wet and cool throughout the YD in this region, and was not warm nor dry enough to allow evaporative loss to offset the influx of meltwater and precipitation; thus, the Moorhead phase resulted from changes in the outlet that carried overflow.

463ddf5e-8139-3dd8-b403-d6d164c397b3
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ID: 463ddf5e-8139-3dd8-b403-d6d164c397b3
Tag: []
Abstract: The recent and fossil pollen data obtained under the frame of the multi-disciplinary international El'gygytgyn Drilling Project represent a unique archive, which allows the testing of a range of pollen-based reconstruction approaches and the deciphering of changes in the regional vegetation and climate. In the current study we provide details of the biome reconstruction method applied to the late Pliocene and Quaternary pollen records from Lake El'gygytgyn. All terrestrial pollen taxa identified in the spectra from Lake El'gygytgyn were assigned to major vegetation types (biomes), which today occur near the lake and in the broader region of eastern and northern Asia and, thus, could be potentially present in this region during the past. When applied to the pollen spectra from the middle Pleistocene to present, the method suggests (1) a predominance of tundra during the Holocene, (2) a short interval during the marine isotope stage (MIS) 5.5 interglacial distinguished by cold deciduous forest, and (3) long phases of taiga dominance during MIS 31 and, particularly, MIS 11.3. These two latter interglacials seem to be some of the longest and warmest intervals in the study region within the past million years. During the late Pliocene-early Pleistocene interval (i.e., ∼3.562-2.200 Ma), there is good correspondence between the millennial-scale vegetation changes documented in the Lake El'gygytgyn record and the alternation of cold and warm marine isotope stages, which reflect changes in the global ice volume and sea level. The biome reconstruction demonstrates changes in the regional vegetation from generally warmer/wetter environments of the earlier (i.e., Pliocene) interval towards colder/drier environments of the Pleistocene. The reconstruction indicates that the taxon-rich cool mixed and cool conifer forest biomes are mostly characteristic of the time prior to MIS G16, whereas the tundra biome becomes a prominent feature starting from MIS G6. These results consistently indicate that the study region supported significant tree populations during most of the interval prior to ∼2.730 Ma. The cold- and drought-tolerant steppe biome first appears in the reconstruction ∼3.298 Ma during the tundra-dominated MIS M2, whereas the tundra biome initially occurs between ∼3.379 and ∼3.378 Ma within MIS MG4. Prior to ∼2.800 Ma, several other cold stages during this generally warm Pliocene interval were characterized by the tundra biome. © Author(s) 2013. CC Attribution 3.0 License.

2f328379-d8c3-3c7e-93fa-ecdee29d15e1
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Abstract: Hydroclimate sensitivity simulations were conducted with a lake-catchment hydrologic and isotope mass balance model adapted to two small, closed lakes (Castor and Scanlon) located in the Pacific Northwest. Model simulations were designed to investigate the combined influences of persistent disequilibrium, reddening, and equifinality on lake water and sediment (i.e., biogenic and endogenic carbonate mineral) oxygen isotope (δ18O) values and to provide a basis for quantitative, probabilistic climate reconstructions using lake sediment δ18O records. Simulation results indicate that within closed-basin lakes changes in long-term (i.e., multi-decadal) precipitation amounts produce inconsistent responses in lake water and sediment δ18O values that are strongly influenced by lake basin outseepage and morphometry. Simulations of variable initial conditions in which randomly generated monthly climate data (i.e., precipitation, temperature, and relative humidity) were used to force the model during the equilibration period (which precedes the application of instrumental climate data) demonstrate that Castor Lake and Scanlon Lake have a somewhat limited isotopic ‘memory’ of ∼10 years. Additional tests conducted using a Monte Carlo ensemble (in which random climate data were used to force the model) combined with δ18O analyses of water samples collected from 2003 to 2011 AD, indicate that within small, closed lakes in the Pacific Northwest November–February precipitation is the strongest seasonal, climatic control on sediment oxygen-isotope values. Further, a Monte Carlo based reconstruction of 20 year average November–February precipitation amounts strongly correlates (R2 = 0.66) to instrumental values from the 20th century (with all observed values falling within modeled 95% prediction limits), indicating that probabilistic, quantitative paleoclimate interpretations of lake sediment δ18O records are attainable.

cfad7d88-bafa-368e-b786-39fd7f6d282c
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Abstract: Large glacial lakes modulated the return of meltwater to the ocean during deglaciation, and their drainage may have initiated global climate change. Yet few records of their drainage come from observations within their basins. Sediment cores from nine lakes along a 240-km transect from northwestern Quebec to northeastern Ontario cover a portion of former Lake Ojibway and provide a stratigraphy of the terminal phase of this large glacial lake. Magnetic susceptibility, density, grain size, X-ray fluorescence chemistry and X-ray diffraction data were used to characterize stratigraphic changes within the basin. The basal sequence consists of till and rhythmites, with ice-proximal debris flows overlain by varves. The varves thin up-section and become unrecognizable, which indicates decreased deposition rates. This fine-grained sediment forms the matrix of a clay-pebble conglomerate. The clay-pebbles are ice-rafted debris (IRD). The IRD flux was probably constant, whereas the sedimentation rate of the finer-grained matrix decreased. The end of IRD marks the cessation of icebergs in the lake and is the best indication for drainage of the glacial lake. The conglomerate is capped by laminated to massive gray silt deposited after lake drainage and marks the transition to organic-rich, post-glacial lakes. Such sequences place drainage into the broader context of deglaciation. © 2013 Springer Science+Business Media Dordrecht.

e985fad3-f57f-3246-9c6b-9fbff6254c6c
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Abstract: Lake Chungará(18°15′S, 69°09′W, 4520 m above sea-level) is the largest (22.5 km2) and deepest (40 m) lacustrine ecosystem in the Chilean Altiplano and its location in an active volcanic setting, provides an opportunity to evaluate environmental (volcanic vs. climatic) controls on lacustrine sedimentation. The Late Quaternary depositional history of the lake is reconstructed by means of a multiproxy study of 15 Kullenberg cores and seismic data. The chronological framework is supported by 1014 C AMS dates and one 230Th/234U dates. Lake Chungaráwas formed prior to 12.8 cal kyr bp as a result of the partial collapse of the Parinacota volcano that impounded the Lauca river. The sedimentary architecture of the lacustrine succession has been controlled by (i) the strong inherited palaeo-relief and (ii) changes in the accommodation space, caused by lake-level fluctuations and tectonic subsidence. The first factor determined the location of the depocentre in the NW of the central plain. The second factor caused the area of deposition to extend towards the eastern and southern basin margins with accumulation of high-stand sediments on the elevated marginal platforms. Synsedimentary normal faulting also increased accommodation and increased the rate of sedimentation in the northern part of the basin. Six sedimentary units were identified and correlated in the basin mainly using tephra keybeds. Unit 1 (Late Pleistocene-Early Holocene) is made up of laminated diatomite with some carbonate-rich (calcite and aragonite) laminae. Unit 2 (Mid-Holocene-Recent) is composed of massive to bedded diatomite with abundant tephra (lapilli and ash) layers. Some carbonate-rich layers (calcite and aragonite) occur. Unit 3 consists of macrophyte-rich diatomite deposited in nearshore environments. Unit 4 is composed of littoral sediments dominated by alternating charophyte-rich and other aquatic macrophyte-rich facies. Littoral carbonate productivity peaked when suitable shallow platforms were available for charophyte colonization. Clastic deposits in the lake are restricted to lake margins (Units 5 and 6). Diatom productivity peaked during a lowstand period (Unit 1 and subunit 2a), and was probably favoured by photic conditions affecting larger areas of the lake bottom. Offshore carbonate precipitation reached its maximum during the Early to Mid-Holocene (ca 7.8 and 6.4 cal kyr bp). This may have been favoured by increases in lake solute concentrations resulting from evaporation and calcium input because of the compositional changes in pyroclastic supply. Diatom and pollen data from offshore cores suggest a number of lake-level fluctuations: a Late Pleistocene deepening episode (ca 12.6 cal kyr BP), four shallowing episodes during the Early to Mid-Holocene (ca 10.5, 9.8, 7.8 and 6.7 cal kyr BP) and higher lake levels since the Mid-Holocene (ca 5.7 cal kyr BP) until the present. Explosive activity at Parinacota volcano was very limited between c. >12.8 and 7.8 cal kyr bp. Mafic-rich explosive eruptions from the Ajata satellite cones increased after ca 5.7 cal kyr bp until the present. © 2007 The Authors. Journal compilation 2007 International Association of Sedimentologists.

cefccd52-e8a0-372c-ac6e-09fda32fe795
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Abstract: Sediment records from proglacial lakes between 9 and 10°S in the western Cordillera of the Peruvian Andes document the waxing and waning of alpine glaciers since the end of the Lateglacial stage. These records from the southern tropical Andes provide supporting evidence that the early Holocene (between 12 and 8 ka) was relatively warm and dry, and the middle Holocene (between 8 and 4 ka) was marked by a shift to cooler, and possibly wetter conditions in certain regions, leading to glacial advances. Although there were multiple periods of brief ice advances that interrupted the overall trend, glaciers in multiple valleys generally retreated from ∼4.0 ka through the Medieval Climate Anomaly (1.0–0.7 ka). This late Holocene pattern of ice retreat occurred during a period when lake level studies, and both lacustrine and speleothem stable isotopic records indicate wetter conditions relative to the middle Holocene, suggesting that higher temperatures contributed to the pattern of ice retreat. Following this period of glacial retreat, multiple proxy records suggest that the start of the Little Ice Age (∼0.6–0.1 ka) was a colder and wetter time throughout much of the tropical Andes. There appear to be two primary synoptic-scale climatic controls on temperature and precipitation linked to insolation dynamics that drive changes in ice cover in the southern tropical Andes during the Holocene: 1) the strength of the South America Summer Monsoon, which is linked to Northern Hemisphere temperatures and the mean position of the Intertropical Convergence Zone over the Atlantic, and 2) sea surface temperature distributions in the tropical Pacific Ocean and its influence on atmospheric temperature, precipitation and circulation patterns.

1e34e6ef-31b8-31c1-9ae7-e23e130eacf9
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Abstract: Climate-driven variations in lake-groundwater exchange are recorded by sediments in groundwater-dominated lakes. A groundwater flow-through lake in west-central Montana (USA) registers latest Pleistocene and Holocene hydroclimatic variation in fluid and solute balance, as controlled by rates and timing of groundwater recharge. Early Holocene warming occurred under conditions of relative aridity and low groundwater throughflow, punctuated by a c. 450-yr episode of lake dilution centered on 11 000 cal. yr BP. Maximum evaporative concentration of lake waters, registered in both delta O-18 values and mineralogy of endogenic carbonates, coincided with the early-Holocene peak in insolation seasonality at about 9750 cal. yr BP. Subsequently, progressively decreasing lake residence time drove a sustained long-term decline in salinity while having a very subdued effect on mean delta O-18 values. e explain this decoupling by (1) limits placed on oxygen isotope sensitivity by groundwater throughflow, and (2) a shift toward greater summer rain contribution to lake inflow after mid-Holocene time. Superimposed multidecadal- and century-scale variation in lake-groundwater exchange generated high-frequency but low-amplitude isotopic oscillations throughout the record. High rates of groundwater throughflow maintaining low lake salinity similar to that observed today were established around 1400 cal. yr BP. e infer reduced regional stream baseflow, decrease in permanent wetlands (relative increase in ephemeral wetlands) and enhanced lake and wetland salinity prior to this time, relative to the late Holocene.

01d96d89-b6ed-31f8-a5d1-7d3b278044d2
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Abstract: The Lake Bosumtwi sediment record represents one of the longest and highest-resolution terrestrial records of paleoclimate change available from sub-Saharan Africa. Here we report a new sediment age model framework for the last ~ 45 cal kyr of sedimentation using a combination of high-resolution radiocarbon dating, Bayesian age-depth modeling and lamination counting. Our results highlight the practical limits of these methods for reducing age model uncertainties and suggest that even with very high sampling densities, radiocarbon uncertainties of at least a few hundred years are unavoidable. Age model uncertainties are smallest during the Holocene (205 yr) and the glacial (360 yr) but are large at the base of the record (1660 yr), due to a combination of decreasing sample density, larger calibration uncertainties and increases in radiocarbon age scatter. For portions of the chronology older than ~ 35 cal kyr, additional considerations, such as the use of a low-blank graphitization system and more rigorous sample pretreatment were necessary to generate a reliable age depth model because of the incorporation of small amounts of younger carbon. A comparison of radiocarbon age model results and lamination counts over the time interval ~ 15–30 cal kyr agree with an overall discrepancy of ~ 10% and display similar changes in sedimentation rate, supporting the annual nature of sediment laminations in the early part of the record. Changes in sedimentation rates reconstructed from the age-depth model indicate that intervals of enhanced sediment delivery occurred at 16–19, 24 and 29–31 cal kyr, broadly synchronous with reconstructed drought episodes elsewhere in northern West Africa and potentially, with changes in Atlantic meridional heat transport during North Atlantic Heinrich events. These data suggest that millennial-scale drought events in the West African monsoon region were latitudinally extensive, reaching within several hundred kilometers of the Guinea coast. This is inconsistent with a simple southward shift in the mean position of the monsoon rainbelt, and requires changes in moisture convergence as a result of either a reduction in the moisture content of the tropical rainbelt, decreased convection, or both.

cebaa3ff-a62e-37a7-9acc-751d7b3b843a
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Abstract: We used an isotopic mass-balance model to examine how the hydrogeologic setting of lakes influences isotopic response of evaporating lake water to idealized hydroclimatic changes. The model uses a monthly water and isotope balance approach with simplified water-column structure and groundwater exchanges. The framework for comparative simulations is provided by lakes in a region of the Northern Rocky Mountains that display high interlake geochemical variability, thought to be controlled by groundwater hydraulics. Our analysis highlights several isotopic effects of flow between aquifers and lakes, leading to possible divergence of isotopic paleorecords formed under a common climate. Amplitude of isotopic variation resulting from simulated climate forcing was greatly damped when high groundwater fluxes and/or low lake volume resulted in low lake fluid residence time. Differing precipitation and evaporation scenarios that are equivalent in annual fluid balance (P-E) resulted in different isotopic signatures, interpreted as a result of evaporation kinetics. Concentrating low-δ groundwater inflow during spring months raised springtime lake δ values, a counterintuitive result of coincidence between times of high groundwater inflow and the evaporation season. Transient effects of reduced fluid balance caused excursions opposite in sign from eventual steady-state isotopic shifts resulting from enhanced groundwater inflow dominance. Lags in response between climate forcing and isotopic signals were shortened by high groundwater fluxes and resulting short lake residence times. Groundwater-lake exchange exerts control over patterns of lake isotopic response to evaporation through effects on lake residence time, inflow composition, and seasonal timing of inflow and outflow. Sediments from groundwater-linked lakes, often used for paleoenvironmental analysis, should be expected to reflect isotopic complexities of the type shown here. © 2007 Springer Science+Business Media, Inc.

2492bffb-df5d-3fd0-8770-d0ce0dfb44d1
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Abstract: The continuous 300-m long drill cores obtained from Lake Bosumtwi, Ghana represent one of the longest, continuous lacustrine sequences obtained from an extant lake, and contain an unprecedented record of late Quaternary climate change in West Africa. However, one of the main challenges associated with generating long paleoclimate time series from terrestrial records such as this is the development of accurate age-depth relationships because unlike marine records, lacustrine sequences cannot be tuned to global ice volume records via δ18O stratigraphy. The Lake Bosumtwi record thus offers an excellent case study for examining the potential and the challenges associated with different geochronological techniques in lacustrine systems. In the present study, we use a combination of radiocarbon, optically stimulated luminescence and U-series dating and paleomagnetic excursions to generate a chronology for the upper ca. 150 ka of sedimentation at Lake Bosumtwi and employ a Bayesian approach to generate a continuous age-depth relationship. The resultant chronology is then used to test the effectiveness of tuning of an environmental magnetic proxy for dust against a well-dated record of high latitude dust. Our approach highlights the advantages of using multiple dating approaches, and the dangers of relying on too few age constraints when dating long sedimentary sequences. However, the excellent agreement between the different approaches over most of the record suggest that well-constrained age-depth models for long sedimentary sequences can be produced using this combination of approaches. Furthermore, our data provide support for extending the chronology beyond the limit of radiocarbon, U-series and OSL in the future using paleomagnetic excursions/reversals and tuning against well-dated high latitude paleoclimate records. © 2012 Elsevier B.V.

eb90b475-d25e-3c83-9a5e-33b813b3439c
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Abstract: A radiocarbon dated sediment record from Laguna de Los Anteojos, a cirque lake in the Mérida Andes of Venezuela, indicates that warmer and wetter atmospheric conditions occurred in the northern tropics at the onset of the Bølling (~ 14,600 cal yr BP), and abruptly colder and drier conditions around the time of the Younger Dryas (YD). Geochemical and clastic sediment analyses from Los Anteojos show that glaciers advanced at ~ 12,850 cal yr BP, reached their YD maximum extent at ~ 12,650 cal yr BP, and then retreated until complete deglaciation of the watershed at ~ 11,750 cal yr BP. The onset of warmer conditions that ended the coldest phase of the YD occurred several hundred years earlier at Los Anteojos than in the high latitudes of the Northern Hemisphere. During the peak YD glacial advance, glacier equilibrium-line altitudes in the region were ~ 360 to 480 m lower, and temperature was ~ 2.2 to 2.9 °C colder than modern. Independent palynological evidence from the Los Anteojos sediment core indicates that the northern Andes were more arid and at least 2.3 °C colder during the YD. The direction and timing of glacial fluctuations in Venezuela are consistent with observations of marine sediment records from the Cariaco Basin that suggest abrupt cooling occurred at ~ 12,850 cal yr BP, followed by a shift to higher temperature after ~ 12,300 cal yr BP. The timing and pattern of climatic changes in northern South America are also consistent with paleoclimate records from the southern Tropical Andes that suggest a southward shift in the position of the Intertropical Convergence Zone occurred at the start of the cooling event, followed by a return to wetter conditions in northern South America during the late stages of the YD. The early warming of the tropical atmosphere and invigoration of the hydrologic cycle likely contributed to the shift to increased temperature in the higher latitudes of the Northern Hemisphere at the end of the late Glacial stage.

825fac02-564b-3877-8472-62dc32c1c1c1
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Abstract: During the middle and late Holocene, the Iberian Peninsula underwent large climatic and hydrologic changes, but the temporal resolution and regional distribution of available palaeoenvironmental records is still insufficient for a comprehensive assessment of the regional variability. The high sedimentation rate in karstic, meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a detailed reconstruction of the regional palaeoecology over the last 5,340 years using diatom analysis, aquatic pollen, sedimentological data, and historic documentary records. Results show marked fluctuations in diatom species assemblage composition, mainly between dominant Cyclotella taxa and small Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta reflects changes in trophic state, while the succession of centric and pennate species most likely reflects changes in the hydrology of the lake. The diatom assemblages were used to identify six main phases: (1) high productivity and likely lower lake levels before 2350 BC, (2) lower lake levels and a strong arid phase between 2350 and 1850 BC, (3) lake level increase between 1850 and 850 BC, (4) relatively high lake level with fluctuating conditions during the Iberian and Roman Epochs (650 BC–350 AD), (5) lower lake levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by increased human activities in the watershed during the Medieval Climate Anomaly (900–1300 AD), and (6) relatively higher lake levels during the LIA (1380–1850 AD) and afterwards. The combined study of diatoms, algae and pollen provides a detailed reconstruction of past climate, which refines understanding of regional environmental variability and interactions between climate and socio-economic conditions in the Pyrenees.

46c09e76-8476-3b79-9091-ef60e4ced69a
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Abstract: Lake-water Mg:Ca responds to endogenesis of carbonate minerals, providing a valuable indicator of paleosalinity when water-column cation ratios are preserved in calcareous lake sediments. Typically Mg:Ca and total dissolved solids (TDS) correlate positively over a broad range of ionic compositions where calcium carbonate precipitation occurs. However, in groundwater-fed lakes where inflow solutes are dominated by HCO3–, Ca2+, and Mg2+ ions, and concentration of conservative solutes is limited by outflow, CaCO3 formation and depletion of major source-water ions results in a negative correlation between Mg:Ca and TDS at low lake salinity. This relationship is promoted by high pCO2 of inflowing groundwater, a common characteristic of groundwater-fed lakes such as our field example, a groundwater flow-through lake in western Montana, United States. Equilibrium modeling of our field example shows that evaporative evolution is expected to reverse the slope of the Mg:Ca/TDS relationship at moderately higher lake concentration. Generally, the TDS at the point of Mg:Ca/TDS reversal will depend on the initial concentration of less-reactive ions, and so on the source lithology for groundwater solutes.

e10d2246-960f-3849-a70a-80c2a4fc1e3a
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Abstract: Proxy evidence at decadal resolution from Late Holocene sediments from Pickerel Lake, northeastern South Dakota, shows distinct centennial cycles (400-700 years) in magnetic susceptibility; contents of carbonate, organic carbon, and major elements; abundance in ostracodes; and δ18O and δ13C values in calcite. Proxies indicate cyclic changes in eolian input, productivity, and temperature. Maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates (MARs), and in abundances of the ostracode Fabaeformiscandona rawsoni. This indicates variable windy, and dry conditions with westerly wind dominance, including during the Medieval Climate Anomaly. Maxima in carbonates, organic carbon, phosphorous, and high δ13C values of endogenic calcite indicate moister and less windy periods with increased lake productivity, including during the Little Ice Age, and alternate with maxima of eolian transport. Times of the Maunder, Spörer and Wolf sunspot minima are characterized by maxima in δ18O values and aluminum MARs, and minima in δ13C values and organic carbon content. We interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4-5 °C associated with decreases in epilimnetic productivity during summer.We propose that the centennial cycles are triggered by solar activity, originate in the tropical Pacific, and their onset during the Late Holocene is associated with insolation conditions driven by precession. The cyclic pattern is transmitted from the tropical Pacific into the atmosphere and transported by westerly winds into the North Atlantic realm where they strengthen the Atlantic Meridional Overturning Circulation during periods of northern Great Plains wind maxima. This consequently leads to moister climates in Central and Northern Europe. Thus, Pickerel Lake provides evidence for mechanisms of teleconnections including an atmospheric link bridging between the different climate regimes from the tropical Pacific to the North Atlantic and onto the European continent. © 2010 Elsevier Ltd.

3c3eb44f-5e0b-3896-b999-48cc2ecbe020
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Abstract: The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after approximately 70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.

7ccbbb4d-ad0f-3d10-aa68-6f162c9a755b
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Abstract: Scientific drill cores recovered from Lake Malawi exhibit a remarkable down-core lithologic variability, and are indicative of radically changing environmental conditions forced by large-amplitude lake-level shifts over the past 150 kyr. Here we present detailed lithologic and sedimentary fabric descriptions of the key sedimentary units, along with down-core physical properties data, down-core organic matter geochemistry (TOC, C/N, and [delta]13C data sets), and images and descriptions from core sections and from sediment smear slide microscopy. These data reveal a fundamental change in Lake Malawi's limnology and regional climate at ca. 60-70 ka. Prior to this time the lake was characterized by large-amplitude variations in lake level and water chemistry, but after 60 ka the lake remained comparatively high, and the central basin drill site accumulated mainly organic-rich, laminated sediments. Organic matter sources changed dramatically during the different lake stages. During major lake high stands, a mixed assemblage of algal (diatom-dominated), woodland and aquatic macrophyte (C4-pathway), and grassland (C3-pathway) organic matter was deposited, whereas during extreme low lake stages (water depths < 200 m), when saline, alkaline lakes persisted in the basin, sediments with minimal amounts of algal-dominated organic matter accumulated and were preserved.

c382b2ae-84d7-35b9-9abc-95534881be69
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Abstract: The Indo-Pacific warm pool houses the largest zone of deep atmospheric convection on Earth and plays a critical role in global climate variations. Despite the region's importance, changes in Indo-Pacific hydroclimate on orbital timescales remain poorly constrained. Here we present high-resolution geochemical records of surface runoff and vegetation from sediment cores from Lake Towuti, on the island of Sulawesi in central Indonesia, that continuously span the past 60,000 y. We show that wet conditions and rainforest ecosystems on Sulawesi present during marine isotope stage 3 (MIS3) and the Holocene were interrupted by severe drying between ∼33,000 and 16,000 y B.P. when Northern Hemisphere ice sheets expanded and global temperatures cooled. Our record reveals little direct influence of precessional orbital forcing on regional climate, and the similarity between MIS3 and Holocene climates observed in Lake Towuti suggests that exposure of the Sunda Shelf has a weaker influence on regional hydroclimate and terrestrial ecosystems than suggested previously. We infer that hydrological variability in this part of Indonesia varies strongly in response to high-latitude climate forcing, likely through reorganizations of the monsoons and the position of the intertropical convergence zone. These findings suggest an important role for the tropical western Pacific in amplifying glacial-interglacial climate variability.

fd12948c-c686-3fa6-bcb7-e15222197cc6
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Abstract: The recovery of detailed and continuous paleoclimate records from the interior of the African continent has long been of interest for understanding climate dynamics of the tropics, and also for constraining the environmental backdrop to the evolution and spread of early Homo sapiens. In 2005 an international team of scientists collected a series of scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from interior East Africa. The paleoclimate records, which include lithostratigraphic, geochemical, geophysical and paleobiological observations documented in this special issue of Palaeo3, indicate an interval of high-amplitude climate variability between 145,000 and ~60,000years ago, when several severe arid intervals reduced Lake Malawi's volume by more than 95%. These intervals of pronounced tropical African aridity in the early Late Pleistocene around Lake Malawi were much more severe than the Last Glacial Maximum (LGM), a well-documented period of drought in equatorial and Northern Hemisphere tropical east Africa. After 70,000years ago climate shifted to more humid conditions and lake levels rose. During this latter interval however, wind patterns shifted rapidly, and perhaps synchronously with high-latitude shifts and changes in thermohaline circulation. This transition to wetter, more stable conditions coincided with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa. © 2010 Elsevier B.V.

b5595853-a905-3624-bbca-3329fadd4327
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Abstract: The Little Ice Age (LIA) is characterized by widespread northern hemisphere cooling during a period of reduced radiative forcing. Sediment records from three crater lakes indicate that the most severe drought of the last 1200 years struck East Java at the end of the LIA. We use 14C and U-series dating applied to carbonate geochemical records from Lakes Lading, Logung, and Lamongan to demonstrate this drought occurred at 1790 Common Era (CE) ± 20 years. Drought occurred during a period of strong El Niño events and Asian monsoon failures in the late 1790s, yet our records indicate that drought conditions persisted well beyond this decade and reached peak intensity in East Java ca 1810 CE ± 30 years. The continuation of severe drought into the 1800s may have resulted from the large volcanic eruptions that occurred in 1809, 1815 and 1835 CE, which likely caused brief, abrupt decreases in Indo-Pacific Warm Pool (IPWP) sea surface temperatures (SSTs), reducing local convection in East Java. Alternatively, broad changes in atmospheric circulation, such as a slowing of the Pacific Walker Circulation in response to decreased solar radiation during the LIA, could have produced several decades of drought in East Java. However, there is a lack of clear supporting evidence for such a change based upon paleohydrological records from the opposite ends of both the Indian and Pacific ocean zonal circulation systems. Based on the available evidence, we suggest severe multidecadal drought in East Java throughout the turn of the 19th century was driven by locally reduced convection resulting from a combination of heightened El Niño activity and volcanic eruptions.

7e1ccca4-cb11-36ca-be64-5092db10382c
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Abstract: The occurrence, or not, of the Younger Dryas cold reversal in the tropical Andes remains a controversial topic. This study reports a clear signal for this event in the Venezuelan Andes, employing high-resolution palynological analysis of a well-dated sediment core from Laguna de Los Anteojos, situated around 3900 m elevation, within grass páramo vegetation. The lake is surrounded by some Polylepis forests which are close to their upper distribution limit. The section of the core discussed here is 150-cm long and dated between about 14.68 and 9.35 cal kyr BP, using a polynomial age-depth model based on six AMS radiocarbon dates. Between 12.86 and 11.65 cal kyr BP, an abrupt shift occurred in the pollen assemblage, manifested by a decline of Podocarpus, Polylepis and Huperzia, combined with an increase in Poaceae and Asteraceae. The aquatic pteridophyte Isoëtes also decreased and disappeard, and the algae remains show their minimum values. Pollen assemblages from the Younger Dryas interval show maximum dissimilarity values compared with today's pollen assemblage, and are more similar to modern analogs from superpáramo vegetation, growing at elevations 400-500 m higher. A lowering of vegetation zones of this magnitude corresponds to a temperature decline of between 2.5 and 3.8 °C. During this colder interval lake levels may have been lower, suggesting a decrease in available moisture. The vegetation shift documented in Anteojos record between 12.86 and 11.65 cal kyr BP is comparable to the El Abra Stadial in the Colombian Andes but it differs in magnitude. The Anteojos shift is better dated and coincides with the Younger Dryas chron as recorded in the Cariaco Basin sea surface temperature reconstructions and records of continental runoff, as well as in the oxygen isotope measurements from the Greenland ice cores. When compared to other proxies of quasi-immediate response to climate, the time lag for the response of vegetation to climate is found to be negligible at a centennial scale. The Polylepis pollen curve is especially noteworthy, as it reproduces the overall pollen trends and matches well with paleoclimatic reconstructions based on other proxies. Hence, Polylepis might be used as a reliable paleoclimatic indicator in lake sediments close to its uppermost distribution boundary.

cd12f3ce-7deb-39e5-bf89-04c76c7b172a
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Abstract: We report vegetation changes of the last millennium inferred from palynological analysis of a sediment core from Lake Montcortès, situated at ~1,000 m elevation in the southern pre-Pyrenean flank. The record begins in the Middle Ages (~AD 800) and ends around AD1920, with an average resolution of ~30 years. The reconstructed vegetation sequence is complex and shows the influence of both climate and humans in shaping the landscape. Pre-feudal times were characterized by the presence of well-developed conifer forests, which were intensely burned at the beginning of feudal times (AD 1000) and were replaced by cereal (rye) and hemp cultivation, as well as meadows and pastures. In the thirteenth century, a relatively short period of warming, likely corresponding to the Medieval Warm Period, was inferred from the presence of a low Mediterranean scrub community that is today restricted to <800 m elevation. This community disappeared during Little Ice Age cooling in the fifteenth century, coinciding with a decline in human activities around the lake. Forest recovery began around AD 1500, at the beginning of the Modern period, coinciding with wetter climate. Forests, however, declined again during the seventeenth century, coinciding with maximum olive and hemp cultivation. This situation was reversed in post-Modern times (nineteenth century), characterized by an intense agricultural crisis and a significant decline in population that favored forest re-expansion. Correlations with nearby Estanya Lake, situated about 350 m below, provide a regional picture of environmental change. Besides some climate forcing evident in both sequences, human activities seem to have been the main drivers of landscape and vegetation change in the southern Pyrenean flank, in agreement with conclusions from other studies in high-mountain environments.

d1d66e93-208c-34dd-9950-b02bf171facc
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ID: d1d66e93-208c-34dd-9950-b02bf171facc
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Abstract: Variations in the location and strength of convection in the Western Pacific Warm Pool (WPWP) have a profound impact on the distribution and amount of global rainfall. Much of the variability in WPWP convection is attributed to variations in the El Niño-Southern Oscillation, for which the long-term trends and forcing mechanisms remain poorly understood. Despite the importance of WPWP convection to global climate change, we have very few paleohydrological reconstructions from the region. Here we present a new paleolimnologic and paleohydrologic record spanning the past 1,400 years using a multi-proxy dataset from Lake Logung, located in East Java, Indonesia that provides insights into centennial-scale trends in warm pool hydrology. Organic matter δ13C data indicate that East Java became wetter over the last millennium until ca. 1800 Common Era (CE), consistent with evidence for the southward migration of the Intertropical Convergence Zone (ITCZ) during this time. Superimposed on this long-term trend are four decade- to century-scale droughts, inferred from organic matter δ13C and calcite abundance data. They are centered at 1030, 1550, 1830, and 1996 CE. The three more recent droughts correlate with hydrologic anomalies documented in other proxy records from the WPWP region on both sides of the equator, and the two most recent droughts correlate in time with historically documented periods of multiple, intense El Niño events. Thus, our record provides strong evidence that century-scale hydrologic variability in this region relates to changes in the Walker Circulation. Human activity within the lake catchment is apparent since 1860 CE. © 2011 Springer Science+Business Media B.V.

9fc506f3-798d-3d65-9f12-ddb50ca03830
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ID: 9fc506f3-798d-3d65-9f12-ddb50ca03830
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Abstract: High resolution multiproxy data (pollen, sedimentology, geochemistry, chironomids and charcoal) from the Basa de la Mora (BSM) lake sequence (42° 32′ N, 0° 19′ E, 1914 m a.s.l.) show marked climate variability in the central southern Pyrenees throughout the Holocene. A robust age model based on 15 AMS radiocarbon dates underpins the first precise reconstruction of rapid climate changes during the Holocene from this area. During the Early Holocene, increased winter snowpack and high snowmelt during summer, as a consequence of high seasonality, led to higher lake levels, a chironomid community dominated by non-lacustrine taxa (Orthocladiinae) related to higher inlet streams, and a forested landscape with intense run-off processes in the watershed. From 9.8 to 8.1 cal ka BP, climate instability is inferred from rapid and intense forest shifts and high fluctuation in surface run-off. Shifts among conifers and mesophytes reveal at least four short-lived dry events at 9.7, 9.3, 8.8 and 8.3 cal ka BP. Between 8.1 and 5.7 cal ka BP a stable climate with higher precipitation favoured highest lake levels and forest expansion, with spread of mesophytes, withdrawal of conifers and intensification of fires, coinciding with the Holocene Climate Optimum. At 5.7 cal ka BP a major change leading to drier conditions contributed to a regional decline in mesophytes, expansion of pines and junipers, and a significant lake level drop. Despite drier conditions, fire activity dropped as consequence of biomass reduction. Two arid intervals occurred between 2.9 and 2.4 cal ka BP and at 1.2–0.7 cal ka BP (800–1300 AD). The latter coincides with the Medieval Climate Anomaly and is one of the most arid phases of the Holocene in BSM sequence. Anthropogenic disturbances were small until 700 AD, when human pressure over landscape intensified, with Olea cultivation in the lowlands and significant deforestation in highlands. Colder and unfavourable weather conditions during the second part of the Little Ice Age caused a temporary cease of high-land management. The most intense anthropogenic disturbances occurred during the second half of 19th century. Last decades are characterized by recovery of the vegetation cover as a result of land abandonment, and lowered lake levels, probably due to higher temperatures.

25c928a3-14a6-31de-b7b1-3a5b271be4e6
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ID: 25c928a3-14a6-31de-b7b1-3a5b271be4e6
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Abstract: Paleolimnological information is often extracted from diatom records using weighted averaging calibration and regression techniques. Larger calibration sample sets yield better inferences because they better characterize the environmental characteristics and species assemblages of the sample region. To optimize inferred information from fossil assemblages, however, it is worth knowing if fewer calibration samples can be used. Furthermore, confidence in environmental reconstructions is greater if we consider the relative importance of (A) similarity between fossil and calibration assemblages and (B) how well fossil taxa respond to the environmental variable of interest. We examine these issues using ~200-year sediment profiles from four Minnesota lakes and a 145-lake surface sediment training set calibrated for total phosphorus (TP). Training set sample sizes ranging from 10 to 145 were created through random sample selection, and models based on these training sets were used to calculate diatom-inferred (DI) TP data from fossil samples. Relationships between DI-TP variability and sample size were used to determine the minimum sample size needed to optimize the model for paleo-reconstruction. Similarly, similarities between fossil and modern assemblages were calculated for each size training set. Finally, fossil and modern assemblages were compared to determine whether older fossil samples had poorer similarity with modern analogs. More than 50-80 samples, depending on lake, were needed to stabilize variability in DI-TP results, and >110 training set samples were needed to minimize modern-fossil assemblage dissimilarities. Dissimilarities appeared to increase with sample age, but only one of the four studied cores displayed a significant trend. We have two recommendations for future studies: (1) be cautious when dealing with smaller training sets, especially if they are used to interpret older fossil assemblages and (2) understand how well fossil taxa are attuned to the variable of interest, as it is critical to evaluating the quality of the diatom-inferred data. © 2013 Springer Science+Business Media Dordrecht.

2f12f84d-8919-3a1f-b5cd-9193dface793
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ID: 2f12f84d-8919-3a1f-b5cd-9193dface793
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Abstract: We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g−1), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 µS cm−1, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g−1, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 µS cm−1, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g−1, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.

c401eff1-23d7-3732-a243-cec01fda4e62
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ID: c401eff1-23d7-3732-a243-cec01fda4e62
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Abstract: Modern lake hydrodynamics, ostracode species autecology, stable isotopes (δ18O and δ13C) of multiple ostracode species, ostracode taphonomy and sediment geochemistry were studied to improve interpretation of the late Pleistocene–early Holocene (∼24–10 ka) stable isotope record of ostracodes in sediment core PI-6 from Lago Petén Itzá, northern Guatemala. Oxygen and carbon stable isotopes in modern and fossil species assemblages of Lago Petén Itzá were used as indicators of changes in the balance between evaporation and precipitation, past lake level and carbon source. Ostracode taphonomy was used to detect past periods of strong currents, high-energy environments, and possible partial or full mixing of the lake. The modern lake water isotopic composition displays clear seasonal differences that are independent of lake level fluctuations. Modern benthic species displayed lower δ18O and δ13C values than nektobenthic species, with differences of 3.0‰ and 5.3‰, respectively. Valves of nektobenthic species display higher values of δ13C because these ostracodes live in shallower environments among abundant algae and aquatic plants, where productivity is high. The benthic species Limnocythere opesta Brehm, 1939 displayed the smallest average offset from δ18O water (+0.3‰) and the largest offset from δ13CDIC values (−4.1‰) among studied ostracode species. Nektobenthic species Heterocypris punctata Keyser, 1975 displayed the smallest difference relative to the δ13CDIC values (−0.1‰). Late Pleistocene–early Holocene climate conditions and water levels in Lago Petén Itzá can be summarized as follows: 1) high lake levels and cold conditions (Last Glacial Maximum [LGM], ∼24–19 ka), 2) fluctuating lake levels and cold conditions (Heinrich Stadial 1 [HS1], ∼19–15 ka), 3) high lake levels and warm and wetter conditions (Bølling-Allerød [BA], ∼15–13 ka), 4) low lake levels and dry conditions (Younger Dryas [YD], ∼13–11.5 ka) and 5) high lake levels and warm and wetter conditions (early Holocene, ∼11.5–10.0 ka). Average lake level fluctuation in Lago Petén Itzá during the late Pleistocene–early Holocene was as much as ∼25 m. Ostracode analyses suggest that the LGM was characterized by relatively low δ18O (+4.7 to +6.0‰), and δ13C values (−7.1 to −6.4‰) in ostracode valves, high inferred water depths and high percentages of broken adult and juvenile valves (>66%), suggesting a high-energy environment, strong currents, partial to full mixing, downslope transport, colder water temperatures and wetter conditions. An increase in the relative abundance of the benthic species L. opesta and higher numbers of broken valves suggest heavy precipitation events during the LGM (∼23.7, 21.7, 20.8 and 20.1 ka). HS1 was predominantly dry, but we identified times when lake levels were slightly higher, at the onset of the deglacial and a brief period (∼17–16 ka) between HS1b and HS1a. All studied climate proxies indicate wetter and warmer conditions and lake system stability during the BA. Lake levels dropped during the YD and gradually increased during the Preboreal and early Holocene. We demonstrate that modern and fossil ostracode isotopic signatures, species assemblages and taphonomy can be used together with physical and geochemical variables in Lago Petén Itzá sediments to make high-resolution inferences about late Pleistocene–early Holocene environmental changes in the lowland Neotropics.

a5d0831d-a4d2-30ea-a804-f368c7c5f2f0
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ID: a5d0831d-a4d2-30ea-a804-f368c7c5f2f0
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Abstract: Sediments in lakes in the Andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates. Despite their scarcity, carbonates along with organic matter provide significant paleoenvironmental information about lake systems. This study focuses on the carbonates in Lake Chungará, their morphologies, distribution and origin deduced from the isotopic markers. These markers reflected changes in the water and the biomass between the onset of the Holocene and around 9.6cal kyr BP. These changes are marked by general increases in TOC, TN, and TN-δ15NAIR, and by fluctuating values of TOC-δ13CVPDB in its sediments and are probably related to major shifts in the lake surface/volume associated with rises in lake level. An increase in salinity around 10cal kyr BP is thought to be linked to a short dry period, giving rise to the onset of carbonate production. The mid-Holocene arid period between 7.3 and 3.5calka BP, with a maximum of aridity around 6.0cal kyr BP, was deduced from δ18OVPDB values in the endogenic carbonates. These results match the reconstructions in Lake Titicaca based on benthic diatoms and paleoshore levels.Offshore sediments mainly consist of a diatomaceous ooze, laminated in the lower half (Unit 1), and banded-massive with tephra layers in the upper half of the sequence (Unit 2). TOC-δ13CVPDB and the C/N ratio confirm that phytoplankton was the main source of organic matter in these sediments. Shallower sediments (units 3 to 5) developed in platform and littoral settings, providing evidence of subaqueous macrophytes and, to a lesser extent, land plants. Carbonate content ranges between 0.1 and 6wt.% in offshore settings (30 to 40m water depth) and reaches the maximum values in the lower part of Unit 2. Carbonate minerals (low magnesium calcite and minor amounts of high magnesium calcite and aragonite) are scarce and are arranged in mm-thick layers, commonly forming cm-thick levels or bioclasts. Carbonate layers are made up of euhedral-to-subhedral spindle-shaped calcite crystals and, to a lesser extent, aragonite needles, all in the μm range. Aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains, where carbonate reaches locally up to 20wt.%. CO2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm-thick carbonate layers.The average values for δ13CVPDB in lake water, plankton and sediments of Lake Chungará (as organic matter or as carbonate) are around 15‰ higher than commonly reported values in other lakes. This 13C enrichment is attributed to carbon assimilation from a DIC affected by methanogenesis, in which HCO3 - is the dominant species. The δ13CVPDB and δ18OVPDB variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the Holocene. © 2011 Elsevier B.V.

8efa1c8d-3d6c-365b-9311-685f59e98bea
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ID: 8efa1c8d-3d6c-365b-9311-685f59e98bea
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Abstract: We have applied the TEX86 paleothermometer to produce a surface water temperature record for Lake Malawi spanning the past 700 years. Over much of the record temperature fluctuates from ~ 24-27 °C with a mean of ~ 25 °C ; however, there has been a substantial increase in temperature of ~ 2.0 °C during the past ~ 100 years. The TEX86 temperature record reveals a strong similarity to the instrumental record; both records demonstrate warming (~ 0.7-1.4 °C) over the past ~ 50 years as well as a cooling anomaly around 1959. Comparison of the TEX86 temperature record with the proxy records of primary productivity suggests that wind induced upwelling and/or precipitation have a strong influence on the surface temperature of Lake Malawi.

ba52ae2e-656a-38c2-8faf-77ea12a63b2f
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ID: ba52ae2e-656a-38c2-8faf-77ea12a63b2f
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Abstract: Ostracod species from at least seven genera were recovered from drill cores recently collected at Lake Malawi, a large (29,500 km2) and deep (706 m) African rift valley lake located in the southern African tropics (9-14° S). These genera include Limnocythere, Candonopsis, Ilyocypris, Sclerocypris, Gomphocythere, possible Allocypria? and an unknown Cypridosine. Taphonomic variables such as the percentage of valve breakage, adult individuals, carbonate and oxidized coatings as well as associated mineralogy, can be used to delineate lake-level low and highstands. This record of lake-level fluctuations is correlated with paleoecological changes in ostracod communities throughout the core record of the past ~ 145 ka. Two major assemblages found within the Lake Malawi cores include a Limnocythere-dominated shallow, saline/alkaline assemblage that occurred between 133 and 130 ka and between 118 and 90 ka and a deeper water Cypridopsine assemblage that lived in waters 10s to 100s of meters in depth and dominated the ostracod assemblage during intervals of lake-level transitions (136-133 ka, 129-128 ka and 86-63 ka), between the occurrence of the littoral assemblage and the appearance of indicators of bottom water anoxia. Changes in occurrences and abundances indicate variations in paleoecological affinities related to lake chemistry and oxygenation of bottom waters. The characteristics of the different lineages influence how that lineage is likely to respond to environmental variability such as lake-level fluctuations. Clades that include large numbers of endemic species in Lake Malawi tend to be specialists and thus more susceptible to environmental perturbations. On the other hand, cosmopolitan species within the lake all appear to be generalists, suggesting that they have high tolerances to environmental variability and therefore, these genera tend to be monospecific within the lake and are less likely to have radiated within the basin. The distribution of these ostracods can be used to evaluate hypotheses about the environmental history at the landscape-scale and their potential influence on species' distribution and diversification histories.

861cab01-6df2-39e5-89f5-e4d9f41a08d8
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ID: 861cab01-6df2-39e5-89f5-e4d9f41a08d8
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Abstract: Lakes on carbonate platform islands such as the Bahamas display wide variability in morphometry, chemistry, and fauna. These parameters are ultimately driven by climate, sea level, and carbonate accumulation and dissolution. The authors propose a model that integrates climatological, geomorphological, and stratigraphic frameworks to understand processes of carbonate-hosted lake formation and limnological characteristics in modern day environments, with applications to carbonate lake sedimentary records. Fifty-two lakes from San Salvador Island and Eleuthera, Bahamas, were examined for water chemistry, basin morphology, conduit development, conductivity, and major ions. Using non-metric, multi-dimensional scaling ordination methods, the authors derived a model dividing lakes into either constructional or destructional formational modes. Constructional lakes were further divided into pre-highstand and highstand types based on whether their formation occurred during a marine regressive or transgressive phase. Destructional lakes are created continually by dissolution of bedrock at fresh/saline water interfaces and their formation is therefore related to changing climate and sea level. This model shows that lake formation is influenced by the hydrologic balance associated with climatic conditions that drives karst dissolution as well as the deposition of aeolian dune ridges that isolate basins due to sea-level fluctuations. It allows for testing and examining the climatic and hydrologic regime as related to carbonate accumulation and dissolution through time, and for an improved understanding of lake sensitivity and response to climate as preserved in the lacustrine sedimentary record.

f91286d1-7f42-38ad-b70b-8121c24dcc99
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Abstract: High sediment accumulation rates in lacustrine and shallow-marine archives around Iceland offer the potential to compare high-resolution paleoclimatic reconstructions from terrestrial and marine archives; however, direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives. Icelandic paleomagnetic secular variation (PSV) has the potential to synchronize these records. Here we compare Holocene PSV from a well-dated marine core on the North Iceland shelf with PSV from two lacustrine archives with comparable sediment-accumulation rates, HVT03–1A, a glacier-dominated lake, and HAK03–1B, in a nonglacial catchment. Geochemically characterized tephra layers combined with unique high-amplitude structures in the PSV records provide secure tie points every ∼200 yr. Once the records are synchronized, the chronology from the marine core can be reliably transferred to the two lacustrine records. The resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every –1 k.y. Sediment accumulation rate changes occur at similar times in both lakes, despite very different catchment properties. Low and regular accumulation rates during the Holocene thermal maximum suggest regionally stable, vegetated catchments, followed by a stepped landscape destabilization during the transition into neoglaciation, culminating with maximum sedimentation rates during the Little Ice Age. PSV allows synchronization between multiple records from nearby marine and lacustrine archives, providing improved age models and a means of assessing leads and lags between marine and terrestrial environments.

6454868c-e8de-3e42-b569-a7653a0fc85d
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ID: 6454868c-e8de-3e42-b569-a7653a0fc85d
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Abstract: We present a 6,000-yr record of changing water balance in the Pacific Northwest inferred from measurements of carbonate δ(18)O and grayscale on a sediment core collected from Castor Lake, Washington. This subdecadally resolved drought record tracks the 1,500-yr tree-ring-based Palmer Drought Severity Index reconstructions of Cook et al. [Cook ER, Woodhouse CA, Eakin CM, Meko DM, Stahle DW (2004) Science 306:1015-1018] in the Pacific Northwest and extends our knowledge back to 6,000 yr B.P. The results demonstrate that low-frequency drought/pluvial cycles, with occasional long-duration, multidecadal events, are a persistent feature of regional climate. Furthermore, the average duration of multidecadal wet/dry cycles has increased since the middle Holocene, which has acted to increase the amplitude and impact of these events. This is especially apparent during the last 1,000 yr. We suggest these transitions were driven by changes in the tropical and extratropical Pacific and are related to apparent intensification of the El Niño Southern Oscillation over this interval and its related effects on the Pacific Decadal Oscillation. The Castor Lake record also corroborates the notion that the 20th century, prior to recent aridity, was a relatively wet period compared to the last 6,000 yr. Our findings suggest that the hydroclimate response in the Pacific Northwest to future warming will be intimately tied to the impact of warming on the El Niño Southern Oscillation.

39cc501f-ff46-3502-96ed-3de5b4859ec3
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ID: 39cc501f-ff46-3502-96ed-3de5b4859ec3
Tag: []
Abstract: The carbon stable isotopic value of dissolved inorganic carbon (δ13CDIC) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. δ13CDIC behavior in three of these lakes departed from the conventional model wherein epilimnetic waters are seasonally enriched relative to all hypolimnetic waters, and in general δ13CDIC values in the water column were not readily correlated to parameters such as lake stratification, algal productivity, hydraulic residence time, or water chemistry. Additionally, the processes implicated in generating the δ13CDIC values of individual lakes differ between lakes with similar δ13CDIC compositions. Each lake thus initially appears idiosyncratic, but when the effects of carbonate mineral equilibria, microbial activity, and lake residence time are viewed in terms of the magnitude of distinct DIC pools and fluxes in stratified lakes, generalizations can be made that allow lakes to be grouped by δ13CDIC behavior. We recognize three modes in the relationship between δ13CDIC values and DIC concentration ([DIC]) of individual lakes: (A) δ13CDIC values decreasing with increasing [DIC]; (B) δ13CDIC values increasing with increasing [DIC]; (C) δ13CDIC values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding δ13CDIC dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary δ13C components in the lacustrine paleorecord. © 2006 Elsevier Inc. All rights reserved.

4beba476-e89e-35b2-b53a-32a03205e31f
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ID: 4beba476-e89e-35b2-b53a-32a03205e31f
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Abstract: The detailed paleohydrological record of Lake Turkana, the largest lake in the eastern branch of the East African Rift, is necessary for determining the connectivity between adjacent watersheds in tropical Africa. The migration of both the Intertropical Convergence Zone (ITCZ) and the Congo Air Boundary (CAB) constrains rainfall amount and duration for most of East Africa. Lake Turkana, in northern Kenya, is the world's largest desert lake and experiences two ITCZ-associated rainy seasons annually, with cumulative rainfall of ~200mm/yr. Evidence from new continuous, high-fidelity sediment core records and high-resolution CHIRP seismic reflection data suggests that Lake Turkana received enough rainfall during the African Humid Period (AHP) to fill the lake to its sill (100m above current lake level) and spilled over into the White Nile River system. An atmospheric configuration with an eastward-shifted CAB over the Turkana region and the northern Kenya Rift is invoked as an additional source of rainfall for the catchment. This configuration began abruptly at ~11ka and lasted until ~5ka when Lake Turkana became a closed basin and was cut off from the White Nile. Prior to the AHP, Lake Turkana experienced at least two desiccation events following the Last Glacial Maximum at 18.5 and 17ka when the lake was at least 100m lower than modern, as evidenced by basin-wide, high amplitude reflections and 14C-dated shallow water facies in sediment cores. Lake level fluctuations generally follow trends in mean solar insolation, however the onset of lake level extremes are much more abrupt than rates of insolation change. Lake Turkana's location in relation to atmospheric convergence and dynamic rainfall patterns makes its susceptible to extreme climate change over relatively short timescales. © 2014 Elsevier B.V.

20d8637d-1a0b-3e40-b0cf-d2a01b295f3c
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ID: 20d8637d-1a0b-3e40-b0cf-d2a01b295f3c
Tag: []
Abstract: Reconstructing fire regimes and fuel characteristics is an important aspect of understanding past forest ecosystem processes. Fuel sources and fire regimes in the upper Midwestern United States have been shown to be sensitive to regional climatic variability, such as drought periods on millennial timescales. Yet, records documenting the connections between disturbance activity and the corresponding fuel source fluctuations in mesic deciduous forests and prairie/oak savanna in this region are limited. Thus, it has been difficult to provide a framework to evaluate changes in moisture availability on fire activity and the relationships with fuel source fluctuations in this region. We present high-resolution charcoal analyses of lake sediments from four sites in southern Wisconsin (USA) to characterize fire activity and fuel source fluctuation in mesic deciduous forests and prairie/oak savanna over the last 10 000 years. We found that fire occurrence across the four study sites has been asynchronous throughout the Holocene, because of site-specific differences that have strongly influenced local fire regimes. Additionally, we found that during periods of high fire activity the primary fuels were from arboreal sources, and during periods of low fire activity the primary fuels were from non-arboreal sources. However, fluctuations in fuel sources did not always correspond to changes in vegetation, or changes in fire frequency. © 2014 John Wiley & Sons, Ltd.

0fa288fd-97e0-377c-81b6-1d261f0ada49
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ID: 0fa288fd-97e0-377c-81b6-1d261f0ada49
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Abstract: Selected multi-proxy and accurately dated marine and terrestrial records covering the past 2000 years in the Iberian Peninsula (IP) facilitated a comprehensive regional paleoclimate reconstruction for the Medieval Climate Anomaly (MCA: 900-1300 AD). The sequences enabled an integrated approach to land-sea comparisons and, despite local differences and some minor chronological inconsistencies, presented clear evidence that the MCA was a dry period in the Mediterranean IP. It was a period characterized by decreased lake levels, more xerophytic and heliophytic vegetation, a low frequency of floods, major Saharan eolian fluxes, and less fluvial input to marine basins. In contrast, reconstruction based on sequences from the Atlantic Ocean side of the peninsula indicated increased humidity. The data highlight the unique characteristics of the MCA relative to earlier (the Dark Ages, DA: ca 500-900 years AD) and subsequent (the Little Ice Age, LIA: 1300-1850 years AD) colder periods. The reconstruction supports the hypothesis of Trouet et al. (2009), that a persistent positive mode of the North Atlantic Oscillation (NAO) dominated the MCA. © 2012 Elsevier Ltd.

85ceda72-f912-3d92-864d-c6973d2c7d2d
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ID: 85ceda72-f912-3d92-864d-c6973d2c7d2d
Tag: []
Abstract: Question: As anthropogenic change pushes ecosystems towards historically novel states, understanding how landscape disturbances interact with climate variability becomes increasingly important. We reconstructed the Holocene vegetation and fire history of a small watershed to investigate the linkages among disturbance regimes, biogeochemical cycling and ecosystem structure. Specifically we asked: (i) how do compositional and structural changes in forest ecosystems modify fire regimes; and (ii) how do changes in fuel type and fire regimes impact biogeochemical cycling? Location: Comstock Lake, Wisconsin, USA, a small kettle lake at the modern prairie-forest ecotone in the Great Lakes Region of North America. Methods: We analysed a lacustrine sediment core from Comstock Lake, Wisconsin, for paleoecological proxies including charcoal quantity and morphotype, pollen composition, magnetic susceptibility, organic matter concentration and stable carbon and nitrogen isotopes. These paleoecological variables were used to reconstruct climate-mediated shifts in vegetation, fire disturbance and biogeochemical cycling during the last 13 200 calendar years before present (cal yr BP). Results: Forest composition in the landscape surrounding Comstock Lake varied among coniferous forest, deciduous forest and savanna over the Holocene. Independent of the vegetation changes, five distinct fire regimes were identified, ranging from frequent, low-intensity surface fires to infrequent, high-intensity crown fires. The main feature of our vegetation reconstruction is a shift from Pinus to Quercus forest that occurred between 10 000 and 11 000 cal yr BP. However, changes in fuel source and forest structure were apparently more important than shifts of species composition in determining fire regimes. We observed an inverse relationship between non-arboreal fuel sources and biomass burning throughout all five fire regimes. The biogeochemical consequences of fires were apparent during some, but not all regime transitions. Generally, the sediments were composed of relatively small clastic inputs from the catchment and mostly autochthonous organic sources. Conclusion: The location of Comstock Lake on the prairie-forest boundary meant that regional climate signals during the Holocene were filtered through vegetation. Both compositional changes in dominant tree taxa and structural changes in canopy openness influenced fire regimes.

baaf9749-7177-3f96-97aa-a54b3467a56f
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ID: baaf9749-7177-3f96-97aa-a54b3467a56f
Tag: []
Abstract: We present the Holocene sequence from Lago Enol (43°16′N, 4°59′W, 1,070 m a.s.l.), Cantabrian Mountains, northern Spain. A multiproxy analysis provided comprehensive information about regional humidity and temperature changes. The analysis included sedimentological descriptions, physical properties, organic carbon and carbonate content, mineralogy and geochemical composition together with biological proxies including diatom and ostracod assemblages. A detailed pollen study enabled reconstruction of variations in vegetation cover, which were interpreted in the context of climate changes and human impact. Four distinct stages were recognized for the last 13,500 years: (1) a cold and dry episode that includes the Younger Dryas event (13,500–11,600 cal. year BP); (2) a humid and warmer period characterizing the onset of the Holocene (11,600–8,700 cal. year BP); (3) a tendency toward a drier climate during the middle Holocene (8,700–4,650 cal. year BP); and (4) a return to humid conditions following landscape modification by human activity (pastoral activities, deforestation) in the late Holocene (4,650–2,200 cal. year BP). Superimposed on relatively stable landscape conditions (e.g. maintenance of well established forests), the typical environmental variability of the southern European region is observed at this site.

ba946c76-b989-3d50-9d97-b2d3a1ca0ca9
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ID: ba946c76-b989-3d50-9d97-b2d3a1ca0ca9
Tag: []
Abstract: High-resolution geochemical analyses obtained using an X-ray fluorescence (XRF) Core Scanner, as well as mineralogical data from the Lago Chungará sedimentary sequence in the northern Andean Chilean Altiplano (18°S), provided a detailed reconstruction of the lacustrine sedimentary evolution during the last 14,000 cal. yr BP. The high-resolution analyses attained in this study allowed to distinguish abrupt periods, identify the complex structures of the early and mid-Holocene arid intervals and to compare their timing with Titicaca lake and Sajama ice records. Three main components in the lake sediments have been identified: (a) biogenic component, mainly from diatoms (b) volcanics (ash layers) from the nearby Parinacota Volcano and (c) endogenic carbonates. The correlation between volcanic input in Lago Chungará and the total particles deposited in the Nevado Sajama ice core suggests the Parinacota Volcano as the common source. The geochemical record of Lago Chungará indicates an increase in siliceous productivity during the early Holocene, lagging behind the rise in temperatures inferred from the Nevado Sajama ice core. The regional mid-Holocene aridity crisis can be characterized as a number of short events with calcite and aragonite precipitation in the offshore lake zones. © 2006 Elsevier Ltd and INQUA.

5cb78075-b05f-3bc6-a3b7-713c89f31722
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ID: 5cb78075-b05f-3bc6-a3b7-713c89f31722
Tag: []
Abstract: A multi-proxy study of short sediment cores recovered in small, karstic Lake Estanya (42°02′ N, 0°32′ E, 670 m.a.s.l.) in the Pre-Pyrenean Ranges (NE Spain) provides a detailed record of the complex environmental, hydrological and anthropogenic interactions occurring in the area since medieval times. The integration of sedimentary facies, elemental and isotopic geochemistry, and biological proxies (diatoms, chironomids and pollen), together with a robust chronological control, provided by AMS radiocarbon dating and 210Pb and 137Cs radiometric techniques, enabled precise reconstruction of the main phases of environmental change, associated with the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the industrial era. Shallow lake levels and saline conditions with poor development of littoral environments prevailed during medieval times (1150–1300 AD). Generally higher water levels and more dilute waters occurred during the LIA (1300–1850 AD), although this period shows a complex internal paleohydrological structure and is contemporaneous with a gradual increase of farming activity. Maximum lake levels and flooding of the current littoral shelf occurred during the nineteenth century, coinciding with the maximum expansion of agriculture in the area and prior to the last cold phase of the LIA. Finally, declining lake levels during the twentieth century, coinciding with a decrease in human pressure, are associated with warmer climate conditions. A strong link with solar irradiance is suggested by the coherence between periods of more positive water balance and phases of reduced solar activity. Changes in winter precipitation and dominance of NAO negative phases would be responsible for wet LIA conditions in western Mediterranean regions. The main environmental stages recorded in Lake Estanya are consistent with Western Mediterranean continental records, and show similarities with both Central and NE Iberian reconstructions, reflecting a strong climatic control of the hydrological and anthropogenic changes during the last 800 years.

63c689f1-0c4a-3b0c-b67d-78eef42e7a41
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ID: 63c689f1-0c4a-3b0c-b67d-78eef42e7a41
Tag: []
Abstract: Lake Karakul in the Pamirs (Tajikistan) is a deep brackish-water lake in a closed basin at almost 4000 m above sea level. Water samples from the catchment area and Lake Karakul, and a 104-cm sediment core from its shallow eastern sub-basin, were investigated and provide a first lake record from the region spanning the last 4200 cal yr BP. Multi-proxy analyses revealed the following: A relatively high meltwater inflow from glaciers, snow fields and frozen ground in the catchment as a result of relatively warm conditions was reconstructed for the period between 4200 and 3500 cal yr BP. The shift to colder climatic conditions around 3500 cal yr BP was probably abrupt; total organic carbon values displayed the most dramatic drop for the entire core within less than 40 years. In contrast, the subsequent re-establishment of warmer conditions occurred gradually over several centuries. A higher meltwater supply to the lake and warmer conditions were recorded since ca. 1900 cal yr BP with two slightly colder intervals between 1200 and 800 cal yr BP and between 400 and 100 cal yr BP. The abrupt shift to significantly colder and drier conditions around 3500 cal yr BP in the eastern Pamirs is seen also in records from the Aral Sea and the Guliya ice core from the western Tibetan Plateau. However, more palaeoclimate studies in this highly continental part of Central Asia are needed to assess the spatial and temporal patterns of the Holocene climate in the region. © 2010 Elsevier B.V. All rights reserved.

5147c5db-ea74-31b0-8db7-2ccd2baa75db
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ID: 5147c5db-ea74-31b0-8db7-2ccd2baa75db
Tag: []
Abstract: We present a palaeoclimatic reconstruction of the last glacial cycle in Iberia (ca. 120,000–11,600 cal yrs BP) based on multi-proxy reconstructions from lake sediments with robust chronologies, and with a particular focus on abrupt climate changes. The selected lake sequences provide an integrated approach from northern Iberia exploring temperature conditions, humidity variations and land-sea comparisons during the most relevant climate transitions of the last glacial period. Thus, we present evidence that demonstrates: (i) cold but relatively humid conditions during the transition from MIS 5 to MIS 4, which prevailed until ca. 60,000 cal yrs BP in northern Iberia; (ii) a general tendency towards greater aridity during MIS 4 and MIS 3 (ca 60,000 to 23,500 cal yrs BP) punctuated by abrupt climate changes related to Heinrich Events (HE), (iii) a complex, highly variable climate during MIS 2 (23,500 to 14,600 cal yrs BP) with the “Mystery Interval” (MI: 18,500 to 14,600 cal yrs BP) and not the global Last Glacial Maximum (LGM: 23,000 to 19,000 cal yrs BP) as the coldest and most arid period. The last glacial transition starts in synchrony with Greenland ice records at 14,600 cal yrs BP but the temperature increase was not so abrupt in the Iberian records and the highest humidity was attained during the Allerød (GI-1a to GI-1c) and not during the Bølling (GI-1e) period. The Younger Dryas event (GS-1) is discernible in northern Iberian lake records as a cold and dry interval, although Iberian vegetation records present a geographically variable signal for this interval, perhaps related to vegetation resilience.

a0ddeacb-1bef-3c11-a69d-390d8bab2eb2
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ID: a0ddeacb-1bef-3c11-a69d-390d8bab2eb2
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Abstract: A number of studies have shown that Fourier transform infrared spectroscopy (FTIRS) can be applied to quantitatively assess lacustrine sediment constituents. In this study, we developed calibration models based on FTIRS for the quantitative determination of biogenic silica (BSi; n Combining double low line 420; gradient: 0.9-56.5%), total organic carbon (TOC; n Combining double low line 309; gradient: 0-2.9%), and total inorganic carbon (TIC; nCombining double low line 152; gradient: 0-0.4%) in a 318 m-long sediment record with a basal age of 3.6 million years from Lake El'gygytgyn, Far East Russian Arctic. The developed partial least squares (PLS) regression models yield high cross-validated (CV) R2CV Combining double low line 0.86-0.91 and low root mean square error of cross-validation (RMSECV) (3.1-7.0% of the gradient for the different properties). By applying these models to 6771 samples from the entire sediment record, we obtained detailed insight into bioproductivity variations in Lake El'gygytgyn throughout the middle to late Pliocene and Quaternary. High accumulation rates of BSi indicate a productivity maximum during the middle Pliocene (3.6-3.3 Ma), followed by gradually decreasing rates during the late Pliocene and Quaternary. The average BSi accumulation during the middle Pliocene was ∼3 times higher than maximum accumulation rates during the past 1.5 million years. The indicated progressive deterioration of environmental and climatic conditions in the Siberian Arctic starting at ca. 3.3 Ma is consistent with the first occurrence of glacial periods and the finally complete establishment of glacial-interglacial cycles during the Quaternary. ©Author(s) 2014.

b2c66f91-9c21-3c1d-87bb-7c0ea815cd58
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ID: b2c66f91-9c21-3c1d-87bb-7c0ea815cd58
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Abstract: Both terrestrial and aquatic ecosystems should respond to abrupt climate changes such as those that affected the midcontinent of North America during the Holocene. A variety of paleorecords indicate that the onset of prolonged dry conditions in this region occurred as rapidly as 300 yr during the early Holocene, with a subsequent increase in moisture occurring rapidly in the late Holocene. Here, we report a 9500 yr multiproxy sediment record from Deming Lake, Minnesota, USA, that demonstrates only subtle dynamics during rapid climate changes that caused vegetation in the catchment to shift among pine forest, open grassland, and deciduous forest. The most substantial changes in ecosystem properties immediately followed deglaciation of the landscape, formation of the lake, and initial development of pine forests. In contrast, there were only muted responses to pronounced mid-Holocene climate changes that caused vegetation in the catchment to switch from pine forest to open grassland (ca. 8000 yr B.P.), and then deciduous forest (ca. 5400 yr B.P.). The flux of organic and inorganic terrigenous material, the processing of carbon, and catchment erosion changed rapidly during the most recent shift at 3300 yr B.P. to the modern pine forest. These changes coincided with the onset of meromictic conditions that influenced internal lake dynamics. However, the terrestrial influence on the lake sedimentary record gradually diminished over time, indicating a trajectory of increasing catchment stabilization that was relatively impervious to dramatic regional climate changes. The relative complacency of the Deming Lake record during the late Holocene indicates relative resistance to abrupt climate change at later stages of ecosystem development.

4500b774-e2fc-3558-9ced-3a9de1a73940
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ID: 4500b774-e2fc-3558-9ced-3a9de1a73940
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Abstract: Glaciated alpine landscapes are sensitive to changes in climate. Shifts in temperature and precipitation can cause significant changes to glacier size and terminus position, the production and delivery of organic mass, and in the hydrologic energy related to the transport of water and sediment through proglacial environments. A sediment core representing a 12,900-yr record collected from Swiftcurrent Lake, located on the eastern side of Glacier National Park, Montana, was analyzed to assess variability in Holocene and latest Pleistocene environment. The spectral signature of total organic carbon content (%TOC) since ~ 7.6 ka matches that of solar forcing over 70-500 yr timescales. Periodic inputs of dolomite to the lake reflect an increased footprint of Grinnell Glacier, and occur during periods when sediment sinks are reduced, glacial erosion is increased, and hydrologic energy is increased. Grain size, carbon/nitrogen (C/N) ratios, and %TOC broadly define the termination of the Younger Dryas chronozone at Swiftcurrent Lake, as well as major Holocene climate transitions. Variability in core parameters is linked to other records of temperature and aridity in the northern Rocky Mountains over the late Pleistocene and Holocene.

a0fce8ed-4c91-3a4d-bdcb-84f3558ac17b
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ID: a0fce8ed-4c91-3a4d-bdcb-84f3558ac17b
Tag: []
Abstract: We used charcoal and fossil pollen to investigate how fire, vegetation and climate have interacted over the past 2300 years at Ferry Lake, located on a sand plain in northwestern Wisconsin. Pollen analysis shows a rapid transition from oak (Quercus spp.)-dominated woodland to a relatively open pine (Pinus spp.) forest at 1450 cal. yr BP, and a more closed-canopy pine forest beginning about 700 cal. yr BP. We calculated accumulation rates of 125-250 ųm charcoal fragments (CHAR) in contiguous 0.5 cm thick sediment samples, each representing 7-10 years. Graminoid charcoal fragments were tallied separately to track the relative abundance of grass charcoal. During the oak period charcoal peaks have relatively weak periodicity and relatively high accumulation rates of grass charcoal. Charcoal peaks are less frequent (with a periodicity of 130-200 years), and larger during the open-canopy pine period, with lower grass CHAR. CHAR of both charcoal types decreases further between 1000 and 850 cal. yr BP and remains low until the period of European settlement. Several hundred years later (700 cal. yr BP) white pine pollen increases and pollen from herbaceous taxa decreases, suggesting a more mesic, closed-canopy forest. Our results demonstrate that the vegetation and fire regime at this sandplain site changed substantially, but apparently not synchronously, during the last 2300 years, a period when millennial-scale regional climate was relatively similar to modern.

7bf7ba93-ca51-3ced-9ea3-d6b5959bff43
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ID: 7bf7ba93-ca51-3ced-9ea3-d6b5959bff43
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Abstract: Central Mexico is a complex area with late Quaternary volcanic activity, climatic diversity and a long history of human occupation. Pollen, microcharcoal and magnetic susceptibility analyses of two sedimentary sequences from Lake Zirahuen in western central Mexico spanning the last 17,000 years provide evidence of a highly dynamic environment. During the late glacial Pinus forest developed around the lake, indicating cold and dry conditions. During the last deglaciation climatic amelioration was recorded at 13.5 ka, driving a strong and rapid change in vegetation composition and increase in lake levels. From the latest Pleistocene to early Holocene a hiatus, probably related to an erosive event, is recognized in the northern sequence. The central sequence spans the entire Holocene and reveals three periods of important ecological changes during the early Holocene. A first episode between 9.5 and 9.0 ka with a decrease in pine forest seems to have been associated with summer insolation increases. A second peak of forest change at 8.2 ka and was probably associated with the cold oscillation documented in the North Atlantic. A third abrupt change was evident from 7.5 to 7.1 ka with an anomalous plant community related to wetter Holocene climates and possible to a volcanic event. An episode of dry conditions was recorded from 4.5 to 4.2 ka, which was related to an increase in ENSO activity. Human influence over the landscape was evident from 3.5 ka to the present. The Zirahuen records offer a complex history of a landscape characterized by short and long term vegetation changes associated with factors ranging from global climate to local disturbances.

e1f16a8b-ba24-3378-a8bc-4d87fa7240ec
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Abstract: A combination of marine (Alboran Sea cores, ODP 976 and TTR 300 G) and terrestrial (Zoñar Lake, Andalucia, Spain) geochemical proxies provides a high-resolution reconstruction of climate variability and human influence in the southwestern Mediterranean region for the last 4000 years at inter-centennial resolution. Proxies respond to changes in precipitation rather than temperature alone. Our combined terrestrial and marine archive documents a succession of dry and wet periods coherent with the North Atlantic climate signal. A dry period occurred prior to 2.7 cal ka BP - synchronously to the global aridity crisis of the third-millennium BC - and during the Medieval Climate Anomaly (1.4-0.7 cal ka BP). Wetter conditions prevailed from 2.7 to 1.4 cal ka BP. Hydrological signatures during the Little Ice Age are highly variable but consistent with more humidity than the Medieval Climate Anomaly. Additionally, Pb anomalies in sediments at the end of the Bronze Age suggest anthropogenic pollution earlier than the Roman Empire development in the Iberian Peninsula. The Late Holocene climate evolution of the in the study area confirms the see-saw pattern between the eastern and western Mediterranean regions and the higher influence of the North Atlantic dynamics in the western Mediterranean. © Author(s) 2010.

c0d718fc-be0c-39b0-a4bb-a04005dd8317
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ID: c0d718fc-be0c-39b0-a4bb-a04005dd8317
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Abstract: Lake Malawi contains a long continuous sedimentary record of climate change in the southern hemisphere African tropics. We develop a stratigraphic framework of this basin over the last ~ 150 ka by integrating several vintages of seismic-reflection data with recently acquired drill cores. In the seismic-reflection data set, we document three lake-level cycles where progradational delta seismic facies and erosional truncation surfaces mark the basal boundary of each sequence. The clinoform packages and their down-dip, time-equivalent surfaces can be mapped throughout each basin, where each major lowstand surface was followed by a transgression and highstand. On several occasions, lake level dropped as much as 500 m below present lake level (BPLL) in the North Basin and 550 m BPLL in the Central Basin, resulting in a 97% reduction of water volume and 89% reduction of water surface area relative to modern conditions. Evidence for these lake-level fluctuations in the drill cores includes major changes in saturated bulk density, natural gamma ray values, and total organic carbon. During lowstands, density values doubled, while total organic carbon values dropped from ~ 5% to 0.2%. Coarse-grained sediment and organic matter flux into the basin were higher during transgressions, when precipitation, runoff, sediment supply, and nutrient input were high. This sedimentation pattern is also observed in seismic-reflection profiles, where coarse-grained seismic facies occur at the bases of sequences, and in the drill-core data where the highest total organic carbon values are observed immediately above lowstand surfaces.

77ae2fb8-e158-34be-8004-9047bd5f464b
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ID: 77ae2fb8-e158-34be-8004-9047bd5f464b
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Abstract: High-resolution geochemical analysis of a 6-m-long sediment core from Zoñar Lake, southern Spain, provides a detailed characterization of major changes in lake and watershed processes during the last 4,000 years. Geochemical variables were used as paleolimnological indicators and complement Zoñar Lakes's paleoenvironmental reconstruction based on sedimentological and biological proxies, which define periods of increasing allochthonous input to the lake and periods of dominant autochthonous sedimentation. Chemical ratios identify periods of endogenic carbonate formation (higher Ca/Al, Sr/Al and Ba/Al ratios), evaporite precipitation (higher S/Al, Sr/Al ratios), and anoxic conditions (higher Mo/Al, U/Th ratios and Eu anomaly). Higher productivity is marked by elevated organic carbon content and carbonate precipitation (Mg/Ca). Hydrological reconstruction for Zoñar Lake was based on sedimentological, mineralogical and biological proxies, and shows that lower lake levels are characterized by Sr-rich sediments (a brackish lake with aragonite) and S-rich sediments (a saline lake with gypsum), while higher lake levels are characterized by sediments enriched in elements associated with alumino-silicates (Al, K, Ti, Fe, trace and rare earth elements), reflecting fresher conditions. Geochemical indicators also mark periods of higher detrital input to the lake related to human activity in the watershed: (1) during the Iberian Roman Humid Period (650 BC-AD 300), around the onset of the Little Ice Age (AD 1400), during the relatively drier Post-Roman and Middle Ages (AD 800-1400), and over the last 50 years, due to mechanized farming practices. Heavy metal enrichment in the sediments (Cu and Ni) suggests intensification of human activities during the Iberian Roman Period, and the use of fertilizers during the last 50 years. © 2009 Springer Science+Business Media B.V.

595c58c8-43d1-360a-bdee-a0ae9bcd1e50
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ID: 595c58c8-43d1-360a-bdee-a0ae9bcd1e50
Tag: []
Abstract: We inferred millennial-scale climate variations and paleohydrological conditions in the northern sector of the American tropics for 30.3–5.5 cal ka BP using geochemical characteristics of sediments from Lake Chalco in central Mexico. The sediment sequence is chronologically constrained with three tephra and nine radiocarbon dates. Temporal variations in titanium, total inorganic carbon, total organic carbon/titanium ratio, carbon/nitrogen ratio, and silica/titanium ratio indicate changes in runoff, salinity, productivity, and sources. Higher concentrations of Ti indicate more runoff during latest Marine Isotope Stage (MIS) 3 (30.3–28.6 cal ka BP). Runoff was lower during the last glacial maximum (LGM; 23–19 cal ka BP) than during the Heinrich 2 event (26–24 cal ka BP). The interval of reduced runoff continued up to 17.5 cal ka BP but increased during the Bølling/Allerød. Trends of decreasing runoff and increasing salinity are observed throughout MIS 1. Lake Chalco received less runoff during the LGM compared to deglaciation, opposite the trend of other North American tropical records. Different amounts of rainfall at different sites are possibly due to shifts in the position of the Intertropical Convergence Zone, changes in the size of the Altlantic warm pool, and varying sea-surface temperatures of the Atlantic and Pacific oceans.

69df97de-7e1b-3b18-b5e6-ebaa919af499
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ID: 69df97de-7e1b-3b18-b5e6-ebaa919af499
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Abstract: The Greenland Ice Sheet is a major component of the Arctic cryosphere and the magnitude of its response to future climate changes remains uncertain. Longer-term records of climate near the ice sheet margin provide information about natural climate variability and can be used to understand the causes of past changes in the Greenland Ice Sheet. As a proxy for Holocene climate near the ice sheet margin, we reconstruct the fluctuations of Bregne ice cap in the Scoresby Sund region of central east Greenland. Bregne is a small ice cap (2.5km2 in area) and responds sensitively to summer temperatures. We employ a multi-proxy approach to reconstruct the ice cap fluctuations using geomorphic mapping, 10Be ages of boulders and bedrock and lake sediment records.Past extents of Bregne ice cap are marked by moraines and registered by sediments in downvalley lakes. 10Be ages of bedrock and boulders outboard of the moraines indicate that Bregne ice cap was within ~250m of its present-day limit by at least 10.7ka. Multi-proxy data from sediments in Two Move lake, located downvalley from Bregne ice cap, indicate that the ice cap likely completely disappeared during early and middle Holocene time. Increasing magnetic susceptibility and percent clastic material from ~6.5 to ~1.9calkaBP in Two Move lake sediments suggest progressively colder conditions and increased snow accumulation on the highlands west of the lake. Laminated silt deposited at ~2.6calkaBP and ~1.9calkaBP to present registers the onset and persistence of Bregne ice cap during the late Holocene. 10Be ages of boulders on an unweathered, unvegetated moraine in the Bregne ice cap forefield range from 0.74 to 9.60ka. The youngest 10Be age (0.74ka) likely represents the age of the moraine whereas older ages may be due to 10Be inherited from prior periods of exposure. This late Holocene moraine marks the second largest advance of the ice cap since deglaciation of the region at the end of the last ice age. The oldest moraine in the forefield dates to ≤2.6calkaBP. The fluctuations of Bregne ice cap were likely influenced by Northern Hemisphere summer insolation throughout the Holocene and abrupt late Holocene cold events. © 2013 Elsevier Ltd.

d6134dcc-4bba-3b95-9c4b-780b1fd80757
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ID: d6134dcc-4bba-3b95-9c4b-780b1fd80757
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Abstract: The nonlinear and complex behavior of glacier dynamic processes (e.g., surging and ice calving) presents major challenges for future estimates of runoff and sea-level change. Because direct observations are temporally limited, reconstructions of past fluctuations from glaciers that undergo dynamic advance and/or retreat are valuable. We constructed a 300 yr, annually resolved record of surges and terminus fluctuations of Langjökull ice cap (central Iceland) using a novel combination of varve counting, multibeam bathymetry, seismic imagery, and multiple sediment cores from targeted sites in Hvítárvatn, a large proglacial lake. Langjökull achieved its maximum Neoglacial extent between ca. A.D. 1700 and ca. 1930, when two outlet glaciers, Norðurjökull and Suðurjökull, advanced into the lake and maintained active calving margins. Norðurjökull advanced into the basin ca. 1720, and remained at or near its maximum extension for most of the 19th century, whereas Suðurjökull underwent a quasi-periodic series of 8 surges between 1828 and 1930, with a recurrence interval of 14 ± 4 yr. Each surge event resulted in fragmentation of the glacier terminus during advances of up to 1.6 km that occurred in <2 yr. Collapse of the expanded ice, iceberg melting, and reestablishment of the ice front at a nearshore grounding line occurred within 1–3 yr of the surge. Surges converted glacier ice to runoff at ∼10× the non-surging rate. Our precise estimates of the timing, duration, and magnitude of Suðurjökull surges provide ideal targets for the next generation of glacier surge models.

bd344b07-dce1-3d2e-9164-34eb1406acb6
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ID: bd344b07-dce1-3d2e-9164-34eb1406acb6
Tag: []
Abstract: Iceland is well situated to monitor North Atlantic Holocene climate variability. Terrestrial sites there offer the potential for well-dated, high-resolution, continuous records of environmental change and/or glacier activity. Laminated sediments from the proglacial lake Hvítárvatn provide a continuous record of environmental change and the development of the adjacent Langjökull ice cap for the past 10.2 ka. Replicate lake sediment cores, collected from multiple locations in the basin, are placed in a secure geochronology by splicing a varve chronology for the past 3 ka with a tephra-constrained, paleomagnetic secular variation derived chronology for older sediments. Multiple proxies, including sedimentation rate, bulk density, ice-rafted debris, sediment organic matter, biogenic silica, and diatom abundance, allow annual to multi-decadal resolution and reveal a dynamic Holocene terrestrial climate. Following regional deglaciation of the main Iceland Ice Sheet, summer temperatures were high enough that mountain ice caps had already melted, or were contributing insignificant sediment to the lake. Pronounced increases in sedimentation rate, sediment density, and the influx of terrestrial organic matter, between 8.7 and 7.9 ka suggest early Holocene warmth was interrupted by two distinct pulses of cold summers leading to widespread landscape destabilization and possibly glacier growth. The Holocene thermal maximum (HTM; 7.9 to 5.5 ka) was characterized by high within-lake productivity and ice-free conditions in the watershed. Neoglaciation is recorded as a non-linear transition toward cooler summers, landscape destabilization, and the inception and expansion of Langjökull beginning ca 5.5 ka, with notable increases in ice cap size and landscape instability at 4.2 and 3.0 ka. The past two millennia are characterized by the abrupt onset of sustained cold periods at ca 550 and 1250 AD, separated by an interval of relative warmth from ca 950 to 1150 AD. The greatest Holocene extent of Langjökull occurred in the nineteenth century and is coincident with peak landscape instability, followed by ice recession throughout the twentieth century.

48b0847b-832e-3926-8a3b-f3490b1c21e7
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ID: 48b0847b-832e-3926-8a3b-f3490b1c21e7
Tag: []
Abstract: The most explosive volcanic event of the Quaternary was the eruption of Mt. Toba, Sumatra, 75,000 y ago, which produced voluminous ash deposits found across much of the Indian Ocean, Indian Peninsula, and South China Sea. A major climatic downturn observed within the Greenland ice cores has been attributed to the cooling effects of the ash and aerosols ejected during the eruption of the Youngest Toba Tuff (YTT). These events coincided roughly with a hypothesized human genetic bottleneck, when the number of our species in Africa may have been reduced to near extinction. Some have speculated that the demise of early modern humans at that time was due in part to a dramatic climate shift triggered by the supereruption. Others have argued that environmental conditions would not have been so severe to have such an impact on our ancestors, and furthermore, that modern humans may have already expanded beyond Africa by this time. We report an observation of the YTT in Africa, recovered as a cryptotephra layer in Lake Malawi sediments, >7,000 km west of the source volcano. The YTT isochron provides an accurate and precise age estimate for the Lake Malawi paleoclimate record, which revises the chronology of past climatic events in East Africa. The YTT in Lake Malawi is not accompanied by a major change in sediment composition or evidence for substantial temperature change, implying that the eruption did not significantly impact the climate of East Africa and was not the cause of a human genetic bottleneck at that time.

46f96df7-62e7-3850-a090-a28720b60da3
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ID: 46f96df7-62e7-3850-a090-a28720b60da3
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Abstract: A suite of environmental proxies in annually laminated sediments from Hvítárvatn, a proglacial lake in the central highlands of Iceland, are used to reconstruct regional climate variability and glacial activity for the past 3000 years. Sedimentological analysis is supported by tephrostratigraphy to confirm the continuous, annual nature of the laminae, and a master varve chronology places proxies from multiple lake cores in a secure geochronology. Varve thickness is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the adjacent Langjökull ice cap. The continuous presence of glacially derived clastic varves in the sediment fill confirms that the ice cap has occupied the lake catchment for the duration of the record. Varve thickness, varve thickness variance, ice-rafted debris, total organic carbon (mass flux and bulk concentration), and C:N of sedimentary organic matter, reveal a dynamic late Holocene climate with abrupt and large-scale changes in ice-cap size and landscape stability. A first-order trend toward cooler summers and ice-cap expansion is punctuated by notable periods of rapid ice cap growth and/or landscape instability at ca 1000 BC, 600 BC, 550 AD and 1250 AD. The largest perturbation began ca 1250 AD, signaling the onset of the Little Ice Age and the termination of three centuries of relative warmth during Medieval times. Consistent deposition of ice-rafted debris in Hvítárvatn is restricted to the last 250 years, demonstrating that Langjökull only advanced into Hvítárvatn during the coldest centuries of the Little Ice Age, beginning in the mid eighteenth century. This advance represents the glacial maximum for at least the last 3 ka, and likely since regional deglaciation 10 ka. The multi-centennial response of biological proxies to the Hekla 3 tephra deposition illustrates the significant impact of large explosive eruptions on local environments, and catchment sensitivity to perturbations. © 2011 Elsevier Ltd.

47e32374-3e66-3a93-8291-16ff10e4d80e
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ID: 47e32374-3e66-3a93-8291-16ff10e4d80e
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Abstract: The climate of tropical Africa transitioned from an interval of pronounced, orbitally-paced megadroughts to more humid and stable conditions approximately 70,000 years ago (Scholz et al., 2007). The regional atmospheric circulation patterns that accompanied these climatic changes, however, are unclear due to a paucity of continental paleoclimate records from tropical Africa extending into the last interglacial. We present a new 140-kyr record of the deuterium/hydrogen isotopic ratio of terrestrial leaf waxes (δDwax) from drill cores from Lake Malawi, southeast Africa, that spans this important climatic transition. δDwax shifts from highly variable and relatively D-depleted to more stable and D-enriched around 56 ka, contemporary with the onset of more humid conditions in the region. Moisture source and transport history dominate the δDwax signal at Lake Malawi, with local rainfall amount playing a secondary role for much of the paleorecord. Analysis of modern moisture sources for Lake Malawi suggests that D-depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier, D-depleted air mass currently located in central southern Africa to the Lake Malawi catchment. This D-depleted air mass is associated with the descending limb of the Hadley cell, which implies significant changes in the Hadley circulation during the megadroughts and related changes in the position of the Intertropical Convergence Zone over Africa. These findings demonstrate the ability of δDwax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance, and elucidate potential mechanisms for pronounced hydrological change in southeast Africa during the late Pleistocene.

b4c4e268-a242-320c-80da-7dff5213bd5a
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ID: b4c4e268-a242-320c-80da-7dff5213bd5a
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Abstract: Sediments from Lake Pepin on the Mississippi River, southeastern Minnesota, are used as provenance tracers to assess variations in hydrology and sediment-transport during the middle Holocene. Three rivers contribute sediment to Lake Pepin, and each catchment is characterized by a distinctly different geologic terrain. The geochemical fingerprint for each drainage basin was determined from the elemental composition of heavy minerals in the silt-sized fraction of modern sediment samples. Down-core elemental abundances were compared with these fingerprints by use of a chemical-mass-balance model that apportions sediment to the source areas. We observed a decreased contribution from the Minnesota River during the interval ∼6700-5500 14C yr BP, which we attribute to decreased discharge of the Minnesota River, likely controlled by a combination of precipitation, snow melt, and groundwater input to the river. This hydrologic condition coincides with the mid-Holocene prairie period recorded by fossil pollen data. The occurrence of this feature in a proxy record for hydrologic variations supports the hypothesis that the mid-Holocene prairie period reflects drier conditions than before or after in midwestern North America. © Springer 2006.

76b273f1-0cec-3ec6-9382-e16a3dacc26d
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ID: 76b273f1-0cec-3ec6-9382-e16a3dacc26d
Tag: []
Abstract: Thermokarst lakes and peat-accumulating drained lake basins cover a substantial portion of Arctic lowland landscapes, yet the role of thermokarst lake drainage and ensuing peat formation in landscape-scale carbon (C) budgets remains understudied. Here we use measurements of terrestrial peat thickness, bulk density, organic matter content, and basal radiocarbon age from permafrost cores, soil pits, and exposures in vegetated, drained lake basins to characterize regional lake drainage chronology, C accumulation rates, and the role of thermokarst-lake cycling in carbon dynamics throughout the Holocene on the northern Seward Peninsula, Alaska. Most detectable lake drainage events occurred within the last 4,000 years with the highest drainage frequency during the medieval climate anomaly. Peat accumulation rates were highest in young (50–500 years) drained lake basins (35.2 g C m−2 yr−1) and decreased exponentially with time since drainage to 9 g C m−2 yr−1 in the oldest basins. Spatial analyses of terrestrial peat depth, basal peat radiocarbon ages, basin geomorphology, and satellite-derived land surface properties (Normalized Difference Vegetation Index (NDVI); Minimum Noise Fraction (MNF)) from Landsat satellite data revealed significant relationships between peat thickness and mean basin NDVI or MNF. By upscaling observed relationships, we infer that drained thermokarst lake basins, covering 391 km2 (76%) of the 515 km2 study region, store 6.4–6.6 Tg organic C in drained lake basin terrestrial peat. Peat accumulation in drained lake basins likely serves to offset greenhouse gas release from thermokarst-impacted landscapes and should be incorporated in landscape-scale C budgets.

63f8bbfb-c4ef-3813-ab11-344fdc121712
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ID: 63f8bbfb-c4ef-3813-ab11-344fdc121712
Tag: []
Abstract: Continuous high-resolution pollen data for the past 225 ka from sediments in Bear Lake, Utah-Idaho reflect changes in vegetation and climate that correlate well with variations in summer insolation and global ice-volume during MIS 1 through 7. Spectral analysis of the pollen data identified peaks at 21-22 and 100 ka corresponding to periodicities in Earth's precession and eccentricity orbital cycles. Suborbital climatic fluctuations recorded in the pollen data, denoted by 6 and 5 ka cyclicities, are similar to Greenland atmospheric temperatures and North Atlantic ice-rafting Heinrich events. Our results show that millennial-scale climate variability is also evident during MIS 5, 6 and 7, including the occurrence of Heinrich-like events in MIS 6, showing the long-term feature of such climate variability. This study provides clear evidence of a highly interconnected ocean-atmosphere system during the last two glacial/interglacial cycles that extended its influence as far as continental western North America. Our study also contributes to a greater understanding of the impact of long-term climate change on vegetation of western North America. Such high-resolution studies are particularly important in efforts of the scientific community to predict the consequences of future climate change.

726cd87b-727a-3d1f-8636-588c8455d77a
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ID: 726cd87b-727a-3d1f-8636-588c8455d77a
Tag: []
Abstract: We describe five common charcoal morphotypes observed in late-Holocene lake sediments from northern Wisconsin and compare them with charcoal produced by burning modern plant material. Our experiments show that grass cuticle, conifer wood and leaves of some broadleaved taxa all produce recognizable charcoal types that are preserved in sediments. We use the identification of charcoal morphotypes to enhance our interpretation of a previously published charcoal record from Ferry Lake, Wisconsin. The occurrence of the different charcoal morphotypes changed as the vegetation and fire regimes changed over the past 2300 yr. Charred grass cuticle was more common before 1300 cal. yr BP when small charcoal peaks were frequent and the pollen assemblage suggests that an open oak savanna surrounded the lake. Charcoal with bordered pits produced from burned conifer wood was more common after 1300 cal. yr BP, when red/jack pine pollen increased and the frequency of charcoal peaks decreased, suggesting a switch from a surface fire regime to one with less frequent crown fires. Our results suggest that stratigraphic changes in the occurrence of charcoal morphotypes can improve our understanding of past vegetation and fire regimes. © 2007 SAGE Publications.

72264215-5d9e-32a1-81f1-f781193da145
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ID: 72264215-5d9e-32a1-81f1-f781193da145
Tag: []
Abstract: The multiproxy study (sedimentology, geochemistry and diatoms) of sediment cores from Sanabria Lake (42°07'30″ N, 06°43'00″ W, 1000m a.s.l.) together with a robust 14C chronology provides the first high-resolution and continuous sedimentary record in the region, extending back the last 26ka. The development of a proglacial lake before 26calka BP demonstrates the onset of deglaciation before the global Last Glacial Maximum, similarly to other alpine glaciers in southern European mountains. Rapid deglaciation occurred at the beginning of the Greenland Interstadial GI-1e (Bølling, 14.6calka BP). Following a short-lived episode of glacier re-advance (14.4-14.2calka BP, GI-1d), a climatic improvement at 13.9calka BP suggests the glaciers retreated from the lake basin during the GI-1c. Another glacier reactivation phase occurred between ca13.0-12.4ka, starting earlier than the onset of GS-1 (Younger Dryas). Rapid deglaciation during the Early Holocene (11.7-10.1calka BP) was followed by a period of higher river discharge (10.1-8.2calka BP). After 8.2ka, the Holocene is characterized by a general decreasing trend in humidity, punctuated by the driest phase during the Mid Holocene (ca6.8-4.8), a wetter interval between 4.8 and 3.3calka BP, and a relatively decline of rainfall since then till present, with a minor increase in humidity during some phases (ca1670-1760) of the Little Ice Age.Discrete silt layers intercalated in the organic-rich Holocene deposits reflect large flooding events of the Tera River (ca10.1, 8.4, 7.5, 6.2, 5.7-5.6, 4.6, 4.2, 3.7, 3.3, 3.1, 2.7, 2.5 and 2.0calka BP). Their synchronicity with a number of cold and humid events described in the Atlantic demonstrates a strong control of NW Iberian climate by North Atlantic dynamics at centennial-millennial scale. Comparison with Western Mediterranean records points to similar regional dynamics during the Holocene, although modulated in the NW Iberian Peninsula by the stronger Atlantic influence. © 2014 Elsevier Ltd.

6beac4df-cb4a-32b8-9df1-5469557ce8dd
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ID: 6beac4df-cb4a-32b8-9df1-5469557ce8dd
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Abstract: Aeolian dust is rarely considered an important source for nutrients in large peatlands, which generally develop in moist regions far from the major centers of dust production. As a result, past studies assumed that the Everglades provides a classic example of an originally oligotrophic, P-limited wetland that was subsequently degraded by anthropogenic activities. However, a multiproxy sedimentary record indicates that changes in atmospheric circulation patterns produced an abrupt shift in the hydrology and dust deposition in the Everglades over the past 4,600 y. A wet climatic period with high loadings of aeolian dust prevailed before 2800 cal BP (calibrated years before present) when vegetation typical of a deep slough dominated the principal drainage outlet of the Everglades. This dust was apparently transported from distant source areas, such as the Sahara Desert, by tropical storms according to its elemental chemistry and mineralogy. A drier climatic regime with a steep decline in dustfall persisted after 2800 cal BP maintaining sawgrass vegetation at the coring site as tree islands developed nearby (and pine forests covered adjacent uplands). The marked decline in dustfall was related to corresponding declines in sedimentary phosphorus, organic nitrogen, and organic carbon, suggesting that a close relationship existed between dustfall, primary production, and possibly, vegetation patterning before the 20th century. The climatic change after 2800 cal BP was probably produced by a shift in the Bermuda High to the southeast, shunting tropical storms to the south of Florida into the Gulf of Mexico.

9324b80c-f75c-3179-b1ca-a727003e5490
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ID: 9324b80c-f75c-3179-b1ca-a727003e5490
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Abstract: 1 The Hudson Bay Lowlands have been rising isostatically for the past 7000 years, creating a regional chronosequence as new land emerges from the sea. Rates of uplift are most rapid in the eastern portion of the lowlands near the lower Albany River study area. 2 The stratigraphy of three raised bogs was investigated to determine rates and pathways of peatland development in the Albany River region. The bogs are distributed evenly along the regional chronosequence from the oldest site at Oldman (5980 ± 100 BP) to progressively younger sites at Albany River (4810 ± 70) and Belec Lake (3960 ± 60). 3 Each bog had the same stratigraphic sequence, beginning with a basal tidal marsh assemblage that was rapidly replaced by a Larix-dominated swamp forest, followed by a Picea-forested bog, and ultimately a non-forested bog. The bog-fen boundary is marked by the disappearance of fen indicators, dominance of bog-forming Sphagna, and a sharp decline in nitrogen. Each of these successional stages was associated with different rates of vertical growth. 4 The rate of successional change was more rapid at the younger sites, and their vertical growth curve was more curvilinear. The formation of a raised bog, for example, was 1.3 times more rapid at Albany River and 5.5 times more rapid at Belec Lake than at Oldman. Belec Lake reached its ultimate successional stage first, although it was the last site to emerge from the sea. 5 The differential rate of isostatic uplift across this region rather than climate was the principal environmental driver for peatland development. The faster rate of uplift on the lower reaches of the drainage basin continues to reduce the regional slope, impede drainage and shift river channels, continually altering the local hydrogeological setting. 6 Groundwater flow simulations based on the Dupuit equation show that the growth of these raised bogs was probably constrained by their local hydrogeological setting. Bog formation was first induced by the creation of interfluvial divides between headwardly eroding streams or shifting river channels, and further bog growth was ultimately constrained by the width of the interfluve and the depth of river incision. The Belec Lake bog was the first to approach its limiting height because its narrow interfluve could only support a low water-table mound. 7 Although peatland succession largely followed the same conservative pathway at each site, both the pace and direction of these pathways were set by geological processes, which are probably the decisive drivers for the evolution of this large peat basin.

6bc1fcce-c52b-31e8-8ce7-77e6fabf29ed
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ID: 6bc1fcce-c52b-31e8-8ce7-77e6fabf29ed
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Abstract: A high resolution multiproxy study (magnetic susceptibility, X-ray diffraction, XRF scanner, gray-colour values, Total Organic Carbon, Total Inorganic Carbon, Total Carbon and Total Biogenic Silica) of the sedimentary infill of Lago Chungará (northern Chilean Altiplano) was undertaken to unravel the environmental forcings controlling its evolution using a number of different multivariate statistical techniques. Redundancy analyses enabled us to identify the main provenance of the studied proxies whereas stratigraphically unconstrained cluster analyses allowed us to distinguish the "outsiders" as result of anomalous XRF scanner acquisitions. Principal Component Analysis (PCA) was employed to identify and isolate the main underlying environmental gradients that characterize the sedimentary infill of Lago Chungará. The first eigenvector of the PCA could be interpreted as an indicator of changes in the input of volcaniclastic material, whereas the second one would indicate changes in water availability. The chronological model of this sedimentary sequence was constructed using 17 AMS 14C and 1 238U/230Th dates in order to characterize the volcaniclastic input and the changes in water availability in the last 12,300 cal years BP. Comparison of the reconstructed volcaniclastic input of Lago Chungará with the dust particle record from the Nevado Sajama ice core suggested that the Parinacota volcano eruptions were the main source of dust during the mid and Late Holocene rather than the dry out lakes as has previously been pointed out. The comparison of the water availability reconstruction of Lago Chungará with three of the most detailed paleoenvironmental records of the region (Paco Cocha, Lake Titicaca and Salar Uyuni) showed an heterogeneous (and sometimes contradictory) temporal and spatial pattern distribution of moisture. Although the four reconstructions showed a good correlation, each lacustrine ecosystem responded differently to the moisture oscillations that affected this region. The variations in the paleoenvironmental records could be attributed to the dating uncertainities, lake size, lake morphology, catchment size and lacustrine ecosystem responses to the abrupt arid events. © 2007 Springer Science+Business Media B.V.

5a8d28b3-ce2b-32f5-abce-551610d462b4
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ID: 5a8d28b3-ce2b-32f5-abce-551610d462b4
Tag: []
Abstract: Late-Holocene climatic conditions in the upper Great Lakes region have changed sufficiently to produce significant changes in vegetation and fire regimes. The objective of this study was to determine how the vegetation mosaic and fire regimes on an oak (Quercus spp.)- and pine (Pinus spp.)-dominated sand plain in northwestern Wisconsin responded to climatic changes of the past 1,200 years. We used pollen and charcoal records from a network of sites to investigate the range of natural variability of vegetation on a 1,500-km 2 landscape on the southern part of the sand plain. A major vegetation shift from jack pine (Pinus banksiana) and red pine (P. resinosa) to increased abundance of white pine (P. strobus) occurred between 700 and 600 calendar years before present (cal yr BP), apparently corresponding to more mesic conditions regionally. A decrease in charcoal accumulation rate also occurred at most sites but was not synchronous with the vegetation change. At some sites there were further changes in vegetation and fire regimes occurring ∼500-300 cal yr BP, but these changes were not as strong or unidirectional as those that occurred 700-600 cal yr BP. Our results suggest that both the composition and the distribution of vegetation of the southern part of the sand plain have been sensitive to relatively small climatic changes, and that the vegetation at the time of European settlement was a transitory phenomenon, rather than a long-term stable condition. © 2007 Springer Science+Business Media B.V.

c60c9c88-b2b9-3d62-bc4d-f5ef3d6a5c98
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ID: c60c9c88-b2b9-3d62-bc4d-f5ef3d6a5c98
Tag: []
Abstract: Oxygen isotopes of diatom silica and petrographical characterisation of diatomaceous laminated sediments of Lago Chungará (northern Chilean Altiplano) have allowed us to establish its palaeohydrological evolution during the Lateglacial–early Holocene (ca. 12 000–9400 cal. yr BP). These laminated sediments are composed of light and dark pluriannual couplets of diatomaceous ooze formed by different processes. Light sediment laminae accumulated during short-term diatom blooms whereas dark sediment laminae represent the baseline limnological conditions during several years of deposition. Oxygen isotope analysis of the dark diatom laminae show a general δ18O enrichment trend during the studied period. Comparison of these δ18Odiatom values with the previously published lake-level evolution suggests a correlation between δ18Odiatom and the precipitation:evaporation ratio, but also with the evolution of other local hydrological factors as changes in the groundwater outflow as well as shifts in the surface:volume ratio of Lago Chungará. The lake expanded (probably increasing this ratio) during the rising lake-level trend due to changes in its morphology, enhancing evaporation. Furthermore, the lake's hydrology was probably modified as the groundwater outflow became sealed by sediments, increasing lake water residence time and potential evaporation. Both factors could cause isotope enrichment. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons, Ltd.

f38b2513-aeaf-3a7f-ba4d-be40362da75f
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ID: f38b2513-aeaf-3a7f-ba4d-be40362da75f
Tag: []
Abstract: Monthly limnological monitoring in Lake Sanabria (Spain) since 1986 provided a unique opportunity to test relationships among climate, hydrology and lake dynamics and how they are recorded in the lake sediments. Four datasets were employed: (1) meteorological (monthly maximum and minimum air temperature and total precipitation), (2) limnological (Secchi disk, water temperature, conductivity, pH, dissolved oxygen, nitrate, silicon, total and reactive phosphorus, and total chlorophylls and chlorophyll a), (3) hydrological (Tera River water input and output), and (4) XRF core scanner measurements carried out in short cores. Linear models between the different dataset variables allowed us to characterize the climate signal transmission from one to the other and cross-correlation analyses permitted us to identify the different response times (if any) between them. Principal Component Analyses (PCA) of the limnological and geochemical datasets allowed us to identify the main processes that link lake dynamics, primarily nutrient supply and organic productivity, with some sedimentological processes, e. g. organic matter and phosphorus accumulation. Sediment chronology was established by gamma spectrometry (210Pb). Water input to Lake Sanabria is controlled mostly by the Tera River input and is linked directly to precipitation. Response of the Lake Sanabria water budget to climate oscillations is immediate, as the strongest correlation between these two datasets occurs with no lag time. PCA of the limnological dataset indicated that most of the variance is related to nutrient input, and comparison with the Tera River water discharge shows that nutrient input was controlled mainly by oscillations in the hydrological balance. The lag time between the hydrological and limnological datasets is 1 month. The PCA of the XRF core scanner dataset showed that the principal process that controls the chemical composition of the Lake Sanabria sediments is related to sediment and nutrient delivery from the Tera River and organic productivity. Comparison of the nutrient input reconstructed using the limnological dataset and the XRF core scanner data indicated that the sediments act as a low-pass filter, smoothing the climate signal. It was, however, possible to establish the link between these datasets, and obtain a quantitative reconstruction of precipitation for the 1959-2005 AD period that captures the regional variability. This quantitative precipitation reconstruction suggests it is possible to obtain accurate climate reconstructions using non-laminated sediments. © 2011 Springer Science+Business Media B.V.

50f7085a-4f4b-3b23-b6aa-67b4851fa6dd
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ID: 50f7085a-4f4b-3b23-b6aa-67b4851fa6dd
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Abstract: Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the Bølling-Allerød warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.

3d0c9d83-7417-38a4-bd5a-3ab51ec00872
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ID: 3d0c9d83-7417-38a4-bd5a-3ab51ec00872
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Abstract: Two high-sediment-accumulation-rate Icelandic lakes, the glacial lake Hvítárvatn and the non-glacial lake Haukadalsvatn, contain numerous tephra layers of known age, which together with high-resolution paleomagnetic secular variations allow synchronization with a well-dated marine core from the shelf north of Iceland. A composite standardized climate record from the two lakes provides a single time series that efficiently integrates multi-proxy data that reflect the evolution of summer temperatures through the Holocene. The first-order trends in biogenic silica (BSi), δ13C, and C:N rise relatively abruptly following deglaciation, reaching maximum values shortly after 8 ka following a complex minimum between 8.7 and 8.0 ka. The Holocene Thermal Maximum (HTM) in the lakes is marked by all proxies, with a sharp transition out of the 8 ka cold event into peak summer warmth by 7.9 ka, and continuing warm with some fluctuations until 5.5 ka. Decreasing summer insolation after the HTM is reflected by incremental cooling, initially ∼5.5 ka, with subsequent cold perturbations recorded by all proxies 4.3 to 4.0 ka and 3.1 to 2.8 ka. The strongest disturbance occurred after 2 ka with initial summer cooling occurring between 1.4 and 1.0 ka, followed by a more severe drop in summer temperatures after 0.7 ka culminating between 0.5 and 0.2 ka. Following each late Holocene cold departure, BSi re-equilibrated at a lower value independent of the sediment accumulation rate. Some of the abrupt shifts may be related to Icelandic volcanism influencing catchment stability, but the lack of a full recovery to pre-existing values after the perturbation suggests increased periglacial activity, decreased vegetation cover, and glacier growth in the highlands of Iceland. The similarity in timing, direction and magnitude of our multi-proxy records from glacial and non-glacial lakes, and from the adjacent marine shelf, suggests that our composite record reflects large-scale shifts in ocean/atmosphere circulation throughout the northern North Atlantic.

44d65e36-ad56-3a67-a482-0c22acb97a98
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ID: 44d65e36-ad56-3a67-a482-0c22acb97a98
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Abstract: Seismic profiles of Far East Russian Lake El'gygytgyn, formed by a meteorite impact some 3.6 million years ago, show a stratified sediment succession that can be separated into subunits Ia and Ib at approximately 167 m below lake floor (Combining double low line∼3.17 Ma). The upper (Ia) is well-stratified, while the lower is acoustically more massive and discontinuous. The sediments are intercalated with frequent mass movement deposits mainly in the proximal areas, while the distal region is almost free of such deposits at least in the upper part. In spring 2009, a long core drilled in the lake center within the framework of the International Continental Scientific Drilling Program (ICDP) penetrated the entire lacustrine sediment succession down to ∼320 m below lake floor and about 200 m farther into the meteorite-impact-related bedrock. Downhole logging data down to 390 m below lake floor show that the bedrock and the lacustrine part differ significantly in their petrophysical characteristics. The contact between the bedrock and the lacustrine sediments is not abrupt, but rather transitional with a variable mixture of impact-altered bedrock clasts in a lacustrine matrix. Physical and chemical proxies measured on the cores can be used to divide the lacustrine part into five different statistical clusters. These can be plotted in a redox-condition vs. input-type diagram, with total organic carbon content and magnetic susceptibility values indicating anoxic or oxic conditions and with the Si/Ti ratio representing more clastic or more biogenic input. Plotting the clusters in this diagram allows identifying clusters that represent glacial phases (cluster I), super interglacials (cluster II), and interglacial phases (clusters III and IV). © Author(s) 2013.

3e817097-53fd-34ee-b348-c3e7ef42e404
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ID: 3e817097-53fd-34ee-b348-c3e7ef42e404
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Abstract: Rock magnetic, biochemical and inorganic records of the sediment cores PG1351 and Lz1024 from Lake El'gygytgyn, Chukotka peninsula, Far East Russian Arctic, were subject to a hierarchical agglomerative cluster analysis in order to refine and extend the pattern of climate modes as defined by Melles et al. (2007). Cluster analysis of the data obtained from both cores yielded similar results, differentiating clearly between the four climate modes warm, peak warm, cold and dry, and cold and moist. In addition, two transitional phases were identified, representing the early stages of a cold phase and slightly colder conditions during a warm phase. The statistical approach can thus be used to resolve gradual changes in the sedimentary units as an indicator of available oxygen in the hypolimnion in greater detail. Based upon cluster analyses on core Lz1024, the published succession of climate modes in core PG1351, covering the last 250 ka, was modified and extended back to 350 ka. Comparison to the marine oxygen isotope (δ18O) stack LR04 (Lisiecki and Raymo, 2005) and the summer insolation at 67.5 °N, with the extended Lake El'gygytgyn parameter records of magnetic susceptibility (κLF), total organic carbon content (TOC) and the chemical index of alteration (CIA; Minyuk et al., 2007), revealed that all stages back to marine isotope stage (MIS) 10 and most of the substages are clearly reflected in the pattern derived from the cluster analysis. © Author(s) 2013.

98d70ff3-898d-317e-bedf-58b3a963b51a
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ID: 98d70ff3-898d-317e-bedf-58b3a963b51a
Tag: []
Abstract: Multiproxy climate records from Iceland document complex changes in terrestrial climate and glacier fluctuations through the Holocene, revealing some coherent patterns of change as well as significant spatial variability. Most studies on the Last Glacial Maximum and subsequent deglaciation reveal a dynamic Iceland Ice Sheet (IIS) that responded abruptly to changes in ocean currents and sea level. The IIS broke up catastrophically around 15 ka as the Polar Front migrated northward and sea level rose. Indications of regional advance or halt of the glaciers are seen in late Alleröd/early Younger Dryas time and again in PreBoreal time. Due to the apparent rise of relative sea level in Iceland during this time, most sites contain evidence for fluctuating, tidewater glacier termini occupying paleo fjords and bays. The time between the end of the Younger Dryas and the Preboreal was characterized by repeated jökulhlaups that eroded glacial deposits. By 10.3 ka, the main ice sheet was in rapid retreat across the highlands of Iceland. The Holocene thermal maximum (HTM) was reached after 8 ka with land temperatures estimated to be 3 °C higher than the 1961-1990 reference, and net precipitation similar to modern. Such temperatures imply largely ice-free conditions across Iceland in the early to mid-Holocene. Several marine and lacustrine sediment climate proxies record substantial summer temperature depression between 8.5 and 8 ka, but no moraines have been detected from that time. Termination of the HTM and onset of Neoglacial cooling took place sometime after 6 ka with increased glacier activity between 4.5 and 4.0 ka, intensifying between 3.0 and 2.5 ka. Although a distinct warming during the Medieval Warm Period is not dramatically apparent in Icelandic records, the interval from ca AD 0 to 1200 is commonly characterized by relative stability with slow rates of change. The literature most commonly describes Little Ice Age moraines (ca AD 1250-1900) as representing the most extensive ice margins since early Holocene deglaciation, with temperature depressions of 1-2 °C compared to the AD 1961-1990 average. Steep north-south and west-east temperature gradients are reconstructed in the Holocene records of Iceland, suggesting a strong maritime influence on the terrestrial climate of Iceland. © 2009 Elsevier Ltd.

a36fe035-03de-35fc-9505-2f12310b5029
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ID: a36fe035-03de-35fc-9505-2f12310b5029
Tag: []
Abstract: Lake El'gygytgyn, located in the Far East Russian Arctic, was formed by a meteorite impact about 3.58 Ma ago. In 2009, the International Continental Scientific Drilling Program (ICDP) at Lake El'gygytgyn obtained a continuous sediment sequence of the lacustrine deposits and the upper part of the impact breccia. Here, we present grain-size data of the past 2.6 Ma. General downcore grain-size variations yield coarser sediments during warm periods and finer ones during cold periods. According to principal component analysis (PCA), the climate-dependent variations in grain-size distributions mainly occur in the coarse silt and very fine silt fraction. During interglacial periods, accumulation of coarser material in the lake center is caused by redistribution of clastic material by a wind-induced current pattern during the ice-free period. Sediment supply to the lake is triggered by the thickness of the active layer in the catchment and the availability of water as a transport medium. During glacial periods, sedimentation at Lake El'gygytgyn is hampered by the occurrence of a perennial ice cover, with sedimentation being restricted to seasonal moats and vertical conduits through the ice. Thus, the summer temperature predominantly triggers transport of coarse material into the lake center. Time series analysis that was carried out to gain insight into the frequency of the grain-size data showed variations predominately on 98.5, 40.6, and 22.9 kyr oscillations, which correspond to Milankovitch's eccentricity, obliquity and precession bands. Variations in the relative power of these three oscillation bands during the Quaternary suggest that sedimentation processes at Lake El'gygytgyn are dominated by environmental variations caused by global glacial-interglacial variations (eccentricity, obliquity), and local insolation forcing and/or latitudinal teleconnections (precession), respectively. ©Author(s) 2013.

72c8dff7-e181-3329-adaf-76dec2da0495
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ID: 72c8dff7-e181-3329-adaf-76dec2da0495
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Abstract: In order to better constrain the limnological impacts from recent climate change relative to those of the Holocene, we developed a high-resolution multi-proxy paleoenvironmental record from a small lake on eastern Baffin Island, Arctic Canada. Carbon and nitrogen elemental and isotopic compositions from sediment organic matter, algal pigments, and diatom assemblages are integrated to provide robust indices of paleoclimatic variability. In particular, the ratio between individual carotenoid pigments (lutein:diatoxanthin) reveals a shift in dominant primary production from ‘green’ taxa (chlorophytes, higher plants, and bryophytes) during the Holocene Thermal Maximum (HTM) to ‘brown’ taxa (diatoms and chrysophytes) over the mid- to late Holocene. Green pigment abundance appears most sensitive to mean summer temperatures, and their increased relative abundance in the past serves as an indicator of warm times. Regionally, the HTM occurred shortly after local deglaciation (10 ka), persisting until ~7 ka. This timing agrees with that revealed by chironomid assemblages and ice-core records elsewhere in the Canadian Arctic, but is significantly earlier than suggestions from palynology on Baffin Island. This study provides additional evidence that this discrepancy represents the ecesis for higher plant dispersal and colonization on distal, freshly deglaciated landscapes. Pigment and diatom data indicate that mid Holocene cooling began between 7 and 6 ka, intensifying after 3 ka. All proxies show pronounced change after 1.5 ka, with the greatest divergence from average Holocene values occurring during the Little Ice Age (LIA), supporting the growing consensus that the LIA was the coldest multi-centennial interval of the Holocene. In the twentieth century, most proxies, including sedimentary carotenoid ratios, abruptly returned to a similar state as the Holocene Thermal Maximum, while diatom species assemblages present a more muted response. This underscores that anthropogenic alteration of the Earth system has created conditions with no exact analog in the past 10,000 years. Collectively, these results add new information on the dimensions of Arctic lake responses to Holocene climate change, which in turn can be used to reconcile paleoclimate reconstructions from diverse proxies.

f6ebc2ef-d8ce-36aa-b221-ec95665de129
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ID: f6ebc2ef-d8ce-36aa-b221-ec95665de129
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Abstract: Modern process studies of the hydrologic balance of Lake El'gygytgyn, central Chukotka, and the sediment income from the catchment were carried out during a field campaign in spring and summer 2003. Despite high uncertainties due to the limited data, the results provide important first estimates for better understanding the modern and past sedimentation processes in this basin. Formed ca. 3.6 million years ago as a result of a meteorite impact, the basin contains one of the longest paleoclimate records in the terrestrial Arctic. Fluvial activity is concentrated over the short snowmelt period (about 20 days in second part of June). Underground outflow plays a very important role in the water balance and predominates over surface outflow. The residence time of the lake water is estimated to be about 100 yr. © 2013. CC Attribution 3.0 License.

b9186eb5-34a8-3d80-838e-1ac51c8e864e
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ID: b9186eb5-34a8-3d80-838e-1ac51c8e864e
Tag: []
Abstract: We present a continuous ostracod isotope (δ18O and δ13C) record from Lake Petén Itzá, Petén, Guatemala, in the northern, lowland Neotropics that spans the last ∼43 cal ka BP. Variations in oxygen and carbon isotopes closely follow lithologic variations, which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate, respectively. During the last glacial period, the greatest δ18O and δ13C values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with North Atlantic Heinrich events. In contrast, interstadials and the entirety of the Last Glacial Maximum (∼24–19 cal ka BP) are marked by clay deposition and lower δ18O and δ13C values, reflecting higher lake levels and relatively moister climate. Isotope results and pollen data, along with independently inferred past water levels, show the early deglacial period (∼19–15 cal ka BP) was the time of greatest aridity and lowest lake stage of the past 43 ka. This period occurred during Heinrich Stadial 1 (HS 1), when an extensive tropical megadrought has been postulated (Stager et al., 2011). Heinrich Stadial 1 is represented by two episodes of gypsum precipitation and high δ18O and δ13C values in Petén Itzá, interrupted by an intervening period of lower δ18O and δ13C and clay deposition centered on ∼17 cal ka BP. The two periods of inferred maximum cold and/or arid conditions at ∼17.5 and 16.1 cal ka BP coincide approximately with two pulses of ice-rafted debris (IRD) recorded off southern Portugal (Bard et al., 2000). At ∼15 cal ka BP, coinciding with the start of the Bolling-Allerod period, δ18O and δ13C decrease and gypsum precipitation ceases, indicating a transition to warmer and/or wetter conditions. Gypsum precipitation resumed while δ18O and δ13C increased at the start of the Younger Dryas at 13.1 cal ka BP and continued until 10.4 cal ka BP, near the onset of the Holocene. Precipitation changes during the last glacial period in the northern hemisphere Neotropics were closely linked with freshwater forcing to the high-latitude North Atlantic, and sensitive to changes in the location of meltwater input. Climate was coldest/driest when meltwater directly entered the high-latitude North Atlantic, permitting sea ice expansion and weakening of Atlantic Meridional Overturning Circulation (AMOC), which resulted in a more southerly position of the Intertropical Convergence Zone (ITCZ). Upon deglaciation, when meltwater was directed to the Gulf of Mexico, at ∼17 ka and during the Bolling-Allerod period (15–13 ka), precipitation increased in the northern hemisphere Neotropics as North Atlantic sea ice retreated and the ITCZ shifted northward. Results from Lake Petén Itzá offer some support for the meltwater routing hypothesis of Clark et al. (2001).

a4c2b044-4ed0-30db-8379-64a54bb3a2e2
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ID: a4c2b044-4ed0-30db-8379-64a54bb3a2e2
Tag: []
Abstract: The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, 'dust-bowl-like' megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C(4) plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4-6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

7f3c3008-3366-3b40-920d-bfbf0bdc11fb
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ID: 7f3c3008-3366-3b40-920d-bfbf0bdc11fb
Tag: []
Abstract: Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) was used to identify the qualitative differences between dissolved organic matter (DOM) in fen and bog porewaters from the Red Lake II system in the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota. Approximately 80% of the molecular composition in surface porewater was maintained throughout the upper portion of the bog profile (0.17-2.50. m). The qualitative stability of the composition of the DOM was accompanied by a quantitative increase in dissolved organic carbon (DOC) with depth. The composition of DOM in the fen was significantly different at depth, with slightly varying DOC levels. Aromaticity index (AI) values were used to identify condensed aromatic and phenol-type compounds in the porewater of both peatlands. Surface bog and deep fen DOM had surprisingly similar molecular composition. Differences in enzymatic degradation rates via phenol oxidase in the bog and surface fen horizons, slower transport down the bog vertical profile and the presence of a stratum of Sphagnum-woody peat at depth in the fen are suggested as being responsible for the observed variations in DOM composition. © 2010 Elsevier Ltd.

6797f871-f60e-3d4d-b660-f4b7b04ce5d8
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ID: 6797f871-f60e-3d4d-b660-f4b7b04ce5d8
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Abstract: The Arctic region is subject to a great amplitude of climate variability and is currently undergoing large-scale changes due in part to anthropogenic global warming. Accurate projections of future change depend on anticipating the response of the Arctic climate system to forcing, and understanding how the response to human forcing will interact with natural climate variations. The Svalbard Archipelago occupies an important location for studying patterns and causes of Arctic climate variability; however, available paleoclimate records from Svalbard are of restricted use due to limitations of existing climate proxies. Here we present a sub-decadal- to multidecadal-scale record of summer temperature for the past 1800 yr from lake sediments of Kongressvatnet on West Spitsbergen, Svalbard, based on the first instrumental calibration of the alkenone paleothermometer. The age model for the High Arctic lake sediments is based on 210Pb, plutonium activity, and the first application of tephrochronology to lake sediments in this region. We find that the summer warmth of the past 50 yr recorded in both the instrumental and alkenone records was unmatched in West Spitsbergen in the course of the past 1800 yr, including during the Medieval Climate Anomaly, and that summers during the Little Ice Age (LIA) of the 18th and 19th centuries on Svalbard were not particularly cold, even though glaciers occupied their maximum Holocene extent. Our results suggest that increased wintertime precipitation, rather than cold temperatures, was responsible for LIA glaciations on Svalbard and that increased heat transport into the Arctic via the West Spitsbergen Current began ca. A.D. 1600.

c0bcd182-98f4-3acc-a68f-2644a4abcb6e
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ID: c0bcd182-98f4-3acc-a68f-2644a4abcb6e
Tag: []
Abstract: To date, terrestrial archives of long-term climatic change within the Arctic have widely been restricted to ice cores from Greenland and, more recently, sediments from Lake El'gygytgyn in northeast Arctic Russia. Sediments from this lake contain a paleoclimate record of glacialinterglacial cycles during the last three million years. Lowresolution studies at this lake have suggested that changes observed during Transition IV (the transition from marine isotope stage (MIS) 10 to MIS 9) are of greater amplitude than any observed since. In this study, geochemical parameters are used to infer past climatic conditions thus providing the first high-resolution analyses of Transition IV from a terrestrial Arctic setting. These results demonstrate that a significant shift in climate was subsequently followed by a rapid increase in biogenic silica (BSi) production. Following this sharp increase, bioproductivity remained high, but variable, for over a thousand years. This study reveals differences in the timing and magnitude of change within the ratio of silica to titanium (Si/Ti) and BSi records that would not be apparent in lower resolution studies. This has significant implications for the increasingly common use of Si/Ti data as an alternative to traditional BSi measurements. © Author(s) 2013.

18347166-6a9c-3c9e-90d3-934b74653441
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ID: 18347166-6a9c-3c9e-90d3-934b74653441
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Abstract: The close interplay between hydraulic and biotic processes controls the evolution of ecosystems in large, circumboreal peatlands and the degree to which they lose, gain or sequester carbon. In peatlands, biota significantly alters surficial acid–base equilibrium and solute chemistry by releasing dissolved organic compounds into surface and pore waters, which then mix through advection and dispersion through pore water flow paths. We report herein the results of new geochemical mixing models that incorporates organic acid dissociation constants to understand how organic acids control the acid–base chemistry in mixtures of dilute acidic bog pore waters and circum-neutral groundwater (typical of carbonate terrains) that upwells or disperses into peat deposits. In our new mixing models, at least twice as much groundwater is required to neutralize bog water acidity when dissolved organic carbon concentrations exceed 10 mmol/L than in their absence. Although it remains uncertain how future climatic change will alter the composition of dissolved organic matter in peatlands, organic acids should remain an important factor with respect to potential neutralization of peatland waters by groundwater sources. The acid base equilibria in large peat basins not only has important implications for vegetation patterning but also for biogeochemical cycles in these globally important reservoirs for carbon.

7ae89bcf-1aa5-31f3-8e70-6babf78ee195
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ID: 7ae89bcf-1aa5-31f3-8e70-6babf78ee195
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Abstract: Lake Arreo sequence (western Ebro Basin, Spain) illustrates the century-scale climatic variability and human interactions in the landscape during the last 2.5 kyr in the low lands of northern Spain. Two sediment cores from shallow-water and deep-water environments were analyzed using sedimentological, geochemical, mineralogical, biological –diatoms, pollen and charcoal content-, and radiometric techniques for absolute dating. The shallow-water sequence indicates a rapid evolution from an alluvial-influenced wetland prior to 7th century BC to a wetland during the Ibero-Roman Humid Period (BC 630 – AD 465) and a deeper, carbonate producing lake during the Dark Ages Cold Period (AD 465-890). The deep-water core shows the transition from a more saline lake during the arid Medieval Climate Anomaly (MCA, AD 890-1300) to less saline, meromictic conditions, particularly since the onset of the Little Ice Age (LIA, AD 1300-1870). During the last 2.5 kyr, arid conditions occurred prior to 1st century AD, during the MCA and late 19th- mid 20th century while colder temperatures and relatively more humid conditions were more frequent during the the Dark Ages, particularly the 7th century AD and the LIA. The evolution of the lake also reflects changes in grazing and agricultural practices since the Roman Period associated to the exploitation of nearby salt mining. Periods of intense human pressure on the lake watershed occurred during the High Middle Ages (AD 890-1180) and the Modern Period (AD 1600-1830).

659962eb-1f30-3d0d-b1cb-fd0a0c0ef49b
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ID: 659962eb-1f30-3d0d-b1cb-fd0a0c0ef49b
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Abstract: We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes that affect the lake catchment. Chronology is based on floating discontinuous varve counts and 137Cs and 14C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans.

d3331208-efe5-36ad-a9bb-89ce0cafafcb
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ID: d3331208-efe5-36ad-a9bb-89ce0cafafcb
Tag: []
Abstract: Vegetation composition and fire frequency are tightly linked in North American grasslands and have varied considerably throughout the Holocene in response to different drivers. Yet, detailed records of both long-term changes in grassland vegetation composition and diversity, coupled with fire history, are still relatively sparse. In this study, we examine a sediment core from Fox Lake, Minnesota, using pollen, charcoal, magnetic susceptibility, organic carbon (%C), and silica (%Si) records with the aim of understanding grassland structure and function during the Holocene, particularly in the context of vegetation composition and diversity, erosion, and fire activity. Nonarboreal pollen comprises between 37% and 86% of the assemblage throughout the record with the largest percentages occurring during the mid-Holocene (~8000–4000 yr BP). The pollen record also suggests that at 8200 yr BP, there was an abrupt shift from oak-elm woodland to a more open landscape of grassland or savanna, which remained throughout the mid-Holocene. Additionally, the pollen data suggest that vegetation composition exhibited little change in diversity through time despite recurring fire. Charcoal concentrations varied from 30 to nearly 1200 particles cm−3, indicating changes in relative amount of biomass burned, but the morphotypes of charcoal pieces indicate that woody fuels persisted during the mid-Holocene despite the apparent grassland-dominated landscape. Magnetic susceptibility in the sediment ranges from −0.9 to 22.4 (×10−5 SI) throughout the record, with the biggest increase occurring as the vegetation shifted from woodland to grassland entering the mid-Holocene. Organic carbon ranges from 4.6% to 20.0% and exhibits a slow but steady increase after the 8200 yr BP event. Silica decreases slightly but remains generally high between 20.4% and 22.5%.

411fcaa5-634b-3143-abe9-d1120ef73bdb
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ID: 411fcaa5-634b-3143-abe9-d1120ef73bdb
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Abstract: The karstic Lake Montcortès sedimentary sequence spanning the last 1548. yr constitutes the first continuous, high-resolution, multi-proxy varved record in northern Spain. Sediments consist of biogenic varves composed of calcite, organic matter and detrital laminae and turbidite layers. Calcite layer thickness and internal sub-layering indicate changes in water temperature and seasonality whereas the frequency of detrital layers reflects rainfall variability. Higher temperatures occurred in Lake Montcortès in AD 555-738, 825-875, 1010-1322 and 1874-present. Lower temperatures and prolonged winter conditions were recorded in AD 1446-1598, 1663-1711 and 1759-1819. Extreme and multiple precipitation events dominated in AD 571-593, 848-922, 987-1086, 1168-1196, 1217-1249, 1444-1457, 1728-1741 and 1840-1875, indicating complex hydrological variability in NE Spain since AD 463. The sedimentary record of Lake Montcortès reveals a short-term relation between rainfall variability and the detrital influx, pronounced during extended periods of reduced anthropogenic influences. In pre-industrial times, during warm climate episodes, population and land use increased in the area. After the onset of the industrialization, the relationship between climate and human activities decoupled and population dynamics and landscape modifications were therefore mostly determined by socio-economic factors. © 2012 University of Washington.

3419691f-e526-3afc-96cb-686ebf210067
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ID: 3419691f-e526-3afc-96cb-686ebf210067
Tag: []
Abstract: Multiple analyses of dissolved organic carbon (DOC) from pore waters were conducted to define the processes that govern carbon balance in peatlands: (1) source, reactivity, and transport of DOC with respect to vegetation, peat, and age of carbon substrate, (2) reactivity of DOC with respect to molecular size, and (3) lability to photoxidation of surficial DOC. We found that surface organic production fuels heterotrophic respiration at depth in advection-dominated peatlands, especially in fens. Fen DOC was Δ14C enriched relative to the surrounding fen peat, and fen respiration products were similar to this enriched DOC indicating that DOC was the main microbial substrate. Bog DOC was more variable showing either enrichment in ∆14C at depth or ∆14C values that follow peat values. This variability in bogs is probably controlled by the relative importance of vertical transport of labile carbon substrates within the peat profile versus DOC production from bog peat. These results extended our set of observations to 10 years at one bog-fen pair and add two additional bog-fen pairs to our series of observations. Anaerobic incubations of peat, rinsed free of residual DOC, produced DOC and respiration products that were strikingly similar to the peat values in a bog and two fens. This result demonstrated conclusively that downward advection is the process responsible for the presence of modern DOC found at depth in the peat column. Fen DOC has lower C/N values and up to twice as much LMW (<1 kDa) DOC as bogs due to differences in organic inputs and greater microbial processing. Fluorescence irradiation experiments showed that fen DOC is more photolabile than bog DOC.

215d33b4-f6ed-3b47-b8f5-5f0b4940b04c
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ID: 215d33b4-f6ed-3b47-b8f5-5f0b4940b04c
Tag: []
Abstract: Long sediment cores were collected in spring 2006 from Lake Petén Itzá, northern Guatemala, in water depths ranging from 30 to 150 m, as part of an International Continental Scientific Drilling Program project. The sediment records from deep water consist mainly of alternating clay, gypsum and carbonate units and, in at least two drill sites, extend back >200 kyr. Most of the lithostratigraphic units are traceable throughout the basin along seismic reflections that serve as seismic stratigraphic boundaries and suggest that the lithostratigraphy can be used to infer regional palaeoenvironmental changes. A revised seismic stratigraphy was established on the basis of integrated lithological and seismic reflection data from the basin. From ca 200 to ca 85 ka, sediments are dominated by carbonate-clay silt, often interbedded with sandy turbidites, indicating a sediment regime dominated by detrital sedimentation in a relatively humid climate. At ca 85 ka, an exposure horizon consisting of gravels, coarse sand and terrestrial gastropods marks a lake lowstand or partial basin desiccation, indicating dry climate conditions. From ca 85 to ca 48 ka, transgressive carbonate-clay sediments, overlain by deep-water clays, suggest a lake level rise and subsequent stabilization at high stage. From ca 48 ka to present, the lithology is characterized by alternating clay and gypsum units. Gypsum deposition correlates with Heinrich Events (i.e. dry climate), whereas clay units coincide with more humid interstadials. © 2010 The Authors. Journal compilation © 2010 International Association of Sedimentologists.

68ce8c86-d097-323b-8f3c-38a77838b64a
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ID: 68ce8c86-d097-323b-8f3c-38a77838b64a
Tag: []
Abstract: Lakes may store globally significant quantities of organic carbon (OC) in their sediments, but the extent to which burial rates vary across space and time is not well described. Using 210Pb-dated sediment cores, we explored patterns of OC burial in 116 lakes spanning several ecoregions and land use regimes in Minnesota, USA during the past 150-200 years. Rates for individual lakes (across all time periods) range from 3 to 204g C m-2yr-1 (median 33g C m-2yr-1) and show strong geographic separation in accordance with the degree of catchment disturbance and nutrient enrichment. Climate and basin morphometry exercise subordinate control over OC burial patterns, and diagenetic gradients introduce little bias to estimated temporal trends. Median burial rates in agricultural lakes exceed urban lakes and have increased fourfold since Euro-American settlement. The greatest increase in OC burial occurred prior to the widespread adoption of industrial fertilizers, during an era of land clearance and farmland expansion. Northern boreal lakes, impacted by historical logging and limited cottage development yet comparatively undisturbed by human activity, bury OC at rates 3X lower than agricultural lakes and exhibit much smaller increases in OC burial. Scaling up modern OC burial estimates to the entire state, we find that Minnesota lakes annually store 0.40 Tg C in their sediments, equal to 1.5% of annual statewide CO2 emissions from fossil fuel combustion. During the period of Euro-American settlement (circa 1860-2000), cumulative OC burial amounted to 36 Tg C, 40% of which can be attributed to anthropogenic enhancement.

bfac0b3f-f646-35f3-b911-38ebb1e8e327
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ID: bfac0b3f-f646-35f3-b911-38ebb1e8e327
Tag: []
Abstract: Fossil pollen and charcoal analyses of sediments from Lake Titicaca, Peru/Bolivia, provide a record of palaeoclimatic variation spanning four full glacial cycles. Pollen, aquatic microfossils, and charcoal, as well as previously published data including diatom assemblages, carbonate content, and stable carbon isotopic ratios of organic carbon, indicate that interglacials were warm and dry whereas the peaks of glacials were cold and wet. Each of the interglacials documented in the record are somewhat different, with those of MIS 5e and MIS 9 inducing lower lake levels and a drier vegetation signature than those of MIS 7 and 1. The presence of charcoal particles in sediments deposited during previous interglacials provides evidence of the long-term role of fire in shaping Andean ecosystems. © 2011 Elsevier B.V.

aff2268c-b5c9-3dd3-8b82-9819b0e6dbbc
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ID: aff2268c-b5c9-3dd3-8b82-9819b0e6dbbc
Tag: []
Abstract: A 3000-year varve-thickness record from Hvítárvatn, a glacier-dominated lake in central Iceland, preserves inter-annual variations in the delivery of glacially eroded sediment to the lake. The first-order low-frequency trend in varve thickness reflects increased glacial erosion through the Late Holocene, reaching a peak during the Little Ice Age (LIA). Superimposed on this trend are large inter-annual to decadal fluctuations in varve thickness that we suggest reflect variability in climate parameters that determine the efficiency of the fluvial transport system to deliver glacially eroded sediment to the lake each year. We use spectral analysis to test whether regular high-frequency cyclicity in varve thickness exists in the 3-ka record after removing the low-frequency variability. Spectral analyses from three sediment cores recovered from the lake show essentially the same periods of 2.8-3.4, 13, 35-40 and 85-93, for the overlapping ~900-year period. Additionally, cycles of 55, 130 and 290 years are found in the spectrum for the 3000-year record that do not show up in the spectra for the shorter cores. Some of these cycles show similar variability to those of the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO). This relationship is supported by a significant correlation between varve thickness and both the NAO (precipitation) and AMO (summer temperature) indices over the 180-year instrumental period. NAO cyclicities (2-15 years) are weakly expressed in the first half of the record, increase between 600 and 1000 AD, decrease in strength during medieval time, and are most strongly expressed between 1300 AD and the early 20th century. AMO cyclicities (50 to 130 years) are also relatively weak in the first half of the record, becoming quite strong between 600 and 1000 AD and again between 1100 and 1500 AD, but are essentially absent through the peak of the LIA, between 1500 and 1900 AD, a time when strong cyclicities of about 35 years appear. © 2013 Elsevier Ltd.

1a554780-ce4f-3abb-a122-7e1a5c5b3dd7
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ID: 1a554780-ce4f-3abb-a122-7e1a5c5b3dd7
Tag: []
Abstract: New molecular proxies of temperature and hydrology are helping to constrain tropical climate change and elucidate possible forcing mechanisms during the Holocene. Here, we examine a ∼14,000 year record of climate variability from Lake Victoria, East Africa, the world's second largest freshwater lake by surface area. We determined variations in local hydroclimate using compound specific δD of terrestrial leaf waxes, and compared these results to a new record of temperature utilizing the TEX 86 paleotemperature proxy, based on aquatic Thaumarchaeotal membrane lipids. In order to assess the impact of changing climate on the terrestrial environment, we generated a record of compound specific δ 13C from terrestrial leaf waxes, a proxy for ecosystem-level C 3/C 4 plant abundances, and compared the results to previously published pollen-inferred regional vegetation shifts. We observe a general coherence between temperature and rainfall, with a warm, wet interval peaking ∼10-9 ka and subsequent gradual cooling and drying over the remainder of the Holocene. These results, particularly those of rainfall, are in general agreement with other tropical African climate records, indicating a somewhat consistent view of climate over a wide region of tropical East Africa. The δ 13C record from Lake Victoria leaf waxes does not appear to reflect changes in regional climate or vegetation. However, palynological analyses document an abrupt shift from a Poaceae (grasses)-dominated ecosystem during the cooler, arid late Pleistocene to a Moraceae-dominated (trees/shrubs) landscape during the warm, wet early Holocene. We theorize that these proxies are reflecting vegetation in different locations around Lake Victoria. Our results suggest a predominantly insolation-forced climate, with warm, wet conditions peaking at the maximum interhemispheric seasonal insolation contrast, likely intensifying monsoonal precipitation, while maximum aridity coincides with the rainy season insolation and the interhemispheric contrast gradient minima. We interpret a shift in conditions at the Younger Dryas to indicate a limited switch in insolation-dominated control on climate of the Lake Victoria region, to remote teleconnections with the coupled Atlantic and Pacific climate system. © 2012 Elsevier Ltd.

6b08ce7f-5024-3f10-a28d-455480dcfef1
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ID: 6b08ce7f-5024-3f10-a28d-455480dcfef1
Tag: []
Abstract: The geochemical fingerprint of biogenic calcite is largely governed by the physical environment and water chemistry at the place and time of calcification, which is in turn related to both the ecology and phenology of the target study species. We present data on the valve chemistry (δ18O, δ13C, Mg/Ca, Sr/Ca) of living Limnocythere inopinata, a common and eurytopic Holarctic ostracode species, and on simultaneously collected water chemistry data from 15 water bodies in western Mongolia. These selected lake and pond study sites provide a fair representation of the regional trend in solute evolution towards pronounced calcium depletion, which is further characterized by good correlation between Mg/Cawater and total dissolved solids. Ostracode valve chemistry analysis showed that the Sr/Ca ratio (Sr/Cavalve) correlates better with Sr/Cawater than Mg/Cavalve does with Mg/Cawater, perhaps because Sr/Ca is less affected by calcification rate and temperature. The difference in δ18O between the collected valves and a hypothetical ostracode calcite formed during mid-summer reveals an overall coherent trend of evaporative enrichment during summer in the sampled perennial water bodies, whereas smaller and/or ephemeral water bodies appear to be stronger impacted by stochastic rain events. Among-lake variation in δ13C likely reflects the regional gradient in aquatic productivity, and the uniform pattern of relatively small mean offset between the δ13C of ostracode valves and of surface-water dissolved inorganic carbon suggests epibenthic molting behavior in a well-mixed littoral habitat. The observed patterns in water and valve chemistry further suggest that females of L. inopinata, in this particular climatic setting, molt to adult in early summer. This information on calcification time and environment contributes to more rigorous paleo-environmental interpretation of lake-sediment records of this common species in central Asia. Our results also suggest potential for quantitative salinity reconstruction based on ostracode Mg/Cavalve in western Mongolian lakes, however this needs to be constrained by insight in the local hydrology and hydrochemistry of the reconstruction site. © 2011 Elsevier B.V.

d473f17b-b5db-3c2a-b1d1-ca2234797f17
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ID: d473f17b-b5db-3c2a-b1d1-ca2234797f17
Tag: []
Abstract: The low-Arctic region of western Hudson Bay in interior Canada is one of the most poorly described areas of North America in terms of Holocene climate history. Here, we present new data from a well-dated lake sediment core from northern Manitoba, Canada. We assemble one of the richest multi-proxy datasets to date for a low-Arctic lake and characterize terrestrial and lake processes and exchanges between them. These proxies include fossil pollen and diatom assemblages, charcoal, magnetic properties (susceptibility and remanance), mineral grain size, bulk density, organic-matter content, elemental geochemistry, sediment cation (K +, Mg 2+, Ca 2+, Fe 2+/Fe 3+) and macronutrient (P, N, C) contents, biogenic-silica content, basal peat dates (wetland initiation), and stable isotopes (δ 13C, δ 15N). The sediment proxies record both broad- and fine-scale (millennial and sub-millennial) climate change. We find indirect evidence for a cool and dry post-glacial period from 9,000 to 6,500 cal yr BP, a warm and moist mid-Holocene period from 6,500 to 2,500 cal yr BP, and a cool and moist late-Holocene period from 2,500 cal yr BP to present. High-resolution geochemical data suggests 300- to 500-year-long dry periods at ~6,500-6,100, 5,300-5,000, 3,300-2,800, and 400-0 cal yr BP. These results suggest that terrestrial and aquatic ecosystem dynamics in the western Hudson Bay region are sensitive to past climate change and are likely to respond to future changes in temperature and precipitation. © 2012 Springer Science+Business Media B.V.

73bef6dd-1ce6-3e66-aac5-84caa7b64bea
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ID: 73bef6dd-1ce6-3e66-aac5-84caa7b64bea
Tag: []
Abstract: Groundwater dominated lakes are an important feature of many landscapes. Their sediments are a particularly valuable source of paleoenvironmental information in semiarid regions where perennial lakes may otherwise be scarce. Where groundwater and lake composition are favorable, carbonate mineral precipitation, evaporative concentration of lake water, and microbial processes can combine to strongly deplete dissolved Ca relative to influent groundwaters. The authigenic carbonate flux (ACF) can then become limited by water column cation availability and thereby be coupled to groundwater inflow rates and aquifer recharge. Here we analyze sedimentary records from two marl-producing, groundwater-controlled lakes and demonstrate a link between one-dimensional ACF and the Palmer Drought Severity Index (PDSI), a measure of land surface wetness. In a restricted outflow lake with high-carbonate alkalinity, ACF is enhanced during historically wet climatic periods in response to increased aquifer recharge rates. ACF in this lake declines during droughts. A neighboring dilute lake with a high rate of groundwater outflow shows comparatively weak coupling between ACF and PDSI history. Ionic chemistry, carbonate mineral equilibria, and δ13C patterns of dissolved inorganic carbon show that the sensitivity of the ACF signal depends on the degree of evaporative evolution of lake water and the mineral saturation state of the water column under conditions of stratification and ice cover. Copyright © 2005 Elsevier Ltd.

40bb30a2-55d0-314c-bfba-92b05378919b
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ID: 40bb30a2-55d0-314c-bfba-92b05378919b
Tag: []
Abstract: We reconstructed vegetation and fire histories from four sites located on a sandy outwash plain in northwestern Wisconsin (USA) to test whether lakes and wetlands have influenced how vegetation and fire regimes in pine–oak forests responded to late-Holocene climatic changes. Because of positive feedbacks between jack pine (Pinus banksiana Lamb.) and fire, communities with few fire breaks should be more resilient to changing climatic conditions. Pollen and charcoal from lakesediment cores were used to reconstruct vegetation changes at 50- to 100-year intervals and forest fire history at decadal time scales for the past 2500 years. The presence of fire breaks affected both fire regimes and the response of vegetation to climatic changes. Areas with more fire breaks had smaller charcoal peaks and the vegetation was more responsive to climate changes. The vegetation in areas with few fire breaks was more resilient, maintaining higher amounts of jack pine and (or) red pine than the more protected sites. We interpret these findings as evidence that positive feedbacks between fire and jack pine forests stabilized vegetation at sites where fire breaks were absent, and that such sites may be relatively resilient to future climate changes, until jack pine is no longer able to regenerate under the regional climatic conditions.

108ba265-22d0-3251-9a20-99424e0c41e4
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ID: 108ba265-22d0-3251-9a20-99424e0c41e4
Tag: []
Abstract: Millennial-scale climate variability has not been well documented in arid northwest China due to the scarcity of high-resolution, well-dated paleoclimate records. Here we present multi-proxy records from sediment cores taken in freshwater Hurleg Lake on the northeastern Tibetan Plateau, which reveal millennial-scale lake-level and climate variations over the past 8,000 years. This high-elevation region is very sensitive to large-scale climate change, thus allowing us to better understand Holocene climate variations in East Asia. The lake-level record, derived from lithology, magnetic mineralogy, carbonate isotopes, ostracode shell isotopes and trace elements, X-ray fluorescence (XRF), and gray scale data, indicates a highly variable and generally dry climate from 7.8 to 1 ka (1 ka = 1,000 cal year BP), and a relatively stable and wet climate after 1 ka. Superimposed on this general trend, six dry intervals at 7.6-7.2 ka, 6.2-5.9 ka, 5.3-4.9 ka, 4.4-3.8 ka, 2.7-2.4 ka, and 1.7-1.1 ka were detected from the high-resolution carbonate content and XRF data. The generally dry climate between 7.8 and 1 ka was almost synchronous with the decrease of East Asian and Indian monsoon intensities shortly after 8 ka. The six dry intervals can be correlated with weak monsoon events recorded in the East Asia and Indian monsoon regions, as well as the North Atlantic cold events. Our data suggest that millennial-scale monsoon variations could cause highly variable climate conditions in arid northwest China during the Holocene. These millennial-scale climate variations may reflect changes in solar variation and/or changes in oceanic and atmospheric circulation. © 2010 Springer Science+Business Media B.V.

c2c8ce8b-3567-3733-9de5-6a1e27cfeb9f
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ID: c2c8ce8b-3567-3733-9de5-6a1e27cfeb9f
Tag: []
Abstract: Sediment records from the northern basin of Lake Malawi provide a means of evaluating the lake basin's response to climate change over the past 75 ky, notably to increased precipitation at the terminations of droughts. Transitions from drier to wetter conditions provide an opportunity to evaluate the system's response to climate shifts. Upon termination of drought episodes at 62 and 72 ka, enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters. These processes appear to have left their mark in the sedimentary record, bringing about a spike of deposition of organic matter (probably of terrestrial origin) at times of increased rainfall. This was immediately followed by a period of deposition of chemically-weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation. After this altered material was removed (perhaps a thousand years after the transition to wetter conditions), fresher material, richer in soluble elements including nutrients, was exposed to chemical weathering, leading to substantial diatom blooms. The lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system. However, biological cycling of silica, including formation and dissolution of phytoliths, may have played a role in mobilization of the silica necessary for the diatom productivity.

cca129ae-3939-356d-b69e-509bc4d36a86
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ID: cca129ae-3939-356d-b69e-509bc4d36a86
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Abstract: A high-resolution record of pollen, charcoal, diatom, and lithologic data from Dailey Lake in southwestern Montana describes postglacial terrestrial and limnologic development from ice retreat ca. 16,000 cal yr BP through the early Holocene. Following deglaciation, the landscape surrounding Dailey Lake was sparsely vegetated, and erosional input into the lake was high. As summer insolation increased and ice recessional processes subsided, Picea parkland developed and diatoms established in the lake at 13,300 cal yr BP. Closed subalpine forests of Picea, Abies, and Pinus established at 12,300 cal yr BP followed by the development of open Pinus and Pseudotsuga forests at 10,200 cal yr BP. Increased planktic diatom abundance indicates a step-like warming at 13,100 cal yr BP, and alternations between planktic and tychoplankic taxa suggest changes in lake thermal structure between 12,400 and 11,400 cal yr BP. An increasingly open forest, in combination with increased benthic diatoms, indicates warm dry summers during the early Holocene after 11,400 cal yr BP, in contrast to nearby records in northern Yellowstone that register prolonged summer-wet conditions until ca. 8000 cal yr BP. Because of its low elevation, Dailey Lake was apparently sensitive to the direct effects of increased summer insolation on temperature and effective moisture, registering dry summers. In contrast, higher elevations in northern Yellowstone responded to the indirect effects of an amplified seasonal insolation cycle on atmospheric circulation, including elevated winter snowpack and/or increased summer convective storms as a result of enhanced monsoonal circulation.

68dee044-7d94-3c10-b8eb-2db6f547a119
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ID: 68dee044-7d94-3c10-b8eb-2db6f547a119
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Abstract: A multi-proxy analysis of two sediment cores from Rantin Lake are used to reconstruct past lake-level changes and to make inferences about millennial-scale variations in precipitation/evaporation (P/E) balance in the southern Yukon, Canada between 10,900 and 3,100 cal yr BP. Analyses of calcium carbonate and organic matter concentration, magnetic susceptibility, titanium content, dry bulk density, and macrofossils are used to reconstruct water-level changes. The development of sand layers and deformed sediments at the deep-water core site (i.e. Core A-06) prior to ~10,900 cal yr BP suggest that lake level was lower at this time. Fine-grained organic sediment deposited from 10,600 to 9,500 cal yr BP indicates a rise in lake level. The formation of an unconformity at the shallow cores site (Core C-06) and the deposition of shallow-water calcium carbonate-rich facies at the Core A-06 site between ~9,500 and ~8,500 cal yr BP suggest lower lake levels at this time. Shallow-water facies gradually transition into a sand layer that likely represents shoreline reworking during an extreme lowstand that occurred at ~8,400 cal yr BP. Following this low water level, fine-grained organic-rich sediment formed by ~8,200 cal yr BP, suggesting deeper water conditions at core site A-06. Calcium carbonate concentrations are relatively low in sediment deposited from ~6,300 to 3,100 cal yr BP in Core A-06, indicating that lake level was comparatively higher during the middle and late Holocene. In general, results from this study suggest that the early Holocene was characterized by high P/E from ~10,500 to 9,500 cal yr BP, low P/E from ~9,500 to 8,400 cal yr BP, and return to higher P/E from ~8,200 to 3,100 cal yr BP.

6676889d-12ef-3afe-9801-ff79e58cef6e
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ID: 6676889d-12ef-3afe-9801-ff79e58cef6e
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Abstract: We investigated the mineral-magnetic behavior of sediments from Lake Towuti located in the Eastern Sulawesi Ophiolite belt, Indonesia. Rock magnetic analysis supplemented by X-ray diffraction and scanning electron microscopy analysis were performed on sediment core TOW10-9B from the north basin of Lake Towuti to give insights on the environmental and sedimentary processes controlling the magnetic properties of the sediment and its paleoclimatic significance. The results show that the core has three distinct zones of varying magnetic properties. Careful examination demonstrates that these zones correspond to varying levels of iron oxide dissolution and magnetite precipitation that are climatically and environmentally dependent. The magnetically strongest zone is characterized by weak iron oxide dissolution and intense magnetite precipitation, likely driven by changes in the stratification and/or water level of the lake during dry conditions in Marine Isotope Stage 2 (MIS2) period, whereas the two magnetically weaker zones are characterized by signs of dissolution and correspond to relatively wet conditions, respectively, during Marine Isotope Stage 3 (MIS3) and the Holocene. Although our data show that major changes in concentration dependent parameters, such as magnetic susceptibility and saturation isothermal remanent magnetization (SIRM), in Lake Towuti sediment correlate with changes in regional rainfall, many of the concentration changes are more strongly affected by in situ chemical processes than by changes in erosion and terrestrial sediment supply. These findings urge caution in the interpretation of magnetic mineral concentration profiles as indicators of clastic sediment inputs.

aa66e6fc-ed3c-33f0-87ee-1f2abe212dc4
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ID: aa66e6fc-ed3c-33f0-87ee-1f2abe212dc4
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Abstract: The Manoomin "wild rice" Science Camp program, a partnership between the University of Minnesota, the Fond du Lac Tribal and Community College, and the Fond du Lac Band of Lake Superior Chippewa is an example of how a communitybased participatory research project can become the catalyst for STEM learning for an entire community, providing effective learning opportunities for grades 5-12 and undergraduate students, elementary and secondary school teachers, and scientists from the reservation, tribal college, and university. Focusing the research on a resource (wild rice) that has important economic, cultural and spiritual meaning for a community, we promote place-based education and support the development of strong science/teacher/community partnerships. Key components of this approach are the Circle of Learning, a conceptual framework that emphasizes trust- and relationship-building between researchers, teachers, students, and American Indian community members, and the Seven Elements of STEM Learning, a pedagogical framework derived from an extensive review of the literature on American Indian education that focuses on a holistic approach to learning that emphasizes the whole student. © 2014 National Association of Geoscience Teachers.

fb16f8c9-4ab4-3719-9ecd-92f7c5fec4d3
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ID: fb16f8c9-4ab4-3719-9ecd-92f7c5fec4d3
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Abstract: We examined long-term rates of dry peat accumulation in 32 14C-dated cores from poor fens in Alaska, to bogs and fens in midcontinental North Dakota and Minnesota, to oceanic bogs in Maine and the Atlantic Provinces of Canada. Sites along this belt transect exhibit mostly linear relationships between cumulative mass and age. Long-term rates of peat accumulation range from 16 to 80 g·m-2·year-1, with a median rate of 47 g·m-2·year-1 and a mean rate of 50 g·m-2·year-1. Rate of accumulation is inversely correlated with mean annual precipitation, but is not correlated with the area of the peat basin, basal age, or mean annual temperature. Four of the five highest rates are from relatively dry midcontinental locations in North Dakota and Minnesota; the other is for a coastal site in Newfoundland. The two lowest rates are from extremely rainy sites on Pleasant Island in the Alaskan panhandle. Individual accumulation rates between adjacent dates are quite variable within the peat cores, and across the transect, they do not correlate significantly with immediately previous rates. The same is true of the four sites with the greatest numbers of dates. There is a small but significant negative correlation within the Red Lake Peatland.

b05a2688-ec4b-35e8-8e33-c1d653a9f4e0
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Abstract: Numerous biological and chemical paleorecords have been used to infer paleoclimate, lake level fluctuation and faunal composition from the drill cores obtained from Lake Malawi, Africa. However, fish fossils have never been used to examine changes in African Great Lake vertebrate aquatic communities nor as indicators of changing paleolimnological conditions. Here we present results of analyses of a Lake Malawi core dating back ~144ka that describe and quantify the composition and abundance of fish fossils and report on stable carbon isotopic data (δ13C) from fish scale, bone and tooth fossils. We compared the fossil δ13C values to δ13C values from extant fish communities to determine whether carbon isotope ratios can be used as indicators of inshore versus offshore pelagic fish assemblages. Fossil buccal teeth, pharyngeal teeth and mills, vertebra and scales from the fish families Cichlidae and Cyprinidae occur in variable abundance throughout the core. Carbon isotopic ratios from numerous fish fossils throughout the core range between -7.2 and -27.5%, similar to those found in contemporary Lake Malawi benthic and pelagic fish faunas. These results are the first paleo-record of fish fossils from a Lake Malawi sediment core and the first reported δ13C values from Lake Malawi fish fossils. This approach provides a new methodology and framework for interpreting pelagic versus inshore fish faunas, lake level fluctuations and the evolution of the Lake Malawi fish assemblages. © 2010 Elsevier B.V.

a79e3d3a-0af6-3c2d-91f8-ac62b1449ac1
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Abstract: We present a source-to-sink environmental magnetic study of a sediment core from Lake Sanabria (north-west Iberian Peninsula) and rocks of its catchment. The results indicate the occurrence of magnetite, and probably also pyrrhotite, in sediments accumulated between ca. 26 and 13 cal ka BP in a proglacial lake environment. These minerals also appear to dominate the magnetic assemblage of Palaeozoic rocks from the lake catchment. This indicates that sedimentation was then driven by the erosion of glacial flour, which suffered minimal chemical transformation due to a rapid and short routing to the lake. A sharp change in magnetic properties observed in the lake sediments between 13 and 12.6 cal ka BP reflects the rapid retreat of glaciers from the lake catchment. Sediments from the upper half of the studied sequence, accumulated after 12.6 cal ka BP in a lacustrine environment with strong fluvial influence, contain magnetite and smaller amounts of maghemite and greigite. We suggest that greigite grew authigenically under anoxic conditions caused by enhanced accumulation of organic matter into the lake. The occurrence of maghemite in these sediments suggests pedogenic activity in the then deglaciated lake catchment before the erosion and transportation of detrital material into the lake.

8610be8b-d26a-3efd-9305-3422c06bda37
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Abstract: For an accurate assessment of the anthropogenic impacts on evolutionary change in natural populations, we need long-term environmental, genetic and phenotypic data that predate human disturbances. Analysis of c. 1600 years of history chronicled in the sediments of South Center Lake, Minnesota, USA, revealed major environmental changes beginning c. 120 years ago coinciding with the initiation of industrialised agriculture in the catchment area. Population genetic structure, analysed using DNA from dormant eggs of the keystone aquatic herbivore, Daphnia pulicaria, suggested no change for c. 1500 years prior to striking shifts associated with anthropogenic environmental alterations. Furthermore, phenotypic assays on the oldest resurrected metazoan genotypes (potentially as old as c. 700 years) indicate significant shifts in phosphorus utilisation rates compared to younger genotypes. Younger genotypes show steeper reaction norms with high growth under high phosphorus (P), and low growth under low P, while 'ancient' genotypes show flat reaction norms, yet higher growth efficiency under low P. Using this resurrection ecology approach, environmental, genetic and phenotypic data spanning pre- and post-industrialised agricultural eras clearly reveal the evolutionary consequences of anthropogenic environmental change.

7e3734d3-6c4d-3192-9705-4f82f634ab22
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Abstract: 

56f3ba28-700f-3bfc-9d86-8f0147b364de
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Abstract: Lake Nicaragua, the largest lake in Central America, is a promising site for paleolimnological study of past climate change, tectonic and volcanic activity, and pre-Columbian agriculture in the region. It is near the northern limit of the Intertropical Convergence Zone (ITCZ), which brings the rainy season to the tropics, so effects of decreasing precipitation due to southern migration of the ITCZ through the Holocene should be observable. Because fault zones and an active volcano lie within the lake, the long-term impact of tectonic and volcanic activity can also be examined. Finally, the fertile volcanic soils near the lake may have encouraged early agriculture. We analyzed diatoms, biogenic silica (BSi), total organic carbon (TOC), water content, volcanic glass, and magnetic susceptibility in a sediment core from Lake Nicaragua with eleven accelerator mass spectroscopy radiocarbon dates, spanning ~5,700 years. Sediment accumulation rates decreased from the bottom to the top of the core, indicating a general drying trend through the Holocene. An increase in eutrophic diatom abundance suggests that pre-Columbian agriculture impacted the lake as early as ~5,400 cal yr BP. Above a horizon of coarser grains deposited sometime between ~5,200 and 1,600 cal yr BP, planktonic diatoms increased and remained dominant to the top of the core, indicating that water depth permanently increased. Although magnetic susceptibility peaked and water content dipped at the coarse horizon, volcanic glass fragments did not increase, suggesting that the coarse horizon and subsequent increase in water depth were caused by tectonic rather than by volcanic activity. Decreased accumulation rates of BSi and TOC indicate that water became clearer when depth increased. © 2013 Springer Science+Business Media Dordrecht.

aa76e406-9d88-3e33-9a9f-e2563d620ad0
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Abstract: The abundance of sedimentary organic material from two lakes was used to infer past Holocene storminess on Adak Island where frequent storms generate abundant rainfall and extensive cloud cover. Andrew and Heart Lakes are located 10 km apart; their contrasting physical characteristics cause the sedimentary organic matter to respond differently to storms. Their records were synchronized using correlated tephra beds. Sedimentation rates increased between 4.0 and 3.5 ka in both lakes. Over the instrumental period, Andrew Lake biogenic-silica content (BSi) is most strongly correlated with winter sunlight availability, which influences photosynthetic production, and river input, which influences the dilution of BSi by mineral matter. Heart Lake BSi is likely affected by wind-driven remobilization of sediment, as suggested by correlations among BSi, the North Pacific Index, and winter storminess. The results indicate relatively stormy conditions from 9.6 to 4.0 ka, followed by drying between 4.0 and 2.7 ka, with the driest conditions from 2.7 to 1.5 ka. The stormiest period was between AD 500 and 1200, then drying from 1150 to 1500 and more variable until 1850. This record of Holocene storminess fills a major gap at the center of action for North Pacific wintertime climate. © 2014 University of Washington.

42d6ae22-33fa-376b-a7db-43a35a5f150a
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Abstract: Interactions among multiple causes of ecological perturbation, such as climate change and disturbance, can produce "ecological surprises." Here, we examine whether climate-fire-vegetation interactions can produce ecological changes that differ in direction from those expected from the effects of climate change alone. To do so, we focus on the "Big Woods" of central Minnesota, USA, which was shaped both by climate and fire. The deciduous Big Woods forest replaced regional woodlands and savannas after the severity of regional fire regimes declined at ca. AD 1300. A trend toward wet conditions has long been assumed to explain the forest expansion, but we show that water levels at two lakes within the region (Wolsfeld Lake and Bufflehead Pond) were low when open woodlands were transformed into the Big Woods. Water levels were high instead at ca. 2240-795 BC when regional fire regimes were most severe. Based on the correlation between water levels and fireregime severity, we infer that prolonged or repeated droughts after ca. AD 1265 reduced the biomass and connectivity of fine fuels (grasses) within the woodlands. As a result, regional fire severity declined and allowed tree populations to expand. Tree-ring data from the region show a peak in the recruitment of key Big Woods tree species during the AD 1930s drought and suggest that low regional moisture balance need not have been a limiting factor for forest expansion. The regional history, thus, demonstrates the types of counterintuitive ecosystem changes that may arise as climate changes in the future. © 2009 by the Ecological Society of America.

f0fd2995-661f-37a8-9cf0-14fe6334fd81
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Abstract: Paleolimnological and plant physiological literature were reviewed to determine which types of aquatic plant macrofossils are suitable for radiocarbon dating, with a particular focus on the uptake of reservoir-aged dissolved inorganic carbon (DIC) by emergent plants. Submerged aquatic plants utilize large amounts of DIC and are clearly not suitable for radiocarbon dating. Under certain environmental conditions, some emergent aquatic plants can metabolize DIC in quantities large enough to introduce old-carbon error to radiocarbon dates acquired from their remains (plant macrofossils). Over 300 plant macrofossil images are included in the online resource Tool for Microscopic Identification; http://tmi.laccore.umn.edu) along with guidance on identification and suitability for radiocarbon dating.

a4dfd141-fbc2-3b4e-8030-65207ef93187
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Abstract: For more than a century it has been postulated that the Holocene vegetation of western Europe has changed in significant ways. A half-century ago a lively debate revolved on whether there were one or two dry intervals causing bogs to dry out and become forested, or whether instead the climate warmed to a maximum and then cooled. Today none of these climatic schemes is accepted without reservation, because two nonclimatic factors are recognized as significant: the differential immigration rates of dominant tree types (e.g., spruce in the north and beech in the south) brought unexpected changes in forest composition, and Neolithic man cleared the forest for agriculture and thereby disrupted the natural plant associations. In North America some of the same problems exist. In the hardwood forests of the Northeast, which are richer than but otherwise not unlike those of western Europe, the successive spread of white pine, hemlock, beech, hickory, and chestnut into oakdominated forests provides a pollen sequence that may yield no climatic message. On the other hand, on the ecotone between these hardwood forests and the conifer forests of the Great Lakes-St. Lawrence area, the southward expansion of spruce, fir, and tamarack in the late Holocene implies a climatic cooling of regional importance, although the progressive conversion of lakes to wetlands favored the expansion of wetland forms of these genera. In the southeastern states the late-Holocene expansion of southern pines has uncertain climatic significance. About all that can be said about the distribution and ecology of the 10 or so species is that some of them favor sandy soils and are adapted to frequent fires. In coastal areas the expansion of pines was accompanied by development of great swamps like Okefenokee and the Everglades-perhaps related to the stabilization of the water table after the early Holocene rise of sea level. The vegetation replaced by the pines in Florida consisted of oak scrub with prairie-like openings, indicating dry early Holocene conditions, which in fact had also prevailed during the time of Wisconsin glaciation. In the Midwest the vegetation history provides a clearer record of Holocene climatic change, at least along the prairie border in Minnesota. With the withdrawal of the boreal spruce forest soon after ice retreat, pine forest and hardwood forest succeeded rapidly, as in the eastern states. But prairie was not far behind. By 7000 years ago the prairie had advanced into east-central Minnesota, 75 miles east of its present limit. It then withdrew to the west, as hardwoods expanded again, followed by conifers from the north. The sequence easily fits the paleoclimatic concept of gradual warming and drying to a maximum, followed by cooling to the present day. It is supported by independent fossil evidence from lake sediments, showing that lakes were shallow or even intermittently dry during mid-Holocene time. Here we have a paleoclimatic pattern that is consistent with the record from glaciers in the western mountains-a record that involves a late-Holocene Neoglaciation after a mid-Holocene interval of distant glacial recession. Just as the Neoglaciation is time-transgressive, according to the review of its evidence by Porter and Denton, so also is the mid-Holocene episode of maximum warmth, and they are thus both geologicclimate units. The warm episode is commonly termed the Hypsithermal, which, however, was defined by Deevey and Flint as a time-stratigraphic unit that is supposed to have time-parallel rather than time-transgressive boundaries. It was defined on the basis of pollen-zone boundaries in western Europe and the northeastern United States that have a sound biogeographic but questionable paleoclimatic basis. Perhaps it should be redefined as Porter and Denton suggest, as a geologic-climate unit with recognizable time-transgressive boundaries that match the gradual geographic shifts in the general circulation of the atmosphere and the resulting location of storm tracks and weather patterns. Holocene glacial and vegetational progressions provide a good record of climatic change, if one can work out the lag effects related to the glacial economy and the geographic factors controlling tree migration. The terminology for the Holocene, where so much time control is available, should indicate the dynamic character not only of the climate but also of the geologic and biogeographic processes controlled by climate. © 1976.

ee813076-f764-348b-9cb2-ba1ce64beb92
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Abstract: Hydrologic phenomena are dynamic, so their understanding and prediction are difficult and challenging. Many are cyclic, ranging from diurnal, seasonal, and annual cycles to climatic fluctuations of many hundreds or thousands of years. The purpose of this paper is to examine stratigraphic studies of lake sediments for their potential (1) documenting changes in lake level, sedimentation rate, and water quality throughout the life of a lake and (2) extending the record of climatic fluctuations to regions of natural lakes where other indirect methods have fround little use.

0907f3e3-7cc4-33a1-985b-204bc6105211
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Abstract: Kirchner Marsh and Lake Carlson are located 3 miles apart in Dakota County about 15 miles south of Minneapolis in the St. Croix moraine, which was formed by the Superior lobe during the Gary phase of the Wisconsin glaciation. During the Mankato phase that followed, the Des Moines lobe advanced to within a few miles of the sites. The region today is in a mixed-oak forest, with a maplebasswood forest 15 miles to the west and a re-entrant of the prairie on the sand plain south of the moraine. The general limit of coniferous trees is about 50 miles northeast of the sites, although outliers, especially of Pinus strobus, may be found along the Mississippi Valley a few miles to the east. One sediment core 12-13 m long from each site was analyzed for pollen content at 5-25-cm intervals. Diagrams based on percentage of total pollen (trees, shrubs, wind-pollinated herbs) show essentially identical sequences at the two sites, starting with the late-glacial phase of ice retreat. The diagrams have been subdivided into pollen zones according to the A-B-C sequence introduced by Deevey for New England. The late-glacial pollen record starts at Kirchner Marsh with a short Picea-Cyperaceae-Gramineae phase (Zone K), believed to represent a spruce parkland. Its C-14 date of 13,270 BP and the stratigraphy indicate a pre-Two Creeks and post- Gary correlation. Apparently the Kirchner site did not become established as a lake until this time owing to persistence of dead ice in the moraine. The absence of pollen of specific tundra indicators and the presence of pollen of such thermophilous plants as Fraxinus, Quercus, Corylus, Ambrosia, Humulus, and Typha latifolia imply that the climate was cool rather than cold. Zone A-a, which follows, correlates with the Two Creeks interstade. It is marked by the dominance of Picea, with appreciable percentages of Fraxinus and Ambrosia and with minor amounts of other thermophilous plants and the normal boreal associates of spruce like Betula, Larix, and Salix. Zone A-b, starting 12,050 C-14 years ago, correlates with the Valders ice advance. It is represented at both Kirchner and Carlson and shows the withdrawal of Fraxinus and Ambrosia and the slight rise of Artemisia. Except for the absence of pine in the late-glacial assemblage the vegetation implied by these three zones seems to have its closest modern counterpart in the southern fringe of the Boreal Forest of the Riding Mountain region of southwest Manitoba. It is concluded that pine did not migrate southward with the spruce during the Wisconsin glaciation, at least in the western Great Lakes region, and was thus eliminated from this region. During the lateglacial phases of ice retreat, herbs and spruce pioneered on the deglaciated terrain; pine did not follow until the destruction of the spruce forest at the end of the late-glacial phase. Zone B introduces postglacial time. It represents the time of rapid Vegetational succession following the deterioration of the spruce forest. Simultaneous maxima of Betula, Alnus, Fraxinus, and Abies occurred 10,230 years ago at Kirchner Marsh. These were followed rapidly by a Pinus maximum and then a rise of Ulmus, Quercus, and other deciduous types, dated as 9300 years ago at the correlative site of Madeha. This succession may represent differential rates of migration from refuges south and east of Minnesota. Deciduous trees dominate the C Zones. Zone C-a shows Ulmus and Ostrya /Carpinus followed by Quercus; it probably represents principally a mesic maple-basswood forest changing to oak. Zone C-b represents the advance of prairie into the region at the expense of the oak woodland or savanna. The large and abrupt fluctuations in the curves for Ambrosia-type and Chenopodiineae, especially at the Carlson site, may record encroachment of annual weeds onto intermittently dried lake bottoms. C-14 dates place Zone C-b between 7100 and 5100 years ago. In Zone C-c the Quercus again dominates until the abrupt increase in Ambrosiatype and Chenopodiineae that marks the time of forest clearance and land settlement 50-75 years ago. © 1963, The Geological Society of America, Inc.

b493f640-6306-3721-8039-54939b47cfce
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Abstract: A late Pleistocene Artemisia steppe, implying a cool, dry climate, changed about 13,000 years ago to an oak-pistachio savanna, as the climate became warmer. About 5500 years ago the savanna thickened to an oak forest, presumably reflecting an increase in precipitation or decrease in temperature to modern levels.

c526f101-069c-35e3-9248-d870220b2b5e
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Abstract: Laguna Junin is a large lake 675 km long but only 4 m deep, located in the grass-covered altiplano at an altitude of 4,100 m, northeast of Lima. Pollen analysis of a 30-m core indicates that from about 12 000 yr ago until some time after 3000 yr ago, about 30%; of the pollen was carried in by easterly winds from the east Andean forests, which today are at least 600 m lower in elevation. Contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the Junin plain was dominated by local grassland types and had little contribution from this easterly source. At some time after 3000 yr ago (perhaps about 1200 yr ago), the pollen influx to Laguna Junin from this distant forest source greatly diminished, perhaps because of deforestation by increased human populations. Between about 12 000 and 24 000 yr ago in the Junin core, the sparse pollen concentration was dominated by grassland types, supplemented by Polylepis/Acaena, Alnus, and Compositae blown in from the sub-puna shrublands that may have occupied much of the east Andean slopes in place of the humid forest that occurs there today. The silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain, especially from the cordillera to the west. These glaciers left a distinctive pattern of moraines and outwash fans, which served to dam the lake basin. An unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago, when the lake was probably dry. Before this time, the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation, recorded on the landscape by an older set of moraines and outwash fans, which initially dammed the lake basin. -Authors

01c40c0f-b357-3190-ba88-eb33dbc899db
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Abstract: New fossil pollen records from the central Peruvian Andes at elevations above 4000 m including pollen-concentration data and additional radiocarbon dates, provide information on late Quaternary vegetation change for the region. A pollen assemblage typical of superpuna plus low pollen concentrations, and % organics in the late glacial section of Laguna Tuctua indicate a cold/dry climate, but strong evidence for a late glacial climate reversal, i.e., the Younger Dryas, is not presently confirmed. An abrupt increase in Urticales pollen as Compositae and Polylepis-Acaena type pollen decline signals the beginning of the Holocene. Subsequent increases in Urticales and Plantago rigida type pollen point to increased moisture as well as higher temperatures from about 11,000 to 7000 yr B.P. A possible correlation exists between the rapid expansion of puna and closed continuous forest and the rise in importance of camelids in the early Holocene. In the mid-Holocene Plantago rigida cushion bogs and continuous montane forest elements decline, suggesting a drier climate. Grasses and Cheno-Ams then increase, reaching maxima about 4000 yr B.P. Pastoralism and agriculture are thought to have developed at this time, according to ethnobotanical studies from this area, and pollen evidence seems to support these findings. Between 3000 and 2000 yr B.P. pollen percentages of Poaceae, Plantago rigida type, and other local taxa decline as glacial activity was renewed in the higher mountains. Zea mays is present by 2000 yr B.P., and pollen of weeds and secondary successional plants associated with increasing agricultural disturbance are evident. © 1994.

ba3f0674-81e6-3e18-a13f-e0bcfe11f925
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Abstract: Paleorecords from Minnesota and adjacent areas have often been used to evaluate large-scale climatic processes in the mid-continent of North America. However, most of these records are compromised by chronological flaws, making problematic any comparisons with climatic interpretations based on other records (e.g., GISP2 in Greenland). We report here a high-resolution pollen record with a secure chronology constrained by 26 14C dates on terrestrial macrofossils from Steel Lake, central Minnesota. About 11.2ka cal BP (11,000 calibrated yr before present) the late-glacial Picea forest near Steel Lake was succeeded abruptly by Pinus banksiana and/or resinosa. The Pinus forest began to open 9.4ka cal BP with the expansion of prairie taxa, and a pine parkland or savanna prevailed until about 8ka cal BP, when Quercus replaced Pinus to become the dominant tree in the prairie areas for 4500 yr. The close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern Dakotas and in Minnesota, suggesting that the abrupt decline of the spruce forest was time-transgressive from southwest to northeast during 2000 yr, and that the development of prairie was time-transgressive in the same direction over 2600 yr. Correlation of key pollen horizons at Steel Lake with those in the high-resolution pollen profiles of Elk Lake, ca. 50km northwest of Steel Lake, suggests that the well-known Elk Lake varve chronology for the early Holocene is about 1000 yr too young. © 2003 Elsevier Ltd. All rights reserved.

3f5d4bc3-615f-397b-8c13-a9f95ca6a613
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Abstract: Abundant macrofossils of Picea and other forest plants, mixed with sand and other glacial material, occur at the base of the fine organic sediment of many lakes in Minnesota and adjacent areas at the margin of the Laurentide ice sheet in central North America. Such accumulations are interpreted as representing forest-floor litter washed into depressions from superglacial forest on stagnant ice. An example of a trash layer in Minnesota is described from Steel Lake, which contains not only macrofossils of Picea from the superglacial forest but also much older fragments representing forest overridden by the advancing ice. A modern analogue is on the surge moraines of the Klutlan glacier in the Canadian Rocky Mountains. Trash layers have not been described from similar dead-ice landscapes in Europe, perhaps because tundra plants rather than forest invaded the stagnant ice and left little litter. The closest example is found in Poland, where a basal peat of Allerød age is interpreted as of superglacial origin. The trash layer in the Minnesota sites is abruptly overlain by many meters of typical Holocene fine organic sediment. At the same level the Picea pollen zone changes to the Pinus zone, marking a prominent climatic change at nearly the same date as the end of the Younger Dryas cool interval, which is registered especially in areas around the North Atlantic Ocean. In Minnesota, however, the Picea/Pinus transition is time-transgressive from southwest to northeast over about 2000 years, in synchrony with the retreat of the Laurentide ice sheet.

20de9a8c-53bd-3acb-a008-e8d0ae9afef9
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Abstract: Glaciolacustrine rhythmites within sediment cores from Lake Superior record the regional recession of the Laurentide Ice Sheet (LIS) from 10,700 to 8900 cal ybp [ca. 9.5-8.0 14C ka]. LIS retreat from Superior opened eastern Lake Agassiz outlets so that the rhythmites reflect the combined impacts of sediment-laden meltwater and Lake Agassiz discharge. Multiple rhythmite stratigraphies, a time series analysis of the thickness measurements, and high-resolution inorganic carbonate data demonstrate that this is an annual record (varved). The varve thickness records primarily document regional ice margin dynamics; correlative thick varve sequences at 9100 cal ybp [∼ 8.1 14C ka] and 10,400-10,200 cal ybp [∼ 9.2-9.0 14C ka] record two periods of enhanced glaciofluvial discharge, most likely moraine formation (the Nakina and Nipigon). General varve cessation is associated with the circumvention of Lake Agassiz and glacial meltwater into Lake Ojibway at 9040 cal ybp [∼ 8.1 14C ka], although adjacent to the inlets from Lake Nipigon, rhythmic sedimentation persisted for 200 years. Positively identifying Lake Agassiz catastrophic discharge events remains speculative but seems feasible. Following retreat of Marquette ice that had re-advanced to fill the basin, the initial influx of Lake Agassiz water is expected at around 10,600 cal ybp [∼ 9.4 14C ka], but at this time, most of northeastern Lake Superior was covered by ice. Three sets of thick-thin varves in western Lake Superior perhaps record influxes of Lake Agassiz at around 10,630, 10,600, and 10,570 cal ybp [∼ 9.4 14C ka]. Varve formation in Superior coincides with high lake levels in Lake Huron, suggesting that high lake levels in Huron correspond to periods of high Agassiz and/or meltwater flow into Lake Superior. © 2007 Elsevier B.V. All rights reserved.

5186811f-f9b9-3c73-abf6-3a8adcb4b60e
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ID: 5186811f-f9b9-3c73-abf6-3a8adcb4b60e
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Abstract: The Late Glacial and early-Holocene vegetational history of a newly dated pollen and macrofossil diagram from Besbog, a cirque lake at 2250 m just above the forest limit in the Pirin Mountains of southwestern Bulgaria, is compared with a newly dated pollen diagram for the mire Shiroka Polyana at 1400 m in the conifer forest of the nearby Rhodope Mountains in order to investigate the chronology of major changes in the vegetation at different elevations. In the Lake Besbog record the non-arboreal pollen assemblage of the Late Glacial changed abruptly to that of Betula, Quercus and other deciduous types. The date for this change is about 11.6 ka cal b.p. The Quercus assemblage may be composed of pollen blown from intermediate elevations, to which deciduous forest had expanded because of higher summer temperatures related to high summer insolation. At Shiroka Polyana (1400 m) in the modern conifer belt, a similar change did not occur until about 8.8 ka cal b.p. The persistence of the dry steppe or steppe forest in the early Holocene at this lower site can also be attributed to high summer insolation. Thus as atmospheric temperature increased at the end of the Late Glacial, deciduous forests expanded first at intermediate elevations in the Pirin Mountains and only later in the Rhodope Mountains at lower elevations as summer insolation decreased. © Springer-Verlag 2006.

5a05e9f0-7a8d-3674-89a7-f29c74b9c439
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ID: 5a05e9f0-7a8d-3674-89a7-f29c74b9c439
Tag: []
Abstract: The sediments of numerous cirque lakes in the Pirin Mountains have yielded several generalized pollen diagrams, but low stratigraphic and temporal resolution has hampered detailed reconstruction of the vegetational history, especially for the Lateglacial. New pollen and macrofossil analyses of high stratigraphic resolution combined with well-controlled time-depth curves for Lake Besbog and Lake Kremensko-5 now permit the firm identification of the Lateglacial interstadial interval (Bølling-Allerød), dated at 13.8-12.6 ka cal. BP at Besbog and 14.1-12.8 kacal. BP at Kremensko-5 and correlated with the Greenland Interstadial GI-1 (14.5-12.6 ka cal. BP). Macrofossil analyses indicate that during the Bølling-Allerød period Betula and Juniperus occurred around the lakes (at 2124-2250 m a.s.l). Following the Younger Dryas a Betula woodland covered the mountains between 1900 and 2250 m in the early Holocene. Because of higher summer insolation at this time, the temperate deciduous forest reached up to 1900 m, or 800 m above their present limit, producing a regional pollen rain that dominated pollen deposition in the cirque lakes. After about 7000 cal. yr BP conifer forests replaced the birch woodland at middle and high elevations. © 2006 SAGE Publications.

ccb4ec7a-cf7b-3655-8958-705e3da7aada
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Abstract: Pollen analysis of sediments from core BL96-2 at Bear Lake (42°N, 111°20'W), located on the Utah-Idaho border in America's western cordillera, provides a record of regional vegetation changes from full glacial to the late Holocene. The reconstructed vegetation records are mostly independent of Bear Lake's hydrologic state and are therefore useful for identifying times when climate forcing contributed to lake changes. The Bear Lake pollen results indicate that significant changes in the Bear Lake vegetation occurred during the intervals 15,300-13,900, 12,000-10,000, 7500-6700, 6700-5300, 3800-3600, and 2200-1300 cal yr B.P. These intervals coincide with regional shifts in vegetation and climate, documented in pollen, isotope and biogeographic records in the Basin and Range region, suggesting that large-scale climate was the primary forcing factor for these intervals of change. Maximum aridity and warmth is indicated from 12,000 to 7500 cal yr B.P., followed by intervals of generally more mesic and cool conditions, especially after 7500 cal yr B.P. Copyright © 2009 The Geological Society of America.

f8e5f730-47f8-3565-bc8e-4668490c11ad
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Abstract: Study of Lake Pepin and Lake St. Croix began more than a century ago, but new information has permitted a closer look at the geologic history of these two riverine lakes located on the upper Mississippi River system. Drainages from large proglacial lakes Agassiz and Duluth at the end of the last glaciation helped shape the current valleys. As high-discharge outlet waters receded, tributary streams deposited fans of sediment in the incised river valleys. These tributary fans dammed the main river, forming riverine lakes. Lake Pepin was previously thought to be a single long continuous lake, extending for 80 km from its dam at the Chippewa River fan all the way up to St. Paul, with an arm extending up the St. Croix valley. Recent borings taken at bridge and dam locations show more than a single section of lake sediments, indicating a more complex history. The Minnesota and Mississippi Rivers did not always follow their current paths. Valleys cut into bedrock but now buried by glacial sediment indicate former river courses, with the most recent of these from the last interglacial period marked at the surface by chains of lakes. The morphology of the Mississippi valley bottom, and thus the morphology of Lake Pepin as it filled the valley, is reflect in part by the existence of these old valleys but also by the presence of glacial outwash terraces and the alluvial fans of tributary streams. A sediment core taken in Lake Pepin near Lake City had a piece of wood in gravels just below lake sediments that dated to 10.3 ka cal. BP, indicating that the lake formed as the Chippewa River fan grew shortly after the floodwaters of Lakes Agassiz and Duluth receded. Data from new borings indicate small lakes were dammed behind several tributary fans in the Mississippi River valley between the modern Lake Pepin and St. Paul. One tributary lake, here called Early Lake Vermillion, may have hydraulically dammed the St. Croix River, creating an incipient Lake St. Croix. The tributary fans from the Vermillion River, the Cannon River, and the Chippewa River all served to segment the main river valley into a series of riverine lakes. Later the growth of the Chippewa fan surpassed that of the Vermillion and Cannon fans to create a single large lake, here called late Lake Pepin, which extended upstream to St. Paul. Sediment cores taken from Lake Pepin did not have significant organic matter to develop a chronology from radiocarbon dating. Rather, magnetic features were matched with those from a Lake St. Croix core, which did have a known radiocarbon chronology. The Pepin delta migration rate was then estimated by projecting the elevations of the top of the buried lake sediments to the dated Lake Pepin core, using an estimated slope of 10 cm/km, the current slope of Lake Pepin sediment surface. By these approximations, the Lake Pepin delta prograded past Hastings 6.0 ka cal BP and Red Wing 1.4 ka cal BP. © 2009 Springer Science+Business Media B.V.

fa3c29ba-332d-3b22-9582-95402b910422
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Abstract: Lacustrine carbonate deposits (tufa) record variations in terrestrial hydrology and are preserved in many now-arid regions of the world, but are challenging to date with precision and accuracy. Many contain detritus and/or unsupported thorium (Th) that degrades or prevents the measurement of precise uranium-series (U-series) ages, and radiocarbon ages are frequently affected by both the reservoir (hard-water) effect and contamination with younger atmospheric carbon. The usual method of testing the accuracy of carbonate 14C measurements, comparison with U/Th ages or organic carbon dates, does not separate the reservoir and modern contamination effects, allowing for only relatively imprecise age estimates in samples undatable by U/Th.We have separated the modern contamination problem and the reservoir effect using a step dissolution technique on a variety of carbonate materials from Mono Lake, California, a long-lived closed-basin lake sensitive to regional precipitation variability. New dissolution experiments focus on the deglacial sediments of the Wilson Creek Formation, which preserve ostracodes and fans and mounds of thinolite, a cold-water, hydrated calcium carbonate (CaCO 3.6H 2O). Stepped-dissolution ages of thinolite crystals and dense calcites increased by 500-1000 years over the bulk age, and produced a plateau of analytically indistinguishable ages, indicating nearly complete removal of modern carbon. Repeated experiments on ostracodes from a single sample showed an increase of >3500 years over the bulk age and >6500 years from first to last step, but ages increased up to the last ~5% of the CO 2 evolved, without forming a plateau. This may be due to the extremely large surface area-to-volume ratio of ostracodes, and inhomogeneous dissolution of the hundreds of individual shells required for the experiments. Further experiments are planned to test the effects of modern carbon on tufa, gastropods, and other shells, with the goal of systematically testing the precision and reliability of chronologies for dramatic changes in lake level in arid regions of the world. © 2012 Elsevier B.V.

4bc0dcf0-0908-3544-a05c-7cb18e1d58a5
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Abstract: Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment-water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorus concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10% of the inflowing total bioavailable silica (TSib = DSi + ASi) while Lake Pepin traps closer to 20% of its inflowing TSib, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land-ocean continuum. © 2008 Springer Science+Business Media B.V.

089cfedb-318a-3bbf-b1c5-3fd2ca0dc2fb
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ID: 089cfedb-318a-3bbf-b1c5-3fd2ca0dc2fb
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Abstract: Oxygen isotope (δ18O) measurements of authigenic carbonate from Cleland Lake (southeastern British Columbia), Paradise Lake (central British Columbia), and Lime Lake (eastern Washington) provide a ~9000 year Holocene record of precipitation-evaporation balance variations in the Pacific Northwest. Both Cleland Lake and Paradise Lake are small, surficially closed-basin systems with no active inflows or outflows. Lime Lake is surficially open with a seasonally active overflow. Water isotope values from Cleland and Paradise plot along the local evaporation line, indicating that precipitation-evaporation balance is a strong influence on lake hydrology. In contrast, Lime Lake water isotope values plot on the local meteoric water line, signifying minimal influence by evaporation. To infer past hydrologic balance variations at a high temporal resolution, we sampled the Cleland, Paradise, and Lime Lake sediment cores at 1-60 mm intervals (~3-33 years per sample on average) and measured the isotopic composition of fine-grained (<63 μm) authigenic CaCO3 in each sample. Negative δ18O values, which indicate wetter conditions in closed-basin lakes, occur in Cleland Lake sediment from 7600 to 2200 years before present (yr BP), and are followed by more positive δ18O values, which suggest drier conditions, after 2200 yr BP. Highly negative δ18O values in the Cleland Lake record centered on ~2400 yr BP suggest that lake levels were high (and that the lake may have been overflowing) at this time as a result of a substantially wetter climate. Similarly, Paradise Lake sediment δ18O values are relatively low from 7600 to 4000 yr BP and increase from ~4000 to 3000 yr BP and from ~2000 yr BP to present, indicating that climate became drier from the middle through the late Holocene. The δ18O record from Lime Lake, which principally reflects changes in the isotopic composition of precipitation, exhibits less variability than the closed-basin lake records and follows a generally increasing trend from the mid-Holocene to present. These results are consistent with several proximal reconstructions of changes in lake-level, precipitation amount, and precipitation isotopic composition and may also reflect the establishment of modern El Niño Southern Oscillation (ENSO) variability in the late Holocene, as inferred from proxy evidence of synoptic ocean-atmosphere changes in the Pacific basin. Results from mid-Holocene (6000 yr BP) climate model simulations conducted as part of the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3) indicate that in much of western North America, the cold season (October-March) was wetter and the warm season (April-September) was considerably drier relative to the late Holocene, leading to an overall drier climate in western North America with enhanced hydroclimatic seasonality. This is consistent with inferences from the Cleland and Paradise δ18O records, which lake modeling experiments indicate are strongly influenced by cold season precipitation-evaporation balance. This also explains apparent inconsistencies between the lake δ18O records and other proxies of hydroclimatic change from the greater Pacific Northwest region that are less sensitive to cold season climate and thus indicate relatively drier conditions during the mid-Holocene. The abrupt negative excursion at ~2400 yr BP in the Cleland Lake δ18O data, as well as the marked shift to more positive values after this time, demonstrate that gradual changes in ocean-atmosphere dynamics can produce abrupt, non-linear hydroclimate responses in the interior regions of western North America.

c44c27ca-9650-3223-bed7-fcad58850159
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Abstract: We show how sedimentary charcoal records from multiple sites within a single landscape can be used to compare fire histories and reveal small scale patterns in fire regimes. Our objective is to develop strategies for classifying and comparing late-Holocene charcoal records in Midwestern oak- and pine-dominated sand plain ecosystems where fire regimes include a mix of surface and crown fires. Using standard techniques for the analysis of charcoal from lake sediments, we compiled 1000- to 4000-yr-long records of charcoal accumulation and charcoal peak frequencies from 10 small lakes across a sand plain in northwestern Wisconsin. We used cluster analysis to identify six types of charcoal signatures that differ in their charcoal influx rates, amount of grass charcoal, and frequency and magnitude of charcoal peaks. The charcoal records demonstrate that while fire histories vary among sites, there are regional patterns in the occurrence of charcoal signature types that are consistent with expected differences in fire regimes based on regional climate and vegetation reconstructions. The fire histories also show periods of regional change in charcoal signatures occurring during times of regional climate changes at ~. 700, 1000, and 3500. cal yr BP. © 2010 University of Washington.

239ad278-6908-36e0-8f20-bbb17e5a946c
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ID: 239ad278-6908-36e0-8f20-bbb17e5a946c
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Abstract: Reconstruction of soil weathering is one of the more difficult problems for palaeoecologists interested in landscape-scale environmental changes. Here we use scanning electron microscopy and energy dispersive x-ray analysis (SEM/EDS) to study the pitting of feldspars in soils and lake sediments at two sites in the Great Lakes region, USA. A semi-quantitative feldspar weathering index (FWI) allows description of the weathering status for large numbers of individual grains, and reveals differences among soil horizons and strata of lake sediment as well as between the two sites. Feldspars in upper soil horizons generally have greater FWI than those in lower horizons, but analyses also suggest considerable weathering in the soil C horizon at the site with more rapid water infiltration. At one site, the FWI of upper soil horizons and surface lake sediment are similar, implying that lake sediments have the potential to record changes in soil over time. However, differences between upper soil horizons and surface sediment at the other site highlight the need to examine more sites and more profiles within sites to better relate catchment and sediment characteristics. We interpret increases in the FWI over time within lake sediments as evidence of progressive weathering of soils and conclude that the sites had different histories of soil development. However, the complexities of sediment deposition in lakes and the paucity of information relating FWI to soil characteristics prevent straightforward reconstruction of soil development. Variability within each record appears tied to initial grain chemistry, depositional environment, changes in lake level and progressive degradation of grains. As more information relating micromorphology to soil characteristics becomes available, this technique should become a more useful complement to chemical studies of lake sediment.

36b7d4ec-f7a0-3681-9726-a660caed4e0f
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ID: 36b7d4ec-f7a0-3681-9726-a660caed4e0f
Tag: []
Abstract: The effect of eutrophication on particulate amorphous silica (ASi) sequestration was isolated and quantified in Lake St. Croix and Lake Pepin, two natural, human-impacted impoundments of the upper Mississippi River. In contrast to impoundments behind engineered dams, where silica (Si) fluxes may be changed by various aspects of dam construction, these two riverine lakes have long (9,000+ years) sedimentary sequences that record the entire span of cultural eutrophication and the resulting silica sequestration. The concentrations of dissolved silicate (DSi) and ASi in the lake inflows were measured for 1 year to obtain the total flux of bioavailable silica (TSi b = DSi + ASi) to each impoundment. Historical rates of Si sequestration in each lake were determined using ASi burial in multiple sediment cores and modeled estimates of historical TSi b fluxes. The Si trapping efficiency of each lake was found to have increased exponentially with cultural eutrophication (estimated two- to fivefold increase in Lake St. Croix and 9- to 16-fold increase in Lake Pepin over the last 100 years), indicating the degree to which eutrophication of impoundments can reduce silica export to downstream coastal and marine ecosystems. Because these two lakes presently exhibit different degrees of eutrophication, together they depict a relationship between phosphorus concentration and Si trapping efficiency that may be applied to other impoundments, including human-made reservoirs. © 2011 Springer Science+Business Media B.V.

a1504ecc-5211-34ec-94d8-2235e00a3b5d
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Abstract: Two geochemical techniques from soil mineralogy were used with lake sediment to reconstruct soil development in the catchments of two lakes-one on outwash and the other on till-in the Great Lakes region. A sodium pyrophosphate extraction provides information about leaching of exchangeable and organically complexed cations from terrestrial sources, while an acidified ammonium oxalate extraction removes mineral materials without well-developed crystalline structure that are indicative of podzolization. More evidence of pedogenesis is preserved in sediment of the lake on outwash- likely a result of greater retention of material within the soil profile in the catchment on till. This difference between the record suggests that excessively well-drained (leaky) catchments may provide better records of soil development than those with higher water-holding capacity. Evidence of acidification and podzolization appeared first at the lake on till and 4500 years later at the lake on outwash. Both records reveal a late Holocene intensification of weathering that coincided with a regional climatic change to greater moisture availability. The variability in weathering rates observed presents a more complex picture of weathering than the simple exponential functions often assumed in chronosequence studies. Use of lake sediment records provides better temporal resolution of significant events in soil formation than could be achieved using a chronosequence approach. © 2002 University of Washington.

872142a5-d703-35de-b9cf-184a15e43a78
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ID: 872142a5-d703-35de-b9cf-184a15e43a78
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Abstract: A series of 34 layers of struvite (hydrous Mg-ammonium-phosphate) were deposited in laminated sediments of Kettle Lake, North Dakota, in the Northern Great Plains, USA. Sedimentologic, mineralogic and nitrogen isotopic evidence suggest that struvite was deposited during nutrient-enriched eutrophication events within a Mg-rich lacustrine environment. The struvite layers are dispersed between 4650 and 8700 cal. yr BP, with particularly high frequency between 8068 and 8700 cal. yr BP. The youngest struvite layer dates to 2734 cal. yr BP. Key features of the struvite-forming events were (a) relatively low water stage and consequent elevated water salinity associated with the dry mid Holocene, (b) disappearance of most lakes and wetlands in the region, and (c) focused, but brief, visitations by large populations of migratory waterfowl. The lack of more frequent occurrence of struvite is ascribed to the rarity with which this combination of conditions was achieved, in combination with difficulty of preservation for this salt. 15N isotopes in struvite (mean 7.51‰) are heavy relative to 15N in sediment and also extremely uniform over time. The isotopes are interpreted to reflect a water-fowl waste source without extensive NH4 volatilizaton. The timing of the struvite events does not closely correspond to century-scale mid-Holocene drought cycles inferred from oscillating aragonite concentrations, although there is a weak preferred occurrence in the humid phase of these cycles. Thus the struvite events are ephemeral prairie features of this generally arid period but tend not to occur at precisely the most arid intervals. © 2007 SAGE Publications.

9c88241d-cdd2-36ce-9b31-6e6c8595cc11
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Abstract: The fire history of the northern Great Plains has been largely reconstructed from the historical record. To clarify this history, segments of 1-m cores from four lakes in North and South Dakota and northeastern Montana were examined for charcoal fragments. The cores spanned a time interval of 172 to 380 yr. Samples integrating 5-10 yr of deposition were taken from 2-cm sections along each core and sieved. Charcoal concentrations ranged from 6-14,799 fragments/gram of sediment, and mean charcoal abundance was negatively correlated with longitude. Results suggest periods of increased fire from A.D. 1700-1740 and A.D. 1850-1900. These peaks were broadly synchronous across the region and ranged in duration from 20-40 yr. Postsettlement patterns of charcoal deposition were highly variable but generally much lower than presettlement intervals.

7eb87308-5ce7-3a68-9c1b-84735bb02d7f
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Abstract: We propose a model that explains variations in magnetic parameters of lake sediments as a record of Holocene climate change. Our model is based on records from four lakes and incorporates the effects of erosion, dust deposition, and the authigenesis and diagenesis of the magnetic component of the sediment. Once checked against high resolution multi proxy climate records, which are currently being established for some of our study sites, it will allow us to use magnetic proxies to establish high-resolution climate reconstructions on a regional scale. Our model utilizes a combination of concentration-dependent parameters (magnetic susceptibility, IRM) and grain-size-dependent parameters (ARM/IRM, hysteresis parameters). Magnetic mineralogy is characterized by a combination of low-temperature measurements and S-ratios, and our magnetic measurements are complemented by XRD, LOI and smearslide analyses. During periods of forest growth within the watershed, deposition of terrigenous material is low and the sediment magnetic properties are characterized by low concentrations of mainly authigenic minerals (low values of IRM, high ratios of ARM/IRM). During the early to mid-Holocene dry period, deposition of terrigenous material increased due to intensified dust deposition and the erosion of lake margins caused by lowered water levels. Concentration of magnetic minerals increases (high IRM, χ) and so does the grain-size of the magnetic fraction (low ARM/IRM). During the late Holocene, sediment magnetic properties depend on the varied position of the site with respect to the prairie-forest ecotone.

1100e11a-6844-3fe4-821b-ffa49bb87c2c
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Abstract: 1. Treeline ecotones, such as the prairie-forest boundary, represent climatically sensitive regions where the relative abundance of vegetation types is controlled by complex interactions between climate and local factors. Responses of vegetation and fire to climate change may be tightly linked as a result of strong feedbacks among fuel production, vegetation structure and fire frequency/severity, but the importance of these feedbacks for controlling the stability of this ecotone is unclear. 2. In this study, we examined the prairie-forest ecotone in south-central Minnesota using two lake sediment cores to reconstruct independent records of climate, vegetation and fire over the past 12 500 years. Using pollen, charcoal, sediment magnetic analyses and LOI properties, we investigated whether fires were controlled directly by climate or indirectly by fuel production. 3. Sediment magnetic and LOI data suggest four broad climatic periods occurring c. 11 350-8250 BP (cool/humid), c. 8250-4250 BP (warm/dry), c. 4250-2450 BP (warm/ humid), and c. 2450-0 BP (cool/humid), indicating that, since the mid-Holocene, climate has shifted towards wetter conditions favouring greater in-lake production and fuel production on the landscape. 4. The area surrounding both lakes was characterized by boreal forest c. 12 500-10 000 BP, changing to an Ulmus-Ostrya forest c. 10 000-9000 BP, changing to a community dominated by prairie (Poaceae-Ambrosia-Artemisid) and deciduous forest taxa c. 8000-4250 BP, and finally shifting to a Quercus-dominated woodland/savanna beginning c. 4250-3000 BP. 5. Charcoal influx increased from an average of 0.11-0.62 mm2 cm-2 year-1 during the early Holocene forest period (c. 11 350-8250 BP) to 1.71-3.36 mm2 cm-2 year-1 during the period of prairie expansion (c. 8250-4250 BP) and again increased to 4.18-4.90 mm2 cm-2 year-1 at the start of the woodland/savanna period (c. 4250 BP). 6. As a result of the influence of climate on community composition and fuel productivity, changes in fire severity may be the result and not the cause of shifts in vegetation.

f7d17a79-383e-306a-8a86-101ae8c5929b
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Abstract: Easter Island (SE Pacific, 27°S) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Although the general climate evolution of the south Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial-interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by "El Niño-like" dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2-0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake. © 2009 Elsevier Ltd. All rights reserved.

64ab9808-ef5f-3de3-9121-c7a8b3f0a846
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ID: 64ab9808-ef5f-3de3-9121-c7a8b3f0a846
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Abstract: West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat floor in the central part of the basin below 111 m of water. Four cores, 5-11 m in length, were collected using a Kullenberg piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter. Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal, diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence; possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300-9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500-4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to the basin and possibly an increase in productivity. © 2008 Springer Science+Business Media B.V.

50a47da4-fa79-353b-aaf0-ca0e50619254
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Abstract: The Quaternary sediments sampled by continuous 120-m-long drill cores from Bear Lake (Utah-Idaho) comprise one of the longest lacustrine sequences recovered from an extant lake. The cores serve as a good case study for the construction of an age model for sequences that extend beyond the range of radiocarbon dating. From a variety of potential age indicators, we selected a combination of radiocarbon ages, one magnetic excursion (correlated to a standard sequence), and a single Uranium-series age to develop an initial data set. The reliability of the excursion and U-series data require consideration of their position with respect to sediments of inferred interglacial character, but not direct correlation with other paleoclimate records. Data omitted from the age model include amino acid age estimates, which have a large amount of scatter, and tephrochronology correlations, which have relatively large uncertainties. Because the initial data set was restricted to the upper half of the BL00-1 core, we inferred additional ages by direct correlation to the independently dated paleoclimate record from Devils Hole. We developed an age model for the entire core using statistical methods that consider both the uncertainties of the original data and that of the curve-fitting process, with a combination of our initial data set and the climate correlations as control points. This age model represents our best estimate of the chronology of deposition in Bear Lake. Because the age model contains assumptions about the correlation of Bear Lake to other climate records, the model cannot be used to address some paleoclimate questions, such as phase relationships with other areas. © 2006 Elsevier Ltd. All rights reserved.

4e680648-8c30-307d-b179-e427f0c7a970
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ID: 4e680648-8c30-307d-b179-e427f0c7a970
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Abstract: The transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition ( ∼ 11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ∼ 11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep-water cores (>50m below modern water level) indicate several wet-dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1 -4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum-Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high-latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores - Bermuda high-pressure system. This mechanism operated on millennial-to-submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.

9ad47538-fe48-34e8-8f2c-bdb65d453548
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Abstract: We inferred the climate history for Central Asia over the past 20,000 years, using sediments from core QH07, taken in the southeastern basin of Lake Qinghai, which lies at the northeastern margin of the Tibetan Plateau. Results from multiple environmental indicators are internally consistent and yield a clear late Pleistocene and Holocene climate record. Carbonate content and total organic carbon (TOC) in Lake Qinghai sediments are interpreted as indicators of the strength of the Asian summer monsoon. Warm and wet intervals, associated with increased monsoon strength, are indicated by increased carbonate and TOC content. During the glacial period (~20,000 to ~14,600 cal year BP), summer monsoon intensity remained low and relatively constant at Lake Qinghai, suggesting cool, dry, and relatively stable climate conditions. The inferred stable, cold, arid environment of the glacial maximum seems to persist through the Younger Dryas time period, and little or no evidence of a warm interval correlative with the Bølling–Allerød is found in the QH07 record. The transition between the late Pleistocene and the Holocene, about 11,500 cal year BP, was abrupt, more so than indicated by speleothems in eastern China. The Holocene (~11,500 cal year BP to present) was a time of enhanced summer monsoon strength and greater variability, indicating relatively wetter but more unstable climatic conditions than those of the late Pleistocene. The warmest, wettest part of the Holocene, marked by increased organic matter and carbonate contents, occurred from ~11,500 to ~9,000 cal year BP, consistent with maximum summer insolation contrast between 30°N and 15°N. A gradual reduction in precipitation (weakened summer monsoon) is inferred from decreased carbonate content through the course of the Holocene. We propose that changes in the contrast of summer insolation between 30°N and 15°N are the primary control on the Asian monsoon system over glacial/interglacial time scales. Secondary influences may include regional and global albedo changes attributable to ice-cover and vegetation shifts and sea level changes (distance from moisture source in Pacific Ocean). The abruptness of the change at the beginning of the Holocene, combined with an increase in variability, suggest a threshold for the arrival of monsoonal rainfall at the northeastern edge of the Tibetan Plateau.

4d159d4e-9cdf-36d8-9a54-735079dcf176
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ID: 4d159d4e-9cdf-36d8-9a54-735079dcf176
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Abstract: An energy-balanced hydrologic model is used to quantitatively assess atmosphere-water budget relationships across the Lake Malawi catchment, a hydrologically-open lake within the East African Rift System. The model first simulates the historical lake-level record over the last 100 years using climate station and vegetation data as inputs. Atmospheric conditions required to sustain equilibrium water balance are then estimated at known critical lake-levels: modern (700 m maximum water depth), basin closure (696 m maximum water depth), 500 m, 350 m, 200 m, and 150 m maximum water depth. The critical low lake stages were determined from analysis of seismic-reflection and deep lake drill-core data. The model predicts modern precipitation rate to be 955 mm/yr, which is consistent with observed climate station precipitation records. The minimum lowstand observed in geophysical records is 150 m water depth (550 m below present lake-level), and occurred about 95,000 years before present. The precipitation rate required to sustain equilibrium conditions at this low lake stage is 557 mm/yr, assuming modern Lake Malawi temperature and vegetation, and 374 mm/yr using modern temperature and vegetation data from the Little Karoo Basin, an analogue for the Malawi paleo-environment during severe arid intervals that resulted in major lake lowstands. The latter result is consistent with the range of precipitation measured from the Little Karoo Basin (100 to 500 mm/yr), and from interpretations of drill-core data sets (Cohen et al., 2007). The time required to drop lake-level from its modern maximum to the most severe low lake stage determined from paleoclimate data sets (from 700 m to 150 m maximum water depth) is less than 2500 years, even when accounting for additional water volume loss stored as groundwater. A lake-level fall of this magnitude reduces the lake surface area by 94% and reduces the total lake volume by 99%.

8088f1cf-cb61-3c69-8829-284753ad147e
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ID: 8088f1cf-cb61-3c69-8829-284753ad147e
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Abstract: Deposition of distal tephra-fall deposits in lake systems with low watershed sediment fluxes can represent large geochemical anomalies in sediment elemental composition. We examine the persistence of compositional effects of fallout from the 7.63 kyr BP Mt. Mazama caldera-forming event and other tephra deposits in the micro-laminated abyssal sediments of Pettit Lake, central Idaho, which received ~50 kg m−2 of Mazama tephra-fall material. Persistent post-depositional dominance of the Mazama material is reflected in elemental composition and grain size distribution and is related to lake hypsometry and the presence of a large in-lake sediment reservoir subject to remobilization. Decoupling of recycled clastic mineral material with Mazama compositional affinity from both biogenic matter and from clastic material derived from within the watershed is shown by coherence between dated primary tephras and a radiocarbon chronology.

02ae9eb0-7b08-32d8-a0e8-9d893bc03e3e
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ID: 02ae9eb0-7b08-32d8-a0e8-9d893bc03e3e
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Abstract: Marine isotope stages (MIS) 11 has been proposed as an analog for the present interglacial; however, terrestrial records of this time period are rare. Sediments from Lake El'gygytgyn (67 300 N, 172 50 E) in Far East Russia contain a 3.56 Ma record of climate variability from the Arctic. Here, we present the first terrestrial Arctic reconstruction of environmental and climatic changes from MIS 8 through 12 (289 to 464 ka) using organic geochemical proxies. Terrestrial vegetation changes, as revealed by plant leaf wax (n-alkane) indices and concentrations of arborinol (a biomarker for trees), show increased tree cover around the lake during interglacial periods, with higher concentrations observed during MIS 11 as compared to MIS 9. A similar pattern is also observed in records of aquatic productivity revealed by molecular indicators from dinoflagellates (dinosterol), eustigmatophyte algae (long-chain (C28-C32) 1, 15 n-alkyl diols) in addition to short-chain n-alkanes, where aquatic productivity is highest during MIS 11. Changes recorded in these molecular proxies show a similar structure to relative temperature variability as recorded by the MBT/CBT (Methylation of Branched Tetraether/Cyclization of Branched Tetraether) paleothermometer, based on branched glycerol dialkyl glycerol tetraethers (GDGTs). Additionally, relative MBT/CBT temperature changes generally track pollen and diatom 18O temperature estimates, compiled by other studies, which suggest glacial-interglacial temperature changes of 9 to 12 C. These records of environmental and climatic change indicate Arctic sensitivity to external forcings such as orbital variability and atmospheric greenhouse gas concentrations. Overall, this study indicates that organic geochemical analyses of the Lake El'gygytgyn sediment archive can provide critical insight into the response of lake ecosystems and their sensitivity in high latitude regions. © Author(s) 2013.

be9be62e-bf5d-304e-9e92-ac22fb2356a4
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Abstract: A US National Science Foundation-funded workshop occurred 17-19 May 2013 at the University of Oklahoma to stimulate research using continental scientific drilling to explore earth's sedimentary, paleobiological and biogeochemical record. Participants submitted 3-page "pre-proposals" to highlight projects that envisioned using drill-core studies to address scientific issues in paleobiology, paleoclimatology, stratigraphy and biogeochemistry, and to identify locations where key questions can best be addressed. The workshop was also intended to encourage US scientists to take advantage of the exceptional capacity of unweathered, continuous core records to answer important questions in the history of earth's sedimentary, biogeochemical and paleobiologic systems. Introductory talks on drilling and coring methods, plus best practices in core handling and curation, opened the workshop to enable all to understand the opportunities and challenges presented by scientific drilling. Participants worked in thematic breakout sessions to consider questions to be addressed using drill cores related to glacial-interglacial and icehouse-greenhouse transitions, records of evolutionary events and extinctions, records of major biogeochemical events in the oceans, reorganization of earth's atmosphere, Lagerstätte and exceptional fossil biota, records of vegetation-landscape change, and special sampling requirements, contamination, and coring tool concerns for paleobiology, geochemistry, geochronology, and stratigraphy-sedimentology studies. Closing discussions at the workshop focused on the role drilling can play in studying overarching science questions about the evolution of the earth system. The key theme, holding the most impact in terms of societal relevance, is understanding how climate transitions have driven biotic change, and the role of pristine, stratigraphically continuous cores in advancing our understanding of this linkage. Scientific drilling, and particularly drilling applied to continental targets, provides unique opportunities to obtain continuous and unaltered material for increasingly sophisticated analyses, tapping the entire geologic record (extending through the Archean), and probing the full dynamic range of climate change and its impact on biotic history. © Author(s) 2013.

6b037abd-cbce-315f-ad05-d23d5f9f1389
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Abstract: The ca. 13 m long sediment core PG1351, recovered in 1998 from the central part of Lake El'gygytgyn, NE Siberia, was investigated for lithostratigraphy, water content, dry bulk density (DBD), total organic carbon (TOC), total nitrogen (TN), total sulphur (TS) and biogenic silica (opal) contents, and for TOC stable isotope ratios (δ13CTOC). The event stratigraphy recorded in major differences in sediment composition match variations in regional summer insolation, thus confirming a new age model for this core, which suggests that it spans the last 250 ka BP. Four depositional units of contrasting lithological and biogeochemical composition have been distinguished, reflecting past environmental conditions associated with relatively warm, peak warm, cold and dry, and cold but more moist climate modes. A relatively warm climate, resulting in complete summer melt of the lake ice cover and seasonal mixing of the water column, prevailed during the Holocene and Marine Isotope Stages (MIS) 3, 5.1, 5.3, 6.1, 6.3, 6.5, 7.1-7.3, 7.5, 8.1 and 8.3. MIS 5.5 (Eemian) was characterized by significantly enhanced aquatic primary production and organic matter supply from the catchment, indicating peak warm conditions. During MIS 2, 5.2, 5.4, 6.2 and 6.4 the climate was cold and dry, leading to perennial lake ice cover, little regional snowfall, and a stagnant water body. A cold but more moist climate during MIS 4, 6.6, 7.4, 8.2 and 8.4 is thought to have produced more snow cover on the perennial ice, strongly reducing light penetration and biogenic primary production in the lake. While the cold-warm pattern during the past three glacial-interglacial cycles is probably controlled by changes in regional summer insolation, differences in the intensity of the warm phases and in the degree of aridity (changing snowfall) during cold phases likely were due to changes in atmospheric circulation patterns. © 2006 Springer Science+Business Media B.V.

1934a68b-c5ea-30f0-8c83-b606066e47af
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ID: 1934a68b-c5ea-30f0-8c83-b606066e47af
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Abstract: We synthesize African paleoclimate from 150 to 30 ka (thousand years ago) using 85 diverse datasets at a regional scale, testing for coherence with North Atlantic glacial/interglacial phases and northern and southern hemisphere insolation cycles. Two major determinants of circum-African climate variability over this time period are supported by principal components analysis: North Atlantic sea surface temperature (SST) variations and local insolation maxima. North Atlantic SSTs correlated with the variability found in most circum-African SST records, whereas the variability of the majority of terrestrial temperature and precipitation records is explained by local insolation maxima, particularly at times when solar radiation was intense and highly variable (e.g., 150-75 ka). We demonstrate that climates varied with latitude, such that periods of relatively increased aridity or humidity were asynchronous across the northern, eastern, tropical and southern portions of Africa. Comparisons of the archaeological, fossil, or genetic records with generalized patterns of environmental change based solely on northern hemisphere glacial/interglacial cycles are therefore imprecise. We compare our refined climatic framework to a database of 64 radiometrically-dated paleoanthropological sites to test hypotheses of demographic response to climatic change among African hominin populations during the 150-30 ka interval. We argue that at a continental scale, population and climate changes were asynchronous and likely occurred under different regimes of climate forcing, creating alternating opportunities for migration into adjacent regions. Our results suggest little relation between large scale demographic and climate change in southern Africa during this time span, but strongly support the hypothesis of hominin occupation of the Sahara during discrete humid intervals ~135-115 ka and 105-75 ka. Hominin populations in equatorial and eastern Africa may have been buffered from the extremes of climate change by locally steep altitudinal and rainfall gradients and the complex and variable effects of increased aridity on human habitat suitability in the tropics. Our data are consistent with hominin migrations out of Africa through varying exit points from ~140-80 ka.

a97783c8-4d5f-3f82-8aa0-edc8634c634d
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ID: a97783c8-4d5f-3f82-8aa0-edc8634c634d
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Abstract: We document frequent, rapid, strong, millennial-scale paleovegetation shifts throughout the late Pleistocene, within a 100,000+ yr interval (~ 115–15 ka) of terrestrial sediments from the mid-Atlantic Region (MAR) of North America. High-resolution analyses of fossil pollen from one core locality revealed a continuously shifting sequence of thermally dependent forest assemblages, ranging between two endmembers: subtropical oak-tupelo-bald cypress-gum forest and high boreal spruce-pine forest. Sedimentary textural evidence indicates fluvial, paludal, and loess deposition, and paleosol formation, representing sequential freshwater to subaerial environments in which this record was deposited. Its total age–depth model, based on radiocarbon and optically stimulated luminescence ages, ranges from terrestrial oxygen isotope stages (OIS) 6 to 1. The particular core sub-interval presented here is correlative in trend and timing to that portion of the oxygen isotope sequence common among several Greenland ice cores: interstades GI2 to GI24 (≈ OIS2–5 d). This site thus provides the first evidence for an essentially complete series of ‘Dansgaard–Oeschger’ climate events in the MAR. These data reveal that the ~ 100,000 yr preceding the Late Glacial and Holocene in the MAR of North America were characterized by frequently and dynamically changing climate states, and by vegetation shifts that closely tracked the Greenland paleoclimate sequence.

85892eb3-fa95-3b92-ad16-df433bc7e9d2
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ID: 85892eb3-fa95-3b92-ad16-df433bc7e9d2
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Abstract: A 12.7 m long sedimentary record recovered from Lake El'gygytgyn, located in a meteorite impact crater created 3.6 Ma in Late Cretaceous igneous rocks on Chukotka Peninsula, northeast Siberia, has been analysed for its palaeo- and rock-magnetic properties. Continuous high resolution (1 mm) measurements of magnetic susceptibility yielded successions of pronounced lows and highs. Analyses of the rock-magnetic properties by low and high temperature runs of magnetic susceptibility, determination of hysteresis parameters as well as IRM acquisition experiments, yielded a dominance of PSD (pseudo-single domain) magnetite in intervals of high magnetic susceptibility, whereas, due to selective magnetite dissolution associated with anoxic Lake water and/or pore water conditions during times of enhanced deposition of organic matter, haematite dominates within low susceptibility intervals in terms of mass percentage. Here however, magnetic properties are still dominated by magnetite. Five AMS (accelerator mass spectromentry) C-14 ages, eight IRSL (infrared stimulated luminescense) ages together with preliminary pollen data suggest that variations in magnetite content reflect climatic variability of the last similar to300 ka with low (high) susceptibilities representing cold (warm) climates. This pattern, caused by a complex system of deposition, preservation or decomposition of organic matter and/or magnetic minerals, in turn can be correlated in detail to global climate archives such as the oxygen isotope records from Greenland ice cores and marine sediments, respectively. Thus, the sedimentary sequence recovered from Lake El'gygytgyn represents the longest continuous terrestrial climate record now available from the Arctic.

b95d6674-bcbb-3b20-ab95-855e837e7abd
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Abstract: Ash layers from explosive volcanic eruptions (i.e., tephra) represent isochronous surfaces independent from the environment in which they are deposited and the distance from their source. In comparison to eastern Beringia (nonglaciated Yukon and Alaska), few Plio-Pleistocene distal tephra are known from western Beringia (non-glaciated arctic and subarctic eastern Russia), hindering the dating and correlation of sediments beyond the limit of radiocarbon and luminescence methods. The identification of eight visible tephra layers (T0-T7) in sediment cores extracted from Lake El'gygytgyn, in the Far East Russian Arctic, indicates the feasibility of developing a tephrostratigraphic framework for this region. These tephra range in age from ca 45 ky to 2.2My old, and each is described and characterized by its major-, minor-, trace-element and Pb isotope composition. These data show that subduction-zone-related volcanism from the Kurile-Kamchatka-Aleutian Arc and Alaska Peninsula is the most likely source, with Pb isotope data indicating a Kamchatkan volcanic source for tephra layers T0-T5 and T7, while a source in the Aleutian Arc is possible for tephra T6. The location of Lake El'gygytgyn relative to potential source volcanoes (>1000 km) suggests these tephra are regionally distributed over a large area. These deposits provide a unique opportunity to correlate the highresolution paleoenvironmental records of Lake El'gygytgyn to other terrestrial paleoenvironmental archives from western Beringia and marine records from the western North Pacific and Bering Sea, and to move towards the development of a robust integrated framework between the continuous paleoclimatic records of Lake El'gygytgyn and other terrestrial and marine records in NE Eurasia. © Author(s) 2014.

1600ba38-3a8d-3ee5-b8c1-d190d7a1c392
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Abstract: This selective review of the Quaternary paleoclimate of the South American summer monsoon (SASM) domain presents viewpoints regarding a range of key issues in the field, many of which are unresolved and some of which are controversial. (1) El Niño-Southern Oscillation variability, while the most important global-scale mode of interannual climate variation, is insufficient to explain most of the variation of tropical South American climate observed in both the instrumental and the paleoclimate records. (2) Significant climate variation in tropical South America occurs on seasonal to orbital (i.e. multi-millennial) time scales as a result of sea-surface temperature (SST) variation and ocean–atmosphere interactions of the tropical Atlantic. (3) Decadal-scale climate variability, linked with this tropical Atlantic variability, has been a persistent characteristic of climate in tropical South America for at least the past half millennium, and likely, far beyond. (4) Centennial-to-millennial climate events in tropical South America were of longer duration and, perhaps, larger amplitude than any observed in the instrumental period, which is little more than a century long in tropical South America. These were superimposed upon both precession-paced insolation changes that caused significant variation in SASM precipitation and eccentricity-paced global glacial boundary conditions that caused significant changes in the tropical South American moisture balance. As a result, river sediment and water discharge increased and decreased across tropical South America, lake levels rose and fell, paleolakes arose and disappeared on the Altiplano, glaciers waxed and waned in the tropical Andes, and the tropical rainforest underwent significant changes in composition and extent. To further evaluate climate forcing over the last glacial cycle (∼125 ka), we developed a climate forcing model that combines summer insolation forcing and a proxy for North Atlantic SST forcing to reconstruct long-term precipitation variation in the SASM domain. The success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation. In addition, we propose a critical correction for speleothem stable oxygen isotopic ratios, which are among the most significant of paleoclimate proxies in tropical South America for reconstruction of variation of paleo-precipitation (or SASM intensity). However, it is already well known that any particular δ18O value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount—the influence of temperature on carbonate-water isotopic fractionation in the cave and the influence of changing δ18O of seawater. Quantitatively accounting for both “artifacts” can significantly alter the interpretations of speleothem records. In tropical South America, both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo-hydrologic signal (but by different amounts in glacial and inter-glacial stages). These corrections have even graver implications for the interpretation of tropical Northern Hemisphere speleothem records (e.g. Chinese speleothems) where the combined adjustments tend to decrease or even eliminate the “true” signal amplitude.

27366ff9-3cf2-3c6c-be97-a22cb9f1c87f
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ID: 27366ff9-3cf2-3c6c-be97-a22cb9f1c87f
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Abstract: Tropical and subtropical wetlands are considered to be globally important sources of greenhouse gases, but their capacity to store carbon is presumably limited by warm soil temperatures and high rates of decomposition. Unfortunately, these assumptions can be difficult to test across long timescales because the chronology, cumulative mass, and completeness of a sedimentary profile are often difficult to establish. We therefore made a detailed analysis of a core from the principal drainage outlet of the Everglades of South Florida in order to assess these problems and determine the factors that could govern carbon accumulation in this large subtropical wetland. Accelerator mass spectroscopy dating provided direct evidence for both hard-water and open-system sources of dating errors, whereas cumulative mass varied depending upon the type of method used. Radiocarbon dates of gastropod shells, nevertheless, seemed to provide a reliable chronology for this core once the hard-water error was quantified and subtracted. Long-term accumulation rates were then calculated to be 12.1 g m-2 yr-1 for carbon, which is less than half the average rate reported for northern and tropical peatlands. Moreover, accumulation rates remained slow and relatively steady for both organic and inorganic strata, and the slow rate of sediment accretion (0.2 mm yr -1) tracked the correspondingly slow rise in sea level (0.35 mm yr-1) reported for South Florida over the past 4000 years. These results suggest that sea level and the local geologic setting may impose long-term constraints on rates of sediment and carbon accumulation in the Everglades and other wetlands. © 2012. American Geophysical Union. All Rights Reserved.

19aef9b2-2656-3149-a9b1-cbf59339edce
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ID: 19aef9b2-2656-3149-a9b1-cbf59339edce
Tag: []
Abstract: A series of environmental changes from late-glacial ice recession through the early Holocene are revealed in a 7000-yr-long record of pollen, charcoal, geochemistry, and stable isotopes from Blacktail Pond, a closed-basin lake in Yellowstone National Park. Prior to 11,500 cal yr BP, cool conditions dominated, fire activity was low, and alpine tundra and Picea parkland grew on the landscape. A step-like climate change to warm summer conditions occurred at 11,500 cal yr BP. In response, fire activity increased facilitating a transition from Picea parkland to closed Pinus forest. From 11,500 to 8280 cal yr BP, warm summers and abundant moisture mostly likely from high winter snowfall supported closed Pinus contorta forests. Cooler drier summer conditions prevailed beginning 8280 cal yr BP due to decreased summer insolation and winter snowpack, and lower parkland developed. The timing of vegetation change in the Blacktail Pond record is similar to other low- and middle-elevation sites in the northern Rocky Mountains during the late-glacial period, suggesting local plant communities responded to regional-scale climate change; however, the timing of vegetation changes was spatially variable during the early and middle Holocene due to the varying influences of strengthened summer monsoons and subtropical high on regional precipitation patterns.

16913a5e-2867-37d3-808d-a21f169f29f1
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ID: 16913a5e-2867-37d3-808d-a21f169f29f1
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Abstract: Lake sediment records from the Cordillera de Mérida in the northern Venezuelan Andes document the history of local glacial variability and climate changes during the Holocene (~12ka to the present). The valleys that contain these lakes have similar bedrock compositions and hypsometries, but have different headwall elevations and aspects, which makes them useful for investigating the magnitude of past glaciations. There was widespread glacial retreat in the Venezuelan Andes during the early Holocene, after which most watersheds remained ice free, and thus far only valleys with headwalls higher than ~4400masl contain evidence of glaciation during the last ~10ka. There was a pronounced shift in sediment composition for the Montos (headwall: ~4750masl) and Los Anteojos (headwall: ~4400masl) records during the middle Holocene from ~8.0 to 7.7ka when conditions appear to have become ice free and drier. There is tentative evidence that the glacier in the Mucubají valley (headwall: ~4609masl) advanced from ~8.1 to 6.6ka and then retreated during the latter stages of the middle Holocene. Clastic sediment accumulation in other nearby lake basins was either low or decreased throughout most of the middle Holocene as watersheds stabilized under warmer and/or drier conditions. In the Montos record, there was another major shift in sediment composition that occurred from ~6.5 to 5.7ka, similar to other regional records that suggest conditions were drier during this period. Overall, the late Holocene was a period of warmer and wetter conditions with ice extent at a minimum in the northern tropical Andes. There were also punctuated decadal to multi-centennial periods of higher clastic sediment accumulation during the last ~4ka, likely in response to periods of cooling and/or local precipitation changes. In watersheds with headwalls above 4600masl, there is evidence of glacial advances during the Little Ice Age (~0.6-0.1ka). The pattern of glacial variability is generally similar in both the northern and southern tropics during the Little Ice Age, suggesting that ice margins in both regions were responding to colder and wetter conditions during the latest Holocene. The observed pattern of Holocene climate variability in the Venezuelan Andes cannot be explained by insolation forcing alone, and tropical ocean influences were likely associated with the observed glacial and lake level changes. © 2014 Elsevier Ltd.

b42cf571-6119-3ac9-aeb3-c7846bb2a2c6
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ID: b42cf571-6119-3ac9-aeb3-c7846bb2a2c6
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Abstract: Pollen records from Lake Malawi, Africa spanning the last 135 kyr show substantial and abrupt vegetation response to multiple episodes of extreme aridity between 135 and 75 ka. Peaks in both the relative abundance and total production of Podocarpus pollen define the first two of these drought episodes. From 135 to 127 and again from 117 to 105 kyr BP, Podocarpus percentages remain above 16% with peak values as high as 38% indicating a period marked by a cool climate resulting in expansion of montane forest taxa to lower elevations. Marine palynological records from the Angola Margin and Congo Fan show similar peak Podocarpus percentages at this time (oxygen isotope stage 5d) indicating a similar climate across the African continent at this latitude. From 105 to 90 ka, continuing drought resulted in total pollen accumulation rates in Lake Malawi to fall to less than 300 grains/cm2/yr of predominately grass pollen. This episode in African history was severe enough to cause the disappearance of pteridophytes and forest taxa such as Uapaca and Brachystegia as well as montane taxa (Podocarpus, Olea spp. and Ericaceae) within the pollen source area of Lake Malawi. These taxa all remain nearly absent from the surrounding vegetation for the next 18,000 years. The resultant semi-desert vegetation would have been inhospitable for early humans living within or traveling through the Lake Malawi region. Increasing moisture following these arid intervals allowed expansion, creation and maintenance of a more diverse landscape vegetation mosaic around Lake Malawi including Zambezian miombo woodland, humid evergreen woodland and afromontane forests. The relative abundance of each fluctuated in response to either cooling (i.e. afromontane expansion from 60 to 56 ka) or moisture balance (i.e. increasing humid evergreen woodland between 75 and 65 ka). Notably there was no significant change in vegetation composition during the Last Glacial Maximum (LGM) (30-15 ka) as compared to the previous 20,000 years.

cc14bc44-1668-3c7f-ab39-8a6e44bf3e6d
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ID: cc14bc44-1668-3c7f-ab39-8a6e44bf3e6d
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Abstract: We used seismic images and sedimentary data from piston cores to conduct a sequence stratigraphic analysis of sediments in Lake Petén Itzá, northern Guatemala. Our results document lake level fluctuations in this lowland Neotropical region that were related to glacial-to-interglacial climate change during the Late Pleistocene. A bathymetric survey of Lake Petén Itzá (area = 100 km2) revealed a maximum water depth of ~160 m and the existence of a deep cryptodepression that extends 50 m below modern sea level. The great depth suggests that the basin held water even during arid conditions associated with full glacial periods. Lake Petén Itzá may thus possess the only long continuous lacustrine sediment record of Late Pleistocene environmental and climate change in the lowland Neotropics. Two seismic reflection campaigns imaged the subsurface basin sediments that overlie basement. The sediment package was divided into four major seismic sequences (T, G, R, and B). Sequences are separated by unconformities that represent depositional cycles related to lake level fluctuations. Sediments of the uppermost sequence (T) were recovered and radiocarbon-dated in Kullenberg piston cores taken along a water depth transect. Seismic profiles reveal a basin-wide paleoshoreline just below sequence T at ~56 m below present lake level. This constructional feature formed during a lowstand of the last glacial period when the lake was reduced to only ~13% of its present volume. In cores taken landward of the paleoshoreline, Late Glacial-age deposits consist of paleosols, indicating subaerial exposure. Basinward of the shoreline, sediments are composed of dense gypsum sands and interbedded silty clays, reflecting authigenic gypsum formation under arid climate conditions. The top of the soil horizon and cessation of gypsum precipitation are represented by a strong seismic reflection (t). It marks the base of the uppermost seismic sequence T and is dated in several cores between ~11.1 and 10.2 cal kyr BP. Lake level rose quickly at this time in response to a shift from arid-to-humid climate conditions at the Late Glacial/Early Holocene transition. We infer a similar sediment response to climate variations in the older stratigraphic sequences (G, R, and B), related to earlier glacial-to-interglacial and stadial-to-interstadial cycles. Older sequences are also distinguished from one another by erosional unconformities that probably represent major lake level falls. Future recovery of the older stratigraphic record by drilling in Lake Petén Itzá will provide ages for these older units and enable us to test the depositional model inferred from seismic stratigraphy.

7a4213d3-ccb0-3427-b004-55790085873a
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ID: 7a4213d3-ccb0-3427-b004-55790085873a
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Abstract: The Albufera of Valencia, one of the largest oligohaline coastal lagoons of the Iberian Peninsula, has suffered hydrological and landscape modifications since medieval times. This paleolimnological study clears up the controversy about the factors, which drove the lake to its present state. Lithological descriptions, ostracod paleoassemblages and Cyprideis torosa (Jones, 1850) shell morphological variations and geochemistry (δ18O, δ13C and Sr/Ca) have been used to reconstruct the lake ecosystem evolution during the last two centuries. The sandy sediments at the bottom of L’Antina core (63 cm) presented a typical assemblage of brackish water ostracods, with shell-derived δ18O values, inferred salinity and Sr/CaWATER characteristic of waters with marine influence. However, 10 cm above, coinciding with a drastic change in lithology, ostracod palaeoassemblages and decreased shell δ18O indicated a shift to freshwater influences. At the top of the sequence, inferred salinity suggests oligohaline waters and ostracods have almost disappeared. The early brackish conditions of Lake Albufera changed at the beginning of the 19th century when important hydrological modifications were undertaken for irrigation and started the major period of rice field expansion. Later, with the change of traditional rice farming to a more technological agriculture, together with increased domestic and industrial sewage inputs, the Albufera became a highly eutrophicated system 50 years ago. Eventually, the loss of macrophyte cover, cyanobacteria blooms and the increase of grazing by fish forced the loss of ostracod communities, currently almost inexistent.

bf00d37e-80bb-3088-bf6f-e5b6b2781464
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ID: bf00d37e-80bb-3088-bf6f-e5b6b2781464
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Abstract: Apparent changes in vegetation distribution, fire, and other disturbance regimes throughout western North America have prompted investigations of the relative importance of human activities and climate change as potential causal mechanisms. Assessing the effects of Euro-American settlement is difficult because climate changes occur on multi-decadal to centennial time scales and require longer time perspectives than historic observations can provide. Here, we report vegetation and environmental changes over the past ~13,000 years as recorded in a sediment record from Bison Lake, a subalpine lake on a high plateau in northwestern Colorado. Results are based on multiple independent proxies, which include pollen, charcoal, and elemental geochemistry, and are compared with previously reported interpretations of hydroclimatic changes from oxygen isotope ratios. The pollen data indicate a slowly changing vegetation sequence from sagebrush steppe during the late glacial to coniferous forest through the late Holocene. The most dramatic vegetation changes of the Holocene occurred during the ‘Medieval Climate Anomaly’ (MCA) and ‘Little Ice Age’ (LIA) with rapid replacement of conifer forest by grassland followed by an equally rapid return to conifer forest. Late Holocene vegetation responses are mirrored by changes in fire, lake biological productivity, and watershed erosion. These combined records indicate that subsequent disturbance related to Euro-American settlement, although perhaps significant, had acted upon a landscape that was already responding to MCA-LIA hydroclimatic change. Results document both rapid and long-term subalpine grassland ecosystem dynamics driven by agents of change that can be anticipated in the future and simulated by ecosystem models.

3a4d5555-58e9-3858-8a22-3bcd65db5f6b
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ID: 3a4d5555-58e9-3858-8a22-3bcd65db5f6b
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Abstract: The Holocene turbidite systems of Lake Kivu and the Pliocene turbidite systems of Lake Albert in the East African Rift were examined using high-resolution 2-D and 3-D seismic reflection data and sediment core information. Based on investigations of seismic facies and lithofacies, several key turbidity-flow depositional elements were observed, including channels, overbank levees with sediment waves, and depositional lobes. Analyses of the sources of the recent and ancient turbidite systems in these two extensional basins suggest that flood-induced hyperpycnal flows are important triggers of turbidity currents in lacustrine rift basins. From source to sink, sediment dispersal, facies distribution, and depositional thickness of the turbidite systems are strongly influenced by rift topography. The Lake Kivu and Lake Albert rifts serve as excellent analogues for understanding the sedimentary patterns of lacustrine turbidites in extensional basins.

ebd5ac29-137e-38a9-a474-c55759d26d98
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ID: ebd5ac29-137e-38a9-a474-c55759d26d98
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Abstract: We investigate Lake Estanya's diatom and pollen records from the Late Glacial (LG) to the Early Holocene (EH), in order to compare limnological and vegetation responses to common climate forcing. The biotic changes recognized in this study largely agree with the hydrological evolution of the lake described previously for the same period. The diatom record shows high sensitivity to fluctuations in both lake level and salinity concentration as a consequence of climate shifts. In addition vegetation results indicate that the area could have played an important role as regional vegetation refuge. Shallow lake conditions during the Last Glacial Maximum (LGM) were punctuated by relatively deeper freshwaters between 19.3 and 18.6 cal kyr BP and at 18.0 cal kyr BP, as recorded by diatom shifts. A subsequent increasing aridity trend, coinciding with the Mystery Interval (MI), affected the diatom accumulation rates, which dropped to its minimum values between 17.2 and 14.7 cal kyr BP. Particularly dry and cold conditions during the LGM and MI are supported by the largest values of steppic pollen taxa of the whole sequence, which account for up to 40%. However, relatively high values of Betula during the Heinrich Event 1 suggest a plausible regional vegetation refuge. Abrupt cooling and warming episodes within the LG triggered remarkable ecological threshold crossings in the diatom communities, especially during the stadial/interstadial episodes. At this point, the vegetation reflects the onset of warm conditions during the Bølling/Allerød with the partial substitution of Betula by Marcescent and Evergreen Quercus, which probably indicates the arrival of temperate taxa to the area and the likely migration of birch to higher altitudes. The Younger Dryas Stadial shows a complex ecological response. Diatoms are very poorly preserved, but aquatic taxa reach their highest values. An increase in Marcescent Quercus during this cold stage lends further support to the hypothesis that this is a regional vegetation refuge. Low lake levels recorded during the EH affected the development and preservation of diatom communities. A delay in the onset of humid conditions for the EH is also supported by the vegetation composition, characterized by the maximum expansion of Juniperus.

39e73b9f-e2e1-3032-90e7-d3093af75ee3
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ID: 39e73b9f-e2e1-3032-90e7-d3093af75ee3
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Abstract: Sediment records from floodplain lakes have a large and commonly untapped potential for inferring wetland response to global change. The Brazilian Pantanal is a vast, seasonally inundated savanna floodplain system controlled by the flood pulse of the Upper Paraguay River. Little is known, however, about how floodplain lakes within the Pantanal act as sedimentary basins, or what influence hydroclimatic variables exert on limnogeological processes. This knowledge gap was addressed through an actualistic analysis of three large, shallow (<5 m) floodplain lakes in the western Pantanal: Lagoa Gaíva, Lagoa Mandioré and Baia Vermelha. The lakes are dilute (CO32- > Si4+ > Ca2+), mildly alkaline, freshwater systems, the chemistries and morphometrics of which evolve with seasonal flooding. Lake sills are bathymetric shoals marked by siliciclastic fans and marsh vegetation. Flows at the sills likely undergo seasonal reversals with the changing stage of the Upper Paraguay River. Deposition in deeper waters, typically encountered in proximity to margin-coincident topography, is dominated by reduced silty-clays with abundant siliceous microfossils and organic matter. Stable isotopes of carbon and nitrogen, plus hydrogen index measured on bulk organic matter, suggest that contributions from algae (including cyanobacteria) and other C3-vegetation dominate in these environments. The presence of lotic sponge spicules, together with patterns of terrigenous sand deposition and geochemical indicators of productivity, points to the importance of the flood pulse for sediment and nutrient delivery to the lakes. Flood-pulse plumes, waves and bioturbation likewise affect the continuity of sedimentation. Short-lived radioisotopes indicate rates of 0.11-0.24 cm year-1 at sites of uninterrupted deposition. A conceptual facies model, developed from insights gained from modern seasonal processes, can be used to predict limnogeological change when the lakes become isolated on the floodplain or during intervals associated with a strengthened flood pulse. Amplification of the seasonal cycle over longer time scales suggests carbonate, sandy lowstand fan and terrestrial organic matter deposition during arid periods, whereas deposition of lotic sponges, mixed aquatic organic matter, and highstand deltas characterizes wet intervals. The results hold substantial value for interpreting paleolimnological records from floodplain lakes linked to large tropical rivers with annual flooding cycles. © 2011 Springer Science+Business Media B.V.

67fca8cf-b1f0-3f1f-b4d5-d90a5be3974b
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ID: 67fca8cf-b1f0-3f1f-b4d5-d90a5be3974b
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Abstract: The transport of moisture in the tropics is a critical process for the global energy budget and on geologic timescales, has markedly influenced continental landscapes, migratory pathways, and biological evolution. Here we present a continuous, first-of-its-kind 1.3-My record of continental hydroclimate and lake-level variability derived from drill core data from Lake Malawi, East Africa (9-15° S). Over the Quaternary, we observe dramatic shifts in effective moisture, resulting in large-scale changes in one of the world's largest lakes and most diverse freshwater ecosystems. Results show evidence for 24 lake level drops of more than 200 m during the Late Quaternary, including 15 lowstands when water levels were more than 400 m lower than modern. A dramatic shift is observed at the Mid-Pleistocene Transition (MPT), consistent with far-field climate forcing, which separates vastly different hydroclimate regimes before and after ∼800,000 years ago. Before 800 ka, lake levels were lower, indicating a climate drier than today, and water levels changed frequently. Following the MPT high-amplitude lake level variations dominate the record. From 800 to 100 ka, a deep, often overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of extreme drought. Periods of high lake level are observed during times of high eccentricity. The extreme hydroclimate variability exerted a profound influence on the Lake Malawi endemic cichlid fish species flock; the geographically extensive habitat reconfiguration provided novel ecological opportunities, enabling new populations to differentiate rapidly to distinct species.

8a509d30-b94c-353b-92db-2cf94ab5bfce
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ID: 8a509d30-b94c-353b-92db-2cf94ab5bfce
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Abstract: Change in water availability is of great concern in the coastal southwest United States (CSWUS). Reconstructing the history of water pre–1800 AD requires the use of proxy data. Lakes provide long-lived, high-resolution terrestrial archives of past hydrologic change, and their sediments contain a variety of proxies. This study presents geochemical and sedimentological data from Zaca Lake, CA (Santa Barbara County) used to reconstruct a 3000 year history of winter season moisture source (δDwax) and catchment run-off (125–2000 μm sand) at decadal resolution. Here we show that winter season moisture source and run-off are highly variable over the past 3000 years; superimposed are regime shifts between wetter or drier conditions that persist on average over multiple centuries. Moisture source and run-off do not consistently covary indicating multiple atmospheric circulation modes where wetter/drier conditions prevail. Grain-size analysis reveals two intervals of multi-century drought with less run-off that pre-date the “epic droughts” as identified by Cook et al. (2004). A well-defined wet period with more run-off is identified during the Little Ice Age. Notably, the grain size data show strong coherence with western North American percent drought area indices for the past 1000 years. As a result, our data extend the history of drought and pluvials back to 3000 calendar years BP in the CSWUS. Comparison to tropical Pacific proxies confirms the long-term relationship between El Niño and enhanced run-off in the CSWUS. Our results demonstrate the long-term importance of the tropical Pacific to the CSWUS winter season hydroclimate.

60e6be68-5aa3-3f56-966c-ea1dfed8843b
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ID: 60e6be68-5aa3-3f56-966c-ea1dfed8843b
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Abstract: Varve thickness records from glacial Lake Ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the Laurentide Ice Sheet during the early Holocene. The demise of Lake Ojibway occurred when an ice dam of the shrinking Laurentide Ice Sheet failed, perhaps releasing enough freshwater to impede Atlantic meridional overturning circulation and precipitate the 8200 cal BP climatic cooling event. Previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long. Between varves numbered 1 to 1500, average rates of ice margin retreat increased from 140 m/y to as high as 400 m/y. Beginning prior to varve year 1800, and lasting to around year 2000, ice readvanced along a front at least 300 km wide during an episode called the Cochrane. The total distance of the advance was at least 75 km, and averaged around 200 m/y. During the advance, a series of ~50 anomalously thick varves were deposited, the cause of which is uncertain. A widespread unconformity occurred during the end of the Cochrane advance, which was followed at around varve year 2060 by the return of varves, which thicken abruptly. Unlike previous varves, these varves include ice-rafted pellets of Lake Ojibway sediment. These varves may have resulted from rapid melting and calving, rather than a second advance, and may relate to basin re-filling following a lowstand. There are no direct dates on the Lake Ojibway varves, but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater (including Lake Agassiz overflow), diverted from routes to the Lake Superior basin, into the Lake Ojibway basin. Dates from the Lake Superior basin suggest that the diversion of this water occurred at around 9040 cal BP. Applying this date to varve 1528 constrains the age of the Lake Ojibway varves numbered 1 to 2100 to between 10,570 and 8470 (±200) cal BP. © 2011 Elsevier Ltd and INQUA.

8c50344a-2737-3d16-9fcd-ad48619f981b
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Abstract: Novel plant assemblages are a long-recognized feature of late-glacial North America, but identifying their causes has been hampered by inaccurate radiocarbon chronologies and the multiplicity of ecological and climatic events during the late Pleistocene. Recently we reported that the formation of no-analog vegetation may have been linked to declines in Pleistocene megafaunal communities, based on pollen and spores from the coprophilous fungus Sporormiella at sites in Indiana and New York. We present a new, multi-proxy analysis from Silver Lake, OH, which 1) updates the radiocarbon chronology of a classic pollen record with a well-established zone of no-analog vegetation, 2) combines a new sub-centennial pollen record with charcoal, Sporormiella, and x-ray fluorescence (XRF) spectroscopy analyses for an integrated record of landscape change before, during, and after the period of no-analog vegetation, and 3) replicates both the absolute and relative temporal patterns of landscape change at Appleman Lake, IN. At Silver Lake, the decline in Sporormiella at 13.9 ka BP was immediately followed by the formation of novel plant assemblages, as well as the highest-magnitude charcoal peak in the record. Increased Ca and Sr concentrations during the no-analog interval indicate either increased moisture, increased input of nutrients from deciduous litter, or both. The duration of the no-analog assemblages (13.9–11.8 ka BP) roughly corresponds to the period of peak insolation dissimilarity, but is more temporally constrained than previously reported in subcontinental-scale syntheses (17–11 ka BP). We propose a hierarchy of controls on late-glacial plant communities, where biotic interactions such as megaherbivory mediate climate-driven vegetation change.

64434de2-5c41-3c4c-b82b-ceac4c9cc8a6
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ID: 64434de2-5c41-3c4c-b82b-ceac4c9cc8a6
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Abstract: Shorelines and surficial deposits (including buried forest-floor mats and organic-rich wetland sediments) show that Great Salt Lake did not rise higher than modern lake levels during the earliest Holocene (11.5–10.2 cal ka BP; 10–9 14C ka BP). During that period, finely laminated, organic-rich muds (sapropel) containing brine-shrimp cysts and pellets and interbedded sodium-sulfate salts were deposited on the lake floor. Sapropel deposition was probably caused by stratification of the water column — a freshwater cap possibly was formed by groundwater, which had been stored in upland aquifers during the immediately preceding late-Pleistocene deep-lake cycle (Lake Bonneville), and was actively discharging on the basin floor. A climate characterized by low precipitation and runoff, combined with local areas of groundwater discharge in piedmont settings, could explain the apparent conflict between evidence for a shallow lake (a dry climate) and previously published interpretations for a moist climate in the Great Salt Lake basin of the eastern Great Basin.

4cea6cab-ee3e-3843-81de-bd037f0b1887
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ID: 4cea6cab-ee3e-3843-81de-bd037f0b1887
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Abstract: Abrupt climate change in the past is thought to have disrupted societies by accelerating environmental degradation, potentially leading to cultural collapse. Linking climate change directly to societal disruption is challenging because socioeconomic factors also play a large role, with climate being secondary or sometimes inconsequential. Combining paleolimnologic, historical, and archaeological methods provides for a more secure basis for interpreting the past impacts of climate on society. We present pollen, non-pollen palynomorph, geochemical, paleomagnetic and sedimentary data from a high-resolution 2700 yr lake sediment core from central Italy and compare these data with local historical documents and archeological surveys to reconstruct a record of environmental change in relation to socioeconomic history and climatic fluctuations. Here we document cases in which environmental change is strongly linked to changes in local land management practices in the absence of clear climatic change, as well as examples when climate change appears to have been a strong catalyst that resulted in significant environmental change that impacted local communities. During the Imperial Roman period, despite a long period of stable, mild climate, and a large urban population in nearby Rome, our site shows only limited evidence for environmental degradation. Warm and mild climate during the Medieval Warm period, on the other hand, led to widespread deforestation and erosion. The ability of the Romans to utilize imported resources through an extensive trade network may have allowed for preservation of the environment near the Roman capital, whereas during medieval time, the need to rely on local resources led to environmental degradation. Cool wet climate during the Little Ice Age led to a breakdown in local land use practices, widespread land abandonment and rapid reforestation. Our results present a high-resolution regional case study that explores the effect of climate change on society for an under-documented region of Europe.

36d8a0b8-9311-303a-a350-f4768a90a78d
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ID: 36d8a0b8-9311-303a-a350-f4768a90a78d
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Abstract: We have conducted a global survey of archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids in lake sediments in order to develop the TEX86 paleotemperature proxy for application in continental systems. Surface sediments of 46 globally distributed lakes were analyzed for GDGT, but isoprenoid GDGT derived from aquatic Crenarchaeota, were only unambiguously detected in 20 of the 46 lakes analyzed. Aquatic crenarchaeotal GDGT were detected mainly in sediments from large lakes (>4000 km2) and hydrothermal or volcanic lakes, suggesting that in some (mostly smaller) lakes either aquatic Crenarchaeota are present at low abundance resulting in sedimentary lipids below levels of detection, or they are absent. Branched GDGT, thought to be derived primarily from soil bacteria, were identified in all lake sediments analyzed. Correlation of the TEX86 in those lakes with sufficient amounts of putative crenarchaeotal GDGT with annual mean lake temperature is reasonably good (r2 = 0.68, N = 20). In order to reduce the influence of soil derived isoprenoid GDGT on the TEX86 lake temperatures, we have applied a filter based on relative soil derived inputs (as determined by the BIT [Branched and Isoprenoid Tetraether] index) which results in a calibration relationship with r2 = 0.86 (N = 12) and an estimated temperature error of 3.6 °C. Our results suggest that the TEX86 should be applied only in lakes with sufficient production of GDGT by aquatic Crenarchaeota relative to isoprenoid GDGT derived from soil in the watershed or other aquatic sources. © 2009 Elsevier Ltd. All rights reserved.

25696cdc-ebf6-3aa1-8021-dfb2c33650cc
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ID: 25696cdc-ebf6-3aa1-8021-dfb2c33650cc
Tag: []
Abstract: Analysis of sedimentary diatom assemblages (10 to 144. ka) form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of Lake Malawi. Lake-level fluctuations on the order of hundreds of meters were inferred from dramatic changes in the fossil and sedimentary archives. Many of the fossil diatom assemblages we observed have no analog in modern Lake Malawi. Cyclotelloid diatom species are a major component of fossil assemblages prior to 35. ka, but are not found in significant abundances in the modern diatom communities in Lake Malawi. Salinity- and alkalinity-tolerant plankton has not been reported in the modern lake system, but frequently dominant fossil diatom assemblages prior to 85. ka. Large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31. ka. Similarly, prior to 31. ka, common central-basin aulacoseiroid species are replaced by species found in the shallow, well-mixed southern basin. Surprisingly, tychoplankton and periphyton were not common throughout prolonged lowstands, but tended to increase in relative abundance during periods of inferred deeper-lake environments. A high-resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wet-sieved fossil and mineralogical residue records. Prior to 70. ka, fossil assemblages suggest that the central basin was periodically a much shallower, more saline and/or alkaline, well-mixed environment. The most significant reconstructed lowstands are ~. 600. m below the modern lake level and span thousands of years. These conditions contrast starkly with the deep, dilute, dysaerobic environments of the modern central basin. After 70. ka, our reconstruction indicates sustained deeper-water environments were common, marked by a few brief, but significant, lowstands. High amplitude lake-level fluctuations appear related to changes in insolation. Seismic reflection data and additional sediment cores recovered from the northern basin of Lake Malawi provide evidence that supports our reconstruction. © 2010 Elsevier B.V.

42300663-db2f-30c1-b2ea-a78cf76f8a7a
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ID: 42300663-db2f-30c1-b2ea-a78cf76f8a7a
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Abstract: Tropical climate is rapidly changing, but the effects of these changes on the geosphere are unknown, despite a likelihood of climatically-induced changes on weathering and erosion. The lack of long, continuous paleo-records prevents an examination of terrestrial responses to climate change with sufficient detail to answer questions about how systems behaved in the past and may alter in the future. We use high-resolution records of pollen, clay mineralogy, and particle size from a drill core from Lake Malawi, southeast Africa, to examine atmosphere-biosphere-geosphere interactions during the last deglaciation (∼18–9 ka), a period of dramatic temperature and hydrologic changes. The results demonstrate that climatic controls on Lake Malawi vegetation are critically important to weathering processes and erosion patterns during the deglaciation. At 18 ka, afromontane forests dominated but were progressively replaced by tropical seasonal forest, as summer rainfall increased. Despite indication of decreased rainfall, drought-intolerant forest persisted through the Younger Dryas (YD) resulting from a shorter dry season. Following the YD, an intensified summer monsoon and increased rainfall seasonality were coeval with forest decline and expansion of drought-tolerant miombo woodland. Clay minerals closely track the vegetation record, with high ratios of kaolinite to smectite (K/S) indicating heavy leaching when forest predominates, despite variable rainfall. In the early Holocene, when rainfall and temperature increased (effective moisture remained low), open woodlands expansion resulted in decreased K/S, suggesting a reduction in chemical weathering intensity. Terrigenous sediment mass accumulation rates also increased, suggesting critical linkages among open vegetation and erosion during intervals of enhanced summer rainfall. This study shows a strong, direct influence of vegetation composition on weathering intensity in the tropics. As climate change will likely impact this interplay between the biosphere and geosphere, tropical landscape change could lead to deleterious effects on soil and water quality in regions with little infrastructure for mitigation.

a9077245-f3ab-3c26-8b15-c9b74018fcc6
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ID: a9077245-f3ab-3c26-8b15-c9b74018fcc6
Tag: []
Abstract: Vestra Gíslholtsvatn, a non-glacial lake in southern Iceland, provides a continuous high-resolution multi-proxy paleoenvironmental record spanning more than 10 ka. It is characterized by an interplay between volcanic and climatic controls on the lake sediment environmental proxies. Initial lake ontogeny was disrupted by deposition of the Saksunarvatn tephra (∼10.3 ka) followed by unusually slow ontogeny for 2 ka. Principal component analysis defines three climate zones between 8 and 1 ka. Quasi-stabilization and warm summer proxies between ∼8 and 5.7 ka define Holocene Thermal Maximum conditions. A subtle shift to cooler summers between ∼5.7 and ∼4.2 ka is interpreted as early Neoglaciation, intensifying after 4.2 ka. Following settlement of Iceland (∼1.1 ka) anthropogenic signals overprint the climate record. Throughout the record short-term variability is attributed to tephra fall events in the catchment from nearby explosive volcanism, whereas centennial- and millennial-scale changes are attributed to climate. When compared with other Icelandic lakes, environmental proxies from Vestra Gíslholtsvatn indicate less variability albeit similar timing of environmental change, suggesting a relatively stable flow of the Irminger Current along the south coast of Iceland compared with a more variable flow of warm Atlantic surface water along the western and northern coasts of Iceland after 8 ka.

bbd0141c-d8d1-37bc-ae58-3d32e8925ef1
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ID: bbd0141c-d8d1-37bc-ae58-3d32e8925ef1
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Abstract: Expansive playa-lake systems situated in high-altitude piggyback basins are important and conspicuous components of both modern and ancient cordilleran orogenic systems. Extant playa lakes provide vital habitat for numerous endemic species, whereas sediments from these deposystems may record signals of climate change or develop natural resources over geological time. Laguna de los Pozuelos (North-west Argentina) provides the opportunity for an actualistic sedimentological and geochemical assessment of a piggyback basin playa lake in an area of critical interest for understanding Quaternary palaeoclimate dynamics. Silty clays and diatom ooze are the dominant playa-lake centre microfacies, with concentrations of total organic carbon and biogenic silica commonly exceeding 1·5 wt% in this sub-environment. Elemental and stable isotopic analyses point to a mixed organic matter composition in the playa-lake centre, with substantial contributions from algae and transported aquatic macrophytes. Bulk sediment and organic mass accumulation rates in the southern playa-lake centre approach 0·22 g cm−2 year−1 and 2·89 mg cm−2 year−1, respectively, indicating moderately rapid deposition with negligible deflation over historic time. Playa margin facies contain higher percentages of fragmented biogenic carbonate (ostracods and charophytes) and inorganically precipitated aragonite crusts due to seasonal pumping and evaporation of ground water. Organic matter accumulation is limited along these heavily bioturbated wet and dry mud flats. Fluvial–lacustrine transitional environments, which are key waterbird habitats, are either silty terminal splay (northern axis) or sandy deltas (southern axis) containing highly oxidized and partially allochthonous organic matter. Modern analogue data from Laguna de los Pozuelos provide key insights for: (i) environmental reconstructions of ancient lake sequences; and (ii) improving facies models for piggyback basins.

28a09f2c-d4b1-31b2-920a-1fd0ff129364
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ID: 28a09f2c-d4b1-31b2-920a-1fd0ff129364
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Abstract: Understanding the natural mechanisms that control fire occurrence in terrigenous ecosystems requires long and continuous records of past fires. Proxies, such as sedimentary charcoal and tree-ring fire scars, have temporal or spatial limitations and do not directly detect fire intensity. We show in this study that polycyclic aromatic hydrocarbons (PAHs) produced during wildfires record local fire events and fire intensity. We demonstrate that high performance liquid chromatography with fluorescence detector (HPLC-FLD) is superior to gas chromatography–mass spectrometry (GC–MS) for detecting the low concentrations of sedimentary PAHs derived from natural fires. The HPLC-FLD is at least twice as sensitive as the GC–MS in selective ion monitoring (SIM) mode for parent PAHs and five times as sensitive for retene. The annual samples extracted from varved sediments from Swamp Lake in Yosemite National Park, California are compared with the observational fire history record and show that PAH fluxes record fires within 0.5 km of the lake. The low molecular weight (LMW) PAHs (e.g., fluoranthene, pyrene and benz[a]anthracene) are the best recorders of fire, whereas the high molecular weight (HMW) PAHs likely record fire intensity. PAHs appear to resolve some of the issues inherent to other fire proxies, such as secondary deposition of charcoal. This study advances our understanding of how PAHs can be used as markers for fire events and poses new questions regarding the distribution of these compounds in the environment.

cbcb5d24-1128-3a7e-a190-1d0ffe4521b9
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ID: cbcb5d24-1128-3a7e-a190-1d0ffe4521b9
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Abstract: The relationship of fire and climate in explaining the origin and maintenance of the grasslands of the northern Great Plains has long been of interest. I examined the hypothesis that burning was more frequent during wet years using charcoal to reconstruct fire histories near Coldwater Lake in southcentral ND, and Rice Lake in northcentral ND and published records of ostracod Mg/Ca ratios as a proxy for climate at these same two sites. Over the past 10000 years, charcoal influx rates for Coldwater (0.04-5.68 mm2 cm-2 year-1) greatly exceeded influx rates for Rice (<0.01-1.91 mm2 cm-2 year-1). Both sites showed strong, significant periodicities of ∼1500-1800, ∼800-900 and ∼130-140 years but charcoal maxima and minima were only similar at ∼4200 cal years BP. Charcoal influx during the past 2000 years also revealed strong periodicities of between 310-400 and 140 years at both sites. Comparison of smoothed charcoal influxes and ostracod Mg/Ca ratios suggests that both proxies are responding to changes in climate. When smoothed with a 320-400-year window, increases in charcoal influx typically preceded rises in ostracod Mg/Ca ratio by ∼50-100 years, and further smoothing (140-year window) of the residuals suggested a variable relationship between Mg/Ca ratios and charcoal influx. These results do not clearly support or reject the fuel limitation hypothesis, and this lack of clear support may result from (a) climate-driven shifts in the mix of C3 and C4 grasses resulting in maximum productivity at intermediate moisture levels, (b) possibly different responses of charcoal and Mg/Ca ratios to summer vs. winter precipitation, or (c) ground-water driven lags in the response of Mg/Ca ratios to shifts in climate. Comparison of the results from this study with other studies suggests that links between fire and fuels at finer time scales (0-100 years) may be broadly constrained by longer term (∼500-2000 years) patterns of climate in the northern Great Plains. © 2004 Elsevier B.V. All rights reserved.

8c2fe260-7614-3d09-91a4-f587765a5076
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ID: 8c2fe260-7614-3d09-91a4-f587765a5076
Tag: []
Abstract: Sediment magnetic properties of a short core from Sharkey Lake, MN, record the effects of Euroamerican settlement and climate change over the last 150 yr. The onset of European-style farming led to increased erosion, reflected in high values of concentration-dependent parameters such as magnetic susceptibility (k), Isothermal Remanent Magnetization (IRM), and Anhysteretic Remanent Magnetization (ARM). These high values are only partially due to increased supply of terrigenous material to the lake, and recent sediment contains an additional component of authigenic fine (single-domain) magnetite, most likely magnetosomes from magnetotactic bacteria. High organic productivity in the lake during the 1920s to 1940s drought increased this authigenic component resulting in highly magnetic fine-grained sediment. A comparison with older Holocene sediment from the same lake shows that, over time, most of the fine magnetic signal is lost after deposition, leading to decreases in magnetization and a bimodal grain size distribution of ultrafine, superparamagnetic grains and coarser multidomain particles, evident from measurements of ARM/IRM ratios, hysteresis measurements, and low-temperature analyses. The effects of dissolution and the superposition of climate and land-use signals complicate the use of recent sediments as modern analogs for sediment magnetic analyses. © 2004 University of Washington. All rights reserved.

fd3c6845-d6d4-3f03-b021-14585f97c4f1
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ID: fd3c6845-d6d4-3f03-b021-14585f97c4f1
Tag: []
Abstract: Few studies have assessed the relative importance of landscape, land use history, climate, and regional heterogeneity on lake ecosystem processes, despite the likelihood that interactions among these factors must be important for controlling lake dynamics. We used 14 sediment measures from 20 lake cores in a climatically sensitive region of the prairie-forest border in southern Minnesota to (1) assess relationships between modern lake productivity (Carlson's Trophic State Index [TSI]), modern land-use, catchment, and lake morphometry, and (2) contrast regional responses to climatic transitions from the 'Medieval Climatic Anomaly'(1000-1350) to the 'Little Ice Age' (1350-1800) to 'Modern'(~1980-1996 AD). TSI was significantly positively correlated with modern sedimentation rate, and accumulation rates of organic matter (OM), biogenic silica (BSi), and total phosphorus (TP). TSI was not significantly correlated with "modern" land cover, catchment, or lake morphometry characteristics, but total organic N(N) was negatively correlated with percent cultivation in the catchment area and negatively correlated with δ15N of bulk organic matter in sediment. Regional, among-lake heterogeneity was high over the past 1,000 years, but Little Ice Age (LIA) cooling appeared to result in an approximately 20% decline in OM, BSi and TP accumulation, while warming and cultural eutrophication of the past 150 years corresponded to a 200-400% increase in accumulation rates as well as an 80% increase in carbonates and a small but significant 10% drop in C/N ratios consistent with greater in-lake productivity. Our results indicate that climate does have regional effects on lake ecosystems but that among-lake variability is high, reflecting the importance of local factors and suggesting a need for (1) more explicit definition of what 'regional' means, (2) a focus on degree as well as direction of change, and (3) estimating accumulation rates based on multiple lakes and multiple cores within lakes. © 2010 Springer Science+Business Media B.V.

b02d084c-2ba7-3846-92f6-d2599142d60c
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ID: b02d084c-2ba7-3846-92f6-d2599142d60c
Tag: []
Abstract: The mid-Holocene (ca. 8000-4000 cal yr BP) was a time of marked aridity throughout much of Minnesota, and the changes due to mid-Holocene aridity are seen as an analog for future responses to global warming. In this study, we compare the transition into (ca. 9000-7000 yr ago) and out of (ca. 5000-2500 yr ago) the mid-Holocene (MH) period at Kimble Pond and Sharkey Lake, located along the prairie forest ecotone in south-central Minnesota, using high resolution (∼ 5-36 yr) sampling of pollen, charcoal, sediment magnetic and loss-on-ignition properties. Changes in vegetation were asymmetrical with increasing aridity being marked by a pronounced shift from woodland/forest-dominated landscape to a more open mix of grassland and woodland/savanna. In contrast, at the end of the MH, grassland remained an important component of the landscape despite increasing effective moisture, and high charcoal influxes (median 2.7-4.0 vs. 0.6-1.7 mm2 cm- 2 yr- 1 at start of MH) suggest the role of fire in limiting woodland expansion. Asymmetric vegetation responses, variation among and within proxies, and the near-absence of fire today suggest caution in using changes associated with mid-Holocene aridity at the prairie forest boundary as an analog for future responses to global warming. © 2006 University of Washington.

3f2284e3-5863-38aa-862d-475d2db39e9a
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ID: 3f2284e3-5863-38aa-862d-475d2db39e9a
Tag: []
Abstract: Fires are common in grassland regions of the world, and the frequency and severity of fire is linked to climate-driven changes in fuel loads. Western Mongolia is dominated by grasslands but the fire history of this region is largely unknown. We reconstructed modern fire (48 lakes) and historical fires (9 lakes) using sediment charcoal. Modern fuel loads were estimated using a combination of clipped plots, satellite-based estimates of annual aboveground net primary productivity (NPP) and NPP modeled from annual temperature and precipitation. Loss-on-ignition and environmental magnetics of lake sediments were analyzed as proxies for climate. We found little evidence for modern or historical fire in the landscape, as charcoal was absent from the surface sediments of 34 of 48 lakes. Charcoal influxes were uniformly low, averaging from 0.002 to 0.028 mm2/cm2 per yr, over the past 1200 years at nine lakes, and the past 6000ĝ€"5000 years at two of the lakes with longer sediment records. In the modern landscape, livestock grazing has eliminated most of the fuels necessary to carry a fire, as measured fuel loads (27.3±4.9 g/m2) were only ∼20% of aboveground annual NPP estimated using MODIS Imagery or modeled from climate data. The historical absence of fire may indicate a longer history of intensive grazing than sometimes assumed, and cultural prohibitions against burning may also play a role. Regional summary indicated a >50% decrease in charcoal influxes since AD 1600 at most sites which may be related to lower temperatures or greater aridity during the 'Little Ice Age'. Alternatively this decrease in charcoal influxes may reflect increases in livestock numbers or increased local concentrations because of restrictions on the movement of animals coincident with the establishment of Manchu rule in the late seventeenth century.

f0ca7c6c-e93d-33c0-83dd-f83c99eef88d
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ID: f0ca7c6c-e93d-33c0-83dd-f83c99eef88d
Tag: []
Abstract: Aspen parkland in central Canada may change substantially with increased warming and aridity as prairies replace forests, fire return intervals decrease and lake levels decline. We examined the relationships among vegetation, climate, fire and lake-ecosystem properties using lake sediment cores from the current northern and southern boundaries of the aspen parkland in southwestern Manitoba. We analyzed pollen, charcoal, sediment magnetics, biogenic silica, phosphorus, grain size and LOI, and dated the cores using 210Pb and 14C (AMS, calibrated). The Jones Lake record, from the southern edge of the parkland, began considerably earlier (~11 000 cal. BP) than the Mallard Pond record at the northern edge (~8600 cal. BP). These sites were characterized as prairie communities with low fire severity and relatively low lake productivity during the warm, dry period from 9000 to 6000 cal. BP. Beginning around 6500 cal. BP at Jones Lake and 3400 cal. BP at Mallard Pond, conditions appeared to get wetter as indicated by arboreal pollen percentage increases from ~30% to 40-60%, concurrent with a rise in charcoal and proxies for lake productivity (biogenic silica and percent organic phosphorus). Similar to previous studies along the prairie-forest border, we found that charcoal increased during warmer, wetter periods with increased forest cover and fuel loading rather than during warmer, drier periods of prairie dominance. Our results underscore the importance of regional changes in moisture, and its effects on lake levels and forest biomass, as a dominant control of the aspen parkland dynamics. © The Author(s), 2009.

17a8293c-7ea2-3c96-bf10-558559f55e5c
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ID: 17a8293c-7ea2-3c96-bf10-558559f55e5c
Tag: []
Abstract: Microfossil and X-ray analyses of sediment cores from Lake Victoria reveal a history of dramatically shifting environmental conditions over the last 25 000 yr. The diatom record of a 10 m core collected from beneath 66 m of water at an offshore station extends the known history of the lake 10 000 yr beyond the published records from Pilkington Bay and Damba Channel, and shows that maximal late Pleistocene aridity occurred between 15 000 and 13 000 BP. There is no evidence that the lake level fell low enough to confine fishes to refugia in small isolated ponds or around river mouths. -from Authors

43e43c16-e1b0-335b-8c79-19d1b70d91b6
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ID: 43e43c16-e1b0-335b-8c79-19d1b70d91b6
Tag: []
Abstract: Fine-interval (∼30-45 yr) sampling of a core from Lake Victoria's Damba Channel shows that numerous abrupt changes in the lake's diatom assemblages have occurred in response to climatic fluctuations over the past 11,400 
                        14C yr. Four distinct climatic phases bounded by sudden transitions are inferred: (1) variably dry ∼11,400-10,000 yr B.P., (2) humid ∼10,000-7200 yr B.P., (3) more seasonal ∼7200-2200 yr B.P., and (4) more arid ∼2200-0 yr B.P., with a dry "Little Ice Age" event ∼600-200 yr B.P. The diatom-inferred paleoclimatic history for northern Lake Victoria closely resembles that inferred from a well-dated pollen record from Pilkington Bay. Spectral analysis of the diatom record reveals strong periodicities including globally distributed ∼2360-2550, ∼1400, ∼1030-1130, and ∼500 cal-yr cycles. Repeated, rapid shifts between Aulacoseira- and Nitzschia-dominated diatom assemblages suggest that post-1960 changes in the lake's phytoplankton communities have had earlier, climate-driven analogs. © 1997 University of Washington.

e981af25-ca1a-331a-8ce5-fb367df2c497
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ID: e981af25-ca1a-331a-8ce5-fb367df2c497
Tag: []
Abstract: Paleoclimatic records from equatorial East Africa, Antarctica, and Greenland reveal that atmospheric circulation changed abruptly at the early to mid-Holocene transition to full postglacial conditions. A climatic reorganization occurred at all three sites between 8200 and 7800 years ago that lasted 200 years or less and appears to have been related to abrupt transitions in both marine and terrestrial records around the world.

50dcebbb-7c1f-33df-a3c4-bcfb80027409
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ID: 50dcebbb-7c1f-33df-a3c4-bcfb80027409
Tag: []
Abstract: East Africa's enormous Lake Victoria dried out at the close of the last glacial, but the precise timing and origin of that drying event have not been clarified, largely because of uncertainty regarding core stratigraphy and ancient carbon effects on 
                        14C dates from the lake. New dates and re-examination of evidence from widely distributed cores shows that desiccation occurred some time between 15 900 and 14 200 calendar years BP, and perhaps also ca. 18-17 kyr BP. These lake level minima were briefer than has been previously suggested and were synchronous with pronounced global climate disruptions including North Atlantic ice-rafting Heinrich event 1. Less severe declines occurred during the Older and Younger Dryas intervals. Dansgaard-Oeschger type cooling cycles registered in the GISP2 ice core record tracked major lake level regressions in East Africa and weakenings of Afro-Asian monsoons during the late Quaternary, possibly linking the desiccation of Lake Victoria to century-scale reductions in solar radiation output. © 2002 Elsevier Science B.V. All rights reserved.

d0a6c9e4-8142-3f5a-bde1-1cb7290badd6
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ID: d0a6c9e4-8142-3f5a-bde1-1cb7290badd6
Tag: []
Abstract: We present results from multiple stable isotope analyses (δ18O of chironomid larval head capsules, chironomid adult thoraxes and other insect remains and δD, δ13C, δ15N of total organic matter-TOM) of a lake sediment core (04-SVID-03) taken from Stora Vidarvatn in northeastern Iceland to reconstruct past environmental, limnological and δ18O of past lake water changes during the Holocene. Core 04-SVID-03 represents a ∼12,000 cal. yrs BP to present record. Large magnitude changes in δ18O occurred during the Holocene at the site. Downcore shifts in δ18O of chironomids did not correlate with measurements of the δ13C and δ15N of chironomid head capsules, implying that the δ18O changes were not primarily driven by changes in chironomid diet during the Holocene. The δD of TOM provided a proxy of relative lake-water δD changes at the site and also showed large magnitude changes during the record. This approach was supported by analyses of a modern training set where δDTOM analyses were conducted using surface sediments from a suite of freshwater lakes over a large latitudinal gradient. The magnitude of changes in both the δ18O and δD and the relatively negative δ18O values throughout much of the core suggest that the proxies represent more paleoenvironmental information than solely temperature. Additional possible influences on lake-water isotopic composition are discussed, including changes in the seasonality of precipitation, in the patterns of air masses supplying precipitation to Iceland and in the dominant mode of the North Atlantic Oscillation. © 2007 Springer Science+Business Media B.V.

ee5cccd7-f15c-354e-8da5-58e9690a3870
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ID: ee5cccd7-f15c-354e-8da5-58e9690a3870
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Abstract: Here we review palaeoclimatic records from Mexico which have a reasonable chronological framework and are readily accessible. Reconstructions are based on pollen, diatoms, sediment chemistry and isotopes, packrat middens and glacial records and are discussed within the context of Central Mexico, the Yucatan and Northern Mexico, the three regions for which significant information is available. The available data indicate that over the Late Pleistocene and Holocene there have been significant climatic changes in Mexico, although smaller in magnitude than in other parts of the northern hemisphere tropics and subtropics. Maps of conditions at 18,000, 9000, 6000, 5000 and 1000 yr BP are presented. These show that northern Mexico and the Yucatan often show opposite climatic signals, whilst the highlands of central Mexico are more variable. Around the Last Glacial Maximum it seems that the modern summer precipitation regime had collapsed. Northern Mexico, however, was much wetter than today due to increased winter rainfall. Modern rainfall patterns were not fully established until after 9000 yr BP and generally wet conditions prevailed about 6000 yr BP. The mid-Holocene seems to have been a period of great climatic variability with a number of records showing oscillations between 6000 and 5000 yr BP. A period of marked aridity is recorded in the Yucatan and central Mexico about 1000 yr BP. More well-dated, continuous records are needed, especially covering the last glacial-interglacial transition. The changing balance between temperate and tropical climate systems and the influence of the Pacific compared with the Gulf of Mexico/Caribbean warrant further investigation. (C) 2000 Elsevier Science Ltd.

e40eb456-4843-3e93-8f9c-2a643b95ca08
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ID: e40eb456-4843-3e93-8f9c-2a643b95ca08
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Abstract: Quaternary interglacial periods provide glimpses of a warmer Arctic and useful perspectives on possible future conditions, but records of Arctic terrestrial conditions over multiple interglacial periods are rare. Here, we take advantage of a site in the Canadian Arctic where lacustrine sediments representing the past three interglacial periods are preserved in an extant lake. We use subfossil insects (chironomids) preserved in this exceptional sedimentary archive to derive temperature reconstructions through the Holocene up to A.D. 2005, through the Last Interglacial sensu stricto (marine isotope stage or MIS 5e), and a portion of the penultimate interglacial (MIS 7). Chironomidinferred temperatures are warmest for the early Holocene and MIS 5e, two periods with enhanced Northern Hemisphere insolation forcing relative to today. Twentieth-century warming at this site apparently caused the recent extirpation of cold stenothermous chironomid taxa. Assemblages from MIS 5e have close analogs in modern training set data as determined by squared-chord distance, and MIS 5e species assemblages are very similar to Holocene assemblages at this site. MIS 7 sediments record summer temperatures similar to those of the mid- to late Holocene, followed by a descent into glacial conditions. Even MIS 7 chironomid assemblages, dating back ~200,000 yr, have close modern analogs. These lake sediments also provide direct evidence for a period of regional deglaciation between MIS 5e and the Holocene (most likely MIS 5a). To our knowledge, the data presented here represent the longest paleotemperature record thus far generated using chironomids. The existence of close modern analogs for ancient chironomid assemblages at Lake CF8 suggests that this method can provide useful paleotemperature estimates extending back hundreds of millennia. © 2011 Geological Society of America.

da46c121-b56b-3129-98d0-4b00218d4621
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ID: da46c121-b56b-3129-98d0-4b00218d4621
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Abstract: Continuous high-resolution pollen data for the past 225 ka from sediments in Bear Lake, Utah-Idaho reflect changes in vegetation and climate that correlate well with variations in summer insolation and global ice-volume during MIS 1 through 7. Spectral analysis of the pollen data identified peaks at 21-22 and 100 ka corresponding to periodicities in Earth's precession and eccentricity orbital cycles. Suborbital climatic fluctuations recorded in the pollen data, denoted by 6 and 5 ka cyclicities, are similar to Greenland atmospheric temperatures and North Atlantic ice-rafting Heinrich events. Our results show that millennial-scale climate variability is also evident during MIS 5, 6 and 7, including the occurrence of Heinrich-like events in MIS 6, showing the long-term feature of such climate variability. This study provides clear evidence of a highly interconnected ocean-atmosphere system during the last two glacial/interglacial cycles that extended its influence as far as continental western North America. Our study also contributes to a greater understanding of the impact of long-term climate change on vegetation of western North America. Such high-resolution studies are particularly important in efforts of the scientific community to predict the consequences of future climate change. © 2007 Elsevier Ltd. All rights reserved.

15273325-1a8a-3986-b070-3c7e6afa3232
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ID: 15273325-1a8a-3986-b070-3c7e6afa3232
Tag: []
Abstract: Combined magnetic mineral, organic and inorganic carbon content and geochemical analysis were conducted on laminated sediments from Santa Maria del Oro, a crater lake in Nayarit (western Mexico), to build up a model of the palcoenvironmental conditions for the last 2,600years. This sequence is of particular importance as it constitutes a high resolution record of late Holocene climatic and environmental change in west-central Mexico. The analyses attainied in this study allowed to recognize abrupt changes, to identify dry periods, and to compare these variations with available records in central Mexico. Laminations are caused in general by alternations of their main components: lithogenic detritus, biogenic and authigenic carbonates, and amorphous material from biological and volcanic remains. The volcaniclastic accumulations include two tephras, one of them, the Toba Jala, produced by the Ceboruco volcano. Six facies of sill, sand and peat were recognized. Horizons characterized by high inorganic carbon content, authigenic siderite, and the dissolution of the finest ferrimagnetic mineralogy (magnetite) in reductive conditions, are upward followed by an increase in the concentrations of fine grained ferrimagnetic minerals. This sequence represents dissolution-precipitation cycles of magnetic minerals by anoxic/oxic variations in the water-sediment interface during warmer and dryer periods. These environmental conditions are especially present around 600-1140 A.D., and 1410-1830 A.D., which coincide with the archeological Classic period (300-900 A.D.) drought, the Little Ice Age (1350-1800 A.D.) and the droughts of the last 700 years. The effects of climatic variations such as the drought occurred in the archeological Classic period, the Medieval Warm, Period, the Litte Ice Age, and the droughts over the last 700 years, have been documented in sites along central Mexico.

47175ec6-dc8f-3b10-97dc-302ef44c7847
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ID: 47175ec6-dc8f-3b10-97dc-302ef44c7847
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Abstract: Geochemical data obtained from X-ray fluorescence, physical properties, total organic and inorganic carbon content (TOC/TIC), and diatom analysis from a 6.61-m-long sedimentary sequence near the modern northern shore of Lake Zirahuen (101° 44′ W, 19° 26′ N, 2000 m asl) provide a reconstruction of lacustrine sedimentation during the last approximately 17 cal kyr BP. A time scale is based on ten AMS 14C dates and by tephra layers from Jorullo (AD 1759-1764) and Paricutin (AD 1943-1952) volcanoes. The multiproxy analyses presented in this study reveal abrupt changes in environmental and climatic conditions. The results are compared to the paleo-record from nearby Lake Patzcuaro. Dry conditions and low lake level are inferred in the late Pleistocene until ca. 15 cal kyr BP, followed by a slight but sustained increase in lake level, as well as a higher productivity, peaking at ca. 12.1 cal kyr BP. This interpretation is consistent with several regional climatic reconstructions in central Mexico, but it is in opposition to record from Lake Patzcuaro. A sediment hiatus bracketed between 12.1 and 7.2 cal kyr BP suggests a drop in lake level in response to a dry early Holocene. A deeper, more eutrophic and turbid lake is recorded after 7.2 cal kyr BP. Lake level at the coring site during the mid Holocene is considered the highest for the past 17 cal kyr BP. The emplacement of the La Magueyera lava flows (LMLF), dated by thermoluminiscence at 6560 ± 950 year, may have reduced basin volume and contributed to the relative deepening of the lake after 7.2 cal kyr BP. The late Holocene (after 3.9 cal kyr BP) climate is characterized by high instability. Extensive erosion, lower lake levels, dry conditions and pulses of high sediment influx due to high rainfall are inferred for this time. Further decrease in lake level and increased erosion are recorded after ca. AD 1050, at the peak of Purepechas occupation (AD 1300-1521), and until the eighteenth century. Few lacustrine records extend back to the late Pleistocene-early Holocene in central Mexico; this paper contributes to the understanding of late Pleistocene-Holocene paleoclimates in this region. © 2010 Springer Science+Business Media B.V.

a7623e44-babe-3383-863b-f213ffe48e60
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ID: a7623e44-babe-3383-863b-f213ffe48e60
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Abstract: This paper presents high-resolution data of titanium (Ti) and calcium (Ca) concentrations and magnetic susceptibility of lacustrine sediments from Lake Santa Maria de Oro, located in western central Mexico. Major element measurements were carried out with an ITRAX XRF core scanner. The Ti record for the last 700 years (1300-2000 AD) was validated as a marker for wash and runoff events (and therefore precipitation) by comparison with historical information and tree ring chronologies related to major droughts events from several areas in Mexico. The register of Ti identifies 21 drought events, six of which are of major intensity and/or duration (i.e. 1365-1384, 1526, 1655-1670, 1818, 1900 and 1930-2000 AD). Spectral analysis of Ti suggests periodicities of 25, 39, 50, 70 and 117 years for the drought events. These periodicities could be related to solar activity and/or climatic factors such as El Niño Southern Oscillation (ENSO). The drought events documented at Lake Santa Maria del Oro were compared with records of latitudinal displacement of the Intertropical Convergence Zone (the Ti record from Cariaco Basin in the Atlantic Ocean) and the ENSO index. Some of the main drought events (1448-1435, 1485-1474, 1605-1601, 1673-1655 and 1770-1773 AD) correlate with summer southern migration of the Intertropical Convergence Zone (ITCZ), while others, i.e.1818, 1832, 1866, 1900, 1939-1929 and 1958-2000 AD, show a correlation with the ENSO index. The events of 1386-1354, 1536-1510, 1579-1585 and 1780-1808 AD correlate positively both with the ENSO index and southern ITCZ migration.

0ebe210e-bd84-30b5-a1cc-d07972fe1555
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ID: 0ebe210e-bd84-30b5-a1cc-d07972fe1555
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Abstract: Centimeter-sized crystals of struvite (MgNH4PO4·6H2O) were found during the drilling of a core from Chalco paleolake, Mexico for paleoecological and paleoclimatical research. Struvite is a biogenic mineral, unstable in most geological environments. The unusual size and preservation of these struvite crystals allowed us to perform a chemical and mineralogical characterization. This is the first record of struvite from a geological locality in Mexico, and one of the most remarkable in the world.

26d11371-4586-3599-bdde-0fc3c8ade287
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ID: 26d11371-4586-3599-bdde-0fc3c8ade287
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Abstract: Although lakes on the central Tibetan Plateau (TP) expanded significantly in recent decades, causes for the lake growth still have not been well addressed. Based on remote sensing and GIS techniques together with bathymetric survey and water balance analysis, we show how climatic changes and glacier mass loss have influenced the inland lake dynamics on the central TP. Our results show that six closed lakes (Siling Co, Nam Co, Bam Co, Pung Co, Darab Co and Zige Tangco) expanded by 20.2% in area, by 8.7. m in water depth, and by 37.7. Gt in the total storage between 1976 and 2010, with a remarkable acceleration after 1999. The growth rate of lake area, water level and storage between 1999 and 2010 was 5.0, 3.6 and 4.8 times, respectively, than that between 1976 and 1999, corresponding well with the significant climatic changes in the late 1990s. Water balance analysis shows that increased precipitation and runoff, and decreased lake evaporation were the main causes for the coherent lake growth and could contribute by about 70% of total increase in lake storage. Based on modern mass balance results , glacier mass loss between 1999 and 2010 was estimated to contribute to the lake level rise of the three glacial-fed lakes, Siling Co, Nam Co and Pung Co, by ~1.0. m, ~0.7. m and ~1.1. m, respectively, accounting for 11.7%, 28.7% and 11.4% of the total lake level rise. © 2013 Elsevier B.V.

b5e01f48-d136-31cc-a72c-9ad274ae203e
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ID: b5e01f48-d136-31cc-a72c-9ad274ae203e
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Abstract: a number of recent paleoenvironmental studies have argued that abrupt changes in climate have been the primary cause for societal collapse. Many social scientists, including anthropologists and environmental historians, reject environmental explanations as deterministic and overly simplistic. they argue that socio-political decisions contribute to environmental change and that efforts to study societal vulnerability within a human-environment system must include analysis of complex social structures. there is a gap in our understanding of how past societies responded to climate change because there are very few interdisciplinary studies that integrate both physical and behavioral sciences in paleoenvironmental reconstructions. While there is a general sense that modern societies are more insulated than pre-industrial societies from the effects of climate change, this may not prove to be true. a more complete understanding of how both natural and human-caused changes have affected the environment in the past can potentially guide decisions aimed at promoting future sustainability. here we present a project funded by the United States National science foundation that will explicitly integrate paleoenvironmental reconstruction with socioeconomic history in a local context to identify linkages between social and environmental change associated with climate variability.

af72991d-0c43-3a8a-9034-2eca13b4af4a
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ID: af72991d-0c43-3a8a-9034-2eca13b4af4a
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Abstract: Red Lake peatland contains 21 Sphagnum species, 47 other mosses, and 17 liverwort taxa in its present flora, and several other species present only as fossils in the peat. Four broad vegetation types are recognized: bogs, poor fens, forested rich fens, and rich-fen pools (flarks). These vegetation types are differentiated by water chemistry and bryophyte associations. Most of the major peat-forming moss species are common, circumboreally distributed taxa, and they are distinct in their ecology. Shifts in some of their habitat requirements, however, are evident among different geographic regions of the northern hemisphere. In bogs the major peat-formers are Sphagnum species, while on minerotrophic sites in Red Lake peatland they belong mainly to the family Amblystegiaceae.-Authors

cfc445b0-1635-3d0a-9662-dac3b38f6ca8
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ID: cfc445b0-1635-3d0a-9662-dac3b38f6ca8
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Abstract: All the major physiographic features in the mire - a raised bog, spring fen, and intervening water track - are zones of groundwater discharge during parts of the year. Acidic bog waters are neutralized below 0.5 m in the peat profile. A small change in the volume of groundwater discharge in the peatland may therefore be sufficient to neutralize the acidic waters on the bog surface and greatly affect the vegetation.-from Authors

5e5df354-d733-3448-a268-8ae381011bb1
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ID: 5e5df354-d733-3448-a268-8ae381011bb1
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Abstract: A field of nonsorted circles was discovered along the southeast coast of Cape Breton Island, Nova Scotia, that appear to be actively forming. The circles vary in their degree of development from prominent well-developed forms with flat bare centers and vegetated raised rims to incipient circles nearly covered with vegetation. Current activity of the forms is indicated by recently displaced ploughing blocks, sediment displacement along frost cracks, and oriented stones lying within the loosely packed rims. The formation of patterned ground in this relatively mild maritime environment is attributed to a saturated soil layer, similar to the active layer of permafrost, that is maintained by impermeable bedrock and high annual precipitation. The saturated sediment and frequent freeze-thaw cycles during winter provide a mechanism for the formation and maintenance of patterned ground. -from Authors

c287068f-1127-38eb-8294-35181d9d7047
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ID: c287068f-1127-38eb-8294-35181d9d7047
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Abstract: Two peat mounds have developed in the Lost River peatland. One has the chemical and physical properties of an extremely rich fen and the other has the properties of a raised bog. The mounds are separated by a water track with poor-fen vegetation and chemistry. The vegetation at Lost River can be divided into 5 noda that correspond to landform units:spring-fen channel; spring-fen forest, marginal swamp forest, water track, and raised bog. These vegetation types have well-defined ranges for pH and Ca concentration. Ordinations of vascular plant and bryophyte data indicate a close relationship between the vegetation and both moisture and chemical gradients. The link between vegetation and chemistry is also supported by plots of species richness vs pH and Ca concentration in which the peak in species numbers occurs within the rich-fen range. Amount of groundwater within the surface waters ranges from 50% in the spring-fen channels to 1% on the raised bog. The raised bog developed over a depression, which was first filled in with fen peat before minerotrophic sphagna invaded the site c2625 BP. BY 2200 BP all fen indicator species had disappeared and the site was dominated by assemblages similar to those in the present raised bog. The spring-fen mound, however, developed over a rise in the mineral substrate, which was not covered by peat until 3000 BP. The mound was quickly colonized by Sphagnum and no fen indicators appeared until 1160 BP. At this time the bog vegetation was replaced by a sedge fen containing assemblages similar to the present spring-fen channel. This sharp reversal from bog to fen under the spring-fen mound can best be explained by the discharge of alkaline groundwater at the peat surface in sufficient quantities to convert the bog vegetation to a spring-fen channel. -from Authors

c5b3165b-0706-3803-b4e4-5acd431fef3d
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ID: c5b3165b-0706-3803-b4e4-5acd431fef3d
Tag: []
Abstract: 1 Landsat imagery was used to map the distribution of 127 raised bogs in northwestern Minnesota. Bogs collectively cover 1236 km2 (16% of the study area) despite the relatively dry regional climate and periodic droughts. 2. The physical, chemical, and biotic properties of these bogs have no apparent relationship to the westward climatic gradient indicating a high degree of buffering from changes in moisture stress. 3. Most bogs are located where groundwater discharge moderates moisture losses to the atmosphere and may decouple bogs from a direct climatic control. Bogs are also consistently related to physiographic features, such as drainage divides, interfluves of tributary streams, and beach ridges that constrain the surface and groundwater hydrology. 4. During droughts groundwater moves upward through the peat column toward the depressed water table, which is located 1-2 m below the peat surface. During moist periods, however, water-table mounds within these bogs drive surface water downward deflecting the deeper upwardly moving groundwater laterally to the bog margins. 5 Such short-term reversals in flow have little effect on the pore-water chemistry of major cartons, which reflect the predominant downward flow over the past decade. Several chemical species, however, behave nonconservatively and respond more directly to climatic change. 6 Discharge zones for groundwater seem to be an essential prerequisite for bog formation in arid regions. Peat accumulation should be most rapid over discharge zones, which can maintain water table mounds, even during droughts. Once a peat mound has formed, its higher water table will drive local recharge cells, which isolate the vegetation from groundwater and facilitate the development of a raised bog.

852799e4-fed8-3bbe-b7f0-3ce09258e13a
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ID: 852799e4-fed8-3bbe-b7f0-3ce09258e13a
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Abstract: Medicine lake is a small (about 1 km2), shallow (up to 10 m deep), saline (50-170 g l-1) and meromictic lake formed after the retreat of the Wisconsin ice in the north American Great Plains. Based on a detailed sedimentological analysis of cores, we describe and interpret 13 sedimentary subfacies grouped in 9 associations which characterize the following lacustrine subenvironments: clastic littoral (freshwater and saline), springs, microbial mats, bench slope, and pelagial (oxic, alternating oxic-anoxic, anoxic and hypersaline, and organic-dominated). Lateral distribution and vertical evolution of subfacies in our model are controlled by climate fluctuations, climate-related limnological parameters (lake level, TDS and brine composition, and redox conditions), and autocyclic processes (progressive infilling of the basin and higher sedimentation rate in the pclagial realm). Microbial and chemical processes govern deposition in this system, and meromixis plays a decisive role in lake dynamics. Phototropic bacterial plate communites at the chemocline dominated as pelagial organic producers during stable meromictic periods, whereas benthic microbial communities developed during mixed water periods. Water stratification during the Holocene was mainly controlled by three parameters: 1) basin morphometry, 2) lake level, and 3) differences in TDS values between mixolimnion and monimolimnion waters. Sedimentary facies analyses is a powerful descriptive and interpretative tool that greatly contributes to deciphering the high resolution paleoenvironmental information archived in lake sequences. Depositional and paleoenvironmental models provide a dynamic framework for integrating paleolimnological data and other proxy paleorecords. Medicine lake serves as a facies model for shallow, perennial hypersaline, meromictic lakes in modern and ancient lacustrine basins. The sediment sequence from Medicine lake cores is consistent with the general paleoclimatic evolution of the northern Great Plains since the retreat of ice sheets. Our study reveals a plethora of rapid fluctuations in the water cycle both during the middle and the late Holocene. These augment prior paleoclimate reconstructions based on diatom studies of the lower Holocene freshwater to saline transition and on pollen profiles which show little variability during the subsequent long prairie grass episode. © 1995 Kluwer Academic Publishers.

8e0f29d9-7d18-3ae6-a6d6-e3a7c7e19abd
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ID: 8e0f29d9-7d18-3ae6-a6d6-e3a7c7e19abd
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Abstract: Lake Superior represents an important component of the aquatic ecosystem in North America. Along its south shore, ongoing lake-level rise, accelerated erosion, and wetland loss are major environmental concerns to coastal communities. A better prediction of the future of this shore requires placing the instrumental lake-level records in a geological context. However, our knowledge of the late-Holocene history of the world's largest freshwater body remains fragmentary. Here we present a sedimentary record of late-Holocene relative lake-level changes by dating transgressive basal peats resting directly on a sandy substrate along a bathymetric gradient in Bark Bay Slough, Wisconsin. Our record shows a moderate lake-level rise at 1.4±0.2. mm/yr from about 2200 to 1250. cal. yr. BP as a result of relatively slow differential uplift of Bark Bay relative to the controlling outlet at Port Huron. The rise accelerated to 2.3±0.2. mm/yr at about 1250. cal. yr. BP when Sault Ste. Marie emerged as the controlling outlet, thereby separating Superior from Lakes Michigan-Huron and giving rise to the modern hydrographic regime of the upper Great Lakes. The timing of this event in our record is about 1000 yr later than that estimated in most previous studies, but our data complement and confirm the younger age of lake separation inferred using a different methodology. Our results not only provide pertinent information for hydrological regulation, navigation operation, and infrastructural design in the upper Great Lakes, but also provide insight into freshwater wetland succession on flooded coasts. © 2013 Elsevier B.V.

1ef0e003-27a6-3a11-808a-6d8b0f9838fb
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ID: 1ef0e003-27a6-3a11-808a-6d8b0f9838fb
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Abstract: Questions: At millennial scales, is the historic evolution of tropical plant communities the result of 'random walk'? Are presence and abundance of taxa independent of environmental variability? Location: Lake Petén-Itzá, Guatemala, Central American lowlands. Methods: We use an 86 000-yr-long pollen record to study the relative contribution of neutral mechanisms and environmental determinism in configuring vegetation in the Central American lowlands. By using variance ratios, we evaluated the role of neutral drift at community and functional group levels. Additionally, regression analyses were applied to explain persistence and abundance of individual taxa as a function of regional moisture availability and fire frequency. Results: Environmental fluctuations drove vegetation dynamics in the Central American lowlands during the late Pleistocene. When the data set was stratified according to moisture availability and fire frequency, variance ratios further supported our findings. Regression analyses revealed that whereas species' presence was independent of both moisture availability and fire frequency, their abundance was closely associated with both. Conclusions: Neutrality, probably expressed in the face of stressors, through factors such as the existence of microrefugia, survival of relictual individuals and phenotypic and phenologic plasticity, was probably crucial in preventing biodiversity losses. Climate, however, was the main driver of vegetation dynamics in the Yucatan Peninsula during the late Pleistocene. © 2013 International Association for Vegetation Science.

7b7e1450-74b5-3395-bc56-aebc29580334
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ID: 7b7e1450-74b5-3395-bc56-aebc29580334
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Abstract: We reconstruct the Late Holocene sedimentary history of Laguna Chungara (69° 30' W, 18° 10'S, 4520 m.a.s.l., northern Chilean Altiplano) based on high resolution seismic profiling and sedimentologic analyses of cores. Two sedimentary facies associations have been defined and interpreted: i) macrophytedominant littoral, composed of black muds with macrophyte remains and peaty muds, and ii) Characeae-dominant lacustrine self, composed of gray muds and sands with abundant Characeae remains. The two facies associations define three cycles caused by oscillations in the lake level from shallower (macrophyte) to deeper (Characeae) conditions. Changes in the hydrology of Laguna Chungara reflect variations in the effective moisture (precipitation - evaporation) in the Altiplano during the Late Holocene.

bdd6310a-cf46-3633-bbd6-f6eec138b1c4
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Abstract: New geochemical and palynological data and a sedimentological reinterpretation of two cores from Lake Albert allow a revision of the paleoclimatic and paleohydrologic history of northwest Uganda over the last 30,000 14C yr BP. Our findings document one and possibly three periods of significantly reduced inflow to the Nile River system from the White Nile. Based on sediment fabric and mineralogy, shifts in percentages of total organic carbon (TOC) and total organic nitrogen (TON), hydrogen index (HI), and δ13C and δ15N, as well as poor preservation of sporopollenin and siliceous microfossils, we recognize two mature paleosols that formed between 18,000 and 12,500 14C yr BP at the surface of exposed lacustrine muds. A thin fossiliferous zone between the two soils documents a brief period of flooding Lake Albert was below the level of the coring site, and thus a closed basin, for several thousand years during the late Pleistocene. Around 12,500 14C yr BP the younger vertisol was flooded by a major rise in lake level. Relatively deep-water conditions were established soon after this transgression, but an apparent change in sedimentation rate sometime between 8,000 and 3,400 14C yr BP may reflect another, briefer period of low water, which resulted in interruption of sedimentation at the coring site. Palynological data document a rapid and abrupt shift to drier conditions around Lake Albert from 11,400 to 9,900 14C yr BP coincident with the cool Younger Dryas event of the Northern Hemisphere. Geochemical evidence suggests that throughout the terminal Pleistocene and early Holocene, and again in the late Holocene, the water column was relatively well-mixed with little tendency to persistent anoxia.

88d8692a-3a66-3771-9087-4b8c21cd9a79
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ID: 88d8692a-3a66-3771-9087-4b8c21cd9a79
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Abstract: Lake sediments and pollen, spores and algae from the high-elevation endorheic Laguna Miscanti (22°45′S, 67°45′W, 4140 m a.s.l., 13.5 km2 water surface, 10 m deep) in the Atacama Desert of northern Chile provide information about abrupt and high amplitude changes in effective moisture. Although the lack of terrestrial organic macrofossils and the presence of a significant 14C reservoir effect make radiocarbon dating of lake sediments very difficult, we propose the following palaeoenvironmental history. An initial shallow freshwater lake (ca. 22,000 14C years BP) disappeared during the extremely dry conditions of the Last Glacial Maximum (LGM; 18,000 14C years BP). That section is devoid of pollen. The late-glacial lake transgression started around 12,000 14C years BP, peaked in two phases between ca. 11,000 and < 9000 14C years BP, and terminated around 8000 14C years BP. Effective moisture increased more than three times compared to modern conditions (∼ 200 mm precipitation), and a relatively dense terrestrial vegetation was established. Very shallow hypersaline lacustrine conditions prevailed during the mid-Holocene until ca. 3600 14C years BP. However, numerous drying and wetting cycles suggest frequent changes in moisture, maybe even individual storms during the mid-Holocene. After several humid spells, modern conditions were reached at ca. 3000 14C years BP. Comparison between limnogeological data and pollen of terrestrial plants suggest century-scale response lags. Relatively constant concentrations of long-distance transported pollen from lowlands east of the Andes suggest similar atmospheric circulation patterns (mainly tropical summer rainfall) throughout the entire period of time. These findings compare favorably with other regional paleoenvironmental data. © 2001 Elsevier Science B.V.

f20c50d9-4246-3f5d-9dc6-af9043f22e5e
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ID: f20c50d9-4246-3f5d-9dc6-af9043f22e5e
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Abstract: New palynological and sedimentological data obtained from the basal 3 m of core E96-2P from Lake Edward, Uganda-Congo record conditions wetter than present in the Edward basin from ∼11 000 to 6700 yr BP, in phase with other climate and vegetation records of northern hemispheric East Africa. Dominant pollen taxa include Celtis spp., Alchornea spp., Olea spp., and Moraceae indicating a moist semi-deciduous tropical forest. More xeric indicators such as Amaranthaceae and Asteraceae together with Poaceae comprise less than 5% of the pollen sum throughout this interval as compared with between 44 and 50% during a lake lowstand at ∼2000 cal yr BP and at the core top (near modern). The differences between these two assemblages suggest a ∼25 to 60% increase in annual precipitation during the early- to mid-Holocene as compared to modern (1500-2000 vs. 1200 mm/yr today). Early Holocene sediments in E96-2P are composed of laminated diatom oozes with moderately high total sulfur concentrations (2.8-4.7%) and no authigenic calcite, also indicative of conditions wetter than present. Between ∼9000 and 6700 yr BP, palynological and sedimentary proxies indicate sub-millennial-scale events related to changes in riverine discharge and runoff in the Edward basin. We attribute the variability in runoff, and hence precipitation, to Holocene variability in Indian or Atlantic Ocean SSTs or to shifts in the relative contribution of Indian and Atlantic moisture sources to the western Rift of equatorial Africa.

0a42281b-35fe-3868-8b07-5d79e22c5b81
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ID: 0a42281b-35fe-3868-8b07-5d79e22c5b81
Tag: []
Abstract: The Bosumtwi impact crater in Ghana, arguably the best-preserved complex young impact structure town on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1-2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures. Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim. Deep drilling results confirmed the gross structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600-3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. © The Meteoritical Society, 2007. Printed in USA.

dca720e0-0a5b-37b5-99d2-2acb0557250d
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Abstract: Analyses of three sediment cores collected from a tidal salt marsh located on the western edge of San Pablo Bay in the San Francisco Bay estuary have produced long-term records of late Holocene marsh development. The records from these cores include a suite of elements, organic carbon content, fossil seeds, pollen, and stable carbon isotopes. The stratigraphy indicates fresher water conditions than present between 3400 and 2000 cal YBP. A tidal marsh became established at China Camp after about 2000 cal YBP; between 2000 cal YBP and approximately 700 cal YBP, conditions in the estuary were apparently more saline and variable. This interval was terminated by at least three or possibly five flood events, as evidenced by coarse clastic materials most likely washed down from the surrounding uplands. These floods represent high rainfall events, possibly El Niño years, and occurred during the late Medieval Climatic Anomaly. Greater plant diversity and pollen from some species with low salt tolerance are found in a core collected near the upland edge of the marsh and date to about 200 years ago, suggesting fresher conditions than today. Over the past 50 years, the diversity of marsh vegetation has decreased, and salt tolerant plants (especially Salicornia virginica) have become the dominant species. These changes are likely a result of the impacts of water diversions and upstream dams in the San Francisco Bay watershed.

40925179-d001-370e-a7fd-f01c6aae9633
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ID: 40925179-d001-370e-a7fd-f01c6aae9633
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Abstract: The Iberian-Roman Humid Period (IRHP, 2600-1600 cal yr BP), is the most humid phase of the last 4000 yr in southern Spain as recorded in the sedimentary sequence of Zoñar Lake (37°29′00″N, 4°41′22″ W, 300 m a.s.l.). A varve chronology supported by several AMS 14C dates allows study of the lake evolution at annual scale in response to human impact and climate changes. There are four climate phases within this period: i) gradual transition (2600-2500 yr ago, 650-550 BC) from a previous arid period; ii) the most humid interval during the Iberian-Early Roman Epoch (2500-2140 yr ago, 550-190 BC); iii) an arid interval during the Roman Empire Epoch (2140-1800 yr ago, 190 BC AD 150); and iv) a humid period synchronous with the decline of the Roman Empire (1800-1600 yr ago, AD 150-350). Varve thickness and geochemical proxies show a multi-decadal cyclicity similar to modern North Atlantic Oscillation (NAO) (60, 20 years) and solar variability cycles (11 yr). The timing and the structure of this humid period is similar to that described in Eastern Mediterranean and northern European sites and supports the same large-scale climate control for northern latitudes and the Mediterranean region. © 2008 University of Washington.

f2166cfb-054f-31f5-86e1-d0580d93dd43
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Abstract: A principal method for studying past hydroclimatic change is the reconstruction of paleo-lake levels. Here, we provide high-resolution lake-level records from New Long Pond and Rocky Pond in southeastern Massachusetts, which each contain evidence for multiple, sub-centennial-to-millennial scale low stands during the transition between the Late Pleistocene (15.0 ka) and Middle Holocene (ca 7.0 ka). Data from New Long Pond also demonstrate sedimentary evidence for a drop in water levels in the early to mid AD 20th century, when long-term trends in instrumental data show lower-than-average precipitation in the northeastern United States. Local data show the most precipitous declines in precipitation and groundwater levels are concurrent with the most severe drought in the AD 1960s, which occurred during a period of low sea-surface temperatures in the western North Atlantic. Ground penetrating radar and sediment core data indicate five intervals with numerous paleo-shoreline deposits between ca 15.0 and 7.0 ka, similar to the layer deposited in the AD 1960s. Many of the intervals of low lake levels coincide with proposed meltwater release events or abrupt climate oscillations in the circum North Atlantic. For example, we document at least three low stands during the Younger Dryas (12.9-11.6 ka) and in association with the "9.2" and "8.2" ka events. The combined evidence of (1) concurrent paleo-droughts in southeastern New England with documented North Atlantic abrupt cooling events and (2) recent drought with the modern association of low sea-surface temperatures indicates that freshening and cooling of the western North Atlantic is a viable mechanism for decreasing moisture within the region. Large-scale changes in seasonality and ice sheet extent also may have increased the susceptibility of the northeast to dry conditions triggered by changes in the North Atlantic. © 2009 Elsevier Ltd. All rights reserved.

926bf318-d134-365a-8239-b23bbe0ecf28
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ID: 926bf318-d134-365a-8239-b23bbe0ecf28
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Abstract: Open water within Minnesota constitutes 11 830 km2 and represents a significant proportion of the moisture available on the landscape. Because lakes absorb and store heat in the spring and summer, and release heat in the autumn and winter, they also play a significant role in surface energy budgets. Lake areas fluctuate significantly over time, and thus alter surface albedo and energy partitioning in the expanding and contracting lake margin. Observations of lake areas from the 1930s "Dust Bowl" drought were used to approximate the impact of changes in lake area and volume on surface energy budgets. A statewide map of lake area change constructed from historical aerial photos of 620 lakes shows trends in percent area changes that mimic trends in moisture balance. Based on the aerial photos and an analysis of lake bathymetries, the water volume removed from the land exposed around the lakes during the drought was 9.1 ± 3.0 × 109 m3. The total area of exposed land was 3300 ± 800 km2. Based on a balanced energy budget estimate, the transition from standing water to exposed land at lake margins accounted for 1.12 ± 0.31 × 1011 W of energy storage across the state. The decreased heat storage resulted in a statewide average storage flux anomaly of 0.50 ± 0.14 W m-2 with localized anomalies as high as 33.9 ± 7.1 W m-2. Large uncertainties exist, however, regarding the partitioning of the energy because of the wide range of potential albedo values for both land and water. As conditions warmed during the twentieth century, lake volumes have increased. Therefore, the assessment herein of the change in heat storage is relevant to comparisons of high regional temperatures in the 1930s and 2000s. © 2009 American Meteorological Society.

39c35092-6a3f-31e8-8761-97ce494cbe16
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Abstract: We use a series of tests to evaluate two competing hypotheses about the association of climate and vegetation trends in the northeastern United States over the past 15 kyrs. First, that abrupt climate changes on the scale of centuries had little influence on long-term vegetation trends, and second, that abrupt climate changes interacted with slower climate trends to determine the regional sequence of vegetation phases. Our results support the second. Large dissimilarity between temporally close fossil pollen samples indicates large vegetation changes within 500 years across >4° of latitude at ca 13.25-12.75, 12.0-11.5, 10.5, 8.25, and 5.25 ka. The evidence of vegetation change coincides with independent isotopic and sedimentary indicators of rapid shifts in temperature and moisture balance. In several cases, abrupt changes reversed long-term vegetation trends, such as when spruce (Picea) and pine (Pinus) pollen percentages rapidly declined to the north and increased to the south at ca 13.25-12.75 and 8.25 ka respectively. Abrupt events accelerated other long-term trends, such as a regional increase in beech (Fagus) pollen percentages at 8.5-8.0 ka. The regional hemlock (Tsuga) decline at ca 5.25 ka is unique among the abrupt events, and may have been induced by high climatic variability (i.e., repeated severe droughts from 5.7 to 2.0 ka); autoregressive ecological and evolutionary processes could have maintained low hemlock abundance until ca 2.0 ka. Delayed increases in chestnut (Castanea) pollen abundance after 5.8 and 2.5 ka also illustrate the potential for multi-century climate variability to influence species' recruitment as well as mortality. Future climate changes will probably also rapidly initiate persistent vegetation change, particularly by acting as broad, regional-scale disturbances. © 2009.

96cbfcd1-d03d-3e49-af1e-7b1b4469bedb
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ID: 96cbfcd1-d03d-3e49-af1e-7b1b4469bedb
Tag: []
Abstract: Lake-water isotopes can be used to track moisture regimes and water sources at present and in the geologic record. However, the effects of seasonal drought and the seasonal distribution of precipitation on lake-water isotopes are not well documented. To improve our understanding of lake-water isotopes, we analyzed the δD and δ18O values of water from a hundred lakes in the western United States across a broad range of seasonal precipitation regimes. Our results show that the isotopic composition of lake-water inputs is correlated with the isotopic composition of annual precipitation. In areas associated with the summer monsoon in northern New Mexico and southern Colorado, lake-water inputs are skewed toward summer precipitation. These results contrast with published western U.S. river-water isotopic data, which are biased toward winter precipitation, and the paradigm that lakes represent the annual moisture surplus. From the lake-water input compositions, on plots of oxygen versus hydrogen isotopes, evaporative enrichment of individual lakes follows regionally coherent evaporation trends (0.96 < r2 < 0.99). We found that the extent of evaporative enrichment in lakes is controlled by local hydrology and is not directly tied to climate or elevation. Our results suggest that paleoclimate reconstructions based on single isotope records could therefore be confounded by multiple factors (i.e., changes in air mass, seasonality of precipitation, temperature, or evaporation). A spatial network of lake-isotope records, however, could be used to separately assess the influences of these multiple factors. © 2009 Geological Society of America.

b35bce59-31cc-3fe2-b1aa-10567ac498fd
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Abstract: A ∼220,000-year record recovered in a 120-m-long sediment core from Bear Lake, Utah and Idaho, provides an opportunity to reconstruct climate change in the Great Basin and compare it with global climate records. Paleomagnetic data exhibit a geomagnetic feature that possibly occurred during the Laschamp excursion (ca. 40 ka). Although the feature does not exhibit excursional behavior (≥40° departure from the expected value), it might provide an additional age constraint for the sequence. Temporal changes in salinity, which are likely related to changes in freshwater input (mainly through the Bear River) or evaporation, are indicated by variations in mineral magnetic properties. These changes are represented by intervals with preserved detrital Fe-oxide minerals and with varying degrees of diagenetic alteration, including sulfidization. On the basis of these changes, the Bear Lake sequence is divided into seven mineral magnetic zones. The differing magnetic mineralogies among these zones reflect changes in deposition, preservation, and formation of magnetic phases related to factors such as lake level, river input, and water chemistry. The occurrence of greigite and pyrite in the lake sediments corresponds to periods of higher salinity. Pyrite is most abundant in intervals of highest salinity, suggesting that the extent of sulfidization is limited by the availability of SO4
2-. During MIS 2 (zone II), Bear Lake transgressed to capture the Bear River, resulting in deposition of glacially derived hematite-rich detritus from the Uinta Mountains. Millennial-scale variations in the hematite content of Bear Lake sediments during the last glacial maximum (zone II) resemble Dansgaard-Oeschger (D-O) oscillations and Heinrich events (within dating uncertainties), suggesting that the influence of millennial-scale climate oscillations can extend beyond the North Atlantic and influence climate of the Great Basin. The magnetic mineralogy of zones IV-VII (MIS 5, 6, and 7) indicates varying degrees of post-depositional alteration between cold and warm substages, with greigite forming in fresher conditions and pyrite in the more saline conditions. Copyright © 2009 The Geological Society of America.

5b6da743-6d4a-3fd0-90f8-34b7e19dfc31
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ID: 5b6da743-6d4a-3fd0-90f8-34b7e19dfc31
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Abstract: 

097917ff-2a3f-34f0-9feb-3dc017346fe4
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ID: 097917ff-2a3f-34f0-9feb-3dc017346fe4
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Abstract: Surface water oxygen and hydrogen isotopic values are commonly used as proxies of precipitation isotopic values to track modern hydrologic processes while proxies of water isotopic values preserved in lake and river sediments are used for paleoclimate and paleoaltimetry studies. Previous work has been able to explain variability in USA river-water and meteoric-precipitation oxygen isotope variability with geographic variables. These studies show that in the western United States, river-water isotopic values are depleted relative to precipitation values. In comparison, the controls on lake-water isotopic values are not well constrained. It has been documented that western United States lake-water input values, unlike river water, reflect the monthly weighted mean isotopic value of precipitation. To understand the differing controls on lake- and river-water isotopic values in the western United States, we examine the seasonal distribution of precipitation, evaporation and snowmelt across a range of seasonality regimes. We generate new predictive equations based on easily measured factors for western United States lake-water, which are able to explain 69-63% of the variability in lake-water hydrogen and oxygen isotopic values. In addition to the geographic factors that can explain river and precipitation values, lake-water isotopic values need factors related to local hydrologic and climatic characteristics to explain variability. Study results suggest that the spring snowmelt runs off the landscape via rivers and streams, depleting river and stream-water isotopic values. By contrast, lakes receive seasonal contributions of precipitation in proportion to the seasonal fraction of total annual precipitation within their watershed. Climate change may alter the ratio of snow to rain fall, affecting water resource partitioning between rivers and lakes and by implication of groundwater. Paleolimnological studies must account for the multiple drivers of water isotopic values; likewise, studies based on the isotopic composition of fossil material need to distinguish between species that are associated with rivers versus lakes. © 2010 John Wiley & Sons, Ltd.

f4834581-5cb4-3af3-9826-4c088f280580
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ID: f4834581-5cb4-3af3-9826-4c088f280580
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Abstract: 

05c24513-1a69-392e-b110-409c84399d7b
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ID: 05c24513-1a69-392e-b110-409c84399d7b
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Abstract: Geomorphologic mapping, morphometric analysis of glacial features, and sedimentologic characterization of glacial deposits, together with GIS management, have led to reconstruction of the glacial evolution of the Tera valley glacier (Sanabria Lake basin, NW Spain) after the local glacial maximum. Moreover, a structural analysis of the bedrock fracture network based on a detailed map containing 898 fractures has been used to evaluate the influence of bedrock geology in the development of the glacier pattern. A detailed model of the deglaciation has been constructed by complementing the geomorphologic evidence with the chronological and sedimentologic study of a drilling core retrieved from the eastern subbasin of Sanabria Lake in 2004. According to this model, 10 episodes of glacier front retreat and stabilization took place after the local glacial maximum. Subsequently, a proglacial lake was established in the eastern subbasin of current Sanabria Lake, fed by ablation waters coming from the glacier front retreat. The proglacial activity was extended from a minimum age of 25.6 to 14.3. ka BP, in which the sedimentation of organic-rich muds took place until 13.1. ka BP. Subsequently, around 13.1 to 12.2. ka BP, a short advance of the Tera glacier is inferred from the deposition of coarser and less organic sediments. After this date, the glacier front would have retreated to the headwaters of the Tera valley. Sedimentologic and chronological data from another core retrieved in a moraine-dammed deposit indicate that a marginal lake would have been active since at least 22. ka BP until at least up to 10.5. ka BP to the north of Sanabria Lake. These results suggest a local glacial maximum prior to the global LGM and, therefore, new evidence of an early deglaciation in SW Europe. © 2011 Elsevier B.V.

be23f757-df5f-3adb-a071-5c7e01698855
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Abstract: A unique diatom biostratigraphy of the Brunhes chron in Lake Baikal revealed by detailed studies of pelagic diatom paleo-assemblages, indicates that dramatic changes in insolation during the past 800 ka BP produced surprisingly rapid diatom speciation and extinctions. The orbitally-tuned age model allows the diatom assemblages to be compared with individual marine isotopic stages and substages during the Brunhes. Most of the interglacial periods are characterized by explosive speciation and appearances of new pelagic planktonic diatom species. In contrast, most of the glacial periods are characterized by spectacular extinctions. Three broad periods characterized by different degrees of stability in the pelagic diatom assemblages are distinguished during the Brunhes chron in the Lake Baikal sedimentary record: MIS 19-15e, MIS 15a-11 and MIS 9-5, these periods are apparently associated with the long-term changes in the amplitude of precessional insolation variations. Examination of individual interglacial periods further reveals the sensitivity of the Baikal ecosystem to the regional energy balance: virtually every precessional insolation peak during the early Brunhes is characterized by a unique and peculiar paleo-assemblage. For the first time the siliceous microfossil record has demonstrated such a high degree of sensitivity to insolation forcing of regional climate. Despite the dramatic changes in the pelagic plankton driven by extreme climatic changes, diatoms never went fully extinct during any glacial period. Although greatly or completely renewed during each subsequent interglacial, the pelagic diatom assemblage still managed to return dominated by the new Baikal endemic species. The diatom assemblage observed in modern Lake Baikal, despite being mostly endemic, has formed during the past 50-100 ka and is therefore rather young. These results demonstrate the remarkable capacity of this large freshwater system to sustain the dynamic stability on the large time scales. © 2001 Elsevier Science Ltd and INQUA. All rights reserved.

db9a1d23-40bf-3134-8fd0-8e4237a7b263
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Abstract: Records of the tectonic and climatic evolution of continental interiors are important for understanding the dynamics of the Earth's climate system, evolutionary processes within the terrestrial biosphere, and human origins. Sediment drill cores recovered from Lake Baikal provide essential records not only for comparison with oceanic records of marine processes, but also benchmarks which can be used to help interpret other continental records including other lake archives scheduled to be drilled in the near future. Drilling of Lake Baikal made it possible for the first time to have a continental archive with the same scientific and chronostratigraphic integrity as marine records to address critical questions of the Quaternary and Pliocene. The Lake Baikal drilling project (BDP) rapidly progressed from piston coring and seismic reflection studies to conducting the first scientific drilling in 4 short years and to very deep drilling in over 8 years. BDP has taken advantage of the harsh Siberian winters by using the frozen surface of Lake Baikal as a drilling platform. The positioning of the drill sites was selected using seismic and piston coring surveys. By continuously improving the drilling operations and technology, BDP has achieved new core recovery and depth records over the last ten years and become the world's leader in pioneering the recovery of high-quality, extremely long lacustrine sediment sequences from deep water. The success of BDP came at a time of growing interest in lake drilling among members of the paleoclimate community with few recent large-scale coordination efforts to draw upon. At the organizational, technological and financial levels, some recent changes are favorable for the development of a global lake drilling initiative, which could become as successful and efficient as the ocean drilling program. © 2001 Elsevier Science Ltd and INQUA. All rights reserved.

97763c38-f8f4-39c6-a616-33767ec0c9fe
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ID: 97763c38-f8f4-39c6-a616-33767ec0c9fe
Tag: []
Abstract: Loss-on-ignition analysis of Quaternary lake sediments provides an inexpensive and easy way to investigate past environmental changes. The mass loss on ignition at 550°C (LOI) from lake sediment cores may vary because of temporal changes in: (1) sediment composition controlled by factors such as productivity, inorganic inputs, and decomposition; and (2) the patterns of sediment accumulation controlled by factors such as basin morphology and water level. Climatic changes can alter both. Here, modern surface samples and transects of sediment cores, collected across small (<10 ha), shallow (<4 m) lakes in the northeastern United States, show that LOI varies little (2-5%) across the deep portions of these small lakes at a given time. Large changes in LOI occur only at the transition into the littoral (shallow) zone. LOI variations in sediment cores that exceed 2-5%, therefore, appear to represent meaningful environmental changes. However, because of the many possible controls, changes in the LOI of a single core are often hard to interpret. Multiple cores increase the interpretability. At lakes studied here, similar LOI trends among several cores confirm that some LOI changes resulted from basin-wide shifts in sediment composition. Differences among cores, however, developed during the early- and mid-Holocene and indicate that the edge of the littoral zone moved towards the centers of the lakes during two periods of low lake levels, at ca. 11 000-8000 and ca. 5400-3000 cal yr B.P. The basin-wide balance of sediment sources controlled the LOI from deep-water sediments, but sedimentation patterns, which changed as lake levels changed, were also important. LOI differences among cores may therefore help identify past lake-level changes in other lakes.

ef6ed4a9-3ab3-35dd-a7ac-557928f04475
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Abstract: 

a8709215-c1c0-38c5-85c4-4332a54eb33f
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Abstract: Changes in the concentrations of atmospheric greenhouse gases constitute an important part of global climate forcing. Here we present the first continental evidence for climatically caused changes in a methane gas hydrate reservoir. The organic carbon stable isotope record from Lake Baikal during the past 130 000 years registers regular emissions of isotopically light carbon by the occurrence of distinct negative shifts of 3-5‰ at every major orbitally forced cold-to-warm climatic transition during the past 130 000 years, including marine oxygen isotope stage boundaries 6/5e, 5d/5c, 5b/5a and 2/1. We conclude that these emissions were associated with decomposition of sedimentary clathrates, widespread in the Baikal basin. Among potential hypotheses to account for these methane episodes, the most probable appears to be hydrate dissociation due to deglacial warming of lake water. We estimate that as much as 12-33 Tg of methane could have been released with each episode. By recording the systematically recurring episodes of massive methane clathrate decomposition closely linked with the northern hemisphere temperatures during major orbital warmings, the new Baikal δ13C record provides further evidence for the potential involvement of clathrate reservoir in rapid deglacial rises of atmospheric methane levels. © 2003 Elsevier B.V. All rights reserved.

8d34ec08-abfa-3534-a87c-194fa5487441
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Abstract: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a novel proxy for mean annual air temperature (MAAT) and have the potential to be broadly applicable to climate reconstruction using lacustrine sediments. Several calibrations have been put forth relating brGDGT distributions to MAAT using a variety of linear regressions, including the methylation (MBT) and cyclization (CBT) indices of brGDGTs, the relative abundances of the major, non-cyclized brGDGTs (MbrGDGTs), and best subsets regression (BSR) of the fractional abundances of the nine most common brGDGTs. However, these calibrations have rarely been applied to lake sediment cores to reconstruct temperatures and test the applicability of this proxy as a paleothermometer.We present an expanded East African lakes surface sediment brGDGT dataset based upon 111 lakes and examine three methods of calibrating brGDGTs to MAAT. These methods include recalculations of the East African lake MBT/CBT calibration and MbrGDGTs calibrations, as well as a new stepwise forward selection (SFS) calibration that uses the four combined brGDGTs that explain the most variance in temperature in our calibration set. We apply these new calibrations as well as five previously published lacustrine brGDGT calibrations to the brGDGT distributions of our surface sediment dataset and a 48. kyr sediment core from Sacred Lake, Mt. Kenya, producing the first brGDGT temperature reconstruction available from a small tropical lake. We compare the reconstructed temperatures to previously published paleotemperature records from East Africa to help us assess the performance of the brGDGT calibrations. We find that the SFS calibration has a consistently low root mean squared error of prediction (RMSEP) over the entire range of MAAT, while the MBT/CBT and MbrGDGT calibrations have relatively large RMSEPs, particularly between lakes with similar temperatures but variable pH. This suggests that these techniques do not properly deconvolve the temperature and pH signals recorded in the distributions of the brGDGTs. We further find that only the SFS calibration produces a credible reconstructed temperature history from Sacred Lake when compared to other last glacial maximum paleotemperature estimates from East Africa. Thus, we advocate for the use of the SFS calibration when reconstructing paleotemperatures from brGDGTs in East Africa. © 2012 Elsevier B.V.

e506433d-ab92-37c3-b3fb-3b1ed02465c9
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ID: e506433d-ab92-37c3-b3fb-3b1ed02465c9
Tag: []
Abstract: Sediments from lakes in the northeastern United States ("New England") document climatic changes over the past 15000 years that may, in turn, explain the long-term history of regional forest development. A rise in New England temperatures ∼14 600 yr BP (calendar years before present) coincided with the initial increase in spruce (Picea spp.) populations after deglaciation. Later temperature fluctuations correlated with changes in spruce forest composition until 11 600 yr BP, when evidence for a shift to warm, dry conditions agrees with the replacement of spruce by pine (Pinus spp.) populations. Raised lake levels indicate increased moisture availability by 8200 yr BP when mesic hemlock (Tsuga canadensis) and beech (Fagus grandifolia) populations replaced the dry-tolerant pines. Cooler-than-modern temperatures, however, persisted until 6000 yr BP and appear to have limited the expansion of hickory (Carya spp.) populations. Similarly, moisture-dependent chestnut (Castanea dentata) populations did not increase until ∼3000 yr BP, when moisture availability rose to modern levels. Pathogens played a key role in a dramatic decline in hemlock populations around 5400 yr BP, but the decline also corresponds with low lake levels indicating that moisture availability may have been a factor in the decline and recovery. Our analysis, therefore, demonstrates that changes in both the abundance of existing taxa and the arrival of new taxa closely correlate with independently documented changes in climate. Temperature trends do not explain all aspects of the vegetation history, but when both temperature and moisture balance are considered, we see that forest composition consistently changed within centuries or less of climatic changes.

ae220c8d-c70e-3f57-9723-aaf56b750aea
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Abstract: The sedimentary record from Lake Baikal (Siberia, Russia) has been an important source of information about paleoclimatic variability in the northern hemisphere and dynamics of continental rift development. A lack of reliable chronology has, however, been a major obstacle to fully utilizing the Baikal archive for time scales beyond about 4-5 Myr. In this paper we use the distribution of 10Be to establish a new chronology for the longest core drilled in Lake Baikal so far. The 10Be-based chronology spans the last 8 Myr and provides better constraints on sedimentation rates and consequently on an east-west tectonic extension in the lake, which has been apparently coeval with other rifts in Asia that are related to the Tibetan plateau uplift. Our data also show higher 10Be flux in the sediment section older then 5 Myr compared with the younger period. This can be explained partly by warm and humid climatic conditions of the Miocene and partly by a high cosmic ray flux to the Earth resulting from possible low geomagnetic field intensity during that time.

c0e3c7c7-d3cd-32eb-b593-fd0d64d589eb
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Abstract: Stable oxygen and carbon isotope geochemistry of ostracode valves, abundance and assemblages of ostracode species, and sedimentological parameters from cores taken in Williams and Shingobee Lakes in north-central Minnesota show changes in climatic and hydrologic history during the Holocene. Isotopic records are consistent with the following scenario: Before 9800 yr B.P. the two lakes were connected. Increasing evaporation through the jack/red pine period (9800-7700 yr B.P.) led to lower lake levels, leaving small separated basins. The prairie period (7700-4000 yr B.P.) reflects high aridity, and lake levels reached low stands shortly before 6500 yr B.P. Low lake levels are associated with groundwater discharge between 6500 and 6000 yr B.P. The hardwood period (4000-3200 yr B.P.) corresponds to long cold winters and warm to cool summers with lower evaporation rates and slower sedimentation. During the white pine period (<3200 yr B.P.) evaporation increased and/or precipitation shifted to the summer months. These changes can be related to shifting atmospheric circulation patterns. Zonal flow was probably dominant during the early Holocene until the end of the prairie period (c. 4000 yr B.P.). During the hardwood period a combination of zonal and meridional flow patterns caused long and cold winters and wetter summers. During the white pine period wintners were shorter and the meridional flow pattern more significant. Today meridional flow dominates the circulation pattern. © 1995 Kluwer Academic Publishers.

114450a8-7e2d-314a-a97d-1933866d2876
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ID: 114450a8-7e2d-314a-a97d-1933866d2876
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Abstract: Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age. Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka.

76de0231-5277-387e-b48d-e2ddecad80b5
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Abstract: The sediments in Pickerel Lake, northeastern South Dakota, provide a continuous record of climatic and environmental change for the last 12000 yr. Sediments deposited between 12 and 6 ka (radiocarbon) show extreme variations in composition, oxygen and carbon isotopic composition of bulk carbonate, carbon isotopic composition of organic matter, and magnetic susceptibility. These variations reflect changes in sources of moisture, regional vegetation types, precipitation-evaporation balance, ground- and surface-water influx, water residence time, erosion, lake productivity, water level, and water temperature. The total carbonate content of late Pleistocene sediments steadily increased from <20% at the base of the core to as much as 80% in sediments deposited between 11 and 9 ka. By about 8 ka, the total carbonate content of the sediments had declined to about 40% where it remained with little variation for the past 8 kyr, suggesting relatively stable conditions. There are marked increases in values of δ13C and δ18O in bulk carbonate, and δ13C of organic matter, in sediments deposited between 10 and 6 ka as evaporation increased, and the vegetation in the watershed changed from forest to prairie. This shift toward more 18O-enriched carbonate may also reflect a change in source or seasonality of precipitation. During this early Holocene interval the organic carbon (OC) content of the sediments remained relatively low (2-3%), but then increased rapidly to 4.5% between 7 and 6 ka, reflecting the rapid transition to a prairie lake. The OC content fluctuates slightly between 4 and 6% in sediments deposited over the past 6 kyr. Like OC and total carbonate, most variables measured show little variation in the 13 m of sediment deposited over the past 6 kyr, particularly when compared with early Holocene variations. Although the magnetic susceptibility of this upper 13 m of sediment is generally low (<10 SI units), the upper six meters of the section is marked by striking 1 m cycles (ca. 400-500 yr periodicity) in susceptibility. These cycles are interpreted as being due to variations in the influx of eolian detrital-clastic material. Century-scale cyclic variations in different proxy variables for aridity and eolian activity from sediments deposited over the past 2000 yr in other lakes in the northern Great Plains, as well as in sand dune activity, suggest that aridity cycles were the dominant feature of late Holocene climate of the northern Great Plains. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.

ad919a11-1e1b-35db-81f8-99cce3baac97
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Abstract: Ratios of the stable isotopes of oxygen and carbon in benthic ostracodes and marl from cores taken from two lakes in north-central Minnesota reflect Holocene hydrological and vegetation changes. Oxygen isotopes show that Williams and Shingobee Lakes, located in the same watershed but with different positions along a hydrologic gradient, were connected before 9.8 ka as part of a larger lake, Lake Willobee. From 9.8-7.7 ka, the level of Lake Willobee fell as a result of glacial retreat and increasing evaporation, leaving small separated basins. Further decreases in lake level after 7.7 ka due to increasing aridity triggered the inflow of ground water in Williams Lake at about 7 ka, and in Shingobee Lake at about 5 ka. After 4 ka effective moisture increased. The carbon-isotope record reflects changes in vegetation with higher δ13C values during the prairie period (7.7-4 ka) and lower values during preceding and succeeding forest periods. The differences in timing of hydrological events show that the biotic and geochemical response of the lakes to climate variations is mediated by their hydrologic systems. The response may be strongly spatially heterogeneous and can result in contrasting information from geochemical and biotic proxies from the same paleorecord. © 2002 Elsevier Science Ltd. All rights reserved.

cedc3a3d-b97a-3bde-84ef-d68b2ab2a0e9
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ID: cedc3a3d-b97a-3bde-84ef-d68b2ab2a0e9
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Abstract: The accumulation rates of CaCO 3 and organic carbon (OC) in lake sediments are delicately balanced between production in the epilimnion and destruction in the hypolimnion. The cycling of these two forms of carbon makes a "carbon pump" that greatly affects the biogeochemical cycles of other elements. To further understand these biogeochemical dynamics, the lakes, streams, and wetlands of the Shingobee River headwater area of north-central Minnesota have been subjected to intensive hydrologic and biogeochemical studies. Williams Lake, situated close to the highest point in the regional flow system, is hydrologically closed, with no surface inlet or outlet, and ground water and precipitation as the only sources of water. Shingobee Lake, situated at the lowest point in the regional flow system, has the Shingobee River as an inlet and outlet. The surface waters of both lakes are oversaturated, and the bottom waters undersaturated, with respect to CaCO 3 during the summer. The small amount of CaCO 3 that is precipitated in the epilimnion of Williams Lake during the summer is dissolved in the undersaturated hypolimnion and sediments with the result that no CaCO 3 is incorporated into the profundal surface sediments. Because of the high phytoplankton productivity of Shingobee Lake, sufficient CaCO 3 is produced in the epilimnion that large amounts survive the corrosive hypolimnion and sediments, and an average of 46 wt. % accumulates in surface sediments. Another consequence of higher phytoplankton productivity in Shingobee Lake is that the hypolimnion becomes oxygen deficient within a month after overturn in both the spring and fall. Because of reducing conditions that develop in the hypolimnion of Shingobee Lake, high concentrations of dissolved Fe and Mn accumulate there during summer stratification. Precipitation of Fe and Mn oxyhydroxides during periods of fall and spring overturn results in high concentrations of Fe and Mn in surface sediments. In Williams Lake, high concentrations of Fe and Mn do not build up in the hypolimnion. The concentration of CaCO 3 is about 80 wt. % in lower Holocene sediments of both lakes. The lower Holocene sediments in both lakes also contain high concentrations of Fe and Mn, and the lower Holocene sediments of Shingobee are laminated. The waters of both lakes had identical values of δ 13C and δ 18O during the early Holocene, but the waters of Williams Lake "evolved" during the early Holocene, increasing about 10‰ in both δ 13C and δ 18O. Deposits of lacustrine marl occur as much as seven meters above the present elevation of Williams Lake, the highest of the two lakes. Taken together, these observations suggest that the lakes were once connected to form a larger lake called Lake Willobee with a hypolimnion that was anoxic, at least seasonally.

d3af3879-57fb-3936-b610-78ffa7852bf2
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Abstract: Ostracode taxonomy has been of great interest because of their possible use as indicator species in climate and ecosystem changes. In Central and South America, few studies have been carried out and this study includes a contribution to the group. Eleven ostracode species were collected in Lago Petén Itzá (∼100km2), the second largest lowland lake in Guatemala, and from its inflow tributary, Río Ixlú in November 2005 and February 2008. Twenty-seven surface sediment samples were retrieved at water depths from the littoral zone to the lake's maximum depth (>160m). Hard and soft parts of ostracodes were analyzed, and each ostracode species was described for taxonomy, size, ecology, biology and geographic distribution. Species recorded include: Candonocypris serratomarginata?, Cypretta brevisaepta?, Cypridopsis okeechobei, Cytheridella ilosvayi, Darwinula stevensoni, Heterocypris punctata, Limnocythere opesta, Physocypria globula, Pseudocandona sp., Stenocypris major and Strandesia intrepida. Most of the species have a neotropical distribution, two are distributed world-wide (D. stevensoni and S. major), and C. okeechobei and P. globula display nearctic and neotropical distributions. We present new records of C. brevisaepta?, C. serratomarginata?, S. major, and S. intrepida in Guatemala. Physocypria globula was misidentified previously in Lago Petén Itzá as Cypria petenensis, Pseudocandona sp. was misidentified as Candona sp., and C. okeechobei was identified in the past as C. vidua. Limnocythere opesta is the only endemic species of the Petén Lake District, Guatemala. The most abundant and widely distributed species in the lake are P. globula, C. okeechobei, and Pseudocandona sp. Species restricted to the littoral zones and water depths <15m are C. brevisaepta?, D. stevensoni, H. punctata, and S. intrepida. Limnocythere opesta, C. ilosvayi, C. okeechobei, and Pseudocandona sp. are distributed from the littoral zone to a depth of 40m. Species collected only in the Ixlú tributary and in a littoral zone on the west side of the lake were C. serratomarginata? and S. major. During November, live adult L. opesta and C. okeechobei were abundant, but no C. brevisaepta? or C. serratomarginata? adults were found. Adult specimens of C. ilosvayi were more abundant in February. In general, ostracodes collected were smaller than those reported in the literature. An accurate taxonomy will improve the use of ostracode fossil assemblages in long sediment cores when reconstructing past climatic and environmental changes in the northern lowland Neotropics.

ce390779-f75d-34e9-8495-7d3d1e27db8f
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ID: ce390779-f75d-34e9-8495-7d3d1e27db8f
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Abstract: Two ~40-cm-long sediment cores, PI-SC-1-10m and PI-SC-2-40m, were recovered at 10 and 40 m water depth, respectively, from Lago Petén Itzá, in the Department of Petén, northern Guatemala. The cores span the last ~525 years of sediment accumulation in the basin. This study explores lake level and trophic state changes that Lago Petén Itzá has experienced since European contact in the early 1500s. We inferred past environmental variability using changes in sediment geochemistry and fluctuations in relative species abundances of ostracode and chironomid fossil assemblages. Changes in concentrations of organic matter (OM), carbonate, total carbon (TC), total nitrogen (TN), C/N ratios, bromine (Br), and faunal relative abundances were used to infer changes in the trophic status of the lake. Cultural eutrophication began in the 1930s, and anthropogenic impact increased significantly after ~1970. Higher linear sedimentation rates, up to 9.1 mm yr-1, began recently. They are attributed to increases in riparian settlement, deforestation and associated land use changes, which led to accelerated soil erosion. An ostracode-based transfer function was applied to assemblages in core PI-SC-1-10m, which enabled us to identify periods when lake level fluctuated. Such historical fluctuations in lake levels were driven primarily by changes in rainfall. Past lake levels can be summarized as follows: (1) fluctuating, high lake levels from ~1550s to the 1730s and from the early 1940s to 2005, and (2) stable, lower lake levels from about 1750 to the early 1900s. Higher relative abundance of the ostracode Physocypria globula and higher rubidium (Rb) concentrations indicate higher lake levels than today. Chironomids also show sharp fluctuations along the cores that could be related to water level changes. The presence of chironomid assemblages Chironomus, Procladius, and Einfeldia from 1960-2000 AD shows high productivity levels. The Lago Petén Itzá sediment record indicates a generally arid Little Ice Age (LIA), with exceptions around 1580 and 1650 when high lake levels similar to those of the 20th century, i.e. ~5 m higher than today, indicate more humid conditions.

3121188f-f041-3e2b-b64f-d0aa509833c2
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Abstract: 

94d220d0-8d31-3863-842f-adc26442c8a6
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ID: 94d220d0-8d31-3863-842f-adc26442c8a6
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Abstract: We analyzed modern ostracode species assemblages and water column physico-chemical characteristics in Lago Péten Itzá, Guatemala. Lake waters are dominated by sulfate, bicarbonate, calcium and magnesium and display a total ion concentration of ~11 meq L-1. Eleven extant ostracode species were identified. We found higher abundances of living ostracodes, as well as relatively higher species richness (eight species) and biodiversity (H of 1.6) between the littoral zone and a water depth of <20 m. At water depths >40 m, these variables all decreased. The thermocline is located at a water depth of ~20-40 m. Cluster analysis revealed three water depth ranges in the lake of importance to ostracode distribution: 1) littoral zone (0.1-3 m), 2) water depths from the base of the littoral zone to the base of the thermocline (3-40 m), and 3) water depths below the thermocline (40-160 m). The assemblage Darwinula-Heterocypris-Pseudocandona-Strandesia is characteristic of waters <15 m. The Cypridopsis-Cytheridella-Limnocythere assemblage characterizes waters <40 m and Physocypria indicates waters >40 m. Ostracode taxa from Lago Petén Itzá show specific ecological preferences related to water depth and associated physico-chemical characteristics, thus illustrating the potential of ostracodes as indicators of lake level changes and hydrodynamics.

d173e272-a832-31a0-a0ce-2d3b05f2960e
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Abstract: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids found in soils and sediments and their relative abundance correlates with temperature and pH, enabling them to be used as proxies in reconstructing past climatic and environmental conditions. However, the potential for other environmental variables, such as nutrient concentration, to affect brGDGT distributions remains largely unexplored. We have examined the influence of a suite of environmental factors, including temperature, lake water and sediment chemistry, and lake morphometry on brGDGT concentration and distributions in the surface sediments of 111 lakes in East Africa. We found that temperature was the major control on the distributions, while the influence of pH was relatively minor. Water depth also had a minor but statistically significant influence, perhaps due to the relationship between lake depth and deep water anoxia. Water column nutrient concentration did not have a significant effect on the distributions or concentration. We further explored the potential for these variables to affect brGDGT temperature reconstruction by examining the correlation between them and the residuals of our brGDGT temperature calibration. We found that, while the distribution of some cyclized brGDGTs may be influenced by pH and other environmental variables, they are necessary in brGDGT calibration equations in order to accurately reconstruct temperature, likely due to covariation between temperature and other environmental variables. While surface water pH correlated with the relative abundance of certain brGDGTs, caution should be exhibited when using brGDGTs as a pH proxy because of systematic calibration errors. © 2013 Elsevier Ltd.

a2f4071d-348b-35ba-9045-72c52b30e6c2
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Abstract: To determine how grassland, woodland, and bordering forests respond to increased aridity, we used paleoecological methods to examine past responses along a transect of three sites at the eastern boundary of the Northern Plains of North America. Our study region corresponds to the confluence of three air streams that control central North American climates and, hence, should be sensitive to climate change. Sediment cores were analyzed for evidence of Holocene vegetation and fire from tall-grass prairie in eastern North Dakota (Moon Lake), from mixed forest near the prairie border in northwestern Minnesota (Deming Lake), and from mixed forest more remote from prairie in western Wisconsin (Dark Lake). Together with pollen and charcoal analysis, we present a new method for determining δ13C of terrestrial (charred) organic matter and, thus, the relative importance of C3 and C4 photosynthetic pathways in past vegetation. Paleorecords were supplemented with surface charcoal accumulation and δ13C from 21 North American lakes that span boreal, deciduous, pine, and mixed forest to tall- and mixed-grass prairie. Surface charcoal and δ13C follow vegetation and climate gradients, with high charcoal accumulation and δ13C (-20‰) in the Plains (Dakotas, Nebraska, and southwest Minnesota) and decreases to the east, west, and north. The δ13C pattern is consistent with observed patterns of C3:C4 dominance across the region. Sediment, pollen, charcoal, and terrestrial δ13C show that vegetation response to climate change varied substantially among tall-grass prairie, bordering woodland, and forest. During maximum aridity (8000-4000 yr BP) prairie vegetation in eastern North Dakota showed a demise of woody vegetation followed by a fluctuating dominance of grasses (40% C4) and forbs. Meanwhile, prairie expanded eastward into northwestern Minnesota, where it produced a shifting dominance between mostly C4 grasses and woody vegetation until more humid conditions and mixed forest developed after 4000 yr BP. Mixed forest in southwestern Wisconsin showed little response to mid-Holocene aridity. Elevated δ13C values from 5000 to 3000 yr BP suggest that composition of grasses changed (to increased C4), although pollen data indicate that the total abundance of grasses remained constant. The increase in C4 grasses at this time is consistent with previous studies suggesting a delayed dry interval in eastern Iowa. Reduced aridity of the last 2000 yr brought increased fire to tall-grass prairie as higher primary productivity led to increased fuel load. Meanwhile, forest expanded in northwestern Minnesota, leading to decreased ignition and fine fuels, in turn resulting in decreased fire at the woodland margin.

8be0892b-cad8-30cb-be54-83e0f763b4e3
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Abstract: Information on the timing and direction of climatic and environmental change on a millennial scale exists for many regions of North America, whereas little is known about decadal- to centennial-scale variability. Here we present a high-resolution analysis of diatom-inferred salinity from a site in the Northern Great Plains to reconstruct multidecadal- and centennial-scale climatic patterns during the early Holocene. The diatom-inferred salinity indicates a transition from fresh to highly saline conditions between c. 13400 and 7700 cal. yr BP, which suggests a major shift in climate from wet to dry conditions. The overall trend toward increasing salinity is interrupted by several freshwater intervals between c. 9800 and 7950 cal. yr BP, which may be the result of an increase in the frequency of monsoonal flow from the Gulf of Mexico. The early Holocene is considered to be a time of rapid change in climate and vegetation within the Holocene. Although rates of change in the Moon Lake diatom assemblages were high during parts of the early Holocene, in general the rate of change was as great or greater during the last two millennia. This finding may be the result of a generally directional change in climate in the early Holocene, in contrast to shorter-term fluctuations and little directionality in the late Holocene.

116c4989-8f23-3a95-8979-2ba07201d537
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Abstract: The role of climate and fire in the development, maintenance, and species composition of prairie in the eastern axis of the tallgrass Prairie Peninsula intrigued early North American ecologists. However, evaluation of the long-standing hypotheses about the region's environmental history has been hampered by the scarcity of paleorecords. We conducted multiproxy analyses on early and middle Holocene sediments from two Illinois, USA, lakes to assess long-term climatic, vegetational, and fire variability in the region. Sediment mineral composition, carbonate δ18O, ostracode assemblages, and diatom assemblages were integrated to infer fluctuations in moisture availability. Pollen and charcoal δ13C were used to reconstruct vegetation composition, and charcoal influx was used to reconstruct fire. Results indicate that fire-sensitive trees (e.g., Ulmus, Ostrya, Fraxinus, and Acer saccharum) declined and prairie taxa expanded with increased aridity from 10 000 yr BP to 8500 yr BP. Between ∼8500 yr BP and ∼6200 yr BP, aridity declined, and prairie coexisted with fire-sensitive and fire-tolerant (e.g., Quercus and Carya) trees. After ∼6200 yr BP, prairie taxa became dominant, although aridity was not more severe than it was around 8500 yr BP. Along with aridity, fire appears to have played an important role in the establishment and maintenance of prairie communities in the eastern Prairie Peninsula, consistent with the speculations of the early ecologists. Comparison of our data with results from elsewhere in the North American midcontinent indicates that spatial heterogeneity is a characteristic feature of climatic and vegetational variations on millennial time scales. © 2006 by the Ecological Society of America.

ff2c47a4-7795-352b-a788-811160e62644
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Abstract: High-resolution analyses of a late Holocene core from Kettle Lake in North Dakota reveal coeval fluctuations in loss-on-ignition carbonate content, percentage of grass pollen, and charcoal flux. These oscillations are indicative of climate-fuel-fire cycles that have prevailed on the Northern Great Plains (NGP) for most of the late Holocene. High charcoal flux occurred during past moist intervals when grass cover was extensive and fuel loads were high, whereas reduced charcoal flux characterized the intervening droughts when grass cover, and hence fuel loads, decreased, illustrating that fire is not a universal feature of the NGP through time but oscillates with climate. Spectral and wavelet analyses reveal that the cycles have a periodicity of approximately = 160 yr, although secular trends in the cycles are difficult to identify for the entire Holocene because the periodicity in the early Holocene ranged between 80 and 160 yr. Although the cycles are evident for most of the last 4,500 yr, their occasional muting adds further to the overall climatic complexity of the plains. These findings clearly show that the continental interior of North America has experienced short-term climatic cycles accompanied by a marked landscape response for several millennia, regularly alternating between dual landscape modes. The documentation of cycles of similar duration at other sites in the NGP, western North America, and Greenland suggests some degree of regional coherence to climatic forcing. Accordingly, the effects of global warming from increasing greenhouse gases will be superimposed on this natural variability of drought.

1dff2a30-9e28-3f2d-b9bc-c66966080527
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Abstract: A high-resolution age model was developed for Kettle Lake, North Dakota, USA, from a series of 53 accelerator mass spectrometry (AMS) radiocarbon ages calibrated with Bayesian statistical methods, which provide a monotonically increasing series of calibrated ages with depth. Evident in the sediment are several slumps, debris flows, or landslides, which are confirmed by 14C dating. Removal of these facies produces a continuous sedimentary sequence for the past 13,000 yr with exception of one ~260-yr hiatus associated with a 1.5-m-thick slump deposit. All ages except one are on terrestrial macrofossils and charcoal. A test age on aquatic organic detritus shows a hardwater effect of 600 yr at ~2000 cal BP. Two ages from the same level on herbaceous charcoal and Chenopodium seeds are statistically the same, which further demonstrates the suitability of charcoal from grassland environments for AMS 14C age control. However, 2 specimens of wood charcoal are too old relative to bracketing ages and glacial geologic history. These ages confirm the sedimentary interpretation of redeposition and provide a caution about the longevity of wood charcoal in the environment and its suitability for age control in lacustrine sediments. © 2011 by the Arizona Board of Regents on behalf of the University of Arizona.

50f9e867-e633-33d3-9951-1d8cb8a0bb68
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ID: 50f9e867-e633-33d3-9951-1d8cb8a0bb68
Tag: []
Abstract: The succession of sedimentary subfacies in the Medicine Lake basin (South Dakota, USA) documents numerous changes in total dissolved solids (TDS), lake level, and the nature and stability of meromixis during the Holocene. The lake formed about 10.6 ka B.P. when coarse clastics and massive fine-grained muds were deposited in a well-mixed freshwater system. Subsequently the lake became saline and meromictic, depositing a finely laminated unit composed of variegated and gray muds, organic facies, gypsum and aragonite laminae. Phototrophic bacterial plate communities (BPC) at the chemocline were significant producers of pelagic organic matter. Relatively high lake levels (at least 10 m higher than today) but saline and meromictic conditions predominated through the early to mid Holocene. At the end of this phase, lake level dropped and stable meromixis was disrupted. Sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities (BMC) spread over the lake, replacing BPCs as the predominant microbial producers. When lake level rose again, massive and laminated units were deposited under less saline conditions. Another pronounced episode of low lake level and high salinity followed. The most recent sediments are sapropels, massive or banded muds, and littoral sands. The δ18O record from authigenic aragonite shows more positive values during the arid mid-Holocene interval, with an abrupt increase at the end of this period, followed by generally more negative values during the cooler and wetter late Holocene. The small range of δ18O values (3%.) suggests that the lake was well buffered against high-frequency climatic cycles, probably as a result of groundwater modulation. We postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system-either BPCs or BMCs-exerted major controls on the carbon cycle, and consequently, on the development of isotopic trends linking δ13C and δ18O values. More negative δ13C values during the early and mid Holocene are interpreted as a consequence of the predominance of 12C-enriched microbial carbon from BPCs. Examples of both covariant and inverse covariant isotope trends between δ13C and δ18O values occur in the Medicine Lake record. Inverse covariant trends characterize long periods of stable meromixis when BPCs predominate over BMCs. We hypothesize that well-developed water stratification caused by greater influx of relatively 18O-depleted freshwater induces higher productivity at the chemocline and, consequently, more positive δ13C values in the precipitated carbonates. Unstable phases of meromixis or holomictic conditions are characterized by a predominance of BMCs or other biogenic producers over BPCs, and a number of other factors combine to promote covariant trends. The Medicine Lake record is consistent with the paleoclimatic interpretation for the northern Great Plains of a more arid early and middle Holocene followed by an abrupt transition to a relatively cooler and wetter late Holocene. The sedimentary succesion in Medicine Lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late Holocene climate punctuated by arid episodes. © 1995.

5c771505-ec62-3867-bccf-518f890ccf56
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Abstract: The geochemistry of ostracode shells and bulk carbonates in a 19-meter sediment core documents at century-scale resolution the evolution of water chemistry in Coldwater Lake, North Dakota, providing a continuous paleohydrologic record of Holocene climate change in the northern Great Plains. A combination of δ18O, δ13C, Mg/Ca and Sr/Ca in ostracode calcite aided by Sr/Ca in bulk carbonates are used to constrain the paleoclimatic reconstructions. A fresh-water phase in the early Holocene, indicated by the absence of Candona rawsoni and low concentrations of Sr/Ca in bulk carbonate, was followed by a sharp increase in salinity between 10 800 and 8900 yr B.P. The climate was predominately dry during the late part of the early Holocene and most of the middle Holocene (8900-5000 yr B.P.), when the lake was very sensitive and recorded a series of dry and wet oscillations. Maximum salinity occurred around 5500 yr B.P. and was followed by a gradual decrease between 5000 and 2400 yr B.P. From 2400 yr B.P. the δ18O, Mg/Ca, and Sr/Ca in the ostracodes indicate generally wet conditions interrupted by a series of lesser salinity and temperature oscillations lasting until 600 yr B.P. Ostracode geochemistry indicates that a warm and dry climate returned at about the time of the Little Ice Age (600-150 yr B.P.). Ostracode δ13C shows a long-term increasing trend during the Holocene, which suggests that lake productivity and atmospheric CO2 exchange made greater contributions to the hypolimnetic carbon pool as the lake became shallower with time.

e112c774-9a90-3d07-b954-123448a5b57f
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Abstract: Oxygen-isotope profiles for the Late-Glacial carbonate sediments from Red Bog and adjacent Lough Gur in County Limerick in western Ireland are readily correlated with the classical biozones delineated on pollen diagrams for the same cores. The estimated summer temperatures of the Bølling/Allerød were as high as those in the early Holocene and are correlated with increasing Milankovitch summer insolation. This warm phase was abruptly terminated in the Younger Dryas cold episode by a depletion of 4‰ in δ<sup>18</sup>O, suggesting a summer atmospheric temperature decrease of about 12°C, comparable to that inferred from fossil beetle data. The Younger Dryas phase is attributed to a major cooling of the sea-surface temperature by a postulated discharge of icebergs similar to that of the Heinrich events, for the icebergs were much more effective than simple meltwater in cooling the sea surface and thus the climate over Europe. Shorter-term cool phases (Older Dryas, Gerzensee, Preboreal oscillation) are also recognized.

f46ccfb7-c264-3964-b457-d30db2e502c4
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Abstract: Stable isotope and pollen analyses of a sediment core from Deep Lake, Minnesota, provide new information on the climatic effects of glacial Lake Agassiz in Minnesota and insights into the cause of the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region. Bulk-carbonate δ18O exhibited large fluctuations between 12.0 and 9.1 ka (dates are in calendar years throughout this paper unless indicated otherwise), probably reflecting the effects of glacial Lake Agassiz superimposed on climatic warming related to large-scale climatic controls. In particular, a 3‰( decrease in δ18O 11.2-10.2 ka interrupted the δ18O enrichment of 1‰ from 12.0 to 11.2 ka and 3.5‰ from 10.2 to 9.1 ka. This δ18O decrease coincided with the expansion of Lake Agassiz. We interpret this decrease as a result of decreased summer temperature and increased precipitation derived from the cold and isotopically light meltwaters of Lake Agassiz. During this δ18O decline, Pinus pollen continued to increase at the expense of Picea pollen at Deep Lake, as at other Minnesota sites, providing evidence that climatic cooling induced by Lake Agassiz did not cause a reversal to a Picea-dominated vegetation. The absence of such a vegetational response implies that the prominent Picea recurrence during the Younger Dryas in the southern Great Lakes region was not caused solely by climatic cooling due to increased flux of meltwater from Lake Agassiz into the Great Lakes. Instead the Picea recurrence might have been driven primarily by the westward penetration of the Younger Dryas cooling.

255199e2-79ae-3c54-9b80-5fab4ab47e97
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Abstract: Lake response to climate forcing is commonly regarded as being controlled by within-lake processes. We present a case from the northern Great Plains in which Holocene drought response recorded in lake sediments was determined by changes in chemical characteristics of source ground water. Middle Holocene ostracode assemblages were dominated by halophylic Limnocythere stapling indicating increased lake-water salinity, yet the oxygen isotope values in this period decreased by 2‰ to 3‰, contrary to expectations of evaporative dominance. We infer from these proxy records and ground-water flow-model results that the paleoclimatic record in the lake was mediated by changes in ground-water catchment area and by reduction in evapo-transpiration accompanying desiccation of peripheral lake basins.

b30f4fcc-20fa-3180-875d-723a151788b8
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Abstract: Recent studies of the Greenland ice cores have offered many insights into Holocene climatic dynamics at decadal to century timescales. Despite the abundance of continental records of Holocene climate, few have sufficient chronological control and sampling resolution to compare with the Greenland findings. But annually laminated sediments (varves) from lakes can provide high-resolution continental palaeoclimate data with secure chronologies. Here we present analyses of varved sediments from Deep Lake in Minnesota, USA. Trends in the stable oxygen-isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8.9 to 8.3 kyr before present, probably characterized by increased outbreaks of polar air, decreased precipitation temperatures, and a higher fraction of the annual precipitation falling as snow. The abrupt onset of this climate reversal, over several decades, was probably caused by a reorganization of atmospheric circulation and cooling of the Arctic airmass in summer that resulted from the final collapse of the Laurentide ice near Hudson Bay and the discharge of icebergs from the Quebec and Keewatin centres into the Tyrell Sea. The timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8,200 years ago in Greenland and other regions. No shifts in the oxygen-isotope composition of sediment carbonate occurred at 8.2 kyr before present at Deep Lake, but varve thickness increased dramatically, probably as a result of increased deposition of aeolian dust. Taken together, our data suggest that two separate regional-scale climate reversals occurred between 9,000 and 8,000 years ago, and that they were driven by different mechanisms.

ed6d718d-a54b-3c7f-b910-f411eb4e7b91
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Abstract: A 2100-yr decadal-resolution salinity and aridity proxy record of lacustrine ostracode-shell Mg/Ca ratios from a closed-basin lake in the northern Great Plains shows statistically significant periodicities of ~400, 200, 130, and 100 yr. These periodicities are similar to the three principal solar-oscillation periods (420, 218, 143 yr) as inferred from the atmospheric radiocarbon record, suggesting strong solar forcing of century-scale drought frequency with a fundamental 400-yr period and its 2nd, 3rd, and 4th harmonics. Our proxy record correlates visually and statistically (cross-spectral analysis) with the atmospheric Δ14C record (solar proxy) and the GISP2 (Greenland Ice Sheet Project Two) δ18O record (climate proxy), showing that solar minima are in phase with drought periods in the northern Great Plains and cold periods in Greenland. This spectral similarity together with phase correlation indicates a possible teleconnection of century-scale global climate fluctuations through common solar forcing.

25dea364-4be2-339d-b9d1-3a5d875815af
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Abstract: The effect of 20th-century drought on groundwater and lakes in a climatically sensitive area of the Northern Great Plains (Grant County, western Minnesota) was investigated by analysis of lake sediments and historical air photos, in conjunction with groundwater flow simulations. Drought caused an observed 4.0 to 5.1m lake-head decline between 1923 and 1938; all but three either dried completely or declined to < 1.7 m depth. From the deepest (Elk Lake), a 210Pb-dated sediment core was analyzed for ostracode species distribution and geochemistry of C. Rawsoni shells (δ18O, δ13C, Mg/Ca, Sr/Ca). Mg/Ca and δ13C increased during drought at the same time that salinity-tolerant ostracodes thrived, suggesting increasing salinity. However, δ18O decreased during drought, anti-correlative with Mg/Ca. A numerical model for transient response of groundwater flow to drought, modeled after that of 1923-38, was constrained by drought-end recessional strand lines observed on air photos of one lake, with strand line elevations inferred from bathymetry. Simulated lake-water-level declines in the first 15 drought years were consistent with strand line elevations. The rate of decline was exponential at drought onset, slowing as water in lakes and wetlands receded to below land surface and lake evaporation declined. A near-steady state was attained between 40 and 50 simulation years after onset of drought, with the water table from 1-5 m below the level of dry lakebeds and only the two deepest lakes still holding water. They were, however, greatly constricted in area and depth. Simulated evaporation fluxes were reduced by over 50% within 15 years, despite increased ET rates, due to lake area constriction and water table declines. Both relate to land surface morphology beneath and around lakes. Model results suggest that lake-bed elevations exert control on the rate and ultimate level to which groundwater is depressed by drought. The deepest lakes in any region will tend to become the focus of groundwater flow during sustained drought once lakes around it dry out. The unexpected finding of decreasing shell δ18O in response to drought is interpreted as some combination of source and/or evaporation - reduction effects, such as seasonality of recharge/precipitation or reduced ET flux from wetlands that seep back into lakes. © 2002 Published by Elsevier Science Ltd.

490e3da3-d01b-38ee-a848-a6fc13a6b529
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Abstract: Reconstructions of lake-water salinity at decadal resolution for the last 2000 yr are compared among three lakes in North Dakota to infer regional patterns of drought. The intersite comparisons are used to distinguish local variation in climate or hydrology from regional patterns of change. At one lake, diatom-inferred salinity and lake-water Mg/Ca inferred from ostracode shell chemistry are coherent, both in terms of direction and magnitude of change, indicating that each is a robust technique for reconstructing lake-water chemistry. The data show that the last 2000 yr have been characterized by frequent shifts between high and low salinity, suggesting shifts between dry and moist periods. Long intervals of high salinity suggest periods of multiple decades when droughts were intense and frequent, thus indicating times when drought was more persistent than in the 20th century. Both the Medieval Period and Little Ice Age were hydrologically complex, and there is no clear evidence to suggest that either interval was coherent or unusual in effective moisture relative to long-term patterns. Differences among the three sites may be attributed to variation in local hydrology, and these differences emphasize the need for multiple sites in deriving regional climate interpretations from paleoecological data. (C) 2000 University of Washington.

d59da827-4baf-3d25-8212-f139a9fd2f0d
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Abstract: Oxygen and carbon isotopes, trace element concentrations, and ostracode paleoecology are used with ground water flow modeling to assess the magnitude of climate-driven hydrologic changes during mid-Holocene time for Elk Lake, Grant County, Minnesota. During mid-Holocene time, two extended droughts were recorded in the sediment record from Elk Lake at 5385 and 5221 calendar yr BP. A drop in lake level to 14.9 m below the 1994 level and 10.0 m below the maximum 1930s decline at about 5385 calendar yr BP is interpreted for Elk Lake and its water table, based on Heterocypris fretensis occurrence and higher δ18O, δ13C, and Mg/Ca values. Numerical modeling suggests this "low-water" event could have resulted from conditions drier than those of the 1930s drought, sustained over a time frame of approximately a century. The later drought event at approximately 5221 calendar yr BP is marked by a shift in the dominant source of water to Elk Lake from surface run-off to ground water from the till aquifer. The shift in source water is identified by lower δ18O values and the presence of Limnocythere staplini. Estimates of climatic sensitivity for Holocene lakes should not be directly based on modern conditions, but must take into account the changing shape of the lake basin through time and its effect on the paleohydrology. © 2002 Elsevier Science Ltd. All rights reserved.

2215d90a-67b2-342f-86d4-1eccc4728c6b
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Abstract: Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals. Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High δ18O and low δ13 C values are associated with abundant Mn-carbonates. Low δ18O and high δ13C values are associated with only minor concentrations of Mn-carbonates. The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.

9d650e8e-63d4-311b-8147-d8ede3af73c4
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Abstract: A 2100-year multiple proxy record at decadal resolution from a topographically closed (but hydrologically open) lake basin was used to reconstruct changes in lake salinity and evaporative intensity and to evaluate the reliability of different proxies for inferring climatic change in the glaciated Northern Great Plains (NGP). At Rice Lake, North Dakota, the ostracode-Mg/Ca ratios show good correlation with other ostracode, diatom and aeolian proxy records from other sites in the NGP. All of these records show significant century-scale periodicities, which have been related to solar variability as inferred from atmospheric radiocarbon records. This coherent and consistent pattern suggests that these sites record climatic variability at regional scales. The interval of the Mediaeval Climatic Anomaly from ∼900 to 550 cal. yr BP contains two pronounced dry periods. During the following 'Little Ice Age', a single period of frequent drought that peaked at ∼300 cal. yr BP was bracketed by wet periods before and after. In contrast, the ostracode-Sr/Ca ratios and ostracode- and mollusc- δ18O records from Rice Lake have different patterns and do not correlate well with each other or with the same proxies at other sites. This lack of consistent pattern within a site and among several regional sites suggests seasonal and/or site-specific influences on these proxies. Variable contributions of groundwater, with low δ18O values because of selective recharge from snowmelt, may have played an important role in determining the isotopic composition of the lake. The poor correlation of Sr/Ca with other proxies is caused by mediation of inorganic carbonate mineralogy, owing to active uptake of Sr in the formation of inorganic aragonite. Thus, our sediment data along with present-day lakewater chemistry suggest that carbonate-δ18O records in the NGP cannot be interpreted simply in terms of climate because of significant local groundwater influences. Ostracode-Mg/Ca ratios tend to be a better indicator of past salinity because of the conservative nature of Mg in oligosaline waters (<10‰ salinity).

f9d914fd-00ca-3c4b-9f5a-fe5fdd23d2b3
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Abstract: This paper analyzes the effects of geology and geomorphology on surface-water/-groundwater interactions, evapotranspiration, and recharge under conditions of long-term climatic change. Our analysis uses hydrologic data from the glaciated Crow Wing watershed in central Minnesota, USA, combined with a hydrologic model of transient coupled unsaturated/saturated flow (HYDRAT2D). Analysis of historical water-table (1970-1993) and lake-level (1924-2002) records indicates that larger amplitude and longer period fluctuations occur within the upland portions of watersheds due to the response of the aquifer system to relatively short-term climatic fluctuations. Under drought conditions, lake and water-table levels fell by as much as 2-4 m in the uplands but by 1 m in the lowlands. The same pattern can be seen on millennial time scales. Analysis of Holocene lake-core records indicates that Moody Lake, located near the outlet of the Crow Wing watershed, fell by as much as 4 m between about 4400 and 7000 yr BP. During the same time, water levels in Lake Mina, located near the upland watershed divide, fell by about 15 m. Reconstructed Holocene climate as represented by HYDRAT2D gives somewhat larger drops (6 and 24 m for Moody Lake and Lake Mina, respectively). The discrepancy is probably due to the effect of three-dimensional flow. A sensitivity analysis was also carried out to study how aquifer hydraulic conductivity and land-surface topography can influence water-table fluctuations, wetlands formation, and evapotranspiration. The models were run by recycling a wet year (1985, 87 cm annual precipitation) over a 10-year period followed by 20 years of drier and warmer climate (1976, 38 cm precipitation). Model results indicated that groundwater-supported evapotranspiration accounted for as much as 12% (10 cm) of evapotranspiration. The aquifers of highest hydraulic conductivity had the least amount of groundwater-supported evapotranspiration owing to a deep water table. Recharge was even more sensitive to aquifer hydraulic conductivity, especially in the lowland regions. These findings have important implications for paleoclimatic studies, because the hydrologic response of a surface-water body will vary across the watershed to a given climate signal. © 2005 Elsevier B.V. All rights reserved.

e4a083c9-07bc-3e70-90c8-237886ff6c7b
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Abstract: A hydrologifc model of the Shingobee Watershed in north-central Minnesota was developed to reconstruct mid-Holocene paleo-lake levels for Williams Lake, a surface-water body located in the southern portion of the watershed. Hydrologic parameters for the model were first estimated in a calibration exercise using a 9-yr historical record (1990-1998) of climatic and hydrologic stresses. The model reproduced observed temporal and spatial trends in surface/groundwater levels across the watershed. Mid-Holocene aquifer and lake levels were then reconstructed using two paleoclimatic data sets: CCM1 atmospheric general circulation model output and pollen-transfer functions using sediment core data from Williams Lake. Calculated paleo-lake levels based on pollen-derived paleoclimatic reconstructions indicated a 3.5-m drop in simulated lake levels and were in good agreement with the position of mid-Holocene beach sands observed in a Williams Lake sediment core transect. However, calculated paleolake levels based on CCM1 climate forcing produced only a 0.05-m drop in lake levels. We found that decreases in winter precipitation rather than temperature increases had the largest effect on simulated mid-Holocene lake levels. The study illustrates how watershed models can be used to critically evaluate paleoclimatic reconstructions by integrating geologic, climatic, limnologic, and hydrogeologic data sets. © 2002 University of Washington.

5ff42dfe-16c0-3739-b9fa-e9cd7d27a21d
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Abstract: In this study, we have integrated a suite of Holocene paleoclimatic proxies with mathematical modeling in an attempt to obtain a comprehensive picture of how watersheds respond to past climate change. A three-dimensional surface-water-groundwater model was developed to assess the effects of mid-Holocene climate change on water resources within the Crow Wing Watershed, Upper Mississippi Basin in north central Minnesota. The model was first calibrated to a 50 yr historical record of average annual surface-water discharge, monthly groundwater levels, and lake-level fluctuations. The model was able to reproduce reasonably well long-term historical records (1949-1999) of water-table and lake-level fluctuations across the watershed as well as stream discharge near the watershed outlet. The calibrated model was then used to reproduce paleogroundwater and lake levels using climate reconstructions based on pollen-transfer functions from Williams Lake just outside the watershed. Computed declines in mid-Holocene lake levels for two lakes at opposite ends of the watershed were between 6 and 18 m. Simulated streamflow near the outlet of the watershed decreased to 70% of modern average annual discharge after ∼200 yr. The area covered by wetlands for the entire watershed was reduced by ∼16%. The mid-Holocene hydrologic changes indicated by these model results and corroborated by several lake-core records across the Crow Wing Watershed may serve as a useful proxy of the hydrologic response to future warm, dry climatic forecasts (ca. 2050) made by some atmospheric general-circulation models for the glaciated Midwestern United States. © 2007 Geological Society of America.

8d10da37-0bbb-3055-89ad-c78d08fa90b2
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Abstract: After the pronounced fluctuation of 4‰ in δ18O during the Younger Dryas and the 1‰ dip during the Preboreal Oscillation, values for δ18O decreased very steadily from -2.5 to -5.3‰ from 9300 to about 8000 14C yr BP (uncal.). This early-Holocene decrease of almost 3‰ is interpreted as a result of a decline of almost 6°C in the temperature of precipitation. The previous high temperature of much of the Lateglacial and the Preboreal (12000-9300 yr BP uncal.) may reflect warm summers related to greater insolation and warm winters caused by the influence of the Laurentide ice sheet on atmospheric circulation. After 8000 yr BP a disconformity interrupts the sequence, but after about 5000 yr BP the δ18O and δ13C are irregular because of a low lake level, as shown by the occurrence of littoral shells and woody detritus in the littoral core, and because of anthropogenic factors, as inferred from pollen indicators of forest disturbance and fanning found in both the littoral and offshore cores.

af69711d-8598-3baf-bc7c-809b93c8b923
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Abstract: Ostracodes (Ostracoda, Crustacea) are aquatic micro-crustaceans with a significant representation in the fossil record. If the environmental influence on the species composition of their communities is robustly quantified, past changes in ostracode communities reflected in fossil assemblages can be used for paleo-environmental reconstruction. We analyzed ostracode assemblages in recently deposited surface sediments from 56 lakes in western and central Mongolia, and simultaneously recorded local water chemistry and solute concentration in order to elucidate the distribution of individual ostracode species in relation to these broad environmental gradients. Multivariate analysis indicated that the species variation in ostracode assemblages could be mainly attributed to variations in percent calcium (%Ca) relative to total cation content, mean annual precipitation, calcium concentration, alkalinity, percent bicarbonate relative to total anion content, and mean July temperature. This matches well with the results of a similar analysis on presence/absence data of living ostracodes in nearshore samples, even though some differences exist between the faunal composition of both datasets. The documented response of ostracode species to environmental variation tracks the typical solute evolutionary pathway for surface waters in this region, characterized by calcite precipitation and consequent depletion in dissolved calcium. Hence, the best quantitative inference model (WA-PLS model with Rjack2=0.70, RMSEP = 0.40) for paleolimnological application was obtained for %Ca. Comparison between this model and a specific conductance (SC) inference model based on the same dataset, and based on ostracode datasets from different regions, indicated that the %Ca inference model suffers less than the SC inference model from a step-change in reconstructed values. The statistical power of different inference models based on Mongolian ostracodes are variously affected by the common dominance of a single euryhaline species (Limnocythere inopinata), limited faunal turnover in the freshwater portion of the salinity gradient, and the bimodal frequency distribution of SC among regional lakes. The latter probably represents true scarcity of lakes with intermediate salinity rather than a biased representation in our dataset. In a broader context of ostracode ecology, and with respect to regional paleolimnological applications, we highlight the potential of fossil Mongolian ostracode assemblages to trace past hydrological shifts associated with changes in groundwater inflow. © 2011 Elsevier Ltd.

fd2d2347-6e46-37fe-83e1-9bc009d306b1
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Abstract: 

c0430f12-4efe-34c2-bec0-b5c5930b729a
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Abstract: Lake Turkana is an important modern analog to ancient rift environments in east Africa and elsewhere. Carbonate is second in abundance after the detrital silicate fraction of the sediments in the lake. It consists of ostracod tests and a silt-sized micrite. Scanning electron microscopy and X-ray diffraction analysis revealed the micrite to be euhedral crystals of low-Mg calcite about 10 microns in length. Carbon isotope values of the micrite indicate that it forms in the upper water column rather than in the sediments after burial. The oxygen isotope analyses indicate that the micrite could be at isotopic equilibrium with the modern lake waters at temperatures of 36°C. This suggests that the micrite forms either in the surface waters of the open lake or in isolated shallow bays, and is then transported to the deep lake. © 1989 Pergamon Press plc.

14915217-da19-36e8-ac80-fa2d0345c02e
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Abstract: Profiles of carbonate content and lamination thickness in an 11.3 m core from Lake Turkana are interpreted as a record of climatic change for the past 4 ka. On a time scale of millennia, the data agree with other paleohydrologic records from sub-Saharan east Africa. Time-series analysis of both data sets reveal periods of about 270,200, 165, and 100 yr. The thickness data suggest additional periods of about 78, 44, 31, 25, and possibly 20 yr. The sediments consist of laminated light and dark couplets, which are interpreted as a response to an average 4-yr variability in the hydrology of the Ethiopian Pla- teau. We suggest that this may be a feedback from the El Niño-Southern Oscillation. © 1988 Geological Society of America.

c692ccb2-1b8f-355e-99b3-809e589529e1
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Abstract: Authigenic calcite was analyzed for δ13C and δ18O in a 12 m long core from Lake Turkana that spans the last 4200 yr. The δ18O record is interpreted to reflect lake level history, and indicates high stands prior to 4000 yr B.P., from 1800 to 2000 yr B.P., and within the last century. This coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for Lake Turkana based on 14C dates of exposed shoreline deposits. The δ13C curve roughly parallels the δ18O curve, and may reflect the influence of the Omo River plume. Both of the isotope records are strongly cyclic, with periodicites common to both at 80, 120, 150 and 225 yr. Other, somewhat longer periodicites are nearly equal in the two records, and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same Turkana core. © 1991.

28a68fa7-c93d-3171-a3f8-c356000def48
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Abstract: Suggests that the lamination couplets are annual varves, representing biannual sedimentation sequences. The light laminae result from diatom blooms occurring during the dry, windy season (April-October) when the upper 250m of the lake is well-mixed, localized upwelling occurs, and the epilimnion becomes enriched with nutrients. The dark laminae represent sedimentation in the rainy, low-wind velocity season (November-March) when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum. -from Authors

e62db71e-f1a7-3a9d-946d-0a39c181394c
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Abstract: The diatom record of V95-2P, a 7.6 m long sediment core collected from 67 m depth, is the first from the east central portion of Lake Victoria. A soil horizon developed at the coring site shortly before 12,400 14C yr BP due to a lakewide desiccation event. The radiocarbon chronology of the older half of this core is problematic, but if our estimated chronology is correct, then aridity and evaporative concentration of the lake are indicated by the presence of Thalassiosira rudolfi from > 12,400 BP to about 10,000 BP. Abundant Aulacoseira and generally rising biogenic silica concentrations may reflect enhanced water column mixing and rising precipitation:evaporation ratios ca. 10,000-8400 BP. A lakewide reorganization of diatom assemblages occurred around 7200 BP, when Aulacoseira nyassensis largely replaced A. granulata and after which rainfall and wind-driven mixing became more seasonally restricted. Pronounced reductions in offshore diatom deposition, from about 8300-5800 BP and after 3500 BP, may have been related to declines in the duration and/or intensity of wind-driven mixing. Increased aridity, and possibly deforestation in the lake basin, is indicated after 1400 BP.

961b35f9-1537-3c2a-b1b2-88d2120b4478
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Abstract: A 12 m piston core (LT 84-8P) taken in the northern basin of Lake Turkana has been dated by 6 AMS radiocarbon dates on large ostracods and analysed for its pollen content. During the past 2500 years, the pollen assemblages were dominated by Chenopodiaceae/Amaranthaceae (Cheno/Am) and Gramineae, representing the local pollen signal of the surrounding vegetation under arid conditions. Percentage fluctuations of Cheno/Am are interpreted as reflecting lake level variations in good correspondance with oxygen isotope from the same core. A rise in lake level is recorded at about 1600 yr B.P., preceded by a relative lowstand during much of the period from 2500 to 1600 yr B.P. The top of the core is estimated to be about 600 yr old, and shows a shift of unknown duration towards a more humid condition. The high lake level documented in this paper roughly coincides with that reconstructed from other cores in the lake based on diatom and carbonate analyses. © 1995 Elsevier Science B.V. All rights reserved.

c61189f0-9b85-3607-aa4f-b73be372308a
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Abstract: This paper announces the launch of the first National Lacustrine Core Repository (LacCore) in the United States under the sponsorship of the National Science Foundation (NSF), Earth Systems History program, in cooperation with the University of Minnesota, Limnological Research Center (LRC) and Large Lakes Observatory (LLO). Policies are being developed in consultation with a national External Advisory Group (EAG), with requested input from the international paleolimnological community. The dynamic growth of national and international coring programs brings exciting new developments in paleolimnology and also brings a growing need for multi-proxy, cost-efficient usage of core material, similar to practices common in the marine sciences. The goals of this shared program and repository are to: (1) curate cores as a material documentation of scientific analysis and basin archives; (2) increase the quality and scope of paleoecological science by developing procedural standards and by facilitating interdisciplinary interaction among scientists; (3) provide community access to cores; and (4) create web-based access to an ongoing, developing data base. LacCore is a community venture and input is welcome in these early stages. The central idea is that curation of lake cores is significantly more than storage. A curation repository requires professional expertise and advice to assure consistent and coherent conventions for Initial Core Descriptions (ICD), data management, sample oversight, and pro-active dissemination of information on opportunities. Careful curation of cores allows maximum core utilization because maintained and documented cores are available for re-sampling, for comparative, interdisciplinary investigations, for application of new proxy techniques, and for correlation of proxies of environmental change across the continents.

1ceb9860-c266-3242-9a8d-f9f784b5576d
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Abstract: Sedimentologic and geochemical analyses of four cores from Lake Edward, Uganda-Congo, document a complex record of moisture balance in the Edward basin from 11 000 cal yr BP to present. Highly organic, diatomaceous muds provide evidence for shifts in wind intensity and stratification within an early Holocene wet phase. Lake level variations within this period may have shifted due to tectonic lowering of the lake's outlet level. The onset of mid-Holocene aridity, as documented by the appearance of authigenic calcite at 5200 cal yr BP, initiated a period of falling lake levels that culminated in a late Holocene lowstand between 4000 and 2000 cal yr BP. This lowstand is documented by coarse sediments whose fabric and mineralogy depend on the core site. Although different cores yield different ages for this regression, it appears that declining lake levels culminated in a maximum lake lowstand of -14 m. Lake levels then rose rapidly, attaining modern positions by 1700 cal yr BP. This lake level history suggests that although many paleoclimatic changes in Africa are apparently synchronous throughout northern hemispheric Africa, other events may be spatially heterogeneous. These patterns highlight the need for a well-dated network of paleoclimate sites within the African continent. © 2003 Elsevier Science B.V. All rights reserved.

b3f01e2a-945c-3af6-8f3d-d4f6f7b18b0d
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Abstract: Recent investigations by the International Decade for the East African Lakes (IDEAL) have significantly advanced our knowledge on the history of Lake Victoria. Seismic reflection profiles confirm the origin of the lake as a result of regional tilting and provide an estimated age of 400 000 years for the lake basin. Three major desiccation events are recorded in the seismic records that may reflect the 100 000 year Milankovitch cycle of climate forcing. The most recent arid period resulted in complete desiccation of the pre-existing lake. Lake Victoria arose from a dry landscape 14 600 calendar years ago (14.6 ka). Primary production was extremely high as lake level rose in its first 500 years, nourished by the high input of nutrients from the flooded landscape. A few species of cichlids and other fish swam out of their fluvial refugia to colonize the new lake, generating hundreds of new endemic species over the ensuing 14 000 years. Lake level rose until a brief overflow event at about 14.2 ka to 13.6 ka. Closed-basin conditions returned during the Younger Dryas until 11.2 ka, when major outflow and open-basin conditions were permanently established. The lake experienced progressively stronger stratification and water-column stability through the first half of the Holocene, and diatom productivity dropped to a minimum from 9.8 to about 7.5 ka. This period coincides with, but is much longer than, the 8.2 ka climatic event that has been observed in many Holocene records from throughout the world. The degree of water-column mixing appears to have steadily increased over the last 6000 years. Short cores from the open lake, document a shift in lake conditions beginning in the 1930s that progressed to the major ecosystem collapse of the early 1980s. The coincidence of the shift in sediment properties in the 1930s with the beginning of rapid expansion of human population and agricultural activity suggests cause and effect. It is conceivable that the lake experienced similar conditions due to natural causes between about 9800 and 7500 years ago.

8c4346eb-4b09-3ad0-bf32-b550cca44b18
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Abstract: The isotopic and material composition of bottom deposits, examined in the Kara-Bogaz-Gol Gulf, Lake Issyk-Kul, and the Aral Sea, is considered. The obtained data are used to characterize variations in the regime of the water bodies in the Holocene.

86200bdb-95be-3f1d-99a4-dbf2c00d77e7
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Abstract: A silica budget was constructed for Lake Malawi by measuring major cycling processes, including river input/output, atmospheric deposition, sinking, burial, and vertical flux within the water column. A large proportion (> 80%) of the annual input to the lake occurred in the December to March rainy season. During this period, biogenic Si (BSi) made up 61% of Si input from rivers. Microscopic examination indicated that a large fraction of BSi was phytoliths, which likely originated from cultivated maize and terrestrial grasses. Over an annual cycle, river input made up approximately 25% of total Si input to the epilimnion, while 75% resulted from vertical exchange with Si-rich deep water. Due to its long hydraulic residence time, Lake Malawi retains nearly all of the Si that enters it. Despite high temperatures, a relatively high proportion (7 to 11%) of diatom production is permanently buried. A Si flux model is used to demonstrate that river discharge and vertical mixing can have similar effects on BSi burial rate over short time scales (one to two decades), but very different effects over longer time periods. This temporally dependent response must be taken into account when interpreting sediment records.

7d81040e-ba93-3810-b69a-7a2aeca5e2b7
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Abstract: High-resolution analyses of the chemical and stable isotope composition of endogenic calcite from Lake Edward, an equatorial rift lake in central Africa, reveal large oscillations in African climate and moisture balance over the past 5400 yr. These oscillations indicate a series of droughts from 4900 to 400 yr B.P. that define a drought period of ∼725 yr. This period may be correlated to variations in the Indian Ocean monsoons, as documented by similar periods in sea-surface salinity records from the northern Indian Ocean. The similarities in periodicity of the Lake Edward and Indian Ocean records may indicate that low-frequency variability within the Indian and Pacific Oceans dominates millennial-scale African moisture balance and the Holocene climate of the tropical Indian Ocean.

a7e908a0-4adc-3dcb-a596-be4bc21f1714
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Abstract: Paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy. A novel sea-surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota (the TEX86-tetraether index of tetraethers with 86 carbon atoms). Analysis of archaeal membrane lipids in core-top sediments from a diverse suite of lacustrine systems suggests the ubiquitous occurrence of low-temperature crenarchaeota in lake systems as well. Application of the TEX86 proxy in these lacustrine systems indicates that it is valid in lake environments, and the calibration of TEX86 in lacustrine systems is similar to that in the marine environment. This proxy will provide an independent continental paleotemperature tool that will allow palleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change. © 2004 Geological Society of America.

a30be23e-da70-364e-8e1e-d614555b4ebe
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Abstract: High-resolution analyses of the elemental composition of calcite and biogenic silica (BSi) content in piston cores from Lake Edward, equatorial Africa, document complex interactions between climate variability and lacustrine geochemistry over the past 5400 years. Correlation of these records from Lake Edward to other climatically-forced geochemical and lake level records from Lakes Naivasha, Tanganyika, and Turkana allows us to develop a chronology of drought events in equatorial East Africa during the late Holocene. Major drought events of at least century-scale duration are recorded in lacustrine records at about 850, 1500, ∼2000, and 4100 cal year BP. Of these, the most severe event occurred between about 2050 and 1850 cal year BP, during which time Lake Edward stood about 15 m below its present level. Numerous additional droughts of less intensity and/or duration are present in the Lake Edward record, some of which may be correlated to other lacustrine climate records from equatorial East Africa. These events are superimposed on a long-term trend of increasingly arid conditions from 5400 to about 2000 cal year BP, followed by a shift toward wetter climates that may have resulted from an intensification of the winter Indian monsoon. Although the causes of decade- to century-scale climate variability in the East African tropics remain obscure, time-series spectral analysis suggests no direct linkage between solar output and regional rainfall. Rather, significant periods of ∼725, ∼125, 63-72, 31-25, and 19-16 years suggest a tight linkage between the Indian Ocean and African rainfall, and could result from coupled ocean-atmosphere variability inherent to the tropical monsoon system. © 2004 Elsevier Ltd. All rights reserved.

11e66283-6afa-3535-b682-5337146c50e9
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Abstract: Climate proxy data and numerical models suggest that latitudinal displacements of the Intertropical Convergence Zone (ITCZ) occur during millennial-scale cold events in the North Atlantic region, but the potential effects of these ITCZ movements on the climate of equatorial regions are unclear. Here we present a 5400-year geochemical record of rainfall and drought from Lake Edward in equatorial Africa. We observe a non-linear correlation in which drought in equatorial Africa occurs during both cold and warm extremes in the North Atlantic's 1500-year quasi-cycle. We propose that this relationship occurs due to northward/ southward displacement of the ITCZ from its equatorial mean position during warm/cold events. Our results show that millennial-scale high-latitude climate events are linked to changes in equatorial terrestrial climate even during the late Holocene and suggest important constraints on the mechanisms linking tropical and extratropical climate variability. Copyright 2005 by the American Geophysical Union.

00fd95a4-054b-36b0-897e-596e87f64b02
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Abstract: Luke Edward, Uganda-Congo, is one of the least studied of the great lakes of East Africa, and little is known of its physical hydrology. Stable isotope data and modeling and previously published estimates of Lake Edward's water balance are used to constrain the physical hydrology of the lake, and particularly the relative proportion of surface outflow to evaporative water losses. Stable isotope calculations suggest that Lake Edward loses roughly 50% of its water income by evaporation, while reviews of published hydrologic data together with our calculations suggest that evaporation comprises 54% of water losses. The similarity of these two sets of calculations lends credence to their validity, and provides a new water budget for the lake. Our results have important implications for the chemistry and hydroclimatic sensitivity of Lake Edward.

f3a5fa2d-f066-38df-a453-cb936b88ead6
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Abstract: We present a synthesis of available estimates of primary production, organic carbon burial, and lake-atmosphere carbon dioxide exchange data for large lakes of the world. All three fluxes showed significant relationships with latitude and related climate variables, with lower production, higher evasion of carbon dioxide, and higher burial efficiency at higher latitudes. There was no relationship between raw organic carbon mass accumulation rates and latitude. Our estimates suggest that an order of magnitude more carbon is lost to the atmosphere by evasion than is buried in sediments at a global scale, with total global production, evasion, and burial fluxes of approximately 250, 90, and 7 Tg C yr-1. Finally, the data suggest a trend from autotrophy in low-latitude large lakes to heterotrophy and increasing reliance on allochthonous carbon sources in lakes at higher latitudes. Copyright 2007 by the American Geophysical Union.

781629f6-516c-3243-a100-9c3560762f0f
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Abstract: Lingering debate among evolutionary biologists over whether or not Lake Victoria dried out during the late Pleistocene focuses on perceived conflicts between biological and geological evidence for the age of its endemic species. This article reviews and updates the geophysical and paleoecological evidence for lake-wide desiccation and describes the environmental conditions that aquatic species likely experienced during the low stand. Lake Victoria was at its lowest between 18,000 and 14,000 calendar years ago, and it dried out at least once during that time. There is no evidence of remnant ponds or marshes persisting within the desiccated basin. If such features existed, then they would have been small, shallow, turbid, and/or saline, and therefore markedly different from the lake to which today's species are adapted. The existence of Lake Victoria's diverse endemic biota must be reconciled with the incontrovertible geophysical and paleoecological evidence of a ca. 15,000 year age for the lake, and not vice versa. © 2007 Springer Science+Business Media B.V.

8d4ecb7f-81e5-3844-8ac7-c5005fa2c48d
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Abstract: We analyzed a varved sequence of sediment from a 350-m depth in the north basin of Lake Malawi, East Africa, for the size distribution of the sortable silt fraction (10-64 μm). Mean size of the sortable silt (SS) varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of terrigenous silt and clay (TMAR) over much of the interval. Most of the silt and clay is delivered to the offshore basin in benthic nepheloid plumes of unknown duration and frequency. TMAR appears to be related to annual rainfall (which is related to the North Atlantic Oscillation) because it roughly tracks the historical record of lake level that extends back to 1860. SS should be related to density or thickness of underflow, thus related to resuspension intensity or river flood loading. It also tracks lake level and regional wind strength as determined by National Center for Environmental Prediction (NCEP) Reanalysis. © 2008, by the American Society of Limnology and Oceanography, Inc.

2782dd23-8213-3e0f-b2a2-7ffe5cdbd43f
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Abstract: 

50bf55a7-716a-30fe-89d5-39455caf86cb
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Abstract: Affected by the shifting Intertropical Convergence Zone, Lake Malawi holds an outstanding limnic sediment archive storing palaeo-environmental information of a climatically highly responsive landscape in the East African Rift Valley. Reliable chronological data are essential to interpret the lake-bottom sediments and optically stimulated luminescence (OSL) dating is an important method to build up the chronology. We report on material-property studies and OSL-dating results for polymineral fine grains extracted from piston cores obtained from the northern basin of Lake Malawi during the 1998 campaign of the International Decade of the East African Lakes (IDEAL). OSL-dating is based on a post-IR blue (pIRB) light stimulation single aliquot regeneration (SAR) protocol. Our results support earlier publications demonstrating the general applicability of OSL-dating on Lake Malawi sediments of Holocene and late Pleistocene age. However, the study also shows that dose rate estimations imply major challenges which need to be investigated in more detail to improve future OSL-dating results. © 2012 Elsevier B.V.

3b2710ef-59c9-3b4d-a74e-0045b841f750
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Abstract: The Red Lake Peatland Complex in Northern Minnesota is one of the largest and most studied peatlands in North America (140 km2), but the logistical impediments of working in such large remote wetlands limit direct subsurface data collection to a few widely spaced samples. This mid-continent forested bog complex is comprised of three major peat landforms, each of which was surveyed using ground penetrating radar (GPR), electrical resistivity, and induced polarization (IP): (1) a raised, ombrotrophic, wooded crest; (2) a sphagnum lawn down slope of the bog crest; (3) a spring fen water track where water flows across the peat surface around ovoid wooded islands. GPR was used to obtain peat thickness and to distinguish the peat stratigraphy as a function of variable water content between organic soil horizons. Resistivity data was collected to characterize the peat thickness and structure and to image the hydrogeologic framework beneath the organic soil. The results form a novel data set of a well studied bog complex, offering new insights into the peat structure and hydrogeologic framework. These observations have implications for general models of peatland development and carbon cycling.

64f79f8a-6c52-38a9-bded-b1641935e4c8
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Abstract: Millennial-scale shifts in aridity patterns have been documented during the Holocene in the western United States, yet the precise timing, severity, and regional extent of these shifts prompts further study. We present lake sediment core data from Fallen Leaf Lake, a subalpine system at the southern end of the Lake Tahoe basin for which 80% of the contemporary inflow is derived from snowpack delivered by Pacific frontal storm systems. A high quality age model has been constructed using 14C ages on plant macrofossils, 210Pb, and the Tsoyowata tephra datum (7.74–7.95 cal kyr BP). One core captures the transition from the Late Tioga-younger Dryas glaciolacustrine package to laminated opaline clay at 11.48 cal kyr BP. Early Holocene sedimentation rates are relatively high (∼1.9 mm/year) and cooler winter temperatures are inferred by the presence of pebbles interpreted to be transported out into the lake via shore ice. There is a geochemically distinct interval from ∼4.71 to 3.65 cal kyr BP that is interpreted as a late Holocene neopluvial, characterized by depleted δ13C and lower C:N that point to reduced runoff of terrigenous organic matter, increased winter precipitation, and increased algal productivity. The largest Holocene signal in the cores occurs at the end of the neopluvial, at 3.65 cal kyr BP, and marks a shift into a climate state with variable precipitation, yet is overall more arid than the neopluvial. This new climate state persists for ∼3 ka, until the Little Ice Age. Low sedimentation rates (0.5 mm/year), the homogeneous opaline sediment, and steadily increasing contributions of terrestrial vs. algal organic matter in these cores suggest that the lowstand state of Fallen Leaf Lake may have been the norm from 3.65 to 0.55 cal kyr BP, punctuated by short term high precipitation years or multi-year intervals capable of rapid short duration lake level rise. Fallen Leaf Lake is strongly influenced by changes in winter precipitation and temperature, manifested largely by the geochemical proxies, and highlights unique advantages of subalpine lakes in regional paleoclimate reconstructions.

262129e8-f32b-3d70-9af5-ae15ca8dcacc
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ID: 262129e8-f32b-3d70-9af5-ae15ca8dcacc
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Abstract: 

863e9050-f6f4-3a2b-ac68-ba035980cb69
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ID: 863e9050-f6f4-3a2b-ac68-ba035980cb69
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Abstract: The transfer function, derived from canonical correspondence analysis (CCA) of the surface-sample data, is applied to a core from Devils Lake in sediments encompassing the last century. The accuracy of the calibration equation is tested by comparing the diatom-inferred salinity with documented salinity and water-level data recorded for the lake over the last 100 yr. The diatom-based reconstruction shows good agreement with the historical data, particularly in freshwater and at low to moderate salinities. Discrepancies between the measured and diatom-inferred salinity during high-salinity periods may reflect inadequacies with the inference method itself or more likely problems in the sedimentary record of the lake related to poor diatom preservation, sediment mixing and reworking, and dating inaccuracies. -from Author

a51648d5-973f-3a62-817c-6bfa654d640f
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Abstract: Estimates of past lake-water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains. A good correspondence between diatom-inferred salinity and historical records of mean annual precipitation minus evapotranspiration (P - ET) strongly suggests that the sedimentary record from Moon Lake can be used to reconstruct past climatic conditions. Century-scale analysis of the Holocene diatom record indicates four major hydrological periods: an early Holocene transition from an open freshwater system to a closed saline system by 7300 B.P., which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate; a mid-Holocene period of high salinity from 7300 to 4700 B.P., indicating low effective moisture (P - ET); a transitional period of high salinity from 4700 to 2200 B.P., characterized by poor diatom preservation; and a late Holocene period of variable lower salinity during the past 2,200 yr, indicating fluctuations in effective moisture.

22dd44a7-39ee-3d7b-8a2b-56a09ae24ee2
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ID: 22dd44a7-39ee-3d7b-8a2b-56a09ae24ee2
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Abstract: Seismic stratigraphy, sedimentary facies, pollen stratigraphy, diatom-inferred salinity, stable isotope (δ18O and δ13C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 and 9500 14C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 7100 14C yr B.P. A change at about 7100 14C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 4000 14C yr B.P., three arid phases occurred at 6600-6200 14C yr B.P., 5400-5200 14C yr B.P., and 4800-4600 14C yr B.P. Effective moisture generally increased after 4000 14C yr B.P., but periods of low effective moisture occurred between 2900-2800 14C yr B.P. and 1200-800 14C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic, and chemical proxies were occassionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition. © 1997 University of Washington.

5520cc2e-1563-33ec-b375-73f7790ee2f9
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ID: 5520cc2e-1563-33ec-b375-73f7790ee2f9
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Abstract: Devils Lake, North Dakota is a closed-basin lake in which water chemistry is responsive to hydrological changes, making it a valuable resource for documenting paleoclimate conditions. Ostracode trace-element and bulk-carbonate geochemistry in a core are used to generate a 12,000-year record of salinity fluctuations. The relative amounts of Sr, Ca and Ba in the bulk-carbonate fraction can be linked mechanistically to changes in sediment mineralogy (calcite/aragonite) and/or solute chemistry, whereas the elemental geochemistry of ostracode valves reflect solute chemistry. Associated minima in Sr/Ca, and maxima in Mg/Ca, in ostracode valves from the Devils Lake core may be due to aragonite mediation of Sr chemistry during the precipitation of multiple mineral phases during more saline intervals. The Devils Lake record can be divided into four hydrological periods on the basis of the geochemical proxies: (a) 12.5-9 k.y.: Low salinity; (b) 9.5 4.5 k.y.: Fluctuating, saline conditions with a maximum at about 8 k.y.; (c) 4.5-3.5 k.y.: Low salinity phase, but probably not as fresh as at 12.5-9 k.y.; (d) 3.5 k.y.-Present: Saline phase, although generally less so than 9.5-4.5 k.y. However, a large degree of variability characterizes the entire record, suggesting the absence of long-term climatic stability in this region.

93b96819-32d0-3b1d-90cb-b54bc261afa1
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ID: 93b96819-32d0-3b1d-90cb-b54bc261afa1
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Abstract: Extreme large-scale droughts in North America, such as the 'Dust Bowl' of the 1930s, have been infrequent events within the documented history of the past few hundred years, yet this record may not be representative of long-term patterns of natural variation of drought intensity and frequency. In the Great Plains region of central North America, historical droughts have persisted longer than in any other part of the United States, but no detailed records of drought patterns in this region have hitherto been obtained that extend beyond the past 500 years. Here we present a reconstruction of drought intensity and frequency over the past 2,300 years in the Northern Great Plains, based on lake salinity fluctuations inferred from fossil diatom assemblages. This record, of sub-decadal resolution, suggests that extreme droughts of greater intensity than that of the 1930s were more frequent before AD 1200. This high frequency of extreme droughts persisted for centuries, and was most pronounced during AD 200-370, AD 700-850 and AD 1000- 1200. We suggest that before AD 1200, the atmospheric circulation anomalies that produce drought today were more frequent and persistent.

8ab71ecb-8d5c-33a0-ab9b-a08560621143
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Abstract: Opal, organic C and N contents, and organic δ 13 C and δ 15 N stable isotopes were analyzed on a sedimentary sequence spanning the past ~12,000 years from Dune Lake, a shallow groundwater-fed lake in the boreal forest region of central interior Alaska. The chronology, based on 210 Pb and 15 AMS radiocarbon dates on terrestrial materials, indicates that 5 m of biogenic sediment accumulated following dune stabilization at ~10,000 cal yr BP. Previous studies on lake level and pollen were synthesized with these data to assess Holocene changes in hydrogeology and climate. Variability in groundwater level and flux has a strong control on lake level and are negatively correlated with the δ 13 C of organic matter produced within the lake. This relationship results from the input of groundwater with high pCO 2 and with dissolved inorganic carbon depleted in 13 C, which subsequently affects lake phytoplankton δ 13 C signatures. A rapid rise in lake level between ca. 11,000 and 9,500 cal yr BP coincided with stabilization of the dune field adjacent to the lake and expansion of vegetation around the lake, including white spruce. Organic δ 13 C values are relatively low and constant between about 10,000 and 6,000 cal yr BP, and then steadily increase until about 2,000 cal yr BP, suggesting a pattern of high followed by dropping lake levels generally consistent with results from core-transect studies. Higher-frequency fluctuations of 2–3 ‰ in δ 13 C are common, and suggest variability in aspects of the lake-carbon cycle over multi-decadal timescales. Recent trends in groundwater and lake level, river discharge, and precipitation suggest an important role for wintertime precipitation in the regional groundwater system. The unusual hydrology of this system relative to most other lakes studied for paleoclimate in this region provides a novel insight into different seasonal aspects of Alaskan paleoclimatology. The record is consistent with a general long-term decrease in wintertime precipitation, possibly accompanied by an increase in permafrost extent, over the Holocene.

f83da927-fc6c-3f74-9190-55c127545143
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Abstract: Carbonates are the main components of Iberian Quaternary lake sediments. In this review we summarize the main processes controlling carbonate deposition in extant Iberian lakes located in Mesozoic and Tertiary carbonate-dominated regions and formed through karstic activity during the Late Quaternary. The lakes, relatively small (1 ha to 118 ha) and relatively shallow (Zmax = 11 to 40 m) provide examples of the large variability of sedimentary facies, depositional environments, and carbonate sources. Hydrology is dominated by groundwater inflow except those directly connected to the fluvial drainage. Nine lakes have been selected for this review and the main facies in palustrine, littoral and profundal environments described and interpreted. Clastic carbonates occur in all Iberian lakes due to the carbonate composition of the bedrocks, surface formations and soils of the watersheds. Low temperatures and dilute meteoric waters seem responsible for the low carbonate content of sediments in high elevation lakes in the glaciated terrains in the Pyrenees and the Cantabrian Mountains. Clastic carbonates are dominant in small karst lakes with functional inlets where sediment infill is dominated by fining upward sequences deposited during flood events. Re-working of littoral carbonates is common in shallow environments and during low lake level stages. In most lakes, endogenic carbonate production occurs in two settings: i) littoral platforms dominated by Chara and charophyte meadows and ii) epilimnetic zone as biologically-mediated calcite precipitates. Continuous preservation of varves since the Mid-Holocene only occurs in one of the deeper lakes(Montcortès Lake, up to 30 m) where calcite laminae textures (massive, fining upward and coarsening upward) reflect seasonal changes in limnological conditions. However, varves have been formed and preserved in most of the lakes during short periods associated with increased water depth and more frequent meromictic conditions. Most Iberian lakes are in a mature stage and karstic processes are not very active. An outstanding example of a lake with intense karstic activity is Banyoles Lake where increased spring discharge after long rainy periods causes large remobilization and re-suspension of the sediments accumulated in the deepest areas, leading to the deposition of thick homogeneous layers (homogeinites). The Iberian karst lake sequences underline the large variability of facies, carbonate sources, and depositional environments in small lake systems. They illustrate how lake types evolve through the existence of a lake basin at centennial or even smaller time scales. Hydrology is the paramount control on facies and depositional environment patterns distribution and lake evolution and, consequently, a lake classification is proposed based on hydrology and sediment input. A correct interpretation of carbonate sources and depositional history is a key for using lake sequences as archives of past global changes.

4dcd208d-61fd-3a55-ba24-0a7655e41a7f
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Abstract: Scientists have begun a new project to recover and study sediment cores in the Bighorn Basin of Wyoming to investigate the high-frequency climatic and biotic variability of a continental depositional system during past episodes of greenhouse conditions. The project, called the Bighorn Basin Coring Project (BBCP), focuses on the early Paleogene (∼50-65 million years ago), the most recent interval of Earth history characterized by greenhouse climate conditions. During the early Paleogene, global mean annual temperatures were up to 10C higher than today, and carbon dioxide (CO 2) concentrations may have been more than 1000 parts per million [Zachos et al., 2008] compared with the roughly 390 parts per million today. Superimposed on this background greenhouse climate state were a series of short-term extreme warming events called hyperthermals. The best known early Paleogene hyperthermal event is the Paleocene-Eocene Thermal Maximum (PETM; ∼56 million years ago), which is characterized by a large global carbon isotope excursion and coincides with major changes to marine and continental ecosystems [McInerney and Wing, 2011]. At present, the causes of these hyperthermal events remain unknown, although several hypotheses exist.

c8e6e1c0-f665-363c-bd00-89fa99d49270
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ID: c8e6e1c0-f665-363c-bd00-89fa99d49270
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Abstract: Diatom assemblages preserved in sediment cores from closed-basin lakes can provide high-resolution records of past hydrologic and climatic conditions, including long-term patterns in the intensity, duration, and frequency of droughts. At Moon Lake, a closed-basin lake in eastern North Dakota, a comparison of diatom-inferred salinity and the precipitation-based Bhalme-Mooley Drought Index (BMDI) over the last 100 years was highly significant, suggesting that the diatom record contains a sensitive archive of past climatic conditions. A sub-decadal record of inferred salinity for the past 2300 years indicates that extreme droughts of greater intensity than those during the 1930s 'Dust Bowl' were more frequent prior to A.D. 1200. This high frequency of extreme droughts persisted for centuries and was most pronounced from A.D. 200-370, A.D. 700-850 and A.D. 1000-1200. A pronounced shift to generally wetter conditions with less severe droughts of shorter duration occured at A.D. 1200. This abrupt change coincided with the end of the 'Medieval Warm Period' (A.D. 1000-1200) and the onset of the 'Little Ice Age' (A.D. 1300-1850).

82d89f72-4955-3ae1-8ec4-5032a4a6d746
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ID: 82d89f72-4955-3ae1-8ec4-5032a4a6d746
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Abstract: Marine Isotope Stage 11 has been proposed as an analogue for the present interglacial interval; yet, terrestrial climate records from both this region and time interval are rare. The sediments deposited at Lake El'gygytgyn (67°30′N, 172°5′E) in Far East Russia contain a 3·56 Ma record of climate variability. This study presents a high-resolution record of sediment colour change from Marine Isotope Stage 8 to 12 (ca 275 to 475 ka) and demonstrates the link between lake catchment processes and climate variability. The hue colour parameter, calculated from data collected via colour reflectance spectroscopy in the visible spectrum (380 to 720 nm), exhibits correspondence with global climate records. Determining the source of sediment colour changes was achieved through detailed mineralogical and sedimentological methods, and linked to colour changes through a series of colour sensitivity tests. Mineralogical data, measured by X-ray diffraction, reveal fluctuations in concentrations of clay minerals corresponding to colour changes. Further analyses of the clay mineral assemblages show no change in relative clay mineral abundances, yet demonstrate a lake catchment dominated by physical weathering processes. Using measured mineral abundances, reconstructions of sediment colour based on colour reflectance mineral standards link mineral and clay mineral content to overall sediment colour. Colour sensitivity tests demonstrate the ability of iron oxide minerals to stain sediments red. Additionally, colour sensitivity to organic matter content was tested, suggesting that organic content drives variability in the red portion of the spectrum and darkens the overall colour signal. Sediment colour is then ultimately linked to physical weathering of bedrock minerals, with small amounts of chemical weathering producing iron oxides during wet intervals. Fluctuations in the sediment colour reveal a high-resolution record of wet/dry cycles, and provide new information about wet periods for the Russian Arctic region not yet understood from other lake proxy records.

41b21e0c-a3e8-3895-9a8c-e613162bd7bb
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ID: 41b21e0c-a3e8-3895-9a8c-e613162bd7bb
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Abstract: The impact of the ca. 75 ka Mount Toba (Indonesia) supereruption on regional and global climate has been controversial. East Africa is pivotal for qualifying this impact, specifically in addressing a hypothesis that this eruption caused a volcanic winter leading to a genetic bottleneck in anatomically modern human populations. If the Toba eruption caused a volcanic winter in East Africa, effects should be recorded in sediments from climatologically sensitive ecosystems. In deep tropical lakes, cooler temperatures would have reduced the density contrast between the epilimnion and hypolimnion, causing increased mixing, hypolimnion oxygenation, and enhanced nutrient advection. Such environmental changes produce noticeable and predictable responses in lake fossil assemblages and sediment loading. Here we present the first paleoecological evidence from East Africa of environmental conditions surrounding the eruption in a high-resolution (~8-9 yr) analysis of two cores collected by the Lake Malawi Drilling Project. These cores contain an undisturbed record of sedimentation before, during, and after deposition of the youngest Toba Tuff. Concentrations of climate-sensitive ecological indicators such as phantom midges, diatoms, and other algae through a >270 yr interval bracketing the Toba cryptotephra show no unusual or sustained deviations from background variability within Lake Malawi associated with the hypothesized post-Toba cooling. We find no evidence for significantly enhanced mixing or ecosystem disturbance that would be anticipated following a volcanic winter.

65e4f5cd-1835-35d0-ac1a-161bfdaf956f
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ID: 65e4f5cd-1835-35d0-ac1a-161bfdaf956f
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Abstract: Discerning the timing and pattern of late Quaternary glacier variability in the tropical Andes is important for our understanding of global climate change. Terrestrial cosmogenic nuclide (TCN) ages (48) on moraines and radiocarbon-dated clastic sediment records from a moraine-dammed lake at Nevado Huaguruncho, Peru, document the waxing and waning of alpine glaciers in the Eastern Cordillera during the past ∼15 k.y. The integrated moraine and lake records indicate that ice advanced at 14.1 ± 0.4 ka, during the first half of the Antarctic Cold Reversal, and began retreating by 13.7 ± 0.4 ka. Ice retreated and paraglacial sedimentation declined until ca. 12 ka, when proxy indicators of glacigenic sediment increased sharply, heralding an ice advance that culminated in multiple moraine positions from 11.6 ± 0.2 ka to 10.3 ± 0.2 ka. Proxy indicators of glacigenic sediment input suggest oscillating ice extents from ca. 10 to 4 ka, and somewhat more extensive ice cover from 4 to 2 ka, followed by ice retreat. The lack of TCN ages from these intervals suggests that glaciers were less extensive than during the late Holocene. A final Holocene advance occurred during the Little Ice Age (LIA, ca. 0.4 to 0.2 ka) under colder and wetter conditions as documented in regional proxy archives. The pattern of glacier variability at Huaguruncho during the Late Glacial and Holocene is similar to the pattern of tropical Atlantic sea-surface temperatures, and provides evidence that prior to the LIA, ice extent in the eastern tropical Andes was decoupled from temperatures in the high-latitude North Atlantic.

cc9c5e9c-3cef-325b-a1cf-85ce887f7af2
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ID: cc9c5e9c-3cef-325b-a1cf-85ce887f7af2
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Abstract: The cosmogenic radionuclide 10Be was measured from drill core sediments from Lake Malawi in order to help construct a chronology for the study of the tropical paleoclimate in East Africa. Sediment samples were taken every 10 m from the core MAL05-1C to 80 m in depth and then from that depth in core MAL05-1B to 382 m. Sediment samples were then later taken at a higher resolution of every 2 m from MAL05-1C. They were then leached to remove the authigenic fraction, the leachate was processed to separate out the beryllium isotopes, and 10Be was measured at the TAMS Facility at the University of Arizona. The 10Be/9Be profile from Lake Malawi sediments is similar to those derived from marine sediment cores for the late Pleistocene, and is consistent with the few radiocarbon and OSL IR measurements made from the same core. Nevertheless, a strong correlation between the stable isotope 9Be and the cosmogenic isotope 10Be suggests that both isotopes have been well mixed before deposition unlike in some marine sediment cores. In addition, the correlation of beryllium isotopes to a proxy of lake level TOC (Total Organic Matter) from Lake Malawi indicates that the concentrations of 10Be in the lake sediments result from the combined effects of global and local climates on lake level, local hydrology, and sediment transport in the Lake Malawi basin rather than as a direct response to its production in the atmosphere modulated by the intensity of the Earth's dipole. Therefore, a direct correlation of the 10Be/9Be to a chronology derived from the paleomagnetic variations measured from marine sediments was not possible. Nevertheless, a comparison of the 10Be/9Be chronology, allowing for decay, at Lake Malawi to that of the global marine paleomagnetic record suggests that the bottom of core MAL05-1B is no more than 750 ka in age.

ce82b08a-07ae-335d-993b-80bfad484864
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Abstract: A sedimentological and geochemical study of the Lago Enol sequence (Cantabrian Mountains, northern Spain), together with detailed geomorphological mapping, provides a first record of glacier evolution and climate change over the last 40 ka in the Picos de Europa National Park. The Enol glacier retreated from its maximum extent prior to 40 ka BP as demonstrated by the onset of proglacial lacustrine sedimentation in two glaciated depressions: the Comella hollow to the north (before 40 ka BP) and the Lago Enol (before 38 ka BP). These results support previous evidence that the maximum extent of southern European glaciers occurred earlier than in northern Europe. Alternation of homogeneous and laminated proglacial sediments during the glacier retreat illustrate a dynamic glacial evolution during the Marine Isotope Stage (MIS) 3 (40–26 ka BP). A slight warming is detected at 26 ka ago with the change from proglacial sediments (in a lake located in contact to the glacier) to glaciolacustrine sedimentation (in a non-contact or distal lake). Finally, the onset of organic-rich sediments took place at 18 ka ago. This last transition occurred in two phases, similarly to the North Atlantic Last Termination, suggesting a link between North Atlantic Deep Water formation oscillations and palaeohydrological variability in the Cantabrian Mountains. Copyright © 2009 John Wiley & Sons, Ltd.

ae7c93d5-b17a-3826-a1e5-fd0fc6c4ffbb
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ID: ae7c93d5-b17a-3826-a1e5-fd0fc6c4ffbb
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Abstract: Glacial–interglacial climate cycles are known to have triggered migrations and reassortments of tropical biota. Although long-term precessionally-driven changes in temperature and precipitation have been demonstrated using tropical sediment records, responses to abrupt climate changes, e.g. the cooling of Heinrich stadials or warmings of the deglaciation, are poorly documented. The best predictions of future forest responses to ongoing warming will rely on evaluating the influences of both abrupt and long-term climate changes on past ecosystems. A sedimentary sequence recovered from Lake Petén-Itzá, Guatemalan lowlands, provided a natural archive of environmental history. Pollen and charcoal analyses were used to reconstruct the vegetation and climate history of the area during the last 86,000 years. We found that vegetation composition and air temperature were strongly influenced by millennial-scale changes in the North Atlantic Ocean. Whereas Greenland warm interstadials were associated with warm and relatively wet conditions in the Central American lowlands, cold Greenland stadials, especially those associated with Heinrich events, caused extremely dry and cold conditions. Even though the vegetation seemed to have been highly resilient, plant associations without modern analogs emerged mostly following sharp climate pulses of either warmth or cold, and were paralleled by exceptionally high rates of ecological change. Although pulses of temperature change are evident in this 86,000-year record none matched the rates projected for the 21st Century. According to our findings, the ongoing rapid warming will cause no-modern-analog communities, which given the improbability of returning to lower-than-modern CO2 levels, anthropogenic barriers to migration, and increased anthropogenic fires, will pose immense threats to the biodiversity of the region.

6d63c1c0-e6a1-36b4-9fd8-59519acb8122
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ID: 6d63c1c0-e6a1-36b4-9fd8-59519acb8122
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Abstract: Drill cores obtained from Lake Petén Itzá, Petén, Guatemala, contain a ~85-kyr record of terrestrial climate from lowland Central America that was used to reconstruct hydrologic changes in the northern Neotropics during the last glaciation. Sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions, respectively. From ~85 to 48 ka, sediments were dominated by carbonate clay indicating moist conditions during Marine Isotope Stages (MIS) 5a, 4, and early 3. The first gypsum layer was deposited at ~48 ka, signifying a shift toward drier hydrologic conditions and the onset of wet-dry oscillations. During the latter part of MIS 3, Petén climate varied between wetter conditions during interstadials and drier states during stadials. The pattern of clay-gypsum (wet-dry) oscillations during the latter part of MIS 3 (~48-23 ka) closely resembles the temperature records from Greenland ice cores and North Atlantic marine sediment cores and precipitation proxies from the Cariaco Basin. The most arid periods coincided with Heinrich Events when cold sea surface temperatures prevailed in the North Atlantic, meridional overturning circulation was reduced, and the Intertropical Convergence Zone (ITCZ) was displaced southward. A thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake, and pollen accumulated during the same period indicates vegetation consisted of a temperate pine-oak forest. This finding contradicts previous inferences that climate was arid during the Last Glacial Maximum (LGM) chronozone (21±2 ka). At ~18 ka, Petén climate switched from moist to arid conditions and remained dry from 18 to 14.7 ka during the early deglaciation. Moister conditions prevailed during the warmer Bolling-Allerod (14.7-12.8 ka) with the exception of a brief return to dry conditions at ~13.8 ka that coincides with the Older Dryas and meltwater pulse 1A. The onset of the Younger Dryas at 12.8 ka marked the return of gypsum and hence dry conditions. The lake continued to precipitate gypsum until ~10.3 ka when rainfall increased markedly in the early Holocene.

b76cbf5c-b3ad-3893-b9e6-f807ba09ce9a
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ID: b76cbf5c-b3ad-3893-b9e6-f807ba09ce9a
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Abstract: When discharged into surface waters via wastewater effluents, triclosan, the antimicrobial agent in handsoaps, and chlorinated triclosan derivatives (CTDs, formed during disinfection with chlorine) react photochemically to form polychlorinated dibenzo-p-dioxins. To evaluate the historical exposure of waters to these compounds, the levels of triclosan, CTDs, and their derived dioxins were determined in sediment cores collected from wastewater-impacted Minnesota lakes. The accumulation rates and temporal trends of triclosan, CTDs, and dioxins in aquatic sediments were found to be a function of historical wastewater treatment operations and lake system scale. Cores collected from large-scale riverine systems with many wastewater sources recorded increasing concentrations of triclosan, CTDs, and their derived dioxins since the patent of triclosan in 1964. In small-scale lakes with a single wastewater source, the trends were directly attributed to increased triclosan use, local improvements in treatment, and changes in wastewater disinfection since the 1960s. In the lake with no wastewater input, no triclosan or CTDs were detected. Overall, concentrations of triclosan, CTDs, and their dioxins were higher in small-scale systems, reflecting a greater degree of wastewater impact. In cores collected in northern MN, the four dioxins derived from triclosan are present prior to the patent of triclosan, suggesting a secondary source. It is clear, however, that triclosan and CTDs are the dominant source of these congeners after 1965 in systems impacted by wastewater.

fe455907-8fc0-3885-acc6-159260187640
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Abstract: Lake Matano (2°29.1′S 121°20′E) in central Sulawesi, the deepest and one of the largest lakes in Indonesia, provides an excellent opportunity to reconstruct long-term hydrological and vegetation changes at the heart of the Maritime Continent within the tropical Western Pacific Warm Pool. We measured the carbon isotopic composition of terrestrial leaf waxes (δ13Cwax) in new sediment cores from Lake Matano to understand regional variations in vegetation and their interactions with local and high latitude climate forcings during the past 53 kyr. Variations in δ13Cwax record the expansion and contraction of C3 versus C4 vegetation as well as changes in plants' carbon and water use efficiency and, by inference, regional hydroclimate. During Marine Isotope Stages 1 and 3, more negative δ13Cwax values indicate that closed-canopy C3 rainforests predominated in Sulawesi, reflecting a wetter, less seasonal climate. In contrast, more positive δ13Cwax values record a terrestrial ecosystem with more abundant open canopy vegetation and possible expansion of C4 grasses between 29 and 14 ka BP, indicating that during Marine Isotope Stage 2, including the Last Glacial Maximum, the climate was more arid, likely with increased precipitation seasonality, in central Sulawesi. Comparison of our record with other paleovegetation and paleohydrological records from Sulawesi and nearby islands suggests coherent long-term hydrological changes in central Indonesia that appear to be sensitive to high latitude climate across glacial-interglacial boundaries. However, variations in the amplitude of the response of vegetation to these glacial-interglacial forcings highlight the importance of regional topographic control on climate and vegetation and suggest an important role for higher elevations in maintaining rainforest refugia during regionally arid time intervals when C4 savannas and grasslands expanded at lower elevations.

ddc2bc87-3d5f-3a66-8bd5-e948c41890e9
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Abstract: We applied a new method to reconstruct paleotemperature in the tropics during the last deglaciation by measuring oxygen isotopes of co-occurring gypsum hydration water and biogenic carbonate in sediment cores from two lakes on the Yucatan Peninsula. Oxygen and hydrogen isotope values of interstitial and gypsum hydration water indicate that the crystallization water preserves the isotopic signal of the lake water, and has not undergone post-depositional isotopic exchange with sediment pore water. The estimated lake water δ18O is combined with carbonate δ18O to calculate paleotemperature. Three paired measurements of 1200-yr-old gypsum and gastropod aragonite from Lake Chichancanab, Mexico, yielded a mean temperature of 26 °C (range 23–29.5 °C), which is consistent with the mean and range of mean annual temperatures (MAT) in the region today. Paired measurements of ostracods, gastropods, and gypsum hydration water samples were measured in cores from Lake Petén Itzá, Guatemala, spanning the Late Glacial and early Holocene period (18.5–10.4 ka). The lowest recorded temperatures occurred at the start of Heinrich Stadial (HS) 1 at 18.5 ka. Inferred temperatures from benthic ostracods ranged from 16 to 20 °C during HS 1, which is 6–10 °C cooler than MAT in the region today, whereas temperatures derived from shallow-water gastropods were generally warmer (20–25 °C), reflecting epilimnetic temperatures. The derived temperatures support previous findings of greater tropical cooling on land in Central America during the Late Glacial than indicated by nearby marine records. Temperature increased in two steps during the last deglaciation. The first occurred during the Bolling-Allerod (B-A; from 14.7 to 13 ka) when temperature rose to 20–24 °C towards the end of this period. The second step occurred at 10.4 ka near the beginning of the Holocene when ostracod-inferred temperature rose to 26 °C, reflecting modern hypolimnetic temperature set during winter, whereas gastropod-derived temperature attained 30 °C, reflecting modern summer epilimnetic temperature.

a12a0324-5c0f-308b-94cc-5c58a6f52e75
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ID: a12a0324-5c0f-308b-94cc-5c58a6f52e75
Tag: []
Abstract: The hydrogen isotopic composition of plant molecular markers in modern vegetation and marine sediments in southern California have been intensively studied. Here we report a late Holocene reconstruction from Zaca Lake in coastal southern California, together with modern hydrological and vegetation studies in the catchment. The small catchment and decadal sampling resolution throughout the 9m, 3000yr sedimentary record provided a high resolution terrigenous counterpart of nearby marine records from the Santa Barbara Basin. Today, δD values of precipitation average -51.5‰±18 (1σ, n=7). Modern plant leaf wax δD values for the C28 n-alkanoic acid averaged -141‰±11 (1σ, n=10) for Quercus agrifolia, with a calculated fractionation relative to precipitation (εwax/precip) of -94‰±22 (1σ, n=10); in contrast, there was negligible production of the C28 acid by co-dominant Pinus coulteri. Downcore, the C28 acid δD values ranged between -101‰ and -177‰ (mean -150‰±8, 1σ, n=490). Abundance distributions suggested that the sedimentary C28 acid was dominated by Quercus, implying that paleoprecipitation varied between extremes of -8‰ and -92‰ (mean -63‰±14, compound 1σ, n=490). The 3000yr leaf wax D/H record from Zaca Lake revealed substantial temporal variability, greater than observed in a speleothem reconstruction of similar resolution. We suggest that the plant-based proxy may magnify the variability by sampling spring precipitation preferentially. Centennial-duration positive isotopic excursions were associated with more sub-tropical moisture sources and drier conditions including during the Medieval Climatic Anomaly, and negative excursions were associated with N Pacific sources and wetter conditions including during the Little Ice Age. © 2013 Elsevier Ltd.

b87cf738-ad64-377b-adbf-b9394385f981
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ID: b87cf738-ad64-377b-adbf-b9394385f981
Tag: []
Abstract: Peatlands of Southeast Asia store large pools of carbon but the mechanisms of peat accumulation in tropical forests remain to be resolved. Patch dynamics and forest disturbance have seldom been considered as drivers that can amplify or dampen rates of peat accumulation. Here we used a modified piston corer, noninvasive geophysical measurements, and geochemical and paleobotanical techniques to establish the effect of tree fall on carbon accumulation rates in a peat swamp forest dominated by Shorea albida in Brunei (Borneo). Carbon initially accumulated in a mangrove forest at over 300 g C m−2 yr−1 but declined to less than 50 g C m−2 yr−1 with the establishment of a peat swamp forest. A rapid accumulation pulse of 720–960 g C m−2 yr−1 occurred around 1080 years ago as a tip-up pool infilled. Tip-up pools are common in the peatlands of northwest Borneo where windthrow and lightning strikes produce tree falls at a rate of 4 trees ha−1 every decade. A simulation model indicates that tip-up pools, which are formed across the entire forested peat dome, produce local discontinuities in the peat deposit, when peat is removed to create a pool that is rapidly filled with younger material. The resulting discontinuities in peat age at the base and sides of pool deposits obscure linkages between carbon accumulation rates and climate and require new approaches for paleoenvironmental reconstructions. Our results suggest that carbon accumulation in tropical peat swamps may be based on fundamentally different peat-forming processes than those of northern peatlands.

8d12b275-0f56-31af-a564-29a05fd594c1
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ID: 8d12b275-0f56-31af-a564-29a05fd594c1
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Abstract: Oxygen isotope ratios of authigenic calcite (δ18Ocal) measured at annual to decadal resolution from Laguna Pumacocha document Andean precipitation variability during the last 11,200 years. Modern limnological data show that Pumacocha δ18Ocal reflects the average annual isotopic composition of the lake's surface waters (δ18Olw), and that δ18Olw tracks the isotopic composition of precipitation (δ18Oprecip), which is largely controlled by the intensity of the South American summer monsoon (SASM). Based on these relationships we use down-core δ18Ocal measurements as a proxy for δ18Oprecip that varies with the intensity of SASM precipitation. Pumacocha δ18Ocal increased rapidly between 11,200 and 10,300 yr B.P. from − 14.5‰ to − 10.5‰, reaching a maximum of − 10.3‰ by 9800 yr B.P. After 9800 yr B.P., δ18Ocal underwent a long-term decrease that tracked increasing Southern Hemisphere summer insolation, suggesting that enhanced SASM precipitation was linked to precessional forcing. Higher-frequency trends did not follow insolation and therefore represent other variability in the climate system. Millennial-scale trends from Pumacocha strongly resemble those from lower-resolution tropical Andean ice and lake core isotopic records, particularly the Huascaran ice core, and low elevation speleothems. These relationships suggest that tropical Andean isotopic records reflect variations in precipitation intensity related to precessional forcing rather than tropical temperatures. They also demonstrate a coherent pattern of SASM variability, although with differences between low elevation and Andean records during the late Glacial to Holocene transition and the late Holocene. Centennial and decadal SASM precipitation variability is also apparent. Reduced SASM rainfall occurred from 10,000–9200, 7000–5000, 1500–900 yr B.P. and during the last 100 years. Intensifications of the SASM occurred at 5000, 2200–1500, and 550–130 yr B.P. with the amplitude of variability increasing after 2200 yr B.P. These periods may represent SASM responses to ocean–atmosphere variability related to orbital and radiative forcing (e.g., El Niño-Southern Oscillation and the Intertropical Convergence Zone).

d4c44dad-d090-3b1a-a8de-b6dbdba224d0
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ID: d4c44dad-d090-3b1a-a8de-b6dbdba224d0
Tag: []
Abstract: A record of the hydrogen isotopic composition of terrestrial leaf waxes ( δ D wax ) in sediment cores from Lake Titicaca provides new insight into the precipitation history of the Central Andes and controls of South American Summer Monsoon (SASM) variability since the last glacial period. Comparison of the δ D wax record with a 19-kyr δ D record from the nearby Illimani ice core supports the interpretation that precipitation δ D is the primary control on δ D wax with a lesser but significant role for local evapotranspiration and other secondary influences on δ D wax . The Titicaca δ D wax record confirms overall wetter conditions in the Central Andes during the last glacial period relative to a drier Holocene. During the last deglaciation, abrupt δ D wax shifts correspond to millennial-scale events observed in the high-latitude North Atlantic, with dry conditions corresponding to the Bølling–Allerød and early Holocene periods and wetter conditions during late glacial and Younger Dryas intervals. We observe a trend of increasing monsoonal precipitation from the early to the late Holocene, consistent with summer insolation forcing of the SASM, but similar hydrologic variability on precessional timescales is not apparent during the last glacial period. Overall, this study demonstrates the relative importance of high-latitude versus tropical forcing as a dominant control on glacial SASM precipitation variability.

83ab7f3f-0ce8-3007-a735-ffc89b343d51
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ID: 83ab7f3f-0ce8-3007-a735-ffc89b343d51
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Abstract: Bear Lake (Utah-Idaho, USA) has been producing large quantities of carbonate minerals of varying mineralogy for the past 17,000 years. The history of sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, percent organic carbon, percent CaCO3, X-ray diffraction mineralogy, HCl-leach inorganic geochemistry, and magnetic properties on samples from three piston cores. Historically, the Bear River, the main source of water for Great Salt Lake, did not enter Bear Lake until it was artificially diverted into the lake at the beginning of the 20th century. However, during the last glacial interval, the Bear River did enter Bear Lake depositing red, calcareous, silty clay. About 18,000 years ago, the Bear River became disconnected from Bear Lake. A combination of warmer water, increased evaporation, and increased organic productivity triggered the precipitation of calcium carbonate, first as calcite. As the salinity of the lake increased due to evaporation, aragonite began to precipitate about 11,000 years ago. Aragonite is the dominant mineral that accumulated in bottom sediments of the lake during the Holocene, comprising an average of about 70 wt.% of the sediments. Aragonite formation in a large, cold, oligotrophic, high latitude lake is highly unusual. Lacustrine aragonite usually is found in small, saline lakes in which the salinity varies considerably over time. However, Bear Lake contains endemic ostracodes and fish, which indicate that the chemistry of the lake has remained fairly constant for a long time. Stable isotope data from Holocene aragonite show that the salinity of Bear Lake increased throughout the Holocene, but never reached highly evolved values of δ18O in spite of an evaporation-dominated water balance. Bear Lake hydrology combined with evaporation created an unusual situation that produced large amounts of aragonite, but no evaporite minerals.

a24dff19-bebd-38f9-9d81-85082fdac3cf
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ID: a24dff19-bebd-38f9-9d81-85082fdac3cf
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Abstract: Scientific drilling to recover sediment core and fossil samples is a promising approach to increasing our understanding of species evolution in ancient lakes. Most lake drilling efforts to date have focused on paleoclimate reconstruction. However, it is clear from the excellent fossil preservation and high temporal resolution typical of lake beds that significant advances in evolutionary biology can be made through drill core studies coordinated with phylogenetic work on appropriate taxa. Geological records can be used to constrain the age of specific lakes and the timing of evolutionarily significant events (such as lake level fluctuations and salinity crises). Fossil data can be used to test speciation and biogeographic hypotheses and flesh out phylogenetic trees, using a better-resolved fossil record to estimate timing of phylogenetic divergences. The extraordinary preservation of many fossils in anoxic lake beds holds the hope of collecting fossil DNA from the same body fossils that improve our understanding of morphological character evolution and adaptation. Moreover, fossils allow calibration of molecular clocks, which are currently largely inferential. Lake Malawi Drilling Project results provide some guideposts on what might be expected in a drilling project for studies of evolution. The extreme variability in lake level and environmental history that most ancient lakes experience (exemplified by the Lake Malawi record) demonstrates that no one drilling locality is likely to provide a complete record of phylogenetic history for a radiating lineage. Evolutionary biologists should take an active role in the design of drilling projects, which typically have interdisciplinary objectives, to ensure their sampling needs will be met by whatever sites in a lake are ultimately drilled. © 2011 Springer Science+Business Media B.V.

c570e6c0-4a2a-3ce1-9187-233ab35254a3
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ID: c570e6c0-4a2a-3ce1-9187-233ab35254a3
Tag: []
Abstract: Magnetic susceptibility (MS) is commonly measured on lake sediments and used as a proxy for clastic input and soil erosion, for correlating among cores in the same lake basin and aligning successive overlapping drives of the same core. There are several common techniques for measuring MS, each with its own advantages. Here we compare three such techniques measured on a sediment core from Lake Pepin: (a) loop-sensor MS logging on wet sediment of the intact core; (b) point-sensor MS logging on wet sediment of a split (lengthwise) core; and (c) discrete MS measurements of dried subsamples using a susceptibility bridge. To obtain further information about the origin of downcore MS variability, additional magnetic measurements were performed on discrete samples to model ferrimagnetic sedimentary components. Overall trends and individual features in MS curves agree reasonably well between techniques; however, the amplitude of local minima and maxima varies according to the technique used. All three MS techniques captured distinct events c. 1900 and 1940, attributed to increases in allochthonous ferrimagnetic components. The ferrimagnetic particle flux has declined over the past half-century despite increasing sediment accumulation in Lake Pepin, suggesting a possible shift in sediment sources from fields to stream banks.

e546a125-6931-325d-b632-b5b49ca5c040
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ID: e546a125-6931-325d-b632-b5b49ca5c040
Tag: []
Abstract: In eastern North America, large prehistoric settlements were concentrated in and along the floodplains of the midcontinent, but few sedimentary records have been examined adjacent to these sites to evaluate the impacts of Native American land use on terrestrial ecosystems. Here we report a high-resolution and multiproxy paleoecological record from Horseshoe Lake, an oxbow lake in the central Mississippi River valley that is adjacent to the Cahokia site (Illinois, USA), the largest prehistoric settlement north of Mexico. Palynological and carbon isotope data document pronounced vegetation changes over the past 1700 yr driven primarily by land use, including 900 yr (450–1350 CE) of sustained prehistoric human impacts. Rapid forest clearance was followed closely by the proliferation of indigenous seed crops of the Eastern Agricultural Complex beginning ca. 450 CE, centuries before the emergence of Cahokia at 1050 CE. Agricultural intensification that included the use of maize (Zea mays subsp. mays) followed this initial clearance, with peak land use intensity between 900 and 1200 CE. A large flood event ca. 1200 CE marks the onset of agricultural contraction and Cahokia’s decline. Reforestation follows the abandonment of the Cahokia region at ca. 1350 CE. The Horseshoe Lake record thus indicates that regional agricultural activity began abruptly at 450 CE and intensified over the following centuries, well before the formation of Cahokia and other large prehistoric settlements. The evidence that a major flood coincided with the onset of Cahokia’s decline is noteworthy, but will require corroboration from additional records.

a2f8ab6a-1c47-33ab-b5c2-60e515e7d540
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ID: a2f8ab6a-1c47-33ab-b5c2-60e515e7d540
Tag: []
Abstract: It can be advantageous to revisit coring locations in lakes years after an initial paleolimnological study is completed, to assess environmental changes in the intervening time interval. We revisited sediment core sites in Lake Pepin (Minnesota, Wisconsin) more than a decade after an original set of 10 cores was collected, dated radiometrically, and studied in 1996. Prominent magnetic susceptibility features were used to align the new core set with the older set, such that traditional radiometric dating was not necessary to obtain a chronology for the new cores. The procedure used to align the two core sets accounted for compaction of former surface sediments by burial with new sediment. The amount of new sediment, mercury, and phosphorus accumulated at each core site was determined and extrapolated to the depositional area of the lake to estimate recent (1996-2008) whole-basin loads. Recent sediment accumulation in Lake Pepin compared well (within 3%) with monitored inflow data from a gauging station on the upper Mississippi River just before it enters the lake. Bulk sediment accumulation rate remained very high (772,000 t/year) for the recent period (1996-2008), down slightly from the peak in 1990-1996 (876,000 t/year), and almost an order of magnitude above pre-settlement rates. Total phosphorus deposition remained constant since a peak in the 1960s, but was also well above pre-settlement rates. Mercury continued its precipitous decline since peaking in the 1960s. © 2013 Springer Science+Business Media Dordrecht.

4e4832b4-d33f-3058-ba8e-b78bdcf9b922
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ID: 4e4832b4-d33f-3058-ba8e-b78bdcf9b922
Tag: []
Abstract: Mesic tree species such as Fagus grandifolia and Tsuga canadensis experienced multiple abundance declines in eastern North America during the last 8000 years, but the causes remain unclear. This paper presents a new sub-centennial record of Holocene vegetation, fire and sedimentological changes at Spicer Lake, IN, to test hypotheses about the role of fire and hydrological variations on shifts in vegetation composition. Four pollen zones are reported: Abies–Picea forests (15–11.8 ka BP), Pinus-dominated mixed forest (11.8–10.6 ka BP), transitional mixed forest (10.6–6.8 ka BP), and deciduous forest characterized by the expansion and high variability of F. grandifolia (after 6.8 ka BP). Macroscopic charcoal indicates five to seven fires between 6.1 and 4.4 ka BP and no fires between 4.4 and 2 ka BP, despite several large declines in F. grandifolia, and more fires after 1.8 ka BP likely linked to declining F. grandifolia abundances after 1.1 ka BP. Six peaks in mineralogenic sediments are suggestive of hydroclimate variability, but do not consistently correspond to shifts in F. grandifolia abundances. A Bayesian change-point analysis of 15 regional F. grandifolia pollen records identifies peak probabilities of events at 4.8 and 1.1 ka BP, similar in timing to variations in T. canadensis at other sites. Hence, fire can be ruled out as a driver of the mid-Holocene declines of F. grandifolia, but more work is needed to confidently establish the regional timing of F. grandifolia declines and to link them to past droughts and T. canadensis declines in eastern North America.

aaac5bcf-422e-340a-878a-a34d8a288be1
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ID: aaac5bcf-422e-340a-878a-a34d8a288be1
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Abstract: Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. Emissions of Hg from mineral reservoirs and recycling between soil/biomass, oceans, and the atmosphere are fundamental to the global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 μg m-2 a-1). Past Hg deposition does not correlate with changes in regional temperature and precipitation or with most large volcanic events that occurred regionally (~300-400 km from Yanacocha) and globally. In 1450 B.C. (3.4 ka), Hg fluxes abruptly increased and reached the Holocene-maximum flux (6.7 μg m-2 a-1) in 1200 B.C., concurrent with a ~100 year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from 1200 to 500 B.C. suggest a protracted mining-dust source near Yanacocha that is identical in timing to documented pre-Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450-1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 μg m-2 a-1) are 3.0 ± 1.5 times greater than preanthropogenic fluxes. Key Points Hg deposition did not vary with past precipitation, temperature, and volcanism Maximum Holocene Hg fluxes occurred ~3 thousand years ago Modern Hg fluxes are 3 times greater than natural fluxes ©2014. American Geophysical Union. All Rights Reserved.

196e43d0-1ed5-3ee4-9cf8-b6e5911a1bf6
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ID: 196e43d0-1ed5-3ee4-9cf8-b6e5911a1bf6
Tag: []
Abstract: Major components of lacustrine sediments, such as carbonates, organic matter, and biogenic silica, provide significant paleoenvironmental information about lake systems. Fourier transform infrared spectroscopy (FTIR) and scanning X-ray fluorescence (XRF) techniques are fast, cost effective, efficient methods to determine the relative abundances of these components. We investigate the potential of these techniques using sediments from two large lakes, Lake Malawi in Africa and Lake Qinghai in China. Our results show statistically significant correlations of conventionally measured concentrations of carbonate (%CaCO3), total organic carbon (%TOC), and biogenic silica (%BSi), with absorbance in the corresponding FTIR spectral regions and with XRF elemental ratios including calcium:titanium (Ca/Ti), incoherent:coherent X-ray scatter intensities (Inc/Coh), and silicon:titanium (Si/Ti), respectively. The correlation coefficients (R) range from 0.66 to 0.96 for comparisons of FTIR results and conventional measurements, and from 0.70 to 0.90 for XRF results and conventional measurements. Both FTIR and XRF techniques exhibit great potential for rapid assessment of inorganic and organic contents of lacustrine sediments. However, the relationship between XRF-ratios or FTIR-absorbances and abundances of corresponding sedimentary components can vary with sediment source and lithology. © 2013 Springer Science+Business Media Dordrecht.

95ca1ea6-77e3-3356-b6fa-66f7604e451c
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ID: 95ca1ea6-77e3-3356-b6fa-66f7604e451c
Tag: []
Abstract: Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates of drought variability over the past several thousand years are based largely on tree-ring records. We produced a 1,500-y record of winter precipitation in the Pacific Northwest using a physical model-based analysis of lake sediment oxygen isotope data. Our results indicate that during the Medieval Climate Anomaly (MCA) (900-1300 AD) the Pacific Northwest experienced exceptional wetness in winter and that during the Little Ice Age (LIA) (1450-1850 AD) conditions were drier, contrasting with hydroclimatic anomalies in the desert Southwest and consistent with climate dynamics related to the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These findings are somewhat discordant with drought records from tree rings, suggesting that differences in seasonal sensitivity between the two proxies allow a more compete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.

532bf39a-8e02-34aa-a374-04db4e861f8f
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ID: 532bf39a-8e02-34aa-a374-04db4e861f8f
Tag: []
Abstract: Here we establish the timing of major flood events of the central Mississippi River over the last 1,800 y, using floodwater sediments deposited in two floodplain lakes. Shifts in the frequency of high-magnitude floods are mediated by moisture availability over midcontinental North America and correspond to the emergence and decline of Cahokia—a major late prehistoric settlement in the Mississippi River floodplain. The absence of large floods from A.D. 600 to A.D. 1200 facilitated agricultural intensification, population growth, and settlement expansion across the floodplain that are associated with the emergence of Cahokia as a regional center around A.D. 1050. The return of large floods after A.D. 1200, driven by waning midcontinental aridity, marks the onset of sociopolitical reorganization and depopulation that culminate in the abandonment of Cahokia and the surrounding region by A.D. 1350. Shifts in the frequency and magnitude of flooding may be an underappreciated but critical factor in the formation and dissolution of social complexity in early agricultural societies.

e39e4bc6-91af-34d9-828f-8d61fcc92d97
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ID: e39e4bc6-91af-34d9-828f-8d61fcc92d97
Tag: []
Abstract: Knowledge about vegetation dynamics during the last glacial and deglacial periods in southeastern North America is under-constrained owing to low site density and problematic chronologies. New pollen records from two classic sites, Anderson Pond, TN, and Jackson Pond, KY, supported by AMS 14C age models, span 25.2–13.7 ka and 31.0–15.4 ka, respectively. A transition from Pinus dominance to Picea dominance is recorded at Jackson Pond ca. 26.2 ka, ~ coincident with Heinrich Event H2. Anderson and Jackson Ponds record a transition from conifer to deciduous-tree dominance ~ 15.9 and 15.4 ka, respectively, marking the development of no-analog vegetation characterized by moderate to high abundances of Picea, Quercus, Carya, Ulmus, Fraxinus, Ostrya/Carpinus, Cyperaceae, and Poaceae, and preceding by ~ 2000 yr the advent of similar no-analog vegetation in glaciated terrain to the north. No-analog vegetation developed as a time-transgressive, south-to-north pattern, mediated by climatic warming. Sporormiella abundances are consistently low throughout the Jackson and Anderson Pond records, suggesting that megafaunal abundances and effects on vegetation varied regionally or possibly that the Sporormiella signal was not well-expressed at these sites. Additional records with well-constrained chronologies are necessary to assess patterns and mechanisms of vegetation dynamics during the last glacial and deglacial periods.

0aac9978-6041-38b3-9d80-8e3aa4b855c7
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ID: 0aac9978-6041-38b3-9d80-8e3aa4b855c7
Tag: []
Abstract: The Indo-Pacific Warm Pool (IPWP) is an important driver of global climate, but its response to and involvement in paleoclimate change is poorly constrained. We generated a new record of sediment geochemistry from Lake Towuti (2.5°S, 121.5°E), Indonesia, located in the heart of the IPWP, to investigate changes in hydrological connectivity with upstream lakes and the extent of lake mixing and oxygenation during paleoclimate changes over the last 60,000 years BP (60 ka). Lake Towuti is located at the downstream end of the Malili Lakes, a chain of large, ancient, and biologically diverse tectonic lakes occupying a geologically heterogeneous terrain in central Sulawesi, Indonesia. Major and trace element data from river and lake sediments suggest no changes in sediment provenance during the Last Glacial Maximum (LGM), indicating that some of the Malili Lakes remained hydrologically open despite a regionally drier climate. However, samples from the LGM are uniformly less enriched in trace elements than samples from the Holocene and Marine Isotope Stage 3 (MIS3), which suggests a decrease in weathering intensity during the LGM, likely in response to decreased precipitation and temperature. Changes in Fe and other redox-sensitive trace element concentrations indicate changes in water column oxygenation, with the highest oxygen availability occurring during the LGM (15–35 ka) likely due to more frequent and/or deeper mixing of Lake Towuti's water column. The glacial–interglacial trend in lake oxygenation corresponds with changes in regional precipitation and associated changes in the seasonal cycle. The high degree of faunal endemism in these lakes may be related to changes in the lake geochemistry associated with glacial–interglacial environmental variability driven by changing inputs and redox variability.

c178b686-32e6-35fa-b970-642574c0edcd
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ID: c178b686-32e6-35fa-b970-642574c0edcd
Tag: []
Abstract: In recent years, deep drilling undertaken as part of the International Continental Drilling Program has generated multiple long lacustrine sedimentary records to reconstruct continental paleoclimate. In many cases, the tectonic and geomorphic history of these basins is under-constrained and poorly known, which affects the interpretation of climate history from geophysical, geochemical, and paleobiotic proxies in the sedimentary record. In addition, non-analog biotic assemblages that reflect evolutionary processes may constrain the reconstruction of past environments. In the drill-core record of Lake Titicaca, spanning the last ~. 370. ka, the diatom stratigraphy reflects both the influence of climate and the long-term evolution of the lake basin and its biota. In the upper part of the drill-core sequence, glacial intervals were deep and dominated by freshwater planktic taxa, and peak interglacial intervals were shallow and dominated by benthic species, some with saline affinities. In the basal sections of the drill-core record, benthic diatoms are dominant in both glacial and interglacial units, with freshwater taxa dominating the glacial strata. This suggests that the ancient lake basin was shallower during intervals of both wet and dry climate, and that the modern deep lake may result from a progressive subsidence and deepening of the basin over time. In addition, morphological evolution in one of the major lineages of planktic diatoms, Cyclostephanos, indicates substantial change in the limnological environment that affected species morphology and may have driven speciation. © 2011 Elsevier B.V.

31122eed-5eb2-347e-9c4d-ff859beb93b3
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ID: 31122eed-5eb2-347e-9c4d-ff859beb93b3
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Abstract: Droughts are recurrent features in sedimentary records at the prairie–forest ecotone of northern U.S.A. It is therefore important to understand the duration of such events, as well as their severity and consequences beyond the century-scale instrumental record. The existing drought records can be complemented with additional proxies that clearly document the interactions between external factors and the depositional environment. Here we attempt to reconstruct the last millennium drought history of Deming Lake, a small lake at Minnesota's prairie–forest ecotone, using sediment magnetism. In particular, this approach allowed us to explore the interactions between lake and catchment, and test the hypothesis that periods of moderate aridity result in decreased hydrologic sediment transport from the catchment to the lake, as a consequence of reduced erosion and overland flow. Concordantly, during dry episodes we find that the mass fractions of in-lake produced organic matter and biogenic magnetic particles increase relative to allochthonous component fractions. We identify several episodes of aridity at Deming Lake consistent with existing regional drought reconstructions. The most prolonged dry event occurred at the end of the Little Ice Age, suggesting that the second half of this cold interval was drier in the region than previously thought.

61be91a5-ed7c-3a1b-95f2-fcc3d1d91094
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ID: 61be91a5-ed7c-3a1b-95f2-fcc3d1d91094
Tag: []
Abstract: A growing body of paleoclimate data indicates that periods of severe drought affected the Maya Lowlands of southeastern Mexico and northern Central America, especially during the Terminal Classic period (ca. 800-950 CE), raising the possibility that climate change contributed to the widespread collapse of many Maya polities at that time. A broad range of paleoclimate proxy methods have been applied in the Maya Lowlands and the data derived from these methods are sometimes challenging for archeologists and other non-specialists to interpret. This paper reviews the principal methods used for paleoclimate inference in the region and the rationale for climate proxy interpretation to help researchers working in the Maya Lowlands make sense of paleoclimate datasets. In particular, we focus on analyses of speleothems and lake sediment cores. These two paleoclimate archives have been most widely applied in the Maya Lowlands and have the greatest potential to provide insights into climate change impacts on the ancient Maya. We discuss the development of chronologies for these climate archives, the proxies for past climate change found within them, and how these proxy variables are interpreted. Finally, we present strategies for improving our understanding of proxy paleoclimate data from the Maya Lowlands, including multi-proxy analyses, assessment of spatial variability in past climate change, combined analysis of climate models and proxy data, and the integration of paleoclimatology and archeology.

8eb761bd-3df2-30a7-aa46-c328d8cd927c
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Abstract: Simulations conducted using a coupled lake-catchment isotope mass balance model forced with continuous precipitation, temperature, and relative humidity data successfully reproduce (within uncertainty limits) long-term (i.e., multidecadal) trends in reconstructed lake surface elevations and sediment core oxygen isotope (δ18O) values at Castor Lake and Scanlon Lake, north-central Washington. Error inherent in sediment core dating methods and uncertainty in climate data contribute to differences in model reconstructed and measured short-term (i.e., sub-decadal) sediment (i.e., endogenic and/or biogenic carbonate) δ18O values, suggesting that model isotopic performance over sub-decadal time periods cannot be successfully investigated without better constrained climate data and sediment core chronologies. Model reconstructions of past lake surface elevations are consistent with estimates obtained from aerial photography. Simulation results suggest that precipitation is the strongest control on lake isotopic and hydrologic dynamics, with secondary influence by temperature and relative humidity. This model validation exercise demonstrates that lake-catchment oxygen isotope mass balance models forced with instrumental climate data can reproduce lake hydrologic and isotopic variability over multidecadal (or longer) timescales, and therefore, that such models could potentially be used for quantitative investigations of paleo-lake responses to hydroclimatic change. © 2012 Elsevier Ltd.

68bb058d-a800-3314-aeff-8184262c0a2c
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Abstract: Permafrost-related processes drive regional landscape dynamics in the Arctic terrestrial system. A better understanding of past periods indicative of permafrost degradation and aggradation is important for predicting the future response of Arctic landscapes to climate change. Here, we used a multi-proxy approach to analyse a~4m long sediment core from a drained thermokarst lake basin on the northern Seward Peninsula in western Arctic Alaska (USA). Sedimentological, biogeochemical, geochronological, micropalaeontological (ostracoda, testate amoebae) and tephra analyses were used to determine the long-term environmental Early-Wisconsin to Holocene history preserved in our core for central Beringia. Yedoma accumulation dominated throughout the Early to Late-Wisconsin but was interrupted by wetland formation from 44.5 to 41.5ka BP. The latter was terminated by the deposition of 1m of volcanic tephra, most likely originating from the South Killeak Maar eruption at about 42ka BP. Yedoma deposition continued until 22.5ka BP and was followed by a depositional hiatus in the sediment core between 22.5 and 0.23ka BP. We interpret this hiatus as due to intense thermokarst activity in the areas surrounding the site, which served as a sediment source during the Late-Wisconsin to Holocene climate transition. The lake forming the modern basin on the upland initiated around 0.23ka BP and drained catastrophically in spring 2005. The present study emphasises that Arctic lake systems and periglacial landscapes are highly dynamic and that permafrost formation as well as degradation in central Beringia was controlled by regional to global climate patterns as well as by local disturbances.

788b41ce-7ec5-3ad9-832b-8edcd1bf7e88
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Abstract: Sediment cores from Lakes Punta Laguna, Chichancanab, and Petén Itzá on the Yucatan Peninsula were used to (1) investigate "within-horizon" stable isotope variability (δ18O and δ13C) measured on multiple, single ostracod valves and gastropod shells, (2) determine the optimum number of individuals required to infer low-frequency climate changes, and (3) evaluate the potential for using intra-sample δ18O variability in ostracod and gastropod shells as a proxy measure for high-frequency climate variability. Calculated optimum sample numbers ("n") for δ18O and δ13C in the ostracod Cytheridella ilosvayi and the gastropod Pyrgophorus coronatus vary appreciably throughout the cores in all three lakes. Variability and optimum "n" values were, in most cases, larger for C. ilosvayi than for P. coronatus for δ18O measurements, whereas there was no significant difference for δ13C measurements. This finding may be explained by differences in the ecology and life history of the two taxa as well as contrasting modes of calcification. Individual δ18O measurements on C. ilosvayi in sediments from Lake Punta Laguna show that samples from core depths that have high mean δ18O values, indicative of low effective moisture, display lower variability, whereas samples with low mean δ18O values, reflecting times of higher effective moisture, display higher variability. Relatively dry periods were thus consistently dry, whereas relatively wet periods had both wet and dry years. This interpretation of data from the cores applies to two important periods of the late Holocene, the Maya Terminal Classic period and the Little Ice Age. δ18O variability during the ancient Maya Terminal Classic Period (ca. 910-990 AD) indicates not only the driest mean conditions in the last 3,000 years, but consistently dry climate. Variability of δ13C measurements in single stratigraphic layers displayed no relationship with climate conditions inferred from δ18O measurements. © 2009 Springer Science+Business Media B.V.

ee5cd9b2-a99e-3079-9f2a-39699abd4d3a
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Abstract: Leaf-wax hydrogen isotope composition (δD wax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δD wax in the tropics remain poorly understood. We measured δD wax and the stable carbon isotope composition of leaf-waxes (δ 13C wax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δD wax. In this region the estimated hydrogen isotope composition of meteoric water (δD w) varies by only 25‰, and variability in δD w does not explain the relatively large variance in δD wax (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (ε wax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in ε wax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ 13C wax, provides a secondary control on ε wax/w for n-alkanoic acids (ε n-acid/w). Low δ 13C n-acid values are associated with high ε n-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C 4 grasses and C 3 trees and shrubs. A similar relationship between δ 13C n-alkane and ε n-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δD wax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies. © 2012 Elsevier Ltd.

a770fb31-fd73-35a0-9e61-edde26dada2d
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Abstract: This study presents limnological and morphological characteristics, physical and chemical properties of waters, and geochemistry of surface sediments for 63 aquatic ecosystems located on the karst Yucatán Peninsula and surrounding areas of Belize and the Guatemalan highlands and eastern lowlands. Our principal goal was to classify the aquatic systems based on their water variables. A principal component analysis (PCA) of the surface water chemistry data showed that a large fraction of the variance (29%) in water chemistry is explained by conductivity and major ion concentrations. The broad conductivity range, from 168 to 55,300 μS cm-1 reflects saline water intrusion affecting coastal aquatic environments, and the steep NW-S precipitation gradient, from ~450 to>3,200 mm year-1. Coastal waterbodies Celestún and Laguna Rosada displayed the highest conductivities. Minimum surface water temperatures of 21.6°C were measured in highland lakes, and warmest temperatures, up to 31.7°C, were recorded in the lowland waterbodies. Most lakes showed thermal stratification during the sampling period, with the exception of some shallow (<10 m) systems. Lakes Chichancanab, Milagros, and Bacalar displayed sulfate-rich waters. Waters of sinkholes had relatively high conductivities (<3,670 μS cm-1) and a broad range of δ18O values (-4.1 to +3.8%). Ca, HCO3, and SO4 dominated the waters of the lowland lakes, whereas Na was the dominant cation in highland lakes. Coastal aquatic ecosystems were dominated by Na and Cl. Cluster analysis based on surface water variables classified aquatic environments of the lowlands and highlands into three groups: (1) lowland lakes, ponds, wetlands, and coastal waterbodies (2) highland lakes, and (3) sinkholes and rivers. A broad trophic state gradient was recorded, ranging from the eutrophic Lake Amatitlán and the Timul sinkhole to oligotrophic Laguna Ayarza, with the highest water transparency (11.4 m). We used major and trace elements in surface sediments to assess pollution of waterbodies. Lakes Amatitlán, Atescatempa, El Rosario, Cayucón, Chacan-Lara, La Misteriosa, rivers Subín and Río Dulce, the wetland Jamolún, and the sinkhole Petén de Monos showed evidence of pollution and urban development. Their surface sediments displayed high concentrations of As, Cu, Fe, Ni, Pb, Se, Zn, and Zr, which suggest moderate to strong pollution. © Springer Science+Business Media B.V. 2010.

81e28009-790f-31cd-87fd-a3ce310208a0
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ID: 81e28009-790f-31cd-87fd-a3ce310208a0
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Abstract: Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using 210Pb and 137Cs, and analyzed for historical changes (c. 1840-present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l-1 c. 1910 to ~60 μg l-1 since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century. © 2009 Springer Science+Business Media B.V.

c269b120-7e7a-3b63-be37-ea7596bb7417
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ID: c269b120-7e7a-3b63-be37-ea7596bb7417
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Abstract: Paleolimnological studies show that phosphorus (P) loads to the federally protected St. Croix River, a tributary of the Upper Mississippi River, have increased about threefold over the last century. Ongoing management efforts to protect and restore the river hinge on the question of whether the increased nutrient load results from point-source discharges or nonpoint runoff from agricultural intensification and urban expansion. Here we determine the historical contribution of point source phosphorus (P) loads to the St. Croix watershed from 1900-2000 A.D. Historical point source loads were estimated based on discharge volumes, demographics, industrial sources, wastewater technologies, and facility discharge records, where available. Sewering in the basin began in 1905, and since that time, there have been as many as 169 permitted point source dischargers basinwide, including municipal, industrial, and agricultural facilities. Early wastewater management typically discharged untreated sewage; technological advances had secondary treatment in place at most facilities by the 1960s-1970s and much of the municipal population was served by tertiary treatment by the 1990s. Peak nutrient discharges from point sources occurred in the 1960s-1970s. Detergent phosphorus bans instituted in the late 1970s for Minnesota and Wisconsin, greater use of land and groundwater effluent disposal, and improvements in treatment technology brought about decreases in P loads in the 1980s and 1990s. Point-source discharges were compared to historical total phosphorus loads estimated in a whole-basin phosphorus mass balance to calculate the historical contribution of point sources, anthropogenic nonpoint sources, and natural or background sources. We estimated 1990s point source loads at 48 t P yr-1, which represents about 10% of the total phosphorus load (459 t P yr-1, flow-corrected to 412 t P yr-1) to the basin. Without further controls on nutrient inputs to the St. Croix River, annual flow-corrected P loads are projected to increase to 498 t P yr-1 by the 2020s with point source phosphorus loading contributions at 65 t P yr-1 or 13% of the total load. However, if we exclude background P loads to the St. Croix (166 t P yr -1), recent nutrient loads are primarily from anthropogenic nonpoint sources. Point sources also contribute over 19% of the current and future phosphorus load that can be attributed to human activities in the watershed. Interstate and federal efforts to decrease P loading to the St. Croix River by 20% will need to target both point and nonpoint sources. © 2009 Springer Science+Business Media B.V.

0908812c-6342-3680-8ea2-c13d81988d8e
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ID: 0908812c-6342-3680-8ea2-c13d81988d8e
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Abstract: Mongolia is an arid land with limited freshwater availability. Recent changes in climate as well as changes in livestock management in the post-Soviet era may threaten the quality and availability of these freshwater resources and emphasize the need to characterize current conditions relative to a long-term baseline. Because diatoms are responsive to changes in water chemistry and are commonly preserved in lake sediments, they can be used as indicators of both present and past water quality. To these ends, we have developed a diatom-based calibration set and quantitative inference models for the region. Physical and chemical factors and modern diatom communities were sampled at 54 sites in western Mongolia in the Altai and Khangai mountains and the Valley of the Great Lakes. Canonical correspondence analysis (CCA) demonstrated that both salinity and total phosphorus accounted for statistically significant fractions of the variation in 151 diatom taxa across the 54 sampled sites. Diatom-based inference models were developed with weighted averaging (WA) techniques and showed strong predictive capabilities and low prediction errors for salinity and total phosphorus. These models are now available to help identify and interpret historical and future disturbances to this sensitive and globally important ecoregion. © Springer Science+Business Media B.V. 2008.

21a21312-4b0e-304d-bd63-8db98d3e7b37
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ID: 21a21312-4b0e-304d-bd63-8db98d3e7b37
Tag: []
Abstract: Climate warming and major land-use changes have profoundly affected the Mongolian landscape in the past several decades. Previous studies have recognized the impacts of a warmer, more arid climate and Mongolia's 1991 transition from a command to a market economy on terrestrial ecosystems, including impaired sustainability of subsistence herding and threats to wild animals. In this study, we examined the combined effects of changing climate and herding practices on lake eutrophication in Western Mongolia. We sampled 65 lakes for modern nutrients and found the majority of lakes were eutrophic to hyper-eutrophic. Sediment cores were taken from five of the lakes to compare current lake status to paleolimnologial measures of lake eutrophication over the past 100-2000 years, including changes in diatom assemblages, diatom-inferred total phosphorus, biogenic silica, organic matter, and sediment accumulation rates. Variance partitioning analysis showed that recent shifts in diatom assemblages were related to changes in both climate and herding practices. The results presented here demonstrate a need for further study and long-term monitoring of water quality in Mongolia to understand the complicated interactions of climate and land use on aquatic resources and to preserve water quality in this remote and ecologically important region. © 2009 Springer Science+Business Media, LLC.

f2c12ddc-2f8e-3282-8dbc-1e654e0c91fc
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Abstract: A new Amphora species, Amphora soninkhishigae sp. nov. is described from the saline lakes, Oigon Nuur and Uvs Nuur, in western Mongolia. Amphora soninkhishigae is characterized by its small size (valves 12-28 μm long, 2.9-3.8 μm wide), fine ornamentation, and a broad, internally thickened central area on the dorsal side of the valve (dorsal stauros) that branches along the dorsal margin. Among the amphoroid diatoms, Amphora soninkhishigae belongs in the subgenus Oxyamphora Cleve, where it is allied with other small Amphora taxa bearing a dorsal stauros including Amphora staurophora Juhlin-Dannfelt, Amphora abludans Simonsen, Amphora laevissima var. perminuta Grunow, and Amphora laevis var. minuta Cleve.

bb0c4b06-ab7d-354b-bf09-69890fd06eb4
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Abstract: The Valley of the Great Lakes in western Mongolia is a unique ecosystem comprising a wide variety of terrestrial and aquatic habitats. Relatively little is known about the long-term climate history of the area or the relative impact of changes in climate and land use in the region. Tree-ring records have established an understanding of regional temperature fluctuations; however, the few records of moisture balance from Mongolia offer little insight into potential changes in the precipitation-to-evaporation balance in the region with warming temperatures. Modern and paleoclimatic records from across arid Central Asia show substantial temporal and spatial variability in the correlation between temperature and moisture availability and climate models are inconsistent in predictions for future changes with continued warming. In order to refine the understanding of moisture balance in the region, we developed six diatom-inferred records of lake salinity between 200 and 2000 years in length, demonstrating a negative correlation between temperature and effective moisture (warm-dry and cool-wet conditions) over most of this time period. In addition to climate warming, Mongolian grasslands have seen substantial human impacts over the last several decades. Recent core records show evidence of eutrophication and must be interpreted carefully to understand the individual and combined impacts of climate and land use on sedimentary records and the potential for human-environment interactions to confound inferences about climate changes. © The Author(s), 2010.

408ef5c9-307e-30ed-8266-2ff15274e443
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ID: 408ef5c9-307e-30ed-8266-2ff15274e443
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Abstract: The multidisciplinary study of sediment cores from Laguna Zoñar (37°29′00″ N, 4°41′22″ W, 300 m a.s.l., Andalucía, Spain) provides a detailed record of environmental, climatic and anthropogenic changes in a Mediterranean watershed since Medieval times, and an opportunity to evaluate the lake restoration policies during the last decades. The paleohydrological reconstructions show fluctuating lake levels since the end of the Medieval Warm Period (ca. AD 1300) till the late 19th century and a more acute dry period during the late 19th century - early 20th century, after the end of the Little Ice Age. Human activities have played a significant role in Laguna Zoñar hydrological changes since the late 19th century, when the outlet was drained, and particularly in the mid-20th century (till 1982) when the spring waters feeding the lake were diverted for human use. Two main periods of increased human activities in the watershed are recorded in the sediments. The first started with the Christian conquest and colonization of the Guadalquivir River Valley (13th century) particularly after the fall of the Granada Kingdom (15th century). The second one corresponds to the late 19th century when more land was dedicated to olive cultivation. Intensification of soil erosion occurred in the mid-20th century, after the introduction of farm machinery. The lake was declared a protected area in the early 1980s, when some agricultural practices were restricted, and conservation measures implemented. As a consequence, the lake level increased, and some littoral zones were submerged. Pollen indicators reflect this limnological change during the last few decades. Geochemical indicators show a relative decrease in soil erosion, but not changes in the amount of chemical fertilizers reaching the lake. This study provides an opportunity to evaluate the relative significance of human vs. climatic factors in lake hydrology and watershed changes during historical times. Paleolimnological reconstructions should be taken into account by natural resources agencies to better define lake management policies, and to assess the results of restoration policies. © Springer 2006.

0af77acb-564f-3b91-aef3-07f3bc7c2a3d
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ID: 0af77acb-564f-3b91-aef3-07f3bc7c2a3d
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Abstract: A sedimentological, geochemical and palynological study of the Taravilla Lake sequence (Central Iberian Range, NE Spain) provides a detailed record of allochthonous terrigenous layers that intercalate within the lacustrine sediments over the last 2000 years. These terrigenous layers are interpreted as the result of extreme hydrological events that caused higher clastic input to the basin. Anthropogenic influence caused by fires or deforestation is rejected as the main factor generating these layers because human impact, inferred from the pollen reconstruction, was minimal when the terrigenous layers reached their greatest frequency. The reconstructed occurrence of these events in the Taravilla Lake record is coherent with the paleoflood history of the Tagus River, characterized by a notable increase of extreme events at the beginning of the Little Ice Age. The Taravilla record suggests a relationship between the occurrence of extreme hydrological events, solar variability, and the North Atlantic Oscillation for the NE Iberian Peninsula. © 2008 Springer Science+Business Media B.V.

76f086ce-6a6a-3bb4-9c6b-9ffee331aea1
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ID: 76f086ce-6a6a-3bb4-9c6b-9ffee331aea1
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Abstract: A multiproxy study of sediment cores from Zoñar Lake (37°29′00″N, 4°41′22″W, 300 m a.s.l.) supported by 11 14C AMS dates provides the first high-resolution centennial-scale reconstruction of past humidity changes in southern Spain during the last 4000 years. Arid periods occurred prior to 2.9 cal. kyr BP and during 1.3-0.6 cal. kyr BP ('Mediaeval Climate Anomaly'). The most humid period occurred during 2.6-1.6 cal. kyr BP encompassing the late Iron Age-Iberian and Roman epochs. Two humid periods of lower intensity occurred between 0.8 and 0.6 cal. kyr BP (AD 1200-1400) and about 400 cal. yr BP (around AD 1600) coinciding with the onset of the 'Little Ice Age'. Humid conditions are synchronous with a decline in solar output and seem to correspond to atmospheric patterns similar to negative NAO phases. Arid conditions show better correlation with northern Africa climate evolution, suggesting a possible link to subtropical dynamics. The geographic location of Zoñar Lake and the robust chronology provides an opportunity to improve our understanding of the climate evolution in mid latitudes during the late Holocene and to evaluate subtropical and high latitude factors in Mediterranean climate evolution. © 2008 SAGE Publications.

f6a36f96-58e5-3dab-98f8-73caf76f0a99
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Abstract: This interdisciplinary study represents an approximation towards understanding how regional human cultural systems may have been affected by climate change in the northernmost Chilean Altiplano (>3600 m) over the last ca. 11 500 cal a BP. We compare the archaeological record from Hakenasa cave with the lake record from Lago Chungará sediment cores, located 50 km to the south. By integrating both of these archives in conjunction with regional palaeoclimate and archaeological data, we provide new evidence for the role of changing environmental and climatic conditions in human settlement patterns. The first human occupation of the entire Altiplano occurs at Hakenasa and is dated to 9980±40 14C a BP (11 265-11 619 cal. a BP), and took place under wetter regional climate conditions. An archaeologically sterile deposit occurs at Hakenasa between 7870 and 6890 cal. a BP. Constituted by sands and gravels, these sediments are interpreted as a flood event. This time period is synchronous with alternating short dry and wet events recorded in the Lake Chungará sedimentary sequence. Human activity resumes and increases in importance at Hakenasa by ca. 6000 cal. a BP. This corresponds to wetter conditions indicated by the Chungará record. Even though the lake record indicates intense volcanic activity over the last 6000 cal. a BP, this had little or no impact on the human population present at Hakenasa. This study shows that even in this extreme environment human settlement patterns do not always respond in a linear fashion to environmental change. Copyright © 2008 John Wiley & Sons, Ltd.

6ccd81bc-7917-3fab-b968-e8243afdf908
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ID: 6ccd81bc-7917-3fab-b968-e8243afdf908
Tag: []
Abstract: Low and high frequency climatic fluctuations in northern Iberian Peninsula during the last glacial maximum (LGM) and deglaciation are documented in a stalagmite using δ18O and δ13C and hydrologically sensitive trace metal ratios Mg/Ca and Ba/Ca. U/Th dating indicates speleothem growth commenced at 25 kyr BP (Present = year 1950) and extended to 11.6 kyr BP making this one of few European speleothem growing during the last glacial period. Rapid climatic fluctuations as Heinrich event 2 (H2) and Greenland Interstadial (GI-) 2 are well characterized in this record by more arid and cold conditions and by more humid conditions, respectively. Speleothem growth ceased from 18.2 to 15.4 kyr BP (the so-called Mystery Interval) likely reflecting the driest and potentially coldest conditions of this record, coincident with the 2 kyr duration shutdown of the North Atlantic Meridional Overturning Circulation (MOC). A major gradual increase in humidity and possibly in temperature occurred from 15.5 to 13.5 kyr BP, beginning in the Bølling and culminating in the Allerød period. This gradual humidity change contrasts with more abrupt humidity shifts in the Mediterranean, suggesting a different climate threshold for Mediterranean vs. Atlantic margin precipitation. © 2009 Elsevier B.V. All rights reserved.

c91638ee-0ea0-3610-b682-2c61ca8606e8
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ID: c91638ee-0ea0-3610-b682-2c61ca8606e8
Tag: []
Abstract: We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800-1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970-2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem. © 2011 Springer Science+Business Media B.V.

31d8831d-f8b7-3cd2-bac1-3ce3afeac60d
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ID: 31d8831d-f8b7-3cd2-bac1-3ce3afeac60d
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Abstract: Lake sediments from a closed basin in southern Patagonia (Argentina) provide a continental archive with to reconstruct climate change and to test the interhemispheric synchroneity of abrupt events. High-resolution sub-bottom seismic profiles of Lago Cardiel indicate substantial lake-level changes since the late Pleistocene, which were identified and dated in a series of long piston cores. These data allow the reconstruction of the regional water balance at 49'S since the late glacial. The seismic stratigraphy reveals a dry late glacial climate with a desiccation of the basin around 11 220 yr BP (14C). Lake level rapidly increased by 135 m at the Holocene transition. Following the early Holocene highstand at + 55 m, lake level never dropped significantly below modern level. The palaeoclimate changes implied by the Lago Cardiel record are out-of-phase with those implied by records from tropical South America and demonstrate considerable latitudinal asynchroneity in the climate evolution of this continent.

7c4e8fc6-428f-3d40-8861-42c9e20f0372
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ID: 7c4e8fc6-428f-3d40-8861-42c9e20f0372
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Abstract: 

605d1332-1716-3735-8414-5b2638d02dee
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ID: 605d1332-1716-3735-8414-5b2638d02dee
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Abstract: 

4106a490-2eda-3dad-a419-18ef44ea48d8
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ID: 4106a490-2eda-3dad-a419-18ef44ea48d8
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Abstract: 

93e0c7d0-77e2-35bb-a692-9dc4a6161224
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ID: 93e0c7d0-77e2-35bb-a692-9dc4a6161224
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Abstract: The results of a seismic stratigraphic analysis of a closed lake basin, Lago Cardiel, in southernmost South America are reported. Very few high-resolution, continental records spanning the Late Quaternary have been obtained from this region. Seismic sequence stratigraphic analysis allows a reconstruction of lake level variations. Two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period. Following the development of a relatively shallow lake during the late Pleistocene and a short desiccation pulse around 11 220 14C yr BP, a transgression of over 135 m occurred at the beginning of the Holocene. The transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern Lago Cardiel at four different elevations. The preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level. There is no seismic evidence of a major lowering of the lake below modern level during the entire Holocene. Deposition since the mid-Holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution. Seismic reflection geometries, as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80-100 km2 . As Lago Cardiel is under the influence of westerly winds, these most likely drove lake circulation. The identification of drowned beach ridges and of contourite drifts illustrates that high-resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins, but it can also provide information about the rate of lake level changes and the presence and strength of lake currents. © 2004 International Association of Sedimentologists.

cfe842d5-5f41-3158-a562-f6a35d1a32fa
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ID: cfe842d5-5f41-3158-a562-f6a35d1a32fa
Tag: []
Abstract: The paleoclimatic evolution of southern South America is characterized to a large extent by the behavior (strength and latitudinal position) of the storm tracks of the Southern Westerlies. Our study site, Lago Cardiel (49°S), lies within the modern influence of the Southern Westerlies and, therefore, is ideally located to track the past migrations of these storm tracks. With a coring strategy taking into account the lateral differences in sedimentation and an excellent core-to-core correlation using tephra layers, a composite sedimentological record of almost 25 m was established covering the last ∼16,000 cal yr. Sedimentological and petrophysical analysis of the cores revealed the establishment of a dominant lake current since 6800 cal yr BP leading to a drift deposition, which is especially well-expressed in the sedimentary record by an increase in magnetic susceptibility values. As this pattern of currents is most likely induced by wind activity, we propose that the observed increase in magnetic susceptibility documents an intensification of the westerly storm tracks. This intensification occurred slightly earlier than previously suggested based on palynological evidence. The strengthening in the Southern Westerlies during the mid-Holocene is most likely caused by an increase in the temperature gradient as a result of enhanced influence and/or southward migration of the Southeast Pacific anticyclone and a larger Antarctic sea-ice extent. © 2005 Elsevier B.V. All rights reserved.

d63b154f-20e7-3fb7-a9e2-1d7552a12cee
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ID: d63b154f-20e7-3fb7-a9e2-1d7552a12cee
Tag: []
Abstract: Changes in heat balance, wind stress, and lake level can affect the efficiency of mixing and depth of oxygen penetration in deep tropical lakes. This study examines how transient changes in these parameters, as reflected by the redox state of bottom waters, are recorded in the sedimentary record of Lake Malawi. We measured concentration profiles of redox-sensitive metals across a layer of oxidized sediment deposited within a sequence composed of reduced sediments in a core taken at 298 m water depth, ~100 m below the present-day chemocline. By examining a range of metals that exhibit different geochemical behaviors- mobilization under reducing (Fe, Mn) versus. oxidizing (U) conditions; formation of insoluble sulfides (Cd, Mo, Pb, Zn); susceptibility to scavenging onto Fe or Mn oxyhydroxide precipitates (Co, Mo, Pb, V, Zn)-we are able to outline a sequence of events consistent with the formation of this feature: (1) At some time before 11 ky BP there was an episode of penetration of oxygenated waters to depths close to or greater than that of this core, i.e., substantially deeper in the lake basin than at present. At this time diagenetically remobilized Fe and Mn were oxidized and precipitated in a near-surface sediment horizon. (2) Fe and Mn oxyhydroxides that precipitated from the water column as metal-rich anoxic waters were mixed upward into oxygenated waters, were deposited in an immediately overlying layer of the sediment. Such material continued to accumulate in surface sediments for a few hundred years until oxic conditions no longer prevailed in overlying waters. (3) As the depth of the chemocline rose (relative to the present-day lake surface) this authigenic material was, in turn, overlain by reducing sediments. (4) Subsequent diagenetic remobilization at the top and bottom of this 'redox sandwich' resulted in formation of horizons enriched in, U, Mo, and sulfide-forming metals (notably Cd) along the redox gradients both above and, to a lesser extent, below the layer of oxidized sediment. These enrichments, immobilized under reducing conditions, will remain as permanent markers of the episode of deepening of the chemocline even after reduction and remobilization of the oxidized Fe and Mn solids. In addition, such markers may be useful in determining whether observed episodic interruptions in deposition of varved sediments in the deep anoxic basins of Lake Malawi are due to bioturbation brought on by episodes of deeper oxygen penetration in the water column. Copyright (C) 2000 Elsevier Science Ltd.

b0128a3a-bd11-3612-9e3c-30ee71e2cdf9
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ID: b0128a3a-bd11-3612-9e3c-30ee71e2cdf9
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Abstract: In the present study, we compare modern and Quaternary ostracods from two lacustrine basins: Laguna Cari-Laufquen (41°S) and Lago Cardiel (49°S) in Patagonia. Taxonomic and quantitative analyses along with isotopic and chemical studies of the extant ostracod fauna indicate that distinct ostracod associations can be identified as a function of conductivity. Three ostracod associations can be distinguished: (1) springs, ponds and small creeks, characterized by low conductivity (e.g. 1015μscm-1); (2) lakes and permanent ponds, characterized by medium conductivity (e.g. 1625μscm-1) and (3) ephemeral lacustrine environments, generally characterized by higher conductivity (e.g. 16480μscm-1) These modern ostracod associations were also identified in older sequences from sediments outcropping in the Laguna Cari-Laufquen current shoreline, as well as in sediment cores from Lago Cardiel. The predominance of Limnocythere rionegroensis Cusminsky & Whatley in the Cari-Laufquen sections suggests the development of a saline and turbid lake during the Late Pleistocene and Early Holocene, and thus higher precipitation at these latitudes. Changes in ostracod abundance and associations have been observed in Lago Cardiel during the last approximately 16000 calibrated years BP. Conductivity is known to change as a function of the ratio of precipitation to evaporation and a decrease in conductivity from the Late Pleistocene to the Middle Holocene suggests substantial hydrological variations (i.e. increase of the precipitation/evaporation ratio suggests minor conductivity). These two examples show that ostracods provide an excellent proxy for interpreting palaeoclimatic and palaeoenvironmental changes in Patagonia. © 2011 The Linnean Society of London.

26550557-fdc2-3fcc-9975-186169cac2f0
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Abstract: A suite of piston cores recovered from Lake Malawi (9-14°S, 34-35°E), east Africa in 1986 has been analyzed for major and minor elements, organic C and N, calcium carbonate and diatoms. An internally consistent stratigraphy was constructed from calcium carbonate abundance and variations in the two most abundant diatom genera, Stephanodiscus and Melosira, with age control obtained primarily from 14C dating of the carbonate. Differences with time in Fe abundance in a transect of cores from different water depths have been interpreted to reflect changes in chemocline depth. The depth to the chemocline was on the order of 100 m shallower than present prior to 3500 yr B.P., indicating less seasonality. Carbonate production and preservation appears to be related to climatically induced changes in both salinity and chemical distributions in the water column. The carbonate, which precipitates from surface waters, is most abundant during the interval from about 10,000 to 6000 yr B.P. This micrite most likely represents periods of low lake level when salinity increased and carbonate precipitation was enhanced. Sedimentary evidence suggests that lake levels were 100-150 m lower than present during this period. This record is different from climatic trends in northern intertropical Africa, but appears to also be related to changes in insolation and monsoon circulation. This is the northernmost basin in Africa reported to exhibit a "southern hemisphere" response to the early Holocene northern hemisphere summer insolation maximum. The climatic hingeline north of Lake Malawi (∼ 9°S) implied by our results is significantly south of that indicated by general-circulation model simulations, however. The cores show evidence for periods of abrupt climate change during the interval of generally arid climate. © 1991.

a8b5b2f5-cfab-387a-a530-d72ed0ef7c16
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Abstract: Comparison of high-resolution paleoclimate records from Lake Malawi and the Cariaco Basin reveals a consistent tropical behavior during the Little Ice Age. These records, which are interpreted to represent southward excursions of intertropical convergence zone position over southern tropical East Africa and northeastern South America, are characterized by subcentennial variability superimposed on a ramp of increasing intensity from CE 1400 to 1800. This response appears similar to that of " cold" episodes during the Late Glacial. The synchrony of these records suggests a teleconnection in tropical climate that may be a manifestation of persistent ENSO-like conditions. Copyright 2005 by the American Geophysical Union.

cc574af9-a027-35d5-b49c-da44b1fc4376
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Abstract: Lake Victoria, the largest tropical lake in the world, suffers from severe eutrophication and the probable extinction of up to half of its 500+ species of endemic cichlid fishes. The continuing degradation of Lake Victoria's ecological functions has serious long-term consequences for the ecosystem services it provides, and may threaten social welfare in the countries bordering its shores. Evaluation of recent ecological changes in the context of aquatic food-web alterations, catchment disturbance and natural ecosystem variability has been hampered by the scarcity of historical monitoring data. Here, we present high-resolution palaeolimnological data, which show that increases in phytoplankton production developed from the 1930s onwards, which parallels human-population growth and agricultural activity in the Lake Victoria drainage basin. Dominance of bloom-forming cyanobacteria since the late 1980s coincided with a relative decline in diatom growth, which can be attributed to the seasonal depletion of dissolved silica resulting from 50 years of enhanced diatom growth and burial. Eutrophication-induced loss of deep-water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for certain deep-water cichlids. Conservation of Lake Victoria as a functioning ecosystem is contingent upon large-scale implementation of improved land-use practices.

9e02499d-caeb-3576-8106-f9bb9ce6cd1d
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Abstract: High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi. The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. 14C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr BP for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr BP, 28 000 to >40 000 yr BP, and prior to 78 000 yr BP. Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg l-1, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment. -from Authors

38b57512-9ae2-3e32-851f-59339983b379
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ID: 38b57512-9ae2-3e32-851f-59339983b379
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Abstract: High-resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern Lake Malawi provide a climate signal for this part of tropical Africa spanning the past 25,000 years. The biogenic silica mass accumulation rate was low during the relatively dry late Pleistocene, when the river flux of silica to the lake was suppressed. Millennial-scale fluctuations, due to upwelling intensity, in the late Pleistocene climate of the Lake Malawi basin appear to have been closely linked to the Northern Hemisphere climate until 11 thousand years ago. Relatively cold conditions in the Northern Hemisphere coincided with more frequent north winds over the Malawi basin, perhaps resulting from a more southward migration of the Intertropical Convergence Zone.

4785e876-2178-3259-b3c1-cef9e168f10a
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Abstract: New AMS radiocarbon dates and stratigraphic correlations of 7 piston cores recovered from Lake Turkana, East Africa, show that the frequency of cyclic variability is greater than previously reported for a single piston core. Almost 60 new AMS dates of various carbonate fractions indicate that large (150 μm) ostracode carapaces yield the most accurate chronology for the sediment record due to the input of "old" fine-grained carbonate. Stratigraphic correlations among the cores by carbonate abundance and magnetic susceptibility profiles provide supporting evidence for the ostracode-based chronology. This chronology refines earlier estimates of sediment accumulation that impacts the recurrence interval of lamination deposition from one lamination every two years to one lamination per year in two north basin cores. The record of carbonate content within the north basin of the lake reflect its dilution with terrigenous sediments input from the Omo River but direct paleoclimatic interpretations are complicated by lateral migrations of the delta channels with changing lake levels. Time-series analysis of both individual cores and a combined carbonate profile show significant spectral peaks at 1000, 76, 32, 22, 18.6 and 11 yr. The latter three suggest an important link between the sediments accumulating in Lake Turkana and global cycles of climatic variability. © 1994.

4a4d7a8f-f93f-3937-b803-d21fb4a392bc
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Abstract: Three ice cores recovered on or near Mount Logan, together with a nearby lake record (Jellybean Lake), cover variously 500 to 30 000 years. This suite of records offers a unique view of the lapse rate in stable isotopes from the lower to upper troposphere. The region is climatologically important, being beside the Cordilleran pinning-point of the Rossby Wave system and the Aleutian Low. Comparison of stable isotope series over the last 2000 years and model simulations suggest sudden and persistent shifts between modern (mixed) and zonal flow regimes of water vapour transport to the Pacific Northwest. The last such shift was in A.D. 1840. Model simulations for modern and "pure" zonal flow suggest that these shifts are consistent regime changes between these flow types, with predominantly zonal flow prior to ca. A.D. 1840 and modern thereafter. The 5.4 and 0.8 km asl records show a shift at A.D. 1840 and another at A.D. 800. It is speculated that the A.D. 1840 regime shift coincided with the end of the Little Ice Age and the A.D. 800 shift with the beginning of the European Medieval Warm Period. The shifts are very abrupt, taking only a few years at most.

1f7ad15f-7a58-3c04-a6b9-74bc4a433363
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Abstract: 

80d06c43-302d-3dd1-8b74-ed0d15baeb39
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ID: 80d06c43-302d-3dd1-8b74-ed0d15baeb39
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Abstract: The Pantanal is the world's largest tropical wetland and a biodiversity hotspot, yet its response to Quaternary environmental change is unclear. To address this problem, sediment cores from shallow lakes connected to the Upper Paraguay River (PR) were analyzed and radiocarbon dated to track changes in sedimentary environments. Stratal relations, detrital particle size, multiple biogeochemical indicators, and sponge spicules suggest fluctuating lake-level lowstand conditions between ~11,000 and 5300. cal. yr BP, punctuated by sporadic and in some cases erosive flood flows. A hiatus has been recorded from ~5300 to 2600. cal. yr BP, spurred by confinement of the PR within its channel during an episode of profound regional drought. Sustained PR flooding caused a transgression after ~2600. cal. yr BP, with lake-level highstand conditions appearing during the Little Ice Age. Holocene PR flood pulse dynamics are best explained by variability in effective precipitation, likely driven by insolation and tropical sea-surface temperature gradients. Our results provide novel support for hypotheses on: (1) stratigraphic discontinuity of floodplain sedimentary archives; (2) late Holocene methane flux from Southern Hemisphere wetlands; and (3) pre-colonial indigenous ceramics traditions in western Brazil. © 2012.

9bb52eaf-65d7-386f-ad86-b59ea84e5008
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Abstract: Analyses of pollen and plant macrofossils from a new core spanning the past 60,000 years from Lake Tulane, Florida show a strong antiphase relationship in temperature between Florida and the North Atlantic region. During the Pleistocene, oak-scrub and prairie phases were coeval with long, intense Dansgaard-Oeschger interstadials (warm periods) that initiated Bond cycles. Pine phases were coeval with the North Atlantic long stadials (cold periods) that ended Bond cycles and were terminated by Heinrich events. Lake levels were higher during pine phases, and climate was wetter. However, climate in Florida was also warmer during these phases, which were cold periods in the North Atlantic. Perhaps diminution of thermohaline circulation before and during Heinrich events reduced northward heat transport and retained warmth in the subtropical Atlantic and Gulf of Mexico. © 2006 Elsevier Ltd. All rights reserved.

b3cd350d-c81e-3455-a5fd-ce8475fd93f3
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Abstract: Geological evidence for modeled Younger Dryas ice expansion in northern Maine is assessed in conjunction with temperature and precipitation estimates from chironomids and pollen, and plant macrofossil and lake-level analyses from lake sediment. Pollen and chironomid temperature and precipitation transfer-function estimates for the Allerød warming period indicate colder winters, precipitation levels half that of modern times, and summer temperatures near modern levels. The combination of cold winters and low precipitation prevented forest establishment in northern Maine along the Maine/New Brunswick border. While winter temperatures and precipitation remained stable, summer temperatures decreased as much as 7.5 °C during the Younger Dryas stadial, forcing a shift from shrub-dominated to sedge-dominated tundra. Summer and winter temperatures, as well as annual precipitation, increased rapidly at the Holocene onset.

f3ae712b-ac7d-359a-8e48-45ee13fe9692
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Abstract: The fossil diatom record from Elk Lake (Grant County, Minnesota) was used to reconstruct salinity and brine type between 2640 and 4645 14C yr BP. This lake was selected for a brine-type reconstruction because a previous study using fossil-ostracode assemblages indicated a shift in anion composition during the mid-Holocene (Smith et al., 1997). Salinity was reconstructed using a transfer function developed for the Northern Great Plains (NGP) of North America; the reconstruction revealed that salinity was higher (1.5-6.2 g l-1) between ~4000 and 4645 14C yr BP and dropped to 0.35-1.2 g l-1 after 4000 14C yr BP. The anion composition of the system was investigated by passively plotting fossil diatom assemblages onto a canonical correspondence analysis (CCA) biplot of the NGP modern samples to determine where core assemblages fell with respect to brine type. The biplot suggests that Elk Lake was mainly a bicarbonate system, but temporarily shifted to sulfate domination at 4080 14C yr BP. Both the salinity and brine-type reconstructions essentially agree with results from Smith et al. (1997), but the diatom record provides less-definitive information on anion proportions as compared to anion concentrations. Because shifts in the relative abundances of anion-associated diatom taxa generally tracked the ostracode-inferred changes in brine type, we conclude that fossil diatom assemblages can reveal information on shifts in brine type over time and provide insight into brine evolution and groundwater behavior in a lake system. (C) 2000 Elsevier Science Ltd and INQUA. All rights reserved.

341dcdf5-7e0b-3d72-aaa2-78796690927d
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Abstract: Sediment cores from Lake Zeribar in the Zagros Mountains of western Iran provide a record of changes in the diatom flora since about 40 ka. Conductivity transfer functions applied to diatom records from the lake indicate generally higher conductivity during the Last Glacial Maximum and Lateglacial, especially between 15.0 and 10.5 ka. This observation is consistent with previous interpretations of relatively arid conditions during this interval based on the Lake Zeribar pollen records. Oscillations between high- and low-conductivity states during this interval additionally suggest changes in precipitation associated with millennial-scale climate events, with relatively dry conditions during the Younger Dryas. Diatom-inferred conductivity is relatively low throughout the Holocene. Shifts in diatom composition occur in the Holocene record; however, minor variations in reconstructed conductivity are not considered significant.

0e81cad7-470d-3a46-85a5-a726515afccb
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Abstract: Widespread drought is among the most likely and devastating consequences of future global change. Assessment of drought impacts forecast by atmospheric models requires an understanding of natural drought variability, especially under conditions more arid than today. Using high-resolution lake-sediment records from the northern Great Plains, we show pronounced 100- to 130-yr drought cycles during the arid middle Holocene (8000 calendar yr BP). During drought phases, grass productivity declined, erosion and forbs increased, and fuel limitation reduced fire importance. Intervening humid decades saw grass production rise, with stabilization of soils and renewed fire as fuels became abundant. Although both C3 and C4 grasses declined during droughts, a lasting shift to C3 dominance occurred during a single drought ∼8200 calendar yr BP. During the more humid Late Holocene (2800 calendar yr BP), climate was less variable and without evident drought cyclicity. Consequently, drought severity during past, and possibly future, arid phases cannot be anticipated from the attenuated climate variability evident during contemporary humid phases. Our study demonstrates that agriculturally important grassland ecosystems respond sensitively to drought variability, uncertainty in which has profound implications for the future of these ecosystems.

81f0a424-7c6c-3a3b-bc50-22ade4ddba42
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Abstract: We present and compare AMS-14C geochronologies for sediment cores recovered from Lake Titicaca, South America. Radiocarbon dates from three core sites constrain the timing of late Quaternary paleoenvironmental changes in the Central Andes and highlight the site-specific factors that limit the radiocarbon geochronometer. With the exception of mid-Holocene sediments, all cores are generally devoid of macrophyte fragments, thus bulk organic fractions are used to build core chronologies. Comparisons of radiocarbon results for chemically defined fractions (bulk decalcified, humate, humin) suggest that ages derived from all fractions are generally coherent in the post-13,500 yr BP time interval. In the pre-13,500 yr BP time interval, ages derived from humate extracts are significantly younger (300-7000 years) than ages from paired humin residues. Gross age incoherencies between paired humate and humin sub-fractions in pre-13,500 yr BP sediments from all core sites probably reflect the net downward migration of humates. Ages derived from bulk decalcified fractions at our shallow water (90 m) and deep water (230 m) core sites consistently fall between ages derived from humate and humin sub-fractions in the pre-13,500 yr BP interval, reflecting that the bulk decalcified fraction is predominantly a mixture of humate and humin sub-fractions. Bulk decalcified ages from the pre-13,500 yr BP interval at our intermediate depth core site (150 m) are consistently older than humate (youngest) and humin sub-fractions. This uniform, reproducible pattern can be explained by the mobilization of a relatively older organic sub-fraction during and after the re-acidification step following the alkaline treatment of the bulk sediment. The inferred existence of this 'alkali-mobile, acid-soluble' sub-fraction implies a different depositional/post-depositional history that is potentially associated with a difference in source material. While internally consistent geochronologies can be developed for the Lake Titicaca sequence using different organic fractions, mobile organic sub-fractions and fractions containing mobile sub-fractions should generally be avoided in geochronology studies. Consequently, we believe humin and/or bulk decalcified ages provide the most consistent chronologies for the post-13,500 yr BP interval, and humin ages provide the most representative ages for sedimentation prior to 13,500 yr BP interval. Using the age model derived from the deep water core site and a previously published isotope-based lake-level reconstruction, we present a qualitative record of lake level in the context of several ice-core records from the western hemisphere. We find the latest Pleistocene lake-level response to changing insolation began during or just prior to the Bølling/Allerød period. Using the isotope-based lake-level reconstruction, we also find the 85-m drop in lake level that occurred during the mid-Holocene was synchronous with an increase in the variability of ice-core δ18O from a nearby icecap, but was not reflected in any of the polar ice-core records recovered from the interior of Antarctica and Greenland. © 2003 Published by Elsevier B.V.

9f6f4e9e-1720-3e11-b524-5c6f35c2ed00
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ID: 9f6f4e9e-1720-3e11-b524-5c6f35c2ed00
Tag: []
Abstract: Despite the hypothesized importance of the tropics in the global climate system, few tropical paleoclimatic records extend to periods earlier than the last glacial maximum (LGM), about 20,000 years before present. We present a well-dated 170,000-year time series of hydrologic variation from the southern hemisphere tropics of South America that extends from modern times through most of the penultimate glacial period. Alternating mud and salt units in a core from Salar de Uyuni, Bolivia reflect alternations between wet and dry periods. The most striking feature of the sequence is that the duration of paleolakes increased in the late Quaternary. This change may reflect increased precipitation, geomorphic or tectonic processes that affected basin hydrology, or some combination of both. The dominance of salt between 170,000 and 140,000 yr ago indicates that much of the penultimate glacial period was dry, in contrast to wet conditions in the LGM. Our analyses also suggest that the relative influence of insolation forcing on regional moisture budgets may have been stronger during the past 50,000 years than in earlier times. © 2003 University of Washington. All rights reserved.

2a2ed2de-b331-31fc-944d-19c5e5d47888
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ID: 2a2ed2de-b331-31fc-944d-19c5e5d47888
Tag: []
Abstract: A diatom-based transfer function was used to reconstruct water chemistry before European settlement in 55 Minnesota lakes. The lakes span three natural ecoregions, which differ in their history of land use, as well as in surficial geology, climate, and vegetation. Postsettlement trends were compared with water chemistry change reconstructed from two presettlement core sections (circa 1750 and 1800) as a measure of natural variability. Presettlement water quality changes were generally small and nondirectional in all three ecoregions. In contrast, half of the urban lakes showed a statistically significant increase in chloride, whereas 30% of urban and 30% of agricultural region lakes record a statistically significant increase in total phosphorus between 1800 and the present. These changes, which are attributed to road salt and nutrient runoff, are strongly correlated with the percentage of watershed area that is developed (residential or urban) in the case of chloride increases and the percentage of developed (metropolitan areas) or agricultural (agricultural areas) land in the case of nutrient increases. Water quality has changed little since 1800 for lakes in the forested regions of northeastern Minnesota. The few changes that are seen in this region are likely related to natural variations in climate or catchment soils.

0f228f6b-181d-3a60-82ab-c96b3d81cc86
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ID: 0f228f6b-181d-3a60-82ab-c96b3d81cc86
Tag: []
Abstract: A growing number of sites in the Northern Hemisphere show centennial- to millennial-scale climate variation that has been correlated with change in solar variability or with change in North Atlantic circulation. However, it is unclear how (or whether) these oscillations in the climate system are manifest in the Southern Hemisphere because of a lack of sites with suitably high sampling resolution. In this paper, we reconstruct the lake-level history of Lake Titicaca, using the carbon isotopic content of sedimentary organic matter, to evaluate centennial- to millennial-scale precipitation variation and its phasing relative to sites in the Northern Hemisphere. The pattern and timing of lake-level change in Lake Titicaca is similar to the ice-rafted debris record of Holocene Bond events, demonstrating a possible coupling between precipitation variation on the Altiplano and North Atlantic sea-surface temperatures (SSTs). The cold periods of the Holocene Bond events correspond with periods of increased precipitation on the Altiplano. Holocene precipitation variability on the Altiplano is anti-phased with respect to precipitation in the Northern Hemisphere monsoon region. More generally, the tropical Andes underwent large changes in precipitation on centennial-to-millennial timescales during the Holocene. Copyright © 2005 John Wiley & Sons, Ltd.

debeb6ff-13af-3a7a-9b3c-9ecec2c174aa
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ID: debeb6ff-13af-3a7a-9b3c-9ecec2c174aa
Tag: []
Abstract: A 90,000-yr record of environmental change before 18,000 cal yr B.P. has been constructed using pollen analyses from a sediment core obtained from Salar de Uyuni (3653 m above sea level) on the Bolivian Altiplano. The sequence consists of alternating mud and salt, which reflect shifts between wet and dry periods. Low abundances of aquatic species between 108,000 and 50,000 yr ago (such as Myriophyllum and Isoëtes) and marked fluctuations in Pediastrum suggest generally dry conditions dominated by saltpans. Between 50,000 yr ago and 36,000 cal yr B.P., lacustrine sediments become increasingly dominant. The transition to the formation of paleolake "Minchin" begins with marked rises in Isoëtes and Myriophyllum, suggesting a lake of moderate depth. Similarly, between 36,000 and 26,000 cal yr B.P., the transition to paleolake Tauca is also initiated by rises in Isoëtes and Myriophyllum; the sustained presence of Isoëtes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level. Polylepis tarapacana-dominated communities were probably an important component of the Altiplano terrestrial vegetation during much of the Last Glacial Maximum (LGM) and previous wet phases. © 2004 University of Washington. All rights reserved.

e8f37a3d-03ef-3d81-bfe4-ddb86019ccc2
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ID: e8f37a3d-03ef-3d81-bfe4-ddb86019ccc2
Tag: []
Abstract: Time series analysis of a diatom-inferred drought record suggests that Holocene hydroclimate of the northern Rocky Mountains has been characterized by oscillation between two mean climate states. The dominant climate state was initiated at the onset of the Holocene (ca. 11 ka); under this climate state, drought was strongly cyclic, recurring at frequencies that are similar to twentieth century multidecadal phase changes of the Pacific Decadal Oscillation. This pattern remained consistent throughout much of the mid-Holocene, continuing until ca. 4.5 ka. After this time the mean climate state changed, and drought recurrence became unstable; periods of cyclic drought alternated with periods of less predictable drought. The timing of this shift in climate was coincident with widespread severe drought in the mid-continent of North America. Overall, the strongest periodicity in severe drought occurred during the mid-Holocene, when temperatures in the northern Rocky Mountains were warmer than today. © 2006 Geological Society of America.

c7e0e070-a286-30bb-9703-62d45a94c5eb
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ID: c7e0e070-a286-30bb-9703-62d45a94c5eb
Tag: []
Abstract: Several abrupt climate events during the Holocene, including the widely documented oscillation at 8.2 thousand years before present (ka), are attributed to changes in the North Atlantic thermohaline circulation. Additional mechanisms, such as interactions between atmospheric circulation, ice-sheet dynamics, and the influence of solar irradiance, also have been proposed to explain abrupt climatic events, but evidence remains elusive. This study presents evidence from multi-proxy analyses on the Holocene sediments of Eleanor Lake, interior British Columbia. Climatic inferences from our decadal-resolution record of biogenic silica (BSi) abundance are supported by changes in diatom and pollen assemblages from the same core and correlations with existing regional climate records. The BSi record reveals abrupt and persistent climatic shifts at 10.2, 9.3, and 8.5. ka, the latter two of which are coeval with major collapses of the Laurentide Ice Sheet. The record also reveals a short-term cooling at 8.2. ka that is distinct from the 8.5. ka event and similar in magnitude to several other late-Holocene coolings. BSi is correlated with solar-irradiance indices (r=0.43-0.61), but the correlation is opposite in sign to that expected from direct solar forcing and weakens after 8. ka. Possible mechanisms causing the abrupt and persistent climate changes of the early Holocene include 1) sudden losses of ice and proglacial lake extent, causing a shift in the meridional structure of atmospheric circulation, 2) a possible link between solar minima and El Niño-like conditions that are correlated with warm spring temperature in interior British Columbia, and 3) the influence of solar irradiance variability on the position of the polar jet, possibly via effects on the strength of the glacial anticyclone. © 2011 Elsevier Ltd.

22367ac3-c2ac-35a3-8636-1919a5b0e5ac
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ID: 22367ac3-c2ac-35a3-8636-1919a5b0e5ac
Tag: []
Abstract: 

049907fa-6206-30ab-9bac-ba6753f52500
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ID: 049907fa-6206-30ab-9bac-ba6753f52500
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Abstract: Between October 2008 and May 2009, the International Continental Scientific Drilling Program (ICDP) co-sponsored a campaign at Lake El'gygytgyn, located in a 3.6-Ma-old meteorite impact crater in northeastern Siberia. Drilling targets included three holes in the center of the 170-m-deep lake, utilizing the lake ice cover as a drilling platform, plus one hole close to the shore in the western lake catchment. At the lake's center. the entire 315-m-thick lake sediment succession was penetrated. The sediments lack any hiatuses (i.e., no evidence of basin glaciation or desiccation), and their composition reflects the regional climatic and environmental history with great sensitivity. Hence, the record provides the first comprehensive and widely timecontinuous insights into the evolution of the terrestrial Arctic since mid-Pliocene times. This is particularly true for the lowermost 40 meters and uppermost 150 meters of the sequence, which were drilled with almost 100% recovery and likely reflect the initial lake stage during the Pliocene and the last ~2.9 Ma, respectively. Nearly 200 meters of underlying rock were also recovered; these cores consist of an almost complete section of the various types of impact breccias including broken and fractured volcanic basement rocks and associated melt clasts. The investigation of this core sequence promises new information concerning the El'gygytgyn impact event, including the composition and nature of the meteorite, the energy released, and the shock behavior of the volcanic basement rocks. Complementary information on the regional environmental history, including the permafrost history and lake-level fluctuations, is being developed from a 142-m-long drill core recovered from the permafrost deposits in the lake catchment. This core consists of gravelly and sandy alluvial fan deposits in ice-rich permafrost, presumably comprising a discontinuous record of both Quaternary and Pliocene deposits.

ea1d8d03-050d-30cc-a8a0-5a59fdcdf3ab
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ID: ea1d8d03-050d-30cc-a8a0-5a59fdcdf3ab
Tag: []
Abstract: The hydrological position of a lake within the landscape can affect a number of lake chemical, physical, and biological features, as well as how lakes respond to environmental change. We present a paleolimnological test of the model for landscape position and lake response to climate change proposed by Webster et al. (2000). To investigate how diatom communities have responded to drought relative to landscape position, we examined sedimentary diatom profiles extending through the twentieth century from an upland site (Crystal Lake) and a lowland site (Allequash Lake) in the Northern Highlands region of north-central Wisconsin (USA). To explore changes in diatom communities at each site, we developed a calibration set and transfer functions from 48 lakes in Wisconsin’s Northern Highland Lake District. We further determined planktic:benthic ratios in the two target lakes, developed lake level models, and investigated the sensitivity of planktic:benthic diatom ratios to climatic variability over the past century. In the upland lake, diatom communities responded indirectly to climate via drought-induced changes in lake level, which resulted in shifts in planktic versus benthic habitat availability. This response of diatoms to changes in habitat availability provides an alternative approach for tracking climate change in upland lakes, though careful consideration must be given to the effect of the bathymetry and its relationship to lake level change and habitat zonation at individual sites. In the lowland lake, changes in diatom communities were related to temperature (and possibly lakewater chemistry) and physical changes secondarily. These results are consistent with the model by Webster et al. (2000), with chemical changes occurring in the lowland system and little chemical response in the upland system. However, the biological changes in sediment records presented here provide additional insight into how lake response to climatic change is shaped by landscape position, contributing to a clearer understanding of potential changes in ecosystem structure and function during drought conditions.

0c7bcb7a-c106-3ec4-a37d-c26c17f39924
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ID: 0c7bcb7a-c106-3ec4-a37d-c26c17f39924
Tag: []
Abstract: Lake sediment oxygen isotope records (calcium carbonate-δ18O) in the western North American Cordillera developed during the past decade provide substantial evidence of Pacific ocean-atmosphere forcing of hydroclimatic variability during the Holocene. Here we present an overview of 18 lake sediment δ18O records along with a new compilation of lake water δ18O and δ2H that are used to characterize lake sediment sensitivity to precipitation-δ18O in contrast to fractionation by evaporation. Of the 18 records, 14 have substantial sensitivity to evaporation. Two records reflect precipitation-δ18O since the middle Holocene, Jellybean and Bison Lakes, and are geographically positioned in the northern and southern regions of the study area. Their comparative analysis indicates a sequence of time-varying north-south precipitation-δ18O patterns that is evidence for a highly non-stationary influence by Pacific ocean-atmosphere processes on the hydroclimate of western North America. These observations are discussed within the context of previous research on North Pacific precipitation-δ18O based on empirical and modeling methods. The Jellybean and Bison Lake records indicate that a prominent precipitation-δ18O dipole (enriched-north and depleted-south) was sustained between ~3.5 and 1.5ka, which contrasts with earlier Holocene patterns, and appears to indicate the onset of a dominant tropical control on North Pacific ocean-atmosphere dynamics. This remains the state of the system today. Higher frequency reversals of the north-south precipitation-δ18O dipole between ~2.5 and 1.5ka, and during the Medieval Climate Anomaly and the Little Ice Age, also suggest more varieties of Pacific ocean-atmosphere modes than a single Pacific Decadal Oscillation (PDO) type analogue. Results indicate that further investigation of precipitation-δ18O patterns on short (observational) and long (Holocene) time scales is needed to improve our understanding of the processes that drive regional precipitation-δ18O responses to Pacific ocean-atmosphere variability, which in turn, will lead to a better understanding of internal Pacific ocean-atmosphere variability and its response to external climate forcing mechanisms.

17231005-2020-3929-ab68-d18e4488e2d8
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ID: 17231005-2020-3929-ab68-d18e4488e2d8
Tag: []
Abstract: Middle-Holocene (8 to 4ka BP) warmth and aridity are well recorded in sediment archives from midcontinental North America. However, neither the climatic driver nor the seasonal character of precipitation during this period is well understood because of the limitations of available proxy indicators. For example, an important challenge is to distinguish among the interacting effects of evaporation, temperature, or precipitation seasonality in existing δ18O records from the region. Here we combine hydrogen isotopes of palmitic acid and oxygen isotopes of carbonate to derive lake-water isotopic values during the Holocene at Steel Lake in north-central Minnesota. In combination, these data enable us to separate variations in evaporation from variations in the isotopic composition of input-waters to lake. Variations in evaporation are used as a proxy for aridity and lake-water input isotopic values are used as a proxy for the isotopic values of meteoric precipitation. Our results suggest that lake-water input isotopic values were more negative during the middle Holocene than at present. To test whether these more negative values are related to temperature or precipitation seasonality, we compare pollen-inferred temperatures and the expected isotopic value of precipitation resulting from these temperatures to the reconstructed precipitation isotopic values. Results suggest that middle Holocene warmth and aridity were associated with increased evaporation rates and decreased summer precipitation. These inferences are consistent with climate simulations that highlight the role of seasonal insolation and sea surface temperatures in driving variations in precipitation seasonality during the Holocene. Results also suggest that changes in Holocene precipitation seasonality may have influenced the expansion of the prairie-forest border in Minnesota as well as regional variations in grassland community composition. This study demonstrates the efficacy of the dual hydrogen and oxygen isotope approach for distinguishing changes in evaporation and precipitation seasonality in the paleolimnological record. © 2010 Elsevier B.V.

028bf3f0-fc33-3804-83aa-3d7106200601
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ID: 028bf3f0-fc33-3804-83aa-3d7106200601
Tag: []
Abstract: The prairie-forest transition in midcontinental North America is a major physiognomic boundary, and its shifts during the Holocene are a classic example of climate-driven ecotonal dynamics. Recent work suggests asymmetrical Holocene behavior, with a relatively rapid early Holocene deforestation and more gradual reforestation later in the Holocene. This paper presents a new synthesis of the Holocene history of the Great Plains prairie-forest ecotone in the north-central US and central Canada that updates prior mapping efforts and systematically assesses rates of change. Changes in percent woody cover (%WC) are inferred from fossil pollen records, using the modern analog technique and surface-sediment pollen samples cross-referenced against remotely sensed observations. For contemporary pollen samples from the Great Plains, %WC linearly correlates to percent arboreal pollen (%AP), but regression parameters vary interregionally. At present, %AP is consistently higher than %WC, because of high background levels of arboreal pollen. Holocene maps of the eastern prairie-forest ecotone agree with prior maps, showing a rapid decrease in %WC and eastward prairie advance between 10,000 and 8000 ka (1 ka = 1000 calibrated years before present), a maximum eastward position of the ecotone from 7 to 6 ka, and increased %WC and westward prairie retreat after 6 ka. Ecotone position is ambiguous in Iowa and southeastern Minnesota, due to a scarcity of modern analogs for early-Holocene samples with high Ulmus abundances and for samples from alluvial sediments. The northern prairie-forest ecotone was positioned in central Saskatchewan between 12 and 10 ka, stabilized from 10 to 6 ka despite decreases in %WC at some sites, then moved south after 6 ka. In both east and north, ecotonal movements are consistent with a dry early Holocene and increasing moisture availability after 6 ka. Sites near the ecotone consistently show an asymmetric pattern of abrupt early Holocene deforestation (< 300 years) and gradual reforestation after 6 ka. Early Holocene decreases in %WC are faster than the corresponding drops in %AP, because the analog-based %WC reconstructions correct for the high background levels of arboreal pollen types that blur temporal variations in %AP. For example, at Elk Lake, the %AP decline lasts 1000 years, whereas the %WC decline occurs between adjacent pollen samples, approximately 300 years apart. Thus, early Holocene deforestation may have been even more abrupt than previously recognized. Rapid deforestation likely was promoted both by rapid climate changes around 8.2 ka and positive fire-vegetation feedbacks. Non-linear vegetational responses to hydrological variability are consistent with 1) other paleorecords showing rapid die-offs of some eastern tree species in response to aridity and 2) observations of threshold-type ecological responses to recent climate events. The 21st-century trajectory for the Great Plains prairie-forest ecotone is uncertain, because climate models differ over the direction of regional precipitation trends, but future drying would be more likely to trigger threshold-type shifts in ecotone position. © 2009 Elsevier B.V. All rights reserved.

99e6a934-a385-357b-9266-ec79fcd17c73
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ID: 99e6a934-a385-357b-9266-ec79fcd17c73
Tag: []
Abstract: Palynological studies document forest disappearance during the late Holocene in the tropical Maya lowlands of northern Guatemala. The question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation, as previously suggested, or whether it was partly attributable to climate changes. We report multiple palaeoclimate and palaeoenvironment proxies (pollen, geochemical, sedimentological) from sediment cores collected in Lake Petén Itzá, northern Guatemala. Our data indicate that the earliest phase of late Holocene tropical forest reduction in this area started at ∼ 4500 cal yr BP, simultaneous with the onset of a circum-Caribbean drying trend that lasted for ∼ 1500 yr. This forest decline preceded the appearance of anthropogenically associated Zea mays pollen. We conclude that vegetation changes in Petén during the period from ∼ 4500 to ∼ 3000 cal yr BP were largely a consequence of dry climate conditions. Furthermore, palaeoclimate data from low latitudes in North Africa point to teleconnective linkages of this drying trend on both sides of the Atlantic Ocean. © 2008 University of Washington.

5b04288b-1d43-372e-acbe-d72b0c5794c2
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ID: 5b04288b-1d43-372e-acbe-d72b0c5794c2
Tag: []
Abstract: 

f8f67746-a1c5-3bdb-a51f-3ff570c8f426
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ID: f8f67746-a1c5-3bdb-a51f-3ff570c8f426
Tag: []
Abstract: We employed paleolimnological methods to investigate tropical forest recovery and soil stabilization that followed abandonment of agricultural systems associated with disintegration of Classic Maya polities ca. A.D. 800-1000. We used lithological, geochemical, magnetic, and palynological data from sediment cores of Lake Petén Itzá in the Maya Lowlands of northern Guatemala. Sediment core chronology was developed using radiocarbon dates on terrestrial wood and charcoal fragments. Our results indicate that in the absence of large human populations and extensive farming activities, Petén forests recovered under humid climate conditions within a span of 80-260 yr. Soil stabilization postdates pollen evidence of forest regrowth stratigraphically, and required between 120 and 280 yr. We conclude that the tropical forest ecosystem in the watershed of Lake Petén Itzá had been reestablished by the early Postclassic Period (A.D. 1000-1200). © 2010 Geological Society of America.

e6032c83-2f88-31c1-a170-8e75d215c4c7
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ID: e6032c83-2f88-31c1-a170-8e75d215c4c7
Tag: []
Abstract: Reconstructing climate from lake sediments can be challenging, because the response of lakes and various components of lake systems are mediated by non-climatic factors, such as geomorphic and hydrologic stetting. As a result, the magnitude of lake response to climatic forcing may be non-linear. In addition, changes in the lake system associated with the aging process or non-climatic influences may alter the response to a given climate perturbation. These non-linear and non-stationary characteristics can produce spatial heterogeneity in the pattern and timing of inferred change. One approach for generating regionally robust climatic interpretations from lakes is to increase coordinated efforts to generate and synthesize large data sets, so that localized influences can be more clearly distinguished from broad-scale regional patterns. This approach will be most successful for evaluating climate variation at multi-decadal or longer temporal scales; the climatic interpretation of higher frequency limnological variation can be more complicated, because of dating uncertainties and differential response times of individual proxies and systems. © 2007 Springer Science+Business Media B.V.

ebb3c7dd-6d16-3c5e-b2c1-ae0ac5442ee6
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ID: ebb3c7dd-6d16-3c5e-b2c1-ae0ac5442ee6
Tag: []
Abstract: High-resolution paleohydrological reconstructions were carried out in five shallow lakes in the Nebraska Sand Hills across an east-west transect in order to 1) determine whether long-term droughts of the past 4000 years were spatially and temporally coherent across the region, 2) distinguish local variation in climate or hydrology from regional patterns of change, and 3) compare the paleolimnological results with the existing dune-inferred drought records. Diatom-inferred lake-level was reconstructed for all sites and compared with other regional records. Alterations between high and low lake-levels were frequent during the past 4000 years, which suggests that shifts between dry and wet periods were prevalent across the Sand Hills. Extended multi-decadal to centennial-scale droughts were more common prior to 2000 years BP, while the last two millennia were hydrologically more variable and climate conditions alternated on shorter timescales. Despite some discrepancies among the five records, the paleohydrological reconstructions refine the Holocene drought history of the Nebraska Sand Hills, particularly between ~2200 and 4000 cal a BP. Many of the observed drought events are contemporaneous with severe droughts documented at sites in the northern Great Plains and Rocky Mountains, lending support for the severity and regional significance of these events in western North America. © 2011 Elsevier Ltd.

22f06fae-f46d-3466-9344-2fe666aa0d04
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ID: 22f06fae-f46d-3466-9344-2fe666aa0d04
Tag: []
Abstract: Millennial-scale climate variation during the Last Glacial period is evident in many locations worldwide, but it is unclear if such variation occurred in the interior of tropical South America, and, if so, how the low-latitude variation was related to its high-latitude counterpart. A high-resolution record, derived from the deep drilling of sediments on the floor of Lake Titicaca in the southern tropical Andes, is presented that shows clear evidence of millennial-scale climate variation between ∼60 and 20 ka BP. This variation is manifested by alternations of two interbedded sedimentary units. The two units have distinctive sedimentary, geochemical, and paleobiotic properties that are controlled by the relative abundance of terrigenous or nearshore components versus pelagic components. The sediments of more terrigenous or nearshore nature likely were deposited during regionally wetter climates when river transport of water and sediment was higher, whereas the sediments of more pelagic character were deposited during somewhat drier climates regionally. The majority of the wet periods inferred from the Lake Titicaca sediment record are correlated with the cold events in the Greenland ice cores and North Atlantic sediment cores, indicating that increased intensity of the South American summer monsoon was part of near-global scale climate excursions. © 2010 Elsevier Ltd. All rights reserved.

f726f30e-6171-3faa-8bab-2a0379fb0d3d
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ID: f726f30e-6171-3faa-8bab-2a0379fb0d3d
Tag: []
Abstract: The sulfur (S) isotope difference between sedimentary sulfate and sulfide phases preserved in sedimentary rocks (Δ34S) has been utilized to reconstruct marine sulfate concentrations and inferentially the redox evolution of Earth's surface. These interpretations are largely based on experimental studies that indicate that microbial sulfate reduction is accompanied by a substantial kinetic isotope effect (up to 66‰), but only at sulfate concentrations >~200μM. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (~100-350M) and find that the calculated kinetic isotope effect associated with microbial sulfate reduction (34S) is relatively large (~23.5‰), but preserved Δ34S values are considerably smaller (4.7‰-9.9‰).Δ34S values in this system are controlled by the fraction of the sulfate reservoir that is consumed during sulfate reduction and the location of pyrite formation. This reservoir effect strongly influences the S isotope composition of sulfide preserved in the rock record such that Δ34S values increase as a function of sulfate levels, even when sulfate concentrations are >200μM and the kinetic isotope effect is expressed. These findings have important implications for reconstructing the chemical evolution of the ocean-atmosphere system throughout Earth history-not just for the Precambrian © 2013 Geological Society of America.

f138c26e-97d8-3766-a56d-570e863d1a16
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ID: f138c26e-97d8-3766-a56d-570e863d1a16
Tag: []
Abstract: Two long sediment records (cores LTO1-2B and LT01-3B) from Lake Titicaca, Bolivia/Peru, are compared with a previously analysed Holocene record from this lake (core NE98-1 PC). The Holocene records of LT01-2B and NE98-1PC are similar. There are striking differences, however, between the MIS 5e sections of the long cores and the Holocene records. In these records, temperature is probably the dominant parameter that determines the total fossil pollen concentration and is used to time the onset and termination of deglaciation. In contrast, the relative and absolute abundance of specific taxa (e.g. Polylepis/Acaena, Chenopodiaceae) are indicators of relative moisture availability. Although the Holocene contains a period of aridity between ca. 8000 cal. yr BP and 4300 cal. yr BP, it is a minor event compared with the more extreme aridity of MIS 5e. Core LT01-3B showed similar trends during MIS 5e when compared to LT01 -2B, as did NE98-1PC when comparing Holocene records. MIS 5e and the Holocene are markedly different interglacials, depicted by shifts in pollen concentration and taxa representation over time. Copyright © 2005 John Wiley & Sons, Ltd.

eaac5aef-239e-3889-bfca-b03b7eaa6810
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ID: eaac5aef-239e-3889-bfca-b03b7eaa6810
Tag: []
Abstract: Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200-2100, 1700-1600, 1350-1200, 800-600, and 250. cal. yr. BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800-600. cal. yr. BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography. © 2011 University of Washington.

5980e78b-cc82-3cc5-93fe-6dc80e60285a
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ID: 5980e78b-cc82-3cc5-93fe-6dc80e60285a
Tag: []
Abstract: A palynological investigation of the last glacial-interglacial cycle in the southern hemisphere tropical Andes reveals changes in the moisture balance as the main driver in vegetation change. Thirty accelerator mass spectrometry radiocarbon dates, biostratigraphy and tephra correlation reveal that a 119 m sediment core recovered from the Huiñaimarca sub-basin of Lake Titicaca (16.0° to 17.5° S, 68.5° to 70° W; 3810 masl) contains sediments covering > 151,000 years. Correlation of aridity indicators with precessional variations in insolation is used to fine tune the structure of the age-depth curve within this period. Variations in Isoëtes concentration (above/below 10,000 grains/cm3) identify the extent of shallow water environments. Examination of another palaeolimnological indicator (Pediastrum) and consideration of the bathymetry of the Huiñaimarca sub-basin allow the reconstruction of lake-level fluctuations. These data indicate five wet/dry cycles between c. 151,000 and 14,200 cal yr BP. High stands are suggested during the transition into (c. 134,000 cal yr BP), and out of (c. 114,000 and 92,000 cal yr BP), the last interglacial, and during full glacial conditions (c. 70,000 and 45,000 cal yr BP). These cycles are superimposed on a general trend of deepening lake levels through the glacial period. This interpretation is supported by correlation with sediments from Salar de Uyuni (20°S, 68°W; 3653 masl). The youngest wet episode is concurrent with palaeolake Minchin (c. 45,000 cal yr BP), with further evidence for an additional wet period commencing c. 28,000 cal yr BP, concomitant with palaeolake Tauca. The timing of lake level fluctuations is also supported by palaeoshoreline reconstructions from the Uyuni-Poopó region. However, our data do not suggest a major peak in lake level in Huiñaimarca during the Ouki lake cycle (c. 120,000-98,000 cal yr BP) as inferred from U-Th ages obtained from palaeoshorelines around Lago Poopó. The most extreme dry event occurs during the last interglacial period and resulted in a sedimentary hiatus tentatively dated to c. 121,000-129,000 cal yr BP. The observed wet/dry cycles are shown to have a marked and rapid impact on the vegetation. The aridity of the last interglacial promoted a community dominated by Chenopodiaceae/Amaranthacae, with no modern Andean analogue. Polylepis/Acaena pollen is also shown to fluctuate markedly (0-20%), particularly during the transitions into, and out of, the last interglacial. It is probable that this pollen taxon is primarily representative of the high altitude arboreal genus Polylepis, which is a key component of highly biodiverse Andean woodlands today. Rapid fluctuations indicate the sensitivity of this ecosystem to natural environmental pressure and potential vulnerability to future human impact and climate change. The 100,000 year (eccentricity) solar cycle is shown to be the major controlling factor in moisture balance and vegetation over the last glacial-interglacial cycle. However, significant fluctuations in moisture balance are also evident on timescales considerably shorter than the full glacial-interglacial cycle. We have linked these to precessional (21,000 year) forcing. Nevertheless, precise independent dating during the full glacial cycle is required to confirm the importance of this forcing mechanism. © 2007 Elsevier B.V. All rights reserved.

4fc9c696-185f-383b-a28b-1755a8f016d7
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Abstract: Molecular organic geochemical proxies are increasingly being utilized to reconstruct past environmental conditions as new tools continue to be discovered and developed. To date, organic geochemical proxies have been developed mainly for use in marine systems and are widely used in paleoceanography. In contrast, organic proxies have been less commonly used on lacustrine sedimentary records. One reason for this is that the wide range in the physical and chemical properties of lakes complicates application of some organic geochemical proxies in lacustrine settings. Furthermore, in comparison to marine studies, presently only a small number of studies have conducted or are currently conducting fundamental research aimed at developing organic geochemical proxies for use in lacustrine settings. Despite this, an increasing number of (paleo)limnological studies are currently applying organic geochemical techniques to examine present and past environmental conditions. In this manuscript we review the use of a number of commonly utilized organic geochemical and isotopic proxies and discuss their potential for environmental reconstruction in Quaternary lacustrine deposits. © 2011 Elsevier Ltd.

f7b96ce7-8bf5-35a4-a1fb-04441a17e685
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ID: f7b96ce7-8bf5-35a4-a1fb-04441a17e685
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Abstract: 

676e451b-bd98-309f-8403-c4291c9a70fa
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Abstract: A terrestrial sediment sequence exposed in an eroding pingo provides insights into the late-Quaternary environmental history of the northern Seward Peninsula, Alaska. We have obtained the first radiocarbon-dated evidence for a mid-Wisconsin thermokarst lake, demonstrating that complex landscape dynamics involving cyclic permafrost aggradation and thermokarst lake formation occurred over stadial-interstadial as well as glacial-interglacial time periods. High values of Picea pollen and the presence of Larix pollen in sediments dated to 50-40 ka BP strongly suggest the presence of forest or woodland early in MIS 3; the trees grew within a vegetation matrix dominated by grass and sedge, and there is indirect evidence of grazing animals. Thus the interstadial ecosystem was different in structure and composition from the Holocene or from the preceding Last Interglacial period. An early Holocene warm period is indicated by renewed thermokarst lake formation and a range of fossil taxa. Multiple extralimital plant taxa suggest mean July temperatures above modern values. The local presence of spruce during the early Holocene warm interval is evident from a radiocarbon-dated spruce macrofossil remain and indicates significant range extension far beyond the modern tree line. The first direct evidence of spruce in Northwest Alaska during the early Holocene has implications for the presence of forest refugia in Central Beringia and previously assumed routes and timing of post-glacial forest expansion in Alaska. © 2012 Elsevier Ltd.

6402caf8-2698-3f6f-a853-edb18f149f91
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Abstract: Pronounced transient global warming events between 60 and 50 million years ago have been linked to rapid injection of isotopically-light carbon to the ocean-atmosphere system. It is, however, unclear whether the largest of the hyperthermals, the Palaeocene-Eocene Thermal Maximum (PETM; ref.), had a similar origin as the subsequent greenhouse climate events, such as the Eocene Thermal Maximum 2 and H2 events. The timing and evolution of these events is well documented in marine records, but is not well constrained on land. Here we report carbon isotope records from palaeosol carbonate nodules from the Bighorn Basin, Wyoming, USA that record the hyperthermals. Our age model is derived from cyclostratigraphy, and shows a similar structure of events in the terrestrial and marine records. Moreover, the magnitude of the terrestrial isotope excursions is consistently scaled with the marine records, suggesting that the severity of local palaeoenvironmetal change during each event was proportional to the size of the global carbon isotope excursion. We interpret this consistency as an indication of similar mechanisms of carbon release during all three hyperthermals. However, unlike during the PETM (refs,), terrestrial environmental change during the subsequent hyperthermals is not linked to substantial turnover of mammalian fauna in the Bighorn Basin. © 2012 Macmillan Publishers Limited. All rights reserved.

3d5c1113-65c9-327a-8824-2b82da2240c6
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Abstract: The reliability of Arctic climate predictions is currently hampered by insufficient knowledge of natural climate variability in the past. A sediment core from Lake El'gygytgyn in northeastern (NE) Russia provides a continuous, high-resolution record from the Arctic, spanning the past 2.8 million years. This core reveals numerous "super interglacials" during the Quaternary; for marine benthic isotope stages (MIS) 11c and 31, maximum summer temperatures and annual precipitation values are ~4° to 5°C and ~300 millimeters higher than those of MIS 1 and 5e. Climate simulations show that these extreme warm conditions are difficult to explain with greenhouse gas and astronomical forcing alone, implying the importance of amplifying feedbacks and far field influences. The timing of Arctic warming relative to West Antarctic Ice Sheet retreats implies strong interhemispheric climate connectivity.

15cbaee3-e5f1-3d86-b571-17236b84870f
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Abstract: We constructed a radiometrically calibrated proxy record of Late Pleistocene and Holocene climate change exceeding 230,000 yr duration, using pollen profiles from two cores taken through age-equivalent dry lakes - one core having greater age control (via 230Th alpha mass-spectrometry) and the other having greater stratigraphic completeness. The better dated of these two serial pollen records (Searles Lake) served as a reference section for improving the effective radiometric age control in a nearby and more complete pollen record (Owens Lake) because they: (1) are situated ~ 90 km apart in the same drainage system (on, and immediately leeward of, the eastern flank of the Sierra Nevada), and (2) preserved strikingly similar pollen profiles and concordant sequences of sedimentological changes. Pollen assemblages from both lakes are well preserved and diverse, and document serial changes in Late Pleistocene and Holocene plant zone distribution and composition in the westernmost Great Basin; they consist of taxa now inhabiting montane forest, woodland, steppe, and desert-scrub environments. The studied core intervals are interpreted here to be the terrestrial equivalent of marine δ18O stages 1 through 9; these pollen profiles now appear to be among the best radiometrically dated Late Pleistocene records of terrestrial climate change known.

dd3c1631-8c99-3147-aa8f-a86f83dfb4cb
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Abstract: 

fe3184a8-66df-3b8d-850e-22fd82e4f05e
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Abstract: A long sequence of fossil palynomorph assemblages from a 323-m-long core taken at Owens Lake has enabled us to evaluate the gross vegetational trends for the Owens Valley region of California over the past ∼800,000 years. Shifts in vegetation composition and abundance in the study area during the Pleistocene were indicated in core sediments by marked fluctuations in the pollen frequencies of pines, junipers, and, to a lesser extent, of big sagebrush, composites, and chenopods/amaranths. The modern vegetation distribution and modern pollen rain on the eastern flank of the Sierra Nevada indicate that maximal abundances of these taxa generally characterize higher elevation subalpine and montane coniferous forests, lower elevation coniferous woodland, steppe, and desert scrub environments. Pollen frequencies in the upper part of core OL-92 corroborate vegetational trends documented previously from late Wisconsin and Holocene Neotoma middens in the Great Basin. These trends and evidence from this study suggest that woodland taxa expanded their range down the slope of the eastern flank of the Sierra Nevada and were established in (and immediately adjacent to) Owens Valley during moderated climates of the late Wisconsin, apparently in response to decreases in temperature and increases in precipitation, but retreated upslope toward their present position starting as long ago as ca. 20 ka. More importantly, pollen evidence from core OL-92 documents that the southern Sierra Nevada has experienced nine major cool-to-warm vegetation shifts (in addition to the late Wisconsin-early Holocene warming) during the time interval spanning the middle Pleistocene to early Holocene (Brunhes Normal Polarity Chron). We believe that at least six consecutive cool-to-warm shifts (the most recent ones) represent transitions from full-glacial to full-interglacial conditions on the basis of the magnitude of vegetation change in this portion of the pollen record. These marked changes in the frequency curves of dominant palynomorph taxa enabled us to identify boundaries that define 19 (?20) pollen zones in OL-92. The excursions of the pollen frequency curves within and across zone boundaries approximate the nature, duration, and timing of the middle and late Pleistocene climatic trends documented by geochemical (δ18O) evidence from OL-92 and from Devils Hole (DH-11) in the Amargosa Desert of Nevada.

cdc1eded-b7c8-39e3-af44-34966ce2faa6
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Abstract: At Owens Lake, California, paleomagnetic data document the Matuyama/Brunhes polarity boundary near the bottom of a 323-m core (OL-92) and display numerous directional fluctuations throughout the Brunhes chron. Many of the intervals of high directional dispersion were previously interpreted to record magnetic excursions. For the upper ~120 m, these interpretations were tested using the anisotropy of magnetic susceptibility (AMS), which typically defines a subhorizontal planar fabric for sediments deposited in quiet water. AMS data from intervals of deformed core, determined from detailed analysis of sedimentary structures, were compared to a reference AMS fabric derived from undisturbed sediment. This comparison shows that changes in the AMS fabric provide a means of screening core samples for deformation and the associated paleomagnetic record for the adverse effects of distortion. For that portion of core OL-92 studied here (about the upper 120 m), the combined analyses of sedimentary structures and AMS data demonstrate that most of the paleomagnetic features, previously interpreted as geomagnetic excursions, are likely the result of core deformation. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

ae266620-03a2-3095-a006-1b3d9ca79076
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Abstract: In 1872, a magnitude 7.5-7.7 earthquake vertically offset the Owens Valley fault by more than a meter. An eyewitness reported a large wave on the surface of Owens Lake, presumably initiated by the earthquake. Physical evidence of this event is found in cores and trenches from Owens Lake, including soft-sediment deformation and fault offsets. A graded pebbly sand truncates these features, possibly over most of the lake floor, reflecting the 'tsunami' wave. Confirmation of the timing of the event is provided by abnormally high lead concentrations in the sediment immediately above and below these proposed earthquake deposits derived from lead-smelting plants that operated near the eastern lake margin from 1869-1876. The bottom velocity in the deepest part of the lake needed to transport the coarsest grain sizes in the graded pebbly sand provides an estimate of the minimum initial 'tsunami' wave height at 37 cm. This is less than the wave height calculated from long-wave numerical models (about 55 cm) using average fault displacement during the earthquake. Two other graded sand deposits associated with soft-sediment deformation in the Owens Lake record are less than 3000 years old, and are interpreted as evidence of older earthquake and tsunami events. Offsets of the Owens Valley fault elsewhere in the valley indicate that at least two additional large earthquakes occurred during the Holocene, which is consistent with our observations in this lacustrine record. (C) 2000 Elsevier Science B.V. All rights reserved.

99943f1d-ee7c-3590-874a-cde1e3e76786
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ID: 99943f1d-ee7c-3590-874a-cde1e3e76786
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Abstract: The headwaters of the Bear River drainage were occupied during the Last Glacial Maximum (LGM) by outlet glaciers of the Western Uinta Ice Field, an extensive ice mass (∼685 km2) that covered the western slope of the Uinta Mountains. A well-preserved sequence of latero-frontal moraines in the drainage indicates that outlet glaciers advanced beyond the mountain front and coalesced on the piedmont. Glacial deposits in the Bear River drainage provide a unique setting where both 10Be cosmogenic surface-exposure dating of moraine boulders and 14C dating of sediment in Bear Lake downstream of the glaciated area set age limits on the timing of glaciation. Limiting 14C ages of glacial flour in Bear Lake (corrected to calendar years using CALIB 5.0) indicate that ice advance began at 32 ka and culminated at about 24 ka. Based on a Bayesian statistical analysis of cosmogenic surface-exposure ages from two areas on the terminal moraine complex, the Bear River glacier began its final retreat at about 18.7 to 18.1 ka, approximately coincident with the start of deglaciation elsewhere in the central Rocky Mountains and many other alpine glacial localities worldwide. Unlike valleys of the southwestern Uinta Mountains, deglaciation of the Bear River drainage began prior to the hydrologie fall of Lake Bonneville from the Provo shoreline at about 16 ka. © 2007 Regents of the University of Colorado.

dfe52987-66d1-396b-a729-161b83b717fa
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Abstract: Bear Lake, in northeastern Utah and southern Idaho, lies in a large valley formed by an active half-graben. Bear River, the largest river in the Great Basin, enters Bear Lake Valley ∼15 km north of the lake. Two 4-m-long cores provide a lake sediment record extending back ∼26 cal k.y. The penetrated section can be divided into a lower unit composed of quartz-rich clastic sediments and an upper unit composed largely of endogenic carbonate. Data from modern fluvial sediments provide the basis for interpreting changes in provenance of detrital material in the lake cores. Sediments from small streams draining elevated topography on the east and west sides of the lake are characterized by abundant dolomite, high magnetic susceptibility (MS) related to eolian magnetite, and low values of hard isothermal remanent magnetization (HIRM, indicative of hematite content). In contrast, sediments from the headwaters of the Bear River in the Uinta Mountains lack carbonate and have high HIRM and low MS. Sediments from lower reaches of the Bear River contain calcite but little dolomite and have low values of MS and HIRM. These contrasts in catchment properties allow interpretation of the following sequence from variations in properties of the lake sediment: (1) ca. 26 cal ka-onset of glaciation; (2) ca. 26-20 cal ka-quasicyclical, millennial-scale variations in the concentrations of hematite-rich glacial fl our derived from the Uinta Mountains, and dolomite- and magnetite-rich material derived from the local Bear Lake catchment (reflecting variations in glacial extent); (3) ca. 20-19 cal ka-maximum content of glacial fl our; (4) ca. 19-17 cal ka-constant content of Bear River sediment but declining content of glacial fl our from the Uinta Mountains; (5) ca. 17-15.5 cal ka-decline in Bear River sediment and increase in content of sediment from the local catchment; and (6) ca. 15.5-14.5 cal ka-increase in content of endogenic calcite at the expense of detrital material. The onset of glaciation indicated in the Bear Lake record postdates the initial rise of Lake Bonneville and roughly corresponds to the Stansbury shoreline. The lake record indicates that maximum glaciation occurred as Lake Bonneville reached its maximum extent ca. 20 cal ka and that deglaciation was under way while Lake Bonneville remained at its peak. The transition from siliciclastic to carbonate sedimentation probably indicates increasingly evaporative conditions and may coincide with the climatically driven fall of Lake Bonneville from the Provo shoreline. Although lake levels fluctuated during the Younger Dryas, the Bear Lake record for this period is more consistent with drier conditions, rather than cooler, moister conditions interpreted from many studies from western North America. Copyright © 2009 The Geological Society of America.

8d5ccfb0-fda9-3ba3-9d82-a0af2523dab5
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Abstract: A variety of sedimentological evidence was used to construct the lake-level history for Bear Lake, Utah and Idaho, for the past ∼25,000 years. Shorelines provide evidence of precise lake levels, but they are infrequently preserved and are poorly dated. For cored sediment similar to that in the modern lake, grain-size distributions provide estimates of past lake depths. Sedimentary textures provide a highly sensitive, continuous record of lake-level changes, but the modern distribution of fabrics is poorly constrained, and many ancient features have no modern analog. Combining the three types of data yields a more robust lake-level history than can be obtained from any one type alone. When smooth age-depth models are used, lake-level curves from multiple cores contain inconsistent intervals (i.e., one record indicates a rising lake level while another record indicates a falling lake level). These discrepancies were removed and the multiple records were combined into a single lake-level curve by developing age-depth relations that contain changes in deposition rate (i.e., gaps) where indicated by sedimentological evidence. The resultant curve shows that, prior to 18 ka, lake level was stable near the modern level, probably because the lake was overflowing. Between ca. 17.5 and 15.5 ka, lake level was ∼40 m below the modern level, then fluctuated rapidly throughout the post-glacial interval. Following a brief rise centered ca. 15 ka ( = Raspberry Square phase), lake level lowered again to 15-20 m below modern from ca. 14.8-11.8 ka. This regression culminated in a lowstand to 40 m below modern ca. 12.5 ka, before a rapid rise to levels above modern ca. 11.5 ka. Lake level was typically lower than present throughout the Holocene, with pronounced lowstands 15-20 m below the modern level ca. 10-9, 7.0, 6.5-4.5, 3.5, 3.0-2.5, 2.0, and 1.5 ka. High lake levels near or above the modern lake occurred ca. 8.5-8.0, 7.0-6.5, 4.5-3.5, 2.5, and 0.7 ka. This lake-level history is more similar to records from Pyramid Lake, Nevada, and Owens Lake, California, than to those from Lake Bonneville, Utah. Copyright © 2009 The Geological Society of America.

a8495f4d-05a4-3849-9985-044488cfd63e
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Abstract: Bear Lake is a long-lived lake filling a tectonic depression between the Bear River Range to the west and the Bear River Plateau to the east, and straddling the border between Utah and Idaho. Mineralogy, elemental geochemistry, and magnetic properties provide information about variations in provenance of allogenic lithic material in last-glacial-age, quartz-rich sediment in Bear Lake. Grain-size data from the siliciclastic fraction of late-glacial to Holocene carbonate-rich sediments provide information about variations in lake level. For the quartz-rich lower unit, which was deposited while the Bear River fl owed into and out of the lake, four source areas are recognized on the basis of modern fluvial samples with contrasting properties that reflect differences in bedrock geology and in magnetite content from dust. One of these areas is underlain by hematite-rich Uinta Mountain Group rocks in the headwaters of the Bear River. Although Uinta Mountain Group rocks make up a small fraction of the catchment, hematite-rich material from this area is an important component of the lower unit. This material is interpreted to be glacial fl our. Variations in the input of glacial flour are interpreted as having caused quasi-cyclical variations in mineralogical and elemental concentrations, and in magnetic properties within the lower unit. The carbonate-rich younger unit was deposited under conditions similar to those of the modern lake, with the Bear River largely bypassing the lake. For two cores taken in more than 30 m of water, median grain sizes in this unit range from ∼6 μm to more than 30 μm, with the coarsest grain sizes associated with beach or shallow-water deposits. Similar grain-size variations are observed as a function of water depth in the modern lake and provide the basis for interpreting the core grain-size data in terms of lake level. Copyright © 2009 The Geological Society of America.

6b036bba-a09c-3d80-91da-b7f0b04e96b4
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Abstract: Radiocarbon analyses of pollen, ostracodes, and total organic carbon (TOC) provide a reliable chronology for the sediments deposited in Bear Lake over the past 30,000 years. The differences in apparent age between TOC, pollen, and carbonate fractions are consistent and in accord with the origins of these fractions. Comparisons among different fractions indicate that pollen sample ages are the most reliable, at least for the past 15,000 years. The post-glacial radiocarbon data also agree with ages independently estimated from aspartic acid racemization in ostracodes. Ages in the red, siliclastic unit, inferred to be of last glacial age, appear to be several thousand years too old, probably because of a high proportion of reworked, refractory organic carbon in the pollen samples. Age-depth models for five piston cores and the Bear Lake drill core (BL00-1) were constructed by using two methods: quadratic equations and smooth cubic-splinefits. The two types of age models differ only in detail for individual cores, and each approach has its own advantages. Specific lithological horizons were dated in several cores and correlated among them, producing robust average ages for these horizons. The age of the correlated horizons in the red, siliclastic unit can be estimated from the age model for BL00-1, which is controlled by ages above and below the red, siliclastic unit. These ages were then transferred to the correlative horizons in the shorter piston cores, providing control for the sections of the age models in those cores in the red, siliclastic unit. These age models are the backbone for reconstructions of past environmental conditions in Bear Lake. In general, sedimentation rates in Bear Lake have been quite uniform, mostly between 0.3 and 0.8 mm yr-1 in the Holocene, and close to 0.5 mm yr-1 for the longer sedimentary record in the drill core from the deepest part of the lake. Copyright © 2009 The Geological Society of America.

9ea07337-2e36-316a-91be-19a666eba336
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Abstract: Earlier studies indicated that the general pattern of the Holocene climate in the northeastern United States changed from cool and dry (11.6-8.2ka; 1ka=1000 cal yr BP) to warm and wet (8.2-5.4ka) to warm and dry (5.4-3ka) to cool and wet (after 3ka). A new ∼35-year resolution stable isotope record of endogenic calcite from a sediment core for Lake Grinnell in northern New Jersey provided a chance to examine the Holocene climate variations of the region in a finer detail. After the Younger Dryas cold climate reversal, the δ18O fluctuated around a constant value of -7.4‰ until 5.8ka, thereafter shifted to a steadily decreasing trend to the most recent value of -8.2‰ Responding to this shift, the widely observed hemlock decline in the northeastern USA occurred about ∼350-500 (±143.5) years later. Detrended δ18O and δ13C records show a clear covariance at 910-year periodicity. The amplitudes of centennial-scale δ18O variations became much smaller after 4.7ka. At the same time, the dominant frequency of these variations changed from 330 to 500 years. We suggest that a non-linear response of atmospheric circulation to the gradual decrease in insolation is responsible for the shift in the climate trend at 5.8ka as indicated by the deceasing δ18O values. A dominant frequency shift in solar forcing and the decreased seasonal contrast of insolation might have caused the change in climate variability at 4.7ka through modulating ocean and atmosphere circulations. © 2010 Elsevier Ltd.

aad99861-5b02-35f7-9dbc-1210bbdc7858
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Abstract: A continuous, 120-m-long core (BL00-1) from Bear Lake, Utah and Idaho, contains evidence of hydrologic and environmental change over the last two glacial-interglacial cycles. The core was taken at 41.95°N, 111.31°W, near the depocenter of the 60-m-deep, spring-fed, alkaline lake, where carbonate-bearing sediment has accumulated continuously. Chronological control is poor but indicates an average sedimentation rate of 0.54 mm yr-1. Analyses have been completed at multi-centennial to millennial scales, including (in order of decreasing temporal resolution) sediment magnetic properties, oxygen and carbon isotopes on bulk-sediment carbonate, organic- and inorganiccarbon contents, palynology; mineralogy (X-ray diffraction), strontium isotopes on bulk carbonate, ostracode taxonomy, oxygen and carbon isotopes on ostracodes, and diatom assemblages. Massive silty clay and marl constitute most of the core, with variable carbonate content (average = 31 ± 19%) and oxygen-isotopic values (δ18O ranging from -18‰ to -5‰ in bulk carbonate). These variations, as well as fluctuations of biological indicators, reflect changes in the water and sediment discharged from the glaciated headwaters of the dominant tributary, Bear River, and the processes that influenced sediment delivery to the core site, including lake-level changes. Although its influence has varied, Bear River has remained a tributary to Bear Lake during most of the last quarter-million years. The lake disconnected from the river and, except for a few brief excursions, retracted into a topographically closed basin during global interglaciations (during parts of marine isotope stages 7, 5, and 1). These intervals contain up to 80% endogenic aragonite with high δ18O values (average = -5.8 ± 1.7‰), indicative of strongly evaporitic conditions. Interglacial intervals also are dominated by small, benthic/tychoplanktic fragilarioid species indicative of reduced habitat availability associated with low lake levels, and they contain increased high-desert shrub and Juniperus pollen and decreased forest and forest-woodland pollen. The 87Sr 86Sr values (>0.7100) also increase, and the ratio of quartz to dolomite decreases, as expected in the absence of Bear River in flow. The changing paleoenvironments inferred from BL00-1 generally are consistent with other regional and global records of glacialinterglacial fluctuations; the diversity of paleoenvironmental conditions inferred from BL00-1 also reflects the influence of catchment-scale processes. Copyright © 2009 The Geological Society of America.

e955b1c1-e78c-3bf5-9603-f69e5d902e10
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ID: e955b1c1-e78c-3bf5-9603-f69e5d902e10
Tag: []
Abstract: Aquatic biota in Central Asia witnesses and faces a changing environment. Because ostracodes contribute to both extant and fossil lacustrine diversity, they can be used to track evolution in water quality. Living ostracode communities in a variety of aquatic habitats of western Mongolia were analyzed in relation to environmental and hydrochemical variables of those habitats, based on presence/absence data from net samples. The sampled water bodies represent broad gradients in ionic concentration and composition. Ostracode community composition of springs differed from all other sampled habitats, which was also reflected in CCA analyses. Our data indicated that the ostracode fauna of western Mongolia shows a high response to calcium content, alkalinity, salinity, temperature, nutrients, and altitude. Species composition and diversity seem to be determined by solute evolution dominated by Ca-depletion. In general, this seems to be an important regulator of lacustrine ostracode species composition in several semi-arid regions. Binary logistic regression was used to predict the occurrence of the most common species with a minimal set of environmental variables. The resulting models showed on average low performance, and mainly demonstrated the potential of such modeling to predict the distribution of typical bio-indicator species. © Springer Science+Business Media B.V. 2010.

387130e4-61d9-3aa4-8a67-9983b7985130
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ID: 387130e4-61d9-3aa4-8a67-9983b7985130
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Abstract: Quantification of intra-specific morphological variability of aquatic biota along environmental gradients can produce biological proxies that can be applied to paleoenvironmental reconstructions. This morphology-derived proxy information can be especially valuable when dealing with low-diversity fossil assemblages, i. e. in situations when paleoenvironmental inference based on species composition of the assemblage is less effective. We analyzed valve size and outline shape of the widespread and highly environmentally tolerant ostracode species Limnocythere inopinata collected in 15 lakes and ponds of Western Mongolia. We quantified shape variability among and within these living populations in relation to water chemistry and physical habitat variables. Our results indicate that: (1) a population's mean valve outline is related to habitat type, (2) surface water temperature, the alkalinity to sulphate ratio, specific conductance and total phosphorus together explain a high portion of the variance in mean valve outline between populations, and (3) a quantitative model inferring the alkalinity to sulphate ratio from mean valve outline has an R2 of 0.88 and RMSEP of 0.17. These results corroborate the hypothesis that high morphological variability in this ostracode species is due to both ecophenotypic variance and high clonal diversity associated with a mixed reproductive strategy (a combination of sexual and parthenogenetically reproducing lineages), and underline the value of morphometric techniques in paleoecology. © 2010 Springer Science+Business Media B.V.

ce673e5d-f371-37df-810a-789a0e0e6c19
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ID: ce673e5d-f371-37df-810a-789a0e0e6c19
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Abstract: Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

f1836354-623e-3911-be58-b1b29616e76f
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ID: f1836354-623e-3911-be58-b1b29616e76f
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Abstract: To understand the nitrogen (N) cycle in sediments with deep oxygen penetration, we measured pore-water profiles to calculate N fluxes and rates at 13 locations in Lake Superior in water depths ranging from 26 to 318 m. Sediments with high oxygen demand, such as in nearshore or high-sedimentation areas, contribute disproportionally to benthic N removal, despite covering only a small portion of the lake floor. These sediments are nitrate sinks (average 0.16 mmol m-2 d-1) and have denitrification rates (average 0.76 mmol m-2 d-1) that are comparable to those in coastal marine sediments. The deeply oxygenated (4 to > 12 cm) offshore sediments are nitrate sources (average 0.26 mmol m-2 d-1) and generate N2 at lower rates (average 0.10 mmol m-2 d-1). Ammonium is nitrified with high efficiency (90%), and nitrification supports > 50% of denitrification nearshore and ~ 100% offshore. Oxygen consumption by nitrification accounts for 12% of the total sediment oxygen uptake. About 2% of nitrate reduction is coupled to oxidation of iron, a rarely detected pathway. Our Lake Superior N budget indicates significant contributions from sediment-water exchanges and N2 production and is closer to balance than previous budgets. Our results reveal that sediment N cycling in large freshwater lakes is similar to that in marine systems. They further suggest that denitrification rates in slowly accumulating, well-oxygenated sediments cannot be described by the same relationship with total oxygen uptake as in high-sedimentation areas; hence, global models should treat abyssal ocean sediments differently than coastal and shelf sediments. © 2014, by the Association for the Sciences of Limnology and Oceanography, Inc.

bb378a29-192e-3769-9014-01a46c4f91f5
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ID: bb378a29-192e-3769-9014-01a46c4f91f5
Tag: []
Abstract: Sediment records from closed-basin lakes in the Northern Great Plains (NGP) of North America have contributed significantly to our understanding of regional paleoclimatology. A high-resolution (near decadal) fossil diatom record from Kettle Lake, ND, USA that spans the last 8500 cal. yr BP is interpreted in concert with percent abundance of aragonite in the sediment as an independent proxy of groundwater flow to the lake (and thus lake water level). Kettle Lake has been relatively fresh for the majority of the Holocene, likely because of the coarse substrata and a strong connection to the underlying aquifer. Interpretation of diatom assemblages in four groups indicative of low to high groundwater flow, based on the percent of aragonite in sediments, allow interpretations of arid periods (and probable meromictic lake conditions) that could not be detected based on diatom-based salinity reconstructions alone. At the centennial-millennial scale, the diatom record suggests humid/wet periods from 8351 to 8088, 4364 to 1406 and 872 to 620 cal. yr BP, with more arid periods intervening. During the last ~ 4500 years, decadal-centennial scale periods of drought have taken place, despite the generally wetter climate. These droughts appear to have had similar impacts on the Kettle Lake hydrology as the 'Dust Bowl' era droughts, but were longer in duration. © SAGE Publications 2011.

c965aca7-a9f3-34f2-9f4a-d236877433be
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ID: c965aca7-a9f3-34f2-9f4a-d236877433be
Tag: []
Abstract: A decadal-scale multiproxy record of minerals, pollen, and charcoal from Kettle Lake, North Dakota provides a high-resolution record of climate and vegetation change spanning the entire Holocene from the northern Great Plains (NGP) in North America. The chronology is established by over 50 AMS radiocarbon dates. This record exhibits millennial-scale trends evident in other lower-resolution studies, but with much more detail on short-term climate variability and on the rapidity and timing of major climatic shifts. As a proxy for precipitation, we utilize the rate of endogenic carbonate sedimentation, which depends on groundwater inflow, which in turn depends on precipitation. Independent cluster analyses of mineral and pollen data reveal major Holocene mode shifts at 10.73 ka (ka = cal yr BP), 9.25 ka, and 4.44 ka. The early Holocene, 11.7-9.25 ka, was generally wet, with perhaps a trend to higher evaporation associated with warming temperatures. A switch from calcite to aragonite deposition associated with a severe, but brief drought occurred at 10.73 ka. From 10.73 ka to 9.25 ka, climate was generally humid but punctuated at 100-300 yr intervals by brief droughts, including the most severe drought of the entire Holocene at 9.25 ka. This event was coeval with the 9.3-9.2 ka event in the Greenland ice cores and observed at a number of sites worldwide. In contrast, the prominent 8.2 ka event in Greenland is not remarkable at Kettle Lake. The prominence of the 9.25 event locally in the NGP may be due to a major drawdown and northward retreat of Lake Agassiz at this time, reducing its mesoclimatic effect on the NGP and thrusting the region into an insolation controlled regime. The mid-Holocene, 9.25-4.44 ka, was characterized by great variability in moisture on a multi-decadal scale, with severe droughts alternating with more humid periods. The high abundance of the weedy but drought intolerant Ambrosia generally during the mid-Holocene and specifically during the multi-decadal drought periods is seemingly paradoxical, but can be explained by high interannual variability of moisture overlaid on the multi-decadal variability. The late Holocene, 4.44 ka-present, was also characterized by multi-decadal variability in moisture, but was generally wetter than the mid-Holocene and the magnitude of variability was less. The trends in wet-dry mineral, pollen, and charcoal proxies were similar to the mid-Holocene, but late Holocene mineral-pollen assemblages are distinct from mid-Holocene. The shift to wetter climate in the late Holocene was more gradual than the abrupt shift to arid conditions 9.25 ka, which may explain the asymmetric retreat and readvance of forest along the eastern margin of the NGP. Precipitation variations in the NGP have been linked with Pacific and Atlantic sea-surface temperatures, and mid-Holocene drought in the NGP has been linked with sustained La Niña-like conditions in the Pacific. These linkages may explain the decadal- and millennial-scale variations seen in the NGP, but cause of the prominent century-scale variations remains elusive. © 2011 Elsevier Ltd.

c8cda328-151c-35ab-b6d7-bd5d69a80596
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ID: c8cda328-151c-35ab-b6d7-bd5d69a80596
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Abstract: Aim To investigate the effects of abrupt climate change in the North Atlantic on the vegetation history of lowland Central America. We use palynological evidence from a Central American lake on the Yucatan Peninsula to evaluate the effects of rapid climate changes during the last ice age, between 65 and 8ka. Location Lake Petén-Itzá, lowlands of northern Guatemala. Methods Sediment core PI-6 was sampled at c.170-year resolution for pollen and charcoal analysis in order to construct a temporal sequence of environmental change. Uni- and multivariate statistical analyses were performed on the pollen dataset to test whether there was an association between Heinrich events in the North Atlantic and vegetation changes in the Central American lowlands. Results Pollen analysis revealed that the composition of plant assemblages on the Yucatan Peninsula varied in phase with rapid changes in North Atlantic climate. Pine savannas were the main vegetation type between c.60 and 47ka. These savannas gave way to pine-oak (Pinus-Quercus) forests in the latter half of the last ice age. Marked episodes of replacement of the pine-oak assemblage by xeric-tolerant taxa occurred during Heinrich events. The Last Glacial Maximum (LGM) was characterized by mesic conditions. Main conclusions The pollen record from Lake Petén-Itzá showed that vegetation changes associated with Heinrich events were more significant than those associated with the LGM. Each Heinrich event produced a characteristic shift towards xeric taxa. Although colder than Heinrich events, the LGM on the Yucatan Peninsula was relatively moist, and the presumed savannization of the landscape during the maximum cooling of the last glacial was not supported by our data. Our findings suggest alternative scenarios for plant diversification and genetic interchange during glacial times, and also indicate that vegetation in tropical continental settings was not as stable as previously thought. © 2011 Blackwell Publishing Ltd.

f7929613-1053-32eb-8fda-7529f0c3668f
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ID: f7929613-1053-32eb-8fda-7529f0c3668f
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Abstract: During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.

19e0d624-a5cc-39eb-912f-46df77d3c055
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ID: 19e0d624-a5cc-39eb-912f-46df77d3c055
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Abstract: Lake-level changes since ca. 3.6 kilo-annum (ka) at Emerald Lake in the Upper Arkansas River Basin of west-central Colorado coincide in time with changes of the opposite direction at Lake of the Woods in northwestern Wyoming. The contrast provides evidence of a multi-centennial moisture dipole across the Southern and Central Rocky Mountains' region similar to one associated with the efects of the El Niño Southern Oscillation (ENSO) on annual to decadal time scales today. Cores and ground penetrating radar (GPR) profles from Emerald Lake show that deep-water muds accumulated as extensively across the lake basin as today at ca. 3.6-3.0 and 1.4-0.8 ka, and nearly as extensively at 2.5-1.9 ka. The extensive muds indicate episodes of high water at Emerald Lake and date to times when Lake of the Woods was low. Nearshore sand layers at Emerald Lake indicate that water levels fell during the intervening centuries, including when bristlecone pine chronologies have documented repeated multi-decadal droughts in the Upper Arkansas River Basin. Water levels were also low, based on the absence of nearshore mud accumulation, before ca. 3.6 ka, and dramatically lower (>2 m in the currently 4.5-m deep lake) before a sharp rise in water levels by ca. 5.7 ka. A basin-wide change in sediment accumulation patterns, consistent with an expansion and deepening of the lake at ca. 5.7 ka, correlates with regional cooling and similar evidence of increased efective moisture at Lake of the Woods and other sites throughout central North America. The step increase in moisture availability may relate to a global-scale reorganization of climatic patterns, which developed as the mid- and high-latitudes cooled in response to a decline in summer insolation.

9466911a-ab21-3966-adcc-8d87be91072f
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ID: 9466911a-ab21-3966-adcc-8d87be91072f
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Abstract: New evidence in the NW region of the Iberian Peninsula (~42°N 6°W) of a glacial advance coeval with the global Last Glacial Maximum (LGM) of the Marine Isotope Stage 2 has been identified through a dataset of exposure ages based on 23 10Be concentration measurements carried out on boulder samples taken from a set of latero-frontal moraines. Results span the interval 19.2-15.410Beka, matching the last deglaciation period when Iberia experienced the coldest and driest conditions of the last 25ka, and are consistent with Lateglacial chronologies established in other mountain regions from SW Europe. The extent of the LGM stade identified in this work is similar to the local maximum ice extent stade recorded and dated as prior to 33ka using radiocarbon and optically stimulated luminescence. This work showcases how multiple-dating approaches and detailed geomorphological mapping are required to reconstruct realistic palaeoglacier evolution models. © 2013 Elsevier Ltd.

bcdd1518-f9a4-313b-8553-c39d146e191f
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ID: bcdd1518-f9a4-313b-8553-c39d146e191f
Tag: []
Abstract: 

89dce860-036d-398c-a6f6-237e31afbbdf
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ID: 89dce860-036d-398c-a6f6-237e31afbbdf
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Abstract: Quantitative records of pre-industrial temperature changes are fundamental for understanding long-term natural climate variability. We used visible reflectance spectroscopy to measure chlorophyll content (and its derivatives) in a sediment core from Kurupa Lake, north-central Brooks Range, Alaska, to reconstruct summer temperature and the number of annual non-freezing days over the past 5.7 ka. A calibration-in-time approach was used to convert downcore changes in chlorophyll content to the climate variables, and an ensemble approach was used to integrate age and calibration uncertainties. The strongest correlation (rmedian = 0.69, pmedian = 0.02, RMSEP = 1.9°C) is for summer (June through September) temperature using the 20th Century Reanalysis Project dataset. The chlorophyll-inferred 3-year-mean summer temperature shows that the warmest century (3.0–2.9 ka BP) was about 3.0°C (90% range of the ensemble members = 2.3–4.0°C) higher and that the coldest century (1.4–1.3 ka BP) was about 5.5°C lower (90% range = −7.6°C to −5.0°C) than during the reference period (AD 1961–1990). Century-to-century temperature changes over the past 5.7 ka at Kurupa Lake have been large (90% range = −2.8°C to 3.1°C shifts in centennial mean), including the shift between the 19th and 20th centuries, which was above the 90th percentile of temperature changes across all representations of the reconstruction. In contrast to most Northern Hemisphere temperature reconstructions, Kurupa Lake shows no overall millennial-scale cooling trend. We suggest that increased summer duration (by 4.3 days during the last 6 ka) along with no long-term increase in sea-ice cover over the adjacent Chukchi Sea counter-balanced the influence of decreased insolation intensity on the aquatic productivity in Kurupa Lake.

4b02a923-886c-3bbd-ba32-d575a46f5265
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ID: 4b02a923-886c-3bbd-ba32-d575a46f5265
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Abstract: We assessed tropical montane cloud forest (TMCF) sensitivity to natural disturbance by drought, fire, and dieback with a 7300-year-long paleorecord. We analyzed pollen assemblages, charcoal accumulation rates, and higher plant biomarker compounds (average chain length [ACL] of n-alkanes) in sediments from Wai'a¯napanapa, a small lake near the upper forest limit and the mean trade wind inversion (TWI) in Hawai'i. The paleorecord of ACL suggests increased drought frequency and a lower TWI elevation from 2555-1323 cal yr B.P. and 606-334 cal yr B.P. Charcoal began to accumulate and a novel fire regime was initiated ca. 880 cal yr B.P., followed by a decreased fire return interval at ca. 550 cal yr B.P. Diebacks occurred at 2931, 2161, 1162, and 306 cal yr B.P., and two of these were independent of drought or fire. Pollen assemblages indicate that on average species composition changed only 2.8% per decade. These dynamics, though slight, were significantly associated with disturbance. The direction of species composition change varied with disturbance type. Drought was associated with significantly more vines and lianas; fire was associated with an increase in the tree fern Sadleria and indicators of open, disturbed landscapes at the expense of epiphytic ferns; whereas stand-scale dieback was associated with an increase in the tree fern Cibotium. Though this cloud forest was dynamic in response to past disturbance, it has recovered, suggesting a resilient TMCF with no evidence of state change in vegetation type (e.g., grassland or shrubland).

4484ca97-16bf-373d-8307-c55d1077d0ed
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ID: 4484ca97-16bf-373d-8307-c55d1077d0ed
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Abstract: Manganoan siderite ([Fe,Mn]CO3) occurs in abundance of up to 19% (dry weight) as the sole endogenic carbonate within a succession of Holocene, organic-rich, varved sediments from freshwater Otter Lake (OL), Michigan. Radiocarbon dating and varve counts from a 7-m piston core constrain periods of major siderite accumulation to sediments older than 1200calyr BP. Sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet. Bulk-sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial-scale cycles of siderite accumulation. Siderite δ13C and δ18O are enriched when the mineral is abundant and depleted when it is least abundant. Samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water, but are δ13C enriched relative to modern waters. Samples from intervals of low siderite abundance are δ13C and δ18O depleted relative to equilibrium with modern lake water. These data suggest that abundant siderite precipitation occurred when the OL water column was ferruginous (iron meromictic), allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon (DIC) enriched in δ13C below the chemocline, where methanogenesis in waters and sediments influenced DIC composition. Seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution. Manganese substitution in siderite lowered kinetic barriers to low-temperature mineral precipitation. Climate changes and basin filling influenced lake stratification and the rate of siderite precipitation. The siderite occurrence reported here displays remarkable similarity to its occurrence in Paleogene maar lake deposits, suggesting that further studies of Holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments.

bcc3ea3d-0147-3f2b-ab44-644110a8e10b
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ID: bcc3ea3d-0147-3f2b-ab44-644110a8e10b
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Abstract: Alpine lakes receive a large fraction of their nutrients from atmospheric sources and are consequently sensitive to variations in both the amount and chemistry of atmospheric deposition. In this study we explored the spatial changes in lake water chemistry and biology along a gradient of dust deposition in the Wind River Range, Wyoming. Regional differences were explored using the variation in bulk deposition, lake water, sediment, and bedrock geochemistry and catchment characteristics. Dust deposition rates in the Southwestern region averaged 3.34 g m-2 year-1, approximately three times higher than deposition rates in the Northwestern region (average 1.06 g m-2 year-1). Dust-P deposition rates ranged from 87 μg P m2 day-1 in the Northwestern region to 276 μg P m2 day-1 in the Southwestern region. Subalpine and alpine lakes in the Southwestern region had greater total phosphorus (TP) concentrations (5-13 μg L-1) and greater sediment phosphorus (SP) concentrations (2-5 mg g-1) than similar lakes elsewhere in the region (1-8 μg L-1 TP, 0.5-2 mg g-1 SP). Lake phosphorus concentrations were related to dissolved organic carbon (DOC) across vegetation gradients, but related to the percent of bare rock, catchment area to lake area, and catchment steepness across dust deposition gradients. Modern phytoplankton and zooplankton biomasses were two orders of magnitude greater in the Southwest than in the Northwest, and alpine lakes in the Southwest had a unique diatom species assemblage with relatively higher concentrations of Asterionella formosa, Pseudostaurosira pseudoconstruens, and Pseudostaurosira brevistriata. These results suggests that catchment controls on P export to lakes (i.e. DOC) are overridden in dominantly bare rock basins where poor soils cannot effectively retain dust deposited P. © 2014 Springer International Publishing Switzerland.

099c5b53-bf5b-34bd-a19b-03f275d48756
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ID: 099c5b53-bf5b-34bd-a19b-03f275d48756
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Abstract: Growing evidence suggests short-duration climate events may drive community structure and composition more directly than long-term climate means, particularly at ecotones where taxa are close to their physiological limits. Here we use an empirical habitat model to evaluate the role of microclimate during a strong El Niño in structuring a tropical montane cloud forest's upper limit and composition in Hawai'i. We interpolate climate surfaces, derived from a high-density network of climate stations, to permanent vegetation plots. Climatic predictor variables include (1) total rainfall, (2) mean relative humidity, and (3) mean temperature representing non-El Niño periods and a strong El Niño drought. Habitat models explained species composition within the cloud forest with non-El Niño rainfall; however, the ecotone at the cloud forest's upper limit was modeled with relative humidity during a strong El Niño drought and secondarily with non-El Niño rainfall. This forest ecotone may be particularly responsive to strong, short-duration climate variability because taxa here, particularly the isohydric dominant Metrosideros polymorpha, are near their physiological limits. Overall, this study demonstrates moisture's overarching influence on a tropical montane ecosystem, and suggests that short-term climate events affecting moisture status are particularly relevant at tropical ecotones. This study further suggests that predicting the consequences of climate change here, and perhaps in other tropical montane settings, will rely on the skill and certainty around future climate models of regional rainfall, relative humidity, and El Niño.

c5eb713e-e114-3605-8d1b-0eeb9219cbb0
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ID: c5eb713e-e114-3605-8d1b-0eeb9219cbb0
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Abstract: The cause and age of the Moorhead low-water Phase of glacial Lake Agassiz remains uncertain. Newgeochemical (X-ray fluorescence (XRF) and elemental analysis) and chronological (optically stimulated luminescence (OSL)) data are used to test for evaporative enrichment within lacustrine sediment from Rabbit Lake, a small basin just above the highest Lake Agassiz strandline, and from two Lake Agassiz sediment cores at Fargo, North Dakota. Increases in quartz and gypsum interpreted to be of aeolian origin suggest increased aridity at Rabbit Lake sometime after 13 540-13 750 cal years BP. From the Fargo cores, lacustrine sediment of the Brenna and Sherack formations did not show convincing evidence for evaporative enrichment. However, this result is complicated by an erosional contact at the top of the Brenna Formation. A thin middle sand unit between the Brenna and Sherack formation clays is stratigraphically equivalent to the Poplar River Formation, West Fargo Member, but its properties differ from the fluvial sand of the West Fargo Member. Four OSL ages from the organic-poor, middle sand unit at Fargo range between 12.8 ± 0.2 and 13.5 ± 0.2 ka (with ±1.6 ka uncertainty) and suggest lake level fell at Fargo at, or before, 13.1 ±0.2 ka, the average of the OSL ages. With different sedimentological properties, and a difference of—1750 years between the new OSL ages and previously published ages on the West Fargo Member sand, additional work is required to determine whether the middle sand unit is a new member of the Poplar River Formation, recording an earlier and different depositional environment than the West Fargo Member. From a plot of available ages for the Moorhead Phase, the regression remains poorly constrained in time.

7ddd8558-6511-3271-adab-6fb3f3802f60
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ID: 7ddd8558-6511-3271-adab-6fb3f3802f60
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Abstract: Aim: To review and synthesize multiple lines of evidence that describe the spatial patterns of land use associated with prehistoric and early historical Native American societies in eastern North America in order to better characterize the type, spatial extent and temporal persistence of past land use. Location: Temperate forests of eastern North America, and the Eastern Woodlands cultural region. Methods: Ethnohistorical accounts, archaeological data, historical land surveys and palaeoecological records describing indigenous forms of silviculture and agriculture were evaluated across scales ranging from local (10<sup>0</sup> km) to regional (10<sup>2</sup> km) to produce a synthetic description of land-use characteristics. Results: Indigenous land-use practices created patches of distinct ecological conditions within a heterogeneous mosaic of ecosystem types. At all scales, patch location was dynamic, and patches underwent recurrent periods of expansion, contraction and abandonment. Land-use patches varied in their extent and persistence, and are broadly categorized as silvicultural (management of undomesticated woodland taxa) or agricultural (cultivation of domesticated taxa). Silvicultural patches persisted for centuries and extended kilometres to tens of kilometres around settlements and travel corridors. The dynamics of agricultural patches varied among groups, with persistence ranging from decades to centuries and extent ranging from less than a kilometre to tens of kilometres around settlements. Beyond patch boundaries, human impacts on ecosystems become indistinguishable from other drivers of environmental heterogeneity. These characteristics of patches are evident across scales and multiple lines of evidence. Main conclusions: Our findings challenge the view that prehistoric human impacts on vegetation were widespread and ubiquitous, and build on previous work showing these impacts to be more localized and heterogeneous by providing quantitative descriptions of land-use patch characteristics. Collaborative efforts that combine multiple data sources are needed to refine these descriptions and generate more precise measures of land-use pattern to further investigate the history of human impacts on the Earth system.

6e6896ce-cdc2-3d74-8a63-4384574c47bb
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ID: 6e6896ce-cdc2-3d74-8a63-4384574c47bb
Tag: []
Abstract: The water-level record from the 300. m deep paleo-lake Suguta (Northern Kenya Rift) during the African Humid Period (AHP, 15-5. ka BP) helps to explain decadal to centennial intensity variations in the West African Monsoon (WAM) and the Indian Summer Monsoon (ISM). This water-level record was derived from three different sources: (1) grain size variations in radiocarbon dated and reservoir corrected lacustrine sediments, (2) the altitudes and ages of paleo-shorelines within the basin, and (3) the results of hydro-balance modeling, providing important insights into the character of water level variations (abrupt or gradual) in the amplifier paleo-Lake Suguta. The results of these comprehensive analyses suggest that the AHP highstand in the Suguta Valley was the direct consequence of a northeastwards shift in the Congo Air Boundary (CAB), which was in turn caused by an enhanced atmospheric pressure gradient between East Africa and India during a northern hemisphere insolation maximum. Rapidly decreasing water levels of up to 90. m over less than a hundred years are best explained by changes in solar irradiation either reducing the East African-Indian atmospheric pressure gradient and preventing the CAB from reaching the study area, or reducing the overall humidity in the atmosphere, or a combination of both these effects. In contrast, although not well documented in our record we hypothesize a gradual end of the AHP despite an abrupt change in the source of precipitation when a decreasing pressure gradient between Asia and Africa prevented the CAB from reaching the Suguta Valley. The abruptness was probably buffered by a contemporaneous change in precession producing an insolation maximum at the equator during October. Whether or not this is the case, the water-level record from the Suguta Valley demonstrates the importance of both orbitally-controlled insolation variations and short-term changes in solar irradiation as factors affecting the significant water level variations in East African rift lakes. © 2013 Elsevier B.V.

36e4bf38-061d-34cd-9cd5-76e43b387ecf
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ID: 36e4bf38-061d-34cd-9cd5-76e43b387ecf
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Abstract: Fire has an influence on regional to global atmospheric chemistry and climate. Molecular markers of biomass burning archived in lake sediments are becoming increasingly important in paleoenvironmental reconstruction and may help determine the interaction between climate and fire activity. Here, we present a high performance anion exchange chromatography-mass spectrometry method to allow separation and analysis of levoglucosan, mannosan and galactosan in lake sediments, with implications for reconstructing past biomass burning events. Determining mannosan and galactosan in Lake Kirkpatrick, New Zealand (45.03°S, 168.57°E) sediment cores and comparing these isomers with the more abundant biomass burning markers levoglucosan and charcoal represents a significant advancement in our ability to analyze past fire activity. Levoglucosan, mannosan and galactosan concentrations correlated significantly with macroscopic charcoal concentration. Levoglucosan/mannosan and levoglucosan/(mannosan. +. galactosan) ratios may help determine not only when fires occurred, but also if changes in the primary burned vegetation occurred. © 2014 The Authors.

94c44bad-8185-3e07-afbc-af17a646d946
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ID: 94c44bad-8185-3e07-afbc-af17a646d946
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Abstract: Human-caused forest transitions are documented worldwide, especially during periods when land use by dense agriculturally-based populations intensified. However, the rate at which prehistoric human activities led to permanent deforestation is poorly resolved. In the South Island, New Zealand, the arrival of Polynesians c. 750 years ago resulted in dramatic forest loss and conversion of nearly half of native forests to open vegetation. This transformation, termed the Initial Burning Period, is documented in pollen and charcoal records, but its speed has been poorly constrained. High-resolution chronologies developed with a series of AMS radiocarbon dates from two lake sediment cores suggest the shift from forest to shrubland occurred within decades rather than centuries at drier sites. We examine two sites representing extreme examples of the magnitude of human impacts: a drier site that was inherently more vulnerable to human-set fires and a wetter, less burnable site. The astonishing rate of deforestation at the hands of small transient populations resulted from the intrinsic vulnerability of the native flora to fire and from positive feedbacks in post-fire vegetation recovery that increased landscape flammability. Spatially targeting burning in highly-flammable seral vegetation in forests rarely experiencing fire was sufficient to create an alternate fire-prone stable state. The New Zealand example illustrates how seemingly stable forest ecosystems can experience rapid and permanent conversions. Forest loss in New Zealand is among the fastest ecological transitions documented in the Holocene; yet equally rapid transitions can be expected in present-day regions wherever positive feedbacks support alternate fire-inhibiting, fire-prone stable states.

a52a5f5b-204e-321e-a0b2-8077ef272d2b
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ID: a52a5f5b-204e-321e-a0b2-8077ef272d2b
Tag: []
Abstract: Proxy records of summer monsoon moisture at Lake Qinghai on the northeastern Tibetan Plateau reveal a late Quaternary climate history that is subtly different from that of speleothems from southern and eastern China. Total organic carbon and authigenic carbonate in two independently analyzed and dated cores indicate (1) relative stability and aridity during the glacial interval, (2) small variations during the Bølling-Allerød and the Younger Dryas intervals, (3) comparatively abrupt change at the late Pleistocene/Holocene transition, and (4) relatively high variability during a wet early Holocene. Taken together, the data suggest that a climate threshold exists for penetration of Asian monsoon rainfall onto the Tibetan Plateau, a threshold that was crossed at the beginning of the Holocene. Conceptually, the threshold simply may be related to the topographic barrier that the eastern margin of the Tibetan Plateau presents to the landward penetration of the monsoon, or it may be related to nonlinearities in the climate system itself, such as sudden shifts in the configuration of the Westerly jet stream. Different mechanisms for producing a threshold are not mutually exclusive and may have combined to affect the dynamics of the Asian monsoon. In any case, the threshold is related to the presence of the Tibetan Plateau, which has a profound influence on the Asia monsoon system. Key Points Proxy records at Lake Qinghai reveal a different climate history A climate threshold exists for penetration of Asian monsoon rainfall The threshold is related to the presence of the Tibetan Plateau © 2014. American Geophysical Union. All Rights Reserved.

ff9167a0-b2c3-3481-a165-800cf933b66e
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ID: ff9167a0-b2c3-3481-a165-800cf933b66e
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Abstract: Tropical montane forests house unusual and diverse biota and are considered highly vulnerable to climate change, particularly near the trade wind inversion (TWI) - the upper end of the cloud belt that defines tropical montane cloud forest (TMCF). The upper cloud belt has two possible futures: one hypothesis postulates a 'lifting cloud base', raising both the upper and lower ends of the cloud belt; the other expects the upper end of the cloud belt will change independently, with a 'shifting TWI'. We used a 5900-year-long palaeorecord of vegetation and fire from a small forest hollow at 2455 m in the Cordillera Central, Dominican Republic. The site sits near the upper limit of TMCF taxa and the TWI and allows us to evaluate the relationship between vegetation dynamics and two potential drivers of TWI elevation - the Intertropical Convergence Zone (ITCZ) and the El Niño/Southern Oscillation (ENSO). Vegetation changed from cloud forest (5900-5500 cal. years BP) to alpine grassland (4300-1300 cal. years BP), to pine savanna (1300-600 cal. years BP) and finally to closed pine forest (after 600 cal. years BP). Habitat distribution models for TMCF and pine forest taxa show that these state changes were strongly associated with position of the ITCZ (cloud forest xR<sup>2</sup> = 0.63; pine forest xR<sup>2</sup> = 0.53), providing support for the shifting TWI hypothesis. We find a negative relationship between fire and TMCF and a hump-shaped relationship between fire activity and pine. Synthesis. Shifts up- and downslope of the upper limit of the cloud belt over the last 5900 years produced major vegetation changes. Fire also played a significant role, in particular when pine occupied the site after 1300 years ago and from 1965 AD when fire suppression led to a rapid return of cloud forest taxa. Our results strongly suggest that latitudinal shifts in the ITCZ position have controlled the upper limit of cloud forest in the Caribbean and understanding how the ITCZ will respond to climate change will be critical for tropical montane conservation strategies. Shifts up- and downslope of the cloud belt's upper limit over the last 5900 years produced major vegetation changes on high mountains in the Caribbean. Habitat distribution models show that vegetation state changes were strongly linked to latitudinal position of the ITCZ, providing support for the shifting TWI hypothesis over a lifting cloud base hypothesis for tropical high elevations.

45f8f411-dcd3-3b18-97f2-854aabe5c8a2
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ID: 45f8f411-dcd3-3b18-97f2-854aabe5c8a2
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Abstract: The relative importance of people and climate in shaping prehistoric fire regimes is debated around the world, and this discussion has helped inform our understanding of past and present ecosystem dynamics. Evidence for extensive anthropogenic burning of temperate closed-canopy forests prior to European settlement is geographically variable, and the factors responsible for this variability are not well resolved. We set out to explain the differences in the influence of prehistoric human-set fires in seasonally dry forest types in the Pacific Northwest, New Zealand, and northern Patagonia by comparing the fire traits of dominant taxa, postfire vegetation recovery, long-term climate trends, and human activities that may have motivated burning. Our analysis suggests that ecological and climatic factors explain much of the differences in how these mesic-dry forests responded to prehistoric anthropogenic burning. Understanding past human-environment interactions at regional scales is an important step for assessing the impact of biomass burning at all scales.

9fc53226-2ea3-3c21-999f-f60c3363cc55
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Abstract: Over the last glacial cycle, climate variations in the Neotropics were related to diverse forcing mechanisms whose understanding has been limited because of the scarcity of continuous long records. Here we use high-resolution charcoal particle accumulation, inorganic carbon precipitation and magnetic susceptibility data from Lake Chalco, located in high-altitude tropical central Mexico, to assess the fire regimes and drought periods between ∼85 and 10.8k cal a BP. Overall, drought intervals were characterized by high carbonate deposition and were contemporary with high spring insolation. Most of the high fire activity periods were coupled with droughts, whereas some of them seem to have responded to volcanic activity in the basin. Periods of more fire activity during the latest part of Marine Isotope Stage 3 were associated with a long interval of higher spring insolation. Increase in drought frequency during 47-27k cal a BP were possibly linked to a more active Laurentide Ice Sheet influencing the oceanic and atmospheric controls of the tropical climatic system. In fact, Chalco and Cariaco records show similar millennial-scale trends, suggesting a role of the Intertropical Convergence Zone in modulating moisture availability in the region.

f8f31518-905e-3d66-a86b-032f2588a75e
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ID: f8f31518-905e-3d66-a86b-032f2588a75e
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Abstract: To better understand aquatic-terrestrial linkages in the sub-Arctic, and specifically the relative importance of landscape position versus land cover, we surveyed lakes, soils, land cover, and lake/basin characteristics in a 14000 km2 region of acidic forest-tundra landscape near northern Manitoba, Canada (59.56°N, 97.72°W) in 2009. We analyzed 39 different biological, chemical, and physical variables for lakes and soils. We used a remote-sensing-based classification to determine that the landscape was 21% water, 46% peat-forming lowland, and 24.9% open tundra, and we assigned lake order to all lakes based on the order of the outlet stream for each lake. Lakes were oligotrophic to mesotrophic (median total phosphorus: TP = 11.8 μg L-1), N-limited (median dissolved inorganic nitrogen: TP = 1.6), acidic (median pH 5.7), and had moderate amounts of dissolved organic carbon (median DOC = 5.2 mg L-1). We identified 2 principle groups of variables represented by DOC and conductivity/ cations, respectively, that captured major axes of lake variation. DOC, 2 measures of DOC quality (a250/a365 [a proxy for molecular weight and aromaticity] and specific ultraviolet absorbance), and Fe and were significantly correlated with percent cover of lowland forest, but conductivity/cations were not correlated with variation in land cover. Soils were generally acidic (pH 2.7-4.4) and nutrient-poor, and wetland soils contained more carbon and higher concentrations of calcium, magnesium, and other cations than upland open tundra. Landscape position of lakes (measured as lake order) did not capture systematic differences in land cover or lake biogeochemistry. Our results highlight the importance of lowland export of DOC to lakes and further suggest the need for additional regional studies of aquatic-terrestrial connections in Arctic and sub-Arctic landscapes.

18498fac-0156-3db7-afc5-0586d8c7380f
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ID: 18498fac-0156-3db7-afc5-0586d8c7380f
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Abstract: Lacustrine sedimentary sequences are natural archives of past climatic and environmental changes. Lake level fluctuations in these sedimentary records are frequently associated with climatic changes. However, in active volcanic and tectonic settings, lacustrine sedimentation may be altered by these processes by affecting the place of deposition and controlling the volume and type of detrital material available for removal by surface processes. In consequence, it is necessary to analyze volcanic and tectonic processes in order to achieve a better understanding of climatic and environmental changes in the lacustrine sedimentary sequences in this type of environment. Additionally, volcanic material preserved in lacustrine sequences document the recurrence of volcanic activity in a region. Lake Chalco basin, located in the central part of the Transmexican Volcanic Belt, offers an ideal site to evaluate the impact of volcanic and tectonic activity on lacustrine sedimentation. In this work, we analyze the uppermost 26.7 m of the lacustrine sequence collected in seven cores drilled in different parts of Lake Chalco. In the obtained sequences, we determined the sedimentary facies succession, established their lateral distribution, and analyzed the sedimentary architecture and the impact of volcanotectonic activity on the lacustrine system. The time scale was constructed from radiocarbon determinations and well dated marker tephras from the region. The composite sequence spans the last ca.60000 yr B.P. (MIS 3-MIS 1). Thirteen facies were recognized and grouped into three main categories: 1) detritic, 2) volcaniclastic, and 3) biogenic. Volcaniclastic deposits comprise 18 individual layers 1-50 cm thick, two of which are from well documented eruptions from Popocat&#233;petl and Nevado de Toluca volcanoes. From our cores and other available information, we suggest that the monogenetic volcano Teuhtli, in the southwestern part of the Chalco lacustrine plain, is the origin of the so called Gran Ceniza Bas&#225;ltica tephra, dated between ca. 27000 y 41000 cal B.P. The accumulation of Teuhtli derived products altered the basin morphology and affected the lacustrine sedimentation, which is observed in the diatom record. We suggest that an ancient depocentre was located south of the modern lake, and that its location was controlled by synsedimentary normal faulting, active until ca. 26500 years ago.

f6110450-4db1-34f2-a5ac-cf3ce3f4fb53
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ID: f6110450-4db1-34f2-a5ac-cf3ce3f4fb53
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Abstract: Both modern anthropogenic emissions of mercury (Hg), primarily from artisanal and small-scale gold mining (ASGM), and preindustrial anthropogenic emissions from mining are thought to have a large impact on present-day atmospheric Hg deposition. We study the spatial distribution of Hg and its depositional history over the past ∼400 years in sediment cores from lakes located regionally proximal (∼90-150 km) to the largest ASGM in Peru and distal (>400 km) to major preindustrial mining centers. Total Hg concentrations in surface sediments from fourteen lakes are typical of remote regions (10-115 ng g(-1)). Hg fluxes in cores from four lakes demonstrate preindustrial Hg deposition in southeastern Peru was spatially variable and at least an order of magnitude lower than previously reported fluxes in lakes located closer to mining centers. Average modern (A.D. 2000-2011) Hg fluxes in these cores are 3.4-6.9 μg m(-2) a(-1), compared to average preindustrial (A.D. 1800-1850) fluxes of 0.8-2.5 μg m(-2) a(-1). Modern Hg fluxes determined from the four lakes are on average 3.3 (±1.5) times greater than their preindustrial fluxes, similar to those determined in other remote lakes around the world. This agreement suggests that Hg emissions from ASGM are likely not significantly deposited in nearby down-wind regions.

6a97eafe-74f7-3a36-8cfe-15ec484fa74f
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ID: 6a97eafe-74f7-3a36-8cfe-15ec484fa74f
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Abstract: Changing rainfall patterns resulting from climate change are predicted to influence cyanobacterial blooms and associated production of toxins and other metabolites, but few studies have demonstrated this link. We explored seasonal and interannual trends in 2-methylisoborneol (MIB) concentrations and the correlation of these data with rainfall and hydrologic data from 2000 to 2013. We also conducted a detailed study of cyanobacterial abundances and MIB during 2 hydrologically extreme years, a prolonged drought in 2007 followed by an extremely wet year in 2008. The goal was to assess whether cyanobacterial abundances and explanatory variables that are often correlated with cyanobacterial abundances were useful predictors of MIB concentration. From 2000 to 2013, mean annual rainfall was negatively related to mean annual MIB (r =-0.50, p = 0.08) and explained 25% of the variation in MIB. In 2007, MIB and cyanobacterial abundance were high, whereas in 2008, MIB and cyanobacterial abundance were low. The total N:total P (TN:TP) mass ratio was strongly related to MIB concentration during the drought and explained 44% of the variation in MIB concentration (r =-0.66, p = 0.003). Our results suggest that rainfall and the TN:TP mass ratio may be useful predictors for MIB in drinking-water reservoirs. © 2014 by The Society for Freshwater Science.

a6e6daaf-eeca-399a-9b5f-322892751f8f
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Tag: []
Abstract: For two watersheds in the northern Midwest United States, we show that landscape depressions have a significant impact on watershed hydrology and sediment yields and that the Soil and Water Assessment Tool (SWAT) has appropriate features to simulate these depressions. In our SWAT models of the Willow River in Wisconsin and the Sunrise River in Minnesota, we used Pond and Wetland features to capture runoff from about 40% of the area in each watershed. These depressions trapped considerable sediment, yet further reductions in sediment yield were required for calibration and achieved by reducing the Universal Soil Loss Equation (USLE) cropping-practice (P) factor to 0.40 to 0.45. We suggest terminology to describe annual sediment yields at different conceptual spatial scales and show how SWAT output can be partitioned to extract data at each of these scales. These scales range from plot-scale yields calculated with the USLE to watershed-scale yields measured at the outlet. Intermediate scales include field, upland, pre-riverine, and riverine scales, in descending order along the conceptual flow path from plot to outlet. Sediment delivery ratios, when defined as watershed-scale yields as a percentage of plot-scale yields, ranged from 1% for the Willow watershed (717 km) to 7% for the Sunrise watershed (991 km). Sediment delivery ratios calculated from published relations based on watershed area alone were about 5 to 6%, closer to pre-riverine-scale yields in our watersheds.

1cf3caf9-ed9e-3ac7-9e46-9330255e5f58
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ID: 1cf3caf9-ed9e-3ac7-9e46-9330255e5f58
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Abstract: Question: How do canopy disturbance and soil properties structure vascular plant community species composition and resilience to encroachment by exotic species in a tropical montane wet forest? Location: Hawai'i Experimental Tropical Forest (HETF), a tropical montane wet forest, on Mauna Kea, Hawai'i Island, Hawai'i, USA. Methods: Previous studies employing airborne LiDAR were used to define three zones across an elevation gradient from 900 to 1500 m. Within each zone, a ~1000-m block transect was selected to cross two different volcanic substrates: one derived from surface lava and one derived from thick ash deposits. Non-metric multidimensional scaling (NMS) scores of vegetation data were related to independently-derived environmental NMS scores and spatial location with generalized additive models (GAM). Results: Vascular plant species composition in all elevation zones consists of three NMS axes, which are best modelled by one of three possible environmental NMS axes or by location. The first NMS axis of species composition in the lowest elevation zone (40% variance explained (VE)) is a function of location on volcanic substrates (61% deviance explained (DE)). The second lowest elevation axis (27% VE) is a function of unexplained spatial heterogeneity (31% DE). The third lowest elevation NMS axis (24% VE) is a function of the spatial mosaic of canopy disturbance (16% DE). In the middle elevation zone, species composition most strongly relates to the interaction between volcanic substrate and the condition of the soil surface for all three NMS axes (41%, 27%, 24% VE; 70%, 16%, 24% DE). The primary axis of species composition in the highest elevation zone (41% VE) corresponds with substrate and soil condition (55% DE) but the second and third axes of species composition (27% and 25% VE) relate to canopy dieback disturbance (36%, 14% DE). Counts of exotic species and 0-2 m height class native tree species respond to the type of volcanic substrate and soil surface condition in all three elevation zones. Lava-derived substrates have a higher incidence of exotic species and less native tree regeneration; whereas ash-derived substrates have higher numbers of native tree species regenerating and many fewer exotic species. Discussion: The tropical montane forests on Mauna Kea reflect a native-dominated plant community response to disturbance on both lava- and ash-derived volcanic substrates, and a higher propensity for exotic species to occur on the lava-derived substrate. Native plant communities on ash-derived soils may have higher resilience to exotic invasion than communities on lava-derived substrates. Our results indicate resource managers should explicitly account for variation in soils and substrate type when prioritizing, implementing and monitoring management interventions to foster native plant assemblages and control the spread of exotic and invasive species. © 2013 International Association for Vegetation Science.

9b9a80ad-6932-3f1f-a1c9-ecaf254e7e8d
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ID: 9b9a80ad-6932-3f1f-a1c9-ecaf254e7e8d
Tag: []
Abstract: Thermokarst (thaw) lakes emit methane (CH<inf>4</inf>) to the atmosphere formed from thawed permafrost organic matter (OM), but the relative magnitude of CH<inf>4</inf> production in surface lake sediments vs. deeper thawed permafrost horizons is not well understood. We assessed anaerobic CH<inf>4</inf> production potentials from various depths along a 590 cm long lake sediment core that captured the entire sediment package of the talik (thaw bulb) beneath the center of an interior Alaska thermokarst lake, Vault Lake, and the top 40 cm of thawing permafrost beneath the talik. We also studied the adjacent Vault Creek permafrost tunnel that extends through ice-rich yedoma permafrost soils surrounding the lake and into underlying gravel. Our results showed CH<inf>4</inf> production potentials were highest in the organic-rich surface lake sediments, which were 151 cm thick (mean ± SD: 5.95 ± 1.67 μg C-CH<inf>4</inf> g dw<sup>-1</sup> d<sup>-1</sup>; 125.9 ± 36.2 μg C-CH<inf>4</inf> g C<inf>org</inf><sup>-1</sup> d<sup>-1</sup>). High CH<inf>4</inf> production potentials were also observed in recently thawed permafrost (1.18 ± 0.61 μg C-CH<inf>4</inf>g dw<sup>-1</sup> d<sup>-1</sup>; 59.60± 51.5 μg C-CH<inf>4</inf> g C<inf>org</inf><sup>-1</sup> d<sup>-1</sup>) at the bottom of the talik, but the narrow thicknesses (43 cm) of this horizon limited its overall contribution to total sediment column CH<inf>4</inf> production in the core. Lower rates of CH<inf>4</inf> production were observed in sediment horizons representing permafrost that has been thawing in the talik for a longer period of time. No CH<inf>4</inf> production was observed in samples obtained from the permafrost tunnel, a non-lake environment. Our findings imply that CH<inf>4</inf> production is highly variable in thermokarst lake systems and that both modern OM supplied to surface sediments and ancient OM supplied to both surface and deep lake sediments by in situ thaw and shore erosion of yedoma permafrost are important to lake CH<inf>4</inf> production.

d049cb8c-cd8b-3ede-8cda-38ec62b608f5
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ID: d049cb8c-cd8b-3ede-8cda-38ec62b608f5
Tag: []
Abstract: 

f01de3cd-46ca-3629-996c-86fd5111f11c
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ID: f01de3cd-46ca-3629-996c-86fd5111f11c
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Abstract: A context for recent hydroclimatic extremes and variability is provided by a ~10 k.y. sediment carbonate oxygen isotope (δ18O) record at 5-100 yr resolution from Bison Lake, 3255 m above sea level, in northwestern Colorado (United States). Winter precipitation is the primary water source for the alpine headwater lake in the Upper Colorado River Basin and lake water δ18O measurements reflect seasonal variations in precipitation δ18O. Holocene lake water δ18O variations are inferred from endogenic sedimentary calcite δ18O based on comparisons with historic watershed discharge records and tree-ring reconstructions. Drought periods (i.e., drier winters and/or a more rain-dominated seasonal precipitation balance) generally correspond with higher calcite δ18O values, and vice-versa. Early to middle Holocene δ18O values are higher, implying a rain-dominated seasonal precipitation balance. Lower, more variable δ18O values after ca. 3500 yr ago indicate a snow-dominated but more seasonally variable precipitation balance. The middle to late Holocene δ18O record corresponds with records of El Niño Southern Oscillation intensification that supports a teleconnection between Rocky Mountain climate and North Pacific sea-surface temperatures at decade to century time scales. © 2011 Geological Society of America.

8903bea2-ff45-36dd-908d-b32f0a6d3dce
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ID: 8903bea2-ff45-36dd-908d-b32f0a6d3dce
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Abstract: Pacific climate is known to have varied during the Holocene, but spatial patterns remain poorly defined. This paper compiles terrestrial and marine proxy data from sites along the northeastern Pacific margins and proposes that they indicate 1) suppressed ENSO conditions during the middle Holocene between ~8000 and 4000 cal BP with a North Pacific that generally resembled a La Niña-like or more negative PDO phase and 2) a climate transition between ~4200 and 3000 cal BP that appears to be the teleconnected expression to a more modern-like ENSO Pacific. Compared to modern day conditions, the compiled data suggest that during the middle Holocene, the Aleutian Low was generally weaker during the winter and/or located more to the west, while the North Pacific High was stronger during the summer and located more to the north. Coastal upwelling off California was more enhanced during the summer and fall but suppressed during the spring. Oregon and California sea surface temperatures (SSTs) were cooler. The Santa Barbara Basin had an anomalous record, suggesting warmer SSTs.Late Holocene records indicate a more variable, El Niño-like, and more positive PDO Pacific. The Aleutian Low became more intensified during the winter and/or located more to the east. The North Pacific High became weaker and/or displaced more to the south. Coastal upwelling off California intensified during the spring but decreased during the fall. Oregon and California SSTs became warmer, recording the shoreward migration of sub-tropical gyre waters during the fall, while spring upwelling (cooler SST) increased in the Santa Barbara Basin. The high-resolution proxy records indicate enhanced ENSO and PDO variability after ~4000 cal BP off southern California, ~3400 cal BP off northern California, and by ~2000 cal BP in southwestern Yukon. A progressively northward migration of the ENSO teleconnection during the late Holocene is proposed. © 2010.

76d48d1c-e175-374c-856f-3b55635fba68
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ID: 76d48d1c-e175-374c-856f-3b55635fba68
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Abstract: Over the period of instrumental records, precipitation maximum in the headwaters of the Colorado Rocky Mountains has been dominated by winter snow, with a substantial degree of interannual variability linked to Pacific ocean-atmosphere dynamics. High-elevation snowpack is an important water storage that is carefully observed in order to meet increasing water demands in the greater semi-arid region. The purpose here is to consider Rocky Mountain water trends during the Holocene when known changes in earth's energy balance were caused by precession-driven insolation variability. Changes in solar insolation are thought to have influenced the variability and intensity of the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North American Monsoon and the seasonal precipitation balance between rain and snow at upper elevations. Holocene records are presented from two high elevation lakes located in northwest Colorado that document decade-to-century scale precipitation seasonality for the past ~. 7000. years. Comparisons with sub-tropical records of ENSO indicate that the snowfall-dominated precipitation maxima developed ~. 3000 and 4000. years ago, coincident with evidence for enhanced ENSO/PDO dynamics. During the early-to-mid Holocene the records suggest a more monsoon affected precipitation regime with reduced snowpack, more rainfall, and net moisture deficits that were more severe than recent droughts. The Holocene perspective of precipitation indicates a far broader range of variability than that of the past century and highlights the non-linear character of hydroclimate in the U.S. west. © 2012 .

60e0be8b-051a-3a6f-828c-12fc0ddf5892
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ID: 60e0be8b-051a-3a6f-828c-12fc0ddf5892
Tag: []
Abstract: Reservoir sediments are used cautiously in paleolimnological studies because of dating uncertainties, possible sediment disturbances and even concerns that indicators of trophic status may behave differently in reservoirs as opposed to natural lakes. We measured loss on ignition (LOI), carbon to nitrogen ratio (C:N), diatom abundance, total nitrogen (TN), total phosphorus (TP), TN:TP ratio, and carbon and nitrogen isotopes (δ13C and δ15N) in an 83-cm sediment core to track recent trophic status changes in Beaver Reservoir, Northwest Arkansas, USA. Measurements showed that LOI, TN, TP and diatom abundance increased significantly from the bottom to the top of the core (p < 0.001). The C:N ratio and δ13C indicated a predominantly algal source for organic matter in the sediments. Increases in TN and TP were positively correlated with human population growth (p < 0.01) and the TN:TP ratio recorded a shift from phosphorus to nitrogen limitation around 1990. This shift may have encouraged cyanobacterial growth that caused episodes of taste and odor problems in the reservoir. This study suggests that despite concerns about sediment dating and disturbance, reservoir sediments can provide valuable information on past water quality changes. © 2013 Springer Science+Business Media Dordrecht.

a3deb6b1-849f-3160-9735-2f1408188ba9
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Abstract: Reconstructing centennial timescale hydroclimate variability during the late Holocene is critically important for understanding large-scale patterns of drought and their relationship with climate dynamics. We present sediment oxygen isotope records spanning the last two millennia from 10 lakes, as well as climate model simulations, indicating that the Little Ice Age was dry relative to the Medieval Climate Anomaly in much of the Pacific Northwest of North America. This pattern is consistent with observed associations between the El Niño-Southern Oscillation (ENSO), the Northern Annular Mode, and drought as well as with proxy-based reconstructions of Pacific and Atlantic ocean-atmosphere variations over the past 1000-years the large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer-term shifts in ENSO-like dynamics and that an improved understanding of the centennial timescale relationship between external forcing and drought is necessary for projecting future hydroclimatic conditions in western North America. Key Points Lake sediment isotope records exhibit coherent centennial timescale variability The Little Ice Age was dry in much of the Pacific Northwest Climate model simulations support drought patterns evinced by lake sediment data © 2014. American Geophysical Union. All Rights Reserved.

5bd030d2-439c-37eb-8d22-6b5626e18155
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ID: 5bd030d2-439c-37eb-8d22-6b5626e18155
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Abstract: New records of paleoenvironmental change from two lakes near Cordova, south central Alaska, combined with analysis of previously reported sediment sequences from the adjacent Copper River Delta, provide quantitative constraints on a range of Earth system processes through their expression in relative sea-level change. Basal sediment ages from Upper Whitshed Lake indicate ice-free conditions by at least 14,140-15,040 cal yr BP. While Upper and Lower Whitshed Lakes provide only upper limits to relative sea-level change, interbedded layers of freshwater peat and intertidal silt extending more than 11 m below present sea level in Copper River Delta indicate net submergence over the last 6000 years and multiple earthquake deformation cycles. In contrast, Lower Whitshed Lake, situated just above present high tide level, records only one episode of marine sedimentation, commencing AD 1120-1500, that we interpret as the result of isostatic subsidence due to Little Ice Age mass accumulation of the Chugach Mountain glaciers. Lower Whitshed Lake also records isostatic uplift at the end of the Little Ice Age before the end of marine sedimentation caused by ~1.5 m coseismic uplift in the great Alaska earthquake of AD 1964. We successfully explain the records of relative sea-level change from both Copper River Delta and the Whitshed Lakes by integrating the effects of eustatic sea-level rise, glacial isostasy, earthquake deformation cycles, sediment loading, sediment compaction and Late Holocene changes in glacier mass into a single model. This approach provides initial quantitative constraints on the individual contributions of these processes. Taking reasonable estimates of eustasy, post-Last Glacial Maximum and Neoglacial glacial isostatic adjustment and a simple earthquake deformation cycle, we demonstrate that sediment loading and sediment compaction are both contributors to relative sea-level rise at Copper River Delta, together producing subsidence averaging approximately 1.2 mm yr<sup>-1</sup> over the mid to Late Holocene. Further isolation basin studies have the potential to greatly improve our understanding of the individual contributions of these processes in this highly dynamic region.

695803b6-6b58-3e6c-9e9c-4f858f1abed9
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ID: 695803b6-6b58-3e6c-9e9c-4f858f1abed9
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Abstract: Landscape-scale vulnerability assessment from multiple sources, including paleoecological site histories, can inform climate change adaptation. We used an array of lake sediment pollen and charcoal records to determine how soils and landscape factors influenced the variability of forest composition change over the past 2000 years. The forests in this study are located in northwestern Wisconsin on a sandy glacial outwash plain. Soils and local climate vary across the study area. We used the Natural Resource Conservation Service's Soil Survey Geographic soil database and published fire histories to characterize differences in soils and fire history around each lake site. Individual site histories differed in two metrics of past vegetation dynamics: the extent to which white pine (Pinus strobus) increased during the Little Ice Age (LIA) climate period and the volatility in the rate of change between samples at 50-120 yr intervals. Greater increases of white pine during the LIA occurred on sites with less sandy soils (R2=0.45, P < 0.0163) and on sites with relatively warmer and drier local climate (R2=0.55, P < 0.0056). Volatility in the rate of change between samples was positively associated with LIA fire frequency (R2=0.41, P < 0.0256). Over multi-decadal to centennial timescales, forest compositional change and rate-of-change volatility were associated with higher fire frequency. Over longer (multi-centennial) time frames, forest composition change, especially increased white pine, shifted most in sites with more soil moisture. Our results show that responsiveness of forest composition to climate change was influenced by soils, local climate, and fire. The anticipated climatic changes in the next century will not produce the same community dynamics on the same soil types as in the past, but understanding past dynamics and relationships can help us assess how novel factors and combinations of factors in the future may influence various site types. Our results support climate change adaptation efforts to monitor and conserve the landscape's full range of geophysical features.

7b2ccfc9-0141-33ff-8ba5-2730f9d2eb12
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ID: 7b2ccfc9-0141-33ff-8ba5-2730f9d2eb12
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Abstract: This multi-proxy study of sediment cores from karstic Lake Estanya (Pre-Pyrenean Range, NE Spain) provides the first complete, continuous record of the hydrological evolution in the northeastern Iberian Peninsula over the last 9500 yr. Six sedimentary facies and four main sedimentary units have been defined after integration of sedimentological, mineralogical and geochemical analyses. The use of total sulphur content and sedimentary facies as paleohydrological proxies allows reconstruction of relative changes in lake level. The Estanya record shows a large increase in water availability after 9.2 ka, fluctuating lake levels and salinity during the period 9.2-4.2 ka; and generally higher lake levels after 1.7 ka. Periods of increased run-off and sediment delivery and less saline conditions occurred at 8.5-8.2, 6.7-5.9, and 4.9-4.2 ka. Dominant lower lake levels and concentrated waters during the period 4.2-0.8 ka were punctuated by a higher lake level, higher clastic input episode ca. 1.7-1 ka. Fluctuating, but higher lake levels occurred during the last 800 years. The main hydrological phases in Lake Estanya are coherent with Western Mediterranean and North Atlantic Holocene reconstructions, but they also show similarities with northern African records. © 2007 Elsevier Ltd and INQUA.

2f5fdf74-cb99-3ce1-9845-207a27e5a42e
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ID: 2f5fdf74-cb99-3ce1-9845-207a27e5a42e
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Abstract: Ninety plant macrofossil taxa from the Ziegler Reservoir fossil site near Snowmass Village, Colorado, record environmental changes at high elevation (2705 m asl) in the Rocky Mountains during the Last Interglacial Period. Present-day vegetation is aspen forest (Populus tremuloides) intermixed with species of higher (Picea, Abies) and lower (Artemisia, Quercus) elevations. Stratigraphic units 4-13 contain montane forest taxa found near the site today and several species that today generally live at lower elevations within (Abies concolor, Lycopus americanus) and outside Colorado (Najas flexilis). These data suggest near-modern climatic conditions, with slightly warmer summer and winter temperatures. This montane forest period was succeeded by a shorter treeless interval (Unit 14) representing colder and/or drier conditions. In units 15-16, conifer trees reoccur but deciduous and herb taxa are lacking, suggesting a return to warmer conditions, although cooler than during the earlier forest period. Comparison of these inferred paleoclimatic changes with the site's geochronologic framework indicates that the lower interval of sustained warmth correlates with late MIS 6-early 5b (~ 138-94 ka), the cold interval with MIS 5b (~ 94-87 ka), and the uppermost cool assemblages with MIS 5a (~ 87-77 ka).

c84d5c57-f65e-35f9-8d2e-d32613c88bc4
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ID: c84d5c57-f65e-35f9-8d2e-d32613c88bc4
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Abstract: Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20m, located in the Pre-Pyrenean Ranges (North-eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub-environments can be recognized as: (i) a carbonate-producing 'littoral platform'; (ii) a steep 'talus' dominated by reworking of littoral sediments and mass-wasting processes; and (iii) an 'offshore, distal area', seasonally affected by anoxia with fine-grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a 'basal unit', seismically transparent and restricted to the depocentres of both sub-basins; (ii) an 'intermediate unit' characterized by continuous high-amplitude reflections; and (iii) an 'upper unit' with strong parallel reflectors. Several mass-wasting deposits occur in both sub-basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry 14C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite-producing lake during full glacial times (21 to 17·3 kyr bp); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp); (iv) a saline lake with gypsum-rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic-dominated deposition during the Holocene (9·4 to 0·8 kyr bp); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800years. Climate-driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake-level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass-flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.

0299aecf-2ffc-3662-8bb7-57c938201cf5
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Abstract: The multi-proxy analysis of sediment cores recovered in karstic Lake Estanya (42°02′ N, 0°32′ E; 670 m a. s. l., NE Spain), located in the transitional area between the humid Pyrenees and the semi-arid Central Ebro Basin, provides the first high-resolution, continuous sedimentary record in the region, extending back the last 21 000 years. The integration of sedimentary facies, elemental and isotopical geochemistry and biogenic silica, together with a robust age model based on 17 AMS radiocarbon dates, enables precise reconstruction of the main hydrological and environmental changes in the region during the last deglaciation. Arid conditions, represented by shallow lake levels, predominantly saline waters and reduced organic productivity occurred throughout the last glacial maximum (21-18 cal kyrs BP) and the lateglacial, reaching their maximum intensity during the period 18-14.5 cal kyrs BP (including Heinrich event 1) and the Younger Dryas (12.9-11.6 cal kyrs BP). Less saline conditions characterized the 14.5-12.6 cal kyrs BP period, suggesting higher effective moisture during the Bölling/Allerød. The onset of more humid conditions started at 9.4 cal kyrs, indicating a delayed hydrological response to the onset of the Holocene which is also documented in several sites of the Mediterranean Basin. Higher, although fluctuating, Holocene lake levels were punctuated by a mid Holocene arid period between 4.8 and 4.0 cal kyrs BP. A major lake-level rise occurred at 1.2 cal kyrs BP, conducive to the establishment of conditions similar to the present and interrupted by a last major water level drop, occurring around 800 cal yrs BP, which coincides with the Medieval Climate Anomaly. The main hydrological stages in Lake Estanya are in phase with most Western Mediterranean and North Atlantic continental and marine records, but our results also show similarities with other Iberian and northern African reconstructions, emphasizing peculiarities of palaeohydrological evolution of the Iberian Peninsula during the last deglaciation. © 2009 Elsevier Ltd. All rights reserved.

7489e175-18a6-32cb-9180-5f94e5b2de98
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ID: 7489e175-18a6-32cb-9180-5f94e5b2de98
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Abstract: A revised chronological framework for the deglaciation of the Lake Michigan lobe of the south-central Laurentide Ice Sheet is presented based on radiocarbon ages of plant macrofossils archived in the sediments of low-relief ice-walled lakes. We analyze the precision and accuracy of 15 AMS 14C ages of plant macrofossils obtained from a single ice-walled lake deposit. The semi-circular basin is about 0.72km wide and formed of a 4- to 16-m-thick succession of loess and lacustrine sediment inset into till. The assayed material was leaves, buds and stems of Salix herbacea (snowbed willow). The pooled mean of three ages from the basal lag facies was 18 270±50 14C a BP (21 810cal. a BP), an age that approximates the switch from active ice to stagnating conditions. The pooled mean of four ages for the youngest fossil-bearing horizon was 17 770±40 14C a BP (21 180cal. a BP). Material yielding the oldest and youngest ages may be obtained from sediment cores located at any place within the landform. Based on the estimated settling times of overlying barren, rhythmically bedded sand and silt, the lacustrine environment persisted for about 50 more years. At a 67% confidence level, the dated part of the ice-walled lake succession persisted for between 210 and 860cal. a (modal value: 610cal. a). The deglacial age of five moraines or morainal complexes formed by the fluctuating margin of the Lake Michigan lobe have been assessed using this method. There is no overlap of time intervals documenting when ice-walled lakes persisted on these landforms. The rapid readvances of the lobe during deglaciation after the last glacial maximum probably occurred at some point between the periods of ice-walled lake sedimentation. © 2011 John Wiley & Sons, Ltd.

e2a3b048-ba0f-358c-97f6-ff3c5812aa90
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Abstract: Climatic changes during the late Quaternary have resulted in substantial, often abrupt, rearrangements of terrestrial ecosystems, but the relationship between these environmental changes and prehistoric human culture and population size remains unclear. Using a database of archaeological radiocarbon dates alongside a network of paleoecological records (sedimentary pollen and charcoal) and paleoclimatic reconstructions, we show that periods of cultural and demographic change in the northeastern United States occurred at the same times as the major environmental-climatic transitions of that region. At 11.6, 8.2, 5.4, and 3.0 kyr BP (10(3) calendar years before present), changes in forest composition altered the distribution, availability, and predictability of food resources which triggered technological adjustments manifested in the archaeological record. Human population level has varied in response to these external changes in ecosystems, but the adoption of maize agriculture during the late Holocene also resulted in a substantial population increase. This study demonstrates the long-term interconnectedness of prehistoric human cultures and the ecosystems they inhabited, and provides a consolidated environmental-cultural framework from which more interdisciplinary research and discussion can develop. Moreover, it emphasizes the complex nature of human responses to environmental change in a temperate region.