Lake Record Research Articles

Varve microfacies and chronology from a new sediment record of Lake Gościąż (Poland)

The varved sediment of Lake Gościąż (Central Poland) is one of the most detailed and complete climate archives of the Late Glacial and the Holocene in Central Europe. Here, we present microfacies analyses in combination with μXRF core scanning and a detailed varve chronology of a new and continuous GOS18 sediment record.This record presents six lithozones that mark the most prominent depositional and geochemical changes during the Holocene and Late Glacial. Varve boundaries and five main varve microfacies types were distinguished under petrographic microscope. Analysis provides detailed insights into depositional processes and its changes since the Late Glacial. Microfacies components were used to interpret processes leading to varve formation. A new and independent chronology is obtained by triple varve counting on petrographic thin sections that is complemented by 137Cs measurements and 14C AMS dating of terrestrial plant remains. The age-depth model consists of three parts: (1) the upper part (0–520 cm) that is primarily based on varve counting, (2) the middle part (520–758 cm) obtained through age-depth modelling and (3) the lower part (758–1897 cm) developed by varve counting. The bottom of the composite profile coincides with the onset of lacustrine sedimentation in the late Allerød at 12,834 +134/-233 varve yr BP.The largest shift in geochemistry, expressed by log(Ca/Ti) and log(Si/Ti) ratios show a rapid increase of calcite precipitation and primary productivity at 7940 +112/-168 varve yr BP. Possible triggers for this include local changes in hydrology as the formation of “Na Jazach” system due to the Ruda River development and fluctuations of lake water level.

A high-resolution record of Holocene primary productivity and water-column mixing from the varved sediments of Lake Żabińskie, Poland

Eutrophication and anoxia are increasing in lakes worldwide. However, our understanding of variations of primary productivity and anoxia in lakes over thousands of years is limited. Long-term records are needed to understand the natural variability of lake ecosystems and to improve our understanding of drivers of productivity and anoxia. In this study, we used the varved sediment record of Lake Żabińskie, Poland to answer the following research questions: 1) How have primary production and water column oxygen concentrations varied during the past 10,800 years?; 2) what role did natural and anthropogenic forces have in driving changes in primary production or lake mixing regime? Recently developed hyperspectral imaging (HSI) techniques were used to quantify sedimentary chloropigments-a and bacteriopheopigments-a (Bphe-a) at sub-annual resolution. These data, combined with elemental data from micro X-ray fluorescence (μ-XRF) and pigment assemblage data from high-performance liquid chromatography (HPLC) measurements, were used to reconstruct paleolimnological conditions. Bphe-a was used as an indicator of anoxia, and its presence suggests that an extensive anoxic zone was present nearly continuously from 10.8 to 2.8 ka BP. Anoxic conditions, driven by thermal stratification, were promoted by closed forest cover during that time, which limited wind-driven mixing of the water column. After 2.8 ka BP, water column oxygenation occurred more frequently, particularly during periods of increased human agricultural activity and forest opening. Pronounced anoxia was again present continuously from ~610 to 1470 CE, concurrent with a period of reforestation. After ~1610 CE, deforestation caused increases in erosion rates, algal production, and water column oxygenation. Pigment assemblages indicate that the algal community during the past 150 years was different from any other time during the Holocene. This study demonstrates a clear link between lake biogeochemical processes and forest cover and shows the potential of HSI to produce extremely high-resolution records of past productivity and redox conditions from varved lake sediments.

The Industrial Pollution History Inferred by Stable Pb Isotope in the Chronological Sedimentary Record of a Plateau Lake, Yunnan Province, Southwestern of China

Zhang, Y.; Chang, F.Q.; Duan, L.Z.; Wu, H., Zhang, H.C., 2020. The industrial pollution history inferred by stable Pb isotope in the chronological sedimentary record of a plateau lake, Yunnan Province, Southwestern of China. In: Bai, X. and Zhou, H. (eds.), Advances in Water Resources, Environmental Protection, and Sustainable Development. Journal of Coastal Research, Special Issue No. 115, pp. 641-647. Coconut Creek (Florida), ISSN 0749-0208.Lake Yangzong, a plateau deep water lake, located in the in Yunnan Province where was the one of the most industrialized regions in China, was suffered seriously mining pollution threat in recent decades. In the present research, based on the 210Pb and 137Cs methods, the chronological sequence of the sediment record collected from the lake basin was established. Combined with the stable isotopic compositions, Pb content and geochemical enrichment index (Igeo), the background of industrial activities and anthropogenic lead (Pb) pollution history were investigated. These results revealed that pollution history could divided into four stages. The contribution of Pb was primarily affected by regional emissions since the open reforming period, and local metal production industries was more involved in the late 1990s. Furthermore, the regional pollution pathways of anthropogenic Pb were mainly affected by atmospheric precipitation and transportation, which were predominately related to mining consumption and smelting industry.

U-Th dating of lake sediments: Lessons from the 700 ka sediment record of Lake Junín, Peru

Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has been often limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ∼100 m drill core from Lake Junín, Peru. The results form the foundation of an age-depth model spanning ∼700 kyrs. High uranium concentrations (0.3–4 ppm) of these sediments allow us to date smaller amounts of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest factor limiting the success of previous U-Th dating efforts in other lake basins. Despite this advantage, the dates from 174 analyses on 55 bulk carbonate samples reveal significant scatter that cannot be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and δ234Uinitial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. Alongside simulations of the isotopic evolution of our samples, we find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. Examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that geologic context is equally as important as the accuracy and precision of analytical measurements. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junín is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates.

Vegetation Changes Associated with the Younger Dryas from the Sediments of Silver Lake, Summit County, Ohio, USA

As the climate was warming during the last deglaciation, a millennium-long return to near-glacial conditions—called the Younger Dryas (YD) stadial—occurred about 12.9 to 11.7 ka ago. Prior studies have characterized the vegetation and climatic impacts of the YD stadial in the US upper Midwest from a network of fossil pollen records. The goal of this study was to locate the pronounced rise in organic matter and Pinus (pine) pollen—associated with the latter part of the YD event—in a sediment core from Silver Lake, a kettle lake in Summit County, Ohio. Based upon radiocarbon dating of the Silver Lake sediment core, an increase in sediment organic matter at 1,070 cm (35.1 ft) core depth dates to 12,130 calibrated years (cal yr) BP within the latter part of the YD stadial. A pollen study spanning the change in organic matter displays a gradual decline in Picea (spruce) pollen as well as a pronounced increase in Pinus pollen, similar to published records of the latter part of the YD event. Thus, the Silver Lake pollen record adds a new site to the network of fossil pollen sites, and further refines the vegetation changes within the Allegheny Plateau of Ohio that occurred during the latter part of the YD event. Furthermore, identifying the pronounced rise in Pinus pollen allows for biostratigraphic correlation between the Silver Lake record and another local dated pollen record. This biocorrelation adds further support to the Silver Lake radiometric age model.

The Late Pleistocene–Early Holocene palaeoenvironmental evolution in the SE Baltic region: a new approach based on chironomid, geochemical and isotopic data from Kamyshovoye Lake, Russia

The Kamyshovoye Lake sedimentary record in the southeastern Baltic Sea region was studied to reconstruct climatic fluctuations and the abiotic responses to them during the Lateglacial and Early Holocene. New results from chironomid, isotopic, palaeomagnetic and geochemical data analyses were correlated with earlier evidence of lithological and palynological changes in the Kamyshovoye Lake record. The section of the record that was studied covered the interval between 15 200 and 6500 cal. a BP. Palaeoclimatic reconstructions showed that during the Younger Dryas there was a two‐step decrease in the mean July temperature. The temperature dropped by 3 °C in the period from ̃12 650 to 12 300 cal. a BP, and then it dropped by another 0.5 °C to a minimum of 11.5 °C at 11 900 cal. a BP. During the Younger Dryas–Holocene transition, a temperature increase of 3 °C can be seen over a period of several centuries, inferred from the chironomid data. Further, the temperature curve showed that significant fluctuations continued until ̃9500 cal. a BP. During the coolings, the average July temperature dropped to values that were typical for the Younger Dryas, while values characteristic for the Allerød were only reached at around 9700 cal. a BP. After 9500 cal. a BP, a more stable, gradual increase in temperature was recorded. The short‐term Early Holocene climatic oscillations are clearly traced in the Kamyshovoye sequence, although the responses of the natural components are sometimes asynchronous. In the case of the Kamyshovoye study, the geochemical data seem to be a sensitive indicator of the climatic and environmental changes despite the absence of an evident response to the Holocene onset at ̃11 700 cal. a BP. Considerable changes in the geochemical pattern are recorded later, at ̃11 500 cal. a BP, coinciding with noticeable changes in the development of vegetation. The results obtained in this study contribute to a deeper understanding of how global climatic trends are manifested on a local scale.

Late Holocene history of Tokotan Lake (Kuril Archipelago, Russian Far East): The use of lacustrine records for paleoclimatic reconstructions from geologically dynamic settings

A high-resolution, multiproxy analysis of a core from Tokotan Lake, Urup Island, illustrates the complex paleoenvironmental signals that can be preserved in sedimentary deposits from geologically active areas, such as the Kuril Archipelago. Diatom and geochemical analyses of the Tokotan core suggest a shallowing and lessening of the lake area over the last ~3450 cal BP. Without other proxies, the record could be interpreted as showing a general decrease in precipitation over the Late Holocene. However, palynological data indicate no change in vegetation, thereby implying no significant climatic variation. Neither of these paleoclimatic interpretations agrees with Late Holocene reconstructions from the Kuril Archipelago or the Russian mainland. Paleomagnetic data imply variable sediment sources during the lake's history, indicating alterations within the Tokotan catchment (e.g., shifts in gradients or sediment loads in the inflowing streams). Such changes would be particularly important in the Shabalina River, which today has formed a partial dam across the lake's outlet. The results of the Tokotan study indicate that other factors (e.g., volcanism, seismic activity, sea-level changes) rather than climate were more probable determinants of the Late Holocene lake record. Given the geologic dynamism of the region, it is perhaps not surprising that paleo-records from the Kuril Islands display an inconsistency in the numbers and/or duration of proposed Holocene climatic oscillations, underscoring the importance of multiproxy investigations in areas that are or have been geologically dynamic.

Environmental changes in the eastern Arctic 1.6975-1.965 Ma, palynological investigations from El’gygytgyn Lake, northeastern Russia

The 1.6975–1.965 Ma (MIS 60–MIS 74) portion of the El'gygytgyn Lake record includes two key chronostratigraphic markers in the Calabrian–Gelasian boundary (1.806 Ma) and the Olduvai paleomagnetic excursion (1.77–1.95 Ma). It also provides the first paleovegetation information for these times from the eastern Arctic. Palynological data indicate the dominance of Larix forest and forest-tundra during warm and cool intervals, which differs sharply from the herb tundra which characterized glacial intervals of the Middle and Late Pleistocene. These forests likely included tree species of Betula and Alnus and perhaps Populus and Chosenia. The persistence of Larix indicates summer temperatures were warmer than present. Pinus pumila, a typical interglacial taxon, is restricted to MIS 65 and MIS 71, suggesting other intervals experienced dry winters. Cool and warm periods are clearly marked by vegetation changes during the Late Gelasian Stage, but a dominance of Poaceae pollen and reduction in Alnus pollen during the Early Calabrian Stage mutes the climatic fluctuations. The loss of Alnus perhaps relates to decreases in seasonal temperatures.

Late-Holocene climate response and glacial fluctuations revealed by the sediment record of the monsoon-dominated Chorabari Lake, Central Himalaya

We studied a periglacial lake situated in the monsoon-dominated Central Himalaya where an interplay of monsoonal precipitation and glacial fluctuations during the late Holocene is well preserved. A major catastrophe occurred on 16–17 June 2013, with heavy rains causing rupturing of the moraine-dammed Chorabari Lake located in the Mandakini basin, Central Himalaya, and exposed 8-m-thick section of the lacustrine strata. We reconstructed the late-Holocene climatic variability in the region using multi-parametric approach including magnetic, mineralogical and chemical (XRF) properties of sediments, paired with grain size and optically simulated luminescence (OSL) dating. The OSL chronology suggests that the lake was formed by a lateral moraine during the deglaciation phase of Chorabari Glacier between 4.2 and 3.9 ka and thereafter the lake deposited about 8-m-thick sediment sequence in the past 2.3 ka. The climatic reconstruction of the lake broadly represents the late-Holocene glacial chronology of the Central Himalaya coupled with many short-term climatic perturbations recorded at a peri-glacial lake setting. The major climatic phases inferred from the study suggests (1) a cold period between 260 BCE and 270 CE, (2) warmer conditions between 900 and 1260 CE for glacial recession and (3) glacial conditions between ~1370 and 1720 CE when the glacier gained volume probably during the ‘Little Ice Age’ (LIA). We suggest a high glacial sensitivity to climatic variability in the monsoon-dominated region of the Himalaya.

Development and application of an electrical conductivity transfer function, using diatoms from lakes in the Urals, Russia

Despite a relatively large number of paleoenvironmental and archaeological studies in the Southern Urals, Russia, Holocene moisture dynamics in the region remain a subject of debate. Electrical conductivity (EC) of lake waters in the Southern Urals is climate-dependent and associated with effective moisture. I developed a transfer function using a 72-lake regional diatom dataset and inferred past EC using diatoms in sediment cores from three lakes. The diatom-inferred EC inferences were compared with pollen records to validate the climate interpretation and explore the possible influence of vegetation on lake hydrochemistry. Multivariate analysis showed a clear response of diatom communities to electrical conductivity across a range from 55 to 3780 μS cm−1. The best electrical conductivity inference model (r2boot = 0.78, RMSEPboot = 0.21 log10 μS cm−1), suitable for quantitative reconstructions, was developed using weighted averaging with classical deshrinking. Application of the transfer function to sediment cores from mountain lakes in the forest and forest-steppe regions of the Southern Urals revealed relations between EC and variations in pollen contents of herbs, pine (for the forest-steppe lake) and spruce (for the forest lakes). Although a decrease in EC may be attributable to coniferous forest spread in the watersheds, it is more likely that climate fluctuations caused the coincident shifts in vegetation and lake-water hydrochemistry. Forest lakes were marked by a declining trend in EC since the beginning of the Holocene (11.9–11.5 cal ka BP), which was independent of the larch and birch forest distribution (11.5–10.0 cal ka BP) and was interrupted by minor climate-driven EC increases. Electrical conductivity of lake water was higher than present in the period between 6.5 and 3.8 cal ka BP in all three study lakes. Forest lakes were characterized by gradual increases and decreases in EC, with maximum values occurring ca. 5.0–4.0 cal ka BP. Diatoms in the core from the forest-steppe lake record variations in EC through time. In particular, the record shows highest EC values at 5.3 cal ka BP, a decline in EC at 4.1–3.8 cal ka BP, a slight increase about 3.6 cal ka BP and a substantial decrease since 2.8 cal ka BP.

Paleoenvironmental context for the Late Pleistocene appearance of Didymosphenia in a North American alpine lake

The nuisance species Didymosphenia geminata is thought to have been spread by humans across North and South America and New Zealand. This diatom is of interest for its ability to form thick benthic mats in streams, altering ecosystem properties. Little is known, however, about its historic distribution and the parameters that are associated with its growth in absence of human influence. Although it is considered to be native to Arctic regions, its status at lower latitudes is uncertain. We measured the concentration of D. geminata cells in a sediment core from Beauty Lake, WY, a record that dates back to the Late Pleistocene. From approximately 11,233–8750 years before present (ybp), a Didymosphenia maximum persisted, with peak concentrations of 3774±20 valves/mg dry sediment (9985 ybp). The period corresponded to low flux of Al, Fe, and P within the sediments, supporting the importance of low P in controlling distribution. Furthermore, we demonstrate that D. geminata arrived in Beauty Lake shortly after deglaciation of the watershed. This species arrived without the aid of human introduction, implying an inherent ability for dispersal. We also examined the sediment record of nearby Yellowstone Lake, WY for the presence of D. geminata during the same time interval, but cells were not detected. The Yellowstone caldera is composed of rhyolite, a rock rich in silica (Si) and modest amounts of P. We conclude that both watersheds were exposed to the potential colonization by D. geminata, but just as the North Island of New Zealand has not been able to support this diatom because of P concentrations exceeding a threshold, the Yellowstone watershed is not able to support it either.

Astronomically forced climate evolution in a saline lake record of the middle Eocene to Oligocene, Jianghan Basin, China

Following the early Eocene climatic optimum some 50 million years ago, the marine paleoclimate record indicates that a long interval of global cooling took place, punctuated by a series of short-term warming reversals and culminating with the glaciation of Antarctica at the Eocene-Oligocene transition. We investigate a saline lake record from the Qianjiang and Jinghezhen formations, Jianghan Basin, China for the corresponding continental climate response. A 20.25 Myr long astronomical time scale is constructed based on 405-kyr orbital eccentricity cycle tuning of gamma ray (GR) series measured from the Qianjiang and Jinghezhen formations. The halite-rich interval between 41-40.4 Ma correlates well with the Middle Eocene Climate Optimum recorded in the deep-sea δ18O record, confirming predominantly halite deposition during the warming. Silt-mudstone-rich intervals were deposited during ∼2.4 Myr orbital eccentricity minima. The evidence indicates a history of alternating fresh water (humid/cool) and saline water (dry/hot) lake cycles paced by ∼100 kyr orbital eccentricity cycles. Analysis of GR series from the deepest part of the lake indicates strong astronomical forcing of halite deposition throughout the Middle-Late Eocene, and a shift to siliciclastic deposition and cooler climates at ∼33.9 Ma. Early Oligocene wet/cool climate in the Jianghan Basin signals East Asian summer monsoon intensification, and a reduction from northwest winter monsoon in response to Tibetan Plateau uplift. Northern Hemisphere summer insolation minima in the Middle-Late Eocene with temperatures that are 4.6°C higher than present may provide an analogy for near-future climate change in Jianghan Basin.

Glacial history of the Greenland Ice Sheet and a local ice cap in Qaanaaq, northwest Greenland

ABSTRACTIn this study, we present new information on the glacial history of the Greenland Ice Sheet (GrIS) and a local ice cap in Qaanaaq, northwest Greenland. We use geomorphological mapping, 10Be exposure dating of boulders, analysis of lake cores, and 14C dating of reworked marine molluscs and subfossil plants to constrain the glacial history. Our 14C ages of reworked marine molluscs reveal that the ice extent in the area was at or behind its present‐day position from 42.2 ± 0.4 to 30.6 ± 0.3k cal a BP after which the GrIS expanded to its maximum position during the Last Glacial Maximum. We find evidence of early ice retreat in the deep fjord (Inglefield Bredning) at 11.9 ± 0.6 ka whereas the Taserssuit Valley was deglaciated ~4 ka later at 7.8 ± 0.1k cal a BP. A proglacial lake record suggests that the local ice cap survived the Holocene Thermal Maximum but moss kill‐dates reveal that it was smaller than present for a period of time before 3.3 ± 0.1k until 0.9 ± 0.1k cal a BP, following which the ice in the area expanded towards its Little Ice Age extent. Copyright © 2019 John Wiley & Sons, Ltd.

Long-term climate, vegetation and fire regime change in a managed municipal water supply area, British Columbia, Canada

Post-glacial climate, vegetation and fire history were reconstructed from a sediment record from Begbie Lake, British Columbia, Canada, located in a municipal water supply area servicing > 350,000 people. Watershed managers have identified wildfire as a threat to water supply and seek to understand how vegetation and fire have varied through time with climate. In the cold late-glacial, open Pinus woodlands, periodically disturbed by fire, transitioned to mixed conifer forests subject to high-severity fire. The early Holocene is of interest to watershed managers because climate was warmer and drier than present. During this interval, low streamflow, abundant fire-adapted taxa, elevated background charcoal and regional increases in biomass burning indicate that fire seasons were longer and that fire was an important disturbance mechanism. Climate moistened in the mid Holocene, facilitating canopy closure and decreased fire disturbance. However, surface fires prevailed in Quercus ecosystems, which were expanding locally. Charcoal increased between 6180–2500 cal yr BP as climate further cooled and moistened, likely reflecting human activity and/or increased climate variability. Modern conditions arose within the last few millennia, impacted most recently by European settlement. In combination with paleoclimate modelling, modern management practices and forecast simulations, the Begbie Lake record informs about ecosystem changes within the watershed, yielding insights for management.

Holocene environmental change at Inle Lake, Shan State, Myanmar, and its implications for the regional development of agriculture

We report pollen, charcoal and diatom records from the upper 4 m of a core from Inle Lake, Myanmar, spanning the entire Holocene, to investigate the changing palaeoenvironments in which early agriculture developed. From 12,000 to 10,500 cal yr BP a grass swamp existed until it was flooded by rising lake levels, indicated by the onset of marl deposition at the site. The onset of higher lake levels coincided with a transition from conifer-rich forest to mixed deciduous-evergreen forest, and a reduction in local biomass burning. Lake levels remained high until 6400 cal yr BP, thereafter fluctuating with possible dry phases at 4050, 2400 and 1100 cal yr BP. Increased burning is apparent from 2200 cal yr BP inferred to represent the onset of human forest clearance to its very reduced state today. The Inle Lake record compares with Holocene sequences in China, Thailand and India and marine cores south of Myanmar, which similarly show a regional pattern of drier conditions in the early Greenlandian stage, followed by maximum lake levels in the early Northgrippian stage. Fluctuating lake levels are evident in the later Northgrippian and Meghalayan stages after 6000 cal yr BP. This pattern coincides with precessional reduction in summer insolation in the region, which weakens the Asian Monsoon. The present-day bare catchments of Inle Lake probably mostly resulted from forest destruction over the last 2000 years although hints of human presence can be inferred throughout the older record.

Stratigraphic variations in the δ201Hg/δ199Hg ratio of mercury in sediment cores as historical records of methylmercury production in lakes

Total mercury (Hg) profiles of sediment cores from lakes serve as historical records of Hg pollution. They do not, however, provide information about temporal variations in the abundance of the toxic, preferentially bioaccumulated species methylmercury (CH3Hg+), whose production rate is largely a function of environmental variables that control the growth and activities of Hg-methylating and Hg-demethylating bacteria and the bioavailability of the inorganic Hg(II) from which CH3Hg+ is formed. Earlier studies showed that CH3Hg+ in freshwater food webs had anomalously high δ201Hg/δ199Hg ratios and, similarly, that the δ201Hg/δ199Hg ratio of Hg in a core from a severely Hg-polluted lake paralleled temporal changes in CH3Hg+ production. The present study was undertaken to test the hypothesis that variations in the δ201Hg/δ199Hg ratios of sediment cores from lakes reflect historical variations in CH3Hg+ production in the lakes. Core profiles of δ201Hg/δ199Hg ratios and total Hg concentrations from three widely separated, environmentally distinct temperate lakes polluted with Hg from different industrial sources, and one practically pristine Arctic lake, were examined, and changes attributable to known historical events were noted. Hg pollution led to increases in the δ201Hg/δ199Hg ratio, suggesting enhanced production of CH3Hg+, but the lakes showed different patterns of variation over time, implying the influence of site-specific environmental and biotic factors. In one lake the δ201Hg/δ199Hg ratio increased at the onset of each of three pollution episodes but peaked and then declined, suggesting a surge in methylation that was reversed by demethylation. In another lake, mercury pollution initiated a long-term upward trend in the isotope ratio, with secondary maxima and minima superimposed on it, implying a progressive increase in CH3Hg+, with successive shorter-term changes caused by fluctuations in methylation and demethylation rates. In the third lake, moderately elevated isotope ratios and one large maximum were associated with Hg contamination, but the maximum is attributable to accelerated eutrophication that intensified CH3Hg+ production. In the Arctic lake, the isotope ratio varied with climate change, decreasing during warming trends and increasing during cooling phases. These results constitute evidence that δ201Hg/δ199Hg ratios of sediment cores from lakes record historical variations in the net rate of CH3Hg+ production.

Seismic stratigraphical record of Lake Levinson‐Lessing, Taymyr Peninsula: evidence for ice‐sheet dynamics and lake‐level fluctuations since the Early Weichselian

A multi‐channel, high‐resolution seismic reflection survey using a Micro‐GI airgun was carried out in the framework of the Russian‐German project PLOT (Paleolimnological Transect) on Lake Levinson‐Lessing, Taymyr Peninsula, in 2016. In total, ~70 km of seismic reflection profiles revealed in unprecedented detail the glacial and postglacial sedimentary infill of the lake basin. Five main seismic units have been recognized and interpreted as glacial (Unit V), subglacial and proglacial (Unit IV), marine (Unit III), fluvial‐lacustrine (Unit II) and lacustrine (Unit I) sediments. Of particular significance are imbricated, south‐orientated structures present in the southernmost part of the lake basin within Unit V and a large topographic ridge recognized in front of those structures. We interpret these structures as push moraines and an end moraine, respectively, left by the glacier after its retreat. The depositional pattern of the units above the moraines documents past lake‐level fluctuations. We interpret Unit IV, Unit III and Unit I as highstand deposits, and Unit II as lowstand deposits. Gas‐charged sediments dominate the northern part of the lake basin, whilst they occur only sporadically and in limited spatial extent in the central and southern parts of the lake. In the latter areas, the seismic and echo‐sounder data suggest recent tectonic activity. Our study contributes to the reconstruction of environmental conditions in the Taymyr Peninsula directly following the Early Weichselian deglaciation and shows that deep tectonic lake basins affected by several glaciations can preserve important palaeoenvironmental records, which contributes significantly to our understanding of palaeoenvironmental changes in the Taymyr Peninsula and the central Russian Arctic.

Tropical South Atlantic influence on Northeastern Brazil precipitation and ITCZ displacement during the past 2300 years

Recent paleoclimatic studies suggest that changes in the tropical rainbelt across the Atlantic Ocean during the past two millennia are linked to a latitudinal shift of the Intertropical Convergence Zone (ITCZ) driven by the Northern Hemisphere (NH) climate. However, little is known regarding other potential drivers that can affect tropical Atlantic rainfall, mainly due to the scarcity of adequate and high-resolution records. In this study, we fill this gap by reconstructing precipitation changes in Northeastern Brazil during the last 2,300 years from a high-resolution lake record of hydrogen isotope compositions of plant waxes. We find that regional precipitation along the coastal area of South America was not solely governed by north-south displacements of the ITCZ due to changes in NH climate, but also by the contraction and expansion of the tropical rainbelt due to variations in sea surface temperature and southeast trade winds in the tropical South Atlantic Basin.

First records of polycyclic aromatic hydrocarbons and metals in sediments from a shallow lake in the Pampean–Patagonian region (Argentina)

This study evaluated, for the first time, organic and metal contaminants in sediments from Argentinean saline lakes, specifically La Salada shallow lake, a small polymictic lake on the south border of the Pampean region. The concentration of 16 priority polycyclic aromatic hydrocarbons (PAHs) and 8 metals (Cd, Cu, Cr, Fe, Mn, Ni, Pb and Zn) was assessed and the ecotoxicological effects of these pollutants were examined. The total concentration of the 16 PAHs ranged from 12.55 to 10679.48ngg–1 dry weight, with a predominance of four-ringed PAHs. Molecular ratio analysis showed pyrolytic sources as the main source of PAHs. Among the metals, Mn, Fe and Zn had the highest concentrations in sediments. General metal levels revealed a multiplicity of point sources and rock weathering as their main source. Regarding the potential ecotoxicological effects, adverse effects may be expected from PAHs because several compounds exceeded the sediment quality guidelines threshold effect level and likely effect level. Conversely, metal levels were below levels specified in safety guidelines. These results are interpreted in an international context, providing information that addresses a gap in the pollution record of Argentinean lakes, and should have strong implications for future planning management.

Variations in U concentrations and isotope signatures in two Canadian lakes impacted by U mining: A combination of anthropogenic and biogeochemical processes

Temporal and vertical variations in uranium (U) concentrations and U isotope (δ238U, ‰) signatures were examined in sediment cores collected seven times over a one year period, from two lakes in Ontario, Canada, which are contaminated with U by historical mining activities. Bow Lake is holomictic, experiencing seasonal anoxia, while the sediments of meromictic Bentley Lake are permanently anoxic. Average annual peak concentrations of U in Bow Lake subsurface sediments were approximately 300 μg L−1 and 600 μg g−1 in porewater and bulk sediments, respectively. Similar ranges of concentrations (900 μg L−1 and 600 μg g−1, respectively) were observed in Bentley Lake sediments. The exceedingly high levels of U observed in the porewaters of both lakes, as well as the seasonal variability in U levels, challenge the traditional paradigm regarding U chemistry, i.e., that reduced U(IV) should be insoluble under anoxic conditions.The average annual δ238U ‰ values at the sediment-water interface of both lakes were similar (i.e., 0.47 ± 0.09‰ and 0.50 ± 0.16‰, relative to IRMM-184). The deep sediments in both Bentley Lake and Bow Lake record U isotope composition with a typical fractionation of 0.6‰ relative to the surface water, confirming authigenic U accumulation, i.e., negligible contribution of particulate material from the tailings. Also, the δ238U values in porewater have an average offset of ca. −0.1‰ relative to bulk sediments in anoxic zones and are reversed in the oxic sediment layer.