Recent Lake Sediments Research Articles

Shallow lake response to Holocene climate variation in south-central Minnesota, USA

Sedimentary deposits in lakes across the upper Midwest record the co-evolution of climate and biogeochemistry since the retreat of the Laurentide Ice Sheet at the end of the last glacial period. Here, we report on a Holocene lake sediment record from Chub Lake, a shallow (3 m depth) eutrophic lake system in south-central Minnesota. High-resolution elemental data from scanning XRF along with variations in organic matter, carbonate minerals, clastic material, biogenic silica, charcoal, and carbon isotopes reveal internally consistent patterns of hydroclimatic influence on this shallow lake system from 11,300 years BP to present. In particular, authigenic carbonate mineral formation and preservation in Chub Lake appears to be well suited as a moisture proxy beginning around 9700 BP up until ∼2300 BP, when a combination of more humid climates and basin-infilling change the hydrology of Chub Lake. This work emphasizes the importance of evaluating shallow lake sediment records both as important archives of climate proxies and case studies on how changing climates impact aquatic systems.

Postglacial evolution of the Lake Constance shelf and its relation to a 10-km long chain of submerged Neolithic mounds

A recent bathymetric survey of Lake Constance revealed ~ 170 mounds composed of loosely deposited rocks aligned in a ~ 10-km long chain along the southern Swiss shoreline in a water depth of 3–5 m. The mounds are 10–30 m in diameter and up to 1.5 m high. Over their entire length of occurrence, the mounds are estimated to be composed of ~ 60 million individual boulders, with a total weight of ~ 78,000 t. A ground penetrating radar (GPR) survey showed that the mounds are not linked to the glacial substrate but were rather deposited artificially on the edge of a prograding shelf composed of Holocene lake sediments. Here, we present the results of a coring campaign with four piston cores along a GPR transect across one of the mounds. The cores recovered the full Holocene sedimentary succession all the way into the basal till that is overlain by lacustrine sediments dating back to ~ 14,400 cal. yrs BP. The four cores are merged into a ~ 12.4-m long composite section reflecting continuous sedimentation from the siliciclastic-dominated Late Glacial to the carbonate-rich Late Holocene. The stratigraphic horizon representing the mound’s construction was radiocarbon-dated to ~ 5600–5300 cal. yrs BP, placing them in the Neolithic period. This age was confirmed by radiocarbon dating of wood samples collected during underwater excavation of the mounds. Geochemical analysis of the Holocene sedimentary succession shows generally high carbonate contents (average of 69%). The interval from 5750 to 4950 cal. yrs BP, a part of the mound period, is characterized by a Holocene minimum in carbonate content (average of 57%) and by larger mean grain sizes. Comparing these values to those from a recent surface-sediment depth transect indicates that this was a period of rather low lake levels, which might have favoured mound construction. Correlations to nearby archaeological sites and to the general West-Central European lake-level record indicates that the mounds likely were built during a short phase of low lake levels during a general trend of climatic cooling followed by a lake-level transgression.

Geochemical relationships between shells of the gastropod Gyraulus convexiusculus and modern water bodies on the Tibetan Plateau, and their paleoenvironmental significance

Fossil shells of the gastropod Gyraulus sp. are widespread in the sediments of wetlands and lakes, and in fluvial and lacustrine depositional sequences on the Tibetan Plateau (TP). Although aragonite shells of Gyraulus sp. are a potentially valuable archive of information on environmental changes, the living environment of Gyraulus sp. and the significance of its shell geochemistry (e.g., Sr/Cas, Mg/Cas, δ13Cs, δ18Os) are unknown or disputed. In this study we investigated living specimens of the dominant species of Gyraulus sp. (Gyraulus convexiusculus) in rivers, wetlands, and lakes on the TP, together with its fossil shells in the Holocene lacustrine sediments of Nam Co (NC) on the TP. Specifically, we measured the values of Sr/Cas, Mg/Cas, δ13Cs, and δ18Os of the modern and fossil shells. Our results indicate that the hydrochemical composition and electrical conductivity of the ambient water significantly affect the survival and abundance of G. convexiusculus. The environmental tolerance of G. convexiusculus can be ordered as: carbonate-type > sulfate-type > chloride-type. Additionally, we speculate that the upper threshold of electrical conductivity for the survival of G. convexiusculus does not exceed ∼4544 μS/cm. Furthermore, we established the relationship between modern G. convexiusculus shells and its host water and developed Gyraulus-based transfer functions (TFs). To assess their potential for paleoenvironment reconstruction, the TFs were then applied to fossil shells of G. convexiusculus collected from a Holocene lake sediment profile from a terrace around NC. We show that Sr/Cas in G. convexiusculus is a potentially reliable proxy for reconstructing the Sr/Ca of the paleo-water (Sr/CaPaleo-w) and the paleoconductivity. By contrast, Mg/Cas in G. convexiusculus cannot be used for environmental reconstruction. δ13Cs in G. convexiusculus primarily reflects the δ13CDIC of the host water. When the host water is deficient in HCO3− + CO32− (<280 mg/L) there is an increased supply of dissolved carbon derived from organic sources to the shells of G. convexiusculus. Overall, δ18Os in G. convexiusculus is likely one of the most reliable indicators for the δ18Ow of the host water of the lakes on the TP.

Historical Hg accumulation (∼65 cal kyr BP) in upland lakes of the Southeastern Brazilian Amazonia: New evidence of the extent of geogenic and diagenetic control

The historical upland lake sediments in the Brazilian Amazon witnessed significant enrichment of total mercury (Hg). However, its spatio-temporal relationships between lakes and the main factors responsible for this enrichment are still poorly constrained. Given this, we geochemically investigated 12 radiometrically dated (extending back to ∼65 cal kyr BP) sediment cores from the Carajás plateau, Brazil. The Hg level in historical sediments presented a large temporal variability (from 1 to 3200 μg/kg), with maximum accumulation peaks observed between 30 and 45 cal kyr BP in core R2, LB3, and R1. However, the lack of the Hg peak in other cores (LV2 and LTI3) during the same period despite being proximity and non-correlation of these Hg peaks with the onset of major volcanic events indicates that this source has little bearing. Hg enrichment is highly dependent on the type of sedimentary facies, with higher values were associated with detritic facies (MI) and detritic+organic facies (P/M). Principal component analysis shows that aluminosilicate minerals and organic matter are essential hosts of Hg in sediments. The positive correlation between Al, Ti, and Hg in detritic facies and their strong coherence with Hg/TOC in R1, R5, LSL, ST02, and LB3 cores indicate that Hg is primarily of lithogenic origin. This can be substantiated by the higher background threshold value of Hg (574 μg/kg) in historical lake sediments compared to those in recent lake sediments (340 μg/kg). However, the most pronounced Hg peak (3200 μg/kg) in R2 around 45 cal kyr BP, which correlates positively with TOC, S, Se, As, and Mo indicates their diagenetic enrichment in organic-rich sediments under anoxic conditions. Thus, in addition to the lithogenic effect, it can be argued that diagenesis can play a significant role in prompting Hg enrichment in the Carajás lake sediments in Amazonia.

Quantifying the dynamic processes of soil erosion and lake sediment deposition in the Holocene in China

The Holocene lake sediment records are critical indicators of past global change studies. This study attempts to reconstruct soil erosion rates (SERs) quantitatively based on lake sediment records from 35 lakes in China. Environmental sequences, e.g., precipitation, vegetation and human activity, were reconstructed and used to identify the dynamic processes of soil erosion based on the General Additive Model (GAM). The findings were as follows. First, drivers of soil erosion in lake basins in different sub-regions of China differed markedly due to the evolution of different monsoon systems and regional environments. Second, influenced by the evolution of different monsoon systems and regional environments, soil erosion of the Holocene showed different evolutionary processes in China. For example, SERs gradually increased in the northwestern arid zone and remained relatively stable in the monsoon zone until 2.0 ka (1 ka = 1000 calibrated a B.P.). Third, human activity played a more important role in soil erosion in all sub-regions during the Holocene. Human activities through vegetation/land-use change have led to rapid enhancement of soil erosion in the north, south, and southwest China since 2.0 ka, especially in the last 1000 years. This study contributes to our understanding of the evolution of soil erosion under different climate systems in China.

Geochemical Indicators for Paleolimnological Studies of the Anthropogenic Influence on the Environment of the Russian Federation: A Review

Lake sediments are a reliable source of information about the past, including data of the origin of water bodies and their changes. Russia has more than 2 million lakes, so paleolimnological studies are relevant here. This review deals with the most significant studies of sequential accumulation of pollutants, including heavy metals in recent lake sediments in Russia. The key areas are northwestern regions of Russia (Murmansk Region, the Republic of Karelia, Arkhangelsk Region), the Urals (Chelyabinsk Region, the Republic of Bashkortostan), and Siberia. The review presents the data of pollutants accumulation, the sedimentation rate in lakes in the anthropogenic period, and the key sources of pollution of the environment in each of the mentioned regions. The article is divided into three parts (sections): industrial areas, urbanized areas, and background (pristine) areas so that readers might better understand the specifics of particular pollution and its impact on lake ecosystems. The impact of metallurgical plants, mining companies, boiler rooms, coal and mazut thermal power plants, transport, and other anthropogenic sources influencing geochemical characteristics of lakes located nearby or at a distance to these sources of pollution are considered. For instance, the direct influence of factories and transport was noted in the study of lake sediments in industrial regions and cities. In the background territories, the influence of long-range transport of pollutants was mainly noted. It was found that sedimentation rates are significantly lower in pristine areas, especially in the Frigid zone, compared to urbanized areas and industrial territories. In addition, the excess concentrations of heavy metals over the background are higher in the sediments of lakes that are directly affected by the source of pollution. At the end of the article, further prospects of the development of paleolimnological studies in Russia are discussed in the context of the continuing anthropogenic impact on the environment.

Ancient mining pollution in early to middle Holocene lake sediments from the Lake Superior region, USA

Determining the timing and transport mechanisms (i.e. fluvial and/or atmospheric) of pollution associated with ancient copper mining on Isle Royale in Lake Superior (United States) is important for understanding the current and future trajectory of ecosystems affected by human activity. This study reports metal concentrations in sediment from two small, closed-basin lakes located on Isle Royale (Lily Lake and Pond 2) as a measure of the extent of heavy metal emissions from Indigenous, precontact mining and annealing activities associated with one of the world’s oldest metal working industries. Lily Lake and Pond 2 are both located near known mines, but neither lake contains mines in its catchment and thus could not have received pollution via fluvial transport. Elevated concentrations of lead in sediments dating to ∼6000 years ago at both lakes provide evidence of atmospheric transport of pollution associated with mining emissions. The age of ∼6000 years before present is consistent with the timing of elevated lead concentrations in sediment from McCargoe Cove on the northern shore of Isle Royale. Collectively, the data indicate that the peak in Archaic Period mining occurred on southwestern Isle Royale ∼6000 years ago and was preceded by periods that varied in intensity and location across the area. The discovery of pollution signals in lake sediments from the Lake Superior region provides clues as to the timing, spatial patterns, and magnitude of one of the world’s earliest large-scale metal mining efforts. These findings provide a basis for determining the transport mechanisms of ancient mining pollution and are a step toward assessing the full environmental impact of mining and metal working efforts by early human civilizations. • Anthropogenic lead was detected in lakes near prehistoric mines on Isle Royale. • The presence of anthropogenic lead indicates airborne pollution transport • The peak of mining and associated pollution occurred ∼6000 years ago. • Findings are consistent with regional sediment records and copper artifact dates.

Distribution and carbon isotopic composition of long-chain leaf wax n-alkanes from Holocene lake sediments in the Altai Mountains

The Altai Mountains are located at the junction of Asia and Europe and pathways linking East Asian climate with the Arctic and North Atlantic. Here, we report a record of n-alkanes from the sediments of Shuanghu Lake in the Altai Mountains. In this forested region, the vegetation is dominated by C3 plants and the long-chain n-alkanes (C29–C31) in the lake sediments are predominantly derived from leaf wax lipids. The compound-specific carbon isotopic values of the long-chain n-alkanes are sensitive to regional moisture. Both the δ13C29–31 value and the Paq index show a decreasing trend since ∼9.0 kyr BP, implying an increase in moisture. This trend is punctuated by abrupt decreases in isotopic values centered at 8.2, 7.4, 5.6, 3.0, 2.0 and 0.2 kyr BP, which may indicate cold and wet events on decadal–centennial timescales. The Holocene hydrological patterns recorded at Shuanghu Lake are different from those from monsoonal Asia and they may reflect a response to decreasing Arctic sea ice cover that provided an enhanced moisture source and promoted increased heavy snowfall in the study region, or an increase in the temperature gradient and strength of the jet stream and extratropical cyclones at mid-latitudes, as proposed previously.

Vertical and spatial distribution of plutonium and radio-cesium in lake sediment of China

Anthropogenic radionuclides of plutonium and 137Cs in the environment were usually applied for tracing the environmental impact of nuclear activities as well as establishing the chronology of recent lake sediment. In this study, the vertical distribution and spatial differentiation of plutonium and 137Cs in lake sediments of China were systematically contrastive analyzed to elucidate the regional influence by the local fallout from Chinese nuclear tests. The results demonstrated that the inventory of 137Cs and 239,240Pu, and their isotopic composition in lake sediments in northwestern China were obviously different from other regions, indicating that a portion of radionuclides in lakes in northwest China were originated from the Chinese Lop Nor Nuclear Test site (CNTs), and radionuclides in lakes of other regions may be completely originated from global fallout. Usually, the anthropogenic radionuclides recorded in lake sediments showed a single peak corresponding to the maximum deposition of 1964. However, two or three accumulation peaks of radionuclides were also observed in a few lakes. For the application of radionuclides on the dating of recent sediment, the accumulative peak of 239,240Pu and 137Cs signed to the period of 1963–1964 is the only practicable time marker for lake sediments in other regions of China. Isotopic information, such as 240Pu/239Pu ratio and 239,240Pu/137Cs activity ratio, may be useful for accurate sediment chronology, as well as used as a fingerprint to identify the source of the associated radioactive substance in the environment.

Varved lake sediments as indicators of recent cultural eutrophication and hypolimnetic hypoxia in lakes

Varved sediments provide a unique opportunity to track the history of eutrophication and oxygen conditions in lacustrine environments. The well-preserved varves indicate hypoxic/anoxic conditions in hypolimnetic waters. Sediment records from three lakes (Dubie, Salno and Wąsoskie) located in northern Poland were investigated to determine the drivers that led to the development of varves in the topmost parts of the sediment profiles. We distinguished lithozones characterized by different sedimentary structures in each sediment core. Age-depth models for each sediment record were established using varve counting and isotope dating. Additionally, an independent marker of the Askja-1875 CE tephra supported the dating of sediments from Lake Wąsoskie. Ordination analysis applied to a set of geochemical data enabled identification of three groups of proxies representing the variability in deposition of different material within the lakes: i) terrestrial minerogenic material, ii) organic matter, and iii) carbonates. Our data show that the onset of varve preservation occurred asynchronously in the investigated lakes (from the late 19th century to the mid-20th century) and that the transition from homogeneous to varved sediments was gradual and occurred over several decades. These transformations were consistent with the variability of the proxies, indicating cultural eutrophication and showing a strong relationship with the history of human-induced land-use changes in the catchments. This study shows that well-preserved varves in the topmost sediments of many lakes are geological evidence of human activity. • Preservation of varves in recent lake sediments indicates local land-use transformations. • The onset of varve preservation is asynchronous in different lakes. • The transition zones between homogenous to varved sediments indicate gradual response of lakes.

Asbestos mining waste impacts on the sedimentological evolution of the Bécancour chain of lakes, southern Quebec (Canada)

More than a century (1877–2011 CE) of asbestos mining activities in the Thetford Mines region have resulted in the accumulation of gigantic mineral waste piles on the banks of the Bécancour River (southern Quebec, Canada). This river widens downstream from the mining sites to form a chain of lakes, successively: Stater Pond, Trout Lake, Lake William and Lake Joseph. A previous paleolimnological investigation revealed that waste erosion and transport strongly modified and polluted Trout Lake. However, questions remain about the extent of the mining contamination within the Bécancour River Basin and its impacts at other spatial scales. Here, we aimed to address this lack of knowledge by analyzing the sedimentological evolution of Stater Pond and lakes William, Joseph and Bécancour (upstream reference site). Radiometric dating (210Pb, 137Cs, 14C) and analyses of geochemical composition (ICP-AES/ICP-MS), computed tomography, magnetic susceptibility, loss-on-ignition and grain size were performed on sediment cores retrieved at these sites. In contrast to Lake Bécancour, yet similar to Trout Lake, we found that Stater Pond and Lake William have received high mineral matter loads since the creation of the Lake Asbestos Mine during the 1950s. Recent lake sediments at these downstream sites were highly enriched in magnesium, chromium and nickel. Comparison of their geochemical signature with that of sedimentary source materials from within the drainage basin demonstrated that they predominantly originate from mining waste erosion. Because of this issue, Stater Pond and lakes Trout and William are nowadays exposed to very high sediment accumulation rates (up to 1.4 cm yr−1; 0.6 g cm−2 yr−1) and heavy metal enrichment. Evidence for contamination was also found in Lake Joseph, indicating that wastes are transported and deposited over ≥25 km downstream from the mining sites. Our study highlights the high risks and dangers associated with asbestos pollutants in aquatic ecosystems.

Unraveling the Holocene Eruptive History of Flores Island (Azores) Through the Analysis of Lacustrine Sedimentary Records

Lacustrine sequences from active volcanic settings usually hold a rich and continuous record of tephra layers, providing a critical source of information to reconstruct a most complete eruptive history of a region. Lake sedimentary records on volcanic islands are particularly useful as the typical small size of these islands and their steep subaerial and submarine slopes lead to a lower preservation of potential erodible pyroclastic deposits. Here we explore the lacustrine sedimentary record of Lagoa da Lomba, a crater lake in the central upland area of Flores Island (Azores), to gain insight into the recent eruptive history of this island. The strategic location of Lagoa da Lomba, half distance between the two clusters of recent volcanic activity of the island, together with its long-lasting record, back to 23.52 cal kyr BP, makes this lake a privileged site to investigate the Holocene volcanic history of Flores. Based on a detailed stratigraphic characterization of sediments from a lake transect of three cores, supported by glass shard geochemistry and radiocarbon dating, we recognized four Holocene eruptive events taking place between 6.28 and 2.36 cal kyr BP, demonstrating that the Holocene volcanic activity at Flores Island may have lasted longer than previously reported. Glass shard geochemistry from the different tephra layers suggests three populations, basaltic to trachybasaltic in composition, where the last eruption is the least evolved endmember. Two of the four eruptive events correlate with subaerially-exposed pyroclastic sequences, in terms of stratigraphy and geochemistry. The most recent event recorded at Lagoa da Lomba was constrained to 3.66 – 2.36 cal kyr BP and linked to an eruption sourced from Lagoa Comprida Volcanic System. The second most recent eruptive event was sourced from Lagoa Funda Volcanic System and dated at 3.66 cal kyr BP. Our observations show that Flores experienced vigorous volcanic activity during the Late Holocene. Therefore, contrary to what is assumed, the possibility of future eruptions should be properly considered, and the volcanic hazard here should not be underestimated. Moreover, we highlight the importance of tephrostratigraphy in recent lake sediments to reconstruct past volcanic activity, especially at small volcanic islands, such as Flores, where exposure is poor due to erosion within the limited subaerial area and the dense vegetation.

Tracking mineral and geochemical characteristics of Holocene lake sediments: the case of Hotagen, west-central Sweden

PurposeIntact lake sediments reflect the development of terrestrial ecosystems. This development can be understood by decoding mineral and geochemical information of sedimentary archives. Therefore, we characterized a Holocene lake sediment core and revealed bulk to micro-scale variations via a combination of geochemical techniques and statistical methods.MethodsA 2.3 m sediment core was collected from Hotagen, a lake in west-central Sweden; a sediment sample was collected every 5 cm. A part of each sediment sample was kept untreated (named bulk) and another part was size-fractionated into < 4, 4–16, 16–64, and > 64 µm subsamples. Characterization was then made with respect to grain size distribution (GSD), physico-chemical parameters, geochemical properties, organic composition, and mineralogy. The sediments were investigated at bulk, micro-, and elemental scales using powder X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM–EDX).ResultsThe deepest sediment was identified as glacial till dating back to the Late Pleistocene. The bulk sediments showed a clear distinction between 0–195 cm (unit 1, U1) and 200–225 cm (unit 2, U2) depths. Quartz and feldspar minerals decreased and organic matter and clay minerals increased from the till towards the lower limit of U1. The development in the sedimentary properties marked the transformation of the terrestrial ecosystem from glacier-covered land to vegetated areas. This development was also well reflected by the appearance of X-ray amorphous materials and the formation of distinct organo-mineral aggregates; chlorite was the predominant clay mineral in these aggregates. The geochemical variation between U2 and U1 sediments was further established by resolving the DRIFT spectral components through multivariate curve resolution alternating least square (MCR-ALS). The U1 sediments settled over a period of ~ 7500 years and showed comparable mineral, geochemical, and organic composition. However, the size-fractionated sediments, mainly < 4 µm, showed diverse mineral and geochemical composition. Indeed, these sediments were distinct by containing relatively higher amounts of X-ray amorphous materials and clay minerals, the latter had variable Na, Mg, and K contents.ConclusionThe combined use of geochemical and statistical approaches used in this study followed the mineral and geochemical development of sediments that had settled during the Late Pleistocene and Early Holocene Epochs. Finally, the U2 sediments marked the terrestrial ecosystem development that occurred during the late glaciation, deglaciation, and post-glaciation periods.Graphical abstract

Effects of Pleistocene to Holocene seismicity on the landforms and fluvial-lacustrine sequences of the Ixtlahuaca paleobasin, and their possible relation with the Acambay graben: Implications for the seismic hazard assessment of central Mexico

We mapped Plio-Pleistocene lake deposits in the Ixtlahuaca paleobasin in Central Mexico, located 51 km south of the Acambay Graben and 69 km west of Mexico City. Within the Acambay basin, Pleistocene to Holocene lake sediments record primary and secondary environmental effects of strong local earthquakes as a consequence of Quaternary activity of the major normal border faults. Likewise, the Ixtlahuaca paleobasin has been controlled mainly by the Quaternary evolution of the Perales and Ixtlahuaca faults, even if there is no record of local historical earthquakes, and therefore the area has not been considered as potentially seismic. However, the application of the ESI 2007 Scale to deformation structures as effects of seismic events on the geological-natural environment in the Tierras Blancas - San Bartolo Lanzados basins and San Pedro El Alto, within the Acambay graben, and its comparison with seismites identified in the Ixtlahuaca paleobasin, show that past earthquakes similar to the Mw 6.5 to 7 surface faulting events generated by the main faults of the Acambay graben have also occurred in the Ixtlahuaca area.

Small shards and long distances — three cryptotephra layers from the Nahe palaeolake including the first discovery of Laacher See Tephra in Schleswig‐Holstein (Germany)

ABSTRACTInvestigations of Lateglacial to Early Holocene lake sediments from the Nahe palaeolake (northern Germany) provided a high‐resolution palynological record. To increase the temporal resolution of the record a targeted search for cryptotephra was carried out on the basis of pollen stratigraphy. Three cryptotephra horizons were detected and geochemically identified as G10ka series tephra (a Saksunarvatn Ash), Vedde Ash and Laacher See Tephra. Here we present the first geochemically confirmed finding of the ash from the Laacher See Eruption in Schleswig‐Holstein—extending the so far detected fallout fan of the eruption further to the north‐west. These finds enable direct stratigraphical correlations and underline the potential of the site for further investigations.

Development of an in situ branched GDGT calibration in Lake 578, southern Greenland

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are abundant in lake sediments and can potentially be used as proxies for evaluating past environmental conditions. The degree of methylation of brGDGTs has a strong relationship with the ambient temperature, and numerous brGDGT temperature reconstructions have been generated ranging from the tropics to high latitudes and over a variety of timescales. However, this proxy still requires calibration and validation in Arctic settings, which are among the most climatically sensitive regions on Earth. Here, we investigate Lake 578, a dimictic lake located in southern Greenland, and examine brGDGTs in a sediment core, surface sediments, catchment soils, and settling particulate matter (SPM). Our results suggest that sedimentary brGDGTs are mainly produced in the water column, and the SPM MBT′5ME (Methylation of Branched Tetraethers) values are significantly correlated with depth-integrated summer water temperature. Here, we provide the first lake water temperature-brGDGT calibration based on the MBT′5ME index, with an improved root mean square error (RMSE) of 0.52 °C. Like previous studies, we find that watershed soils differ in brGDGT distributions from those of the SPM or lake sediments, and we conclude that most brGDGT production in Lake 578 is occurring in situ. In the sediment core, we observe a significant cooling trend in the uppermost 7 cm, but this result is unrealistic because it conflicts with the instrumental temperature record from this region. This “core-top cooling” is associated with increasing relative abundances of brGDGT-IIa and IIIa, and with decreasing brGDGT-Ia, and we speculate that it may be driven by diagenetic effects or changing microbial communities in the sediment subsurface. The anomalous “core-top cooling” phenomenon, which is noted in numerous other published lacustrine brGDGT records, highlights the need for further studies of brGDGTs in modern and recent lake sediments. Constraining the source organisms of brGDGTs and better understanding the relationship between brGDGTs and environmental factors are essential, prior to their use in paleoclimate reconstructions.

How to Deal With Multi-Proxy Data for Paleoenvironmental Reconstructions: Applications to a Holocene Lake Sediment Record From the Tian Shan, Central Asia

Multi-proxy investigations on geological archives provide valuable information about environmental variations in the past. As opposed to single-proxy studies, the combination of several proxies can reveal more detailed information and strengthen subsequent paleoenvironmental reconstructions. However, there is still no consensus about how to deal with resulting highly dimensional datasets in a statistical manner. In many cases, the interpretations of multi-proxy datasets rely on visually matching several proxy records, which can lead to incorrect or insufficient interpretations. Here we report an innovative approach that combines the novel dimension reduction technique Uniform Manifold Approximation and Projection (UMAP) and the time series analysis R package asdetect to identify and characterize Holocene environmental phases and phase boundaries in a sediment core from Lake Chatyr Kol, southern Kyrgyzstan. Despite the fact that the Holocene climate evolution of Central Asia has been intensively studied during the last decades, knowledge about regional climate development during the Holocene and the underlying mechanisms is still relatively scarce. We particularly focus on phase transitions and differentiate between event-based shifts as opposed to gradual phase transitions. For this study, long-chain alkenones were used as a paleotemperature proxy and variations in long-chain alkyl diol distributions were ascribed to relative changes of algal input. The compound-specific stable hydrogen isotope compositions (δD) of individual n-alkanes were utilized as paleohydrological proxies, with the δD of mid-chain n-alkanes reflecting changes in the δD of the lake water and the δD of long-chain n-alkanes recording the δD of the meteoric water. We show the potential of modern analysis tools for data-driven paleoenvironmental reconstructions and advocate for their more frequent implementation in multi-proxy studies.

N-Alkane Characteristics of Thawed Permafrost Deposits Below a Thermokarst Lake on Bykovsky Peninsula, Northeastern Siberia

Rapid permafrost thaw by thermokarst mobilizes previously frozen organic matter (OM) down to tens of meters deep within decades to centuries, leading to microbial degradation and greenhouse gas release. Late Pleistocene ice-rich Yedoma deposits that thaw underneath thermokarst lakes and refreeze after lake drainage are called taberal sediments. Although widespread, these have not been the subject of many studies. To study OM characteristics and degradability in thawed Yedoma, we obtained a 31.5 m long core from beneath a thermokarst lake on the Bykovsky Peninsula, northeastern Siberia. We reported radiocarbon ages, biogeochemical parameters (organic carbon (OC) content and bulk carbon isotopes) and n-alkane distributions. We found the most degraded OM in frozen, fluvial sediments at the bottom of the core, as indicated by the lowest n-alkane odd-over-even predominance (OEP; 2.2). Above this, the thawed Yedoma sediments had an n-alkane distribution typical of emergent vegetation, suggesting a landscape dominated by low-centered polygons. These sediments were OC poor (OC content: 0.8 wt%, 60% of samples <0.1 wt%), but the OM (OEP~5.0) was better preserved than in the fluvial sediments. The upper part of the Yedoma reflected a transition to a drier, grass dominated environment. Furthermore, this unit’s OM was least degraded (OEP~9.4). The thermokarst lake that formed about 8 cal ka BP thawed the Yedoma in the talik and deposited Holocene lake sediments containing well-preserved OM (OEP~8.4) with the highest n-alkane concentrations (20.8 μg g-1 sediment). Old, allochthonous OM was found in the thawed Yedoma and frozen fluvial deposits. Using an n-alkane endmember model, we identified a mixed OM input in all units. In our study, the thawed Yedoma sediments contained less OC than reported in other studies for still frozen Yedoma. The Yedoma OM was more degraded compared to previous biomarker research on frozen Yedoma. However, this signal is overprinted by the input signal. The fluvial deposits below the Yedoma contained more OM, but this OM was more degraded, which can be explained by the OM input signal. Continued talik deepening and expansion of this thermokarst lake and others similar to it will expose OM with heterogeneous properties to microbial degradation.

Time marker of 137Cs fallout maximum in lake sediments of Northwest China

Caesium-137 (137Cs) has no natural source and is commonly employed in establishing the chronology of recent lake sediments in the Anthropocene epoch. Because of their location in the northern mid-latitude Westerlies circulation region, downwind of the Chernobyl accident and their proximity to the Lop Nur Chinese Nuclear Testing (CNT) site, lake sediments in NW China record a137Cs fallout maximum of global atmospheric thermonuclear weapon tests (GTWT) in 1963–1964 Common Era (CE), with probably local fallout subpeaks due to the Chernobyl accident in 1986 CE and CNT in 1976 CE. This complexity means that the use of 137Cs fallout maximum in lake sediments as a time marker in NW China is thus far not well constrained, impeding assessment of recent and future changes in paleoclimate and human activities. To identify the 137Cs time marker and provide a reliable chronology of surface lake sediments for investigating paleoclimatic changes and for defining the Anthropocene epoch in NW China during the last 50–100 years, here we describe chronologies of 6 near-surface sediment cores from NW China based on 137Cs and 210Pb activities, and review the chronology of 23 lake sediments and 10 ice cores from NW China and central Asia based on 137Cs, 210Pb, 239+240Pu, 3H, 241Am, and beta-activity dating methods, and annual layer counting, as well as 240Pu/239Pu isotopic ratios. Based on the potential influence of radioactivity transport from the Chernobyl accident and CNT, and the comparison with deposition records of 26 European lake sediments and 5 Alpine ice cores, we propose that the 137Cs fallout maximum of lake sediments in NW China and central Asia is primarily attributable to GTWT and that there is no unambiguous evidence to confirm the Chernobyl- and CNT-derived 137Cs local-fallout subpeaks. Taking into account the approximately one-year delay between injection of 137Cs into the atmosphere and its incorporation in lake sediment records, we conclude that the 137Cs fallout maximum in lake sediments of NW China provides a reliable GTWT time marker for 1964 CE.

Seasonal deposition processes and chronology of a varved Holocene lake sediment record from Chatyr Kol lake (Kyrgyz Republic)

Abstract. Microfacies analysis of a sediment record from Chatyr Kol lake (Kyrgyz Republic) reveals the presence of seasonal laminae (varves) from the sediment base dated at 11 619±603 BP (years Before Present) up to ∼360±40 BP. The Chatvd19 floating varve chronology relies on replicate varve counts on overlapping petrographic thin sections with an uncertainty of ±5 %. The uppermost non-varved interval was chronologically constrained by 210Pb and 137Cs gamma spectrometry and interpolation based on varve thickness measurements of adjacent varved intervals with an assumed maximum uncertainty of 10 %. Six varve types were distinguished, are described in detail, and show a changing predominance of clastic-organic, clastic-calcitic or clastic-aragonitic, calcitic-clastic, organic-clastic, and clastic-diatom varves throughout the Holocene. Variations in varve thickness and the number and composition of seasonal sublayers are attributed to (1) changes in the amount of summer or winter/spring precipitation affecting local runoff and erosion and/or to (2) evaporative conditions during summer. Radiocarbon dating of bulk organic matter, daphnia remains, aquatic plant remains, and Ruppia maritima seeds reveals reservoir ages with a clear decreasing trend up core from ∼6150 years in the early Holocene, to ∼3000 years in the mid-Holocene, to ∼1000 years and less in the late Holocene and modern times. In contrast, two radiocarbon dates from terrestrial plant remains are in good agreement with the varve-based chronology.