Lake Cores Research Articles

Salinity impacts on n-alkanes in lake sediments of the Badain Jaran Desert, Northwestern China: Implications for paleoclimate reconstruction

n-Alkanes in lake sediments are essential biomarkers for paleoclimate research, yet the influence of lake salinity on their distribution remains unclear. This study investigates this relationship by analyzing sediments from eight lakes in the Badain Jaran Desert, all subject to similar climatic conditions but varying in salinity. Soil samples from the shoreline vegetation belt and suspended particle matter (SPM) were collected to identify the origin of plant input in the lake sediments. Additionally, sediment samples from the littoral zone to the lake center were all gathered to evaluate the salinity effects at different water depths. Key indexes such as content, the proportion of aquatic macrophyte (Paq), average chain length (ACL), and carbon preference index (CPI) of n-alkanes were examined to elucidate salinity effects. The result showed that the surrounding vegetation belt is the primary source of n-alkanes in lake sediments, with minimal contributions from SPM. Increased salinity was found to decrease CPI in the littoral zone, while also reducing Paq and increasing ACL in the lake bottom sediments. Changes of Paq, ACL and CPI in sediments are not attribute to variations in vegetation belt input but rather suggest that high salinity enhances n-alkane degradation, particularly favoring the degradation of mid-chain compounds in lake bottom sediments. Consequently, the reliability of n-alkane indexes as indicators of aquatic macrophyte in deep lake cores may be compromised by salinity. This study underscores the importance of accounting for salinity effects when using n-alkanes in reconstructing paleoclimate and suggests that their potential as indicators of lake salinization merits further attention.

Sediment Core DNA-Metabarcoding and Chitinous Remain Identification: Integrating Complementary Methods to Characterise Chironomidae Biodiversity in Lake Sediment Archives.

Chironomidae, so-called non-biting midges, are considered key bioindicators of aquatic ecosystem variability. Data derived from morphologically identifying their chitinous remains in sediments document chironomid larvae assemblages, which are studied to reconstruct ecosystem changes over time. Recent developments in sedimentary DNA (sedDNA) research have demonstrated that molecular techniques are suitable for determining past and present occurrences of organisms. Nevertheless, sedDNA records documenting alterations in chironomid assemblages remain largely unexplored. To close this gap, we examined the applicability of sedDNA metabarcoding to identify Chironomidae assemblages in lake sediments by sampling and processing three 21-35 cm long sediment cores from Lake Sempach in Switzerland. With a focus on developing analytical approaches, we compared an invertebrate-universal (FWH) and a newly designed Chironomidae-specific metabarcoding primer set (CH) to assess their performance in detecting Chironomidae DNA. We isolated and identified chitinous larval remains and compared the morphotype assemblages with the data derived from sedDNA metabarcoding. Results showed a good overall agreement of the morphotype assemblage-specific clustering among the chitinous remains and the metabarcoding datasets. Both methods indicated higher chironomid assemblage similarity between the two littoral cores in contrast to the deep lake core. Moreover, we observed a pronounced primer bias effect resulting in more Chironomidae detections with the CH primer combination compared to the FWH combination. Overall, we conclude that sedDNA metabarcoding can supplement traditional remain identifications and potentially provide independent reconstructions of past chironomid assemblage changes. Furthermore, it has the potential of more efficient workflows, better sample standardisation and species-level resolution datasets.

Evidence of lake-level fluctuations during the late Holocene based on diatoms from Lake Vransko, Cres Island (north-central Mediterranean)

An investigation of Lake Vransko, located on the island of Cres in the Adriatic Sea, was performed in order to reconstruct the palaeoenvironmental changes that occurred during the late Holocene, based on diatoms. Fluctuations in the lake level were mostly driven by the intensification of climatic oscillations (wet periods during North Atlantic Oscillation intervals and summer drought during Azores High intervals) and anthropogenic influences (e.g. the extraction of drinking water). The anthropogenic impact of local deforestation is encountered, associated with a peak in phosphorous in the deeper parts of the lake cores. As a consequence of these impacts ten diatom abundance zones (DZs) and the water/sediment interface (WSI) were recorded from the deeper core (CS-51) and two from the shallower (CS-52) core, along with the WSI, based on which an abrupt fluctuation in the level of the lake was interpreted. A dominance of Epithemia adnata in the deeper core, and Gomphonema pumilum in the CS-51, and in the CS-52 indicated a shallow phase connected with the summer drought recorded in the lake. A dominance of Amphora inariensis, Pseudostaurosira spp., Cyclotella spp. and Pantocsekiella ocellata in the rest of the cores indicated deeper, more alkaline lake conditions during the wet periods. The DZs accord with depth variations in Adriatic coastal lakes that are key sites of more extreme recent changes in the local climate.

Centennial Records of Microplastics in Lake Cores in Huguangyan Maar Lake, China.

Microplastic records from lake cores can reconstruct the plastic pollution history. However, the associations between anthropogenic activities and microplastic accumulation are not well understood. Huguangyan Maar Lake (HML) is a deep-enclosed lake without inlets and outlets, where the sedimentary environment is ideal for preserving a stable and historical microplastic record. Microplastic (size: 10-500 μm) characteristics in the HML core were identified using the Laser Direct Infrared Imaging system. The earliest detectable microplastics appeared unit in 1955 (1.1 items g-1). The microplastic abundance ranged from n.d. to 615.2 items g-1 in 1955-2019 with an average of 134.9 items g-1. The abundance declined slightly during the 1970s and then increased rapidly after China's Reform and Opening Up in 1978. Sixteen polymer types were detectable, with polyethylene and polypropylene dominating, accounting for 23.5 and 23.3% of the total abundance, and the size at 10-100 μm accounted for 80%. Socioeconomic factors dominated the microplastic accumulation based on the random forest modeling, and the contributions of GDP per capita, plastic-related industry yield, and total crop yield were, respectively, 13.9, 35.1, and 9.3% between 1955-2019. The total crop yield contribution further increased by 1.7% after 1978. Coarse sediment particles increased with soil erosion exacerbated microplastics discharging into the sediment.

Palaeoclimatic shifts in the Central Ganga Basin during the Middle- to Late Holocene: Exploring the 4.2 ka arid event and its implications in northern India

The Central Ganga Basin is one of the most densely populated regions of India. It is agriculturally diverse and contributes much to the Indian economy. The region has housed numerous ancient and mediaeval empires. This study presents a continuous record of the paleomonsoon from the Chandrika Devi lake, Lucknow district of Uttar Pradesh, India which is linked with paleo vegetational shifts over the last ~6000 years (5871–75 cal yr BP). The chronology of the lake core is based on three accelerated mass spectrometry (AMS) radiocarbon and two Optically Stimulated Luminescence (OSL) dates. The multiproxy data (grain size, major and trace element ratio, total organic carbon (TOC wt%), carbon isotopes (δ13Corg‰) and pollen), suggest that the lake was initially a part of the Gomti river that began to transform into a lake at ~5000 cal yr BP with weakening of the Indian summer monsoon (ISM) in the Central Ganga Basin. The lake formation was completed at ~4100 cal yr BP under the influence of the 4.2 ka arid event. This phase marks the beginning of human presence as well as agricultural activities in the lake region with the appearance of Cerealia pollen and other agricultural taxa. The agricultural activity surrounding the lake catchment peaked at ~3000 cal yr BP. The lake gradually shrank and became a marshy lowland at ~75 cal yr BP. Our study is significant because it is the first comprehensive multiproxy study from the Lucknow region in the Central Ganga Basin on paleomonsoonal variability and its relationship to human activity, agricultural practices during the Late-Holocene with a focus on the 4.2 Ka arid event. Also, pollen record suggests that the changes in agriculture and human activity began just after 4.2 ka arid event in the study area.

Hydrologic remobilisation of tephra‐fall deposits: A sedimentological analysis throughout fluvio‐lacustrine systems of North‐West Patagonia

AbstractThis paper explores the response to Holocene ash inundation in different sub‐environments of two fluvio‐lacustrine systems, Las Piedritas and Totoral, focussing on the processes of volcaniclastic remobilisation. Sediment cores and outcrops were used to recognise three deposit types, noting how some differ from lahars, through sedimentological and stratigraphic analysis: (1) thick, laterally discontinuous, pumice‐dominated deposits occur overlying alluvial plain and lower terrace surfaces across both watersheds; (2) chaotic, extremely poorly sorted and matrix‐supported beds composed of both epiclastic and pyroclastic material occur as two conspicuous deposits at Las Piedritas outcrops, in a low‐gradient confined channel setting; and (3) layers with an exclusively pyroclastic composition and no flow sedimentary structures or fragment roundness were found draping each of the two chaotic intervals and intercalating with the background sediment in Las Piedritas prodelta. Thick pumice packages capping the floodplains resulted from the overflow of streams carrying a floating pumice load. Due to its positive buoyancy, the pumice is transported as a slowly moving overlying mantle without mixing with the streamflow. The disorganised pattern and poor sorting of the matrix‐rich polymictic beds are consistent with laminar, gravity‐driven, high‐concentration flows. They constitute secondary lahars, originating upstream from large run‐off processes that reworked a pyroclastic substrate. The lack of reworking features in the vitric‐rich units identified in the lake core retrieved from Las Piedritas prodelta, as well as the continuous and widespread occurrences of those interbedded in upstream outcrops, indicate an airfall origin. No reworked tephra unit is recognised in the prodelta sediment record, although various deposits from the hydrologic remobilisation of airfall tephra are found along the subaerial portion of both watersheds. Apparently, large amounts of flotation‐remobilised tephra were retained on the delta plain, while another considerable fraction would have been carried away from the creek mouth under the influence of dominant regional winds.

Investigation of the last two centuries sedimentation dynamics in high-altitude lakes of Southern Carpathians, Romania

This study investigates the last two centuries sedimentation dynamics in four high-altitude lakes located in Southern Carpathians, Romania. Furthermore, a novel approach is proposed for identifying the anthropic or natural underlying causes, by comparison of the acceleration of the change in sedimentation rate with a baseline growth rate trend provided by an isolated peat bog. The high-resolution chronologies were developed using the 210Pb dating technique and the CRS model. 137Cs alternative time-marker validated the age-depth models and reassured the quality of the results. The results indicated several short-interval high sedimentation events within the lake cores, yielding up to five times the average rate for the investigated period. The cause of the high sedimentation episodes was generally attributed to anthropic activities (primarily road construction) and extreme natural events. A first-order derivative equation was employed to plot the acceleration in the sedimentation rate of the lakes with the peat bog baseline. The discrepancies between the acceleration trends highlighted significant deviations from the natural variation tendencies and provided preliminary data regarding the underlying causes of the intense sedimentation periods.

Pollen-based quantitative reconstructions of Holocene climate at Gun Nuur in the northern Mongolian Plateau

This study quantitatively reconstructs Holocene climate based on a palynological analysis of a 745-cm lake core at Gun Nuur in the northern Mongolian Plateau. The core is precisely dated with 49 dates and the dataset comprises 372 pollen samples providing high temporal resolution. We obtained quantitative reconstructions of annual temperature (Tann) and precipitation (Pann) based on comparison with a modern pollen dataset comprising 604 data sites for which climate correlation coefficients are high (R2 = 0.76 for Tann and R2 = 0.70 for Pann) and the uncertainties are relatively small (RMSEP = 1.99 °C for Tann and RMSEP = 46.22 mm for Pann). Results show that the climate in the northern Mongolian Plateau experienced a warm-dry climate in the early Holocene (∼10,300 to ∼7900 cal. yr BP), a stable cool-wet climate in the middle and early-late Holocene (∼7900 to ∼2300 cal. yr BP), and an increasingly cool-wet climate in more recent times (last ∼2300 years). The high temperature in the northern Mongolian Plateau before ∼7900 cal. yr BP is attributable to the thermal sensitivity of the interior Asian continent to high summer insolation. The nearly constant and rather stable temperature during the past ∼7900 years is most likely the result of the thermal insensitivity of the well-developed and densely-covered Taiga forests in south Siberia to declining summer insolation. Increasing precipitation in the northern Mongolian Plateau in the period before ∼7900 cal. yr BP is similar to coeval records from the Altai Mountains, the Yinshan Mountains and the Lake Baikal basin, and most likely resulted from rising sea surface temperature both in the North Atlantic Ocean and in the Western Tropical Pacific Ocean. The stable and nearly constant precipitation during the past ∼7900 years most likely resulted from the combined effect of decreasing water vapor supply from the cooling Western Tropical Pacific Ocean and the increasing water vapor supply from the warming North Atlantic Ocean.

Mid-late holocene climatic reconstruction using core sediments from Khajjiar lake, Himachal Pradesh, India

We present results of our investigations on 7.65 m long core sediments from the Khajjiar lake, Himachal Pradesh, aimed at reconstructing palaeoclimate variability in the climatically sensitive region affected by both ISM (Indian Summer Monsoon) and IWM (Indian Winter Monsoon). Based on the multi-proxy approach, including organic geochemistry (carbon isotopes), magnetic susceptibility as primary proxies and inorganic geochemistry (major oxides) with grain size analysis as secondary proxies, climate of the mid-late Holocene (∼4600 yr) has been established on centennial to millennial scale. The chronology of the sediments is constrained by five AMS radiocarbon dates. The sedimentological data reveal variations in sediment grain size related to palaeo-precipitation. The first high resolution multi-proxy record from the Khajjiar lake core indicates less humid conditions during ∼4600–4185 cal yr BP except an extreme peak of dry and arid climate at around ∼4370 cal yr BP. Intensified monsoon with more wet and humid conditions has been interpreted during ∼4185–3790 cal yr BP. ∼3790–3300 cal yr BP, ∼2845–2115 and ∼1555–405 cal yr BP, and ∼2990–2845 cal yr BP and fluctuations are observed from ∼2115 to 1555 cal yr BP. The results suggest two major climatic phases corresponding with the 4.2 ka and Roman Warm Period (RWP). A regional comparison of mid-late Holocene climate records reveals a broad synchronicity, but with considerable spatial variation. The timing and duration of climate events varied across regions.

THE SIZE INHERITED AGE EFFECT ON RADIOCARBON DATES OF ALLUVIAL DEPOSITS: REDATING CHARCOAL FRAGMENTS IN A SAND-BED STREAM, MACDONALD RIVER, NSW, AUSTRALIA

ABSTRACT Radiocarbon dates on charred plant remains are often used to define the chronology of archives such as lake cores and fluvial sequences. However, charcoal is often older than its depositional context because old-wood can be burnt and a range of transport and storage stages exist between the woodland and stream or lake bed (“inherited age”). In 1978, Blong and Gillespie dated four size fractions of charcoal found floating or saltating in the Macdonald River, Australia. They found larger fragments gave younger age estimates, raising the possibility that taphonomic modifications could help identify the youngest fragments. In 1978 each date required 1000s charcoal fragments. This study returns to a sample from the Macdonald River to date individual charcoal fragments and finds the inherited age may be more than 1700 years (mode 250 years) older than the collection date. Taphonomic factors, e.g., size, shape or fungal infestation cannot identify the youngest fragments. Only two fragments on short-lived materials correctly estimated the date of collection. In SE Australia, this study suggests that wood charcoal will overestimate the age of deposition, taphonomic modifications cannot be used to identify which are youngest, and multiple short-lived materials are required to accurately estimate the deposition age.

Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra

Branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in sedimentary archives are increasingly used for paleotemperature reconstructions due to their strong correlation with mean annual air temperature. However, environmental factors can influence the brGDGT producing bacterial community, potentially affecting the brGDGT-temperature relationship and introducing fundamental errors in reconstructions. Here we assess the reliability of the methylation index of brGDGTs (MBT′5ME) in sediments as a paleotemperature proxy by tracking provenance differences based on brGDGT fractional abundances in a short lake core, two peat cores and surface soils on Sumatra (n = 333 in total). Then, we attempt to reconstruct the Holocene paleotemperatures on Sumatra using the two peat cores. Our results indicate distinct brGDGT and H-shaped brGDGT (H-GDGT) compositions in soil, lake and peat environments, suggesting production by different bacterial communities. We introduce a new index, the isomerization of H-GDGTs (IRH) that can distinguish between these environments. In an 11,000-year long peat core from Diatas, we find that brGDGT composition changes are dominated by bacterial community shifts rather than temperature changes. In contrast, a core from the nearby Padang peatland can be robustly used for a brGDGT-based paleotemperature reconstruction since there are no signs of past environmental or brGDGT source shifts. The results from Padang indicate a gradual warming trend over the past 8,000 years, consistent with climate model simulations and nearby sea surface temperature reconstructions. However, current MBT′5ME calibrations yield larger warming trends compared to simulations and other proxy studies, suggesting the need for tropical and/or peat-specific brGDGT temperature calibrations. Our findings demonstrate the importance of assessing environmental shifts and bacterial source community changes when employing brGDGT paleothermometry. The methodological framework outlined in this study can be used in future research for reliable down-core brGDGT temperature reconstructions. Our proxy reconstruction over the past 8,000 years offers novel insights into the Holocene temperature evolution from a region with low climate seasonality.

Harnessing the composition of dissolved organic matter in lagoon sediment in association with rare earth elements using fluorescence and UV–visible absorption spectroscopy

Dissolved Organic Matter (DOM) plays a pivotal role in influencing metal binding and mobility within lagoon sediments. However, there exists a gap in understanding the compositional alterations of DOM concerning Rare Earth Elements (REEs) across varying pollution gradients. This study aimed to characterize DOM and examine its relationship with REEs in sediment cores from different pollution levels in Yundang Lagoon, China using excitation-emission matrix-parallel factor analysis (EEM-PARAFAC). The results raveled four distinct fluorescent components. Among these, two correspond to humic-like substances, while the remaining two are attributed to protein-like substances. Remarkably, the prevalence of protein-like compounds was observed to exceed 58% of the total fluorescence intensity across all the investigated sites. Furthermore, a substantial discrepancy in total fluorescence intensity was detected between the Songbai Lake and the Inner and Outer Lagoon, indicating a variance in DOM content. In terms of REEs, the average concentration of total REEs was notably elevated within the Songbai Lake sediments (318.36 mg/kg) as compared to the Inner and Outer Lagoon sediments (296.36 and 278.05 mg/kg, respectively). Of significance is the enrichment of Light Rare Earth Elements (LREEs), particularly Ce, La, Pr, and Nd, over Heavy REEs (HREEs) across all surveyed locations. Intriguingly, a coherent trend emerged wherein the fluorescence intensity and LREE concentrations exhibited a synchronized increase from Outer to Inner to Songbai Lake core sediments. This observation substantiates a strong correlation between DOM content and pollution levels (p < 0.05). By shedding light on the intricate interplay between DOM and REEs within urban aquatic sediments, this study imparts novel insights which enrich our comprehension of urban environmental dynamics.

Natural Analogue Studies in Support of Post-Closure Safety Assessment of Deep Geological Disposal

Natural analogues are systems that have evolved over geological timescales with features similar to one or several components of a deep geological repository (DGR). Natural analogues complement short-duration laboratory studies since they are existing reflections of many long-term processes that might affect the performance of a repository. Mathematical models are often used for the post-closure safety assessment of a DGR. Confidence in the models’ predictions is enhanced when the models successfully simulate the past evolution of a natural analogue. This paper summarizes the Canadian Nuclear Safety Commission’s (CNSC’s) recent research on natural analogues to inform on (1) glacial erosion, (2) engineered barrier system, and (3) uranium reactive transport in the context of DGRs for radioactive wastes. Glaciation and its erosion are prominent factors impacting the performance of future DGRs at high latitudes in the northern hemisphere. The authors have reviewed the field data from the Greenland Analogue Project, developed a conceptual and mathematical model for the simulation of the thermal conditions within the Greenland ice sheet, as well as the thermal-hydraulic conditions at its base and the ice sheet velocity, and eventually estimated the erosion rate at the site. The Cigar Lake Analogue demonstrates the long-term radionuclide containment capability of the illite clay zone enveloping the ore body, serving as an analogy to the engineered clay barriers. The CNSC and University of Ottawa analyzed 129I in the Cigar Lake core samples, and modeled and correlated the diffusion-dominated transport of radionuclides over the geological evolution of the Cigar Lake deposit. The results provide information on the mobility of fission products and significant radionuclides in conditions analogous to the source, engineered barriers, and near-field host rock of a DGR. The reactive transport and geochemistry of the Kiggavik-Andrew Lake uranium deposit mineralization and remobilization was another natural uranium deposit analogue studied by the CNSC. A reactive transport model was established according to the conceptualized geochemical processes and run under specified boundary and initial conditions to validate the geochemical processes. The geometry, timing, geochemistry, and fluid composition were used as model constraints.

WEPAPIS 70 (Western Pacific paleointensity-stacking for the last 70 ka)

Earth's magnetic field comprises dipoles and non-dipole components. In some cases, the non-dipole magnetic fields can be detected only in certain areas. Some changes in the magnetic field might occur and be recorded only in certain regions, so it is necessary to make relative paleointensity stacking for regions to observe events that may not be recorded in other regions. To find out about these changes in the Western Pacific, a record called WEPAPIS-70 (Western Pacific Paleointensity Stacking for the Last 70 ka) was made by stacking the paleointensity record from twelve marine and lake cores. WEPAPIS-70 is comparable to other global and regional paleointensity models but shows features that are possibly specific to the Western Pacific region. WEPAPIS-70 identifies the Tianchi excursion that was not recorded in the western hemisphere. WEPAPIS-70 also identifies short-lived events, such as the Rockall excursion.

Lacustrine evidence reveals spatially and temporally distinct Holocene ruptures on the Sawtooth Fault, Central Idaho, USA

ABSTRACTThe Sawtooth Fault in central Idaho, USA, is a range‐bounding normal fault with a Late Pleistocene–Holocene scarp near the up‐valley ends of several range‐front lakes. Cores from Redfish Lake, which spans the fault, exhibit evidence of catastrophic sediment re‐mobilization in two sequences consisting variably of intraclastic mud‐clast conglomerate, massive homogenite, and graded silt and clay. Event strata of the younger sequence are in unconformable contact with Pleistocene sediments in two cores on the northern lake‐basin slope, suggesting transport from the down‐valley portion of the lake toward the basin center. A silt cap associated with the event strata is preserved at all coring sites. The event sequence, distributed lake‐wide, dates to ca. 4300 cal a bp, while additional event strata in two Redfish Lake cores pre‐date the 7627 cal a bp Mazama ash. We propose seismic triggering of the sediment disruptions. Asynchronous event stratigraphy in Pettit Lake, 15 km south of Redfish Lake, suggests distinct ruptures along two portions of the Sawtooth Fault. The sedimentology and geochemistry of remobilized sediments in both lakes indicates that sources included Late Pleistocene and early Holocene lake‐floor sediments and suggests failures that cut deeply into the sedimentary sequence. Cores collected from Stanley Lake (17 km northwest of Redfish Lake) following the nearby 2020 M6.5 Stanley earthquake exhibit a thin, sandy event horizon associated with documented delta collapse. Subaerial slope failures associated with the 2020 event left no widespread disturbance stratigraphy in Redfish Lake. We conclude that the large‐magnitude lake‐floor sediment remobilization events probably required rupture of fault sections proximal to Redfish and Pettit lakes.

Earthquake, floods and changing land use history: A 200-year overview of environmental changes in Selenga River basin as indicated by n-alkanes and related proxies in sediments from shallow lakes

The Selenga River basin, located in southern Siberia, is an important component of the Lake Baikal ecosystem, and comprises approximately 80 % of the Baikal watershed. Within the Selenga River basin, two localized study regions were chosen. The first, the Selenga Delta, is one of the largest inland freshwater floodplains in the world and plays an important role in the ecosystem functioning of Baikal. It purifies the river waters before they enter the lake and acts as a refuge for many of Baikal's endemic species. The second location, the Gusinoozersk region, is southwest of Lake Baikal and the Selenga Delta, and was chosen as a more heavily industrialized region within the Selenga River basin. Anthropogenic activities, including industry, urban settlements, aquaculture and agriculture, have historically increased ecological damage within this area. We assessed possible drivers of changes in sedimentary organic matter (OM) composition within two shallow lakes (SLNG04 and Black Lake), located in the Selenga Delta and the Selenga watershed, respectively. We focused on individual n-alkanes, one of the most abundant and common lipids used to provide information on past vegetation and used multivariate statistics to disentangle changes in the sources of sedimentary OM over time. The depositional OM history of SLNG04B core can be divided in four zones: (i) major influence of non-emergent vascular plants, typically found in transitional environments (ca. 1835 to ca. 1875); (ii) increased influence of grasses/herbs (ca. 1880 to ca. 1910); (iii) transition from non-emergent vascular plants and grasses/herbs to submerged and floating macrophytes and phytoplankton (ca. 1915 to ca. 1945); (iv) maintenance of autochthonous OM from submerged and floating macrophytes and phytoplankton (ca. 1945 to ca. 2014). The depositional OM history of the Black Lake core can be divided in two main zones: (i) major influence of non-emergent vascular plants and submerged and floating macrophytes (ca. 1915 to ca. 1980); (ii) increased influence of grasses/herbs and phytoplankton (ca. 1980 to ca. 2010). Natural events (e.g., an earthquake in 1862 caused flooding and subsidence of much of the land surrounding SLNG04 lake and a further catastrophic flood event in 1897) and anthropogenic activities (e.g., nutrient pollution from expansion of agricultural and livestock population) changed the composition of sedimentary OM resulting in ecological shifts across trophic levels in the Selenga River basin.

Shed female caribou antlers extend records of calving activity on the Arctic National Wildlife Refuge by millennia

Caribou (Rangifer tarandus) have among the longest annual migrations of any terrestrial mammal as they move from winter ranges to spring calving grounds. Biomonitoring records indicate broad consistencies in calving geography across the last several decades, but how long have herds used particular calving grounds? Furthermore, how representative are modern patterns of calving geography to periods that pre-date recent climatic perturbations and increased anthropogenic stresses? While modern ecological datasets are not long enough to address these questions, bones from past generations of caribou lying on the tundra provide unique opportunities to study historical calving geography. This is possible because female caribou shed their antlers within days of giving birth, releasing a skeletal indicator of calving. Today, the Coastal Plain of the Arctic National Wildlife Refuge (Alaska) is a key calving ground for the Porcupine Caribou Herd (PCH). To test the duration across which caribou have used this area as a calving ground, we radiocarbon dated three highly weathered female antlers collected from tundra surfaces on the Coastal Plain. Calibrated radiocarbon dates indicate that these antlers were shed between ~1,600 and more than 3,000 calendar years ago. The antiquity of these shed antlers provides the first physical evidence of calving activity on the PCH calving grounds from previous millennia, substantiating the long ecological legacy of the Coastal Plain as a caribou calving ground. Comparisons to published lake core records also reveal that dates of two of the antlers correspond to periods with average summer temperatures that were warmer than has been typical during the last several decades of biomonitoring. This finding expands the range of climatic settings in which caribou are known to use the current PCH calving grounds and suggests that the Coastal Plain of the Arctic Refuge may remain an important caribou calving ground during at least portions of predicted future warming. Discarded skeletal materials provide opportunities to assess the historical states of living populations, including aspects of reproductive biology and migration. Particularly in high-latitude settings, these insights can extend across millennia and offer rare glimpses into the past that can inform current and future management policies.

Quantitative tree cover reconstruction during the past 2000 years in the central–northern parts of the Daxing'an Mountains, northeast China

Reconstructing the history of tree cover and exploring the relationship between forest growth and environmental factors are of great significance for predicting future vegetation changes and guiding ecosystem restoration and protection. On the basis of pollen data of two lake cores and three peat profiles in the central–northern parts of the Daxing'an Mountains and using multiple methods, we quantitatively reconstructed the tree cover of the past 2000 years. We found that the reconstruction results of the different methods are quite similar. Among all these methods, random forest had the highest prediction accuracy for modern pollen samples. Therefore, the random forest reconstruction results can better reflect changes in regional tree cover, which was approximately 38%–42% during 2000–200 Cal yr BP. It then obviously increased from a low value of approximately 40% to a modern level of approximately 45%. The precipitation and carbon dioxide (CO2) concentration were the main drivers for the tree cover before and after 200 Cal yr BP, respectively. This indicates that environments with elevated CO2 level in the near future will benefit the growth of vegetation in the central–northern parts of the Daxing'an Mountains.

Paleoenvironmental studies of lakeside watershed settlements of the Tobol-Ishim inter-fluve (Zolotoe 1 settlement, Kurgan Oblast)

During the study of the process of settling people in the territory of the forest-steppe and steppe zones of Western Siberia, both valleyside settlement sites of the Bronze Age and watershed lakeside settlements have been identified. The settlement of Zolotoe 1 is one of the few sites of the Late Bronze Age investigated by excava-tions and confined to the vast water-dividing surface between the rivers Tobol and Ishim in Kurgan Oblast. Two stages of population have been identified, associated with the Alakul and Alekseyevka-Sargary Cultures. Despite the change of the resource base from alluvial to watershed, the archaeological materials do not indicate differ-ences in the economies and traditions of the Alakul population of this watershed settlement from the analogous valley sites. Due to the lack of data on the environment of such watershed settlements of the Bronze Age, the analysis has been carried out on the pollen data from the benthal deposits of Lake Zolotinskoe nearby the settle-ment (off-site data), carbonated macro-residues from various archaeological contexts of the occupational layer (on-site data), and the soil profiles on the settlement and beyond it (on-site data). By means of the radiocarbon dating it was found that the palynological data from the lake core sample show the environmental conditions of only the last third — end of the 2nd millennium BCE, beginning from the 24th century BCE; hence, it has not been possible to reconstruct the specifics of the natural environment surrounding the Alakul population of the site du-ring the first phase of the settlement. The overall environmental background for the time depth of the Alakul Cul-ture has been analyzed on the data from the neighboring regions. It appears that it does not correspond with the arid phase but is related to warm, but moderately humid conditions. However, already in the middle of the 2nd millennium BCE (3,500–3,300 cal. BP), probably, there were changes towards more arid and/or warm climate, which brought about adverse conditions for the forestation in the forest-steppe, advance of the steppe and a low-ering of the groundwater table. This is also confirmed by the results of the study of Lake Zolotinskoe. Beginning from the 24th century BCE, the watershed areas of the Tobol basin were dominated by open meadow-steppe landscapes with small birch forests, while the climate was more arid than it is today. The lower part of the lake waterside was heavily waterlogged; the lake probably shoaled in summer, but did not overdry. Such a natural environment was the background for the second period of population of the settlement by the bearers of the Alek-seyevka-Sargary Culture and, probably, was not changing up until the end of the 2nd millennium BCE. The most part of the carbonized macro-residues of the Alakul and the Sargary cultural layer belongs to ruderal flora, which suggests a long-term inhabitation on the settlement. The analysis of the soil buried underneath the cultural layer suggests that during the emergence of the settlement the natural conditions were similar to modern, but differed in a better water availability.

Radionuclide 239+240Pu for Dating and Sedimentation Rate in Chinese Lakes

Radionuclide 239+240Pu dating and sedimentation rate is a crucial parameter for studying lake sedimentary processes. To comprehensively investigate 239+240Pu vertical distributions and relevant processes in Chinese lakes, this study collects and summarizes 54 sediment core samples from 40 lakes and reservoirs globally. First, the results suggest that two typical vertical distributions of 239+240Pu are summarized in Chinese lakes. The location of the 239+240Pu peak corresponds to the age of global atmospheric nuclear test deposition in 1963 ± 1. Second, 239+240Pu based on 1963 peak is far more suitable for lake dating compared to 137Cs, and a significant linear correlation (R2 = 0.924) between the depth of 239+240Pu maximum value (cm) and sedimentation rate (cm a–1) in Chinese lakes was established. For the first time, this study has found that this significant linear relationship can be generalized globally (R2 = 0.909), regardless of where the lake sediment cores were sampled. Third, we identify three typical vertical distributions for the 240Pu/239Pu atom ratio in Chinese lake sediment cores: invariant distribution, mutation distribution, and gradient distribution. This result implies that 239+240Pu in Chinese lakes mainly originate from the global fallout; yet this does not rule out the potential contribution from local sources of 239+240Pu, such as the Lop Nor nuclear tests, the atomic city nuclear activities, and the Chernobyl nuclear accident. However, the trend in the average 240Pu/239Pu ratio varies independent of latitude. Moreover, a significant positive linear correlation is detected between average 239+240Pu inventory in the soil and lake cores, with a coefficient of 0.538. In particular, the high 239+240Pu inventory background value in Northwest China closely correlates with the deposition of 239+240Pu in some lakes (e.g., Lake Sugan). This manuscript provides insights into 239+240Pu vertical patterns and processes in Chinese lakes and sheds light on future research using 239+240Pu for dating and sedimentation rate in lakes outside China.