The Batagay Megaslump in eastern Siberia exposes one of the oldest permafrost archives and provides insights into Beringian paleoenvironments. However, its geochronology calls for improvement given known difficulties with permafrost dating and current developments in dating techniques. Here, we present the first systematic geochronological study of the Upper Ice Complex (Yedoma) that constitutes more than half of the exposed stratigraphy based on 135 radiocarbon dates and four post infrared infrared stimulated luminescence ages. Newly obtained radiocarbon ages of bulk sediment, wood, and non-wood macrofossils from the same sediment samples span between 55,000 yrs BP and 31,000 cal yrs BP and, supplemented by radiocarbon ages of wedge ice dissolved organic carbon, bulk sediment, and macrofossils, which allow an enhanced interpretation of the dating results. Luminescence ages from the Upper Ice Complex and the units below (149 to 56 ka) provide independent age control and refine the maximum age of the Upper Ice Complex to Marine Isotope Stage 5, supported by an age of 112.8 +/- 8.4 ka and rejecting several finite radiocarbon ages close to detection limit. The present study provides a robust chronology for the interpretation of paleoenvironmental proxy data from the Upper Ice Complex and enhances the overall geochronology of the Batagay Megaslump.

Understanding snow processes in the sea-ice system is essential to improving Arctic sea-ice predictions and climate modeling. We show that the winter snow cover on Arctic sea ice is strongly enriched in heavy isotopes near the snow-sea ice interface, unexplainable by snow metamorphism alone. During the MOSAiC expedition, stratigraphic investigations revealed that large temperature gradients drive water vapor transport and mass transfer from sea ice into the snowpack. We estimate the contributed snow depth equivalent as 39 ± 7 mm (cumulative mass redistribution) and 63 ± 21 mm (isotope two-source model). Despite uncertainties, both highlight the need for detailed snowpack vapor flux modeling. Recognizing this recrystallization process improves understanding of snow stratigraphy, gas exchange, atmospheric chemistry through snow impurity distributions (e.g., sea salt aerosol), and reduces uncertainties in snow mass balance and heat conductivity. With continued Arctic change, evolving snowpack temperature gradients and recrystallized sea-ice snow contributions will further shape these processes.

Paleohydrological connections and pathways within Nettilling Lake, the largest lake in the Canadian Arctic Archipelago (Baffin Island, Canada), remain poorly known. This knowledge gap hinders our ability to reliably infer past environmental changes that could provide clues to the future evolution of this region with rapid environmental change at high latitudes. The lake is currently fed by freshwaters from its catchment and drains westward into the Foxe Basin via the Koukdjuaq River. However, its early hydrological connections following the last deglaciation are thought to have been complex, when marine waters from the postglacial Tyrell Sea initially invaded the Nettilling basin from the west, followed by a brief reversal when Atlantic waters fed the basin from the east via the Cumberland Sound and Nettilling Fjord. To test this hypothesis, we investigated the lithology, sedimentology and geochemistry of various sediment archives from the easternmost sector of the lake. The multi-proxy data revealed three successive phases: an initial glaciomarine phase (8300 - 7300 cal. BP), a brackish phase (7300 - 6000 cal. BP) and a lacustrine freshwater phase (6000 - present). Our results support the hypothesis formulated by Blake (1966) of temporary hydrological connectivity between the Foxe Basin in the west, followed by a link with the Atlantic Ocean in the east via the Cumberland Sound before marine waters retreated due to differential glacio-isostatic uplift that forced the basin to be progressively occupied by the fresh waters of the present-day Nettilling Lake.

Snow on sea ice is crucial in moderating sea ice and atmosphere interactions, yet fully grasping snow's isotopic composition and the processes shaping it presents substantial challenges, including sublimation and wind redistribution. This study utilizes a year of stable water isotope datasets from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019/2020 to explore the complex interactions between snow deposition processes and postdepositional changes affecting snow on Arctic sea ice including seasonal and spatial dynamics. We compare snow data with water vapor isotope measurements by examining 911 individual snow isotope measurements and integrating these discrete snow samples with continuous water vapor isotope data. Autumn shows a pronounced 818O offset between snow and vapor. In winter, 818O and d-excess in surface snow and water vapor diverge sharply, indicating kinetic fractionation under extremely cold temperatures as research vessel Polarstern drifted from the Siberian to the Atlantic Arctic. While water vapor 818O responds rapidly to air temperature and humidity changes, surface snow 818O values are modulated by postdepositional processes like sublimation and wind redistribution. We found that these 2 processes play a key role in isotopic enrichment that is intensified by the snow's prolonged surface residence. Wind-driven snow redistribution, occurring during 67% of the winter period, leads to an average surface snow 818O of-22 parts per thousand across the sea ice by redistributing and mixing fresh snow with more metamorphosed snow. This study provides new insights into how wind-driven redistribution and prolonged surface residence not only alter isotopic values in surface snow but also obscure seasonal isotopic patterns, complicating the interpretation of snow isotope records in the Arctic. Our research to understand the differences between the isotopic values of vapor and the isotopic values of snow provides insight into interactions between snow and the atmosphere, as well as the processes that alter isotopic values internally within the Arctic snowpack. Our study highlights the complexity of surface snow isotope geochemistry across the Arctic from the eastern to the central basin during the MOSAiC expedition window and how the underlying processes of water vapor transport, temperature-isotope relations, and the role of secondary processes, including wind redistribution and sea ice formation all contribute to the horizontal and vertical geochemistry patterns.

During the Late Pleistocene, a severely cold and dry climate strengthened dust production across the Northern Hemisphere. Despite many studies examining aridification and dust production, there is a lack of understanding about dust transportation during global climate variability. Long-range transported (LRT) dust can be traced by comparing global geochemical signatures and constraining provenance relationships. Here we report rare earth element abundances and strontium, neodymium, and oxygen isotope compositions of inorganic substances in ice wedges from Batagay and Central Yakutia (Cyuie and Churapcha), which comprise Yedoma deposits that formed in unglaciated Beringia. Distinct geochemical properties reflect differences between local and LRT dust contributions. Particles in the Batagay ice wedges show higher similarities to Chinese aeolian deposits, while those in Central Yakutia indicate stronger local input. These provenance constraints highlight variability in atmospheric circulation transporting dust to the Arctic during the Late Pleistocene, linking climate changes to aerosol distribution.

Ecological connectivity shapes ecosystem responses to climate change and is thought to underpin stability, yet its millennial-scale dynamics remain poorly resolved. We asked how spatial and temporal connectivity of plant metacommunities changed over the last 40 ka and which processes drove it. We analysed and compiled plant sedimentary DNA from 20 lake cores across Beringia (Siberia, Alaska) to investigate community dynamics and, for a high-resolution subset, applied beta- and zeta-diversity to track connectivity. Vegetation changed coherently across the glacial–Holocene transition, with trait shifts mirroring functional composition. Connectivity peaked during the late MIS3 and Last Glacial Maximum—likely aided by the Bering Land Bridge, mass effects and facilitation—collapsed during the Deglacial with rapid turnover, and rebounded in the Holocene as shrub and boreal communities expanded. Temporal zeta within sites exceeded spatial zeta, indicating strong local persistence and resilience. Tundra sites uninvaded by forest maintained continuous species pools. Overall, these patterns underscore the value of a metacommunity perspective for assessing millennial-scale connectivity changes. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, https://ror.org/0472cxd90 Deutsche Forschungsgemeinschaft, https://ror.org/018mejw64 U.S. National Science Foundation, https://ror.org/021nxhr62 Bundesministerium für Bildung und Forschung

Speleothems from caves in near-permafrost regions can provide unique insights into the climatic history of temperature-sensitive environments. These regions are often remote and only seasonally accessible, which makes cave monitoring-a requirement for speleothembased proxy interpretation-challenging. We document ventilation, the infiltration regime, and the isotopic composition of dripwater in Botovskaya Cave in southern Siberia, a host to speleothems previously used to reconstruct Quaternary permafrost dynamics in continental Eurasia. We explore surface and cave temperature records, cave air CO2 concentration data and stable isotopes in precipitation and dripwater and evaluate infiltration conditions and the sensitivity of speleothems to regional atmospheric dynamics. Locations further into the cave are thermally more stable (T = 1.8-2.5 +/- 0.1 degrees C) than sites closer to the entrances which show inter-seasonal temperature variability of ca. 4 degrees C. Cave CO2 concentration is highest during summer when the cave acts like a cold trap and cave air becomes stagnant and in winter when frozen ground and epikarst and entrances closed by snow subdue ventilation. Accordingly, sub-horizontal Botovskaya Cave ventilates via vertical joints and the entrances during spring whenever cave air is warmer than the surface air. Dripwater isotopic composition (518O and 52H) generally reflects the mean isotopic composition of precipitation, but 518O decreases with distance from the entrances, and in the deeper sections of the cave reflects the mean (multi)annual isotopic composition in precipitation, likely due to an increasing contribution of snowmelt to total infiltration. Slope aspect and vegetation density above the cave section determine the proportion of received seasonal precipitation. On local scale our results provide insights into the environmental processes that govern the microclimate in Botovskaya Cave and facilitate robust interpretation of future speleothem-based proxy reconstructions. On the pan-regional scale we propose a potential mechanism responsible for suboptimal reproducibility of stalagmite records from the same cave.

Melt ponds are a common feature of the Arctic sea-ice environment during summer, and they play an important role in the exchange of heat and water vapor between the ocean and the atmosphere. We report the results of a time-series study of the CO2 dynamics within melt ponds (and nearby lead) and related fluxes with the atmosphere during the summer-to-autumn transition in the central Arctic Ocean during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. In late summer 2020, low-salinity meltwater was distributed throughout the melt ponds, and undersaturation of pCO2 in the meltwater drove a net influx of CO2 from the atmosphere. The meltwater layer subsequently thinned due to seawater influx, and a strong gradient in salinity and low-pCO2 water was observed at the interface between meltwater and seawater at the beginning of September. Mixing between meltwater and underlying seawater drives a significant drawdown of pCO2 as a result of the non-linearities in carbonate chemistry. By the middle of September, the strong stratification within the meltwater had dissipated. Subsequent freezing then began, and cooling and wind-induced drifting of ice floes caused mixing and an influx of seawater through the bottom of the melt pond. The pCO2 in the melt pond reached 300 µatm as a result of exchanging melt pond water with the underlying seawater. However, gas exchange was impeded by the formation of impermeable freshwater ice on the surface of the melt pond, and the net flux of CO2 was nearly zero into the pond, which was no longer a sink for atmospheric CO2. Overall, the melt ponds in this Arctic sea-ice area (both melt ponds and lead water) act as moderate sinks for atmospheric CO2.

The Siberian Arctic is warming rapidly, causing permafrost to thaw and altering the biogeochemistry of aquatic environments, with cascading effects on the coastal and shelf ecosystems of the Arctic Ocean. The Lena River, one of the largest Arctic rivers, drains a catchment dominated by permafrost. Baseline discharge biogeochemistry data are necessary to understand present and future changes in land-to-ocean fluxes. Here, we present a high-frequency 4.5-year-long dataset from a sampling program of the Lena River's biogeochemistry, spanning April 2018 to August 2022. The dataset comprises 587 sampling events and measurements of various parameters, including water temperature, electrical conductivity, stable oxygen and hydrogen isotopes, dissolved organic carbon concentration and 14C, colored and fluorescent dissolved organic matter, dissolved inorganic and total nutrients, and dissolved elemental and ion concentrations. Sampling consistency and continuity and data quality were ensured through simple sampling protocols, real-time communication, and collaboration with local and international partners. The data are available as a collection of datasets separated by parameter groups and periods at https://doi.org/10.1594/PANGAEA.913197 (Juhls et al., 2020b). To our knowledge, this dataset provides an unprecedented temporal resolution of an Arctic river's biogeochemistry. This makes it a unique baseline on which future environmental changes, including changes in river hydrology, at temporal scales from precipitation event to seasonal to interannual can be detected.

Warming can lead to mobilization of organic matter (OM) initially stored in circumarctic permafrost and subsequent greenhouse gas release to the atmosphere. Our understanding remains limited regarding how the extent of carbon release, that is, OM reactivity, varies across terrestrial permafrost types and how it changes during transport from land to marine shelves. In this study, we measured bulk organic (TOC, C/N), isotopic (delta C-13, Delta C-14), and thermogravimetric properties (TGA) as proxies of OM reactivity on bulk and water-soluble fractions (leachates) from terrestrial Holocene and Pleistocene permafrost, bulk surface sediments from the Laptev Sea, and sediment cores from the western Laptev Sea. Bulk OM from terrestrial Pleistocene permafrost exhibited lower reactivity compared to Holocene permafrost, as indicated by its lower thermoreactivity and more advanced degradation state, reflected in higher delta C-13 values and lower C/N ratios. Marine surface sediments showed relatively old radiocarbon ages and reduced OM thermoreactivity in the eastern Laptev Sea shelf compared to the central and western Laptev Sea shelf. This likely resulted from a higher contribution of Pleistocene permafrost-derived OM. In the central and western Laptev Sea, a rapid decrease in OM thermoreactivity was observed near the coast, followed by a more gradual decline offshore. Downcore analyses revealed that the reduction in OM thermoreactivity primarily reflected degradation during cross-shelf transport rather than after burial. Our results advance the understanding of OM reactivity differences between Pleistocene and Holocene permafrost, as well as changes in terrestrial permafrost OM thermoreactivity during transport and post-burial.

Climate is rapidly changing in northern regions, including Central Yakutia, a densely populated area in Siberia. Here, permafrost-thaw lakes in topographical depressions, named “alaas”, are widely distributed. Alaases and the residual lakes within became the traditional home to the indigenous Sakha people, providing critical ecosystem services like fresh water supply, meadows for cattle breeding, or fishing and hunting grounds. Alaas formation is closely related to the Late Glacial and Early Holocene warming, as it was caused by the degradation of permafrost. This makes alaases, and permafrost-thaw lakes in general, highly sensitive to both climatic changes and land use impacts. Global warming is predicted to cause permafrost loss, potentially resulting in new alaas formations and irreversibly changing water quality and biodiversity within the existing alaas lakes. The exact consequences of anthropogenic climate change and land use on these unique landforms are still poorly understood, which may also be a result of lacking data availability. Here, we present a comprehensive new dataset of limnological characteristics of 66 lakes across Central Yakutia Lowland and the Oymyakon Highlands, with a focus on 51 alaas lakes in Central Yakutia. During field work in summer of 2021, we measured lake physical properties (lake depth, pH, specific conductivity) and afterwards we analyzed lake water hydrochemistry including ions, dissolved organic carbon (DOC), isotopic composition (δ18O H20, δD H20), and aquatic and terrestrial plant composition via surface sediment environmental DNA metabarcoding. The majority of alaas lakes are classified as magnesium-bicarbonate types. Isotope concentrations indicate that lakes in the Central Yakutian Lowlands are controlled mainly by evaporation, underlining their sensitivity to future warming. Aquatic vegetation is dominated by submerged macrophytes, whereas terrestrial vegetation mainly consists of graminoids and forbs. Settlements are mostly situated in connected alaas systems, where flowing water results in lower DOC concentration. This “snapshot” of limnological characteristics can be helpful to assess the most critical factors which may be impacted by land use or respond to future warming.

AimThis study investigates the main changes in bryophyte diversity and composition during the last 30,000 years in arctic and boreal ecosystems. Turnover in biological and ecological trait compositions of recorded bryophytes is assessed to explore changes in dominant life-strategies and habitats.LocationThe study area covers Siberia and Alaska (55 degrees-90 degrees N, 50 degrees-150 degrees E and 40 degrees-90 degrees N, 150 degrees E-140 degrees W).TaxonBryophytes.MethodsWe leveraged a dataset of plant DNA metabarcoding (chloroplast, trnL) of sedimentary ancient DNA from 26 lake sediment cores. Our statistical analyses only target DNA reads assigned to bryophytes.ResultsA total of 120 amplicon sequence variants were obtained, which could be assigned 100% to bryophyte taxa. Our results indicate that the dominant families in the palaeo-record reflect well the dominant families observed in Siberia and Alaska, and compositional changes reflect mainly the modern latitudinal gradient. For example, Sphagnum shows a clear post-glacial expansion pattern. Richness was high in times of high habitat diversity. Our data reveal a turnover from biological traits characteristic of an r-strategy (colonists/short-lived, small size, monoicous, long sporophyte season, small spores) during the glacial towards a K-strategy (long-lived, large size, dioicous, short sporophyte season, large spores) during the Holocene. The ecological traits indicate a shift towards taxa with a preference for high summer temperature, forested habitats, soil acidity and wetlands.Main ConclusionsOur results indicated substantial changes in the diversity and taxonomic composition, and dominant biological and ecological traits of bryophyte communities along with past warming and related habitat changes at the transition from glacial period to Holocene. This may to some extent represent an analogue for ongoing ecological change in northern high latitudes. Leveraging bryophyte records in sedimentary ancient DNA reveals promising results which could be enhanced once bryophyte representation in biodiversity and genetic databases has increased and bryophyte-specific ancient DNA methods become established.

Most large terrestrial mammals are declining rapidly under climate change, but whether their loss triggers broader ecosystem collapse remains unresolved. Using sedimentary ancient metagenomics, we reconstructed ecological networks and community turnover of Arctic mammals and plants over the past 25,000 years. We identified rewiring of mammalian interactions between 25–15 ka initiating cascading effects, culminating in the steppe–tundra’s transition to open woodland by 10 ka. Incorporating traits analysis, we revealed this prolonged process was buffered by long-distance seed dispersal—especially via megaherbivores—but loss of dispersal due to defaunation ultimately drove steppe–tundra collapse. Trophic interactions by megaomnivores and small mammals were insufficient to prevent turnover. Our findings identify resilient interactions and key megafaunal functions, offering insights for targeted rewilding to restore Arctic ecosystem functioning. ### Competing Interest Statement The authors have declared no competing interest.

The Transpolar Drift (TPD) plays a crucial role in regulating Arctic climate and ecosystems by transporting fresh water and key substances, such as terrestrial nutrients and pollutants, from the Siberian Shelf across the Arctic Ocean to the North Atlantic. However, year-round observations of the TPD remain scarce, creating significant knowledge gaps regarding the influence of sea ice drift and ocean surface circulation on the transport pathways of Siberian fresh water and associated matter. Using geochemical provenance tracer data collected over a complete seasonal cycle, our study reveals substantial spatiotemporal variability in the dispersal pathways of Siberian matter along the TPD. This variability reflects dynamic shifts in contributions of individual Siberian rivers as they integrate into a large-scale current system, followed by their rapid and extensive redistribution through a combination of seasonal ice-ocean exchanges and divergent ice drift. These findings emphasize the complexity of Arctic ice-ocean transport pathways and highlight the challenges of forecasting their dynamics in light of anticipated changes in sea ice extent, river discharge, and surface circulation patterns.

In this study, we integrate geochronological, cryolithological, paleoecological, and modeling data to reconstruct the Last Interglacial (LIG) climate around the New Siberian Islands, revealing significantly warmer conditions compared to today. New luminescence dating of the lacustrine deposits mostly preserved in ice-wedge pseudomorphs of 1-3 m thickness along the Dmitry Laptev Strait indicates ages consistent with the LIG (Marine Isotope Stage (MIS) 5e). Analysis of plant macrofossils and of pollen and faunal records (beetles and chironomids) from these deposits suggests mean temperatures of the warmest month (MTWAs) of 10.3 to 12.9 degrees C, 9.0 +/- 3.0 degrees C, 8 to 10.5 degrees C, and 9.4 to 15.3 degrees C for Bol'shoy Lyakhovsky and of 12.7 to 15.3 degrees C, 9.7 +/- 2.9 degrees C, 8 to 14 degrees C, and 12.0-13.8 degrees C for Oyogos Yar. The fossil-beetle-based mutual climate range for mean temperatures of the coldest month is -34 to -26 degrees C for Bol'shoy Lyakhovsky and -38 to -26 degrees C for Oyogos Yar. Our chironomid-based reconstructions of water table depth suggest 1.7 to 5.6 m for Bol'shoy Lyakhovsky, while previous analysis suggested 1.1 to 3.3 m for Oyogos Yar. Pollen-based reconstruction of mean annual precipitation (MAP) suggests 271 +/- 56 mm for Bol'shoy Lyakhovsky and 229 +/- 22 mm for Oyogos Yar. The first-time application of clumped isotopes to permafrost-preserved biogenic calcite of ostracods and bivalves for Oyogos Yar reconstructed near-surface water temperatures of 10.3 +/- 3.0 degrees C and bottom-water temperatures of 5.3 +/- 1.5 degrees C in thermokarst lakes during summer. In summary, the analyzed proxies suggest summers warmer than today by 5.5 to 12.8 degrees C for Bol'shoy Lyakhovsky and by 0.2 to 7.5 degrees C for Oyogos Yar and winters warmer than today by up to 7.1 and 8.4 degrees C for Bol'shoy Lyakhovsky and Oyogos Yar, respectively. Modern mean annual precipitation values are within the uncertainty range of the reconstructions. Climate model simulations for the LIG from PMIP suggest MTWAs warmer than today for Bol'shoy Lyakhovsky (4.4 +/- 1.0 degrees C compared to 2.5 degrees C) and colder than today for Oyogos Yar (4.5 +/- 1.2 degrees C compared to 7.8 degrees C), underestimating the Eemian warming reconstructed from our multiple paleoecological proxies. The LIG warming mainly affected summer conditions, whereas modern and future warming will rather impact winter conditions. As the LIG annual mean temperature is often used as an analog for the future climate in the High Arctic, the proxy-model mismatch highlights the urgent need for more systematic quantitative proxy-based temperature reconstructions in the Arctic and more sophisticated Earth system models capable of capturing Arctic paleoenvironmental conditions.

Melt pond is a common and important feature of the Arctic in the summer season. Melt ponds provide unique microbial habitats with high light availability, which can promote photosynthesis. Therefore, melt ponds play an important role for nutrient cycling at the ice-ocean interface. However, the changes in nutrient dynamics in and under the sea ice resulting from melt pond formation are poorly understood. To elucidate melt pond nutrient (NO3 -, NO2 -, NH4 +, PO4 3-, and Si(OH)4) dynamics and their relationship with the melt pond bottom ice, which is sea ice right beneath the floor of a melt pond, in the Central Arctic Ocean during late summer, melt pond water and bottom sea-ice samples were collected during the MOSAiC Expedition (2019-2020). Comparison with the dilution line based on winter surface seawater, which is a source of sea ice, suggest that nutrients in the melt ponds are consumed by algae or other organisms, and then remineralized at the pond bottom. Nutrients then percolated downward through the porous bottom ice. Melt pond water was completely exchanged with surrounding seawater (lead or under-ice seawater) and snow derived water. If the surrounding seawater and snow are rich in nutrients, the exchange promotes photosynthesis within the melt pond water and can enhance nutrient accumulation within the pond bottom ice.

Northern Eurasia underwent major hydroclimatic changes since the beginning of the Holocene interglacial. A rapid warming period reaching the Holocene Thermal Maximum (HTM), followed by a general cooling trend until recent times, was observed in Eurasian lacustrine diatom oxygen isotope (delta O-18(diatom)) records. In this study, we present a new Holocene delta O-18(diatom) record from Lake Khamra (59.99 degrees N, 112.98 degrees E, Siberia). Our record aligns with Holocene delta O-18(diatom) records across the Northern Hemisphere, showing a general millennial-scale cooling trend following an initial maximum at 11.2 cal. ka BP and a second maximum at 6.7 cal. ka BP. These maxima correspond to the summer insolation maximum and elevated Northern Hemisphere air temperatures, as well as increased bioproductivity. Variability on centennial scales is likely driven by precipitation changes, which coincide with higher sedimentation rates and overlay the general decreasing trend throughout the Holocene. In addition, we compared two multiproxy datasets with decadal resolution from Lake Khamra, including delta O-18(diatom) data and biogeochemical proxies such as total organic carbon (TOC), total nitrogen (TN), stable carbon (delta C-13) and nitrogen (delta N-15) isotopes, and total mercury (THg). The datasets cover a similar to 210-year period (c. 6.140-6.350 cal. ka BP) at the end of the HTM and a recently published similar to 220-year record (c. 1790-2015 CE) that embraces the anthropogenic times. The comparison of these two warm phases reveals distinct differences in both the absolute values and the variability of the records. Regarding the d18Odiatom data, the recent period shows a nearly threefold increase in range and double the standard deviation, suggesting greater hydroclimatic variability compared to the end of the HTM. Notably, THg levels indicate a sharp increase in recent decades, while delta C-13 declined, contrasting with the observations at the end of the HTM. We attribute these observations partially to far-reaching anthropogenic effects on remote lake systems.

Abstract. Freshwater ecosystems are a major feature of the northern landscapes that are expected to experience significant future changes due to climate change and land-use alterations. In Central Yakutia, abundant lakes in topographic permafrost-thaw depressions, named ‘alaas’, define the traditional cultural landscape that is home to the indigenous Sakha people, with critical ecosystem services like freshwater supply, meadows for cattle breeding, as well as fishing and hunting grounds. In contrast, lakes in the Verkhoyansk mountain region east of Central Yakutia are of glacial origin or developed on glacial moraines and represent deeper and more oligotrophic lake systems much less used as human resources. Here, we analyse the hydrochemistry, sedimentary DNA (sedDNA)-derived aquatic plant diversity, geomorphology, and adjacent land cover of sixty-six lakes across the Central Yakutian lowland permafrost landscape and the Verkhoyansk Oymyakon high mountain plateau to understand their characteristics and environmental drivers. Our hydrochemical analysis reveals a clear distinction between the low-mineralised mountain lakes and the highly variable hydrochemistry of the lowland thermokarst lakes. The lake developmental stage within the thermokarst lake sequence seems to be a key driver of lake hydrochemistry in the lakes of the Central Yakutian lowland. Specifically, the lake’s developmental stage is reflected by dissolved organic carbon (DOC), pH, its stable isotopic composition, and the hydrochemical facies of alkali and earth alkali elements. New thermokarst lakes have a depleted stable isotopic composition, possibly due to contributions from meltwater of adjacent permafrost ground-ice. This thermokarst lake stage is typically located within forest and has the highest DOC. In contrast, the hydrologically open thermokarst lake systems, typically located in large connected alaas systems with settlements and managed land use, have lower DOC and fewer mineralisation than recently formed thermokarst lakes or old alaas lakes. The dilution in the hydrologically connected alaas lakes occurs due to flushing, mainly during high discharge events such as the regular snowmelt. Old alaas lakes show an enriched oxygen isotope composition and have high salinity and mineral content, suggesting processes of evaporation and highlighting their vulnerability to future warming. However, low chloride together with an enriched isotopic composition and elevated fluoride characterise several of the sampled high-salinity lakes. This points to an additional process beyond the current evaporation, such as fluoride leakage from lacustrine sediments or salt deposits. SedDNA-derived macrophyte diversity reflects lake types and reveals the dominance of brackish water-tolerant cosmopolitan submerged macrophytes, particularly Stuckenia and Potamogeton, across all lake types. The macrophytes Myriophyllum and M. verticillatum are exclusively found in freshwater lakes in the lowlands and the mountain regions, supporting their indicator value for freshwater conditions. Our results provide a detailed examination of lake systems in modern conditions within highly climate-sensitive lowland and mountain permafrost landscapes.

Northern latitudes have been significantly impacted by recent climate warming, which has increased the probability of experiencing extreme weather events. To comprehensively understand hydroclimate change and reconstruct hydroclimatic anomalies such as drought periods, appropriate proxy records reaching further back in time beyond meteorological measurements are needed. Here we present a 220-year (2015–1790 CE), continuous, stable oxygen isotope record of diatoms (δ18Odiatom) from Lake Khamra (59.99° N, 112.98° E) in eastern Siberia, an area highly sensitive to climate change and for which there is a demand for palaeohydrological data. This high-resolution proxy record was obtained from a 210Pb–137Cs-dated sediment short core and analysed to reconstruct hydroclimate variability at a sub-decadal scale. The interpretation of the δ18Odiatom record is supported by meteorological data, modern isotope hydrology and geochemical analyses of the same sediment, which is indicative of the conditions in the lake and catchment. A comparison with meteorological data going back to 1930 CE revealed that the δ18Odiatom record of Lake Khamra is primarily influenced by regional precipitation changes rather than the air temperature. We identified winter precipitation, which enters the lake as isotopically depleted snowmelt water, as the key process impacting the diatom isotope variability. We related the overall depletion of δ18Odiatom in recent decades to an observed increase in winter precipitation in the area, likely associated with the global air temperature rise, Arctic sea ice retreat and increased moisture transport inland. Available palaeoclimate proxy records, including a fire reconstruction for the same lake, support the idea that the new record is a valuable hydroclimate proxy that is indicative of precipitation deficits and excludes solar insolation and air temperature as primary driving forces, even before the first meteorological recordings. We propose two possible hydroclimatic anomalies that were detected in the Lake Khamra δ18Odiatom record: one at the beginning of the 19th century and a second prominent event in the 1950s. Both are interpreted as prolonged dry periods associated with enriched δ18Odiatom values likely caused by reduced winter precipitation, which coincide with phases of reconstructed severe wildfires in the region. Despite the apparent pristine lake area, we observed a three- to fourfold increase in mercury concentrations and accumulation rates within the sediment record since the early 20th century, which is partly attributed to human air pollution.