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The land-ocean interface in the Arctic is a sensitive environment facing severe changes due to rising global air temperatures. In particular, Arctic river deltas are rapidly changing permafrost landscapes which will become more dynamic due to sea-level rise, longer thaw periods, changes in river discharge, increased storm-surge flooding and thawing permafrost. As a result, previously frozen river delta deposits are becoming available for microbial decomposition as permafrost thaws. However, very few studies have focused on Arctic deltas and estimates of deltaic carbon stocks are even more limited. Therefore, we compiled 140 soil cores (new and already published soil cores), consisting of more than 1400 samples from 17 different deltas around the Arctic Ocean. In addition, we mapped the spatial extent of more than 250 Arctic deltas in order to accurately assess the carbon and nitrogen stock estimations for Arctic deltas. Our study shows that Arctic river delta deposits contain a considerable amount of carbon and nitrogen. The ongoing thaw and degradation of these permafrost deposits resulting from global climate warming might release additional carbon and nitrogen with implications for Arctic waters and biogeochemical cycles. The additional export of terrestrial carbon and nitrogen will alter biogeochemical processes not only in the nearshore zone, but throughout the Arctic Ocean. With this study we will improve our understanding of changing terrestrial carbon and nitrogen deposits and their contribution to a changing Arctic Ocean.

Lake El'gygytgyn, located in central Chukotka, Russian Arctic, was the subject of an international drilling project that resulted in the recovery of the longest continuous palaeoclimatic and palaeoenvironmental record for the terrestrial Arctic covering the last 3.6 million years. Here, we present the reconstruction of the lake-level fluctuations of Lake El'gygytgyn since Marine Isotope Stage (MIS) 7 based on lithological and palynological as well as chronological studies of shallow-water sediment cores and subaerial lake terraces. Reconstructed lake levels show an abrupt rise during glacial-interglacial terminations (MIS 6/5 and MIS 2/1) and during the MIS 4/3 stadial-interstadial transition. The most prominent lowstands occurred during glacial periods associated with a permanent lake-ice cover (namely MIS 6, MIS 4 and MIS 2). Major triggering mechanisms of the lake-level fluctuations at Lake El'gygytgyn are predominantly changes in air temperature and precipitation. Regional summer temperatures control the volume of meltwater supply as well as the duration of the lake-ice cover (permanent or seasonal). The duration of the lake-ice cover, in turn, enables or hampers near-shore sediment transport, thus leading to long-term lake-level oscillations on glacial-interglacial time scales by blocking or opening the lake outflow, respectively. During periods of seasonal ice cover the lake level was additionally influenced by changes in precipitation. The discovered mechanism of climatologically driven level fluctuations of Lake El'gygytgyn are probably valid for large hydrologically open lakes in the Arctic in general, thus helping to understand arctic palaeohydrology and providing missing information for climate modelling.

Archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) are globally abundant in soils under various climatic conditions, but little is known about their sources, relative distribution, and environmental controls on their diversity in high latitude permafrost deposits. Thus, constraints on GDGT-based proxies, such as methylation of branched GDGTs (MBT) or cyclization of branched GDGTs (CBT) used to infer mean annual temperature or soil pH, are also sparse. We investigated the GDGT diversity in typical North Siberian permafrost deposits including the active layer of polygonal tundra soils (seasonally frozen ground), fluvial terrace/floodplain sediments, Holocene and Pleistocene thermokarst sediments, and late Pleistocene Ice Complex (Yedoma). Our data show that isoprenoid GDGTs are produced by both methanotrophic and methanogenic Euryarchaeota, as well as Thaumarchaeota, but their abundance does not seem to be controlled by the investigated physicochemical parameters including %TOC, %TN, and soil pH. Branched GDGTs (brGDGTs) show similar distributional changes to those observed in other high latitude soil samples, i.e., a dominance of pentamethylated and hexamethylated brGDGTs, likely reflecting the adaptation to low temperatures and a positive correlation of the degree of cyclization with soil pH. Specifically, brGDGT-IIIa correlates positively with %TOC and %TN and negatively with soil pH, while brGDGT-Ib and brGDGT-IIb correlate negatively with %TOC and %TN and positively with pH. Moreover, we observe a negative correlation between 5-methyl and 6-methyl brGDGTs without cyclopentane moieties (except brGDGT-IIIa), but this anticorrelation does not seem to be related to the investigated physicochemical parameters. The observed brGDGT distribution yields a permafrost-specific soil pH calibration, pH'(PF) = 1.8451 x CBT'(PF) + 8.5396 (r(2) = 0.60, RMSE = 0.074; n = 109). The different investigated deposit types show some distinct GDGT distributional change

Arctic river deltas are sensitive polar landscapes at the land-ocean interface. In contrast to lower latitude deltas, Arctic deltas are characterized by low temperatures, a strong seasonality and the presence of permafrost. Seasonal freezing conditions and underlying permafrost hinders runoff for most of the year and leads to typical land forms such as ice wedge polygons, frost mounds and thermokarst lakes. However, compared to other permafrost dominated landscapes, Arctic deltas are more dynamic. The surface morphology is changing constantly due to river ice break up and subsequent spring flooding, coastal and shoreline erosion, thaw slumping, and degradation of ice rich deposits. Deltaic sediments also tend to be highly susceptible to ground-ice aggradation, making them more ice-rich than adjacent nondeltaic landscapes. In addition, Arctic deltas will be severely affected by global climate change through sea level rise, lengthened thaw season, changing river discharge, storm surge flooding and thawing permafrost. We are therefore at risk, to face reactivation of millennia-old soil carbon and nitrogen deposits by the degradation of previously permanently frozen river delta deposits. However, there is a lack of studies on Arctic deltas and only very coarse estimates on Arctic delta carbon and nitrogen stocks exist. Here we present a new data-set of 140 soil cores, including more than 1400 samples from 17 different deltas spread across the Arctic. We combine new and legacy soil core data to estimate for the first time pan-Arctic deltaic carbon and nitrogen stocks and close a knowledge gap for deep permafrost stock estimations. We found that Arctic deltas present a significant pool for organic carbon and nitrogen, thus their change poses risks far beyond the Arctic. Permafrost thaw in such dynamic landscapes will increase nutrient transport from land to ocean with implications on Arctic near-shore zones (e.g. affecting foodwebs and biogeochemical processes) as well as

Permafrost deposits at Bol'shoy Lyakhovsky Island are natural archives dating back to the Eemian (Krest Yuryakh Suite, ~125 kyr BP) with excellent conditions to preserve ancient DNA. The today treeless island is located between the Laptev and East Siberian Seas, but during the last glacial it was part of the Western Beringian landmass due to the marine regression, which exposed the shallow Siberian shelf. Our aim was to reconstruct vegetation changes driven by the strong climatic oscillations from the last interglacial until today. Permafrost coring took place west of the Zimov’e River where four terrestrial permafrost cores were drilled. In total we collected 72 sediment samples and combined pollen analysis with sedimentary ancient DNA metabarcoding for which we amplified the short vascular plant specific P6-loop of the plastid trnL (UAA) intron. Furthermore, we applied two newly developed larch-specific chloroplast SNP markers to assess their suitability in identifying past population dynamics from environmental samples. The new markers were re-sequenced and displayed both SNP variants of each marker in last interglacial samples. Highest diversity and a vegetation containing trees (Picea, Larix, Populus) were inferred covering the island during the last interglacial. During the Mid Weichselian interstadial and the Bølling-Allerød interstadial complex only Larix was detected among trees but disappeared along with most shrub taxa during the Holocene. This suggests that the northern extent of the Siberian treeline was further north than previously inferred from pollen analyses, and that modern larch taiga is differently composed than the taiga during the last interglacial.

In this study the organic matter (OM) in several permafrost cores from Bol´shoy Lyakhovsky Island in NE Siberia was investigated. In context of the observed global warming the aim was to evaluate the potential of freeze-locked OM from different depositional ages to act as a substrate provider for microbial production of greenhouse gases from thawing permafrost. To assess this potential, exemplarily the concentrations of free and bound acetate, which form an appropriate substrate for methanogenesis, are determined. The largest free (in pore water) and bound (organic matrix linked) acetate substrate pools are present in layers that cover interstadial MIS 3 and stadial MIS 4 Yedoma permafrost deposits. In contrast, deposits from the last interglacial MIS 5e (Eemian) contain only a small pool of substrates. The Holocene (MIS 1) deposits reveal a significant bound acetate pool, representing a future substrate potential upon release during OM degradation. Additionally, pyrolysis experiments on the OM allocate an increased aliphatic character to the MIS 3 and 4 Late Pleistocene deposits, which might indicate less decomposed and presumably better degradable OM. Biomarkers for past microbial communities including those for methanogenic archaea show also highest abundance during MIS 3 and 4, which indicates that the OM stimulated microbial degradation and presumably greenhouse gas production during time of deposition. On a broader perspective, Arctic warming will increase and deepen permafrost thaw and favour substrate availability from freeze-locked older permafrost deposits. Therefore, especially the Yedoma deposits show a high potential for providing substrates relevant for microbial greenhouse gas production

The composition of perennially frozen deposits holds information on the palaeo-environment during and following deposition. In this study, we investigate late Pleistocene permafrost at the western coast of the Buor Khaya Peninsula in the south-central Laptev Sea (Siberia), namely the prominent eastern Siberian Yedoma Ice Complex (IC). Two Yedoma IC exposures and one drill core were studied for cryolithological (i.e. ice and sediment features), geochemical, and geochronological parameters. Borehole temperatures were measured for 3 years to capture the current thermal state of permafrost. The studied sequences were composed of ice-oversaturated silts and fine-grained sands with considerable amounts of organic matter (0.2 to 24 wt %). Syngenetic ice wedges intersect the frozen deposits. The deposition of the Yedoma IC, as revealed by radiocarbon dates of sedimentary organic matter, took place between 54.1 and 30.1 kyr BP. Continued Yedoma IC deposition until about 14.7 kyr BP is shown by dates from organic matter preserved in ice-wedge ice. For the lowermost and oldest Yedoma IC part, infrared-stimulated luminescence dates on feldspar show deposition ages between 51.1 +/- 4.9 and 44.2 +/- 3.6 kyr BP. End-member modelling was applied to grain-size-distribution data to determined sedimentation processes during Yedoma IC formation. Three to five robust end-members were detected within Yedoma IC deposits, which we interpret as different modes of primary and reworked unconfined alluvial slope and fan deposition as well as of localized eolian and fluvial sediment, which is overprinted by in situ frost weathering. The cryolithological inventory of the Yedoma IC preserved on the Buor Khaya Peninsula is closely related to the results of other IC studies, for example, to the west on the Bykovsky Peninsula, where formation time (mainly during the late Pleistocene marine isotope stages (MIS) 3 interstadial) and formation conditions were similar. Local freezing conditions on Buor K

Climate models project an annual average temperature increase of about 3-5°C in the terrestrial Arctic until the year 2100, which corresponds to the reconstructed temperatures of the last interglacial (Eemian, ~130-110 kyr BP). Hence, the past vegetation composition might provide an analogue for the future. Bol’shoy Lyakhovsky is a treeless island, framed by the Laptev and East Siberian Seas. Our goal was to reconstruct the vegetation history from four permafrost sediment cores, focusing on terrestrial vegetation during warm phases since the last interglacial. We used sedimentary ancient DNA metabarcoding of thetrnLp6-loop. A total of 264 taxa were detected with 39% being assigned to species and 37% to genus level. During several potentially warmer phases tree and shrub taxa were detected. The Eemian was the most diverse phase, also in recording three different tree taxa (Picea, Larix, Populus), whereas Larix was the only tree taxon recorded afterwards. During the Bølling-Allerød interstadial complex (~14.7-12.7 kyr BP), soon after the Last Glacial Maximum (LGM), Larix had already re-colonized Bol’shoy Lyakhovsky, suggesting the persistence of larches at high latitudes throughout the LGM. Despite warmer temperatures of the Holocene, Larix was not detected. Increasing humidity combined with the disconnection of the island from the mainland due to the global sea level rise might have made persistence and/or re-colonization impossible. As the current coastline position has been suggested to differ from the one of the last interglacial, it is however uncertain to which degree the vegetation composition represents a potential future analogue for north-eastern Siberia.

Permafrost is a sensitive element of the cryosphere, but operational monitoring of the ground thermal conditions on large spatial scales is still lacking. Here, we demonstrate a remote-sensing-based scheme that is capable of estimating the transient evolution of ground temperatures and active layer thickness by means of the ground thermal model CryoGrid 2. The scheme is applied to an area of approximately 16 000 km2 in the Lena River delta (LRD) in NE Siberia for a period of 14 years. The forcing data sets at 1 km spatial and weekly temporal resolution are synthesized from satellite products and fields of meteorological variables from the ERA-Interim reanalysis. To assign spatially distributed ground thermal properties, a stratigraphic classification based on geomorphological observations and mapping is constructed, which accounts for the large-scale patterns of sediment types, ground ice and surface properties in the Lena River delta. A comparison of the model forcing to in situ measurements on Samoylov Island in the southern part of the study area yields an acceptable agreement for the purpose of ground thermal modeling, for surface temperature, snow depth, and timing of the onset and termination of the winter snow cover. The model results are compared to observations of ground temperatures and thaw depths at nine sites in the Lena River delta, suggesting that thaw depths are in most cases reproduced to within 0.1 m or less and multi-year averages of ground temperatures within 1–2 °C. Comparison of monthly average temperatures at depths of 2–3 m in five boreholes yielded an RMSE of 1.1 °C and a bias of −0.9 °C for the model results. The highest ground temperatures are calculated for grid cells close to the main river channels in the south as well as areas with sandy sediments and low organic and ice contents in the central delta, where also the largest thaw depths occur. On the other hand, the lowest temperatures are modeled for the eastern part, which is an area

Organic matter deposited in ancient, ice-rich permafrost sediments is vulnerable to climate change and may contribute to the future release of greenhouse gases; it is thus important to get a better characterization of the plant organic matter within such sediments. From a Late Quaternary permafrost sediment core from the Buor Khaya Peninsula, we analysed plant-derived sedimentary ancient DNA (sedaDNA) to identify the taxonomic composition of plant organic matter, and undertook palynological analysis to assess the environmental conditions during deposition. Using sedaDNA, we identified 154 taxa and from pollen and non-pollen palynomorphs we identified 83 taxa. In the deposits dated between 54 and 51 kyr BP, sedaDNA records a diverse low-centred polygon plant community including recurring aquatic pond vegetation while from the pollen record we infer terrestrial open-land vegetation with relatively dry environmental conditions at a regional scale. A fluctuating dominance of either terrestrial or swamp and aquatic taxa in both proxies allowed the local hydrological development of the polygon to be traced. In deposits dated between 11.4 and 9.7 kyr BP (13.4–11.1 cal kyr BP), sedaDNA shows a taxonomic turnover to moist shrub tundra and a lower taxonomic richness compared to the older samples. Pollen also records a shrub tundra community, mostly seen as changes in relative proportions of the most dominant taxa, while a decrease in taxonomic richness was less pronounced compared to sedaDNA. Our results show the advantages of using sedaDNA in combination with palynological analyses when macrofossils are rarely preserved. The high resolution of the sedaDNA record provides a detailed picture of the taxonomic composition of plant-derived organic matter throughout the core, and palynological analyses prove valuable by allowing for inferences of regional environmental conditions.

Past climatic changes led to the migration of plant species and even whole plant communities, e.g. the replacement of arctic tundra by boreal taiga and vice versa. Sedimentary ancient DNA from natural archives is a valuable proxy to track such changes. Bol’shoy Lyakhovsky is an island within the Laptev Sea but it was part of Western Beringia during the last glacial due to the marine regression. There deposits can be found dating back to the Eemian (Krest Yuryakh Suite, ~125 kyr BP). Paleobotanical records based on macrofossils and pollen are available, but limited in their temporal resolution. Since sedimentary ancient DNA has proven to reveal complementary information not captured by traditional paleobotanical proxies, our aim was to provide a paleobotanical record in high resolution spanning from the Eemian to the present. We collected four terrestrial permafrost sediment cores from dated coastal localities and applied a DNA metabarcoding approach using the universal plant barcode of the trnL P6 loop. In total we recovered 325 taxa of which 244 (75%) were identified to species or genus level. The dataset is dominated by Saliceae, Anthemidae and Agrostidinae. In twelve samples from three different cores we detected sequences of Larix, which has not been detected before by macrofossil analyses and only sporadically by pollen analyses of coastal outcrops at the island. This suggests that the northern extent of the Siberian treeline was further north than previously assumed, and that Larix was indeed present at Bol’shoy Lyakhovsky Island.

Ecosystem boundaries, such as the Arctic-Boreal treeline, are strongly coupled with climate and were spatially highly dynamic during past glacial-interglacial cycles. Only a few studies cover vegetation changes since the last interglacial, as most of the former landscapes are inundated and difficult to access. Using pollen analysis and sedimentary ancient DNA (sedaDNA) metabarcoding, we reveal vegetation changes on Bol'shoy Lyakhovsky Island since the last interglacial from permafrost sediments. Last interglacial samples depict high levels of floral diversity with the presence of trees (Larix, Picea, Populus) and shrubs (Alnus, Betula, Ribes, Cornus, Saliceae) on the currently treeless island. After the Last Glacial Maximum, Larix re-colonised the island but disappeared along with most shrub taxa. This was probably caused by Holocene sea-level rise, which led to increased oceanic conditions on the island. Additionally, we applied two newly developed larch-specific chloroplast markers to evaluate their potential for tracking past population dynamics from environmental samples. The novel markers were successfully re-sequenced and exhibited two variants of each marker in last interglacial samples. SedaDNA can track vegetation changes as well as genetic changes across geographic space through time and can improve our understanding of past processes that shape modern patterns.