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The Batagay megaslump, a permafrost thaw feature in north-eastern Siberia, provides access to ancient permafrost up to ∼650 kyr old. We aimed to assess the permafrost-locked organic matter (OM) quality and to deduce palaeo-environmental information on glacial–interglacial timescales. We sampled five stratigraphic units exposed on the 55 m high slump headwall and analysed lipid biomarkers (alkanes, fatty acids and alcohols). Our findings revealed similar biogeochemical signatures for the glacial periods: the lower ice complex (Marine Isotope Stage (MIS) 16 or earlier), the lower sand unit (sometime between MIS 16–6) and the upper ice complex (MIS 4–2). The OM in these units has a terrestrial character, and microbial activity was likely limited. Contrarily, the n -alkane and fatty acid distributions differed for the units from interglacial periods: the woody layer (MIS 5), separating the lower sand unit and the upper ice complex, and the Holocene cover (MIS 1), on top of the upper ice complex. The woody layer, marking a permafrost degradation disconformity, contained markers of terrestrial origin (sterols) and high microbial decomposition (iso- and anteiso-fatty acids). In the Holocene cover, biomarkers pointed to wet depositional conditions and we identified branched and cyclic alkanes, which are likely of microbial origin. Higher OM decomposition characterised the interglacial periods. As climate warming will continue permafrost degradation in the Batagay megaslump and in other areas, large amounts of deeply buried ancient OM with variable composition and degradability are mobilised, likely significantly enhancing greenhouse gas emissions from permafrost regions. [ABSTRACT FROM AUTHOR]

Pollen records from Siberia are mostly absent in global or Northern Hemisphere synthesis works. Here we present a taxonomically harmonized and temporally standardized pollen dataset that was synthesized using 173 palynological records from Siberia and adjacent areas (northeastern Asia, 42–75 ∘ N, 50–180 ∘ E). Pollen data were taxonomically harmonized, i.e. the original 437 taxa were assigned to 106 combined pollen taxa. Age–depth models for all records were revised by applying a constant Bayesian age–depth modelling routine. The pollen dataset is available as count data and percentage data in a table format (taxa vs. samples), with age information for each sample. The dataset has relatively few sites covering the last glacial period between 40 and 11.5 ka (calibrated thousands of years before 1950 CE) particularly from the central and western part of the study area. In the Holocene period, the dataset has many sites from most of the area, with the exception of the central part of Siberia. Of the 173 pollen records, 81 % of pollen counts were downloaded from open databases (GPD, EPD, PANGAEA) and 10 % were contributions by the original data gatherers, while a few were digitized from publications. Most of the pollen records originate from peatlands (48 %) and lake sediments (33 %). Most of the records (83 %) have ≥3 dates, allowing the establishment of reliable chronologies. The dataset can be used for various purposes, including pollen data mapping (example maps for Larix at selected time slices are shown) as well as quantitative climate and vegetation reconstructions. The datasets for pollen counts and pollen percentages are available at 10.1594/PANGAEA.898616 (Cao et al., 2019a), also including the site information, data source, original publication, dating data, and the plant functional type for each pollen taxa. [ABSTRACT FROM AUTHOR]

Climate change in Siberia is currently receiving a lot of attention because large permafrost-covered areas could provide a strong positive feedback to global warming through the release of carbon that has been sequestered there on glacial–interglacial timescales. Geological evidence and climate model experiments show that the Siberian region also played an exceptional role during glacial periods. The region that is currently known for its harsh cold climate did not experience major glaciations during the last ice age, including its severest stages around the Last Glacial Maximum (LGM). On the contrary, it is thought that glacial summer temperatures were comparable to the present day. However, evidence of glaciation has been found for several older glacial periods. We combine LGM experiments from the second and third phases of the Paleoclimate Modelling Intercomparison Project (PMIP2 and PMIP3) with sensitivity experiments using the Community Earth System Model (CESM). Together, these climate model experiments reveal that the intermodel spread in LGM summer temperatures in Siberia is much larger than in any other region of the globe and suggest that temperatures in Siberia are highly susceptible to changes in the imposed glacial boundary conditions, the included feedbacks and processes, and to the model physics of the different components of the climate model. We find that changes in the circumpolar atmospheric stationary wave pattern and associated northward heat transport drive strong local snow and vegetation feedbacks and that this combination explains the susceptibility of LGM summer temperatures in Siberia. This suggests that a small difference between two glacial periods in terms of climate, ice buildup or their respective evolution towards maximum glacial conditions can lead to strongly divergent summer temperatures in Siberia, allowing for the buildup of an ice sheet during some glacial periods, while during others, above-freezing summer temperatures preclude a multi-year snowpack from forming. [ABSTRACT FROM AUTHOR]

Pollen records from Siberia are mostly absent in global or Northern Hemisphere synthesis works. Here we present a taxonomically harmonized and temporally standardized pollen dataset that was synthesized using 173 palynological records from Siberia and adjacent areas (northeast Asia, 50°–180° E and 42°–75° N). Pollen data were taxonomically harmonized, that is the original 437 taxa were transformed to 106 combined pollen taxa. Age-depth models for all records were revised by applying a constant Bayesian age-depth modelling routine. The pollen dataset is available as count data and percentage data in a table format (taxa vs. samples) with age information for each sample. The dataset has relatively few sites covering the last glacial period between 40 and 11.5 cal ka BP (calibrated thousand years before present 1950 CE) particularly from the central and western part of the study area. In the Holocene period, the dataset has many sites from most of the area except the central part of Siberia. Of the 173 pollen records, 81 % of pollen counts were downloaded from open databases (GPD, EPD, Pangaea) and 10 % were contributions of the original data gatherers, while a few were digitized from publications. Most of the pollen records originate from peatlands (48 %) and lake sediments (33 %). Most of the records (83 %) have ≥ 3 dates allowing the establishment of reliable chronologies. The dataset can be used for various purposes including pollen data mapping (example maps for Larix at selected time-slices are shown) as well as quantitative climate and vegetation reconstructions. The datasets for pollen counts and pollen percentages are available at https://doi.pangaea.de/10.1594/PANGAEA.898616 (Cao et al., 2019). [ABSTRACT FROM AUTHOR]

Deep permafrost deposits that developed during the last glacial period and early Holocene inunglaciated lowlands of the circum-arctic region have been a carbon sink for millennia. Thissignificant carbon pool is now being degraded because of increasing ground temperatures thatparticularly affect the ice-rich sediments in the Yedoma domain, also called ice complexdeposits. The loess-like Yedoma deposits that still cover an area of about 1 million km2contain large syngenetic ice-wedges resulting in very high ground-ice contents (50–80% ofthe ground volume). This makes them particularly prone to disturbances such as thermokarstand thermo-erosion processes causing ground subsidence and sediment relocationsprocesses by which the previously frozen organic matter is exposed to microbialdecomposition. The complexity of processes and changes taking place in the thermokarstlandscape complicates the prediction of greenhouse gas emissions resulting from themineralization of the organic carbon (OC) stored in Yedoma deposits. To investigated thedegradability of OC in thermokarst-affected Yedoma deposits, we designed a modelstudy on a retrogressive thaw slump in the Lena River Delta (NE Siberia). Thethawing of the ice wedges in the Yedoma exposed the sedimentary OC in the form ofthaw mounds and mixes ancient and younger sediments, which contain ancient andyounger organic matter of different age and stage of degradation. Using molecularsieves coupled with respiration chambers as well as depth- samplers, we collectedCO2 at different locations on the thaw slump for radiocarbon analysis providinginformation on different carbon sources (ancient, young) released from the different studysites. Additionally, we incubated sediment samples to identify substrates releasedpreferentially. Higher 14C contents of the bulk OC in the near-surface sediment compared to theunderlying sediment in two thaw mounds indicated the input of younger substrates potentiallyderived from younger sedimentary units or leachate of the ice wedges. Likewise, youngerlayers at another site suggest admixtures of Holocene active-layer material. These youngersubstrates are preferentially mineralized resulting in relatively high 14C contents of the CO2collected with respirations chambers compared to bulk OC. Using depth samplers installednear the still frozen ground (50-60 cm) we collected very old CO2, which has similar 14Cconcentrations like the bulk sedimentary OC, indicating that the ancient (up to 30,000 yrBP old) Yedoma OC is mineralized if no younger substrates are available. Thisobservation is confirmed by first 14CO2 results of incubation experiments. Variable 14CO2results for three successive years further suggest that considerable changes are takingplace in the sediment due to the rapid erosion of the thaw slump causing continuoussediment displacement, the formation of cracks, and the development of a sparsevegetation on the bare sediments. This in turn most probably caused changes in soiltemperature, moisture, oxygen and nutrient contents (to be analysed) mirrored byyounger CO2 emissions at some sites. Overall the 14CO2results reveal the complexityof the different deposits and of effects that may take place during their thawingcomplicating the prediction of future greenhouse gas releases from thawing Yedoma. [ABSTRACT FROM AUTHOR]