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The present study examines the formation history and cryolithological properties of the late-Pleistocene Yedoma Ice Complex (IC) and its Holocene cover in the eastern Lena delta on Sobo-Sise Island. The sedimentary sequence was continuously sampled at 0.5 m resolution at a vertical Yedoma cliff starting from 24.2 m above river level (a.r.l.). The sequence differentiates into three cryostratigraphic units: Unit A, dated from ca. 52 to 28 cal kyr BP; Unit B, dated from ca. 28 to 15 cal kyr BP; Unit C, dated from ca. 7 to 0 cal kyr BP. Three chronologic gaps in the record are striking. The hiatus during the interstadial marine isotope stage (MIS) 3 (36–29 cal kyr BP) as well as during stadial MIS 2 (20–17 cal kyr BP) might be related to fluvial erosion and/or changed discharge patterns of the Lena river caused by repeated outburst floods from the glacial Lake Vitim in southern Siberia along the Lena river valley towards the Arctic Ocean. The hiatus during the MIS 2–1 transition (15–7 cal kyr BP) is a commonly observed feature in permafrost chronologies due to intense thermokarst activity of the deglacial period. The chronologic gaps of the Sobo-Sise Yedoma record are similarly found at two neighbouring Yedoma IC sites on Bykovsky Peninsula and Kurungnakh-Sise Island and are most likely of regional importance. The three cryostratigraphic units of the Sobo-Sise Yedoma exhibit distinct signatures in properties of their clastic, organic, and ice components. Higher permafrost aggradation rates of 1 m kyr -1 with higher organic-matter (OM) stocks (29 ± 15 kg C m -3 , 2.2 ± 1.0 kg N m -3 ; Unit A) and mainly coarse silt are found for the interstadial MIS 3 if compared to the stadial MIS 2 with 0.7 m kyr -1 permafrost aggradation, lower OM stocks (14 ± 8 kg C m -3 , 1.4 ± 0.4 kg N m -3 ; Unit B), and pronounced peaks in the coarse-silt and medium-sand fractions. Geochemical signatures of intra-sedimental ice reflect the differences in summer evaporation and moisture regime by higher ion content and less depleted ratios of stable δ18 O and stable δ D isotopes but lower deuterium excess (d) values during interstadial MIS 3 if compared to stadial MIS 2. The δ18 O and δ D composition of MIS 3 and MIS 2 ice wedges shows characteristic well-depleted values and low d values, while MIS 1 ice wedges have elevated mean d values between 11 ‰ and 15 ‰ and surprisingly low δ18 O and δ D values. Hence, the isotopic difference between late-Pleistocene and Holocene ice wedges is more pronounced in d than in δ values. The present study of the permafrost exposed at the Sobo-Sise Yedoma cliff provides a comprehensive cryostratigraphic inventory, insights into permafrost aggradation, and degradation over the last approximately 52 kyr as well as their climatic and morphodynamic controls on the regional scale of the central Laptev Sea coastal region in NE Siberia. [ABSTRACT FROM AUTHOR]

The late Pleistocene Yedoma Ice Complex is an ice-rich and organic-bearing type of permafrost deposit widely distributed across Beringia and is assumed to be especially prone to deep degradation with warming temperature, which is a potential tipping point of the climate system. To better understand Yedoma formation, its local characteristics, and its regional sedimentological composition, we compiled the grain-size distributions (GSDs) of 771 samples from 23 Yedoma locations across the Arctic; samples from sites located close together were pooled to form 17 study sites. In addition, we studied 160 samples from three non-Yedoma ice-wedge polygon and floodplain sites for the comparison of Yedoma samples with Holocene depositional environments. The multimodal GSDs indicate that a variety of sediment production, transport, and depositional processes were involved in Yedoma formation. To disentangle these processes, a robust endmember modeling analysis (rEMMA) was performed. Nine robust grain-size endmembers (rEMs) characterize Yedoma deposits across Beringia. The study sites of Yedoma deposits were finally classified using cluster analysis. The resulting four clusters consisted of two to five sites that are distributed randomly across northeastern Siberia and Alaska, suggesting that the differences are associated with rather local conditions. In contrast to prior studies suggesting a largely aeolian contribution to Yedoma sedimentation, the wide range of rEMs indicates that aeolian sedimentation processes cannot explain the entire variability found in GSDs of Yedoma deposits. Instead, Yedoma sedimentation is controlled by local conditions such as source rocks and weathering processes, nearby paleotopography, and diverse sediment transport processes. Our findings support the hypothesis of a polygenetic Yedoma origin involving alluvial, fluvial, and niveo-aeolian transport; accumulation in ponding waters; and in situ frost weathering as well as postdepositional processes of solifluction, cryoturbation, and pedogenesis. The characteristic rEM composition of the Yedoma clusters will help to improve how grain-size-dependent parameters in permafrost models and soil carbon budgets are considered. Our results show the characteristic properties of ice-rich Yedoma deposits in the terrestrial Arctic. Characterizing and quantifying site-specific past depositional processes is crucial for elucidating and understanding the trajectories of this unique kind of ice-rich permafrost in a warmer future. Kurzfassung: Der spätpleistozäne Yedoma Eiskomplex ist ein eisreicher und organikhaltiger Permafrosttyp, der in Beringia weit verbreitet ist. Durch den hohen Eisanteil wird der Yedoma Eiskomplex im Zuge des Klimawandels als besonders anfällig für tiefgreifende Störungen betrachtet und damit ein potentieller Kipppunkt des Klimasystems. Um seine Entstehung, die lokalen Eigenschaften und die regionale sedimentologische Zusammensetzung besser zu verstehen, haben wir die Korngrößenverteilung von 771 Proben an 23 Yedoma-Standorten in der Arktis zusammengestellt; räumlich eng zusammenhängende Probenserien wurden zu 17 Untersuchungsstandorten zusammengefasst. Darüber hinaus wurden 160 Proben aus nicht Yedoma-Ablagerungen von drei Eiskeilpolygon- und Überschwemmungsgebieten als holozäne Referenzen untersucht. Die multimodalen Korngrößenverteilungen zeigen, dass eine Vielzahl von Sedimentbildungs-, Transport- und Ablagerungsprozessen an der Yedoma-Entstehung beteiligt waren. Um diese Prozesse zu erkennen, wurde eine robuste Endmembermodellierungsanalyse (rEMMA) durchgeführt. Neun robuste Endmember (rEM) charakterisieren die Yedoma-Ablagerungen über ganz Beringia. Die untersuchten Standorte der Yedoma-Ablagerungen wurden anschließend mittels Clusteranalyse klassifiziert. Die daraus resultierenden vier Cluster umfassen zwei bis fünf Untersuchungsstandorte, die unregelmäßig über den Nordosten Sibiriens und Alaska verteilt sind. Die breite Palette von rEMs zeigt, dass nicht allein äolische Sedimentationsprozesse für die Variabilität in den Korngrößenverteilungen von Yedoma-Ablagerungen verantwortlich sind. Vielmehr wird die Sedimentation der Yedoma-Ablagerungen eher durch lokale Bedingungen wie Ausgangsgesteine, ehemalige Topographie und multiple Transportprozesse gesteuert. Das stützt die Hypothese einer polygenetischen Yedoma-Entstehung, die alluvialen, fluvialen und nival-äolischen Transport und Akkumulation in polygonalen Tümpeln und in-situ Frostverwitterung sowie postsedimentäre Frostverwitterung, Solifluktion, Kryoturbation und Pedogenese beinhaltet. Die charakteristische rEM Zusammensetzung der Yedoma Cluster kann auch helfen korngrößenspezifische Parameter besser in der Kohlenstoffbudgetierung und Permafrostmodellierung zu berücksichtigen. Damit trägt die Charakterisierung und Quantifizierung standortspezifischer Ablagerungsprozesse in der Vergangenheit dazu bei, die charakteristischen Eigenschaften eisreicher Yedoma-Ablagerungen in der terrestrischen Arktis aufzuklären. Dies ist entscheidend für das Verständnis der Fortentwicklung dieses besonderen Permafrosttyps in einer wärmeren Zukunft. [ABSTRACT FROM AUTHOR]

Differentiating thermokarst basin sediments with respect to the involved processes and environmental conditions is an important tool to understand permafrost landscape dynamics and scenarios and future trajectories in a warming Arctic and Subarctic. Thermokarst basin deposits have complex sedimentary structures due to the variability of Yedoma source sediments, reworking during the Late Glacial to Holocene climate changes, and different stages of thermokarst history. Here we reconstruct the dynamic growth of thermokarst lakes and basins and related changes of depositional conditions preserved in sediment sequences using a combination of biogeochemical data and robust grain-size endmember analysis (rEMMA). This multi-proxy approach is used on 10 sediment cores (each 300–400 cm deep) from two key thermokarst sites to distinguish four time slices that describe the Holocene thermokarst (lake) basin evolution in Central Yakutia (CY). Biogeochemical proxies and rEMMA reveal fine-grained sedimentation with rather high lake levels and/or reducing conditions, and coarse-grained sedimentation with rather shallow lake levels and/or oxidizing (i.e. terrestrial) conditions in relation to distal and proximal depositional and post-sedimentary conditions. Statistical analysis suggests that the biogeochemical parameters are almost independent of thermokarst deposit sedimentology. Thus, the biogeochemical parameters are considered as signals of secondary (post-sedimentary) reworking. The rEMMA results are clearly reflecting grain-size variations and depositional conditions. This indicates small-scale varying depositional environments, frequently changing lake levels, and predominantly lateral expansion at the edges of rapidly growing small thermokarst lakes and basins. These small bodies finally coalesced, forming the large thermokarst basins we see today in CY. Considering previous paleoenvironmental reconstructions in Siberia, we show the initiation of thaw and subsidence during the Late Glacial to Holocene transition between about 11 and 9 cal kyrs BP, intensive and extensive thermokarst activity for the Holocene Thermal Maximum (HTM) at about 7 to 5 cal kyrs BP, severely fluctuating water levels and further lateral basin growth between 3.5 cal kyrs BP and 1.5 cal kyrs BP, and the cessation of thermokarst activity and extensive frost-induced processes (i.e. permafrost aggradation) after about 1.5 cal kyrs BP. However, gradual permafrost warming over recent decades, in addition to human impacts, has led to renewed high rates of subsidence and abrupt, rapid CY thermokarst processes. • Small thermokarst lakes and basins grew rapidly during the Holocene Thermal Maximum. • Short-term phases of forcing climate lead to very active thermokarst processes. • Endmember analysis reveals different depositional environments in growing lakes. • Distal and proximal depositional and post-sedimentary conditions are differentiated. • Sedimentological and biogeochemical characteristics are weakly correlated. [ABSTRACT FROM AUTHOR]