THE YEDOMA CLIFF OF SOBO‑SISE ISLAND ‑ INSIGHTS INTO PAST AND MODERN PERMAFROST DYNAMICS AND RELATED ORGANIC MATTER STOCK AND RELEASE Afiliations * Alfred Wegener Institute Helmholtz Centre of Polar and Marine Research, Potsdam, Germany ** Faculty of Geography, Lomonosov Moscow State University, Moscow, Russia *** Polar Geography, Arctic and Antarctic Research Institute, St. Petersburg, Russia References Fuchs et al. Rapid )luvio‐thermal erosion of a yedoma permafrost cliﬀ in the Lena River Delta, Front. Earth Sci., 2020, 8, 336. Margold et al. Repeated mega)loods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years, Quaternary Sci. Rev., 2018, 187, 41‐61. Wetterich et al. The cryostratigraphy of the Yedoma cliﬀ of Sobo‐Sise Island (Lena Delta) reveals permafrost dynamics in the Central Laptev Sea coastal region during the last about 52 ka. TCD, under review, https://doi.org/10.5194/tc‐2020‐179. Figure 1. Profile positions of sediment profiles SOB18‐01, SOB18‐03 and SOB18‐06 as well as ice‐wedge profiles in 2014 and 2018. Lena Delta inset from ESA‐BELSPO 2015, produced by VITO (ESA Earth online). Yedoma (MIS3) 1 The Sobo‑Sise Yedoma Cliﬀ Syngenetic late Pleistocene Ice Complex of the Yedoma type is widespread in East Siberian lowlands and intentively studied in the Lena Delta and the Laptev Sea coastal in recent decades by joint Russian‑German eﬀorts (LENA Expedition). Diagnostic for Yedoma Ice Complex are the presence of syngenetic ice wedges, the oversaturation of the sediment with pore ice and segregated ice (excess ice) forming lenticular and reticulate cryostructures within mainly )ine‐grained deposits. During a )ield campaign in 2018, the entire permafrost sequence of the Sobo‑Sise Yedoma cliﬀ has been sampled in 0.5‐m vertical intervals via rope decending in three overlapping pro)iles (Wetterich et al., 2020; Figure 1). The studied Yedoma cliﬀ on Sobo‐Sise Island is remarkable for its rapid shoreline erosion of up to 22.3 m yr⁻¹ (1965‐2018; Fuchs et al., 2020) over about 1.9 km length with a mean annual retreat rate of 9.1 m yr⁻¹. The present study of the permafrost exposed the Sobo‐Sise Yedoma cliﬀ provides a comprehensive cryostratigraphic and organic matter inventory, insights into permafrost aggradation and degradation over the last about 52 thousand years and their climatic and morphodynamic controls on regional scale of the Central Laptev Sea coastal region in NE Siberia. Sebastian Wetterich*, Matthias Fuchs*, Alexander Kizyakov**, Jens Strauss*, Ingmar Nitze*, Michael Fritz*, Juliane Wolter*, Thomas Opel*, Hanno Meyer*, Aleksei Aksenov***, Lutz Schirrmeister*, Guido Grosse* 2 Geochronology and cryostratigraphy The geochronological record of the Sobo Sise Yedoma spans the last 52 ka cal BP based on radiocarbon dating and age‐height modelling (Figures 2, 3). The sequence diﬀerentiates into three cryostratigraphic units that are MIS 3 Yedoma IC (52–28 ka cal BP), MIS 2 Yedoma IC (28–15 ka cal BP) and MIS 1 Holocene cover (7–0 ka cal BP). The cryostratigraphic sequence is not continuous, but has chronological gaps at 36–29 ka cal BP, at 20–17 ka cal BP and at 15–7 cal ka BP (Figure 2). These gaps represent traces of past changes in climatic conditions as well as in permafrost deposition and/or erosion regimes. Similar observations have been made on adjacent Yedoma IC sites on Bykovsky Peninsula and Kurungnakh‐Sise Island (Figure 4) and are likely related to repeated outburst )loods of glacial Lake Vitim along the Lena Valley into the Arctic Ocean during MIS 3 and MIS 2 as proposed by Margold et al. (2018). The cryostratigraphic units of the Yedoma cliﬀ are built of poorly sorted sandy silt but diﬀer in prevalent grain‐size fractions ranging from )ine silt to middle sand. The organic matter content is highest in the thin MIS 1 (unit C) Holocene cover (TOC of 11.3±9.9 wt%, TN of 0.6±0.3 wt%), but still substantial in MIS 3 (unit A) Yedoma IC (TOC of 4.5±2.5 wt%, TN of 0.3±0.1 wt%) and in MI