The late Pleistocene ice-rich Yedoma permafrost is extremely sensitive to Arctic warming. Warming air temperatures, decreasing sea ice extent lead to an increasing degradation of the Yedoma permafrost and thus to a greater sediment input from coastal shorelines and river floodplains to the Laptev Sea. Thus, so far freeze-locked sediments and any potentially hazardous contaminants contained in them are entering Arctic waters and the biological food chain. Shallow (down to <2m) Arctic permafrost soil layers were found to include high levels of mercury (Hg) due to natural enrichment processes of environmentally available Hg (Schuster et al. 2018). However, opposed to seasonal thaw processes of the active layer and long-term gradual thaw through active layer deepening, abrupt thaw processes such as thermokarst, thermo-erosion, and coastal erosion are capable of mobilising permafrost-soils and stored contaminants from tens of meters depth within years to decades. In this study, we determined Hg concentrations from various deposits in Siberia’s deep permafrost sediments. We studied links between sediment properties and Hg enrichment in order to assess a first deep Hg inventory in late Pleistocene permafrost down to 36 m below surface. To do this, we used sediment profiles from seven sites representing different permafrost degradation states on Bykovsky Peninsula (northern Yakutian coast) and in the Yukechi Alas region (Central Yakutia). We analysed 41 samples for Hg content, total carbon, total nitrogen and organic carbon as well as grain size distribution, bulk density and mass specific magnetic susceptibility. Figure 1: (a) geographical overview and detailed location of the study site at Bykovsky Peninsula (b) and Yukechi Alas in Yakutia (c); (d) stratigraphical transect of the study sites and different states of degrading permafrost in Siberia. The numbers indicate the areas of interest in this study. 1) Talik in Yedoma (unfrozen), 2) late Pleistocene Yedoma (frozen), 3) talik in thermokarst (unfrozen), 4) refrozen drained lake basin = Alas (frozen), 5) talik in thermokarst close to sea (unfrozen), 6) talik below seawater flooded thermokarst basins (= lagoons) (unfrozen). We show that the deep sediments (to 30 meter below surface) are characterized by an Hg concentration of 9.72 ± 9.28 μg kg-1 and an correlation of Hg to organic carbon, total nitrogen, grain-size distribution and mass specific magnetic susceptibility. Hg concentrations are higher in the generally sandier sediment of the Bykovsky Peninsula than in the siltier sediment of the Yukechi Alas. In conclusion, we found that the deep permafrost sediments, frozen since tens of millennia, contain sizeable amounts of Hg. Even though the average amount of Hg is with 9.72 μg/kg below levels immediately critical for life and our median is 85 % less (Schuster et al. 2018) than found in Arctic topsoil outside Siberia. Even if the Hg concentrations are not particularly high compared to other sites, the permafrost’s huge spatial coverage results in a significant amount of Hg that can be introduce into nearby aquatic environments and food webs. As the next step, the consequences of old Hg re-entering the active biogeochemical cycles and food webs with ongoing Arctic warming remain unclear and need to be studied in more detail. References 1. Schuster, P. et al. Geophysical Research Letters, 2018, 45, 1463– 1471, https://doi.org/10.1002/2017GL075571