A record of permafrost conditions for Arctic Siberia from Elgygytgyn Impact Crater

Elgygytgyn Impact Crater in the terrestrial Arctic holds the unique opportunity to trace back frozen ground conditions to the Pliocene/Pleistocene boundary. A meteor hit the Chukotkian mountain belt 3.6 Myr ago and created a basin in Cretaceous volcanic bedrock. Its geological composition produces a fairly uniform provenance signal in the lake sediments, which presumably start to accumulate soon after the impact event. It makes the archive suitable to testvariations of sediment-mineralogical properties as proxy data reflecting the strength of cryogenic weathering in the catchment. The production of (a) silt size debris, (b) a peculiar single grain morphometry, and (c) the enrichment ofquartz grains in the silt fraction with respect to feldspar occurrence and to the sand fraction serve as indicators of frozen ground conditions in near surface deposits. They result from thaw and freeze dynamics in the active layer and cause a selective grain break-up. Around the Crater Lake deposits also accumulate at piedmont settings. A set of surface samples and two five-meter-sections extracted from slope deposits serve as a reference for the modern and the Holocene permafrost situation.Recent ICDP drilling into the permafrost margin of Elgygytgyn Crater has recovered a core 140 m in length and ongoing deep drilling into the lake basin extends beyond 300.000 years, the time our study covers up to now. The presentation sums up current results of both permafrost and lake sediment records when displaying the sedimentmineralogical properties. They illustrate the persistence of cryogenic weathering even in warm periods such as theEemian and the Holocene