ИСТИНА

The processes that deposited syngenetically-frozen ice-rich silts and silt-sand intergrades (yedoma) in western Beringia remain uncertain despite more than half a century of research in NE Siberia. Such uncertainty fundamentally affects our understanding of the depositional history and palaeoecology of western Beringia, the sedimentary processes that lead to sequestration of hundreds of Pg of carbon and whether yedoma provides a globally significant record of ice-age atmospheric conditions or just regional floodplain activity. Here we test the hypotheses of aeolian versus waterlain deposition of yedoma sandy silt; elucidate the palaeoenvironmental conditions; and develop a conceptual model of silt deposition to clarify understanding of yedoma formation during the Late Pleistocene. This is based on a field study in 2009 of the Russian stratotype of the ‘Yedoma Suite’, at Duvanny Yar, in the lower Kolyma River, northern Yakutia, supplemented by observations we have collected there and at other sites in the Kolyma Lowland since the 1970s. Most of the yedoma at Duvanny Yar is cryopedolith (material which has experienced incipient pedogenesis along with syngenetic freezing). Mineralised and humified organic remains dispersed within cryopedolith indicate incipient soil formation, but distinct soil horizons are absent. Five buried palaeosols and palaeosol ‘complexes’ are identified within cryopedolith on the basis of sedimentary and geochemical properties. The cryopedolith-palaeosol sequence accreted incrementally upwards on a vegetated palaeo-landsurface with a relief of at least several metres, preserving syngenetic ground ice in the aggrading permafrost. Three hypotheses concerning the processes and environmental conditions of yedoma silt deposition at Duvanny Yar are tested. The alluvial-lacustrine hypothesis and the polygenetic hypothesis are both discounted on sedimentary, palaeoenvironmental, geocryological and palaeoecological grounds. The loessal hypothesis provides the only reasonable explanation to account for the bulk of the yedoma silts at this site. The Duvanny Yar yedoma is part of a subcontinental-scale region of Late Pleistocene cold-climate loess. One end member remains rich in syngenetic ground ice, the other was ice-poor and subject to complete permafrost degradation at the end of the last ice age. These end members reflect a distinction between enduring cold permafrost conditions leading to stacked transition zones and large syngenetic ice wedges in Beringia vs. intermittent permafrost conditions leading to repeated permafrost thaw and small ice-wedge pseudomorphs in NW Europe.