Late glacial to Holocene environmental dynamics of the terrestrial Arctic new records from thermokarst deposits along the Dmitry Laptev Strait (NE Siberia)

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Wetterich, S. , Schirrmeister, L. , Andreev, A. , Pudenz, M. , Opel, T. , Meyer, H. and Derevyagin, A. Y. (2009): Late glacial to Holocene environmental dynamics of the terrestrial Arctic new records from thermokarst deposits along the Dmitry Laptev Strait (NE Siberia) , APEX, Arctic Palaeoclimate and its EXtremes - beyond the frontier, Third International Conference and Workshop Copenhagen, March 31st - April 3rd .
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The palaeo-landscape and environmental history during the last glacial-interglacial transition was studied in a broader regional context by comparing permafrost sequences from the northern and the southern coasts of the Dmitry Laptev Strait (East Siberian Sea) on Bolshoy Lyakhovsky Island (73°17′N, 141°20′E) and on the Oyogos Yar coast (72°36'N, 143°36'E).Both coastal exposures show the same general stratigraphy pointing to similar landscape and environmental dynamics which were reconstructed using cryolithological, sedimentological and micro-palaeontological (i.e. pollen, freshwater ostracods) methods as well as stable isotope data from ice wedges.The late Glacial and Holocene permafrost records of both coastal sections are subdivided into three horizons that are of taberal (thawed and refrozen), lacustrine, and boggy origin.In both sections, the lowermost horizons consist of taberal deposits of the former late Pleistocene Ice Complex (dated between 46.6 and 36.6 14C kyr BP) which are composed of sandy silt with peat lenses and thaw signs (whitish laminations). The ice content is generally low and the cryostructures are lens-like reticulated or layered, typical for refrozen sediments. The pollen data from the taberal horizons point to a Middle Weichselian interstadial vegetation. Ostracod shells are rare.The taberal horizon is discordantly covered by a lacustrine horizon of late Glacial age (dated between 14.8 and 12.5 14C kyr BP). Its lowermost part is composed of cryoturbated peaty palaeosols in both sections. The overlying lacustrine facies is structured by alternating beds of clayish silts and plant detritus layers which contain numerous mollusc shells, ostracods, and wood fragments. The cryostructure is lens-like or lens-like layered and epigenetic roots of ice wedges are common within the lacustrine horizons. Pollen records are in good accordance with Allerød to Younger Dryas ages indicating a grass-sedge tundra dominated vegetation. The ostracod records point to stable aquatic conditions during the late Glacial where the high numbers and diverse assemblages were found. Thus, thermokarst development and lake formation started some time before the Holocene. The uppermost horizons accumulated under subaerial conditions in a boggy polygonal tundra environment from about 7.5 to 4.0 14C kyr BP at the Bolshoy Lyakhovsky section and between about 10.0 and 3.3 14C kyr BP at the Oyogos Yar section. However, the late Holocene deposits discordantly cover the underlying early Holocene sediments.Numerous peat inclusions or single peat layers were found in a sandy silt matrix with ice bands and a lens-like cryostructure between single bands which indicate syngenetic freezing of the sediments. Stable isotope signatures of ice wedges syngenetically formed in boggy deposits clearly differ from isotope composition of Weichselian ice wedges, pointing to warmer conditions during the Holocene. However, a notable variability and a clear warming trend occurred during the last millennium. An early Holocene shrubby tundra vegetation is reconstructed from the pollen spectra, whereas the late Holocene pollen spectra indicate a shift to modern wetland tundra vegetation. The early Holocene ostracod assemblages are sparse and low diverse.The multi-proxy palaeoenvironmental record of the last glacial- interglacial transition indicates a general pattern of regional landscape development according to climatic changes. Evidence of the three landscape development stages was obtained, including (1) thermokarst-induced formation of basins, (2) accumulation of lacustrine sequences, and (3) transformation of lake-dominated areas into polygonal tundra. The stages are considered to be of stratigraphical significance.

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