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The permafrost record of Elgygytgyn Impact Crater

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Citation:
Schwamborn, G. , Fedorov, G. , Schirrmeister, L. , Meyer, H. and Hubberten, H. W. (2008): The permafrost record of Elgygytgyn Impact Crater , Second International Conference and Workshop of Arctic Palaeoclimate and its Extremes (APEX) Recent Advances, April 1 - 4, 2008, Durham University, Department of Geography, Durham City, UK.. .
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Abstract:

Elgygytgyn Impact Crater on the Chukotka Peninsula provides the unique opportunity to identify recent to Late Pleistocene permafrost conditions in terrestrial deposits and to trace back the permafrost history when using suitable proxy data with the adjacent lake sediment archive. At maximum this may retrieve a palaeoenvironment history containing changes in permafrost conditions back to > 3 Myr BP, the time of the meteor impact. Knowledge about the Late Quaternary changes as verified by terrestrial archives provide an interpretation scheme that can be applied to more ancient portions of the glacial cycles using the lake sediments.Currently, the weathering detritus at Elgygtgyn Crater is created under permafrost conditions. It passes through typical mechanisms of periglacial landscape dynamics (i.e. solifluction, surface wash, thermo erosion, river erosion) into the lake, which is placed in the central basin. Based on field observation and laboratory analysis of frozen ground deposits several conclusions are highlighted describing periglacial dynamics during the Late Quaternary. (1) Subaerial terrace formation resulting from slope debris deposition was initiated during the Late Pleistocene / Holocene transition. During Late Holocene the accumulation rate on the slopes decreases. (2) Ice-wedge architecture within frozen ground allows identifying two generations of Holocene ground ice formation. Near-surface thermal change occurred at 4000 yrs BP creating narrow-meshed ice wedge polygons on top of wide-meshed polygons. (3) Pore ice oxygen isotope signatures reveal that the regional Holocene Thermal Maximum happened at about 9000 yrs BP. (4) The crater undergoes a principal lake level drop in Late Quaternary time. Age determination of pebble bars that surround the lake reveal a minimum age of 13,000 yr BP for the ancient shorelines. Dating is based on analysis of a permafrost core that was extracted behind the raised bars, where slope deposits have accumulated after the bar formation. (5) Mineralogical ratios (quartz to feldspar) and single quartz grain micromorphology have been tested on Holocene frozen ground deposits as proxy data reflecting the strength of cryogenic weathering. The selective cryogenic break-up of grains is particularly related to thaw and freeze dynamics in the active layer. When applied to the lake sediments the mineralogical data illustrate the persistence of cryogenic weathering at least back to about 300,000 yrs BP, the time that is covered by first lake sediment cores. Future ICDP deep drillings into the permafrost and the lake will enable to extend knowledge about permafrost changes back into time. Presumably, this will cover the Pliocene/Pleistocene boundary when northern hemispheric glaciations started to intensify and the onset of permafrost formation can be dated.

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