ABSTRACT: Permafrost and periglacial processes at the easternmost edge of Beringia in the Western Canadian Arctic

CGRG Bibliography of Canadian Geomorphology
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Abstract

Herschel Island - about 70 km east of the Yukon- Alaska border - occurs as the only major elevation on the Yukon Coastal Plain facing the Southern Beaufort Sea. Being accumulated as a terminal moraine during the Early to Middle Wisconsin the island has been intensively affected by periglacial processes for at least the last 50 ka BP. Since Herschel Island most likely remained ice-free during the last glacial maximum (LGM) it became part of the vast unglaciated land mass - Beringia - and is therefore an excellent study area to reconstruct paleoenvironmental dynamics on the easternmost edge of Beringia where records since the Late Pleistocene are still sparse. Multi-proxy analyses on sediments and stable isotope analyses on ground ice samples have been performed to unravel periglacial processes towards sedimentary history, permafrost aggradation and degradation through time as well as to link these processes to distinct periods of climate change. Sediments generallyconsist of clayey diamicton and sandy silts with varying amounts of pebbles, cobbles and organic remains. However, stratigraphic appraisals are difficult due to the deformed nature of Herschel Island sediments by glacial ice thrusting, subsequent cryoturbation and recent mass wasting. Nevertheless, radiocarbon dated peat suggests that until 8.4 ka BP bioproductivity was inhibited due to continuous harsh climate conditions. During the HoloceneThermal Maximum (HTM) thaw lakes developed and a rapid accumulation of peat followed on degrading polygonal ground. An extensive active layer thickening is recorded by a widespread thaw unconformity along the island’s coast at depths between 1.2 to 2 m below surface. Different types of ground ice recovered range widely regarding their isotopic composition, thus reflecting different types of water and strongly variable climatic conditions during ground ice development. The oxygen isotopic signature of Holocene ice wedges varies between -20 to -24 ‰ VSMOW, which generally agrees with the supposition that recent temperatures are supposed to produce d18O values of about -20‰ in the study area. In contrast, a relict wedge truncated at 1.5 m below surface revealed oxygen isotopic values ranging from -27 to -30 ‰. This leads to the assumption that Herschel Island comprises ice wedges that formed likely prior to the Holocene Thermal Maximum (HTM) and afterwards. The isotopic composition of intrasedimental ice recovered from sediment profiles variesbetween -17 to -27 ‰ and mostly decreases with depth, indicating that ground ice in different depths preserves paleo-temperatures during the aggradation of permafrost sequences

Bibliography of Canadian Geomorphology