Yedoma and thermokarst in the northern part of Seward Peninsula, Alaska
Abstract
Yedoma, or late Pleistocene ice-rich syngenetic permafrost with large ice wedges, widely occurs in parts of Alaska that were unglaciated during last glaciation. Such sediment occurs in Interior Alaska, the Brooks Range foothills and Seward Peninsula is often formed in loess. Until recently, Yedoma has been studied mainly in Russia. Since the late Pleistocene, most Yedoma has been destroyed by thermokarst and thermal erosion. It is believed that most of the degradation of Yedoma occurred during the transition from late Pleistocene to early Holocene (Gravis, 1978; Shur, 1988). Yedoma continues to degraded as observed in the Russian Arctic and in Alaska. Degradation of Yedoma occurs in different forms. Thermokarst lakes are the most obvious and one the most damaging forms of Yedoma degradation. The other is shore thermal erosion. Unlike most thaw lakes in permafrost regions, which are usually shallow and are not deeper than 1 to 3 m, thaw lakes in Yedoma can be very deep. We studied lakes in which the water depth reached 10 to 14 m and these lakes frequently have drainage outlets that reduce water levels. Simple calculations show that without drainage, the depth of a lake developing in 40 m thick Yedoma can vary from 25 to 35 m. Thermokarst lakes in Yedoma have several stages of development. The first stage is triggered by the active layer thickening and partial thawing of ice wedges. Small ponds over ice wedges, especially at their intersections, are formed at this stage. In favorable conditions these ponds grow deeper and coalesce to form a shallow initial thermokarst lake. This initial stage of development can be interrupted by drainage associated with sloping relief and by accumulation of organic on the soil surface. In the continuous permafrost zone, remnants of Yedoma are stable when well drained. The second stage of lake formation includes fast melting of ice wedges and ice-rich sediment between them, which results in fast deepening of the lake. Yedoma sections, we have studied, are characterized by total volumetric ice content from 70 to 90% and thaw strain (ratio of thaw settlement to initial thickness before thawing) from 50 to 80%. With such high ice content the formation of thaw lakes in Yedoma is comparable to the formation of kettle lakes in glacial terrain. At this stage, only shallow taliks develop beneath thaw lakes. With time, thawing can reach the base of the ice-rich stratum, and only sudden, complete drainage can prevent Yedoma from thawing entirely. Thickness of Yedoma on the Seward Peninsula varies from a few to up to 50 m and we estimate that it takes dozens to several hundreds of years to achieve full degradation under thaw lakes, depending on its thickness and if thawing proceeds without interruption. The final stage of thermokarst in Yedoma is related to entire thawing of Yedoma, lake drainage and subsequent freezing of soils in a drained lake basin. This stage produces alases (thaw lake basins), alas valleys, and alas plains. All these forms are common in the northern part of Seward Peninsula.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.C41A0443S
- Keywords:
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- 0702 CRYOSPHERE / Permafrost;
- 0708 CRYOSPHERE / Thermokarst;
- 0738 CRYOSPHERE / Ice;
- 0746 CRYOSPHERE / Lakes