Thermokarst Processes 1321 Thermokarst Subsidence

Thermokarst subsidence denotes a lowering of the ground surface following ablation of excess ice in permafrost. Ablation typically occurs by melting caused by heat conduction as the active layer deepens or surface water ponds. In permeable soils, however, it also results from heat convection by percolating rain or ground-water. The subsequent loss of excess water by drainage or evaporation allows the soil to consolidate and ground surface to subside. Subsidence is clearest where it is localised, for example above intersecting ice wedges (Fig. 13.1) or where collapse

Julian B. Murton

Department of Geography, University of Sussex, Brighton BN1 9QJ, UK e-mail: [email protected]

R. Margesin (ed.) Permafrost Soils, Soil Biology 16,

DOI: 10.1007/978-3-540-69371-0, © Springer-Verlag Berlin Heidelberg 2009

Fig. 13.1 High-centred ice-wedge polygons near Johnson Bay, Tuktoyaktuk Peninsula, western Arctic Canada. The pond has developed by thermokarst subsidence above intersecting ice wedges. The fissures marking the polygon margins have developed by thermal erosion. Person for scale. Photo courtesy of Mark Bateman

Fig. 13.1 High-centred ice-wedge polygons near Johnson Bay, Tuktoyaktuk Peninsula, western Arctic Canada. The pond has developed by thermokarst subsidence above intersecting ice wedges. The fissures marking the polygon margins have developed by thermal erosion. Person for scale. Photo courtesy of Mark Bateman pits on the floors of drained lakes develop by melting of an ice-rich layer at the top of permafrost (Mackay 1999). Under very dry soil conditions, however, ice loss can occur directly by sublimation. In the hyperarid cold desert of southern Victoria Land, Antarctica, where soil temperatures may remain < 0°C all year, sublimation of ground ice through a porous, gravelly overburden has caused localised ground subsidence, forming prominent troughs around high-centred ice-wedge polygons (Marchant et al. 2002).

The degree of subsidence depends on the amount and distribution of excess ice prior to thaw and on the thickness of permafrost thawed (Mackay 1970). Subsidence can be predicted if the excess ice content is known, although prediction may be complicated by mass movement (Lawson 1982). The amount of subsidence varies from millimetres to many metres. Subsidence of 20 m or more can result from thaw of thick, ice-rich Pleistocene silts (Yedoma or ice-complex) which underlie more than 1 million km2 of northern Siberia and central Alaska (Kachurin 1962; Zimov et al. 2006).

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