Active-layer deepening is inevitable in areas of ice-rich permafrost — because of interannual or longer term variations in thaw depth — and leads to thermokarst subsidence. But evidence for subsidence is usually clear only where deepening has been substantial, where remnants of the unaffected ground surface remain, or where subsidence has altered the surface hydrology and vegetation. In the cryostratigra-
phy, evidence for former active-layer deepening occurs where a thaw unconformity truncates excess ice below the base of the modern active layer (Fig. 13.3). Pullman et al. (2007), in a study of potential thaw settlement following severe disturbance to vegetation on the tundra of the Alaskan Arctic Coastal Plain, determined values between 0 cm (in sandy soils) and 103 cm (in silty soils). Mackay (1995) estimated that, following a forest-tundra fire near Inuvik, Canada, active-layer deepening of 10-78 cm produced ground subsidence of 5-39 cm during the succeeding 5-20 years. Osterkamp et al. (2000) reported subsidence commonly of 2 m for discontinuous permafrost in the boreal forest of the Mentasta Pass area, southeast Alaska, where active-layer deepening attributed to climate warming has led to the replacement of spruce stands by wet sedge meadows whose surface is typically 1-3 m below that of the original spruce forest.
As the active layer deepens, thaw consolidation produces melt-out horizons and may trigger soft-sediment deformation. Mineral particles released from thawing ice assume a tighter packing than sediment dispersed in the ice, and so form distinctive melt-out horizons that record thaw events (Fig. 13.3). Soft-sediment deformation is most likely to occur during rapid thaw of ice-rich clayey sediments, when the soil is reduced to a fluid-like consistency. Under such conditions, processes associated with water-escape, buoyancy and subsidence form thermokarst involutions (Murton and French 1993; Harris et al. 2000). Other consequences of active-layer deepening
Fig. 13.3 Thaw unconformity marking the base of the early Holocene active layer truncates massive ice and icy sediments (basal Laurentide ice), Summer Island, Tuktoyaktuk Coastlands, Canada. Melt-out till containing thermokarst involutions in a relict active layer overlies the thaw unconformity. Person for scale
Fig. 13.3 Thaw unconformity marking the base of the early Holocene active layer truncates massive ice and icy sediments (basal Laurentide ice), Summer Island, Tuktoyaktuk Coastlands, Canada. Melt-out till containing thermokarst involutions in a relict active layer overlies the thaw unconformity. Person for scale through ice-rich permafrost may include the enlargement of mud hummocks and, on hillslopes, enhanced gelifluction and triggering of active-layer detachment slides.
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