Geology Reference
In-Depth Information
within it, does not occur. The closest comparable features are the shallow thermokarst
depressions and lakes of the Mackenzie Delta region, in which closed (hydrostatic) system
pingos form. However, few of these depressions have the same striking relief as the Yaku-
tian depressions. Moreover, they are primarily the result of lateral growth rather than
subsidence. Probably the most analogous terrain to the alas thermokarst relief is the ice-
cored topography described from parts of the Canadian arctic coastal plain (Mackay,
1963; Rampton, 1974). For example, in the Tuktoyaktuk Peninsula, thermokarst develop-
ment along small creeks has produced “macro-beaded” drainage systems where a series
of depressions, partly lake-fi lled, are connected to each other by narrower sections or
shallow interfl uves.
Much of the lowland terrain of the Yukon Coastal Plain and Pleistocene Mackenzie
Delta regions are essentially thermokarstic in nature and similar to that of the Laptev
Sea coastal lowlands previously described. For example, extensive areas of ice-cored
terrain near Sabine Point, Northern Yukon, are regarded as being remnants of an initial
upland surface, greatly modifi ed by thermokarst processes (Harry et al., 1988). The
bodies of massive ice and icy sediments formed as permafrost aggraded following
ice retreat from the maximum Late-Wisconsinan (Buckland) glacial limit. Then,
multiple episodes of regional, climatically-induced thermokarst occurred between 14 000
and 8000 years ago that resulted in thaw settlement and surface subsidence in a series of
coalescent and polycyclic thaw-lake basins. Thaw mobilization of sediment resulted in
the transport of material from upland areas into the basins and to the progressive isola-
tion of rolling ice-cored uplands areas draped with a variable thickness of surfi cial materi-
als of thermokarst origin.
8.8. HUMAN-INDUCED THERMOKARST
In spite of our understanding of thermokarst processes in permafrost environments,
numerous examples of human-induced thermokarst continue to occur today.
Russia, by virtue of its long history of northern exploration and settlement, has by far
the greatest experience in this respect. As early as 1925, experiments were being under-
taken to determine the effects of vegetation changes on the underlying permafrost, brought
about either by deforestation or plowing (Tikhomirov, 1959; Tyrtikov, 1964) (Table 8.2).
In Alaska, similar experimental studies were subsequently undertaken following the colo-
nization and settlement of the 1920-1940 period. For example, one study in the Fairbanks
region involved the cutting and/or stripping of surface vegetation by the US Army Corps
of Engineers in 1946. In the stripped area, the active-layer thickness increased from 1.0 m
to more than 3.0 m over a ten-year period. Subsequent studies emphasized the thermal
role played by the surface organic layer and/or forest cover (e.g. Babb and Bliss, 1974;
Brown et al., 1969; Haugen and Brown, 1970; Kallio and Reiger, 1969).
8.8.1. Causes
Without doubt, the most common cause of human-induced thermokarst is the clearance
of vegetation for agricultural or construction purposes. However, it must be emphasized
that even small disturbances to the ground surface may be suffi cient to induce thermokarst
activity. J. R. Mackay (1970), for example, describes how an Eskimo dog in the Mackenzie
Delta was tied to a stake with a 1.5 m long chain. In the ten days of tether, the animal
