Geology Reference
In-Depth Information
is discussed in Chapter 11. By contrast, much current thermokarst activity in Arctic
North America is largely the result of local, non-climatic factors. However, a widespread
regional thaw unconformity in the western Canadian Arctic does suggest a period of
regional thermokarst activity between approximately 9000 and 4500 years ago (Burn,
1997; Burn et al., 1986; French, 1999; Mackay, 1975b). Since then, climate has cooled and
regional thermokarst has been limited. For example, on Banks Island, the freezing of
taliks and the growth of pingos, between 4000 and 2500 years ago (Pissart and French,
1976), indicates that climate deteriorated at that time. The impact of the recent climate
warming of the last 30 years in the northern polar region has yet to be fully appreciated
or understood.
It is diffi cult to generalize about the global distribution of thermokarst phenomena
since the climatic, permafrost, and ground ice controls vary from area to area. However,
several general observations are appropriate. First, thermokarst develops best in uncon-
solidated ice-rich sediments rather than in bedrock. Obviously, this refl ects the structural
coherence of bedrock and the fact that fi ne-grained sediments promote ice segregation.
Thus, in certain areas, thermokarst processes may assume dramatic regional importance.
Second, thermokarst is rarely reported from alpine and mountainous regions. Clearly, this
observation is related to the fi rst. Third, thermokarst is largely absent from many of the
ice-free areas of the extreme polar latitudes. This is largely because the aridity of these
regions leads to low ground-ice amounts in the near-surface sediments. An exception to
this particular generalization, the fi ne-grained sediments of the Fosheim Peninsula of
northern Ellesmere Island, has been discussed in Chapter 7. In the case of “ice-free” Dry
Valleys of Southern Victoria Land, Antarctica, buried glacier ice is widespread, and sub-
limation, rather than melt, of buried ice occurs. This must be regarded as a special and
unusual form of thermokarst activity. Fourth, thermokarst is obviously favored in areas
of warm permafrost, where near-surface ground temperatures are close to 0°C. For
example, in discontinuous and sporadic permafrost, the vegetation cover and organic mat
are crucial to permafrost preservation and, if disturbed for any reason, thermokarst may
be initiated. By contrast, in terrain underlain by cold permafrost, thermokarst may be less
important because the active layer is shallow, the period of summer thaw is short, and the
thermal change required to initiate thermokarst is large.
8.2.2. Specifi c
It must be stressed that thermokarst can develop in a stable (i.e. unchanging) cold climate
in response to a variety of situations that may be either natural or human-induced. This
was fi rst emphasized by S. P. Kachurin (1955). The process is sometimes described as
“self-developing thermokarst” (Aleshinskaya et al., 1972).
The widespread presence of ice-wedge polygons in many periglacial landscapes (see
Figure 6.1) means that, in summer, water accumulates not only in the trough above the
ice wedge, but also in the depression formed at the junction of ice wedges and within the
depressed centers of low-centered polygons. These shallow bodies of standing water
invariably favor preferential thaw during summer and impede freeze-up in the autumn.
For example, Table 8.1A provides data that demonstrate the thickness of the active layer
in typical lowland terrain of central Siberia beneath 20-25 cm of standing water is twice
as great as beneath adjacent tundra. Thus, localized thermokarst is a natural and ubiqui-
tous process in many poorly-drained tundra lowlands. Once polygons have developed their
low-center characteristic, further thaw is promoted without any supplementary agent (see
Figure 8.8B).
