Geology Reference
In-Depth Information
12
Evidence for Past Permafrost
The evidence from which the previous existence of perennially-frozen ground can be
deduced is reasonably clear and unambiguous. It can be categorized as that which
indicates either (i) permafrost aggradation or (ii) permafrost degradation. The distinc-
tion between permafrost and deep seasonal-frost can be obscure. There is widespread
evidence for Late-Pleistocene thermokarst activity in the mid-latitudes.
12.1. INTRODUCTION
The periglacial mid-latitude environments of the Pleistocene were characterized by the
presence of either seasonal or perennial frost, intense frost action, and the enhanced
activity of azonal processes such as snow, wind, and running water. This chapter reviews
the evidence that indicates the former existence of permafrost. It warrants treatment as
a separate chapter because the permafrost conditions of high latitudes are a central com-
ponent to present-day periglacial environments.
12.2. PAST PERMAFROST AGGRADATION
12.2.1. The Paleo-Permafrost Table
If permafrost previously existed, it must have formed in association with a ground-surface
layer that was subject to seasonal freezing and thawing. This layer is termed the active
layer. In regions where permafrost is forming today, the base of the active layer constitutes
the permafrost table. Because of annual variations in the maximum depth of thaw, the
base of the active layer can fl uctuate slightly. This interface, between the active layer and
the underlying permafrost, is referred to as the transient zone. Both the active layer and
the transient zone have been discussed earlier in Chapter 5.
If the top of permafrost (or the base of the active layer), so defi ned, can be identifi ed in
unfrozen sediments in non-permafrost regions, this is highly signifi cant in terms of paleo-
climatic reconstruction. This is because it provides a vital distinction between seasonally-
and perennially-frozen ground conditions. For example, if such a layer can be identifi ed, its
thickness can be used, through application of the Stefan equation (see Chapter 5), to
approximate the mean ground surface temperature (MAGST) (see Figure 3.3). If one
then makes assumptions as to the magnitude of the surface offset (see Chapter 3), an
approximation of the mean annual air temperature (MAAT) can be inferred.
A complicating factor is the presence of relict permafrost. Where this occurs, the base
of the seasonally-frozen layer may not be the top of permafrost because the latter may be
