Geology Reference
In-Depth Information
of about 1.5 m. Measurements, made over a 6-year period (1991-1996) in the burn areas,
indicate that the upper 2.4 m of permafrost had degraded by 1997 and ground temperature
throughout the permafrost had warmed by approximately
0.2 °C. A simplifi ed numerical
model suggests that approximately 1000 years would be required for complete degradation
of the permafrost.
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5.7.6. Lakes and Surface Water Bodies
Water has a high volumetric heat capacity (see Table 5.1). It is not surprising, therefore,
that lakes create the greatest natural thermal departure at the ground surface from condi-
tions determined by climate (Brewer, 1958; Lachenbruch, 1957; Mackay, 1962, pp. 33-48).
Numerous fi eld observations, together with theoretical considerations (Brown et al., 1964;
Johnston and Brown, 1964; Kudryavtsev, 1965, pp. 25-27) indicate that an unfrozen layer,
or talik, exists beneath water bodies that do not freeze to their bottoms in winter. The
size of talik varies with area and depth of the water body, water temperature, thickness
of winter ice and snow cover, and nature and compaction of bottom sediments. Lakes are
widespread in many permafrost regions and especially common in deltaic, coastal, and
lowland tundra environments, where they may occupy as much as 15-20% of the total
land area.
Along the southern fringes of the discontinuous permafrost zone, especially in wet-
lands, drainage conditions assume great importance in determining the presence or
absence of permafrost (Brown, 1973b; Zoltai, 1973). This is discussed more fully in
Chapter 6 in the context of palsas and organic terrain.
5.8. THE ACTIVE LAYER
The active layer refers to the layer of ground in areas underlain by permafrost which thaws
during summer. As a rule, it is thinnest in polar regions (as little as 15 cm) and becomes
thicker in sub-arctic regions (as much as 1.0 m or more). In the continuous permafrost
zone, it generally reaches the permafrost table. In the discontinuous permafrost zone, it
may be separated from underlying permafrost by a talik, or residual thaw layer. The thick-
ness of the active layer varies from year to year depending on controls such as ambient
air temperature, slope orientation and angle, vegetation, drainage, snow cover, soil and/or
rock type, and water content.
The active layer includes the uppermost part of the permafrost wherever either the
salinity or clay content of the permafrost allows it to thaw and refreeze annually, even
though the material remains cryotic (i.e. below 0 °C). The term “depth to permafrost” as
a synonym for thickness of the active layer is misleading, especially in areas where the
active layer is separated from the permafrost by a thawed, or non-cryotic (i.e. above 0 °C),
layer of ground.
The introduction of geocryological, or so-called “cryotic,” terminology has confused
the defi nition (see above) of the active layer. As shown in Figure 5.1A, the active layer,
defi ned traditionally, includes a near-surface layer that is seasonally cryotic and a zone
immediately beneath which is subject to thaw in spite of the fact that the temperature is
below 0 °C (i.e. cryotic). This zone is termed “seasonally-active” permafrost. Because the
active layer, so defi ned, varies consistently both in time and space, it is diffi cult to apply
to operational procedures in the fi eld (Burn, 1998b). The CALM monitoring program
uses the traditional defi nition of the active layer.
