Geology Reference
In-Depth Information
period in the Tuktoyaktuk Peninsula. There, and elsewhere, many thaw lakes drain by
the expansion of adjacent basins and fl uvial “tapping” (i.e. erosion) along ice-wedge
systems (Mackay, 1963, 1988b, 1992b; Walker, 1978). This can result in rapid, sometimes
catastrophic, lake drainage and was the method chosen by Mackay to artifi cially induce
the draining of Lake Illisarvik (see Chapter 6). In coastal areas of the western Arctic and
northern Siberia, thaw lakes may drain when truncated by rapid coastal retreat associated
with thermal abrasion and Holocene wave action (Mackay, 1986a; Romanovskii et al.,
2000).
Their growth, expansion, and eventual drainage are mechanisms that are not fully
understood. What is clear, however, is that thaw lakes evolve relatively rapidly through
lateral bank erosion that may average as much as 15-25cm per year. This makes thaw
lakes an especially dynamic feature of the tundra landscape.
D. M. Hopkins (1949) was one of the fi rst to outline a cyclic growth model that
envisaged thaw lakes growing in size by coalescing with adjacent lakes and migrating
across the tundra surface. With time, it was hypothesized that vegetation would grow
upon the newly-exposed lake fl oor while the migrating lake would slowly infi ll with silt
and organic matter. At the same time, permafrost aggradation and lake-bottom heave
on the fl oor of the depression and mass-wasting upon the banks would lead to the ulti-
mate obliteration of both lake and depression from the landscape. In subsequent years,
the concept of a thaw-lake cycle, consisting of sequential stages of initiation, expansion,
and drainage, became well established in the Alaskan literature (Billings and Peterson,
1980; Black, 1969; Britton, 1967; Tedrow, 1969). In support of this model, radiocarbon
dates obtained from organic material within drained thaw-lake basins are used to argue
that the growth, drainage, and rebirth of small thaw lakes and depressions is accom-
plished within a relatively short time span, approximately 2000-5000 years (Black, 1969;
Te d row, 19 69 ) .
While this model may be satisfactory for northern Alaska, where topography, climate,
and geology are reasonably uniform, it is by no means universally applicable. It lacks a
rigorous understanding of thermokarst processes and, specifi cally, of thermokarst lake-
basin sedimentation. Furthermore, in spite of radiocarbon dating of organic material
from within drained thaw-lake depressions, these dates merely indicate a minimal age
for lake drainage and there is no evidence to indicate that the later stages of the cycle
actually exist. It has also been suggested that the processes responsible for the hypothe-
sized thaw-lake migration across the tundra are also the same as those responsible for
thaw-lake orientation (Tedrow, 1969). However, no geomorphic or stratigraphic evidence
for such migration can be found in many areas of oriented thaw lakes. It seems best to
conclude, following J. R. Mackay (1963), that thaw lakes are quasi-equilibrium landscape
elements.
The question of thaw-lake drainage is another interesting aspect of thaw-lakes.
Many are either partially or completely drained. Two contrasting modes of lake
drainage have been suggested: (1) gradual infi lling and sedimentation, as described in
the cyclic model above, and (2) catastrophic outfl ow following lake tapping or trun-
cation by coastal retreat (Mackay, 1979a, p. 31; Walker, 1978; Weller and Derksen,
1979). It can be argued that the latter, a rapid permafrost-related process, is more
appropriate to explain the transition from thaw lakes to drained thaw-lake depres-
sions than gradual sedimentation, lake-bottom heave, and infi ll as suggested in the
Alaskan thaw-lake cycle. In fact, tapping must be regarded as a ubiquitous process
that occurs in poorly-drained tundra terrain wherever polygonal ice-wedge systems
are well developed. Several examples of lake tapping can be seen in Figure 8.11 (see
below).
