Geology Reference
In-Depth Information
Table 5.5.
Ground temperatures and environmental conditions at four adjacent sites near Churchill,
Manitoba (58°45
′
N), 1974-1976.
Site:
Average hole
Active-layer
Organic
Snow depth
Average snow
Permafrost
temperature
thickness
layer
(1974/75/76)
densities
Yes/No
(°C)
(cm)
(cm)
(cm)
(1974/75/76)
Quartzite
−
2.9
−
750
61/51/20
0.23/0.32/0.26
Yes
bedrock
Marine deposits
2.6
75-90
23
46/36/33
0.27/0.31/0.34
Yes
overlying till
Palsa
−
0.9
50
40
56/53/38
0.22/0.33/0.22
Yes
Depression
+
0.4
75 (seasonal
150
69/102/58
0.23/0.24/0.19
No
frost)
Source: Brown (1973a, 1978).
is often diffi cult to predict permafrost conditions in these areas without detailed site
investigations.
5.7.5. Fire
Wildfi res in the taiga and boreal forest are surprisingly common. Many start by lightning
and much, if not all, of these regions have been burned over at least once. In Alaska, over
400 000 hectares burned between 1940 and 1969 (Viereck, 1973a, b) and in northwestern
Arctic Canada, approximately one million hectares burned between 1962 and 1971 (Brown
and Grave, 1979, p. 10). In many ways, the boreal forest can be regarded as a fi re climax
(Payette et al., 1989).
Tu nd r a fi res are less frequent on account of the relative absence of woody materials
and lower summer temperatures. However, they are not unknown (Shilts, 1975; Wein,
1976; Wein and Shilts, 1976). In particular, the tussock-forming cotton grass,
Eriophorum
vaginatum
, is especially prone to fi re (Wein and Bliss, 1973). When the dry top of the
tussock is burnt, the darkened surface promotes increased thaw. Where fi re occurs in areas
at the northern boundary of the boreal forest, and trees are killed, open tundra may
develop. In Siberia, these areas are sometimes termed pyrogenic tundra (Kriuchkov,
1968).
The effect of fi re upon permafrost depends upon the nature and dampness of the veg-
etation, and the speed at which the fi re passes through the area. If the fi re passes rapidly,
and if the surface cover is peat, moss, or lichens, only the trees may burn and the ground
beneath 2-3 cm may remain untouched. In this case, little change will occur to the
permafrost. However, if the surface vegetation is exceptionally dry, and if the fi re
moves slowly, considerable changes in permafrost may result. At Inuvik, NWT, Canada,
for example, the effects of a 1968 forest fi re led to destruction of much of the vegetation,
thaw of ice-rich sediments, rapid gullying and thermal erosion, and numerous earth fl ows
(Heginbottom, 1973). The more long-term effect has been an increase in active-layer
thickness in the burned-over area (Mackay, 1977b, 1995a) (see Table 7.1). Elsewhere in
the Mackenzie Valley, forest fi res have triggered permafrost degradation and slope insta-
bility (Harry and McInnes, 1988). More recently, the long-term degradation of permafrost
following a fi re in 1958 in a spruce forest in the southern Yukon Territory has been docu-
mented by C. R. Burn (1998a). An area which escaped burn in 1958 indicates that, under
prevailing climatic conditions, permafrost has a thickness of about 17 m and an active layer
