Geology Reference
In-Depth Information
Figure 7.4. Typical distribution of ground ice with depth, Richards Island, Pleistocene Mackenzie
Delta, Canada. Note: the curve represents a visual best fi t line. From Pollard and French (1980).
7.3.2. Distribution with Depth
The majority of ground ice is concentrated in the upper 1-3 m of permafrost, in the zone
immediately below the permafrost table (Figure 7.4). J. Büdel (1977; see the English
translation by Fischer and Busche, 1982, pp. 103-105) was one of the fi rst to highlight this,
following observation of shattered ice-rich bedrock beneath stream channels on Svalbard.
As a consequence, he formulated the “ice-rind” concept in which this near-surface ice-rich
layer favored rock shattering, and thus stream incision.
The “ice-rind” concept is not particularly satisfactory in explaining what Büdel termed
“excessive valley deepening” in permafrost. The deep valley incision on Svalbard is best
explained in terms of deglaciation and recent isostatic uplift. The area is in paraglacial
transition (see Chapter 2). On the other hand, Büdel's observation concerning the ice-rich
nature of near-surface permafrost on Svalbard is a valid generalization. In northern
Canada, for example, ice-rich near-surface permafrost has been documented in a number
of environments (Mackay, 1981a; Pollard, 2000a; Stangl, et al., 1982; Williams 1968).
There is also a relationship between near-surface ice content and vegetation. For example,
Table 7.3 gives mean ice content in the top 50 cm of permafrost for six major ecological
terrain types in the Mackenzie Delta. The volume of ice in the top 50 cm of permafrost
ranges from
15% at a spruce/feathermoss forest site to almost 50% at a spruce/
crowberry-lichen site (Kokelj and Burn, 2005).
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