Geology Reference
In-Depth Information
lower than that of alpine permafrost in the Rocky Mountains in North America (0 °C to
−
1 °C) (Péwé, 1983a) and the southern boundary of latitudinal permafrost in Canada
(about
1.1 °C) (Brown and Péwé, 1973). The high average geothermal gradient on the
Plateau (22 m/°C in permafrost; 16 m/°C below permafrost) is due, almost certainly, to the
high geothermal heat fl ux associated with the recent tectonic uplift of the Tibet Plateau.
The regional distribution of permafrost in central Asia is complicated not only by the
mountain ranges and adjacent inter-montane plateaus and basins but also by faulting, lakes
and springs, and highly mobile sand dunes. The effect of snow cover upon the ground
thermal regime is negligible because snow accumulation is often slight (
−
7 cm). Along the
northern fl ank of the Tien Shan, in southern Kazakhstan, boreal forest occurs, but else-
where, and at higher elevations in China, the vegetation is sparse.
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5.5. RELICT PERMAFROST
Since permafrost is defi ned exclusively upon the basis of temperature, permafrost may
occur on the sea fl oor wherever mean annual sea-bottom temperatures are below 0 °C.
Moreover, because of the lower freezing point which exists under saline conditions, sedi-
ments are not necessarily frozen. The term “saline permafrost” is sometimes used. In
addition to sub-sea permafrost, much terrestrial (land) permafrost is unrelated to the
present climate (see earlier).
5.5.1. Sub-Sea Permafrost
Large areas of sub-sea permafrost occurs beneath the waters of the Laptev and East
Siberian seas (Baranov, 1959; Danilov et al., 1998). Permafrost also occurs beneath the
sea fl oor of the southern Beaufort Sea and the Western Canadian Arctic, and may extend
as far as 100 km from shore (Mackay, 1972b; Dyke, 1991). Areas of sub-sea permafrost
are usually identifi ed either by either drilling or acoustic geophysical surveys. The latter
approach only recognizes ice-bonded permafrost; that is, permafrost which is cemented
together by ice crystals, and therefore underestimates the existence of unfrozen, saline
permafrost bodies (Hunter et al., 1976).
Sub-sea permafrost can be in either thermal equilibrium or disequilibrium. In areas
where sea-bottom temperature is negative, and has remained so over a considerable time
period, shallow (
150 m thick) equilibrium permafrost may exist. Its thickness will depend
upon such factors as mean sea-bottom temperature, soil properties, geothermal heat fl ux,
and sedimentation rates. Most sub-sea permafrost probably developed during the colder
periods of the Quaternary when eustatic sea level fl uctuations exposed large areas of the
continental shelves to cold, sub-aerial conditions (see Chapter 12). Today, this permafrost
is relict and slowly degrading. Thawing of ice-bonded sediment below the sea fl oor in the
southern Beaufort Sea is largely controlled by rapid shoreline retreat and the infl ux of
warm spring and summer discharge of the Mackenzie River. This can affect sea-bottom
temperatures to a depth of 10-20 m.
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5.5.2. Relict (Terrestrial) Permafrost
Much relict terrestrial permafrost is Late-Pleistocene in age. In the western Arctic of
Canada, glacially-deformed ground ice and icy sediments are thought to be at least 40 000
years old (Mackay et al., 1972), and in central Alaska there is evidence that permafrost
