Permafrost Zone

(in Soviet usage, “permanent cryolithic zone”), an upper layer of the earth’s crust characterized by a stable long-term negative or zero temperature, which ensures year-round and long-term preservation (not less than two consecutive years) of subterranean ice. The upper part of the permafrost zone is made up of permanently frozen rock and underground ice bodies, which form the frozen zone of the lithosphere; the lower part is composed of frozen rock and horizons of strongly mineralized subterranean waters that do not freeze. The formation of ice inclusions there can only be associated with the appearance of fresh water or weakly mineralized solutions in natural or artificial cavities. This part of the permafrost zone predominates in zones of hindered water exchange and extends into the zone of active water exchange. The upper boundary of the zone in subglacial conditions passes along the ice-rock interface, whereas under subaerial and subaqueous conditions it passes along the foot of the layer of rock that is seasonally thawed or heated to more than 0°C. On this boundary, which is not constant in time and space, the temperature never rises above 0°C during the course of the year. Negative values for the mean annual temperature of the earth’s surface (which in practical terms coincide with the annual mean temperature of the rock at the foot of the seasonally thawed layer) are an essential condition for the appearance of the permafrost zone. Where the annual mean temperatures on the surface of the land or shelf are positive, the permafrost zone can only exist in a degraded state as a relict of harsher climatic conditions in the past.

The lower boundary of the permafrost zone passes along the geoisothermal line of 0°C, which gradually changes its position upon a change in the conditions of heat and water exchange between the upper layers of the rock and the surface of the soil, atmosphere, and bodies of water; this can be detected only over sufficiently large time intervals. The depth of the zero isothermal line from the earth’s surface ranges from a few meters in the moderate latitudes (on the boundaries of the range of distribution of permanently frozen or cold rock) to several kilometers in the high latitudes (more than 4 km in Antarctica and 1.5 km in the subarctic region).

In the southern hemisphere the permafrost zone spreads under the antarctic ice sheet and on its shelf zone with negative annual mean sea-bottom temperatures, and also under glaciers and the seasonally thawed soils of the mountain structures of South America, Africa, and Australia. In the northern hemisphere, the zone encompasses the vast subpolar belt of the continents, which broadens from west to east as the climate becomes increasingly continental; the island and continental mountain structures that rise above the snow line; a significant part of the shelf of the arctic seas; and the rock beneath the ice sheets and seasonally thawed soils of Greenland, Iceland, and the islands of the Arctic Ocean.

The permafrost zone also exists under the thermal sink lakes that are common on the plains of the arctic and subarctic regions. In the high latitudes, the continuity of the zone is broken by open and closed taliks of various origin in which the temperature of the rock is above zero for at least part of the year. In the broad band of plains near the present-day boundary of the permafrost zone, only individual islands of permanently frozen rock are encountered. A broad (more than 400 km) and discontinuous wedge-shaped layer of relict permafrost stretches over the Western Siberian plain to the south of this border (where there is no permanently frozen rock in the subsoil layer) at a significant depth from the surface (down to 100 m and more). It formerly (apparently before the Holocene climatic optimum) merged with the active layer but in the modern age is thawing intensively on the top and bottom. The area of distribution of the permafrost zone, taking into account relict frozen layers, is more than 25 percent of the land area, including 11 percent under ice sheets. On the map of cryogenic formations the areas occupied by the permafrost zone are shown by dark crosshatching (see Figure 1).

Stability of the land at high latitudes and at a sufficient elevation above sea level, as well as a specific type of circulation of the atmosphere and oceanic waters, is required for the emergence of the permafrost zone. The formation of the permafrost zone precedes the development of surface glaciation and encompasses larger areas than the latter. The permafrost has been especially well expressed during periods of global climatic cooling. In the course of the earth’s geological history, periods of aggradation and degradation of the permafrost zone have alternated numerous times.

The term “permanent cryolithic zone” was proposed by P. F. Shvetsov in 1955. Organization of the study of phenomena of the permafrost zone in the USSR began in 1927 and is associated with M. I. Sumgin. Significant contributions to further development of the theory of the permafrost zone were made by N. I. Tolstikhin, V. A. Kudriavtsev, P.-A. Shumskii, I. la. Baranov, B. N. Dostovalov, and A. I. Popov (USSR); S. Muller, T. L. Pewe, A. L. Washburn, and A. Lachenbruch (USA); A. Cailleux (France); G. Taylor (Great Britain); G. Beskow (Sweden); and J. R. McKay (Canada).