associated with elevated bogs. Within foot-hills and 'golets' regions the first permafrost areas still occur at low elevations and appear on the shaded, moss-covered slopes blanketed with clay-silts and rock-debris. The individual permafrost islands may be situated at the foot of a terrace where snow patches lie.

The permafrost in this subzone is characterized as a whole by high negative mean annual temperatures rarely lower than — 1 °C with the most commonly encountered mean temperatures (imean) varying from 0 to

— 0.5°C. The unfrozen ground dominates within all the topographic elements, especially within watersheds and on river terraces. Within the areas most favourable for summer precipitation infiltration, snow accumulations, solar insolation, etc., the mean annual temperatures can be significantly higher and reach +4 or + 5°C.

In the (sub)zone of distribution of massive islands the frozen ground is characterized by an increase in number and size of islands and masses while the thawed ground is characterized by decrease of the area of open, radiation-thermal taliks. This (sub)zone is situated to the north of the previous one with plains and highlands and higher than the previous one in mountain regions. The lower mean annual temperatures of frozen as well as of unfrozen ground are typical. The prevailing temperature varies from +1 to

— 1 °C. At the same time the mean annual ground temperature (imean) can reach + 2°C on the south-facing dry slopes (comprising not more than 25% of the area) and only — 2°C within the peaty north-facing slopes. The permafrost is widespread within the swampy terrain with hillocky peatlands in the boreal-tundra zone of the European part of the former USSR.

In the Northern and subpolar Urals the small permafrost islands give way to larger permafrost masses as the topographic relief and slope steepness increase. From north southwards their hypsometric position rises from 200 to 1000 m. The permafrost masses on the west-facing slopes, being more moist and snowy, are situated 100 m higher in the subpolar Urals and 500 m higher in the Northern Urals, compared with the corresponding east-facing slopes. In the Central Urals the perennially frozen soil masses are situated only on the mountain 'golets' summits, when their altitude exceeds 1000 m.

Within the Western Siberian plain the permafrost is developed in the form of large and small masses of epicryogenic Holocene peatlands characterized by temperatures to —1 and — 2°C. Such permafrost masses are distributed at latitudes in this subzone in which unfrozen soils also occur in the form of islands and masses, and the zone itself extends almost up to the polar circle (see Fig. 15.1). To the east of the Yenisei river, permafrost islands/masses are typical of the lower areas within the Tunguska plateau occupied by light coniferous taiga, and in the Predbaikalskaya plain. To the east of Lake Baikal such permafrost is developed on the steppes of the Uda-Selenga highland as well as on the peaty and swampy Amur-Zeya plain occupied by sparse coniferous-taiga forests.

There is no clear dividing line between the first two geotemperature subzones because of the fact that change in surroundings from south northward is gradual and consequently gradual change in geocryological conditions is typical of the plains. Within the mountain regions permafrost areas increase more sharply because of lowering of the mean annual ground temperatures, with a mean gradient of 0.4-0.6°C for every 100m increase in altitude, and in connection with the sharp change of topographic and landscape-climatic conditions across the altitudinal belts.

The unfrozen ground within this zone occupies from 50 to 25% of the area, and is composed of radiation-thermal and underwater taliks the mean temperature of which is rather low (up to +1 C more seldom + 2 °C. When the natural conditions are disturbed they become thermodynamically unstable and can become frozen within some areas.

The (sub)zone of discontinuous distribution of permafrost is the most severe within the southern geocryological zone. The mean annual permafrost temperatures here vary from 0 to — 2°C and lower still within 25% of the territory. Unfrozen ground occurs only under favourable conditions in the form of islands and, rarely, masses, and decreases in area from south northward from 20 to 5%. It is represented by open taliks of radiation-thermal and water-thermal origin up to +1°C, more seldom +2°C in temperature, and occurring within certain limits in areas of sand flood-plains and terraces.

The permafrost of this subzone is widespread in the north of the boreal tundra of the European part of the former USSR. In Western Siberia it is represented by large masses composed of mixed soils and peats. To the east of the Yenisei river on the interstream area between the Nizhnaya Tunguska and Podkamennaya Tunguska rivers the permafrost of this zone extends south of latitude 60° N, and also occupies low plateaus and highlands within the western part of South Yakutiya and the Aldan-Tympton interstream area. The relationship between permafrost and unfrozen materials in Cambrian carbonate deposits and Jurassic sandstones depends on the warming effect of summer precipitation infiltrating through the karst cavities and in crystalline rocks through the water permeable fracture zones. The mean annual temperature increase due to heat from precipitation infiltration can reach 2-3 °C. As a result, unfrozen masses and linear zones occur among the surrounding high-temperature (from 0 to — 2°C) permafrost on the water shed surfaces of plateaus and highlands composed of fractured rocks. As a rule pine-larch forest, growing on areas with dry sandy silty gravels and loose detritus, is the indicator of absence of permafrost or of the deep seasonal thawing of the ground.

In regions of loose Quaternary deposits open and closed taliks exist under rivers and lakes in sands of eolian, alluvial and fluvioglacial origin, and in flooded lowland swamps. A narrowing of this geotemperature subzone eastward within the Central Siberian highland, compared to the first two subzones, is the noteworthy feature. This is connected with the change from gently rolling, wavy, slightly dissected topography to an elevated surface with diabase intrusions, sharply dissected by deep valleys and affected by glaciation. The latter features were evidently also the limit of a widespread thawing from the surface of permafrost during the climatic optimum of the Holocene. At the present time the combined impact of these factors has brought about a sharp latitudinal transition from intermittent to continuous distribution of permafrost and to a sharp reduction of the areas of unfrozen ground in this region.

The zone of continuous distribution

In this zone of the permafrost (the northern geocryological zone) there is disturbance of the continuity by open taliks developed locally under large river beds, deep lakes and in the areas of ground water discharge. Taliks of radiation-thermal origin are developed only under particularly favourable conditions for their existence, being represented for the most part by closed taliks which occur more often in the southern subzone of the northern geocryological zone where temperatures down to — 2°C, more seldom to — 3°C, are found. As a rule they are developed on weak Jurassic sandstones as well as on alluvial and eolian sands. The permafrost of the northern geocryological zone as a whole is characterized by the mean annual temperatures varying from 0 to — 15°C and lower, decreasing regularly from the southern margin of the zone northward and north-eastward in accordance with landscape-climatic and altitudinal zonality. On plains the intensity of the lowering of the mean annual ground temperature is associated with the landscape and lithological-moisture features of the areas, in highland regions with the continental climate (development of temperature inversions, deep depressions and valleys characterized by complex air-exchange conditions compared with those in the surrounding inter-stream areas.

The mean annual ground temperatures in the south of the northern geocryological zone are characterized by high negative values (for the most part from — 1 to — 3 °C) at present. Such permafrost temperatures are observed in Western Siberia in the latitude of the Arctic circle. They exist also to the north of this latitude along large river valleys in accordance with the distribution of the boreal-tundra landscape. The high temperature continuous permafrost is widespread on the right bank of Yenisey river, in Zabaykal'ye and in Southern Yakutiya, in the south of the Russian Far East (see Fig. 15.1). Its existence depends on the warming effect of snow cover and summer precipitation, on a favourable radiational regime, good drainage and relatively low ice content of the deposits which repeatedly thaw from the surface and freeze again.

To the north of this subzone the mean annual ground temperatures decrease gradually. Thus they decrease from — 2 to — 7°C (in the latitude of Anabar massif) within vast areas of the Central Siberian highland. The lowest ground temperatures are established on the northern plains of Western Siberia (from - 5 to -9°C), of Central Siberia (from -9 to - 13°C), of Eastern Siberia (from —9 to — 15°C) and on the coasts of the Arctic islands (from — 5 to — 15 °C). Within the northern geocryological zone higher mean annual ground temperatures (than the zonal ones) are established in alluvial deposits of river valleys within all the geotemperature subzones. Within mountain systems of the southern part of Siberia, Central Asia and the Caucasus owing to the high incoming solar radiation at the surface in southern latitudes, continuous permafrost is developed only at high altitudes: from 2700 m in Gornyy Altay, from 3000-3500m in Dzungarskiy Alatau, from 3 500-4500 m in Tien Shan and the Pamirs and from 30003500 m in the Caucasus. As the topographic relief increases, the mean annual ground temperatures in mountain massifs decrease from valleys to watersheds, reaching — 15°C and lower on high summits. Within the mountain slopes covered with mobile tills and deposits of large blocks with 'golets' ice the lowest temperatures occur on the steep north-facing slopes. The decrease in mean annual ground temperatures at comparable altitudes is greater with the increase of the topographic dissection and steepness because of the extent of cooling of the steep slope forms and increase in drainage of the hard and semihard rock masses. Glaciers increase the temperature of ground under them leading to glacial talik formation within firn zones and decrease it down to the mean annual air temperature value within the ice zones, thus modifying the permafrost distribution and its temperature regime.

Thus regular decrease of the permafrost temperatures takes place within similar topographic elements over the whole territory of the continuous permafrost. In the case of different ground composition the lowest tempera tures are formed in cohesive and peaty ground. Local increase of the zonal temperature is possible under favourable conditions for the heat exchange in loose clastic deposits; however, the general latitudinal zonation holds with the temperatures decreasing northward.

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