Distribution of main types of seasonal ground thawing and freezing

The main distribution pattern of different types of seasonal ground thawing and freezing on the territory of the former USSR is shown on the general map-diagram (Fig. 15.8) on which we can follow in succession the zonal-regional changes depending on the stability of process (on tmean) and on the climatic type at the surface (on A0). Within every type there exists (almost without exception) areas with azonal conditions of freezing and thawing, typical of more northern or more southern regions because of regional variations of natural conditions. Only the most northern and the most southern parts of the country are characterized by the smallest range of seasonal thawing and freezing depths because the differentiation of the natural conditions in those parts does not cause any marked changes in geocryological environment.

In spite of the fact that considerable variations of the permafrost conditions are noted within one region or even within a part of a region, especially in areas with permafrost, the distribution of various types of seasonal ground freezing and thawing follows the zonal and regional pattern within the predominant landscapes over the former Soviet Union as a whole.

Seasonal thawing of ground

Zonal-belt as well as sectorial changes of mean annual temperature ¿mean» and as a consequence spatial differentiation of various types of seasonal thawing, are typical of the vast areas with permafrost. The successive change of seasonal thawing types from Arctic and Polar to semi-transitional and transitional takes place in connection with a regular increase of mean annual temperature £mean from north southward (within plain topography) and from summits to valleys (in mountain systems) as a whole. Increase of amplitude eastward and with lower altitude in mountains is typical of the permafrost areas. Thus a successive change in the type of seasonal thawing takes place with respect to continentality (through A0) from west eastward from the temperate-maritime type in the north-western part of the European Russia to the extremely sharply continental type in the valleys of Central Yakutiya, Eastern Siberia and Zabaykal'ye.

The range of thickness of the seasonally thawing layer varies from the first centimetres to 3-5 m and more. The increase in the depth of thaw takes place in accordance with increase of summer heat flux through the ground surface, southward as well as eastward and with lower altitudes in mountain regions as a whole. The maximum values of <^th occur in the zone of discontinuous permafrost where transitional and semi-transitional types of seasonal thawing of surface deposits prevail. The thaw depth for low moist ure content coarse-grained soils in conditions of increased continentality can reach 4-5 m. The minimal thawing depths occur in moisture-rich peaty grounds, all other factors being similar.

A brief description is given below of the main types of seasonal thawing within the former USSR.

Arctic and polar types of seasonal thawing occur where the mean annual temperature is below —10 °C, and are observed within European Russia only in the zone of Arctic and ice deserts of the archipelago of Franz Josef Land (Zemlya Frantsa Iosifa) and on the high (800-1000 m) summits of the islands of Novaya Zemlya. Within the Asian part of Russia, they occur on the islands of Severnaya Zemlya, on the northern coastal lowlands and in the most elevated parts of the mountain systems of Central and Eastern Siberia and the Russian Far East. These types occur in the south of the country in the nival zone of the high mountains of the Pamirs and Tien Shan. The temperate-marine and temperate-continental types of seasonal thawing prevail on the Arctic islands, coastal lowlands and on the summits of the Middle Asia mountains. Continental and rather sharply continental types of seasonal thawing are typical of the belt of Arctic mountain deserts and high mountain of Eastern Yakutiya. The depth of thaw of ground within these regions is not very great (from the first centimetres to a few tens of centimetres). Only within high mountains of alpine type can the summer thawing thickness reach 1 m and more on coarse-grained soils in sunny situations. The ground thawing can be absent entirely in extremely cold years.

Stable and long-term stable types of seasonal thawing of ground the mean annual temperature of which varies from —2 to — 5°C, or — 10°C, are developed in the north-eastern European part of Russia, in the high mountains of the Northern and Polar Urals, on the archipelago of Novaya Zemlya, to the north of the Arctic circle on the West Siberian lowland, in the northern and central parts of Central Siberia, practically over the whole territory of Eastern Siberia and the Russian Far East as well as in the mountain part of Zabaykal'ye. The extent of these types increases steadily from west eastward and south-eastward. The amplitudes of the temperature fluctuations at the soil surface caused by growth of the climatic continentality with distance away from the Atlantic Ocean increase in the same direction. Thus in the North-East of European Russia and in Western Siberia temperate-maritime and temperate-continental types of seasonal ground thawing prevail. Continental and rather sharply continental types of thawing occur under natural conditions in Central and Western Siberia. Formation of the extremely sharply continental type is a possibility in valleys with scarce or absent vegetation cover. Thus while the thawing depth of moisture-rich clay-silt deposits with a mean annual temperature of — 5 °C does not exceed 1 m in the European part, the summer thawing depth in Middle Siberia can reach 2 m under the same conditions.

The depth of summer thawing within these regions varies greatly depending on surface and ground-moisture conditions. Thus coarse-debris soils can thaw to a depth of 3 m in Zabaykal'ye under conditions of continental and sharply continental types of heat exchange at the ground surface.

Transitional and semi-transitional types of seasonal thawing are widespread in the region of discontinuous permafrost, the mean annual temperature of which varies from 0 to 2°C. The territory under review includes the northern and central parts of the Kola Peninsula, Malozemel'skaya Tundra, central and southern parts of Bol'shezemel'skaya Tundra, the greater part of the West Siberian lowland, practically all the southern part of Central Siberia, the middle-altitude mountain zones of Zabaykal'ye, the Russian Far East and Kamchatka. These types of seasonal ground thawing and freezing are developed also on the most elevated areas of Sakhalin and the Sikhote-Alin mountains, and prevail in the middle-height mountains of Sayany, Altay and in the southern mountains of Middle Asia and in the high mountains of the Caucasus.

The unstable character of the ground thermal regime, the presence of short-term and discontinuous permafrost, the frequent change of seasonal ground thawing and freezing types over the area and of the maximum values of seasonal thawing and freezing depths of soils being observed, are typical of the territories with such types of seasonal thawing. Thus the thawing depth of coarse-grained deposits is 2 m on the soil surface within Kola Peninsula in the conditions of a temperate-continental climate. The seasonal thawing depth increases to 3 m in dry sands within Western Siberia where a rather sharply continental type of thawing occurs. Within mountain regions of Central Siberia and Zabaikalye under conditions of rather sharply and sharply continental regimes of temperature fluctuations at the ground surface, the thickness of the seasonally thawing layer in coarsegrained materials can reach 3-4 m. The seasonally frozen layer is 5-6 m in thickness within the same region on the areas without permafrost.

Seasonalfreezing of ground

Annual seasonal freezing of the upper horizons of soils and ground takes place practically within the whole region of ground situated south of the permafrost region. According to available data the thickness of the seasonally freezing layer in coarse-grained materials does not exceed 5-6 m, while the seasonally freezing layer in clay ground does not exceed m; the thickness of the seasonally frozen layer in peat does not exceed 1-1.5 m within the former USSR. The lowest values of winter freezing are observed in the southern regions and extend only to the first centimetres.

The distribution of types of seasonal ground freezing outside the permafrost zone according to their mean annual temperatures has a mainly latitudinal pattern. Some tendency for eastward displacement on the European territory as well as in Western Siberia is caused by the general increase in continentality in this direction.

Transitional, semi-transitional and long-term stable types of seasonal freezing of ground with mean annual temperatures not exceeding 5°C are developed on the Kola Peninsula, in Karelia, in the northern part of the Russian plain, in the foot-hills of the Urals Preduralye, in the Middle and Southern Urals as well as in central and southern parts of the West-Siberian plain. In the east of the country the seasonally freezing ground of these types is widespread on the southern foot-hills of the Sredinnyy ridge and the southern part of Kamchatka as well as on the Kurile Islands, in northern and central parts of Sakhalin and in Primorye.

Within the European territory the amplitude of temperature fluctuations at the ground surface corresponds to temperate- maritime and temperate-continental types of seasonal freezing as a whole, with the general tendency for continentality to increase eastward. The freezing depth increases in the same direction, varying, in dry sands, from 2.2 to 2.5 m with mean annual temperature of +1 °C. In similar deposits to the south when the ground temperature increases to + 5°C, the depth decreases to 1-1.5 m.

Some increase of freezing depth (to 3-3.5 m) is noted in Western Siberia southwards within this zone. It is associated with increase in the amplitude of the ground surface temperature fluctuation and decrease of the moisture content in the freezing layers. The same situation is observed within the European territory of the country southwards although the increment in thickness of the seasonally frozen layer does not exceed 0.5 m.

The stable type of seasonal freezing of the soil occurs throughout nearly all the European part of the former USSR, the most southern part of the West-Siberian Plain, Kazakhstan, the southern part of Sakhalin and valley territories of Primorye. A certain successive change in the freezing types from west eastward and south-eastward is noted in connection with climatic continentality. Thus the temperate-maritime and temperate- continental types of seasonal ground freezing prevail in the European territory while the continental and especially continental types prevail on the north of the Caspian region and on the foot-hills of the Southern Urals and in Kazakh stan. Within the latter (mainly in Kazakh melkosopochnik) the maximum thickness of the seasonally frozen layer reaches 2-2.5 m in conditions of stable, high temperature (7-9 °C).

The southern type of seasonal freezing where the mean annual ground temperature varies from 10 to 15°C is developed in Trans-Carpathians, within the Black Sea lowland, in the Crimea, Pre-Caucasus and Trans-Caucasus on the Caspian lowland and in the central part of Central Asia. The depth of seasonal ground freezing varies from a few centimetres to 0.1-0.3 m within the European part in conditions of temperate-maritime and temperate-continental types of climate. As far as the Caspian region and Central Asia are concerned, the depth of winter freezing can reach 1 m under the conditions of a continental and extremely continental-temperature regime.

The subtropical type of seasonal ground freezing occurs where the mean ground temperature exceeds 15°C and is found on the southern coast of the Crimea, in Trans-Caucasus (Kura lowland) and in the southern desert zone of Central Asia. Regular seasonal ground freezing is absent over the greater part of these territories. Only a short-term or daily (nightly) manifestation of the process is noted.

0 0

Post a comment