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Physical Geography of Northern Eurasia


<<< Permafrost (Introduction) | Physical Geography Index | The Active Layer >>>

Distribution, Geothermal Regime, and Thickness of Permafrost

Permafrost occurs in various environments: polar, subarctic, middle latitudes, and alpine (Baranov, 1959; Kudryavtsev et al., 19786; French, 1996). In Northern Eurasia, permafrost is found in polar and temperate zones, in the mountains, and on the Arctic shelf. Permafrost is classified according to its territorial extent, mean annual temperature at the depth of zero annual amplitude, and thickness of frozen ground. In terms of its territorial extent, permafrost is classified into one of the following categories: continuous, discontinuous, massive-island (scattered). [In this chapter, the terms 'massive-island' and 'island permafrost' will be used Ч synonymous terms are 'scattered permafrost' and 'sporadic permafrost'] and island (sporadic) (Kudryavtsev et al., 1978a). The continuous permafrost zone is often referred to as the 'northern geocryological zone' while the discontinuous, massive-island and island zones are known as the 'southern geocryological zone' (Permafrost Science, 1981). In general, the spatial extent and severity of permafrost conditions in Russia increase northwards and eastwards. In the south, permafrost is young and its extent and thickness depend mainly on contemporary climatic conditions; in the north, permafrost conditions are an imprint of the Pleistocene climate (Kudryavtsev et al., 1978b). The regional distribution and thickness of permafrost vary in response to changes in climate, relief, bedrock, extent of water bodies, and vegetation. Mean annual temperature of permafrost is a function of heat exchange in the contemporary landscapes (Kudryavtsev, 1965; Kudryavtsev et al., 1978a).

The southern permafrost boundary extends across Russia from the Kola peninsula in the west to the Russian-Chinese border in the Far East. It crosses different landscapes: tundra in the Kola peninsula, northern taiga, in the European north, northern, and central taiga in Western Siberia, central and southern taiga in Eastern Siberia, steppe near the Mongolian border, and the forest-steppe in the Russian Far East. Within Russia, permafrost reaches its southernmost extent at 49∞N in Transbaikalia. To the west of the Yenisey river, the southern boundary has a sub-latitudal direction which is consistent with climatic zonation (Figure 6.1).

Distribution, temperature, and thickness of permafrost

Fig. 6.1 Distribution, temperature, and thickness of permafrost

To the east of the Yenisey, the boundary takes a sub-meridional direction and sharply deviates to the south. There are two main reasons for this deviation: (1) an orographical contrast between the low depositional plain of Western Siberia where the maximum height does exceed 250 m, and the denuded tableland of the Central Siberian plateau with an average elevation of 600 m; (2) the increasing continentality of climate. The domination of the Siberian anticyclone over Eastern Siberia in winter leads to intensive radiational cooling, which results in extremely low air temperatures, and ensures a small depth of snow cover (see above). Along the southern boundary of permafrost in Western Siberia, the mean winter air temperatures are about -20∞C with snow depth in excess of 0.6 m, whereas at the Central Siberian plateau the average winter temperatures vary between -23∞C and -27∞C and snow depth does not exceed 0.5 m (Gavrilova, 1981). The southern limit of permafrost lies between the -1∞C and -3∞C mean annual air temperature isotherms in the European part of Russia, between the -2∞C and -5∞C isotherms in Western Siberia, and coincides with the -3∞C isotherm in Eastern Siberia and the Far East (Gavrilova, 1981). The percentage of area underlain by permafrost increases northwards from 5-10 per cent (island permafrost) at the southern edge to over 90 per cent (continuous permafrost) along the Arctic coast and in the high mountains (Figure 6.1). Mean annual ground temperature decreases and typical thickness increases from island to continuous permafrost.

The Continuous Permafrost Zone

Continuous permafrost occupies about 50 per cent of the total area of Russia's cryolithozone. It occurs in the arctic, subarctic, and temperate climatic zones; in the landscapes of polar desert, tundra, forest-tundra, northern, and central taiga; on the low depositional and high eroded plains, and in the mountains. The southern limit of the continuous permafrost coincides approximately with the -5∞C mean annual air temperature isotherm in European Russia, with the -7∞C isotherm in Western Siberia, and with the -8∞C isotherm in Eastern Siberia (Gavrilova, 1981). Mean annual ground temperature (MAGT) varies between -5∞C and -15∞C; thermal conditions are particularly diverse in the south. The lowest MAGT is observed the Arctic islands and the Arctic coast of Sakha-Yakutia. Conditions are milder southwards and westwards: both insolation and air temperature increase, particularly in summer, as well as snow depth and the area occupied by lakes and bogs which provide better insulation. Denser vegetation cover assists the accumulation of snow in winter. These factors lead to an increase in permafrost temperature to -10∞C through to -5∞C in the Western Siberian and Taymyr tundras and in the lowlands of northern Sakha-Yakutia. The highest MAGT (between -5∞C and -3∞C) within the continuous permafrost zone is observed in the northern tundra of European Russia, in the forest-tundra zone of Western Siberia and in the taiga of the Lena-Vilyuy plain (Figure 6.1).

The lowest temperatures of mountainous permafrost (between -13∞C and -15∞C) occur in the subarctic mountains of north-eastern Siberia3 (Geocryology of the USSR: Eastern Siberia and the Far East, 1989). In the tundra and taiga regions of the mountains and uplands of Central Siberia MAGT ranges between -10∞C and -5∞C; frequent and intensive temperature inversions and cold air drainage are important controls over permafrost temperatures in these regions. In the sparse forests of the inner and eastern parts of the Central Siberian plateau, MAGT is higher and ranges between -5∞C to -3∞C. It should be pointed out, that permafrost temperature in the mountainous regions varies greatly in time and space as a result of the great variety of local controls such as aspect and steepness of slopes (Kudryavtsev, 1965).

In the continuous zone, the thickness of permafrost varies from 100 m to more than 1000 m not only with latitude and altitude as a function of climate, but also in response to variations in bedrock and Quaternary history. On the plains, frozen ground reaches a maximum thickness of 400-600 m on the Arctic islands and in the narrow zone of the Arctic coastal plain in northern Sakha-Yakutia (Geocryology of the USSR: Eastern Siberia and the Far East, 1989). In tundra, the average thickness of frozen ground ranges between 300 and 500 m though sometimes it can be less, about 100 m as, for example, on the young marine terraces of the Yamal peninsula (Figure 6.1), (Geocryology of the USSR: Western Siberia, 1989). The thickness of mountainous permafrost is often much greater than that on the plain as a result of higher thermal conductivity of rocks (Feldman et al., 1988; French, 1996). In the mountains of northeastern Asia, in the northern part of the Central Siberian plateau, and in the nival belt of Transbaikalia, permafrost exceeds 1000 m in thickness. The thickest known permafrost exists on the Anabar tableland in the north-eastern Central Siberian plateau where it extends to a depth of 1200-1470 m. [An estimated value of 1600 m reported by Nekrasov (1976) and quoted by French (1996) is not supported by factual evidence (Geocryology of the USSR: Eastern Siberia and the Far East, 1989)]. The extreme thickness of permafrost is caused by a number of factors, such as high thermal conductivity of bedrock, low geothermal flux which does not exceed 13-27 MW m-2 (Geocryology of the USSR: Central Siberia, 1989), and paleoclimatic conditions. In these regions, which had never been glaciated, perennial freezing began in response to the very low air temperatures in the Neogene and continued throughout the Pleistocene, lasting in total about 15 million years. Westwards, permafrost thickness becomes smaller and in the Polar Urals it ranges between 300 m and 500 m, while in the taiga regions permafrost develops to a depth of 100-300 m (Geocryology of the USSR: European Territory of the USSR, 1989).

The Discontinuous Permafrost Zone

In the discontinuous zone, bodies of frozen ground exist alongside areas of thawed ground (taliks) which occupy about one-third of the entire territory. The extent of unfrozen ground increases westwards and southwards in response to climatic factors: the increasing amount of received solar radiation, higher summer temperatures, longer summers, and greater depth of snow cover (in the western part of the cryolithozone). In the western regions, discontinuous permafrost occurs in the forest-tundra zone and at places extends to the northern taiga regions. Typical MAGT ranges between -1∞C and -3∞C. Permafrost thickness increases eastwards from 100-150 m in the north-eastern European Russia to 200-300 m in Western Siberia. In Eastern Siberia, discontinuous permafrost underlies the northern taiga and forest belt in the mountains. Here, the average thickness is about 150-200 m while in the mountains of the Far East it is about 100 m (Figure 6.1).

Taliks are a characteristic feature of the discontinuous permafrost zone. They differ in their origins and may form in response to the presence of surface and underground water bodies, the heat effect of increasing solar radiation, and the insulating effect of deep snow cover. Accordingly, two major types of talik are distinguished: hydrothermal and thermal (Kudryavtsev et al., 1978a). The majority of taliks develop in response to the local heat sources, such as water bodies. Taliks located under rivers and large lakes which do not freeze to the bottom in winter are typical of the flat terrain of the western cryolithozone. The intensive accumulation of snow, which protects soils from deep freezing, causes the formation of thermal taliks under depressions and gullies. As a rule these taliks are closed (i.e., entirely surrounded by permafrost) (Brown and Kupsch, 1974). In the eastern part of the discontinuous permafrost zone, hydrothermal taliks form due to the heat supplied by ground water. The release of subpermafrost waters forms open taliks along the cracking zones and in river valleys in the mountains and tablelands of southern Central Siberia, Transbaikalia and the Far East (Geocryology of the USSR: Southern Mountains of the USSR, 1989). In the monsoon regions of the Far East, warm rainwater creates taliks by penetrating coarse-grained filtering sediments. On south-facing slopes of Southern Siberia and the Far East mountains the formation of taliks is controlled by solar radiation.

The Massive-island Permafrost Zone

In the massive-island permafrost zone, terrain conditions are a particularly important control over the formation and distribution of frozen ground, which has a patchy character. It is the variations in terrain that are primarily responsible for the massive-island occurrence of permafrost as well as its temperature and thickness. Peatlands are especially conducive to the formation of frozen ground and about 70 per cent of the total peatland territory is underlain by permafrost. This is a higher share than in the adjacent terrain (Regional Cryolithology, 1989). Peatlands have unique thermal properties due to which, soils, underlying peat, cool faster in winter than they warm in summer. Therefore, MAGT is lower (Feldman et al., 1988; French, 1996). In addition to seasonal variability in thermal conductivity of peat, the insulating effect of moss cover causes mean summer temperatures of soils to decrease by 4-6∞C in comparison to temperatures of the moss-free soils (Kudryavtsev, 1959; Feldman et al., 1988). The formation of permafrost in peatlands is a typical feature of the northern taiga of European Russia and especially of the forested wetlands of Western Siberia where peatlands occupy between 50 per cent and 80 per cent of the land surface. In these regions, MAGT usually varies between -1∞C and -1.5∞C and permafrost thickness reaches 70-100 m (Figure 6.1). In the southern taiga of the Far East, peatlands (called mary) also promote a broad extent of permafrost. Forests are another important control over local permafrost conditions. In the west of the cryolithozone, permafrost islands form under the spruce forests which provide shade in summer and intercept snowfall in winter causing additional cooling of the ground. In the mountains of the Far East, permafrost is typical of the tree-covered northern slopes. Under forests, MAGT is higher than in peatlands and vary between 0∞C to -0.5∞C and thickness does not exceed 50 m (Geocryology of the USSR: Eastern Siberia and the Far East, 1989).

The Island Permafrost Zone

Island permafrost occupies the southernmost part of the cryolithozone. Frozen ground accounts for as little as 5-10 per cent of the total territory of the island permafrost zone. Terrain conditions are by far the most important control over the formation of permafrost here. In northern European Russia, Western Siberia, southern Eastern Siberia, and the Far East, permafrost forms in peat, peaty soils, and under the thick surface moss cover in wetlands. Here, peaty permafrost mounds mark the southern limit of the permafrost zone (Popov, 1962c).

In southern Central Siberia on the watersheds of the Yenisey, Angara, and Lena rivers the agents of permafrost formation are more varied and include geo-morphology. Cold air drainage is typical of the deep river valleys where permafrost forms in response to the lower temperatures. It is also found on the northern slopes covered with mossy forests. Along the eastern frontier of Russia, island permafrost occurs in the mountains of Kamchatka (from an altitude of 1000-1500 m), Sakhalin and the Sikhote-Alin (from 1500 m). MAGT varies within narrow limits from 0∞C to -0.5∞C (Regional Cryolithology, 1989) and the permafrost thickness is 10-20 m in soils, reaching 50 m in rocks (Figure 6.1).

Subsea Cryolithozone

The spatial distribution of the thickness and thermal regime of the submarine cryolithozone depends on a variety of factors, such as climate, the ratio between the thickness of sea ice and sea depth, sea hydrology, salinity and temperature of water and bottom sediments, and sea level oscillations (Mackay, 1972a; Hunter et al., 1976; Rogers and Morack, 1980; Solovjev, 1983). Two types of permafrost develop on the Arctic continental shelf: typical ice-bearing permafrost and the saline unfrozen sediments with temperature below -1.8∞C (Solovjev, 1983), with both frozen and unfrozen marine deposits containing cryopegs (highly saline solutions) which form lenses and layers under and within the marine sediments (Kononova et al., 1971). Both relict and contemporary permafrost are found on the Arctic shelf. Contemporary freezing of subsea deposits occurs only in shallow seas, which remain frozen to the bottom for up to eight or nine months per year (Solovjev, 1983). The perennially frozen sediments form under the ice cover which is approximately 0.5 m thick; when thickness of sea ice reaches 1.0-1.5 m, frozen layers develop on a seasonal basis (Geocryology of the USSR: Western Siberia, 1989). The subsea cryolithozone is characterized by the following features: widespread saline unfrozen sediments and an alternation between perennial frozen layers, saline unfrozen layers, and cryopegs. The area occupied by subsea permafrost and permafrost thickness both decrease northwards. Subsea permafrost is in a condition of thermal disequilibrium (Mackay, 1972a; Solovjev, 1983).

The major area of subsea permafrost, both relict and contemporary, occurs beneath the waters of the Laptev and East Siberian Seas (Baranov, 1959). Relict permafrost developed during the Quaternary marine regressions when areas of the present sea floor were exposed to cold subaerial conditions. Later, during the transgression of 19 000-18 000 BP, permafrost was degraded by thermo-abrasion and geothermal heat (Geocryology of the USSR: Eastern Siberia and the Far East, 1989), and at present continuous relict permafrost occurs only in those parts of the shelf where sea depth does not exceed 10 m. In the East Siberian Sea, relict sub-sea permafrost forms a narrow zone of about 30 km, which extends along the coast, with the permafrost table laying at a depth of 1-8 m beneath the sea floor. The island relict permafrost occurs between the isobaths of 10 m and 60 m, though at these depths the shelf is mainly occupied by unfrozen saline deposits. The contemporary subsea permafrost forms under the shallower waters near the shore and in the shallow deltas of the large rivers (the Lena, Yana, Indigirka, and Kolyma), where the depth does not exceed 2.5 m (Grigoriev, 1966a; French, 1996). Permafrost thickness decreases from 150-200 m near the shore to about 10 m at a distance of 200 m from the coast while the depth at which the permafrost table occurs beneath the sea floor increases from 2 m to 100 m (Geocryology of the USSR: Eastern Siberia and the Far East, 1989). The area occupied by subsea permafrost diminishes westwards. In the Kara Sea continuous relict permafrost, which formed in the late Pleistocene and Holocene, occurs only in the nearshore areas where the sea depth does not exceed 20 m (Geocryology of USSR: Western Siberia, 1989). In the nearshore areas of the Yamal peninsula subsea permafrost may have originated as result of extensive coastal retreat (Grigoriev, 1966b). Subsea permafrost in the Kara Sea lies either straight beneath the sea floor or at a depth of 30-50 m under the floor, with layers of saline unfrozen deposits and lenses of cryopegs underlying or alternating with layers of typical permafrost (Geocryology of the USSR: West Siberia, 1989). Permafrost temperatures normally vary between -1∞C and -3∞C in the southern part of the shelf and -0.5∞C and -2∞C in its northern sector, though in places temperatures may be as low as -10∞C (Geocryology of the USSR: European Territory of the USSR, 1989). Its thickness ranges from 100 m to 200 m along the eastern coast of Novaya Zemlya and the eastern sector of the Kara Sea to less than 100 m in the central sector of the sea. Further west, under the Barents Sea, only sporadic islands of permafrost occur near the Frantz Joseph Land and between the islands of Kanin and Vaygach with unfrozen saline deposits predominating in other areas (Geocryology of the USSR: European Territory of the USSR, 1989). At present, subsea permafrost on the Arctic continental shelf remains poorly researched. Further investigations present a challenging and important task, particularly with respect to the development of natural resources which are abundant on the Arctic shelf.

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