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AbstractPermafrost, or perennially frozen ground, is defined as soil or rock having temperatures below 0oC over at least two consecutive winters and the intervening summer. Much of the permafrost has been frozen since the Pleistocene time. Permafrost occurs in the Arctic, Antarctic and high alpine regions. About one-fifth of the total land area of the world is underlain by permafrost (Burdick et al. 1978). The top layer of the ground in which the temperature fluctuates above or below 0oC during the year is defined as the active layer (Andersland and Ladanyi 1994). Other terms such as seasonally frozen ground, seasonal frost and annually thawed layer are synonyms for the active layer. The thickness of this layer varies spatially and temporally. The upper boundary of permafrost is defined as the permafrost table. In the discontinuous permafrost zone, taliks form between the active layer and the permafrost table. Taliks, or unfrozen ground, are layers of ground that remain unfrozen throughout the year (Andersland and Ladanyi 1994). In the continuous permafrost zone, taliks often occur underneath shallow thermokarst lakes or rivers, where the water below a certain depth may not freeze in winter and, thus, the soil underneath will not freeze either. Other terms, such as thaw lake or cave-in lake, have also been used for a thermokarst lake. Open talik is an area of unfrozen ground that is open to the ground surface but otherwise enclosed in permafrost. Through talik is unfrozen ground that is exposed to the ground surface and to a larger mass of unfrozen ground beneath. Unfrozen ground encased in permafrost is known as a closed talik.
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