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In geology, permafrost or cryotic soil is soil at or below the freezing point of water for two or more years. Most permafrost is located in high latitudes (in and around the Arctic and Antarctic regions), but alpine permafrost may exist at high altitudes in much lower latitudes. Ground ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Permafrost accounts for 0.022% of total water on earth〔 〕 and exists in 24% of exposed land in the Northern Hemisphere.〔 It also occurs subsea on the continental shelves of the continents surrounding the Arctic Ocean, portions of which were exposed during the last glacial period. 〔 〕 A global temperature rise of above current levels would be enough to start the thawing of permafrost in Siberia, according to one group of scientists. ==Extent and manifestations of permafrost== Permafrost is soil, rock or sediment that is frozen more than two consecutive years. In areas not overlain by ice, it exists beneath a layer of soil, rock or sediment, which freezes and thaws annually and is called the "active layer".〔 〕 In practice, this means that permafrost occurs at an average air temperature of -2°C or colder. Active layer thickness varies with the season, but is 0.3 to 4 meters thick (shallow along the Arctic coast; deep in southern Siberia and the Qinghai-Tibetan Plateau). In the Northern Hemisphere, 24% of the ice-free land area, equivalent to 19 million square kilometers,〔Tarnocai et al. 2009. Soil organic carbon pools in the northern circumpolar permafrost region. Global Biogeochemical Cycles 23, GB2023〕 is more or less influenced by permafrost. Most of this area is found in Siberia, northern Canada, Alaska and Greenland. Beneath the active layer annual temperature swings of permafrost become smaller with depth. The deepest depth of permafrost occurs where geothermal heat maintains a temperature above freezing. Above that bottom limit there may be permafrost, whose temperature doesn't change annually—"isothermal permafrost".〔 〕 The extent of permafrost varies with the climate. Today, a considerable area of the Arctic is covered by permafrost (including discontinuous permafrost). Overlying permafrost is a thin ''active layer'' that seasonally thaws during the summer. Plant life can be supported only within the active layer since growth can occur only in soil that is fully thawed for some part of the year. Thickness of the active layer varies by year and location, but is typically thick. In areas of continuous permafrost and harsh winters, the depth of the permafrost can be as much as in the northern Lena and Yana River basins in Siberia.〔 〕 Permafrost can also store carbon, both as peat and as methane. Work investigating the permafrost carbon pool size estimates that 1400–1700 Gt of carbon is stored in the northern circumpolar permafrost region. While a recent study that includes stores of the Tibetan Plateau, estimates total carbon pools in the permafrost of the Northern Hemisphere to be 1832 Gt.〔http://www.the-cryosphere.net/9/479/2015/tc-9-479-2015.pdf〕 This large carbon pool represents more carbon than currently exists in all living things. ===Massive ground ice=== When the ice content of a permafrost exceeds 250 percent (ice to dry soil on a weight basis) it is classified as massive ice. Massive ice bodies can range in composition, in every conceivable gradation from icy mud to pure ice. Massive icy beds have a minimum thickness of at least 2 m, a short diameter of at least 10 m (Mackay, p. 223, 1973). First recorded North American observations were by European scientists at Canning River, Alaska in 1919 (French, 2007). Russian literature provides an earlier date of 1735 and 1739 during the Great North Expedition by P. Lassinius and Kh. P. Leptev, respectively (Shumskiy and Vtyurin, 1966). Two major categories of massive ground ice have been identified including buried surface ice and intrasedimental (Mackay,1989), or constitutional (Shumskiy and Vtyurin, 1966), ice. Buried surface ice includes snow, frozen lake or sea ice, aufies river ice and, probably the most prevalent, buried glacial ice (Astakhov, 1986; Kaplanskaya and Tarnogradskiy, 1986; Astakhov and Isayeva, 1988; French, 1990; Lacelle et al., 2009). Intrasedimental ice forms by in-place freezing of subterranean waters and is dominated by segregational ice which results from the crystallizational differentiation taking place during the freezing of wet sediments, accompanied by water migrating to the freezing front (Shumskiy and Vtyurin, 1966, p 109). Intrasedimental or constitutional ice has been widely observed and studied across Canada and also includes intrusive and injection ice (Shumskiy and Vtyurin, 1966; Mackay, 1971, 1973, 1989; Harry et al., 1988; French and Harry, 1990; Pollard, 1990; Mackay and Dallimore, 1992; French, 1998; Pollard, 1991; Murton et al., 2004). Russian investigators including I.A. Lopatin, B. Khegbomov, S. Taber and G. Beskow formulated the original theories for ice inclusions in freezing soils (Shumskiy and Vtyurin, 1966). In addition, but within a separate category of ground ice, ice wedges are a visually spectacular type of ground ice that produce recognizable patterned ground or tundra polygons. Younger than the surrounding substrate, ice wedges are distinctive massive ice features first investigated by E. Leffingwell in 1919 by Galwitz and Popov (Popov, 1962; Shumskiy and Vtyurin, 1966; Pewe, 1966b; French et al., 1982; Michel, 1990). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Permafrost」の詳細全文を読む スポンサード リンク
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