PRINCIPAL CHARACTERISTICS OF PERMAFROST IN CHINA
Zhou Youwu, Guo Dongxin (Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica)
Permafrost in China has an area of about 2150000km2, occupying 22.3% of total area of our country. According to the zonal regularity of its formation and development, permafrost in China can be divided into high-latitude permafrost and high-altitude permafrost. The former occurring in Northeast China obeys the regularity of latitude zonality. The extent of permafrost continuation reduces from north to south, at a percentage of area occupation varying from 70-80% to under 5%, with its thickness decreasing from 50-100m to 5-20m, and with its mean annual ground temperature increasing from -3.5℃ to 0℃. The south limit of permafrost in Northeast China approximately corresponds to the 0-1℃ isotherm, and partly--to 0--+1℃. The south terminal of this border reaches 46°36'N. The influence of winter inversion, swampation and plantation results in the fact that the permafrost in lowlands has a lower temperature, a larger thickness and a richer ice content than those in higher localities. The high-altitude permafrost distributing in alpine regions in the eastern and the western parts and Qinghai-Xizang Plateau is controlled by the regularity of vertical zonality. The ground tempetature decreases and the permafrost becomes thicker with the rise in elevation. In the alpine region of west China, the minimum value of mean annual ground temperature can be -4--5℃ and the maximum thickness--100-200m; in extensive regions of the Qinghai-Xizang Plateau (elevation 4500-4900m), the mean annnal ground temperature can be -0.5--3.5℃, and the thickness-25-120m or more, and here lower limit of permafrost nearly corresponds to the -2--3℃ isotherm. The lower limit of alpine permafrost is at about 2200-3700m a.s.l. in the west and at about 1500-3000m in the East China, at about 4150-5300m on the Qinghai-Xizang Plateau. It rises with decrease in latitude making a difference of 3000m from north to south, and is generally lower than snowline, with adifference dwindling from 1300m in the north to 800m in the south, while in the south-eastern corner of Qinghai-Xizang Plateau the lower limit is close to the snowline or even higher than the snowline. The frequent Neotectonic movement and the strong geothermal activity, causing the formation of tectonic taliks, may be the chief factors breaking zonality on the plateau. Permafrost in China belongs to the permafrost with higher temperature and smaller thickness. Generally speaking, permafrost on the plateau is more stable. For the influence of local factors, the difference of seasonally frozen (thawed) depth (0.5-2.6m) between these two types of permafrost in fine grain soil is not great. The underground horizontal thick ice layer widely distributes in our country. Its characteristics are as follows: development in the clayey soil or peat of high water content; buried deep underground concentrated within the table of permafrost and the annual temperature variation depth; with thickness varying from several decimeters to 6-7mj belonging to segregated or segregated-cemented ice type. On Qinghai-Xizang Plateau in some pingo the thickness of horizontal thick ice layer at rare cases may reach 13.3m and the buried depth may occasionally reach 43m. Its genetic type is not clear up to now. In syngenetic permafrost of coarse grain soil, the thick ice layer (with conglomerated texture) is of cemented or cemented-segregated origin, with a buried depth reaching 100m. In our country the large vein ice (or wedge ice) as in Siberia and North America has never been discovered. In permafrost regions of our country there are many various cryogenic phenomena. Their distribution and combination corresponds to the latitude and vertical zonality.