中国的多年冻土──过去与现在

PERMAFROST IN CHINA: PAST AND PERSENT

中国多年冻土区的总面积约占中国陆地面积的２２．４％，达２１５００００ｋｍ２。多年冻土的分布特征受气候条件在三度空间的变化所制约。自晚更新世以来，其分布情况已有相当的变化。在东次冰期最盛期，东北地区多年冻土南界曾推进到北纬４１—４２°，在全新世暖期，南界向北退缩，但晚更新世形成的冰楔和多年冻土至今仍存在于大兴安岭北部，全新世中期严寒期冻土有所扩展并形成冰楔。随着气候变化，中国西部高山和高原区高海拔冻土的分布下界已上移８００—１０００ｍ，但高山和高原的主要部分仍处于冰缘环境，有的地方在全新世还发育了共生型多年冻土。

Permafrost regions presently occupy some 2 150 000km2 or 22.4 % of the Chinese territory. Statistics shows that the mean annual air temperature in these regions decreases northwards, eastwards and upwards. In Northeast China, the latitude isthe most significant factor that influences the spatial distribution of air temperature; the altitude is the second. Thus, the distribution of permafrost reveals mainly a latitudinal zonation: the mean annual temperature of permafrost decreases from 0-1℃ at the southern limit down to -2.5℃ (locally -4.2℃) at the north. Correspondingly, the permafrost inceases in thickness from 5-20m to 50-100m, even or more, and varies from sporadic to continuous. In West China, i. e., Xinjiang andthe Qinghai-Xizang(Tibet) Plateau, the altitudinal factor becomes decisive, and the latitudinal one-the second. Thus the development and distribution of permafrost depends mainly upon altitude: it becomes colder, thicker and more stable with rising elevation. The fact that the lower limit of mountain permafrost decends northwards and eastwards must be related to the decrease of the mean annual air temperaturein the same directions. ' The distribution of permafrost has had a considerable change since the Late Pleistocene. In Northeast China, tEe southern limit of permafrost extended to 4142°N during the Last Glaciation Maximum. This could be proved by widely distributed ice-wedge casts and sand-wedges. In the hypsithermal interval of Early Holocene, the southern limit retreated northwards up to 50°N. Inactive ice-wedges found in Wuma show that the permafrost formed in Late Pleistocene remains in the northern Dahingganling Mountains today. Again, inactive ice-wedges found at Yitulihe indicates that there was a colder time of enlarging permafrost during the Middle Holocene. In Mt. Tianshan, the quatz-feldspar coefficient K in sediments shows that thelower limit of mountain permafrost has risen by 900-1000m since Late Pleistocene. Research on buried frozen humus soils reveals that the upper reaches of the Urumqi River has been a rather stable periglacial environment since Holocene, accompanied with the formation syngenetic of permafrost in some swampy lowlands. Iap the Qinghai-Xizang Plateau there are some ice-wedges formed in the coldest stage of Late Pleistocene to be found. At this stage the lower limit of permafrost was 800-l 000m lower than that at present. Since Holocene, the lower limit of permafrost at the fringe of the plateau has risen to some extent. The data of loess susceptibility suggest some fluctuation of climate and some change of permafrost distribution in the plateau. However, the main part of the plateau was still in a periglacial environment. In comparison, from of the Last Glaciation Maximum on, the southern limit of permafrost has retreated northwards by latitudes of 23--24' in northern Europe, 16'to 18' in North America and only 6--8o in East and Northeast China. In (West China, the lower limit of mountain permafrost has risen several hundred meters vertically but moved by several tens of kilometers horizontally.As to the further research, it is suggested that two transects be arranged along 120'E and 90'E, and then excavations and deep borehole drilling be carried out along them to investigate the distribution, composition and structure of frozen ground, to monitor ground temperature and to extract other information stratigraphical, biological, geochemical and gas-geochemical. All of these studies will be helpful to the reconstruction of the paleoclimate and the development history of permafrost, to the evaluation of the stability of frozen ground and to the prediction of the response of cold region environment to the blobal climatic chance.