中国西部末次冰期以来冰川、环境及其变化

晚更新世以来,由于青藏高原及其周围山地的上升,中国西部的气候愈来愈干冷,冰川发育受到抑制。末次冰期最盛时,雪线比今日低300—1500m,古冰缘下限比今日低300—1400m,高山带的气温比今日低3—7℃,高原外围地区低8—10℃。青藏高原从13000aB.P.开始气候变暖,6000aB.P.高温期时冰川强烈退缩或消失,4000—3000aB.P.气候又再次变冷进入全新世新冰期和现代小冰期。目前气候又开始变暖,大部冰川又转入后退时期。

THE GLACIER, ENVIRONMENT AND ITS CHANGES SINCE THE LAST GLACIATION IN WEST CHINA

The climate of the Qinghai-Xizang Plateau and northwest China became colder and drier because of the orogeny of the plateau and the surrounding areas. The development of glaciers had thus been increasingly inhibited since the Late Pleistocene.During the last glaciation, the Qinghai-Xizang Plateau was about 4 000 m a.s.l. and the Himalaya over 5 000 m., forming a barrier to the South Asian Monsoon. Precipitation decreased in, the inner part of the plateau, where developed small ice caps and valley glaciers of about 20—30 km long. In southeastern Xizang, glaciers are about 70—80 km in length, since heavy precipitation from Bengal Monsoon could enter the northern part of the valley. Glaciers in the West Kunlun, Karakorum and West Tian Shan are 50—100 km in length, due to large amount of moisture intercepted from the westerlies.The end moraines of two stages in the last glaciation are evidently in southern Xizang and in Tian Shan. Glaciers of the later stage reached their maximum extent during 27 000—19 000 aB. P. in Mt. Gongga, 25 000—15 000 aB. P. in the West Kunlun and 15 000—14 000 aB. P. in Tian Shan. The highest snowline of the last glaciation was 5 500—5 800 m a.s.l. （in northwestern Xizang）, and the lowest 2 500 m （Altay）. The snowline during the last glaciation was 300—1 500 m lower than that of present. These differences are due to precipitation, temperature and orogeny since the last glaciation.Morphological, sedimentary and palynological data indicate that the climate was cold and dry during the last glaciation, the mean annual temperature in the alpine zone, 3—7℃ lower, and that outside the plateau, 8—10℃ lower than those of today. Large-scale permafrost formed in the Jiantang Plateau. The palaeoperiglacial lower limit was 300—1 400 m lower than that of today, and intermountain basin lake levels were higher, because of the melt water from glaciers and snow.The climate became warmer on the Qinghai-Xizang Plateau 13 000 aB. P. During the Hypsithermal period （7 000—6 000 a B. P.）, the climate was the warmest with the mean annual temperature 1—3℃ warmer than that of today, and the glacier extent was smaller than that of present.During the Neoglaciation there were four eminent advances, i.e. the Chongce advance (¹⁴C 3 983 ±120 aB. P.) and the Xuedang advance (¹⁴C 2 980±150 aB. P.), the Ruoguo advance (¹⁴C 1920 ± 110 a B. P.) and the Hailuoguo advance (¹⁴C 940 ± 50 a B. P).The glaciers have experienced 3—4 fluctuations since the Little Ice Age （16—19th centuries）. Recently, most of the glaciers have been retreating, except the partical advancing glaciers.