0.8 Ma B.P.以来西藏雅鲁藏布江中游地区沙地演化和气候变化

THE SANDFIELD EVOLUTION AND CLIMATIC CHANGES IN THE MIDDLE COURSE AREA OF YARLUNG ZANGBO RIVER IN TIBET, CHINA SINCE 0.80 Ma B.P.

根据地表沉积相特征及其对气候变化的反映，着重讨论了西藏雅鲁藏布江中游沙地形成时代、演化过程和气候变化。沙地早在中更新世中期的０．８ＭａＢ．Ｐ．就已出现，经历了强烈发展、缓慢发展或相对稳定和缩小与固定的多次迭覆更替，并呈不断扩大趋势。与此同时，气候也经历了冷干和暖湿的多次变化，并表现出不断向干冷化方向发展的趋势。

Desertification is an important environmental problem. The results of desertified land investigation show that desertification is very serious in Tibetan autonomous region in China. Therefore we decide to take the middle course area of Yarlung Zangbo River as an example to discuss the sandfield evolution and its relationship with climatic changes. The studied area is located in middle Tibet where climate belongs to aridsemiarid temperate monsoonal region in Tibetan Plateau. The annual precipitation varies from 252 to 508 mm and mainly ocurring in Summer. The other feature of the climate is that strong wind happen frequently and the annual mean wind velocity is above 2.1 m/s. There are many different sedimentary sequences: first is continous eolian sand deposit; second is alternatively depositing eolian sandalluvium soil; third is alternatively depositing loesseolian sandpalaeosol series; last one is alternatively depositing loesseolian coarse silt palaeosoldilluvium and alluvium series.[HX(0,1)40]A timescale for the Quternary deposit sequences may be established by many methods. We take measures of palaeomagnetic event,TL dating, 14C dating and comparision with topical section in Loess Plateau to set up the time series for deposit sequences in the studied area.In order to discuss the sandfield evolution and climatic changes, we take sedimentary facies, magnetic susceptibility and some of chemical elements, debris mineral as analysing indices. The period and model of sandfield evolution and climatic changes were set up as below:(1) 0.80~0.518 Ma B.P. sandfield was developing slowly and its narrowing,fixation intervals existed also, and the climate was characterized by warmhumid dominantly, but warmhumid alternated with coldarid appearance.(2) 0.518~0.08 Ma B.P. the eolian sand deposit increased, it reflected sandfield area was developing acceleratively and the climate was cold, which showed semiarid or arid circumstance.(3) 80~10 ka B.P. eolian sand almost covered anywhere, which indicated sandfield lasting rapidly developing and the climate lasting drought, and the contemporary sandfield pattern was formed in this period.(4) After 10 ka B.P. sandfield had been fixed, which reflected the warmhumid climate in optimum period. Then it revived again and in the period climate fluctuated frequently.The global climatic changes is the main reason which results in the coincidence between the studied area and the Loess Plateaus climatic changes. On one hand,the uplift of Tibet Plateau has helped to set up the southwestern monsoon circulation. On the other hand, it brings about the aridcold change of this area and leads to the difference of climatic change between the study area and the other areas of the whole world.