关键词: 青藏高原, 环境变化, 西南季风

Abstract: Since the Late Cenozoic Era the violent uplift of Tibet Plateau has resulted in the change of planetary wind system,and the appearance of Asia monsoon circulation has changed the pattern of general circulation,which has produced far reaching influence on the natural environment of the earth especially the eastern Asia.The paper studied the southwestern monsoon change and the natural environmental evolution in the middle Tibetan Plateau.The alternatively superincumbent stratum series with aeolian sand-loess-paleosol in the middle reaches of Yarlung Zhangbo River in Tibet has actually reflected the history of natural environmental evolution.Loess and eaolian sand are the deposits in steppe,desert steppe and even desert environments predominated by winter monsoon;while paleosol is formed in warm wet environment when summer monsoon prevails.The susceptibility as substitution indicator of summer monsoon can reflect the summer monsoon change well.The summit values of susceptibility in stratum section correspond to paleosol,the valley values to loess,and the minimum values to aeolian sand.In the susceptibility curves of two sections in the middle reaches of Yarlung Zhangbo river,three high values represent three periods of summer monsoon prevailing,three secondly high values represent three shorter-time and smaller-range periods of summer monsoon prevailing,and the former correspond to the interglacial staircase in the Last Ice Age,the later to the warm period in Holocene;the three valley values representing weak summer monsoon period correspond to ice period or glacial staircase.The further study of comparing the susceptibility reflecting southwestern monsoon with grain size of Arabian Sea deposited materials that reflecting Indian summer monsoon strength and deposition rate reflecting the aridity of material source region as well as the δ¹⁸O record indicating global ice quantity variation proved that the frequency of grain size changing is much higher than that of susceptibility changing,while the susceptibility change of southwestern monsoon corresponds to the deposition rate and δ¹⁸O record.The middle Tibetan Plateau has experienced several times of prosperity and decline since 300 ka B.P.;at the same time,regional environment has experienced several times of warm-wet and cool-dry changes.Around 80 ka B.P.the circulation pattern of the Tibetan Plateau established,the natural environmental features are similar to that of today.

Key words: Tibetan Plateau, environmental change, south western monsoon