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8～ 7Ma左右南亚气候的变干是否由青藏高原隆升到最大高度而引发现代形式季风所造成 ,是一个极具挑战的理论问题 .对位于高原东北部临夏盆地 1 5Ma以来地层中的石英砂扫描电子显微镜分析表明 ,风成砂一直存在并于 8和 6～ 5Ma时含量最高 ;最大风沙颗粒的平均粒径在 1 5～ 3.6 / 2 .6Ma达 0 .44mm ,3.6～ 2 .6Ma之后显著下降 ,平均仅为 0 .1～ 0 .0 7mm左右 .前者可能反映中国西部在 8～ 5Ma气候也十分干旱 ,是全球气候变化的一部分 ,与高原隆起并无直接联系 ;后者表明上新世末研究区近源风沙作用的消失 ,代之以远源风沙作用的盛行 ,可能反映了此时青藏高原的急剧隆起和大气环流的重大改变 .
It is a very challenging theoretical issue whether the drying of the South Asian climate from 8 to 7 Ma is caused by the uplift of the Tibetan Plateau to the maximum height and caused by the monsoon of the modern form.It is very challenging for quartz sand in the strata of Linxia Basin located in the northeastern plateau Scanning electron microscopy analysis showed that the aeolian sand was always present and had the highest contents at 8 and 6 ~ 5Ma. The mean particle size of the largest aeolian sand particles was significant at 1.54 ~ 3.6 / 2 .6Ma up to 0.44mm after 3.6 ~ 2.6Ma Averaging only about 0.1 ~ 0.70mm.The former may reflect the western China in the 8 ~ 5Ma climate is also very dry, is part of the global climate change, and the uplift of the plateau is not directly linked to the latter that the Pliocene The disappearance of near-source sandstorms in the study area, replaced by the prevalence of distant-source sandstorm, may reflect the dramatic uplift of the Qinghai-Tibet Plateau and the significant changes in the atmospheric circulation at this time.