Pronounced Changes In Atmospheric Circulation And Dust Source Areas During The Mid-pleistocene Implicated By The Huining Loess-soil Sequence From The Northeastern Margin Of The Tibetan Plateau

The Quaternary loess-soil sequence in North China has long been considered tobe valuable to trace variations of atmospheric circulation. In this study, anisotropy ofmagnetic susceptibility (AMS) of the Huining loess-soil sequence of last~900kafrom the northeastern margin of the Tibetan Plateau was systematically studied to atfirst to explore the variations of the wind-field in the mid-Pleistocene, and then thesurface texture features of quartz grains and geochemical composition ofrepresentative samples were analyzed to investigate the possible subsequent changesin the dust source areas of the Huining loess.The result of AMS indicates a primary sedimentary magnetic fabric withoutsignificant disturbation by the pedogenesis. At~400ka, the oscillations of foliation (F)and degree of AMS (P) began to increase sharply, indicating the intensification of thewinter surface wind in the northeastern margins of the Tibetan Plateau. Subsequently,the Dec-Kmax of AMS shows a significant shift of the dominant surface wind fromNW-SE to W-E directions since~300ka, while the surface texture features of thequartz grains indicates an increase in contribution of the fluvioglacial sedimentsderiving from the Tibetan Plateau and the geochemical compositions also indicates achange of the main dust source for the Huining sequence from the materials derivingfrom northwestern Gobi and deserts into the fluvioglacial sediments from the TibetanPlateau at the same time. As there is no evidence of significant climatic changesaround300ka, the coeval change of the atmospheric circulation and the dust sourceareas is thus attributed to the uplift of Tibetan Plateau at～400-300ka and thesubsequent corresponding cooling during the mid-Pleistocene. Because the cooling ofTibetan Plateau may not only lead to enhancement of the plateau monsoon but alsocause accumulation of the plateau glacier, which always accelerates the grinding ofrocks and thus provide much more fine-grained fluvioglacial sediments to betransported to the Huining site in the northeastern Tibetan Plateau.