Rock magnetic and grain size evidence for intensified Asian atmospheric circulation since 800,000 years BP Related to Tibetan uplift

Paleomagnetic, rock magnetic, and grain size studies of a thick loess sequence in the West Qin Ling (mountain range) show that loess deposition there began about 800 ka. The data reveal a progressively increasing coarse grain size fraction upwards into the Holocene. The averages of these coarse size fractions are higher than in the central Loess Plateau, which was apparently farther from the source area, and slightly lower than those of the western Loess Plateau and the eastern Tibetan Plateau, which were therefore closer to the source area. The coarsening and source area location suggest (1) that Asian air circulation may have changed and intensified at about 800 ka resulting in dust deposition in West Qin Ling; (2) that dust-carrying winds were driven not only by the Asian winter monsoon, but included also the westerlies and a winter monsoon caused by the Tibetan Plateau High, and (3) that intensification of all these air circulation systems continues to the present. Increased elevation of the Tibetan Plateau so that it reached into the cryosphere by about 800 ka and a subsequent persistent uplift of the plateau may have been the mechanisms to trigger a change and intensify the air circulation system. Moreover, this circulation shift and intensification, simultaneous with a shift in Milankovitch periodicity, may have contributed to large global climate changes such as the 15% increase in global ice volume at ca. 800 ka. (C) 1994 Published by Elsevier Science B.V. All rights reserved.