Monsoon and westerly circulation changes recorded in the late Cenozoic aeolian sequences of Northern China

Systematic grain-size measurements indicate that aeolian sediment in Northern China is generally composed of a coarse component (ca: 10-70 mum) and a fine overlapping component (ca: < 10 mu m), which can be numerically partitioned through grain-size distribution function fitting. Sedimentological consideration, modem dust investigation and the temporal-spatial variations of the two components suggest that the coarse component is produced mainly by low-altitude winds of East Asian winter monsoon circulation, and most part of the fine component probably is transported mainly by high-altitude westerly stream. Grain-size records of the last glacial cycle sections over Chinese Loess Plateau suggest that the strength of winter monsoon and westerly circulation intensified synchronously in glacial stages and weakened in interglacial stages, while the zone axis of the northern westerly probably shifted between about 35 and 37 degrees N in glacial-interglacial cycles. Grain-size records of Quaternary loess and late Tertiary Red Clay indicate that the relative contribution of monsoon/westerly to aeolian accumulation increased progressively in the late Cenozoic, meanwhile, the winter monsoon intensity increased along with simultaneous decreasing of westerly intensity, which are marked by abrupt changes at 3.2 and 1.2-0.9 Ma. These atmospheric changes coincide with the intensification of summer monsoon and drying processes of internal Asia at the two crucial times, revealing a unique high-level forcing of Asian climate evolutions in Late Cenozoic, probably the uplifts of northern Tibet Plateau. Amplified fluctuations of grain-size in the last 2.6 Ma in the orbital time scale likely reflect the dominant role of the global ice volume on the circulation regimes of Northern China in glacial-interglacial cycles. 0 2004 Published by Elsevier B.V.