A LATE MIOCENE - EARLY PLIOCENE RECORD OF ATMOSPHERIC CIRCULATION CHANGE FROM THE LINXIA BASIN, NW CHINA

Grain size spectra of lacustrine and fluvial sediments from the Linxia Basin, northeast Tibetan Plateau, spanning 13.1 to 4.4 Ma, were measured. An analysis of the spectrum types show that sediment sources and deposition processes are complex low in the section, with at least six different spectra types. Moving up section, the variety of spectra gets much simpler, and after 7.8 Ma sediments are dominated by eolian deposition. By comparing grain-size distribution of the Linxia Basin with modern dust, eolian loess, Red Clay, and non-eolian sediments, two primary eolian components are recognized in the Linxia Basin: the 2-10 µm and 10-70 µm fractions. Today, these two components are transported by the Westerlies and the Asian Winter Monsoon. The Mass Accumulation Rate (MAR) of these two eolian components indicates that sediment transport by both the Asian Winter Monsoon and Westerlies intensified at 7.4 Ma and 5.3 Ma. The intensification of atmospheric circulation seen in the increases in MAR of eolian material in the Linxia Basin is correlated with Asian desertification which is likely associated with Arctic ice volume increase and with Late Miocene and Pliocene global cooling. However, at 8.0 Ma the Westerlies are strongest relative to the Asian Winter Monsoon in our record, which suggests that desertification of the basins of Northwest China happened earlier than Arctic ice volume increase, which is more strongly associated with the Asian Winter Monsoon. This may therefore be related to an increase in the height or extent of the Tibetan Plateau. 8.0 Ma also marks the beginning of a new atmospheric circulation pattern which is characterized by the gradual intensification of the Asian Winter Monsoon relative to the Westerlies.