Intensification of the Asian winter monsoon after 7.4 Ma: Grain-size evidence from the Linxia Basin, northeastern Tibetan Plateau, 13.1 Ma to 4.3 Ma

Grain-size studies of eolian deposits (loess and Red Clay) on the Chinese Loess Plateau have produced a detailed history of the Asian winter monsoon covering the last 8 Ma. This paper extends the gain-size record back to 13.1 Ma in the Linxia basin, at the northeast edge of the Tibetan Plateau. By comparing the grain-size distribution of the Linxia basin sediment with known eolian sediments in the region we argue that the 10–70 μm fraction is mostly eolian. This fraction becomes increasingly dominant in the record after 7.4 Ma, with almost all sediments eolian in origin after 6.2 Ma. Given that the 10–70 μm fraction in the Linxia basin are well sorted and have grain-size spectra that are very similar to sediments transported by the Asian winter monsoon, we suggest that the continuous presence of this fraction throughout our record indicates that the Asian winter monsoon has been present in some form since 13.1 Ma. If the assumption that variation in grain-size spectra on the million-year time scale is primarily controlled by the character of eolian input to the basin is valid, the long-term pattern of the eolian fraction reveals two abrupt intensifications of the Asian winter monsoon at 7.4 Ma and 5.3 Ma. These are superimposed on a gradual intensification of the Asian winter monsoon beginning at 8.0–7.4 Ma. We note that the two abrupt intensifications may be related to Arctic ice volume increase, and that the gradual intensification of the Asian winter monsoon after 7.4 Ma is accompanied by central Asia desertification and may therefore be related to increase in the height or extent of the Tibetan Plateau.