Intensification of the East Asian Monsoon in Southern China at about 300–400 kaBP Inferred from Eolian Deposits in the Middle-lower Reaches of the Yangtze River

In the East Asian monsoon region, eolian deposits widely distributed in the middle-lower reaches of the Yantgze River are among the best materials available for studies on Quaternary climate change in the subtropical zone of Southern China. Typical eolian deposits in this region include upper Xiashu Loess (XL) and underlying Vermiculated Red Soil (VRS) layers. In this paper, chronological and paleoclimatic studies are conducted on an eolian deposit sequence near Jiujiang (JJ) city in northern Jiangxi province. A magnetostratigraphic study, combined with optically stimulated luminescence (OSL) dating, is conducted on the JJ section and provides further evidence that eolian deposits in the middle-lower reaches of the Yangtze River have been formed since the late Early Pleistocene, and that the boundary age between the XL and VRS layers is about 300–400 kaBP. In grain-size records of the JJ section, the median grain-size and content of the >30 μm size fraction increase sharply after 300–400 kaBP, representing an East Asian winter monsoon intensification event. Further pollen analysis reveals differing pollen assemblages before and after 300–400 kaBP: there is an evident increase in plants adapted to grow in a warm humid environment after 300–400 kaBP, implying an increase in precipitation caused by intensification of the East Asian summer monsoon. Global ice volume and uplift of the Tibet Plateau (TP) are regarded as crucial factors influencing variations of the East Asian monsoon on a long-term scale. The deep-sea δ¹⁸O record, which reflects variations in global ice volume, shows no obvious change after 300–400 kaBP. Moreover, the influence of global ice volume changes on the East Asian summer and winter monsoons is inverse; the global ice volume increase (decrease) implies a strengthened (weakened) winter monsoon and weakened (strengthened) summer monsoon. We therefore interpret the coupled intensifications of the East Asian summer and winter monsoons at about 300–400 kaBP to the uplift of the TP in the Middle Pleistocene. This climate event is also documented in eolian deposits from the southern margin of the Chinese Loess Plateau (CLP) and from the desert-loess transitional belt. However, it is not recorded in the loess-paleosol sequences from the central part of the CLP, thereby indicating differing climate responses to TP uplift in different regions, which requires further study.