Impact of the East Asian monsoon rainfall changes on the erosion of the Mekong River basin over the past 25,000 yr

Paleohydrological changes in the southern South China Sea (SCS) combined with clay mineralogy have been investigated along core MD01-2393 recovered off the Mekong River mouth in order to assess the impact of sea level and East Asian monsoon rainfall intensity on erosion and weathering during the last 25,000 yr. SSTs and δ18O values determined on Globigerinoides ruber were used to estimate past changes of local seawater oxygen isotope (δ18Ow). The close position of the studied core to the Mekong River mouth at sea level lowstand likely played a role in the δ18Ow fluctuations resulting from changes of the monsoon rainfall and runoff into the Mekong River basin. The smectite/(illite + chlorite) and kaolinite/(illite + chlorite) ratios combined with the illite chemistry index during the Holocene show higher chemical weathering of detrital material originating mainly from the lower reach of the Mekong River. At shorter time scales, periods of strong monsoon rainfall are associated with an intensification of erosion of the Mekong River lowland favoured by the development of incised-valley systems inducing higher inputs of detrital material from the lower relative to the upper reach of the Mekong River. Our findings imply a rapid response of erosion processes of the Mekong River basin to the monsoon rainfall intensity changes.