Late Pleistocene Paleoceanographic Characteristics of the Southwestern South China Sea since 500 ka

Planktonic foraminifera from IMAGES core MD01-2392 near the mouth of the Mekong River were quantitatively studied. Together with SST and thermocline analyses, the faunal results reveal major evolutionary paleoceanographic characteristics in the southwestern South China Sea over the last 500 ka. A higher winter SST was found for glacial periods, especially MIS12, MIS8 and MIS2-4, while interglacials including MIS9, MIS5 and MIS1 with a relatively low SST were documented. As marked by a high abundance of the nutrient-loving, deep dwelling species, the thermocline reached its shallowest depths during MIS5 and MIS1, followed by MIS9, indicating that the estimated low SST for interglacials was due to stronger upwelling. The relatively low estimated SST, however, may also have been influenced by low salinity due to frequent and stronger rainfall during interglacials which was not considered in the SST calculation. The relatively high winter SST during glacial periods favored the prolific growth of Pulleniatina obliquiloculata, a subsurface species preferring warm and saline environments. This implies that during glacial periods, upwelling in the southern South China Sea was suppressed by a winter monsoon flowing from the north, and water exchange weakened substantially because inter-basin passages were shut down during sea level lowstands and the possible development of a freshwater cap, subsequently leading to stronger stratification in the upper water column and the retention of a relatively warmer SST during glacial winters. The main reason why the response of planktonic foraminifera to Late Pleistocene glacial cycles in the southern South China Sea differs from the northern part and open oceans is likely to be because of the semi-enclosed sea basin and monsoon variability during glacial periods.