We provide new evidence for the development of a stable estuarine
circulation characterized by stagnating water bodies, nutrient recycling and increased primary productivity in the South China Sea
during glacial intervals caused by the closure of the shallow and
narrow straits connecting the South China Sea in the south and west. Our main evidence comes from records of Mn concentrations and Mn/Al ratios in two sedimentary cores from the northern and southeastern South China Sea covering the last 500 ky. Concentrations and Mn/Al ratios of the redox sensitive element Mn show clear glacial-interglacial cycles with maxima during interglacial periods and minima during glacial periods. These cycles indicate ventilation cycles of the bottom water connected to the glacial-interglacial changes in sea level. In contrast, total organic carbon (TOC) concentrations display an opposite pattern with pronounced maxima during glacial times, especially in the southern part of the basin. The variations in TOC can be ascribed to two factors. Firstly to variations in primary productivity controlled by variations in the
intensity of the winter monsoon. Secondly to the degree of preservation of TOC controlled by variations in ventilation, ultimately controlled by sea level. Variations in TOC consequently
represent a superimposition of sea level influenced preservation
control and primarily winter monsoon driven variations in primary
productivity intensity. The decrease in Mn correspond to times when sea level dropped below 40-50 m. Larger amplitude of the variations in TOC and Mn in the southern part of the basin compared to the northern sites suggest that oxygen depletion and nutrient recycling was stronger in the parts of the basin situated the furthest from the only remaining opening to the open Pacific, the Luzon strait.