Foraminiferal responses to upwelling variations in the South China Sea over the last 220 000 years

Abstract

Late Quaternary benthic and planktonic foraminiferal and isotopic data were obtained from two sediment cores in the present upwelling areas of the South China Sea: Core 17928 off northwestern Luzon and Core 17954 off eastern Vietnam. Quantitative analyses of foraminifera in the two cores revealed the foraminiferal responses to paleoceanographic changes over the last 220 000 years. High productivity species such as Neogloboquadrina dutertrei, Bulimina acculeata, Uvigerina peregrina, Melonis barleeanus and Chilostomella oviodea displayed high abundance during interglacial periods in Core 17954 and during glacial periods in Core 17928. Correspondingly, variations in benthic foraminifera accumulation rates, the ratio of infaunal to epifaunal benthic foraminifera and the presence of the benthic foraminifers Cibicidoides, Miloilids and Oridorsalis umbonatus, all indicate that bottom water was oxygen-rich and oligotrophic during glacial periods off Vietnam (Core 17954) but such bottom conditions occurred during interglacial periods off Luzon (Core 17928). Sea-surface temperature results from planktonic foraminifera transfer function and variations in the abundance of planktonic foraminifera, typical tropical species Globigerina sacculifer, tropical–subtropical species Globorotalia menardii, Globigerinoides ruber, and Pulleniatina obliquiloculata, transition species Globorotalia inflata and Arctic species Neogloboquadrina pachyderma, indicate an increase in cool waters during marine isotope stage (MIS) 5, except for MIS 5.5, and glacial intervals in the cores 17954 and 17928, respectively. The ratios of mixed-layer/thermocline dwelling species and depth of thermocline (DOT) estimates using transfer function suggest that the DOT was shallower during the interglacial than the glacial periods off Vietnam (Core 17954), but the DOT was shallower during the glacial periods off Luzon (Core 17928). Our results show that in the glacial periods lower sea surface temperature, shoaled thermocline, and high productivity increased off northwestern Luzon (Core 17928), and that, on the contrary, similar event occurred in the interglacial off Vietnam (Core 17954). These differences in the upper water column structure and productivity in the two areas were induced by coastal upwelling, which in turn is driven by monsoons. The intensity of upwellings is also closely related to the evolution of the eastern Asian monsoon. Therefore, we conclude that differences in the upwelling intensity in the two sites indicate strengthening of winter and summer monsoon during the glacial and interglacial periods, respectively.