Planktic foraminiferal δ18O values indicate precipitation variability in the southeastern South China Sea over the last 175 ka BP

Abstract

The South China Sea (SCS) is an important research area of the tropical water cycle and an important pathway by which the Asian monsoon transports water vapor and heat from the tropical ocean to the Asian continent, yet its response to low-latitude tropical processes is poorly resolved. Here, we report high-resolution records of the oxygen isotopes (δ18O) of planktic Globigerinoides ruber and Globorotalia menardii at different depths and Mg/Ca ratios from a core in the southeastern SCS over the last 175 ka BP to discuss the variability of sea surface temperature (SST) and salinity (SSS) patterns, which can provide insights into understanding SCS hydroclimate dynamics. The records show that SST was characterized by glacial-interglacial cycles largely controlled by ice volume and orbital insolation-driven changes in monsoon circulation. Several short high-frequency intervals of high SST in the glacial periods and low SST in the stadial periods corresponded to variations involving enhanced winter or summer monsoons. The variation in δ18O is obviously influenced by monsoon-driven SSS. Although the δ18O values in different areas of the SCS were similar, the comparison shows that the δ18O values in the study area were lower than those in the northern SCS but higher than those in the southern SCS, appearing to reveal the tropical water cycle process of evaporation, transmission and rainfall in the SCS. The negative deviation in δ18O peaks indicated heavy precipitation in the southeastern SCS caused by an enhanced convection intensity and strengthened summer monsoons. A valuable set of surface and subsurface Δδ18OG.r-G.m (Δδ18OG. ruber - Δδ18OG. menardii) records proved that more negative δ18O shifts must be closely related to the surface seawater environment. However, negative shifts in δ18O were controlled by different mechanisms during the glacial and interglacial periods. In the interglacial periods, the northward shift of the intertropical convergence zone (ITCZ) and summer solar insolation played a key role in precipitation, whereas in the glacial periods, El Niño events and differences in the thermal properties between the sea and land dominated.