Stable isotopes in modern planktonic foraminifera: Sediment trap and plankton tow results from the South China Sea

Abstract

This study reports on the stable isotopic composition of modern planktonic foraminifera tests collected from plankton tows and sediment trap moorings in the northern South China Sea. Plankton tow samplings were conducted at water depths of 50, 100, and 200 m during various seasons between December 2002 and August 2008. The sediment trap array was deployed between August 2004 and February 2005 at water depths of 121 m, 619 m, and 3451 m. Four common and widely distributed tropical/subtropical planktonic foraminifer species, including Globigerinoides ruber (white variety), Globigerinoides sacculifer (without sac), Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, were analyzed for δ18O and δ13C within narrow shell size ranges and compared with the concurrent sea surface temperature (SST) and wind stress. Our results show that foraminiferal δ18O is primarily influenced by seawater temperature, while δ13C is affected by surface water nutrients, which in this region can be discerned from wind stress data. The δ18O and δ13C values from foraminifers recovered in plankton tows generally show greater variability and more depleted δ18O than values of tests collected by sediment traps. For foraminifer tests collected by sediment trap, the δ18O of the shallow dwelling G. ruber shows the largest amplitude variation (ca. 1.6‰) among the four species. In contrast, the δ13C of P. obliquiloculata shows relatively constant values throughout the study period. A pattern of enriched δ18O values, associated with marked δ13C depletion, is common to three species collected between late October and late December 2004. This distinct isotopic signal corresponds to a decrease in SST and increase in wind stress, indicating the onset of prevailing northeast winds during the winter season.