Seasonal variations of modern planktonic foraminifera in the South China Sea

Although foraminiferal census and isotope analyses from sediment cores have been widely applied to paleoceanographic reconstructions in the South China Sea (SCS), their modern counterparts, i.e. foraminiferal shells from sediment traps and plankton tows, have been the subjects of few studies in this region. In this investigation, relative abundances of the most popular species used to reconstruct late Quaternary tropical sea-surface conditions have been studied in plankton tows from the SCS in order to offer better constraints on the interpretation of downcore records. In addition to the seasonal differences, the spatial variations in the faunal assemblage are evident from south to north in the SCS basin based on contemporary tows. For example, the species Globigerinella aequilateralis is found to be relatively dominant in the southern (lower latitude) basin but its relative abundance is significantly higher in planktonic tows than in sediments, indicating its proneness to burial dissolution. Pulleniatina obliquiloculata, an index taxon for the Kuroshio Current, is only abundant in the northern part of the SCS basin.The measured oxygen isotope compositions (δ18O) of Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, and P. obliquiloculata all show a good relationship with temperature in the mixed layer (Tmix). The carbon isotope compositions (δ13C) also display a positive distribution pattern relative to Tmix, but with a less significant correlation in winter months than δ18O. Differences of δ18O between shallower (G. sacculifer and G. ruber) and deep dwellers (N. dutertrei and P. obliquiloculata) accurately record the temperature gradient between the surface mixed layer and 100-m water depth, supporting application of this proxy to the reconstruction of upper water-column stratification.