Distribution of planktonic foraminifera in surface sediments and its environmental implication in the Zhongsha waters, South China Sea

To understand the planktonic foraminifera distribution and the environmental factors around Zhongsa Islands, South China Sea, 93 surface sediment samples in the area were collected and analyzed. A total of 28 foraminiferal species were identified. Statistical clusters （Q-mode factor analysis） performed to identify three assemblages （92.3% variance） related to modern water masses: two tropical assemblages different in relative dissolution and one assemblage was mainly influenced by subtropical cold water input. The tropical dissolution-susceptible assemblage is dominated by Trilobatus sacculifer and Globigerinoides ruber and mainly buried in the ridge or carbonate slope sediments. The tropical dissolution-resistant assemblage is dominated by Globorotalia menardii and Pulleniatina obliquiloculata, occurring mainly in the deep-water basin or the Zhongsha trough. The subtropical assemblage is dominated by Globigerina rubescens, depositing mainly in the northern slope of the Zhongsha atoll. The distribution of planktonic foraminifera is controlled by the water depth. Moreover, the impact of invasion of water masses with different temperature and salinity is another factor that should not be ignored, as witnessed by the enrichment of Globigerina rubescens in the northern slope sediment of the Zhongsha carbonate platform, which is induced by the northern cold and high-salinity water invasion. Considering the abundance, diversity, and fragmentation of planktonic foraminifera and the variation with water depth, the carbonate lysocline depth and carbonate compensation depth （CCD） are believed to be 2 750 m and 3 400～3 700 m, respectively.