SEASONAL VARIATION OF PLANKTONIC FORAMINIFERAL ISOTOPIC COMPOSITION FROM THE SEDIMENT TRAPS IN THE SOUTH CHINA SEA

Imprints of seasonal hydrographic change on the two common planktonic foraminiferal shells, Globigerinoides sacculifer and Globigerinoides ruber, were discussed based on specimens collected from the sediment traps deployed in the northern South China Sea. Generally the isotopic composition of G. ruber is more depleted than that of G. sacculifer for both d¹⁸O and d¹³C. The lowest d¹³C values associated with relative heavy d¹⁸O in January and March for both species are indicative of enriched nutrient contents in the surface water induced by winter monsoon. In addition, the d¹³C of G. ruber shows a better (negative) relationship with corresponding particulate mass fluxes than G. sacculifer. G. ruber thus seems to be more sensitive than G. sacculifer in reflecting the sea surface nutrient conditions in terms of paleoceanographic application. The covariance between d¹⁸O and d¹³C values in G. ruber, however, is as two times more than G. sacculifer (with a d¹⁸O/d¹³C slope of -1.29 vs. -0.61). The relationship is different from what would be expected for most biogenic carbonates (0.25~0.33) caused by the enhanced kinetic fractionation at higher ambient carbonate ion concentrations CO₃⁼. Nevertheless, similar regression slopes of d¹⁸O/d¹³C ranging between -0.52~-0.6 are also derived based on G. sacculifer shells collected by plankton tows. It is possible that factors other than carbonate ion effect are responsible for negative correlation between d¹⁸O and d¹³C in this region. In comparison with downcore records, the d¹³C compositions of traps samples are significantly lighter than those from core samples with comparable d¹⁸O values. The shift of d¹³C between trap and core samples should be the consequence of preservation (burial diagenesis).