A test of different factors influencing the isotopic signal of planktonic foraminifera in surface sediments from the northern South China Sea

The stable isotope composition of planktonic foraminifera is one of the most important proxies in paleoenvironmental research. In this study, three parameters affecting the stable isotope values of Globigerinoides ruber from surface sediment from the northern South China Sea were tested: different cleaning methods, different morphotypes, and different size fractions. Our results show that in the small size fraction, there is a small but significant effect on δ13C by oxidizing the tests prior to measurement. Our data also confirm a small but significant difference between different morphotypes of G. ruber. However, the variability caused by the seasonal effect stable isotope value is larger than the effect caused by different cleaning protocols or different morphotypes. The large spread of the isotope values (up to 2‰) have some implications to paleoceanographic reconstructions; when measurements are performed on a small number of foraminiferal tests, the isotope value does not necessarily reflect yearly average or a certain season but is a random value of the seasonal variability in that region.