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The imprint of seasonal hydrographic changes on two widely distributed tropical/subtropical planktonic foraminifer species, Globigerinoides sacculifer (without sac) and Globigerinoides ruber (white variety), has been investigated using specimens collected in sediment traps deployed in the northern South China Sea (SCS; October 2000–October 2002). The faunal assemblage is dominated by G. ruber during the warm summer months, whereas G. sacculifer is consistently present year round at abundances of less than 20%. In general, the measured δ18O and δ13C isotopic compositions of G. ruber are more depleted than those of G. sacculifer, and the δ18O difference between seasons (summer vs. winter) is comparable to that of the local glacial–interglacial cycle (ca. 1‰). The lowest δ13C values are associated with relatively heavy δ18O values in January and March for both species and suggest higher nutrient levels in the surface water that are likely related to the prevailing winter monsoon. This speculation is supported by the increase in terrigenous input to the traps brought by the winter monsoon. The ratio between δ18O and δ13C in G. ruber is approximately two times that of G. sacculifer (with a δ18O/δ13C slope of −1.18 vs. −0.55) and differs from that in most biogenic carbonates (0.25–0.33) based on culture and field studies, which indicate enhanced kinetic fractionation at higher ambient carbonate ion concentrations [CO3=]. However, similar regression slopes of δ18O/δ13C (−0.52 to −0.6) are also derived for G. sacculifer collected from plankton tows. Factors other than the “carbonate ion effect” hence appear to be responsible for the negative relationship between δ18O and δ13C in this region. |
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