Changes in phytoplankton productivity and community structure in the northern South China Sea during the past 260 ka

A high-resolution molecular organic geochemical study of core MD05-2904 reveals changes in the productivity and community structure of paleo-phytoplankton during the last 260 ka in the northern South China Sea (SCS). Multiple biomarkers, including C37 alkenones, dinosterol, brassicasterol and C30 diols, are used as representatives of haptophytes, dinoflagellates, diatoms and eustigmatophytes, respectively. The abundances and mass accumulation rates of biomarkers indicate that phytoplankton productivity was relatively higher during glacial periods than during interglacial periods. However, the phytoplankton community structure, inferred from the relative abundance of biomarkers, does not display any evident glacial/interglacial cycles. In agreement with previous studies, our results also suggest a linkage between the increased paleo-productivity and the enhanced East Asian winter monsoon in the glacial northern SCS. The relatively stable PCS may reflect a lack of sensitivity of the phytoplanktonic groups to variations of nutrient composition on glacial/interglacial and shorter time scales. A long-term decrease in the C37 alkenone abundance since MIS 5 might have been related to the increasing influence of the northward intrusion of Antarctic Intermediate Water and/or sub-Antarctic Mode Water on the SCS thermocline waters. Moreover, the evolution of alkenone producers and changes in the coccolithophore genera on C37 alkenones might also have contributed to the long-term decrease in the C37 alkenone abundance.