Biomarker-derived phytoplankton community for summer monsoon reconstruction in the western South China Sea over the past 450 ka

Marine algal-derived lipid biomarkers (alkenones, brassicasterol, dinosterol, and long-chain diol/keto-ol representing haptophytes, diatoms, dinoflagellates, and eustigmatophytes, respectively) were used to evaluate the phytoplankton productivity and community structure changes in core MD05-2901 from the western South China Sea, which features distinct summer upwelling induced by southwest Asian monsoon. The results revealed substantial differences in the distribution patterns between the four major marine primary producers. Diatom and dinoflagellate biomarkers displayed slightly higher abundances, mostly in interglacials especially after MIS 8, while alkenones exhibited lower values in MIS 12 and MIS 1, with higher values in between especially in the middle of MIS 7, but eustigmatophytes increased in most glaciations, indicating complex responses of different phytoplanktons to paleoclimatic and paleoenvironmental changes over the past 450 ka. The sum of the four phytoplanktons shows subtle glacial–interglacial patterns, probably reflecting the combined hydrological dynamics driven by enhanced summer monsoon during summer/interglacials and enhanced winter monsoon during winter/glacials in the region. The biomarker-based community structure showed relative high contribution from diatoms and dinoflagellates during interglacials, high contribution in the middle part of the section centered at ~210 ka from the coccolithophorids, but varying levels from the eustigmatophytes with high percentages in most glacials. Diatoms show strong nutrient sensitivity and positive relation with other paleo-proxies, and their enrichments during interglacials can be attributed to enhanced nutrient level induced by the East Asian summer monsoon, which could have been coupled with the influence of the global ice volume, the summer insolation and the Southern Hemisphere latent heat.