Abrupt Changes in the Late-Holocene Sedimentary Dinoflagellate Cyst and Geochemical Records of the Northern South China Sea in Response to Regional Climatic or Anthropogenic Impacts

Dinoflagellate cysts and elemental composition were analyzed in a sediment core from the northern South China Sea (SCS) spanning the last 3200 years to examine climatic and anthropogenic impacts on the marine environments, especially with regard to marine primary productivity (PP). Two notable intervals, ~2700-2200 cal yr BP (~770-221 BC) and ~1000-600 cal yr BP (~AD 900-1350), are characterized by decreases in the abundances of total autotrophic taxa, total Impagidinium, Lingulodinium hemicystum, total Spiniferites and Polysphaeridium zoharyi, and increases in the abundances of total Brigantedinium, Selenopemphix nephroides and the ratio of heterotrophic to autotrophic taxa. The older time interval, which corresponds to dynasty collapses in China, coincides with cool and dry regional climatic conditions with higher relative abundance and flux of total Brigantedinium, as well as peaks in Ba/Al, Si/Al and Fe/Al ratios, indicating higher PP in the northern SCS. Higher Pinus flux indicates intensified terrestrial influence resulting from strengthened winter monsoons, which could also account for higher PP. In contrast, the higher relative abundances of total Brigantedinium and higher ratios of heterotrophic to autotrophic taxa observed between ~AD 900 and 1350, which also coincide to a period of human society disruption in China, are associated with a period of warming and dry conditions but not with geochemical evidence for increased PP, suggesting that changes in dinoflagellate cyst assemblages are instead related with local changes in plankton eco-structure rather than regional climatic or anthropogenic impacts on PP.