Late Pleistocene-Holocene distribution of deep-sea benthic foraminifera in the South China Sea and Sulu Sea: Paleoceanographic implications

Variations in the distribution of benthic foraminifera in the South China Sea and Sulu Sea during the last glacial-interglacial cycle have been used to evaluate changes in deep-water conditions in both basins for the last 25,000 years, Bulimina aculeata and Uvigerina spp, were most important at water depths above 3,000 m in the South China Sea during the last glacial, whereas Astrononion pusillum dominates the fauna between 1,500 m and 3,000 m at present. From their modern day distributions, B. aculeata and Uvigerina spp, seem to prefer high sedimentary organic contents and shallow pore water oxygen penetration depths. Below 3,200 m, the glacial fauna was dominated by Planulina wuellersforfi and Oridorsalis umbonatus in the South China Sea, rather than by the agglutinated taxa which dominate the present assemblages below 3,200 m. In the Sulu Sea, the present day Uvigerina (above 1,400 m), Pyrgo murrhina (between 1,400-2,200 m) and Cibicidoides bradyi-O. umbonatus (below 2,500 m) assemblages replaced a glacial assemblage dominated by Pyrgo sphaera and Bulimina robusta. Like Uvigerina and B. aculeata, B. robusta is associated with high sedimentary organic carbon contents and shallow pore water oxygen penetration depths, Below 3,600 m in the Sulu Sea, there is no major difference between the glacial benthic foraminiferal fauna and the Holocene fauna. Two important paleoceanographic conclusions are derived from our observations, First, the increases in abundances and distributional ranges of B. aculeata and Uvigerina spp, in the South China Sea and P. sphaera and B. robusta in the Sulu Sea, indicate an increase in surface productivity during the last glacial. Second, the replacement of the agglutinated species Rhabdammina abyssorum and Cyclammina cancellata by a P. wuellerstorfi -O. umbonatus assemblage in the South China Sea below 3,000 m during the last glacial may reflect a decrease in the carbonate undersaturation of South China Sea and Sulu Sea deep-waters. Our observations suggest that changes in surface primary productivity and bottom water carbonate chemistry may be the two primary factors controlling the distribution of benthic foraminifera in these two basins during the last 25,000 years.