The source of Pleistocene carbonate below the CCD in the central basin of South China Sea: Evidences from coccolith and geochemistry

Carbonate dissolution and deposition in the deep ocean is a critical component of carbon cycle, but detailed pictures of carbonate dynamics below the Carbonate Compensation Depth (CCD) remains poorly understood. In this study, we present a ∼ 420 thousand years record of carbonate content below CCD in IODP Site U1433 at 4.4 km water depth in the South China Sea (SCS). Instead of carbonate-free sediment, we observed that the carbonate content in this deep site could reach as high as 40%. The dissolution-resistant biogenic carbonate, coccoliths, and stable isotope ratio of bulk carbonate were analyzed to trace the carbonate sources. The coccolith assemblage in IODP Site U1433 shows an unrealistic feature of high productivity, which indicates a lateral transport of coccoliths from the northern SCS to the central basin. Coccoliths' contribution to total carbonate was on average 30% and could be as high as ∼80%. Their contribution to carbonate increased at the expense of the dissolution of other biogenic carbonate, such as foraminifera, and decreased with the dilution of other terrestrial carbonate particles. According to the thickness of coccoliths, significant dissolution of coccoliths only happened when the carbonate content dropped below 3%. Marbles from Taiwan, with negative oxygen isotope ratios, and early diagenetic carbonates from carbonate platform nearby, with both negative carbon and oxygen isotope ratios, could be the other important sources besides the marine biogenic carbonate. With help of numerical simulations, we estimated that the lateral transport by turbidity currents could contribute up to 70–90% of the deep-sea sediment in the central basin of SCS. These new findings enrich our knowledge of the carbon fluxes from continental margins to the deep ocean as a key part of the carbon cycle.