Oligocene biogenic siliceous deposits on the slope of the northern South China Sea

The abundance of radiolarian, diatom and sponge spicule and H4SiO4 in pore-waters increase abruptly at the boundary between Early and Late Oligocene (about 30-27.5 Ma) at Site 1148 of the northern South China Sea (SCS), indicating high biogenic silica accumulation during this time. At the same time (about 30-28 Ma), high biogenic silica deposition occurred in the central equatorial Pacific. Comparison of the biogenic silica accumulation at Site 1148 of the SCS with that at Site 929 of the Atlantic verifies that the biogenic silica accumulation between the low latitude Pacific and Atlantic oceans expresses the evident relationship of compensation during the Oligocene. Biogenic silica accumulation decreased in the Atlantic, whereas it increased in the Pacific at the boundary between the Early and Late Oligocene. It resulted from the formation and presence of North Atlantic deep water (NADW) in the Atlantic basin, indicating an intensive basin-basin fractionation. XRD analysis and SEM observation of the sam