Abstract:
The abundance of radiolarian, diatom and sponge spicules and H (sub 4) SiO (sub 4) in pore waters increases abruptly at the boundary between the Early and Late Oligocene (about 30-27.5 Ma) at Site 1148 in 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. A comparison of biogenic silica accumulation at Site 1148 in of the SCS with that at Site 929 in the Atlantic verifies that the biogenic silica accumulation between 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 samples from Site 1148 demonstrate that most of the radiolarian, diatom and sponge spicules have suffered from dissolution and reprecipitation, suggested by the opal-A--opal-CT transformation. As a result of the transformation, porosity increased, but dry and bulk densities decreased, reflecting the consequences of diagenesis on the physical properties of sediments.