Paleoenvironmental Variation and Provenance Records from Neogene Deep Sea Sediment of the South China Sea

Abstract

I employed several paleoenvironmental proxies by utilizing deep sea sediment from the largest marginal basin in east Asia, namely the South China Sea (SCS), to reconstruct the long term paleoenvironmental variation in SE Asia since the Neogene. Sediments deposited within the SCS were mainly delivered by several major fluvial systems whose catchment basins cover several geological blocks in SE Asia. By analyzing the sediment’s geochemical and mineralogical characteristics, several indices which are affected by the source of the sedimentary influx, as well as the weathering state could be built and applied to constrain my paleoenvironmental reconstructions Two deep sea drilling sites, International Ocean Discovery Program (IODP) U1433 and U1435, from the SW and N of SCS were retrieved and used in this study. Bulk geochemistry and isotopic measurements were applied to the samples from Site U1433. Results indicate major provenance changes at 8 Ma. Geochronological U-Pb measurements of detrital zircon grains were conducted on the samples from Site U1435, as well as 11 modern samples from Pearl River tributaries. As the northern SCS’s chemical weathering and erosion variability has been well documented by previous studies, my interest in this site focused on constraining the provenance records, especially in the pre-Oligocene interval. Results indicate that during the Eocene sediment was most likely delivered via several small-scale local rivers from Cathaysia Block, rather than Indochina Block or Palawan,as proposed by several other studies. With both N and SW sub-basin’s provenance record well established, I applied a spectral reflectance proxy (565/435), to reconstruct the paleoenvironmental conditions by quantifying the variation between hematite and goethite. Results of spectral reflectance were also combined with several other commonly used paleoenvironmental proxies, such as K/Al, Chemical Index of Alteration (CIA), clay mineral assemblages and pollen records. Both N and SW of SCS’s paleoenvironmental records were compared and analyzed. The two complete spectral reflectance records imply that the northern and SW SCS underwent two types of climatic conditions with different rainfall intensities since Neogene. Northward migration of the Intertropical Convergence Zone since 10 Ma was considered to lead to this climatic discrepancy.