HOLOCENE SEDIMENT TRANSPORT AND REWORKING IN THE PEARL RIVER AND THE NORTHERN SOUTH CHINA SEA SHELF

The northern margin of the South China Sea receives sediment dominantly from small mountainous rivers on the island of Taiwan and from the Pearl River. Provenance analysis of sediment from ODP Site 1143 in the NE corner of the basin shows an overwhelming flux from Taiwan. Sediment deposited during the Holocene on the continental margin shows more intense chemical weathering compared to that deposited during the LGM. However, hematite/goethite ratios argue for dry conditions despite the known strong summer monsoon at that time. This indicates that the sediment reaching the slope is reworked from the exposed shelf rather than being derived directly from Taiwan. Strong rains eroded the sediments that had been weathering slowly but over a long time period around the Taiwan Strait, until the region was submerged by rising sea levels after ~8 ka. The weathering response to climate change is thus an inherited rather than a direct one. In the Pearl River delta itself less weathering Holocene sediments unconformably overlie strongly weathered sediment that were exposed during the LGM. Sr isotope, major element ratios (e.g., K/Al, K/Rb, CIA) and clay mineral data from the post-9.5 ka section show a clear record with strong monsoon rains correlating with deposition of more altered sediment. We favor enhanced reworking from fluvial terrace deposits as the cause of this chemical perturbation rather than this being a direct response to changing chemical weather rates. Surprisingly the material now in the river and deposited in the delta after 2000 years ago shows a marked rise in weathering intensity despite little change in monsoon strength. We propose that this change is driven by human settlement of the Pearl River region and the erosion of older weathered soils by the initiation of agriculture.