CONTROLS ON MODERN EROSION AND THE DEVELOPMENT OF THE PEARL RIVER BASIN SINCE THE EOCENE

The Pearl River and its tributaries drains large areas of southern China and has been the primary source of sediment to the northern continental margin of the South China Sea since its opening. In this study we use a combination of bulk sediment geochemistry, Nd and Sr isotope geochemistry, and single grain zircon U-Pb dating to understand the source of sediment in the modern drainage, as well as the source of sediment to the rift during pre-oligocne, as sampled by International Ocean Discovery Program (IODP) Expedition 349. A combination of Nd and Sr isotopes show that the Gui, Liu and Dong Rivers are likely not important sources. Single grain zircon dates emphasize the importance of the westernmost tributaries, draining the highest topography and tectonically active areas as the primary sediment producers, namely the Hongshui and Yu Rivers. Our data indicate that climate is not the primary control on erosion patterns and intensities. Zircon dating also shows that the Gui and Liu Rivers are not generating large sediment yields. Discrepancies between this new data and earlier samples make the role of the Dong River hard to determine, although Nd isotopes suggest that it is not dominant. The source of sediment during the Eocene at IODP Site U1435 appears to have been a relatively local basement source, or a regionally restricted river only draining nearby areas of the Cathaysia Block, similar in composition with the modern Dong River. There is no evidence for a large regional river and we exclude sediment transport from the southwest, from Indochina. Our data are consistent with the idea of small drainage systems dominating the basin until the end of the Oligocene (~24 Ma), after which the Pearl River expanded towards its modern state as a result of headwater capture largely towards the West.