CHEMICAL WEATHERING, EROSION AND PROVENANCE RECORD OF SOUTHWESTERN OF THE SOUTH CHINA SEA SINCE MIDDLE MIOCENE

The sediment in the SW South China Sea potentially records the history of erosion, weathering and drainage capture in a tectonically active part of SE Asia, heavily influenced by the monsoon and by the India-Eurasia collision. We analyzed sediment samples from International Ocean Discovery Program (IODP) Site U1433 in the SW South China Sea. Major and trace element compositions, clay mineralogy, Sr and Nd isotope data provide a record of chemical weathering, physical erosion intensity and evolving provenance since 17 Ma, following the end of seafloor spreading. Smectite/(illite+chlorite) and smectite/kaolinite have variable and high values before ~8 Ma after which they are most low and stable. The Chemical Index of Alteration (CIA) increases sharply after 8 Ma, consistent with the clays. ⁸⁷Sr/⁸⁶Sr also changes at 8 Ma suggestive of more alteration but largely driven by a provenance change indicated by Nd isotopes. The Mekong river became the dominant sediment source to Site U1433. Our combined data set implies a change in sediment source to Site U1433 from Borneo to the Mekong especially at 8 Ma. We suggest that this indicates the establishment of the Mekong Delta in its present position, driven by a switch from the Gulf of Thailand, possibly linked to uplift of Central Highlands of Vietnam.