Long-Term Evolution of the East Asian Monsoon and Impact on Erosion and Weathering Around the South China Sea

Abstract:

Deep-water sediment records in the South China Sea have the potential to allow environment in the adjacent landmasses to be reconstructed since the Eocene rifting of the basin if the effects of provenance and drainage capture can be quantified and accounted for. The rifted margin of South China, fed by the Pearl River, likely holds the simplest record, as until the uplift of Taiwan after ~5 Ma the sediment must be dominantly derived from southern China, which is relatively tectonically quiescent. Provenance data suggest that the Pearl River was likely not involved in the major drainage capture events that affect the Yangtze, Red and Mekong Rivers. Choosing the right monsoon proxy is difficult because heavier rain provides moisture, potentially increasing alteration of sediment, yet greater run-off also causes faster transport, which reduces the time available for alteration to occur. The Middle Miocene is generally identified as the time during which the most altered sediment accumulated, while seismic data also finger this period as one with the fastest sediment delivery rates, indicating rapid erosion onshore. Prior to 22 Ma and during the Mid Miocene Climatic Optimum (15.5-17.0 Ma) conditions were wet and tropical but not very seasonal, i.e. the summer monsoon was weak. Chemical weathering traced by clay minerals and geochemical proxies such as K/Al decreased as the Neogene advanced, likely reflecting falling global temperatures rather than weaker summer rainfall. Weathering decreased even faster between 10 and 8 Ma, when the summer monsoon is inferred to have weakened until 5 Ma, consistent with falling sediment delivery rates to the continental margin. Records after that time are more variable, but are also complicated by sediment flux from Taiwan. The onset of Northern Hemispheric Glaciation appears to be linked to short-lived periods of stronger and weaker monsoon. No simple relationship can be inferred between the intensity of the East Asian Monsoon and the uplift of the Tibetan Plateau, which is believed to have grown steadily to the SE since the end of the Oligocene.