Emerging East Asian monsoon triggered by Pacific flooding into the South China Sea at 25 Ma

Abstract:

The development of the Asian monsoon at 25 Ma is one of the most important climate reorganizations in the Late Cenozoic and is thought to be controlled by the uplift of the Tibetan Plateau and closure of the Paratethys. The opening of the South China Sea (SCS), today’s most important water vapor source for the East Asian monsoon is largely disregarded as an important contributor to this climate shift due to SCS seafloor spreading initiating 5-7 myrs prior to the onset of the Asian monsoon. Here, we investigate the bathymetric, sedimentary and oceanographic evolution of the SCS basin by combining Neodymium (εNd) and Strontium isotopic composition of sediments from recent drilling expeditions, high-resolution paleobathymetry reconstructions and ocean circulation models for the time period 30-20 Ma. A prominent shift in εNd from a SCS fluvial environment (εNd -7) to Pacific endmember (εNd -4.5) from 25.5 to 24.5 Ma coupled to ocean circulation models demonstrates that the nascent basin experienced rapid inflow of warm Pacific waters in less than 1 Myr. Strengthening of the Asian monsoon is recorded in SCS sediments by the onset of red clay deposits which reflect increasing northwesterly aeolian transport of Chinese loess sequences by the East Asian winter monsoon. We argue that the rapid inflow of warm, saline Pacific water increased water vapor and east-west humidity gradient and is likely the final trigger for the establishment of a strong East Asian monsoon at 25 Ma.