The flooding of Sundaland during the last deglaciation: imprints in hemipelagic sediments from the southern South China Sea

During the last 30 ka, the South China Sea (SCS) experienced pronounced palaeogeographic changes associated with the postglacial sea level rise, which significantly modified the hydrography of this marginal sea. The most crucial effects in the southern part of the basin were the submergence of Sundaland and the opening of the southern channels connecting the SCS to the tropical Indo-Pacific. Isotopic, sedimentological and organic geochemical parameters determined in two sediment cores from the southern SCS, one in the open sea and the other close to the continental shelf (sites 17961 and 17964, respectively) show that the main hydrographical changes during this period were related to critical thresholds in sea level rise. The main changes occurred at about 15–13.5 ky BP, coincident with Meltwater Pulse (MWP) Ia, when sea surface temperatures (SSTs) at both sites experienced a rapid 1.5°C rise, and the clay content and n-nonacosane concentrations dropped significantly. Both trends reflect a rapid retreat of the coastline and an initial flooding of Sundaland at that time. A second important change, starting with the beginning of MWP Ib at about 11.5 ky BP and culminating at 10 ky BP, involved the establishment of modern hydrographic conditions. This is evident from the rapid convergence of the foraminiferal oxygen isotope records and the establishment of Holocene SST values. These results highlight the need to include the flooding/emergence of Sundaland as an important boundary condition in future modelling studies of Asian palaeomonsoons.