On the significance of sea-level variations and shelf paleo-morphology in governing sedimentation in the southern South China Sea during the last deglaciation

Deglacial sedimentation on the outer Sunda Shelf, the shelf margin and slope shows a close correlation with the stages of flooding of the emerged shelf. It is mainly controlled by the relationship between shelf paleo-physiography and changes in sea level and sediment supply. Based on the sedimentological and geochemical variability at five sites along a transect across the outer shelf and the continental slope covering the last 20 000 years, four intervals of significant depositional changes are identified. The first reorganization in the sedimentation regime, which was established during sea-level lowstand, is associated with the drowning of the lower course of the North Sunda River between 16.5 and 14.5 ka. During and following the rapid rise in sea level at 14.5–14 ka, sedimentation on the slope is characterized by an accelerated decrease in the supply of terrigenous material, associated with the flooding of the middle part of the paleo-valley. This abrupt change in sedimentation is followed by a period of gradual decrease of terrigenous supply to the slope, when the surrounding plains of the river valley were flooded (14–8.5 ka). The final change occurred between ca. 8.5 and 6 ka, when the coastline reached its modern position, and modern sedimentary patterns are established. This change probably marks the end of reworking by transgression. Given the synchroneity of sedimentological changes along the transect across the Sunda Shelf and into the deep sea with the environmental shifts on the central shelf, sedimentation on the margin and slope appears to be mainly controlled by the interrelationship of sea level, shelf paleo-physiography and sediment supply. Moreover, the records display lateral changes through time associated with stages of the deglacial transgression, which mask any potential sedimentological variability due to changes in precipitation, vegetation or atmospheric circulation. This fact cautions the interpretation of near-shore sedimentary records as unequivocal recorders of local climate change (e.g. SE Asian monsoon), and highlights the need to include variations in shelf physiography and sea level in future paleoceanographic studies.