The age and origin of the Straits of Singapore

Consideration of the Quaternary stratigraphy, sea-level history and detailed bathymetry of the Straits of Singapore suggests that a marine connection between the Indian Ocean and South China Sea through the Straits may not have existed until the last interglacial period (oxygen isotope stage; OIS-5e). After this time, a tenuous connection may have also existed during stages 5a and 5c, as well as possibly 5b. The geomorphologic evidence is consistent with flow from west to east during these periods (the opposite direction to modern net flow), along a narrow palaeochannel that can be traced across the floor of the modern straits. Strong currents flowing through the Straits at these times are likely to have been responsible for scouring the 204 m ‘over-deepened’ basin known as the Singapore Deeps, and a similar 102 m deep basin south of the southern outfall of the Riau Straits. The evidence suggests that the region has been undergoing down-warping at a rate of 0.06 to 0.19 mm/year since the beginning of the last interglacial period, and the most likely value is at the lowest end of this range. While a more precise estimate of this rate will require further study, it is likely that the Singapore Straits has not been acted as a significant barrier to migration from mainland Asia to emergent areas of the Sunda Shelf for most of the Quaternary.