PALEOENVIRONMENTAL RECONSTRUCTION OF LATE QUATERNARY SEDIMENTS FROM THE CENTRAL SUNDA SHELF OFF PENINSULAR MALAYSIA

A recent published study suggests that Holocene sediments in the incised Chao-Phraya paleo-river valley, between peninsular Malaysia and southern Vietnam, on the Sunda Shelf (southern South China Sea), are ca.16 m thick and, therefore, would be ideal for investigating Holocene paleoenvironmental changes in the region. In order to address this topic, six 1.25 to 2.2 m long gravity cores were collected at water depths of ca. 70 m along a transect crossing the incised paleo-river valley. We used a multiproxy approach to study these materials: 130 samples for analysis of bulk sediment magnetic susceptibility (BMS), 69 for elemental analysis by means of x-ray fluorescence spectrometry, 54 for analysis of foraminiferal assemblages, and 13 for AMS radiocarbon dating.

The cores are composed of variably sandy mud and one contains an underlying organic-rich, black mud unit. C-14 age estimates range from 11,190 to 1,130 cal. YBP, indicating the sediments are mainly Holocene in age, although the Pleistocene/Holocene boundary occurs in two cores. Foraminiferal data from two contrasting cores provide preliminary inferences of depositional environments. The black mud unit represents a late Pleistocene low energy shallow marine environment with significant organic matter. The combination of low Ca content and low BMS activity supports this inference. In this and other cores, the percent of planktonic foraminifera generally increases up-core suggesting an increase in water depth. This is supported by an increase in Ca content that regionally has been found to be a good proxy for marine carbonate deposition. Indeed, there is a significant increase in Ca content ca. 6,000 years ago which is well dated in one of the cores. This finding correlates very well with the proposed sea-level highstand in this region at ca. 6,500 cal YBP.

Our results indicate that the Holocene sedimentary record in this region is generally less than 2 m thick, as opposed to previous suggestions of a much thicker (ca. 16 m) sedimentary section. Sediments record the environmental response to the flooding of the Sunda Shelf during the post-glacial marine transgression, reaching the maximum marine influence at the time of the sea-level highstand ca. 6,500 cal YBP.