Charu Sharma

The Late Quaternary paleoenvironmental reconstruction of the Sunda Shelf, South China Sea, was done using combined techniques employing foraminifera, radiocarbon chronology, sedimentology, and reflection seismics. 16 sedimentcores ranging in water depths from 71 151 m were used to reconstruct the evolution of the paleoSunda Rivers Delta from the time when the Shelf was subaerially exposed during low sea levels, up to the time when the delta was flooded by postglacial sealevel rise.

A comparison of previous works that documented foraminiferal distribution from the Sunda Shelf and its coastline, with the foraminiferal assemblages identified in the sedimentcores in this study, allowed the delineation of paleoenvironments such as mangrove marsh, estuarine delta, shallow marine delta, baylagoon, coastal, and nearshore zones. AMS radiocarbon ages obtained from faunally defined levels in the sedimentcores were used for reconstructing and revising the sealevel curve for the Sunda Shelf.

A shallow marine (deltaic) foraminiferal assemblage at a depth from 73 m to 75 m, from a sedimentcore in the inner Shelf area was indicative of an open Borneo Strait, thus, an active southwest monsoon, during Marine Isotope Stage 5. An estuarinedeltaic faunal assemblage in 3 sedimentcores from the central Shelf indicated a Marine Isotope Stage 3 shallow marine setting at a depth from 134 m to 138 m, as well as evidenced an active northeast monsoon. The inner and central Shelf remained subaerially exposed during the Last Glacial Maximum (cal BP 19, 250 years) up to a depth of at least 115 m below present mean sea level. Evidence exists for an abrupt termination of the LGM, documented in the sharp transition from a subaerial to a nearshore environment, and indicated by a sealevel rise of at least 1.7 m per 100 years. The sealevel rise in response to the glacial melt water, from cal BP 14, 700 years (+/ 500 years) to cal BP 14, 400 years (+/ 560 years) was at a rate which allowed a mangrovemarsh delta to be maintained, from 95 m to 87 m below present mean sea level. Gradual flooding of the delta led to the formation of shallow coastal environments. Complete flooding of the Shelf followed after cal BP 13, 000 years, which led to drowning and reorganization of the Sunda Rivers Delta System, and reactivated the southwest monsoon through the opening of the Borneo Strait.

The study thus revised the existing sealevel curve for the Sunda Shelf by Hanebuth et al. (2000) in 2 main ways:

1. It documented the abrupt termination of the LGM, thus, an early deglacial melt water pulse, an event overlooked by Hanebuth et al. (2000).
2. It reinterpreted the evolution of the Shelf for the time period from cal BP 14, 700 years (+/ 500 yrs.) to cal BP 14, 400 years (+/ 560 years), as a case of delta development through the formation of a mangrove marsh environment, rather than a case of rapid flooding, as proposed by Hanebuth et al. (2000).

Keywords:
Pages: 189
Supervisor: David Scott