Late Quaternary paleoenvironmental reconstruction of the Sunda Rivers Delta system, Sunda Shelf, south China Sea: Timing of drowning and sea-level changes.

摘要

The Late Quaternary Paleoenvironmental reconstruction of the Sunda Shelf, South China Sea, was achieved using combined techniques employing foraminifera, radiocarbon chronology, sedimentology, and reflection seismics. Sixteen sediment-cores in modern water depths ranging from 71–151 m were used to reconstruct the evolution of the paleo-Sunda 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 post-glacial sea-level rise.; A comparison of previous works that document foraminiferal distributions from the Sunda Shelf and its coastline, with the foraminiferal assemblages identified in the sediment-cores in this study, allowed the delineation of paleoenvironments such as mangrove marsh, estuarine delta, shallow marine delta, bay-lagoon, coastal, and nearshore zones. AMS radiocarbon ages obtained from faunally defined levels in the sediment-cores were used for reconstructing and revising the sea-level curve for the Sunda Shelf.; A shallow marine (−76 m to −77 m), and a deltaic (−71 m to −76 m) foraminiferal assemblage from a sediment-core in the inner Shelf area was indicative of an open Borneo Strait, thus, an active southwest monsoon, during Marine Isotope Stage 5. An estuarine-deltaic faunal assemblage from the central Shelf at a depth from −134 m to −138 m indicated a shallow marine setting and an active northeast monsoon during Marine Isotope Stage 3. The inner and central Shelf remained subaerially exposed during the Last Glacial Maximum (cal BP 19, 250 years) to a depth of at least −115 m below present mean sea level. Evidence exists for an abrupt termination of the LGM, documented by the sharp transition from a subaerial to a nearshore environment, and indicated by a sea-level rise of at least 1.7 m per 100 years. The sea-level rise in response to the input of 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 mangrove-marsh 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 occurred at about cal BP 11,000 years, which led to drowning and reorganization of the Sunda Rivers Delta System, and re-activated the southwest monsoon through the opening of the Borneo Strait.; The study revises a previous existing sea-level curve for the Sunda Shelf in two main ways: (a) It documents the abrupt termination of the LGM, thus, an early deglacial melt water pulse, an event overlooked previously. (b) It re-interprets 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 previously.