Postglacial sea-level changes in the northern South China Sea continental shelf: Evidence for a post-8200 calendar yr BP meltwater pulse

摘要

A postglacial sea-level record reconstructed through the study of 25 cores and 10 grab samples obtained from the siliciclastic-dominated northern South China Sea continental shelf between Hong Kong and Dongsha is presented. The seven sedimentary facies recognized are: (1) Pleistocene overconsolidated mud facies affected by palaeosol development due to sub-aerial exposure during the Last Glacial Maximum (LGM), (2) bioclastic sandy gravel facies occurring at a present-day seabed depth exceeding 120 m, (3) LGM bioclastic beach-dune sand facies at a present-day seabed depth between 110 and 120 m, (4) postglacial siliciclastic beach-dune sand facies formed during at least two episodes of sea-level stillstands at present-day seabed depths between 80 and 90 m, and 30 and 50 m, respectively, (5) postglacial transgressive sand facies of Late Pleistocene to Early Holocene age: radiocarbon dating shows that this facies was deposited in two stages before and after the 8200 calendar yr BP cold event, (6) postglacial sand and gravel-dominated facies with acoustic turbidity at a present-day seabed depth of about 50 m, (7) postglacial mud facies. Based on the radiocarbon ages obtained from samples located above and below the Holocene-Pleistocene hiatus in the two innermost shelf cores, and large variations of atmospheric radiocarbon concentration found during the last glacial period in a submerged stalagmite from the Bahamas (Beck et al., 2001. Science 292, 2453-2458), ages exceeding 8200 calendar yr BP are likely to represent minimum values. Ages postdating this limit, which are less affected by variations of atmospheric radiocarbon concentration, indicate the presence of a post-8200 calendar yr BP cold event meltwater pulse caused by eustatic sea-level rise from approximately 40 m below present to the present level by about 6000 calendar yr BP. Inaccuracies of the pre-8200 calendar yr BP radiocarbon ages may be attributed to large variations of atmospheric Δ~(14)C during the postglacial period with greenhouse gases released from the sub-aerially exposed continental shelf probably accounting for a significant proportion.