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

Abstract

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 14C during the postglacial period with greenhouse gases released from the sub-aerially exposed continental shelf probably accounting for a significant proportion.