AMS 14C DATING AND STABLE ISOTOPE ANALYSIS ON AN 8-KYR OYSTER SHELL FROM TAIPEI BASIN: SEA LEVEL AND SST CHANGES

Seven accelerator mass spectrometry radiocarbon (AMS 14C) dates (7260 +/- 106 similar to 7607 +/- 95 BP averaged 7444 +/- 103 BP) on a giant oyster shell, collected from an ancient shore of the Taipei Basin, are similar to the LSC (liquid scintillation counting) 14C age (7260 +/- 46 BP) of a grass sample inside the shell. The calibrated 14C ages of the C. gigas by Marine20 are 7490 +/- 240 similar to 7805 +/- 230 cal BP (average 7660 +/- 96 cal BP), generally agreed with the calibrated LSC 14C ages of the grass and the oyster shell. Combined with other 14C ages of shoreline samples in the Taipei Basin, it is evident that sea level rose from 8600 to 7600 cal BP and reached a stand higher than modern sea level. During this marine transgression, the sedimentation rate along the shoreline was very high because 14C dating was not able to detect age differences for 4-5 m thick sediment sequences. Sixty-nine analyses of delta 18O and delta 13C from the oldest part of the shell exhibit clear seasonal cycles, with a 4-year period of growth in the 5.5-cm section. According to the delta 18O values, the ancient oyster grew in a warmer-than-present shoreline environment, suggesting that the current absence of the giant oyster in Taiwan is not due to warming conditions.