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∼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∼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 δ¹⁸O and δ¹³C 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 δ¹⁸O 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.