G02_P03

AMS ¹⁴C dating and stable isotope analysis on an 8-kyr oyster shell from Taipei Basin: Sea level and SST changes

Chou C¹,  Kang S¹, Liu T¹, Li H¹

¹Department Of Geosciences, NTU, Taipei, Taiwan

A giant oyster shell ( 42 cm long), Crassostrea gigas, was uncovered from an oyster reef on the ancient shore of eastern Taipei Basin in 2002. The oyster reef was 14.9 m below the ground surface which has an elevation of 10 m a.s.l. Eight AMS ¹⁴C dates along the growth axis of the shell are from 8130±200 yr BP to 8435±155 yr BP, indicating that the shell was formed and deposited about 8300 years ago during the high sea level stand of middle Holocene. A plancenta shell collected 5 m above the oyster reef shows a ¹⁴C age of 7640±60 yr BP, indicating that Taipei Basin was covered by ocean water during 7640±60 ~ 8435±155 yr BP. A total of 79 stable isotope samples were collected in a 5.5-cm section from the oldest part of the shell, giving δ¹⁸O range of -6.03‰ ~ -1.33‰ and δ¹³C range of -2.21‰ ~ -0.31‰ (VPDB). These δ¹⁸O and δ¹³C profiles show clear seasonal cycles, indicating a 4-year growth in the 5.5-cm section. Both δ¹⁸O and δ¹³C values illustrate that the oyster grew in a saline environment. Compared with the δ¹⁸O profiles of modern oyster shells in Taiwan, the ancient oyster grew in a much warmer shoreline water environment. According to the studying section, the oyster reef had an elevation of 0 m at 8300 yr BP, and the Plancenta shell had 3-5 m a.s.l. at 7600 yr BP.

Key words: Taipei Basin, early-Mid Holocene, oyster shell, ¹⁴C dating, stable isotopes