Reconstruction of Environmental Condition Based on Stable Oxygen Isotope Records of Tridacna Maxima Shells

One modern Tridacna maxima shell collected from Nawan Bay (NW_M; 2002), one Kuei-shan site shell (KS_S; 1832~1926 cal BP), one O-luan-pi II site shell (OLPII_S; 3500~3664 cal BP), and three Ken-ting site shells (KT_S01, KT_S02, KT_S03; 3938~4061 cal BP) were sampled to study for their environmental implications. Growth lines, microstructure, and non-luminescent features of all thin sectioned samples were examined under petrographic and cathodoluminescent microscopes to evaluate shell preservation. Using a micromill device, we obtained carbonate powders for isotopic analyses within two-week resolution.

The δ¹³C and δ¹⁸O values of NW_M shell are between 0.57 and 1.61‰ (1.12 ± 0.19‰; mean ± 1σ; N = 143) and between -2.88 and -0.94‰ (-1.69 ± 0.40‰), respectively. The δ¹⁸O values of NW_M shell are generally in equilibrium with ambient seawater between 2000 and 2001. Four exceptions with lower δ¹⁸O values (< mean - 2σ) of NW_M shell can be related to heavy rainfall events during typhoons that affected southern Taiwan between 2000 and 2001. The δ¹⁸O_shell records observed in summer of 2002 are 0.3‰ greater than the expected δ¹⁸O_shell values for normal years. In addition, seasonal amplitude in δ¹⁸O_shell values was reduced. This variation may indicate an enhanced Kuroshio Current which brought relatively ¹⁸O enriched seawater into Nawan Bay in association with ENSO events.

Based on NW_M study, we recognized heavy rainfall events in KT_S01 and KT_S02 δ¹⁸O_shell records and ENSO events in KS_S, KT_S01, KT_S02, and KT_S03 δ¹⁸O_shell records. According to the proxies for the strength of the Asia monsoon of Lake Huguang Maar (Yancheva et al., 2007: Nature, v. 445, p. 74), these archaeological shells lived during the wetter climate/stronger summer monsoon condition. It may imply that the prehistoric human activity in southern Taiwan were mainly under the stronger Asia summer monsoon condition.