2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 6
Presentation Time: 8:00 AM-6:00 PM

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

LIN, Yi-Mei, Department of Earth Sciences, National Taiwan Normal University, 88, Sec. 4, Ting-chou Rd, Taipei, 11677, Taiwan, MII, Horng-sheng, Department of Earth Sciences, National Taiwan Normal Univ, No. 88, Section 4, Ting-Chou Road, Taipei, 11677, Taiwan, LI, Kuang-ti, Institute of History and Philology, Academia Sinica, 128, Sec. 2, Academia Rd.,Nankang, Taipei, 115 and BURR, George S., NSF Arizona AMS Laboratory, Department of Physics, University of Arizona, Tucson, AZ 85721, yamay@ms48.url.com.tw

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 δ18Oshell records observed in summer of 2002 are 0.3‰ greater than the expected δ18Oshell values for normal years. In addition, seasonal amplitude in δ18Oshell 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 δ18Oshell records and ENSO events in KS_S, KT_S01, KT_S02, and KT_S03 δ18Oshell 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.