ENVIRONMENT AND HARVESTING SEASON INDICATED BY STABLE CARBON AND OXYGEN ISOTOPE RECORDS OF ARCHAEOLOGICAL SHELLS FROM SOUTHWESTERN TAIWAN

To unravel the aquatic environment change and shell collected season between 5000 BP and 500 BP in southwestern Taiwan, we studied the stable carbon and oxygen isotope records of 40 Tegillarca granosashells collected from ten different sites (covering five different cultures) in the Tainan Science Park, Taiwan.

A total of 318 isotope analyses were performed. In ascending order, average δ¹³C values of shells collected from Dabenkeng Culture (~5000 BP; Nankuanli site and Nankuanli East site), Niuchuozi Culture (~3800 BP; Youxianfang site), Dahu Culture (~3300 BP; Wangang site, Niuniaogang site, Youxianfang South site, and Wujiancuo site), Niaosong Culture (~1800 BP; Daoye site), and Siraya Culture (~500 BP; Dadaogong site and Shenei site) are -3.54 ± 1.33‰ (± 1σ; N = 103), -5.01 ± 1.00‰ (N = 26), -1.98 ± 1.06‰ (N = 91), -1.95 ± 0.72‰ (N =35), and -3.30 ± 1.07‰ (N =63), respectively. Average δ¹³C values of shells from Dahu Culture and Niaosong Culture were ~1.6‰ greater than those of shells from the earlier and later cultures. Higher δ¹³C values of shells may indicate higher weathering rate/productivity associated with local sea-level fall caused by uplift of Taiwan Island.

Mean δ¹⁸O values of shells are between -3.7‰ and -2.9‰. There is no significant difference in the oxygen isotope records of shells collected from different culture period of time indicating there was no significant hydrological change in this area from 5000 BP to 500 BP. Based on the fluctuation pattern of oxygen isotope records within individual shells, the harvesting seasons were mainly all seasons for Dabenkeng Culture, summer for Niuchuozi Culture, winter for Dahu Culture, mostly winter for Niaosong Culture, and all seasons but no winter for Siraya Culture. Changes in harvesting season of T. granosa may imply different strategies for marine food source.