臺灣臺南社內遺址血蚶與現生血蚶殼體穩定碳氧同位素組成之環境意涵

Abstract

本研究分析了採自台南社內遺址C區T3P3L42~L45F1層位中的60枚血蚶標本，與2010年7月至2011年7月嘉義縣布袋鎮好美里的31枚現生霰石質血蚶（Tegillarca granosa），以及採集地點每月的海水樣本之穩定同位素成分，來探討其於古環境重建之意義。 整體而言，血蚶氧同位素數值振幅與週期明顯。單一標本現生血蚶碳同位素數值平均為-3.01±1.09 ‰ (n=370)。遺址血蚶碳同位素數值平均為-3.45±1.52 ‰ (n=636)。縱貫生長紋方向，氧同位素數值變化曲線的振幅反映出生物生長期間的季節性變化與週期，可推測出貝殼年齡。 遺址血蚶以每4mm、2mm及1mm間距的取樣方式結果中，三者顯示出的死亡季節判斷接近一致，因此在判斷生長週期上，4mm間距取樣已足夠，但是若僅在末端判斷死亡季節，2mm間距取樣可加強振幅且曲線清楚，而1mm間距取樣雖然季節判斷接近，但是震盪較為混亂，2mm間距取樣法於末端取7個樣本點已可觀察到半個至1個週期，並判斷出血蚶死亡季節。經統計分析所有遺址血蚶標本死亡時間於四季分佈情形為：春17%、夏28%、秋35%、冬8%，以及有12%為無法判斷。最後將所有遺址標本做四次的隨機取樣的結果，依季節百分比發現樣本累積數需達25枚時方可看出與60枚樣本相似的血蚶死亡分布季節，而當樣本累積數達40枚後季節百分比曲線可與60枚標本之統計結果一致。 由於研究地點氣候為明顯夏雨冬乾的天氣型態，因此理論上夏季受大量降水影響殼體氧同位素數值會呈現較負值，故僅以冬季的殼體氧同位素數值來討論海水溫度。經由氧同位素數值換算溫度結果，社內遺址（300~500BP）當時冬季海水水溫約為24.8°C，較現今台南七股地區冬季海水水溫（18.4°C; 2008-2011年）為高。In this study, 60 samples of archaeological Tegillarca Granosa from Shenei site in Tainan, and 31 modern blood clams (Tegillarca Granosa) from Chiayi, which sampled during July 2010 to July 2011, were analyzed. At the same time, the isotope content of sea water also collected per month for paleoenvironmental reconstruction. Overall, the isotope analysis shows distinct amplitude and cycle of the oxygen isotope.. The δ18O and δ13C averaged value of modern sample is -3.01 ± 1.09‰ (n = 370). And the δ18O and δ13C averaged value of archaeological sample is -3.45 ± 1.52 ‰ (n = 636). Along the perpendicular transect through growth lines, the amplitude of oxygen-isotope curve were reflecting the influence of seasonal change and cycle during the biological growth, and indicating the age of shell formation. Samples for isotope analysis were taken at 4 mm, 2 mm and 1 mm intervals, the three sampling methods were all lead to corresponding results on the season of death. Therefore, sampling at 4mm is adequate to determine growth cycle. However, if only analyzing the final part to determine the season of death, choosing 2 mm sampling interval will enhance the amplitude and clarify the curve. Although 1mm sampling interval will obtain a better result, the amplitude cycle will also be much more disordered. And sampled at 2 mm interval in the final part with seven samples had already pointed out half to one cycle and determined the season of death. Statistical analysis of archaeological specimens shows 17% were died in spring, 28% in summer, 35% in autumn, 8% in winter, and 12% remained unknown. After conducting 4 times random sampling, coordinating with percentage distribution of deaths in each season, we found the similarity between the results with 60 specimens and with specimens accumulation over than 25, and the season percentage curve would be consistent after specimen accumulation exceeded 40. In the study area, weather pattern was obviously rainy in summer and dry in winter. As heavy rainfall occurred frequently in summer, oxygen isotope values of the specimen tend to be negative. Therefore, we only discuss the seawater temperature in winter through the oxygen isotope values of shell. The calculated minimum oxygen isotope indicate that the temperature of sea water in Shenei site was 24.8°C(300~500BP), and was higher than sea water in Tainan Qigu of modern day, which is 18.4°C(2008-2011).