台灣南部現生牡蠣殼體與水體之穩定同位素記錄及其應用

本研究自2017年10月至2019年8月間，採集養殖於台南市七股地區現生牡蠣以及其所生長的水體標本，共採集101個殼體及208個水體標本，分析其穩定同位素成分，以了解現生牡蠣殼體同位素組成與其生活水文環境間之關係。本研究結果顯示養殖場水體氫同位數值主要介於-74.03‰ ~ 8.17‰間（平均值-2.59 ± 14.46‰，1σ；N＝182；V-SMOW）；氧同位數值主要介於-9.77‰ ~ 0.76‰之間（平均值-0.58 ± 1.90‰；V-SMOW），曾文溪近出海口水體氫同位數值主要介於-82.01‰ ~ -1.73‰間（平均值-19.27 ± 19.68‰，1σ；N＝26；V-SMOW）；氧同位數值主要介於-11.37‰ ~ -0.29‰之間（平均值-3.13 ± 2.90‰；V-SMOW）。養殖場與曾文溪近出海口水體氫、氧同位素有顯著的線性關係，與嘉南天水線之關係相似，可反映出淡水與海水混合環境的特性。養殖場水體氧同位素數值與雨量及鹽度數值亦呈現顯著負相關性，可反映出台灣西南部降水/蒸發之氣候特性。現生牡蠣殼體記錄的氧同位素數值介於-6.72‰~ 0.92‰之間（平均值為-3.38 ± 0.96‰，N＝1091；V-PDB），碳同位素數值介於-4.67‰~0.92‰之間（平均值為 -1.67 ± 0.68‰；V-PDB）。而牡蠣殼體氧同位素數值與海溫及水體氧同位素記錄分別呈現負及正相關性的振盪變化。將海溫與水體氧同位素數據代入氧同位素溫度方程式計算出殼體氧同位素理論平衡值，牡蠣殼體氧同位素數值隨著生長週期的曲線變化與理論平衡值曲線大致符合。在牡犡殼體生長時間的不確定因素下，殼體氧同位素實際數值與理論數值約有67%相符，其他不一致的原因可能為雨季及冬季停止生長之生機效應所影響，因此在春、秋兩季溫度推算較為精確。本研究分析6個南科園區西拉雅文化之大道公遺址考古挖掘出土牡蠣，嘗試探討當時採集牡蠣的季節，及降雨強度與現今的差異。考古遺址牡蠣記錄的氧同位素數值介於-9.45‰~ -1.00‰之間（平均值-4.33 ± 1.50‰，N＝86；1σ）。考古遺址殼體氧同位素記錄亦呈現如現生牡蠣殼體之季節性變化，春、夏、冬季皆有採收，考古遺址殼體與現今殼體比較，當時之夏季氧同位素較現今小1.08‰，冬季則小0.49‰。顯示600~500B.P.時期可能處於較現今溫暖及潮濕的氣候形態。

關鍵字

長牡蠣；穩定同位素；大道公遺址；水文環境

並列摘要

To study the relationship between isotope compositions of modern oyster shells and those of water in which they lived, 101 modern oyster shells and 208 water samples were collected from Chi Ku area monthly from October, 2017 to August, 2019. The hydrogen isotope values of the aquafarm water samples are between -74.03‰ and 8.17‰ （-2.59 ± 14.46‰, 1σ; N = 182; V-SMOW）, and the oxygen isotope values are between -9.77‰ and 0.76‰ （-0.58 ± 1.90‰; V-SMOW）. The D and 18O values of Zengwen River are respectively between -82.01‰ and -1.73‰ （-19.27 ± 19.68‰, 1σ; N = 27） and between -11.37‰ and -0.29‰ （-3.13 ± 2.90‰）. The significant linear relationship between oxygen and hydrogen isotopes values in these water samples shows the characteristics of mixing environment between freshwater and seawater. These water oxygen isotope values negatively and positively correlate with the amount of rainfall and the salinity observed, respectively. The relationfhip between D and 18O values observed is similar to that of the local meteoric water line of SW Taiwan published . The oxygen isotope values of living oyster shells range from -6.72‰ to 0.92‰（ -3.38 ± 0.96‰, N = 1091; V-PDB） and the carbon isotope values range from -4.67‰ to 0.92‰ （-1.67 ± 0.68‰; V-PDB）. Oxygen isotope fluctuation patterns of the oyster shells are similar to that of theoretical equilibrium pattern calculated using observed seawater temperature and water oxygen isotope values. With the uncertainty in the time correlation, the seasonal 18O values of shells are roughly 67% consistent with those of theoretical equilibrium values. Difference between the 18O values of oyster shells and those of equilibrium values might be caused by oyster stop growing during the time of higher precipitation events and lower winter temperature. Therefore, the temperature estimation is more accurate in the spring and autumn seasons. This study analyzes six archaeological oyster shells excavated from Ta Tao Kung archaeological site in the Southern Taiwan Science Park. The oxygen isotope values of these archaeological oysters are ranging from -9.45‰ to -1.00‰ （-4.33 ± 1.50‰, N = 86）. Based on the 18O records, these shells were collected in spring, summer and winter. The average oxygen isotope values of the archaeological shells are 1.08‰ and 0.49‰ less than those of modern shells in summer and winter, respectively. Based on the 18O records, the overall climate in 600 to 500 B.P. may be warmer and wetter than present.