臺灣臺南七股現生牡蠣殼體穩定氧同位素記錄及其於季節性之應用

Abstract

本研究分析2013年3月至2014年7月間養殖於台南七股區135個現生長牡蠣殼體氧同位素組成，探討現生牡蠣殼體氧同位素成分與其生活水體環境之關係。 現生牡蠣殼記錄的氧同位素數值介於-6.92‰~ -0.08‰之間（平均值為 -3.05 ± 1.17‰，N＝2310；1σ；VPDB）。水樣氧同位數值主要介於-0.28‰ ~ 0.74‰之間（平均值0.18 ± 0.29‰，N＝20；1σ；VSMOW），另有一水樣採集於颱風期間，其氧同位素數值為-2.75‰，現生殼體氧同位素數值振盪與海溫變化相似，呈現8個月至3年半之季節性振盪。將水樣氧同位素與將軍測站海溫資料帶入同位素溫度方程式計算出殼體氧同位素理論平衡值，而約有67%之殼體氧同位素實際數值與理論數值相符，而不符合之殼體氧同位素實際數值較理論平衡值輕0.2~0.8‰。 本研究亦分析南科園區西拉雅文化（500~300B.P.)五間厝北遺址挖掘出土之牡蠣，探討古人採集牡蠣的季節，及降雨強度與現今的差異。5個遺址出土牡蠣記錄的氧同位素數值介於-5.98‰~ -1.26‰之間（平均值-3.34 ± 1.37‰，N＝60；1σ）。遺址殼體氧同位素記錄亦呈現季節性變化，主要採收季節為秋冬兩季，與現今殼體比較，遺址殼體在夏季氧同位素較現今高1‰，冬季則低1.2‰。而距今500年前時，全球氣候處於小冰期階段，東亞地區溫度約較現今冷0.3~0.9℃，若溫度變化對殼體氧同位素數值之影響可忽略，則遺址殼體氧同位素與現今相比可反映500~300B.P.台南夏季降雨較現今弱，冬季降雨較現今強。

To exam whether oxygen isotope records of Crassostrea gigas oysters can be used as proxies of environment, 135 cultivated oysters and 21 water samples were collected from Chi Ku area, Tainan City, southern Taiwan in December of 2012, and between March, 2013 and July, 2014. Instrumental air and water temperatures and precipitation records were obtained from a nearest Central Weather Bureau (CWB) station about 16 km north of Chi Ku. The oxygen and carbon isotope values of the ligamental area of the modern oyster shells are from -6.92‰ to -0.08‰ (-3.05 ± 1.17‰, N = 2310; 1σ; VPDB) and from -5.57‰ to 0.63‰ (-1.88 ± 0.81‰), respectively. Oxygen isotope values of the water samples are mainly between -0.28‰ and 0.74‰ (0.18 ± 0.29‰, N = 20; 1σ; VSMOW). However, water oxygen isotope value of -2.75‰ was observed for the water sample collected immediately after a typhoon heavy rainfall .Modern oyster shells present 0.75~3.5 years of seasonal fluctuation pattern. And the pattern is similar to that of CWB instrumental records. 70% values of modern shells are consistent with the predicted δ18O values which calculated from the isotopic equilibrated equation. We analyzed 5 archaeological oyster shells of Siraya culture (500~300B.P.) collected from Wu Chien Tuso North (WCTN) archaeological site of Tainan Science Park to infer the harvest season of mollusks. Oxygen isotope values of the ligamental area of the archaeological oyster shells are between -5.98‰ and -1.26‰ (-3.34 ± 1.37‰, N = 60; 1σ), and carbon isotope values are between -3.21‰ and 0.60‰ (-2.04‰ ± 0.55‰). The oxygen isotope records of archaeological oyster shells also showed clear seasonality. Most of the oysters were collected in autumn and winter. Oxygen isotope values of archaeological oyster shells was 1‰ greater than that of present for summer whereas was 1.2‰ less than that of present for winter. The climate of Global was during the Little Ice Age 500 years ago and it was roughly 0.3~0.9℃ cooler than present. The influence of temperature on oxygen isotope could be negligible and the oxygen isotope of archaeological shells mainly reflect the change of precipitation between past and present. The oxygen isotope of archaeological shells indicate that the precipitation was weaker in summer but heavier in winter in southwestern Taiwan 500-300 B.P.