台灣恆春半島現生蠑螺與墾丁遺址蠑螺口蓋穩定碳氧同位素之環境意義

Abstract

本研究分析恆春半島沿海採集的八個現生蠑螺標本(圓蠑螺、金口蠑螺和台灣蠑螺)，以及墾丁考古遺址的五個口蓋標本(銀口蠑螺)的穩定碳氧同位素組成，探討蠑螺口蓋與殼體之間於碳氧同位素上的關係和現生蠑螺口蓋與殼體的氧同位素成份與其生活海域環境之關連性，並進一步解析墾丁考古遺址的古生態環境。

恆春地區現生蠑螺殼體的13C值介於0.38~2.10‰之間(平均值為1.46±0.30‰，N=365；1σ)、口蓋13C值則在0.65~2.80‰之間(1.76±0.44‰，N=482)。殼體的δ18O值於-2.40~-0.14‰間(-1.54±0.51‰)、口蓋的18O值則在-2.40~0.25‰間(-1.40±0.53‰)。八個現生蠑螺標本中，殼體、口蓋的13C值有七個標本存在明顯的差異，僅有一個金口蠑螺差異甚小；18O值則在兩個圓蠑螺與兩個金口蠑螺出現顯著差異，三個金口蠑螺與一個台灣蠑螺則差異甚小。根據氧同位素數值變化情形顯示，現生蠑螺的生命期約在一年至一年半之間。依據中央氣象局鵝鑾鼻測站的海溫記錄，並代入海水的氧同位素數值(夏季為0‰、冬季為0.14‰)，估計在該環境下達成平衡之霰石氧同位素數值應介於-2.65‰至0.06‰之間，因為殼體、口蓋的18O值多介於此一範圍內，故推論其與水體的18O數值大約可達到平衡，因此蠑螺殼體及口蓋的氧同位素大致可反映海溫季節性的變化。

墾丁考古遺址銀口蠑螺口蓋的18O值在-2.71~-0.28‰之間(-1.54±0.58‰，N=266)，假設4000 B.P.海水的氧同位素數值與今日相似，則恆春半島大約在距今4千年前的溫度估計在21.5~31.5℃之間，比現今溫暖(19.6~29.7℃；2003年1月~2006年12月)。由氧同位素振幅所推論的季節性的變化，當時該蠑螺的採集多在冬春兩季。遺址蠑螺口蓋的13C值在2.45~5.83‰之間(3.33±0.49‰)，整體較現生口蓋的13C值約高1.57‰，顯示當時恆春半島海域營養鹽含量或水團性質與今日有所差別。

To examine when the 18O values of Turban snail shells and opercula reach the isotopic equilibrium with ambient seawater and unravel the paleoenvironment of ~4,000 B.P. in southern Taiwan, this study analyzed the stable isotopic compositions of eight alive Turban snails (two Turbo setosus, five T. chrysostomus and a T. sparverius) collected from Hengchun Peninsula and five opercula of T. argyrostoma collected from Kenting site, Hengchun.

The 13C values of modern snail shells and opercula are between 0.38 and 2.10‰ (1.46±0.30‰; 1σ; N=365) and 0.65 and 2.80‰ (1.76±0.44‰; 1σ; N=482), respectirely. The 18O values of modern snail shells and opercula are between -2.40 and -0.14‰ (-1.54±0.51‰) and -2.40 and 0.25‰ (-1.40±0.53‰), respectirely. 13C values of seven modern opercula are significant different from those of complimentary shell. Based on the sea water temperature from O-luan-pi buoy and measured 18O of seawater (18O = 0‰ SMOW for summer and 18O = 0.14‰ SMOW for winter), the 18O values of aragonite precipitated in equrlibrium should be between -2.65‰ and 0.06‰. Because most of the 18O values of modern Turban snail shells and opercula are within this range, the 18O values of modern Turban snail shells and opercula are in apparently isotopic equibrium with the seawater they lived in.

The 18O values of the five archaeological T. argyrostoma opercula are between -2.71 and 0.28‰ (-1.54±0.58‰; 1σ; N=266). Assuming the 18O values of seawater was simlar to that of present, the calculated oxygen isotope temperature is between 21.5℃ and 31.5℃ for ~4,000 B.P. and was roughly 2℃warmer than present. Based on the 18O records, most of the archaeological snails were collected in winter and spring. 13C values of the archaeological opercula (3.33±0.49‰) are 1.57‰ greater than those of modern opercula suggesting that the hydrological condition was different from present.