台灣恆春半島現生與考古遺址芋螺殼體穩定碳氧同位素所反映之環境記錄

Abstract

本研究分析2008年於恆春半島大光里之潮間帶採集海水標本及現生芋螺標本3枚，墾丁考古遺址標本2枚、鵝鑾鼻第二遺址標本1枚及龜山遺址標本1枚之碳氧同位素組成，探討芋螺遺骸標本所反映之古環境之意義。 現生標本δ18O 數值介於-3.26~-0.72‰ (平均-1.99±0.50‰；1σ，N = 447)、δ13C數值介於-0.14~2.35‰(1.18±0.52‰)之間。其氧同位素值分別呈現6年至9年半之季節性振盪。大光里地區水體氧同位素六個月份平均數值(-0.06±0.23‰)，將現生殼體及水體之氧同位素數值代入氧同位素溫度方程式估算，夏季殼體之氧同位素溫度平均大約為30.6°C，比中央氣象局鵝鑾鼻測站之夏季溫度(28.4°C)略高2°C左右。可能與夏季雨量較多有關。殼體氧同位素之冬季溫度約為24.1°C，與測站冬季均溫(24.2°C)極為相近。但在冬季時，殼體有時可能生長中斷，而無法記錄到較冷的環境。 墾丁遺址(~4000B.P.)之δ18O數值介於-2.70~-0.62‰之間(-1.51±0.04‰，N = 371)；δ13C數值介於0.53~2.79‰之間(2.00±0.04‰)。鵝鑾鼻第二遺址(~3250B.P.)δ18O數值介於-2.36~-0.51‰之間(-1.60±0.44‰，N = 188)之間；δ13C數值介於0.68~2.96‰之間(2.11±0.31‰)。龜山遺址(~1500B.P.)δ18O數值介於-3.21~-0.72‰之間(-2.22±0.55‰，N = 178)；δ13C數值介於1.63~2.87‰之間(2.36±0.30‰)。氧同位素數值分別呈現出3年以上之季節性變化，而碳同位素與現生標本趨勢呈一致性，隨著個體成長，生長速率漸趨緩慢而逐漸變小。 由於4000年來全球冰川體積變化不大，本研究以-0.06‰為當時海水氧同位素數值代入計算，估算當時的氧同位素溫度。在4000年前，年均溫約為26°C，冬季均溫為23.6°C；3250年前年均溫為26.4°C，冬季均溫為23.3°C； 1500年前年均溫為29.1°C，冬季均溫為26.1°C。就大趨勢而言，顯示大約4000年前與3250年前較現今略為寒冷，而1500年前則比現今溫暖。

In order to understand the paleoenvironment of southern Taiwan 4000 B.P., stable isotope composition of Conus shells collected from archaeological sites were analyzed. For comparison, two living Conus shells and sea water samples which they lived in were also collected from Hengchun Peninsula, Taiwan. The δ18O and δ13C values of modern samples are between -3.26‰ and -0.72‰ (-1.99±0.50‰；mean±1σ，N = 447) and between -0.14‰ and 2.35‰ (1.18±0.52‰), respectively. Oxygen isotope record of the modern Conus shells showed six to nine and half seasonal cycles. Six month mean seawater oxygen isotope value is -0.06±0.23‰ (N=42). Modern summer oxygen isotope temperatures (28.4°C) are roughly 2°C higher than those recorded in CWB Eluanbi buoy whereas the winter temperatures (24.1°C) are comparable to those of instrumental records. Deviation in summer temperatures might be due to mixing of seawater and heavier precipitation thus lower δ18O values of the mixture water in summer. Occasionally, minimum winter temperature was not recorded in the shell carbonates possibly due to stop growth of Conus. The δ18O and δ13C values of Kengting site specimens (~4000B.P.) are between -2.70 and -0.62‰ (-1.51±0.04‰，N = 371) and between 0.53 and 2.79‰ (2.00±0.04‰), respectively. The δ18O and δ13C values of O-Luan-Pi II site samples (~3250B.P.) are between -2.36 and -0.51‰ (-1.60±0.44‰，N = 188) and between 0.68 and 2.96‰ (2.11±0.31‰), respectively. The δ18O and δ13C values of Kueishan site samples (1500B.P.) are between -3.21 and -0.72‰ (-2.22±0.55‰，N = 178) and between 1.63 and 2.87‰ (2.36±0.30‰). These δ18O pattern showed at least three year seasonal cycles. The δ13C fluctuation patterns of archaeological Conus shells were similar to those of modern ones. Both the amplitude in δ13C records decreased with ontogenic growth. We assumed the δ18O of 4000B.P. seawater was -0.06‰ because there was no major eustatic sea level change during this period of time. Mean annual temperature (MAT) was approximately 26°C and the mean winter temperature (MWT) was 23.6°C in 4000B.P.. In 3250 years ago, the MAT was 26.4°C and the MWT was 23.3°C. The MAT was 29.1°C and MWT was 26.1°C in 1500B.P.. The temperature of 4000 and 3250 years ago were colder than that of the present, whereas the temperature in 1500 years ago, was warmer than that of present.