以錐蜷殼體之穩定碳氧同位素及元素成分探討古環境—以斯里蘭卡Bolgoda湖泊為例

Abstract

本研究分析斯里蘭卡Bolgoda湖泊現生及湖泊岩芯(長度410 cm；取樣深度9 cm-253.5 cm)中的霰石質軟體動物殼體，進行穩定碳氧同位素以及微量元素成分分析來探討斯里蘭卡地區近2400年以來的環境特徵。 現生標本氧同位素數值介於-6.6‰和-1.7‰ 之間，（平均值為-3.2 ± 0.9‰，N=74；1σ；V-PDB）。岩芯中標本氧同位素數值介於-6.9‰和-1.8‰ 之間，（平均值為-4.0 ± 1.1‰，N=352；1σ；V-PDB）。將推算後的水體氧同位素數值與可倫坡測站30年平均氣溫資料帶入同位素溫度方程式計算出標本一年間的氧同位素理論平衡值，部分標本氧同位素記錄與理論平衡值呈現相似的季節性震盪變化，標本氧同位素數值約可呈現6個月至2年之季節性震盪記錄。 現生標本及岩芯標本的元素分析結果顯示Al/Ca、Fe/Ca、Mn/Ca比值均接近於偵測極限（0.3 mmol/mol），進一步顯示本研究標本的保存度良好，未受成岩作用影響。Bolgoda湖泊水體混和均勻，水體鹽度大致介於2 ppt~ 20 ppt之間，湖水鹽度受到海水及天水影響，理論上水體（殼體）氧同位素數值可以反映淡水注入比例。若殼體Na/Ca 比值的變化主要受到鹽度的影響，標本氧同位素數值較大或較小值時伴隨著較高或低的Na/Ca 比值，進一步反映了乾季及雨季降雨量的差異。 季節性震盪中，現生標本及岩芯標本，於乾季之氧同位素數值多在-2‰左右；而雨季時現生標本約為-4‰；岩芯標本則約為-6‰。假設斯里蘭卡地區近3000年來溫度變化小，因此忽略溫度變化對標本氧同位素數值之影響，則現生標本及岩芯中標本相比可反映2340-190 (cal yr BP)斯里蘭卡西南部雨季降雨較現今強，亦表示較強烈的印度洋西南季風時期；而乾季降雨的降雨量變化不大。

To exam whether stable isotope and element composition records of thiaridae shells can be used as proxies of environment and to unravel the paleoenvironment, this study analyzed 10 shells collected from a core( ~2400 yr B.P. to present ), drilled in Lake Bolgoda and 4 modern samples collected from lakefront of Lake Bolgoda, Sri Lanka. The oxygen and carbon isotope values of modern shells are from -6.6‰ to -1.7‰ ( -3.2 ± 0.9‰, N = 74; average ± 1σ; V-PDB ) and from -11.3‰ to -1.7‰ ( -4.4 ± 2.6‰ ), respectively. Oxygen and carbon isotope values of the core shells are from -6.9‰ to -1.8‰,（-4.0 ± 1.1‰, N = 352）and from -13.0‰ to -1.0‰ ( -5.6 ± 3.1‰ ) , respectively. Compare shell oxygen isotope values with those of predicted values, the seasonal fluctuation pattern is similar to each other and 0.5~2.0 years of seasonal fluctuations are observed. Most of the Al/Ca, Fe/Ca, Mn/Ca values of gastropod shells are low or below the detection limit indicate the samples studies are not altered by diagenesis thus well preserved. Assuming Na/Ca contents of gastropod shells were mainly influenced by the Na/Ca content of water which these gastropods lived in, higher Na/Ca of shells may indicate higher salinity ( low amount precipitation ) of lake water. Thus, Na/Ca of shells can be used to infer the intensity change of Indian Ocean SW monsoon. Seasonality of core shells were more variable, in terms of difference in δ18O values, than those of modern shells indicating ~2400 years B.P. southwestern Sri Lanka was influenced by stronger Indian Ocean southwest monsoon and received more rainfall then present.