貴州雅水剖面石炭紀腕足動物碳氧同位素紀錄之環境意義

Abstract

本研究採集了395個中國貴州省雅水剖面的腕足動物殼體化石殼體，分析其中139個化石標本之穩定碳氧同位素成分來探討華南地塊於石炭紀時期熱帶地區的環境變化。 雅水剖面在早石炭紀地層分為舊司階(早Visean)、上司階(晚Visean)和德塢階(Serpukhovian)，本研究自這139個殼體中共取了369個標本點進行碳氧同位素分析，但僅有89個標本點的數據取自保存良好的部分。舊司階的殼體皆保存不理想；上司階(晚Visean)保存良好殼體的δ18O和δ13C平均值分別為-3.8±0.7‰（1σ，N=52）和2.3±2.4‰；德塢階(Serpukhovian) 保存良好殼體的δ18O和δ13C平均值分別為-3.0±0.5‰（1σ，N=37）和4.1±2.0‰，從上司階到德塢階的δ13C平均值增加1.8‰，可能與碳埋藏量增加有關。 假設早石炭紀海水的δ18O數值為-1‰ (SMOW)。上司階的氧同位素溫度為24~30°C平均（平均值28°C），而在德塢階的氧同位素溫度範圍介於23~26°C之間(平均值25℃)，由上司階(晚期Visean)至德塢階(Serpukhovian)平均溫度呈現約下降3°C的現象，與Mg/Ca比值所計算之溫度變化大致相同。上司階(晚期Visean)至德塢階(Serpukhovian)的降溫與碳同位素數值所呈現的碳埋藏量增加，二氧化碳含量降低的紀錄一致。雅水剖面所呈現之平均值28°C至25°C的溫暖溫度範圍反映出華南地區於石炭紀位於熱帶地區海水溫度較高的特性，類似現今西太平洋暖池的溫度特徵。

This study constructed the Carboniferous tropical environment of Yashui section, Guizhou, South China by analyzing the stable isotope compositions of brachiopod shells. A total of 395 fossil brachiopod shells were collected. One hundred and thirty nine samples were cut, thin-sectioned, and examined under plain light and cathodoluminescence for evaluation of shell preservation. Only 89 out of 369 carbonate powder samples were determined well preserved. In ascending order, mean δ18O values of well preserved brachiopod shells are -3.8 ± 0.7‰ and -3.0±0.5‰ for Shangsian Stage (Late Visean) and Dewaun Stage (Serpukhovian), respectively. Mean 13C values of well preserved brachiopod shells are respectively 2.3 ± 2.4‰ (1σ, N = 52) for Shangsian Stage (Late Visean) and 4.1±2.0‰ (1σ, N = 37) for Dewaun Stage (Serpukhovian). Enrichment in δ13C value of 1.8‰ between Shangsian Stage and Dewaun Stage may correspond to the increased burial rate of organic matter as proposed by previous studies. Assuming the δ18O of Early Carboniferous seawater was -1.0‰, the sea water temperature were 24～30°C (average 28°C) for Shangsian Stage and 23～26°C (average 25°C) for Dewaun Stage. The 3°C decrease in temperature from Shangsian Stage to Dewaun Stage was consistent with the temperature estimation based on the Mg/Ca ratios. The Late Visean to Serpukhovian cooling may be caused by the decrease in atmospheric CO2 concentration as indicated by the increase in carbon isotope value. The overall warm tropical sea surface temperature of South China indicating that modern Western Pacific Warm Pool sea surface temperature characteristic may have existed in Early-Middle Carboniferous.