中國湖北地區晚古生代腕足動物化石殼體之穩定碳氧同位素紀錄與古環境

Late Paleozoic Environment Inferred from Stable Carbon and Oxygen Isotope Records of Brachiopod Shells from Hubei, China

蔡仲元(Jung-Yuan TSAI)

指導教授 ： 米泓生

國立臺灣師範大學/理學院/地球科學系/碩士(2012年)

摘要

本研究採集114個來自中國湖北省晚石炭紀及晚二疊紀的腕足動物化石，分析其穩定碳氧同位素組成，以重建華南地區晚古生代的環境特徵。
化石標本經過整理及拍照後，灌膠固定並製作成岩石薄片。再使用透射光顯微鏡觀察其殼體微細構造，並以陰極射線顯微鏡觀察，判斷化石殼體是否經成岩作用影響。結果顯示部分腕足動物殼體及圍岩在陰極射線下會呈現發藍光或是微弱藍光的現象，再經分析其REEs(Rare Earth Elements)組成，推論可能經過熱液作用，致使其發出藍光。
本研究共分析321個樣品的碳氧同位素成份，保存最好(不發光)的腕足殼體的δ13C及δ18O平均值分別為5.0±2.0‰、-4.5±0.7‰ (N=63)，而在陰極射線下顯示受成岩作用影響(發光)的腕足動物殼體與圍岩的δ18O平均值分別為-6.6±1.4‰ (N=26)與-6.4±1.1‰ (N=30)。由於不發光部位之氧同位素數值顯著大於發光部份之氧同位素數值，可見屬於不發光部份的腕足動物殼體並未受到成岩作用影響，應可用於探討古環境。
除了Kungurian期的碳同位素較輕外(δ13C = 1.7±0.3‰)，其餘的碳同位素紀錄皆偏重(2.9~7.5‰)，顯示華南地區應有較高的生產力。晚石炭紀的氧同位素紀錄為-4.4±0.3‰ (N=6)，而二疊紀Kungurian期的δ18O值為-4.5±0.2‰ (N=4)、Roadian至Capitanian期為-4.2±0.5‰ (N=40)、Wuchiapingian期為-4.2±0.6‰ (N=2)、Changsingian期為-4.6±0.1‰ (N=11)；若假設二疊紀當時海水的氧同位素數值為-1‰ (全球無大陸冰川狀態)，計算出的表層海水氧同位素溫度為32.5±0.6°C，反映出華南地區位於熱帶地區海水溫度高的特性。而Kungurian至Capitanian期的氧同位素數值比同時期烏拉爾(Ural)的標本還要來的小，顯示溫度較高，符合古地理所重建之當時華南陸塊位於古赤道地區熱帶氣候的特性。而華南地區的氧同位素數值比同時期低緯度地區的美國中部(Guadalupe)地區的氧同位素數值輕，反映出美國中部地區為蒸發效應強烈的乾燥環境，但華南陸塊當時位於開放性大洋，而有高溫多雨的氣候環境。
本研究所紀錄的晚二疊紀華南地區海水溫度較高，且有類似現今西太平洋暖池的現象。從腕足殼體沿著生長方向的氧同位素數值變化來看，Roadian至Capitanian期的氧同位素溫度之最高溫及最低溫相差約攝氏4.6~6.1度，至Changsingian期則只相差攝氏了0.9度，反映了當時隨著全球溫度上升，季節性溫差變得較小的特性。

關鍵字

晚古生代 ； 穩定碳氧同位素 ； 腕足動物 ； 中國湖北省

並列摘要

One hundred and fourteen brachiopod shells collected from Hubei,China were analyzed for the stable carbon and oxygen isotope records to infer the tropicalpaleoenvironment for Late Paleozoic. All samples were made into thin sections and were examined for shell preservation under the petrographic and cathodoluminescence(CL) microscopes. Two samples were selected to analyze REEs contents for further evaluation of shell preservation due to the blue light observed under CL. The REEs results show that the blue light might be related to hydrothermal
diagenesis. Three hundred and twenty-one isotopic analyses were performed.Average stable carbon and oxygen isotope of nonluminescent shells(N=63) are 5.0±2.0‰ and -4.5±0.7‰, respectively. Mean δ18O value is-6.6±1.4‰ (N=26) for luminescent shells and is -6.4±1.1‰ (N=30) for luminescent matrix. Therefore, average nonluminescent shell δ18O value
is greater than those of luminescent shells and matrix indicating that the nonluminescent shells were not altered by diagenesis.
IV Except for Kungurian (δ13C = 1.7±0.3‰), carbon isotope values of South China are between 2.9 and 7.5‰, greater than those of contemporary records from other regions. More positive values in carbon isotope records may indicate higher productivity in South China during Late Permian.
Average oxygen isotope values are -4.4±0.3 ‰ (N=6) for late
Carboniferous, -4.5±0.2‰ (N=4) for Kungurian, -4.2±0.5‰ (N=40) for Roadian to Capitanian , -4.2±0.5‰ (N=2) for Wuchiapingian, and -4.6±0.1‰ (N=11) for Changhsingian. Average oxygen isotope values are less than those of mid-continent and Ural in Permian. Assuming the seawater oxygen isotope was -1‰, the calculated average sea surface
temperature was 32.5±0.6°C for Late Permian. This warm sea surface
temperature of South China indicating that modern Western Pacific Warm Pool sea surface temperature pattern was also existed in Late Permian. Based on the oxygen isotope record within in single shells, seasonal temperature fluctuations in South China were significant (4.6~6.1°C) for Roadian to Capitanian but insignificant (0.9°C) in South China for Changhsingian.

並列關鍵字

Paleozoic ； stable carbon and oxygen isotope ； brachiopods ； Hubei

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