中國保山地區晚古生代腕足動物化石穩定碳氧同位素成分及其古環境意義

Abstract

中國大陸雲南省保山地塊在晚古生代時位於南半球溫帶地區，本研究採集此地區晚古生代地層中的腕足動物化石，期藉由分析保存良好的腕足化石殼體碳氧同位素成分以重建此地區的古海洋環境。本研究共分析27個早石炭紀及43個二疊紀腕足標本，將所有標本灌膠固定後，自喙部往前端依最長殼長方向切開製成薄片，以透射光及陰極射線顯微鏡分別觀察腕足殼體微細構造及發光分佈情形；另外也挑選7個早石炭紀的腕足標本以電子微探針分析殼體的元素含量（包括Si、Al、Fe、Mn、Na、S、Sr、Mg、Ca等）以進一步確認標本保存狀態。 我們根據陰極射線下不發光且殼體Si、Al、Fe、Mn等元素含量低於偵測極限（0.3 mmol/mol Ca）的標準辨別出未受成岩作用影響而保存良好的早石炭紀腕足殼體，其Na、S、Sr、Mg的含量則分別為0.1~10.8、0.2~15.0、0.1~1.9、0.6~8.2 mmol/mol Ca（N=17），與前人分析古生代或現生腕足的殼體數值範圍大致相符。保存良好的早石炭紀Tournaisian腕足殼體δ18O及δ13C平均值分別為-1.8 ± 0.7‰、3.3 ± 0.4‰（1σ，N = 35）；而Visean的δ18O及δ13C平均值則分別為-1.8 ± 1.1‰、2.7 ± 0.6‰（1σ，N = 12）。由於早石炭紀地球應處於兩極無大規模冰川發育的環境，故假設全球海水氧同位素數值約-1.0‰，配合保山地區氧同位素記錄換算南緯30度附近冬季海水溫度約為15~17℃左右；與前人研究赤道地區δ18O紀錄比較也呈現約2~8℃的溫差，反映出保山地區於晚古生代位於南半球較高緯度的低溫特性。 保山地區二疊紀的腕足化石殼體則普遍保存較不理想，故僅討論較不受成岩作用影響的碳同位素紀錄，早二疊紀Sakmarine的δ13C平均值為3.5 ± 0.4‰（1σ，N = 6）；二疊紀中期Wordian為4.9 ± 0.7‰（1σ，N = 18）。保山地區晚古生代腕足殼體的碳同位素數值範圍與其他地區的數值大致符合，且有相似的變化趨勢。同時保山地區早二疊紀腕足δ13C值較早石炭紀Visean時期重約1‰，亦可反映出如前人推測之石炭紀中期全球有機碳大量埋藏的事件。This study constructed the Late Paleozoic environment of southern mid latitude by analyzing the stable isotope compositions of brachiopod shells. We collected 27 Early Carboniferous samples and 43 Permian samples from Baoshan Block, Yunnan, China. All samples were cut, thin-sectioned, and examined under plain light and cathodoluminescence for shell preservation. In addition, 7 Early Carboniferous samples were selected to measure the element contents for further evaluation of shell preservation. Only 13 out of 70 samples were determined well preserved. None of the Permian samples were well preserved to provide Permian δ18O records. However, because the carbon isotope of carbonate was more resistant to diagenesis, the Permian δ13C records still indicated environmental condition. Mean δ13C values of well preserved brachiopod shells are 3.3 ± 0.4‰ (1σ, N = 35), 2.7 ± 0.6‰ (N = 12), 3.5 ± 0.4‰ (N = 6), and 4.9 ± 0.7‰ (N = 18) for Tournaisian, Visean, Sakmarine, and Wordian, respectively. The overall δ13C values of Permian are roughly 1‰ greater than those of Early Carboniferous. Enrichment in δ13C values may correspond to the increased burial rate of organic matter as proposed by previous studies. Discussions of δ18O records are limited to Early Carboniferous only. The δ18O values of well preserved samples are respectively -1.8 ± 0.7‰ (1σ, N = 35) and -1.8 ± 1.1‰ (N = 12) for Tournaisian and Visean. Compared to δ18O values of tropic regions, the latitudinal temperature difference was about 3~9℃ for Tournaisian and 3~5℃ for Visean. These latitudinal temperature differences were smaller than that of present. Therefore, the Tournaisian and Visean were most likely under an environment without or with little glaciers. Assuming the δ18O of Early Carboniferous seawater was -1.0‰, the winter sea water temperature were 17℃ for Tournaisian and 15℃ for Visean near 30∘S.