以腕足動物化石殼體穩定碳氧同位素紀錄探究西澳洲二疊紀古環境

Abstract

本研究分析了112個採自西澳洲Carnarvon盆地與Canning盆地二疊紀腕足動物殼體，來重建二疊紀中緯度地區的古環境。殼體均沿著喙部至腹部切割製成薄片，並以岩石薄片顯微鏡觀察其微細構造的保存狀況，同時也觀察薄片在陰極射線的照射下是否有因成岩作用而產生的發光現象，檢查殼體保存狀況。本研究共有249筆數據取自保存良好，不發光(non-luminescent; NL)的部份可供古環境之重建。 保存良好之平均碳同位素分布在由老到輕的地層分別為Callytharra Fm. (CF:4.4±0.5‰, N=28)，Jimba Jimba Calcarenite (JJM:4.5±0.8‰, N=4)，Coyrie/Madeline Fm. (C&MF:4.7±0.8‰, N=10)，Quinnanie Shale/Cundlego Fm. (Qsh:4.9±0.6‰, N=5)，Wandagee Fm. (WF:4.2±0.7‰, N=9)；Noonkanbah Fm. (NF:5.1±1.1‰, N=6)，Lightjack Fm. (LF:5.4±0.2‰, N=2)，Hardman Fm. (HF:4.8±0.3‰, N=3)，與同時期低緯度的盤古東邊副熱帶地區烏拉爾山的數值較為接近，碳同位素趨勢相當，較不同於美國大陸的數值，低於東澳洲的數值，顯示盤古大陸東邊副熱帶地區與古特提斯海地區有著相似的海水循環條件。 氧同位素數值根據地層排列依序為Callytharra Fm. (-0.1±0.5‰, N=28)，Jimba Jimba Calcarenite (-0.1±0.5, N=4)，Coyrie/Madeline Fm. (-0.2±0.4‰, N=10)，Quinnanie Shale/Cundlego Fm. (-0.1±0.4‰, N=5)，Wandagee Fm. (-0.5±0.7‰, N=9)；Noonkanbah Fm. (0.1±0.5‰, N=6)，Lightjack Fm. (-0.6±0.9‰, N=2)，Hardman Fm. (-0.0±0.3‰, N=3)。西澳洲腕足動物殼體的氧同位素數值均大於同時期低緯度及東澳洲的紀錄，顯示西澳洲於二疊紀時期的蒸發效應較為強烈。

To construct the middle paleolatitude environment in the Permian. One hundred and twelve brachiopod shells from two Western Australian basins (Carnarvon and Canning Basins) have been analyzed for oxygen and carbon isotopes to study. All samples were thin sectioned and examined under the petrographic and cathodoluminescence microscopes for evaluating shell preservation. A total of 249 isotopic analyses from well preserved portion (non-luminescent; NL) were found useful for inferring the paleoenvironment. Average δ13C values of NL shells were 4.4 ± 0.5‰ (N = 28) for the Callytharra Formation (late Sakmarian-early Artinskian), 4.5 ± 0.8‰ (N = 4) for the Jimba Jimba Calcarenite (early Artinskian), 4.7± 0.8‰ (N = 10) for the Coyrie/Madeline Formations (both late Artinskian-early Kungurian), 4.9 ±0.6‰ (N =5) for the Quinnanie Shale and Cundlego Formation (both Kungurian), 4.2 ± 0.7‰ (N = 9) for the Wandagee Formation (late Kungurian), 5.1 ± 1.1‰ (N = 6) for the Noonkanbah Formation (Artinskian- Kungurian), 5.4 ± 0.2‰ (N =2) for the Lightjack Formation(Roadian-Capitanian), and 4.8± 0.3‰ (N = 3) for the Hardman Formation (Wuchiapingian). Within the uncertainty of the stratigraphical correlation, the carbon isotope values spanning Late Sakmarian to Kungurian are comparable to those of low latitude Urals. Average δ18O values of NL shells were about 0‰ (-0.2 to 0.2‰) for Callytharra Formation(-0.1±0.5‰), Jimba Jimba Calcarenite(-0.1±0.5), Coyrie/Madeline Formations(-0.2±0.4‰), Quinnanie Shale and Cundlego Formation(-0.1±0.4‰), Noonkanbah Formation(0.1±0.5‰), and Hardman Formation(-0.0±0.3‰); and were -0.5 ± 0.7‰ and -0.6± 0.9‰ for Wandagee Formation and Lightjack Formation, respectively. These oxygen isotope values are overall greater than the coeval values reported for low latitude regions and for eastern Australia. These relatively more positive oxygen isotope values are here interpreted to indicate a possible high evaporation condition for the two studied basins in Western Australia during the Permian.