二疊紀高緯度地區的古環境—澳洲雪梨盆地與塔斯曼島腕足動物化石穩定同位素紀錄

Abstract

本研究利用澳洲東南部雪梨盆地與塔斯曼島，早至中二疊紀地層中腕足動物化石的穩定碳、氧同位素成分來重建南半球高緯度地區的古環境。藉由分析Sakmarian期的Wasp Head Formation (WH)、晚Artinskian期的Pebbly Beach Formation (LoUPB、HiUPB)、Kungurian期的Snapper Point Formation (SP)、晚Kungurian期的Wandrawandian Formation (出露地點Warden Head；WarH)、Roadian期的Wandrawandian Formation (出露地點Penguin Head；PH)、Wordian期的Broughton Formation (BH)以及位於塔斯曼島Artinskian階的Berriedale Limestone (BL)共92個標本，由喙部向最長殼長方向切開製作薄片，觀察其微細構造的保存狀況，並以陰極射線顯微鏡觀察殼體發光分佈情形，評估標本是否受到成岩作用影響。 本研究由二疊紀腕足動物和二枚貝殼體取樣分析共638個分析點，其中401個分析點取自保存良好、不發光(NL)部分，其穩定碳、氧同位素數值可用來重建古環境。另外由標本中挑選20個腕足標本及2個二枚貝標本進行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。各標本NL部分Si、Fe、Mn的元素含量皆低於偵測極限，進一步確認NL部分為未受成岩作用影響而保存良好的腕足殼體部分。 東澳保存良好的平均碳同位素數值(NL)皆大於5‰，與盤古大陸東邊的紀錄有相似的趨勢，和盤古大陸西邊則呈現完全不同的震盪，表示盤古大陸東南部的海域與古地中海有相似的海水循環。最高值的碳同位素7‰位於晚Kungurian期的下部Wandrawandian地層，可能受到湧升流系統的影響。 早至中二疊世在高緯度和低緯度間有氧同位素差值受冰期的影響很明顯。大部分東澳保存良好的平均氧同位素數值(NL)都比低緯度來的大，如早 Sakmarian期(-0.3‰)、晚Artinskian期早期 (-1.1‰)、晚Kungurian期(-1.0‰)、Roadian期(-0.2‰)、Wordian期(-0.7‰)，而早Artinskian期(-2.1‰)和早Kungurian期(-2.5‰)與低緯度地區氧同位素數值相近，晚Artinskian期晚期 (-3.9‰)則低於西盤古大陸低緯度地區的氧同位素數值。假設二疊紀全球海水氧同位素數值約為-1‰，大部分地層可反映出二疊紀高緯度地區較冷的溫度(12-16°C)，而Berriedale Limestone (21°C, 早Artinskian期)、higher Upper Pebbly Beach 地層 (29°C, 晚Artinskian期晚期)、Snapper Point 地層(22°C, 早Kungurian期)與低緯度地區間的溫度梯度較弱，表示此時是屬於冰層體積減少的間冰期。

This study constructed the Early to Middle Permian paleoenvironrnent of southern high latitude by analyzing the stable isotope compositions of brachiopod shells from the Southern Sydney Basin and Tasmania , Australia. Ninety-two brachiopod and bivalve shells we collected from 7 formations (Sakmarian to Wordian) in Sydney Basin. Fossil brachiopods were collected from the Berriedale Limestone Formation (Artinskian) in Tasmania. All sample were thin sectioned and examined under the petrographic and cathodoluminescence microscopes for evaluating shell preservation. In addition, 20 brachiopod shells and 2 bivalve shells were selected to measure the element contents for further evaluation of shell preservation. A total of 638 isotopic analyses were performed. Only 401 isotopic analyses were determined micro-sampled from well preserved portions (non luminescent ; NL) to provide original environment signals. The average carbon isotope values of the NL brachiopod shells from these intervals are greater than 5‰, were comparable to those of eastern Pangea, but different from those of western Pangea. This comparability with eastern Pangea in carbon isotope records is interpreted to indicate similar oceanographic conditions and chemistry between southeastern Gondwana shelf water and that of eastern Pangea. The highest value of 7‰ at Lower Wandrawandian Siltstone (late Kungurian) indicate the possible presence of upwelling systems and thus increasing burial rate of organic matter. Difference in oxygen isotope values between high southern latitudes and low latitudes fluctuated during Early to Middle Permian. Mean oxygen isotope values of NL brachiopod shells were heavier than those of low latitudes in early Sakmarian (-0.3‰), early Late Artinskian (-1.1‰), late Kungurian (-1.0‰), Roadian (-0.2‰), and Wordian (-0.7‰); comparable to those of western Pangea in early Artinskian (-2.1‰) and early Kungurian (-2.5‰); and lighter than those of western Pangea in late Late Artinskian (-3.9‰). Assuming the oxygen isotope value was -1‰ for Permian seawater in southeast Australia, most of the calculated oxygen isotope temperatures (between 12°C and 16°C) reflected southeast Australia's high latitude cool temperature. However, the higher Upper Pebbly Beach Formation (29°C, late Late Artinskian), Berriedale Limestone (21°C, early Artinskian), Snapper Point Formation (22°C, early Kungurian) intervals were apparently relatively warmer, which may be attributed to a combination of warmer temperature and depleted seawater oxygen isotope composition in this region.