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首页> 外文期刊>Journal of the Geological Society >Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic-Jurassic boundary sections in SW Britain
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Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic-Jurassic boundary sections in SW Britain

机译：英国西南部海相三叠纪-侏罗纪界线段碳氧同位素的古环境意义

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摘要

Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric reservoirs. For the Triassic–Jurassic boundary, effectively defined by a first-order mass extinction, major fluctuations in carbon-isotope values have been well documented, but these datasets have generally been derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope data are presented from well-preserved oyster shells (Liostrea) comprising low-magnesium calcite, a mineral species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan, Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie's Bay, Somerset, England. The carbon-isotope signature from St Audrie's Bay, previously defined on the basis of analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an ’initial' negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive excursion, and (3) an extended ’main' negative isotope excursion in the highest part of the section. The data confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the study section, but there are more 13C-depleted values up-section. These lower values probably result from an admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope excursion, and increased by up to 10 °C during the main negative carbon-isotope excursion. The new results are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic Event. The Mg/Ca and Sr/Ca ratios and 18O of investigated Liostrea hisingeri show no correlation, supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by factors other than temperature.
机译：碳同位素地层学是进行地层相关性的有用工具，特别是对于影响了海洋，陆地和 期间沉积的地层>大气储层。对于由一阶质量绝灭有效定义的三叠纪-侏罗纪边界， ，碳同位素值的主要 波动已得到充分记录， 但是这些数据集通常来自bulk-rock 样本。因此，同位素 的特征在多大程度上反映了组成材料的成岩作用变化或比例变化 。在这里，碳和 氧同位素数据来自保存完好的牡蛎壳（Liostrea），该壳包含低镁方解石，矿物 相对抗成岩性改变。样本 是从威尔士格拉摩根市Lavernock Point的一个沿海 剖面中获得的，该沿海地带靠近圣奥德瑞湾侏罗纪地层的候选层型。 ，萨默塞特郡，英国。我们新的 数据证实了圣奥德里湾的碳同位素 签名，该签名先前是基于对大块有机物分析的 定义的。主要特征是（1）该部分最下部的“初始” 负同位素偏移的上部， ，其次是（2）明显的正偏移，以及（3）在该节的最高 部分中扩展了 的'主要'负同位素偏移。数据证实，先前在英格兰西南部的有机有机物 中记录的碳同位素 地层记录了同期 海水的化学成分。在英格兰的 Lyme Regis附近以相同的时间间隔采样的散装碳酸盐显示出与研究部分下部的oyster 方解石相似的趋势，但是有 更多 13 C耗尽值向上部分。这些较低的值可能是 的主要碳酸盐岩和成岩碳酸盐混合物的结果。 从 拉弗诺克角牡蛎壳的氧同位素值计算出的古温度相对较低在正碳-同位素偏移开始时 ，在主要的负碳-同位素偏移过程中增加 至10°C。 新的结果与以下观点相吻合：正碳同位素 偏移对应于低大气二氧化碳 含量的时间，而负碳同位素偏移对应于 到大气中二氧化碳含量高的时代， 也在早侏罗纪（Toarcian）大洋性 缺氧事件中被发现。 Liostrea hisingeri的Mg / Ca和Sr / Ca比值与 18 O没有相关性，支持了来自 现代双壳类动物的数据，表明掺入Mg和Sr 的含量主要受温度以外的因素控制。

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    《Journal of the Geological Society》 |2009年第3期|431-445|共15页
  • 作者

    Christoph Korte; Stephen P. Hesselbo; Hugh C. Jenkyns; Rosalind E.M. Rickaby; Christoph Sp?tl;

  • 作者单位

    Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK;

    Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK;

    Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK;

    Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK;

    Institut für Geologie und Pal?ontologie, Universit?t Innsbruck, Innrain 52, A-6020 Innsbruck, Austria;

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