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Toarcian oceanic anoxic event: An assessment of global causes using belemnite C isotope records

机译：Toarcian海洋缺氧事件：使用贝粒石C同位素记录评估全球原因

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摘要

Two hypotheses have been proposed to explain simultaneous large negative excursions (up to 7% PeeDee belemnite) in bulk carbonate (delta(13)C(carb)) and organic carbon isotope records (delta(13)C(org)) from black shales marking the Toarcian oceanic anoxic event (T-OAE). The first explanation envisions recycling of dissolved inorganic carbon (DIC) with a light isotopic signature into the photic zone from the lower levels of a salinity-stratified water mass, essentially requiring a regional paleoceanographic driver of the carbon cycle. The second involves the rapid and massive dissociation of methane from gas hydrates that effectively renders the T-OAE a global perturbation of the carbon cycle. We present C isotope records from belemnites (delta(13)C(bel)) sampled from two localities, calibrated with high-resolution ammonite biostratigraphy and Sr isotope stratigraphy, in Yorkshire (England) and Dotternhausen (Germany), that can be used to assess which model best explains the observed changes in carbon isotopes. Our records of the delta(13)C composition of belemnite calcite do not show the large negative C isotope excursions shown by coeval records of delta(13)C in sedimentary organic matter or bulk sedimentary carbonate. It follows that isotopically light carbon cannot have dominated the ocean-atmosphere carbon reservoir during the Toarcian OAE, as would be required were the methane release hypothesis correct. On the basis of an evaluation of available carbon isotope records we discuss a model in which the recycling of DIC from the deeper levels of a stratified water body, and shallowing of anoxic conditions into the photic zone, can explain all isotopic profiles. In particular, the model accounts for the higher C isotope values of belemnites that are characteristic of open ocean, well-mixed conditions, and the lower C isotope values of neritic phytoplankton communities that recorded the degree of density stratification and shallowing of anoxia in the photic zone.
机译：提出了两个假设来解释黑色页岩中大量碳酸盐（delta（13）C（carb））和有机碳同位素记录（delta（13）C（org））的同时较大的负偏移（高达7％PeeDee贝伦石）标志着Toarcian海洋缺氧事件（T-OAE）。第一个解释设想将具有轻同位素特征的溶解的无机碳（DIC）从盐度分层水团的较低水平再循环到光化带，这基本上需要碳循环的区域古海洋学驱动力。第二个问题涉及甲烷与天然气水合物的快速大量解离，这有效地使T-OAE成为碳循环的整体扰动。我们介绍了从两个地方取样的贝甜体（del（13）C（bel））的C同位素记录，并用约克夏（英格兰）和Dotternhausen（德国）的高分辨率炸药生物地层学和Sr同位素地层学进行了校准。评估哪种模型最能解释所观察到的碳同位素变化。我们对贝莱石方解石的delta（13）C组成的记录没有显示出同时代记录的沉积有机质或大量沉积碳酸盐中的del（13）C所显示的大的负C同位素偏移。由此可见，在Toarcian OAE时期，同位素轻碳不可能主导海洋-大气碳储层，因为甲烷释放假说是正确的。在对可用碳同位素记录进行评估的基础上，我们讨论了一个模型，在该模型中，DIC从更深的分层水体中循环以及缺氧条件向光化学带的变浅可以解释所有同位素特征。特别是，该模型说明了开放海洋，充分混合条件下特征的贝类动物的较高C同位素值，以及记录了光合层中密度分层和缺氧浅度的浅浮游植物群落的较低C同位素值。区。

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