Bioessential element-depleted ocean following the euxinic maximum of the end-Permian mass extinction
Bioessential element-depleted ocean following the euxinic maximum of the end-Permian mass extinction
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DOI：
10.1016/j.epsl.2014.02.041
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发表时间：
2014-05
影响因子：
5.3
通讯作者：
S. Takahashi;S. Yamasaki;Y. Ogawa;K. Kimura;K. Kaiho;Takeyoshi Yoshida;N. Tsuchiya
中科院分区：
文献类型：
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作者：
S. Takahashi;S. Yamasaki;Y. Ogawa;K. Kimura;K. Kaiho;Takeyoshi Yoshida;N. Tsuchiya
根据日本东北部暴露的最连续的二叠纪-三叠纪远洋深海边界剖面之一的沉积岩样本，我们描述了二叠纪末大灭绝期间古超大洋Panthalassa深海海底微量元素组成的变化。测量结果显示，整个剖面具有低锰（Mn）富集因子（由平均上陆地壳组成归一化）和高铈异常值，表明长兴期（晚二叠世）沉积环境已存在还原条件。其他对氧化还原敏感的微量元素（钒[V]、铬[Cr]、钼[Mo]和铀[U]）富集因子提供了从上二叠统到二叠统末期的详细氧化还原历史。在长兴期上部的岩质层中发现的单次V值增加（代表两步V值减少过程的第一次减少状态）表明，在二叠纪末大灭绝前不到100万年的时间里，形成了一个温和减少的深海环境。随后，在长兴系灰硅质粘土层上发育了Cr、V、Mo含量增加的还原性条件。在最上层的硅质粘土层和最下层的黑色粘土层中，Mo和V的峰值（第二还原状态）被认为是最具还原性的硫化物条件，与二叠纪末的大灭绝事件相一致。在长兴期上部Mo的显著增加导致Mo/U比值高，远高于现代硫化物海洋区域。这一趋势表明，在沉积物-水界面和水柱中都形成了硫化物水条件。在二叠纪末大灭绝层以上，Mo、V、Cr显著减少。根据这一趋势，我们给出了在二叠纪末还原水柱最大发育时期大量元素降水事件后海水中这些元素下降的解释。Mo和U富集的同时Mo/U比值的下降也支持Mo的富集。将Mo和U的析出总量与全球海水清查量进行对比计算表明，当全球海洋中6-10%以上的区域达到研究区域的富氧程度时，大部分溶解元素将沉淀到沉积物中，导致全球海水元素枯竭。钼、钒和铬是初级生产者和动物的生物必需元素。不断减少的水柱和缺乏生物必需元素可能对初级生产者的周转和海洋生物代谢产生相当大的影响，不仅在上层环境中，而且在周围的海洋环境中。
We describe variations in trace element compositions that occurred on the deep seafloor of palaeo-superocean Panthalassa during the end-Permian mass extinction based on samples of sedimentary rock from one of the most continuous Permian–Triassic boundary sections of the pelagic deep sea exposed in north-eastern Japan. Our measurements revealed low manganese (Mn) enrichment factor (normalised by the composition of the average upper continental crust) and high cerium anomaly values throughout the section, suggesting that a reducing condition already existed in the depositional environment in the Changhsingian (Late Permian). Other redox-sensitive trace-element (vanadium [V], chromium [Cr], molybdenum [Mo], and uranium [U]) enrichment factors provide a detailed redox history ranging from the upper Permian to the end of the Permian. A single V increase (representing the first reduction state of a two-step V reduction process) detected in uppermost Changhsingian chert beds suggests development into a mildly reducing deep-sea condition less than 1 million years before the end-Permian mass extinction. Subsequently, a more reducing condition, inferred from increases in Cr, V, and Mo, developed in overlying Changhsingian grey siliceous claystone beds. The most reducing sulphidic condition is recognised by the highest peaks of Mo and V (second reduction state) in the uppermost siliceous claystone and overlying lowermost black claystone beds, in accordance with the end-Permian mass extinction event. This significant increase in Mo in the upper Changhsingian led to a high Mo/U ratio, much larger than that of modern sulphidic ocean regions. This trend suggests that sulphidic water conditions developed both at the sediment–water interface and in the water column. Above the end-Permian mass extinction horizon, Mo, V and Cr decrease significantly. On this trend, we provide an interpretation of drawdown of these elements in seawater after the massive element precipitation event during the end-Permian maximum development of the reducing water column. A decrease in the Mo/U ratio despite enrichment of Mo and U also supports that of Mo. Calculations of the total amounts of these elements precipitated compared with the global seawater inventory suggest that when more than 6–10% of the global ocean became euxinic as much as the study section, most of the dissolved elements would precipitate into sediments, resulting in a global element-depleted seawater condition. Mo, V, and Cr act as bioessential elements for both primary producers and animals. The continuing reducing water column and the lack of bioessential elements could have had a considerable effect on primary producer turnover and marine life metabolism not only in the pelagic environment, but also in surrounding marine environments.