Oceanic water redox conditions of the region between Tethys and Panthalassa during the late early Triassic
Oceanic water redox conditions of the region between Tethys and Panthalassa during the late early Triassic
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DOI：
10.1016/j.palaeo.2020.110143
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发表时间：
2020-12
期刊：
影响因子：
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通讯作者：
K. Yoshizawa;S. Takahashi;Shun Muto;M. Ehiro;T. Tsuihiji
中科院分区：
文献类型：
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作者：
K. Yoshizawa;S. Takahashi;Shun Muto;M. Ehiro;T. Tsuihiji
日本东北部南北上带的大泽组（Osawa Formation）是一个以泥岩为主的奥列尼克阶（Olenekian，Spathian）浅海地层，在二叠纪末生物大灭绝（EPME）事件后，海洋生态系统中存在复杂食物链的化石证据。这一沉积序列为东特提斯-泛海拉萨洋域的古环境提供了重要的记录资料。本文详细记录了大泽组两个剖面的海洋环境，重点是氧化还原条件。平行，未受干扰的纹层表明有限的底栖活动是突出的中上部的大泽组，而扰动纹层的洞穴发生在较低和最高的部分，这表明活跃的底栖活动在富氧的沉积环境。黄铁矿（自形黄铁矿，黄铁矿framboids，和黄铁矿polyframboids）的数量增加，在良好的层压泥岩。一个相对广泛的尺寸分布的黄铁矿framboids表明，这些泥岩沉积在dysocompatibility条件下，并建议水柱是不是硫化物。氧化还原敏感元素如Mn、V和U的值接近于整个大泽组的上陆壳的平均值。钼是适度富集的层状和黄铁矿丰富的泥岩层位，虽然富集程度低于现代硫化物盆地的已知例子。这些数据表明，在这一海洋区域的底层水的条件在Spathian之间的氧化和dysotoxic（弱氧化）变化，从来没有硫化物。这种海洋环境的特点是温和减少底层水和含氧水柱似乎已经恢复和建立的早三叠世海洋生态系统的特点是多样的游泳运动动物和有限的多样性的底栖，in-motile动物的设置。
Fossil evidence of a complex food-chain in the marine ecosystem after the end-Permian mass extinction (EPME) event has been reported from the Osawa Formation, which consists of upper Olenekian (Spathian) shallow marine, mudstone-dominant strata in the South Kitakami Belt in Northeast Japan. This sedimentary sequence offers an important material that records the paleoenvironment of the oceanic region between the eastern Tethys and Panthalassa. This paper reports detailed records of the oceanic environment from two sections of the Osawa Formation focusing on the redox condition. Parallel, undisturbed laminae suggesting limited benthic activity are prominent in the middle to upper parts of the Osawa Formation, while disturbance of laminae by burrows occurs in the lower and uppermost parts, suggesting active benthic activity in an oxygen-rich depositional environment. The amount of pyrites (euhedral pyrite, pyrite framboids, and pyrite polyframboids) increases in the well-laminated mudstones. A relatively broad size distribution of pyrite framboids indicates that these mudstones were deposited under dysoxic conditions and suggest that the water column was not sulfidic. Redox-sensitive elements such as Mn, V, and U show values close to the average values of the upper continental crust throughout the Osawa Formation. Mo is moderately enriched in the well-laminated and pyrite-abundant mudstone horizons although the degree of enrichment is lower than known examples of modern sulfidic basins. These data suggest that the condition of the bottom water in this oceanic region during the Spathian varied between oxic and dysoxic (weakly oxygenated) and was never sulfidic. Such an oceanic environment characterized by mildly reducing bottom water and oxic water column appears to have been the settings for the recovery and establishment of the Early Triassic marine ecosystem characterized by diverse nektonic motile animals and limited diversity of benthic, in-motile animals.