Abstract Fossil evidences of a complex food-chain after the end-Permian mass extinction (EPME) event have 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. It is an important material which 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 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 occur in the lower and uppermost parts, suggesting active benthic activity in an oxygen-rich depositional environment. Pyrites (euhedral pyrite, pyrite framboids, and pyrite polyframboids) increase in the well-laminated mudstones. The relatively large size distributions of pyrite framboids indicate that these mudstones were deposited under dysoxic depositional 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 value of the upper continental crust throughout the Osawa Formation. Mo is moderately enriched in the well-laminated and pyrite-abundant mudstone horizons, but enrichments are lower than known examples of modern sulfidic basins. These data suggest that the bottom waters around this oceanic region during the Spathian varied between oxic and dysoxic (weakly oxygenated), and was never sulfidic. This oceanic environment with mildly reducing bottom water and oxic water-column conditions appears to have been the settings for the recovery and establishment of Early Triassic marine ecosystems characterized by diverse nektonic motile animals and limited diversity of benthic in-motile animals.

中文翻译：

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早三叠世晚期特提斯和潘塔拉萨之间区域的海洋水氧化还原条件

摘要 大泽组报告了二叠纪末大灭绝 (EPME) 事件后复杂食物链的化石证据，该组由日本东北部南北上带的上 Olenekian (Spathian) 浅海相泥岩优势地层组成。它是记录东特提斯和泛塔拉萨之间洋区古环境的重要资料。本文报告了大泽组两个剖面的海洋环境的详细记录，重点是氧化还原条件。大泽组中上部平行未扰动的层状结构表明底栖活动有限，而下部和最上部出现洞穴对层状结构的扰动，表明在富氧沉积环境中底栖活动活跃。黄铁矿（自形黄铁矿，黄铁矿 framboids 和黄铁矿 polyframboids）在层状良好的泥岩中增加。黄铁矿小柱体的相对较大的尺寸分布表明这些泥岩是在缺氧沉积条件下沉积的，并表明水体不是硫化物。Mn、V 和 U 等氧化还原敏感元素的值接近整个大泽组上陆地壳的平均值。Mo 在层状良好且富含黄铁矿的泥岩层位中适度富集，但其富集度低于现代硫化盆地的已知实例。这些数据表明，在斯帕蒂亚时代，这个海洋区域周围的底层水域在含氧和缺氧（弱含氧）之间变化，并且从来没有硫化物。