Abstract It is well documented that Upper Ordovician and Silurian successions record multiple marine turnover events – including the second-largest mass extinction in the Phanerozoic – widespread glaciation, and multiple global carbon cycle perturbations. Whereas causal mechanisms for the Late Ordovician major mass extinction event involving climate, paleoceanographic variation, and δ13C records have been published, similar records remain poorly constrained for subsequent extinction events in the early Silurian. Here, we present new organic matter carbon isotope (δ13Corg) chemostratigraphy and corresponding paleoredox proxies (Fe speciation, [Mn, V, Mo], and pyrite sulfur isotopes) from two organic-rich drill core sections in Sweden and Latvia that span the upper Katian through lower Wenlock stages (446–431 Ma). Pyritized Fe and bulk sedimentary Mn concentrations from Upper Ordovician strata in southern Sweden suggest a local redox shift to more reducing conditions in the late Hirnantian, possibly including euxinic (anoxic and sulfidic water column) conditions that coincide with the second mass extinction pulse. The new high-resolution δ13Corg and δ34Spyr datasets from the late Aeronian (early Silurian) interval within both drill cores show positive excursions that are broadly coincident with the associated Sandvika and sedgwickii extinction events. Independently, Fe speciation and bulk sedimentary trace metal data from this late Aeronian interval record locally euxinic conditions in both the deep basinal (Sweden) and distal shelf (Latvia) settings before and during the late Aeronian positive δ13C excursion. This multiproxy paleoredox dataset provides the first direct evidence for local to regional expansion of reducing marine conditions coincident with this early Silurian (late Aeronian) biotic event and positive δ13C excursion. Additionally, new δ34Spyr values spanning the Llandovery/Wenlock boundary interval in the Latvia core show a positive excursion coincident with Fe speciation and trace metal enrichments that imply a local redox perturbation with intermittently euxinic bottom waters during the rising limb of the Ireviken positive δ13C excursion. The combination of these geochemical data for local- to regional-scale (more data required for global interpretations) changes in marine redox conditions with paleobiological records and evidence for eustatic sea-level rise indicate that environmental stresses related to an expansion of anoxic to euxinic conditions were a probable driver for several extinction events during the latest Ordovician–early Silurian.

中文翻译：

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最新奥陶纪-早志留纪灭绝事件期间波罗的海的海洋氧化还原变化

摘要 上奥陶统和志留纪序列记录了多次海洋周转事件——包括显生宙的第二大物种灭绝——广泛的冰川作用和多次全球碳循环扰动，这是有据可查的。虽然已经发表了涉及气候、古海洋变化和 δ13C 记录的晚奥陶世大灭绝事件的因果机制，但类似的记录对于早期志留纪的后续灭绝事件仍然缺乏约束。在这里，我们展示了来自瑞典和拉脱维亚两个富含有机物的钻芯部分的新的有机质碳同位素 (δ13Corg) 化学地层学和相应的古氧化还原替代物（Fe 形态、[Mn、V、Mo] 和黄铁矿硫同位素） Katian 通过较低的 Wenlock 阶段（446-431 Ma）。来自瑞典南部上奥陶统地层的黄铁矿化 Fe 和大量沉积 Mn 浓度表明，在 Hirnantian 晚期，局部氧化还原转变为更多还原条件，可能包括与第二次大规模灭绝脉冲一致的euxinic（缺氧和硫化水柱）条件。来自两个钻芯内埃伦纪晚期（志留纪早期）区间的新高分辨率 δ13Corg 和 δ34Spyr 数据集显示正偏移，与相关的 Sandvika 和 sedgwickii 灭绝事件大体一致。独立地，来自这个晚期 Aeronian 间隔的 Fe 物种形成和大量沉积痕量金属数据记录了晚期 Aeronian δ13C 正偏移之前和期间深海盆地（瑞典）和远端大陆架（拉脱维亚）环境中的局部优生条件。这个多代理古氧化还原数据集提供了第一个直接证据，证明与这个早期志留纪（埃伦纪晚期）生物事件和正 δ13C 偏移相吻合的海洋条件减少的局部到区域扩展。此外，跨越拉脱维亚核心 Llandovery/Wenlock 边界区间的新 δ34Spyr 值显示出与 Fe 形态和痕量金属富集相一致的正偏移，这意味着在 Ireviken 正 δ13C 偏移的上升支期间，局部氧化还原扰动与间歇性的euxinic 底层水。