Marine redox variability from Baltica during extinction events in the latest Ordovician–early Silurian
Marine redox variability from Baltica during extinction events in the latest Ordovician–early Silurian
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DOI：
10.1016/j.palaeo.2020.109792
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发表时间：
2020-09
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影响因子：
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通讯作者：
Seth A. Young;Emily Benayoun;Nevin P. Kozik;O. Hints;T. Martma;S. M. Bergström;J. Owens
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文献类型：
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作者：
Seth A. Young;Emily Benayoun;Nevin P. Kozik;O. Hints;T. Martma;S. M. Bergström;J. Owens
有充分的证据表明，上奥陶统和志留纪的演替记录了多次海洋更替事件——包括显生宙的第二大物种大灭绝——广泛的冰川作用，以及多次全球碳循环扰动。虽然晚奥陶世大灭绝事件的因果机制涉及气候、古海洋变化和δ13C记录，但类似的记录仍然缺乏对早志留世随后的灭绝事件的约束。本文研究了瑞典和拉脱维亚两个富有机质岩心剖面的有机质碳同位素（δ13Corg）化学地层特征，以及相应的古氧化还原指标（Fe形态、[Mn、V、Mo]和黄铁矿硫同位素）。这两个岩心剖面位于上Katian至下Wenlock阶段（446-431 Ma）。来自瑞典南部上奥陶统地层的黄铁矿化铁和大块沉积Mn浓度表明，在Hirnantian晚期，局部氧化还原向更还原的条件转移，可能包括与第二次大灭绝脉冲相吻合的缺氧和硫化物水柱条件。两个岩心内晚Aeronian（早志留世）段的新高分辨率δ13Corgand δ 34spyrdata数据显示，与相关的Sandvika和sedgwickii灭绝事件大致一致的正偏移。独立地，该晚Aeronian期的铁形态和大块沉积微量金属数据记录了在晚Aeronian正δ13C偏移之前和期间，深海盆（瑞典）和远陆架（拉脱维亚）的局部缺氧条件。这一多指标古还原数据首次提供了与早志留世（晚Aeronian）生物事件和δ13C正偏移相一致的局部到区域的海洋条件减少的直接证据。此外，拉脱维亚岩心横跨Llandovery/Wenlock边界区间的新δ 34spyr值显示出与铁形态和微量金属富集相一致的正偏移，这意味着在Ireviken正δ13C偏移上升段期间，局部氧化还原扰动与间歇缺氧的底部水相吻合。这些局部到区域尺度的海洋氧化还原条件变化的地球化学数据（全球解释需要更多的数据）与古生物学记录和海平面上升的证据相结合，表明与缺氧到缺氧条件扩展相关的环境压力可能是奥陶世晚期-志留纪早期几次灭绝事件的驱动因素。
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 δ13Corgand δ34Spyrdatasets from the late Aeronian (early Silurian) interval within both drill cores show positive excursions that are broadly coincident with the associated Sandvika andsedgwickiiextinction 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 δ34Spyrvalues 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.