Abstract Total reduced inorganic S isotope ratios (δ34SCRS) shift toward more negative values across much of the southern North Atlantic just before the onset of the Cenomanian-Turonian Ocean Anoxic Event (OAE-2). At the same time, there is no parallel isotopic change in the significantly larger pool of kerogen (organic) S, which indicates that the distribution and S-isotope composition of sulfide in the environment likely did not drive the change in δ34SCRS. Here, we investigate possible explanations for the negative shift in δ34SCRS values and their divergence from organic S by isolating iron sulfides for morphological identification and grain-specific isotopic analysis using secondary ion mass spectrometry (SIMS). In pre- and syn-OAE-2 sedimentary rocks from Demerara Rise, we find four distinct morphologies of iron sulfides: pyrite framboids (1–20 μm diameter), irregular pyrite aggregates (1–38 μm diameter), large cemented pyrite aggregates (~60 μm diameter), and irregular and cemented aggregates of the pyrite polymorph marcasite (1–45 μm diameter). These different textural groups have distinct S-isotopic compositions that are largely consistent through the onset of OAE-2. As such, the secular change in bulk δ34SCRS values likely reflects the changing proportions of these phases stratigraphically across OAE-2. All textural groups feature resolvable intra-grain δ34S variability, suggesting that the environments in which they formed were characterized by dynamic sulfide δ34S values and/or by partial closed-system distillation. We use grain-specific δ34S distributions to rule out shoaling of the chemocline within the sediments as a mechanism for the observed decrease in δ34SCRS. Instead, we propose that changes in the reactivity of the iron species delivered to Demerara Rise over the ~200 kyr leading up to the onset of OAE-2 impacted the relative contributions of pyrite with S-isotope signatures reflecting the water column, shallow sediments, and deeper sediments to the bulk sedimentary δ34SCRS value. Specifically, the change in iron reactivity at the onset of OAE-2 favored the production of 34S-depleted large, cemented aggregates and framboids at the expense of more 34S-enriched irregular aggregates. Our results underscore that bulk δ34SCRS measurements integrate multiple reduced phases that form via distinct reaction mechanisms and potentially in different parts of the depositional environment. Grain-specific SIMS analyses dramatically enrich our ability to interpret pyrite isotopic patterns in the geologic record.

中文翻译：

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OAE-2 开始时硫化铁的转变模式驱动黄铁矿 δ34S 记录的区域转移

摘要 在 Cenomanian-Turonian 海洋缺氧事件 (OAE-2) 开始之前，北大西洋南部大部分地区的总还原无机 S 同位素比 (δ34SCRS) 向负值转移。同时，在显着更大的干酪根（有机）S 库中没有平行的同位素变化，这表明环境中硫化物的分布和 S 同位素组成可能没有驱动 δ34SCRS 的变化。在这里，我们通过分离硫化铁进行形态鉴定和使用二次离子质谱法 (SIMS) 进行晶粒特异性同位素分析，研究了 δ34SCRS 值的负移及其与有机 S 的差异的可能解释。在 Demerara Rise 的 pre-OAE-2 沉积岩和 Syn-OAE-2 沉积岩中，我们发现了四种不同的硫化铁形态：黄铁矿小球体（直径 1-20 微米）、不规则的黄铁矿骨料（直径 1-38 微米）、大型胶结黄铁矿骨料（直径约 60 微米），以及黄铁矿多晶型白铁矿的不规则和胶结骨料（直径 1-45 微米）。这些不同的结构组具有不同的 S 同位素组成，这些组成在 OAE-2 的出现期间基本保持一致。因此，大块 δ34SCRS 值的长期变化可能反映了这些阶段在 OAE-2 中地层比例的变化。所有质地组都具有可解析的晶粒内 δ34S 变异性，这表明它们形成的环境以动态硫化物 δ34S 值和/或部分封闭系统蒸馏为特征。我们使用特定于颗粒的 δ34S 分布来排除沉积物中趋化层的浅滩化，作为观察到的 δ34SCRS 减少的机制。相反，我们提出，在 OAE-2 爆发之前，输送到 Demerara Rise 的铁物种的反应性变化影响了黄铁矿的相对贡献，其中 S 同位素特征反映了水柱、浅层沉积物、和更深的沉积物到大块沉积物 δ34SCRS 值。具体而言，在 OAE-2 开始时铁反应性的变化有利于产生 34S 耗尽的大型胶结骨料和小枝状骨料，但会消耗更多富含 34S 的不规则骨料。我们的结果强调，大量 δ34SCRS 测量整合了多个还原相，这些还原相通过不同的反应机制形成，并且可能在沉积环境的不同部分形成。特定于谷物的 SIMS 分析极大地丰富了我们解释地质记录中黄铁矿同位素模式的能力。