Abstract Pyrite formation in marine sedimentary environments plays a key role in the global biogeochemical cycles of carbon, sulfur and iron, regulating Earth’s surface redox balance over geological time scales. The sulfur isotopic composition of pyrite is one of the major geochemical tools for investigating early diagenetic processes in modern marine sediments and substantive changes to the Earth’s surface environment in ancient sedimentary rocks. We studied sulfur–iron diagenesis and the sulfur isotopic evolution in sediments of the Bornholm Basin, southwestern Baltic Sea, to track the formation of pyrite in the near-surface sediments. Pyrite accumulation is observed with depth over the uppermost 100 cm before the extent of pyritization of the highly reactive iron pool (Fepy/FeHR) stays constant at ca. 0.9, suggesting that the use of a single iron-speciation parameter as a proxy for anoxic and sulfidic conditions needs to be supported by other independent indicators in sedimentary records. Stable sulfur isotopic analysis demonstrates that the bulk pools of elemental sulfur and iron monosulfide do not exchange isotopes completely with aqueous sulfide. We suggest that the reactions with polysulfide and aqueous sulfide are probably restricted to the surface of the solid-phase sulfur and iron-sulfur aggregates. Although pyrite is growing throughout the uppermost sediment column, the pyrite at depth has a sulfur isotopic composition similar to that of pyrite formed near the sediment surface. To understand the isotopic discrepancy between pyrite and aqueous sulfide in the deeper sediments, we developed a simple diagenetic model, which reproduces the observed sulfur isotopic composition of pyrite well. Our results suggest that much of the pyrite is rapidly formed near the sediment–water interface, and its δ34S is not as influenced by the 34S-enriched pool of aqueous sulfide in the deeper part of the sediment, allowing 32S-enriched pyrite to be preserved in deeper sediments. This near-surface diagenesis and the associated isotopic pattern are possibly of relevance for many marine sediments with high organic matter content, and high aqueous sulfide but low reactive iron availability. Moreover, our sulfur isotopic data demonstrate that extremely slow pyritization is ongoing in the deep lacustrine clay sediments. These results have implications for the interpretation of sulfur–iron geochemical data in both modern and ancient settings as well as for improving reconstructions of ancient depositional environments and a better understanding of the marine sulfur cycle throughout Earth’s history.

中文翻译：

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博恩霍尔姆盆地沉积物中铁和硫的早期成岩作用：近地表黄铁矿形成的作用

摘要 海洋沉积环境中黄铁矿的形成在全球碳、硫和铁的生物地球化学循环中起着关键作用，在地质时间尺度上调节地球表面氧化还原平衡。黄铁矿的硫同位素组成是研究现代海洋沉积物早期成岩过程和古代沉积岩地表环境实质性变化的主要地球化学工具之一。我们研究了波罗的海西南部博恩霍尔姆盆地沉积物中的硫铁成岩作用和硫同位素演化，以追踪近地表沉积物中黄铁矿的形成。在高反应性铁池 (Fepy/FeHR) 的黄铁矿化程度保持在大约 100 厘米之前，黄铁矿积累的深度超过了最上面 100 厘米。0.9, 表明使用单一铁形态参数作为缺氧和硫化条件的替代需要得到沉积记录中其他独立指标的支持。稳定的硫同位素分析表明，元素硫和一硫化铁的主体池不会与硫化物水溶液完全交换同位素。我们认为与多硫化物和硫化物水溶液的反应可能仅限于固相硫和铁硫聚集体的表面。尽管黄铁矿在整个最上层沉积柱中生长，但深度处的黄铁矿具有类似于沉积物表面附近形成的黄铁矿的硫同位素组成。为了了解更深沉积物中黄铁矿和硫化物水溶液之间的同位素差异，我们开发了一个简单的成岩模型，它很好地再现了观察到的黄铁矿硫同位素组成。我们的研究结果表明，大部分黄铁矿在沉积物-水界面附近迅速形成，其 δ34S 不受沉积物较深部分富含 34S 的硫化物水池的影响，从而使富含 32S 的黄铁矿得以保存在较深的沉积物中。这种近地表成岩作用和相关的同位素模式可能与许多具有高有机质含量、高含水硫化物但低活性铁可用性的海洋沉积物有关。此外，我们的硫同位素数据表明，深部湖相粘土沉积物中的黄铁矿化正在发生极其缓慢。