Sediment-hosted marine sulfur cycling has played a significant role in regulating Earth’s surface chemistry over our planet’s history. Microbially-mediated reactions involving sulfur are often accompanied by sulfur isotope fractionation that, in turn, is captured by sulfate and sulfide minerals, providing the opportunity to track changes in the microbial utilization of sulfur and thus the marine sulfur cycle. Studying sulfur diagenesis within the Bornholm Basin, Baltic Sea, we explore the interplay between carbon, sulfur and iron, focusing on the fate of sulfur and the dynamics of the sulfur and oxygen isotopic response as a function of the varying thickness of the organic carbon-rich Holocene Mud Layer (HML) across the basin. Using a one-dimensional reaction-transport model, porewater sulfate and sulfide profiles were used to calculate net sulfate reduction rates (SRR) and net sulfide production rates, respectively. These calculations suggest a positive relationship between the thickness of the HML and net rates of sulfate reduction and sulfide production. Given that ascending sulfide is enriched in ³⁴S relative to that produced in-situ, a heightened sulfide flux promotes spatially variable precipitation of ³⁴S-enriched pyrite (δ³⁴S ≈ −10‰) close to the sediment–water interface. Modeling results indicate that this isotopically “heavy” sulfide is formed as a consequence of mixing between ascending sulfide (up to +6.3‰) and that produced in-situ (ca. −40‰). Further, we show that the sulfur and oxygen isotopic composition of porewater sulfate is controlled by the net SRR: when the net SRR is high (i.e., in sulfide-replete settings) the downcore increase in δ¹⁸O_SO4 is dampened relative to increase in δ³⁴S_SO4, whereas when net SRR is low (i.e., in iron-rich parts of the basin) downcore δ¹⁸O_SO4 values increase while δ³⁴S_SO4 values remain invariant. We conclude that sedimentation rates and open system diffusion strongly influence the distribution of sulfur species and their sulfur isotopic composition, as well as the oxygen isotopic composition of sulfate, through the interaction between iron, sulfur and methane. This work highlights the importance of considering diffusion to better understand open system diagenesis and the δ³⁴S signatures of sulfate and sulfide in both modern settings and ancient rocks.

沉积物承载的海洋硫循环在我们星球历史上调节地球表面化学方面发挥了重要作用。涉及硫的微生物介导的反应通常伴随着硫同位素分馏，而硫同位素分馏又被硫酸盐和硫化物矿物捕获，提供了跟踪微生物利用硫的变化以及海洋硫循环的机会。研究波罗的海博恩霍尔姆盆地内的硫成岩作用，我们探索碳、硫和铁之间的相互作用，重点关注硫的归宿以及硫和氧同位素响应的动力学，作为有机碳厚度变化的函数-丰富的全新世泥层 (HML) 横跨盆地。使用一维反应输运模型，孔隙水硫酸盐和硫化物曲线分别用于计算净硫酸盐还原率 (SRR) 和净硫化物产率。这些计算表明 HML 的厚度与硫酸盐还原和硫化物生成的净速率之间存在正相关关系。鉴于上升的硫化物富含³⁴ S 相对于原位生产的，更高的硫化物通量促进了³⁴ S 富集黄铁矿（δ ³⁴ S ≈ -10‰）在沉积物 - 水界面附近的空间可变沉淀。建模结果表明，这种同位素“重”硫化物是上升硫化物（高达 +6.3‰）与原位产生的硫化物（约 -40‰）混合的结果。此外，我们表明，孔隙水硫酸盐的硫和氧同位素组成受净 SRR 控制：当净 SRR 高时（即在硫化物充足的环境中），δ ¹⁸ O _SO4的下核增加相对于δ ³⁴ S _SO4，而当净 SRR 较低时（即在盆地的富铁部分），下核 δ ¹⁸ O _SO4值增加，而 δ ³⁴ S _SO4值保持不变。我们得出结论，沉降速率和开放系统扩散通过铁、硫和甲烷之间的相互作用强烈影响硫物质的分布及其硫同位素组成，以及硫酸盐的氧同位素组成。这项工作强调了考虑扩散以更好地了解开放系统成岩作用以及现代环境和古代岩石中硫酸盐和硫化物的 δ ³⁴ S 特征的重要性。