Microbial sulfate reduction (MSR) and associated pyritic sulfide formation are important diagenetic processes in marine sediments. The sulfur isotopic composition of pyrite (δ34Spyr) is proven to be sensitive to changes in sedimentation rates and the content and reactivity of organic carbon, especially on the continental shelves and upper slopes (water depth < 350 m). However, the diagenetic responses of sulfur to variations in climatic and depositional conditions in the deep-sea sediments are still poorly understood. This study combines element contents and isotopes to characterize diagenetic interplays of sulfur, organic carbon, and methane in the continental slope sediments of the northern South China Sea since the mid-Pleistocene. Our data suggest that the total organic carbon (TOC) increased during glacial times, implying enhanced primary productivity due to increased nutrient supply by the East Asian Winter Monsoon, in addition to efficient transfer of organic carbon and better preservation of organic carbon due to reduced bottom water oxygen. Total sulfur and chromium reduction sulfur contents varied concomitantly with the TOC, suggesting an increased burial of organic carbon that enhanced the organoclastic sulfate reduction (OSR) and the formation of authigenic pyrite. The environmental changes did not induce a significant shift in δ34Spyr, due most likely to relatively low sedimentation rates and large fractionation in sulfur isotope through OSR during the glacial-interglacial cycles. Instead, it is hypothesized that the sulfate-driven anaerobic methane oxidation promoted the formation of a higher amount of authigenic pyrite. Consequently, it created a closed diagenetic system leading to positive excursions in δ34Spyr at the sulfate-methane transition zone. Our results suggest the vulnerability of pyrite formation and its sulfur isotopic composition to the changes in monsoon-driven primary productivity and the methane-rich fluid migrations in the continental margin sediments. This study complements the growing evidence for the local diagenetic controls on sedimentary sulfur geochemical records by highlighting the importance of early diagenesis in paleoenvironment reconstruction based on the content and sulfur isotopic composition of pyrite.

中文翻译：

---

更新世中期以来气候变化和富甲烷流体活动对南海北部沉积硫地球化学记录的影响


微生物硫酸盐还原 （MSR） 和相关的黄铁矿硫化物形成是海洋沉积物中重要的成岩过程。黄铁矿的硫同位素组成 （δ34Spyr） 被证明对沉积速率和有机碳含量和反应性的变化很敏感，特别是在大陆架和上部斜坡上（水深 x3C 350 m）。然而，硫对深海沉积物中气候和沉积条件变化的成岩反应仍然知之甚少。本研究结合了元素含量和同位素，以表征自更新世中期以来南海北部大陆斜坡沉积物中硫、有机碳和甲烷的成岩相互作用。我们的数据表明，总有机碳 （TOC） 在冰河时期增加，这意味着由于东亚冬季季风增加了营养供应，此外，由于底层水氧的减少，有机碳的有效转移和有机碳的更好保存，提高了初级生产力。总硫和铬还原硫含量随 TOC 而变化，表明有机碳埋藏增加，从而增强了有机碎屑硫酸盐还原 （OSR） 和自生黄铁矿的形成。环境变化并未引起 δ34Spyr 的显著变化，这很可能是由于在冰期-间冰期循环期间，相对较低的沉降速率和通过 OSR 在硫同位素中的大部分分馏。相反，据推测，硫酸盐驱动的厌氧甲烷氧化促进了更大量的自生黄铁矿的形成。因此，它创建了一个封闭的成岩系统，导致硫酸盐-甲烷过渡带的 δ34Spyr 出现正偏移。 我们的结果表明，黄铁矿的形成及其硫同位素组成对季风驱动的初级生产力变化和大陆边缘沉积物中富含甲烷的流体迁移的脆弱性。本研究通过强调早期成岩作用在基于黄铁矿含量和硫同位素组成的古环境重建中的重要性，补充了沉积硫地球化学记录中局部成岩控制的证据。