Abstract Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.

中文翻译：

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甲烷扩散运移对碳、硫和微量元素自生富集约束下孔隙水和沉积物地球化学的影响——以南海神狐海域为例

摘要 硫酸盐驱动的甲烷厌氧氧化 (SD-AOM) 控制着海洋沉积物向海洋的甲烷释放。沉积物中的各种自生沉淀物已被广泛用于识别过去发生的 SD-AOM。然而，由于缺乏对这些沉淀物形成条件的系统评估，有时会阻碍在过去以甲烷扩散运输为特征的环境中识别 SD-AOM。在南海神狐地区受甲烷向上扩散影响的地点调查了孔隙水和沉积物的碳、硫和微量元素地球化学。在这里，硫酸盐-甲烷过渡区 (SMTZ) 根据硫酸盐和甲烷浓度位于海底约 7.6 m。δ34S 与硫酸盐的 δ18O 值的斜率与甲烷的扩散传输一致。孔隙水物质的浓度和同位素分布表明溶质是扩散而不是对流运输。从硫酸盐的 δ34S 值推断出的硫酸盐还原增强与当地存在相对丰富的、富含 34S 的硫化铁矿物一致。根据硫化铁矿物的数量和目前的硫酸盐扩散通量，在 SMTZ 周围产生观察到的自生硫化铁所需的时间估计为 ~1.1 ka。在研究的沉积物中未检测到自生碳酸盐、钡 (Ba) 和钼 (Mo) 的富集。这一观察结果与当前孔隙水钙、镁、Ba 和 Mo 的通量太低而无法进行自生富集的计算结果一致。低通量主要受扩散控制，这有助于形成富含 34S 的硫化铁矿物。观察到的富集模式——与受平流渗流影响的沉积物不同——预计在现代和古代大陆边缘沉积物中普遍存在，并且可能有助于确定过去的富含甲烷区和上覆的 SD-AOM 区是甲烷汇在地质记录中。