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Sulfate-driven anaerobic oxidation of methane (SD-AOM) and organoclastic sulfate reduction (OSR) are the two processes that consume sulfate in methane-rich marine sediments. Determining the relative contribution of each process to overall sulfate reduction is critical to assess the global carbon and sulfur cycle in marine settings. The analysis of sulfur and oxygen isotope compositions of porewater sulfate (δ18O SO4 and δ34S SO4) provides crucial insight into sulfate reduction pathways and the dynamics of the sedimentary sulfur cycle. The δ18O SO4 /δ34S SO4 ratio is governed by net sulfate reduction rates and has been widely used to distinguish SD-AOM from OSR. Furthermore, δ18O SO4 /δ34S SO4 slopes are believed to vary with changing methane flux. To enhance the applicability of δ18O SO4 vs. δ34S SO4 plots for the tracing of SD-AOM activity and methane flux, we investigate sulfate sulfur and oxygen isotopes of porewaters from three piston cores collected from the Haima seeps of the South China Sea. Reaction-transport modeling previously indicated that sulfate consumption was dominated by SD-AOM under changing methane fluxes (1126 mmol m−2 yr−1 to 26 mmol m−2 yr−1). Our new data demonstrate an increase of the initial δ18O SO4 /δ34S SO4 slope when methane flux decreases. An initial slope of 0.43 corresponds to a methane flux of 194 mmol m−2 yr−1, the lowest currently known methane flux producing a small δ18O SO4 /δ34S SO4 slope diagnostic for SD-AOM in sediments close to the seafloor. When methane flux decreases to 26 mmol m−2 yr−1, a small slope (0.48) is only observed in deeper sediment layers. Our results also reveal an increase of δ18O SO4 /δ34S SO4 slopes with decreasing SD-AOM rates. We estimate that net sulfate reduction rates can be as low as 10−9 mol cm−3 yr−1 to produce slopes smaller than 0.5 in SD-AOM-dominated settings. In OSR-dominated settings, δ18O SO4 /δ34S SO4 slopes tend to be significantly higher than 0.5, while net sulfate reduction rates tend to be lower than 10−5 mol cm−3 yr−1. This study reveals that quantification of methane flux is required to utilize the full potential of δ18O SO4 vs. δ34S SO4 plots in the endeavor of tracing SD-AOM activity and the dynamics of the sulfur cycle in methane-rich environments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]