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Highlights • Porewater geochemistry of two piston cores in hydrate drilling areas was investigated. • A numerical model was applied to quantify involved biogeochemical processes. • Methane consumed mainly originated from external sources rather than local methanogenesis. • The external methane is most likely to originate from local gas hydrate systems. Abstract Methane of various origins is widespread in the sediments of continental margins, representing a key component of the carbon cycle in the ocean. Its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column. However, this type of methane is difficult to track due to complex reactions occurred during its long distance migration. Porewater geochemical analyses of two ∼7 m piston cores (D17-15 and W01-16) combined with a reaction-transport model were applied to quantify methane-related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of Shenhu and Dongsha in the South China Sea (SCS). The model results revealed that anaerobic oxidation of methane (AOM) is the predominant biogeochemical process in both cores, consuming approximately 85% and 74% of sulfate in the modeled domain, respectively. The crossplot of dissolve inorganic carbon (DIC) accumulation versus sulfate depletion of the system suggests that methane consumed during the AOM is mainly originated from external methane sources rather than local methanogenesis. Using a δ13C DIC mass balance calculation together with model-derived fractions of different DIC sources, we estimated that the δ13C value of the external methane in core D17-15 is −66.6‰, which is in accordance with the previous reported δ13C values of hydrate-bound methane in the area. These results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the SCS is mainly originated from local gas hydrate reservoir. The proposed approach has the potential to be used to differentiate the sources of methane, which will provide constraints on dynamics of methane in gas hydrate-bearing marine sediments. [ABSTRACT FROM AUTHOR]

Methane seepage activity caused by gas hydrate dissociation plays an essential role in the global carbon cycle and climate change in Earth's history. However, the triggering mechanisms of gas hydrate dissociation remain elusive because of the lack of a relatively complete history of methane seeps on long-term timescales. Here, we present major and trace elements, stable C–O isotopes, and U–Th dating analyses for cold seep carbonates from drill cores in the South China Sea. Our results reveal a 209,00-year-old history of methane seepage activity, which involves multi-stage changes in seepage intensity and methane source (microbial or thermogenic). Chronological data suggest that hydrostatic pressure reduction and dynamic depositional environment mainly controlled the gas hydrate dynamics in the northwestern South China Sea, which is different from the gas hydrate dissociation in the northeastern South China Sea controlled by bottom water warming. This discrepancy may be caused by the differences in physicochemical conditions (e.g., water depth) in different sea areas. Therefore, the findings of this study help to further clarify the relationship between oceanic hydrate reservoirs and environmental change in other world regions. • Seep carbonates from drill cores in the Qiongdongnan Basin were investigated. • Two methane seepage events were identified in the northwestern South China Sea. • Multi-stage changes in seepage intensity and methane source. • Seepage activity controlled by hydrostatic pressure and depositional environment. [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]