The Impact of Sulfate-Driven Anaerobic Oxidation of Methane on the Carbon Isotopic Composition in Marine Sediments: A Case Study from Two Sites in the Shenhu Area, Northern South China Sea

Abstract

Abstract Authigenic carbonate in seep environments, as a direct byproduct of sulfate-driven anaerobic oxidation of methane (SD-AOM), is usually absent within the sediment column because of the requirement of a strict formation condition. In this case, the lack of a reliable carbon signal may impede the identification of SD-AOM and methane leakage. Here, carbon and oxygen isotopes, elemental compositions, AMS 14C dates in sediments, and porewater geochemistry were investigated from two sites (A27 and SH1) of the Shenhu area, northern South China Sea (SCS), to discuss how SD-AOM affects the carbon isotope in methane-affected marine sediments. Porewater results at both sites indicate the occurrence of methane diffusion from the sulfate-methane transition zone (SMTZ) below. The carbon isotopes of bulk-sediment carbonate and foraminifera show no distinctly negative excursion, reflecting that these signals are invalid in response to SD-AOM in the investigated sites. Then, a mass balance model is adopted to evaluate the δ13C value of authigenic carbonate (δ13CAC). Consequently, three intervals (A2 and A3 from site A27 and S2 from site SH1) are identified, featuring negative δ13CAC values, high TS/TOC ratios, and enhanced contents of authigenic carbonate, which are most likely influenced by SD-AOM. Considering the current SMTZ located at deeper layers, intervals A2 and S2 represent the locations of paleo-SMTZ, while interval A3 is thought to be influenced by the current methane diffusion. Interestingly, the δ13C values of total organic matter (δ13CTOC) show positive excursions within the paleo-SMTZs, which can be explained herein by the diagenetic modification. In the course of SD-AOM at the SMTZ, high rate of (methylotrophic) methanogenesis preferentially consumes lighter carbon atoms in organic matter, with the remainder being gradually more positive. Our results indicate that the exploration of a reliable methane-carbon response in systems lacking seep carbonates plays an important role in constraining SD-AOM and methane release.