Contribution of deep sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from stable and radiocarbon isotopic compositions

Sulfate-driven anaerobic oxidation of methane (AOM) limits the release of methane from marine sediments and promotes the formation of carbonates close to the seafloor in areas of high seepage along continental margins. It has been established that hydrocarbon seeps are a source of methane, dissolved inorganic carbon, and dissolved organic carbon to marine environments. However, questions remain about the contribution of deep-sourced carbon from hydrocarbon seeps to the sedimentary organic carbon pool. In this study, we analyzed the carbon quantity, radiocarbon content (as percent modern carbon, or pMC), stable carbon isotopic content (as 13C of organic matter bound as inclusions within the seep carbonates from the Gulf of Mexico and the South China Sea to determine if sediment organic matter may be used as a proxy for methane seepage intensity. The 13C values of the organic matter (13Corg) exhibited a remarkable range from -81.4 to -23.9. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% percent modern carbon (pMC), suggesting organic matter with the carbonates was an admixture of marine particulate organic matter (13C = -22 VPDB and 90% modern carbon) and seep-derived methane (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity is the most import factor controlling the contribution of methane-derived carbon to the sedimentary column. This study reveals the potential for using 13C values of organic carbon to discern methane-rich environments in ancient sedimentary environments where authigenic carbonate is not present and to constrain the record of AOM through Earth history.