Skip to main content
U.S. flag

An official website of the United States government

Contribution of deep-sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from radiocarbon and stable isotope geochemistry

October 13, 2021

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 seepage areas 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 carbon quantity, radiocarbon content (as percent modern carbon, pMC), stable carbon isotopic compositions (as δ13C) of organic matter enclosed within seep carbonates from the Gulf of Mexico and the South China Sea to assess if sediment organic matter may be used as a proxy for methane seepage intensity. The δ13C values of organic matter (δ13Corg) exhibited a large range from −81.4‰ to −23.9‰. Radiocarbon contents of the carbonate-bound organic matter in seep carbonates ranged from 6% to 28% pMC, suggesting organic matter of the carbonates is a mixture of marine particulate organic matter (δ13C = −22‰ VPDB and 90% modern carbon) and biomass resulting from methane oxidation (assumed to have 0% modern carbon). Assuming constant productivity in the marine photic zone, it is proposed that seepage intensity and duration are the most important factors controlling the contribution of methane-derived carbon to the sedimentary column. This study reinforces the potential for using δ13C 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.

Citation Information

Publication Year 2021
Title Contribution of deep-sourced carbon from hydrocarbon seeps to sedimentary organic carbon: Evidence from radiocarbon and stable isotope geochemistry
DOI 10.1016/j.chemgeo.2021.120572
Authors Dong Feng, John Pohlman, Jorn Peckmann, Yuedong Sun, Yu Hu, Harry Roberts, Duofu Chen
Publication Type Article
Publication Subtype Journal Article
Series Title Chemical Geology
Index ID 70225705
Record Source USGS Publications Warehouse
USGS Organization Woods Hole Coastal and Marine Science Center