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© 2018 Elsevier B.V. Sulfate-driven anaerobic oxidation of methane (SD-AOM) plays a critical role in regulating the global methane budget. Determination of the diagnostic triple isotope exponent θ33 (≡ln33⁡α/ln34⁡α) for SD-AOM can help to identify and quantify microbial sulfate reduction via SD-AOM in the environment. The history of Earth's surface redox conditions can also be examined through the measurement of triple sulfur isotope compositions in sedimentary rocks. Due to difficulties in both culturing anaerobic methanotrophs and sampling pore-water sulfate in SD-AOM-dominated environments, however, the θ33 values for the processes of SD-AOM have not been constrained. We propose that a set of modern cold-seep associated barite samples with low Δδ18O/Δδ34S values bear a record of residual pore-water sulfate during SD-AOM, and therefore the triple sulfur isotope composition of these barites can be used to deduce θ33 values. We applied a 1-D diagenetic reaction–transport model to fit Δ33S and δ′34S results from modern cold seep barites collected from five sites in the Gulf of Mexico. Based on revealed negative correlations (R2=0.77) between Δ33S and δ′34S values we calculated an upper-limit θ33 value of 0.5100 to 0.5112 (±0.0005) given a 1000ln34⁡α value of −30‰ to −10‰. This θ33 value is distinctively lower than that of organoclastic sulfate reduction (OSR) in marine environments where the diagnostic isotope fractionation (1000ln34⁡α) is typically more negative than that of SD-AOM. In addition, cold seep barite data display a negative Δ33S–δ′34S correlation whereas pore-water sulfates of all OSR-dominated settings show a positive one. Therefore, the diagnostic triple-sulfur isotope exponent and associated negative Δ33S–δ′34S correlation may allow for the identification of SD-AOM in sedimentary records.

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Earth and Planetary Science Letters

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