Triple sulfur isotope relationships during sulfate-driven anaerobic oxidation of methane

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 33 e (= ln33 a/ln34 a) for SDAOM can help to identify and quantify microbial sulfate reduction via SD-AOM in the environment. The history of Earth's surface red ox 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 33 e 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 ll8 18 0j !l834S values bear a record of residual pore-water sulfate during SDAOM. and therefore the triple sulfur isotope composition of these barites can be used to deduce 33 e values. We applied a 1-D diagenetic reaction-transport model to fit !l 33S and 8134S results from modern cold seep barites collected from five sites in the Gulf of Mexico. Based on revealed negative correlations (R2 = 0.77) between !l33 S and 8134S values we calculated an upper-limit 33 e value of 0.5100 to 0.5112 (±0.0005) given a 1000ln34 a value of -30%0 to -10%0. This 33 e value is distinctively lower than that of organoclastic sulfate reduction ( OSR) in marine environments where the diagnostic isotope fractionation ( 1000 ln34 a) is typically more negative than that of SD-AOM. In addition. cold seep barite data display a negative !l 33 S-81 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 !l 33S-8134S correlation may allow for the identification of SD-AOM in sedimentary records.