A carbonate-based proxy for sulfate-driven anaerobic oxidation of methane

Sulfate-driven anaerobic oxidation of methane (SD-AOM) supports chemosynthesis-based communities and limits the release of methane from marine sediments. Formation of authigenic carbonates at active methane seeps is promoted by SD-AOM stoichiometry. While distinctively small delta O-18/delta S-34 slopes of pore fluid sulfate have been shown to typify modern methane-rich environments, identification of such environments has been difficult for the geological past due to the lack of sedimentary pore fluids. However, if the isotopic composition of sulfate were archived in authigenic carbonate during early diagenesis, carbonate- associated sulfate (CAS) should display the characteristic delta O-18-delta S-34 pattern. To test this hypothesis, we investigated the delta O-18(CAS), delta S-34(CAS), and Sr-87/Sr-86 signatures of authigenic carbonate minerals from three modern and two ancient methane-seep provinces. The data obtained demonstrate that all deposits regardless of age or location display consistently small delta O-18(CAS)/delta S-34(CAS) slopes (similar to 0.3) and CAS does not represent ambient seawater but pore-water sulfate. This finding confirms the utility of CAS as a recorder of SD-AOM in methane-rich environments. In addition, we report that aragonites bear higher CAS contents, Sr-87/Sr-86 ratios closer to that of contemporary seawater, and a larger delta O-18(CAS)/delta S-34(CAS) slope than calcites, reflecting the shallower formation depth of aragonite where pore-water has a composition close to that of seawater with high concentrations of sulfate. The new proxy can be used to constrain the record of SD-AOM through most of Earth history by measuring the delta O-18 and delta S-34 values of CAS of methane-derived diagenetic carbonates including but not limited to seep carbonates.