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

Sulfate-driven anaerobic oxidation of methane (SD-AOM) supportschemosynthesis-based communities and limits the release of methanefrom marine sediments. Formation of authigenic carbonates at activemethane seeps is promoted by SD-AOM stoichiometry. While distinctivelysmall δ 18 O/δ 34 S slopes of pore fluid sulfate have been shown totypify modern methane-rich environments, identification of such environmentshas been difficult for the geological past due to the lack ofsedimentary pore fluids. However, if the isotopic composition of sulfatewere archived in authigenic carbonate during early diagenesis, carbonate-associated sulfate (CAS) should display the characteristic δ 18 O-δ 34 Spattern. To test this hypothesis, we investigated the δ 18 O CAS , δ 34 S CAS ,and 87 Sr/ 86 Sr signatures of authigenic carbonate minerals from threemodern and two ancient methane-seep provinces. The data obtaineddemonstrate that all deposits regardless of age or location display consistentlysmall δ 18 O CAS /δ 34 S CAS slopes (∼0.3) and CAS does not representambient seawater but pore-water sulfate. This finding confirms theutility of CAS as a recorder of SD-AOM in methane-rich environments.In addition, we report that aragonites bear higher CAS contents, 87 Sr/ 86 Sr ratios closer to that of contemporary seawater, and a largerδ 18 O CAS /δ 34 S CAS slope than calcites, reflecting the shallower formationdepth of aragonite where pore-water has a composition close to thatof seawater with high concentrations of sulfate. The new proxy can beused to constrain the record of SD-AOM through most of Earth historyby measuring the δ 18 O and δ 34 S values of CAS of methane-deriveddiagenetic carbonates including but not limited to seep carbonates.