Reversible anaerobic oxidation of methane, carbon assimilation, and mineral authigenesis in pockmark sediments over the past 40 kyr: Insights from C–S–O isotopes and diagenetic modelling

The long-term effects of deeply sourced methane (CH 4 ) seepage on sediment geochemistry, authigenic mineral formation, and stable isotope signatures were investigated by comparing sediment cores retrieved from a seep and a reference non-seep site on the northwestern continental margin of Madagascar. To constrain the underlying diagenetic processes, measurements of C, S, and Fe in solid and dissolved phases, stable isotope (δ 13 C, δ 34 S, δ 18 O) signatures, and 14 C sediment dating were combined with reaction-transport modeling. This approach enabled distinction between effects related to seepage and anaerobic oxidation of methane (AOM) and those associated with organic matter mineralization. Assimilation of CH 4 -derived carbon can form 13 C-depleted organic carbon (OC), which results in a substantially more negative δ 13 C-OC at the seep than at the control site throughout the 0‒10 m core. Measurements showed a correlation between the concentration of authigenic mineral phases and the isotopic depletion of bulk OC and also stronger depletion in OC isolated from carbonate concretions. Modeling further suggests that the carbonates provide physical protection and that this mechanism is responsible for the preservation of the overall depleted bulk OC signal during burial. The δ 13 C and δ 18 O in carbonate minerals vary substantially with depth, but the C and O isotopic signatures are strongly negatively correlated. This isotopic signature is interpreted to reflect a mixture between a depositional end-member formed in tropical surface water and an authigenic overprint at lower bottom-water temperatures driven by AOM. Additionally, the modeling demonstrates that reversible AOM and gas dissolution effectively shape the dissolved δ 13 C-CH 4 profile, yielding an excellent model fit when the rate of AOM is simulated with a Monod-dependency on both the electron donor (CH 4 ) and terminal acceptor (SO 4 2− ). Finally, our analyses document the effects of millennial-scale seepage on the bulk geochemistry, the dithionite extractable iron pool, the degree of pyritization, and δ 34 S-FeS 2 profiles