Anaerobic oxidation of methane and the stoichiometry of remineralization processes in continental margin sediments

Abstract

In many coastal and continental margin sediments, pore‐water property–property plots yield values of rC:S, the stoichiometric ratio of dissolved inorganic carbon (DIC) produced to sulfate reduced, that are lower than the commonly assumed value of 2. Remineralization of organic matter more reduced than CH2O can cause such observations, as can DIC loss due to authigenic carbonate precipitation. However, through studies of Santa Monica Basin sediments, we have observed that these observations could also be related to the occurrence of anaerobic oxidation of methane (AOM) in sediments. Specifically, using a reactive transport sediment model, we have shown that AOM driven by an “external” methane source (e.g., an upward flux from ancient gas hydrates) can lead to values of rC:S < 2. This contrasts with what is observed when AOM is driven by methane produced during in situ methanogenesis, resulting from the deposition of reactive organic matter to the sediment surface. This situation does not lead to deviations in the value of rC:S from that seen solely for the occurrence of organic matter remineralization by sulfate reduction. With real pore‐water data, if carbonate precipitation is adequately accounted for, observed deviations in values of rC:S from predicted end‐member values for organic matter remineralization and AOM can provide information about the occurrence of AOM and upward methane fluxes in sediments.