Enigmatic Fe-Mn-fueled Anaerobic Oxidation of Methane in sulfidic coastal sediments of the Eastern Arabian Sea

Abstract. Anaerobic oxidation of methane (AOM) coupled with Fe-Mn reduction (Fe-Mn-AOM) is considered a globally important biogeochemical process in marine sediments in addition to sulfate-driven AOM (SO₄²⁻-AOM) responsible for the consumption of methane, a strong greenhouse gas. Most existing studies have emphasized the significance of Fe-Mn-AOM activities in sediments below the depth of the sulfate methane transition zone (SMTZ) with insignificant dissolved sulfide and sulfate concentrations in the porewaters. Here, we report for the first time enigmatic geochemical evidence of focused Fe-Mn-AOM activity across the SMTZ in the presence of high dissolved sulfide concentrations in a sediment core collected within the seasonal coastal hypoxic zone of the Eastern Arabian Sea (West coast of India (WCI)). The Fe-Mn-AOM activity is evident from the concurrent decrease in CH₄ concentrations, d¹³C_CH4 and d¹³C_DIC values coupled with the enrichment of porewater Fe²⁺ and Mn²⁺ concentrations at multiple depths below the seafloor. Since neither CH₄ nor reactive Fe appears to be the limiting factor controlling the Fe-Mn-AOM activity, we hypothesize that the focused Fe-Mn-AOM at multiple depths is likely fueled by the localization of metal-reducing and methanotrophic microbial communities, leading to biogeochemical heterogeneity in a dynamic seasonally hypoxic coastal environment sensitive to climate change. This study highlights new insight into CH₄-S-Fe-Mn biogeochemical cycling with far-reaching implications in climate studies linked to the estimation of sedimentary methane production and consumption.