Comment on bg-2021-58

Abstract

Anaerobic oxidation of methane (AOM) in marine sediments strongly limits the amount of gas reaching the water column and the atmosphere but its efficiency in counteracting future methane emissions at continental margins remains unclear. Small shifts in methane fluxes due to gas hydrate and submarine permafrost destabilization or enhanced methanogenesis in warming Arctic continental shelves may cause the redox boundary in which AOM occurs, known as Sulfate-Methane Transition Zone (SMTZ), to move closer to seafloor, with potential gas release to bottom waters. Here, we investigated the geochemical composition of pore water (SO42− and DIC concentration, δ13CDIC) and gas (CH4, δ13CCH4) in eight gravity cores collected from Ingøydjupet trough, South-Western Barents Sea. Our results show a remarkable variability in SMTZ depth, ranging from 3.5 m to 29.2 m, and that all methane is efficiently consumed by AOM within the sediment. From linear fitting of the sulfate concentration profiles, we calculated diffusive sulfate fluxes ranging from 1.5 nmol cm−2 d−1 to 12.0 nmol cm−2 d−1. AOM rates obtained for two cores using mixing models are 6.5 nmol cm−2 d−1 and 6.7 nmol cm−2 d−1 and account for only 64 % and 56 % of total sulfate reduction at the SMTZ (SRRtot), respectively. The remaining 36 % and 44 % SRRtot correspond to organoclastic sulfate reduction with rates of 3.7 nmol cm−2 d−1 and 5.3 nmol cm−2 d−1. The shallowest SMTZs (< 5 m) and largest SRRtot rates are associated with a shallow subsurface accumulation of gas visible in seismic data, highlighting how small changes in sulfate reduction rates linked to subsurface methane gradients resulted in vertical shifts in SMTZ position of > 20 m. This study provides new insights into the dynamic and biogeochemistry of the SMTZ in marine sediments of continental margins and may help predict the response of the microbial methane filter to future increase in methane fluxes due to ocean warming.