The Crucial Role of Deep-Sourced Methane in Maintaining the Subseafloor Sulfate Budget

Methane (CH4) is a powerful greenhouse gas and its largest reservoir on Earth is held in marine sedi-ments. CH4 in marine sediments is mainly stored in gas-hydrate reservoirs and deep sedimentary strata along continental margins, where large amounts of deep-sourced CH4 ascend to different degrees toward the seafloor. However, the amount of deep-sourced CH4 and its role in subseafloor carbon and sulfur cycling remains poorly constrained. We analyzed sulfate (SO42-) profiles of 157 sites along with previous published 85 sites to determine the regional distribution and amount of SO42-reduction for an area of 1.23 x 105 km2 of the northern South China Sea. Then we compared these obtained results with estimates based on sedimentation rates from the same area. Significantly higher regional SO42-flux estimates based on SO42-profiles (4.26 x 10-3 Tmol a-1), compared to lower estimates based on sedimentation rates (1.23 x 10-3 Tmol a-1), reflect abundant ascending deep-sourced CH4. The difference of the regional SO42-flux estimates (3.03 x 10-3 Tmol a-1) represents the amount of SO42-reduced by CH4 through the anaerobic oxidation of CH4 (AOM). Deep-sourced CH4 contributes 71% to total SO42-consumption in the study area, largely exceeding SO42-consumption by organoclastic sulfate reduction. Our findings substantiate that deep-sourced CH4 governs subseafloor carbon and sulfur cycling to a previously under-rated extent, fueling extensive chemosynthesis-based ecosystems along continental slope and rise.(c) 2022 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).