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 sediments. 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 × 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 × 10−3 Tmol a−1), compared to lower estimates based on sedimentation rates (1.23 × 10−3 Tmol a−1), reflect abundant ascending deep-sourced CH4. The difference of the regional SO42− flux estimates (3.03 × 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 underrated extent, fueling extensive chemosynthesis-based ecosystems along continental slope and rise.