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(SO_(4)^(2-))profiles of 157 sites along with previous published 85 sites to determine the regional distribution and amount of SO_(4)^(2-) reduction for an area of 1.23×10^(5) km^(2) 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 SO_(4)^(2-) flux estimates based on SO_(4)^(2-) 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 SO_(4)^(2-) flux estimates(3.03×10^(-3)Tmol a^(-1))represents the amount of SO_(4)^(2-) reduced by CH_(4) through the anaerobic oxidation of CH_(4)(AOM).Deep-sourced CH_(4) contributes 71%to total SO_(4)^(2-) consumption in the study area,largely exceeding SO_(4)^(2-) 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.