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.