Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation

Marine sediments along continental margins host large quantities of methane (CH4) as gas hydrate, whose stability depends on temperature and pressure. Projected ocean warming is expected to destabilize the feather edge of the gas hydrate stability zone on centennial timescales, mobilizing free gas and potentially increasing seafloor methane emissions. Because methane is a potent greenhouse gas, even partial transfer of mobilized methane to the ocean and atmosphere could enhance warming via positive feedback. A key unresolved question is whether sulfate-dependent anaerobic oxidation of methane (AOM)—which today consumes most methane produced in anoxic marine sediments—can effectively prevent seafloor methane escape under warming-induced hydrate dissociation. Prior studies identify a potential “window of opportunity” for methane escape before the AOM microbial community fully adjusts to increased CH4 fluxes. This study directly addresses the capacity and timing of AOM to mitigate seafloor methane release during hydrate dissociation driven by realistic seafloor warming scenarios, especially in low-permeability sediments where hydraulic fracturing can dominate gas transport.

- AOM is the dominant present-day sink for methane in anoxic marine sediments, often consuming >90% of upward methane flux under diffusive transport, typically at the sulfate-methane transition (SMT). - Hypotheses suggest rapid hydrate dissociation can create fracture pathways that bypass the microbial filter, enabling enhanced seafloor methane escape in low-permeability sediments. - Reaction-transport models (e.g., Dale et al. 2008) indicate AOM can be efficient even in advective systems at steady state, but microbial biomass requires decades (~70 years) to adjust after an abrupt increase in methane flux, creating a transient "window of opportunity" for methane release. - Previous AOM efficiency studies under warming often prescribed constant methane supply and did not couple to hydrate dissociation dynamics or account for free gas transport, which can dominate CH4 transfer during dissociation. - Empirically, the AOM capacity relates to SMT depth (Egger et al. 2018); shallower SMT corresponds to higher sulfate gradients and higher AOM capacity. - Large fractions of global hydrate occur in impermeable muds; in such settings, hydraulic fracturing from overpressure during dissociation likely governs gas migration. - Observational and modeling studies report varied CH4 fluxes to the SRZ and seafloor; models omitting free gas transport generally predict far lower fluxes than multiphase flow models that include gas. These insights motivate a fully coupled thermo-hydraulic-geomechanical model with dynamically evolving SMT and AOM capacity to quantify mitigation potential under realistic warming forcings.

- Model framework: TOUGH+Hydrate (T+H) simulating multiphase heat and mass transport of H2O, CH4, hydrate, and NaCl in gas, liquid, hydrate, and ice phases, coupled to a geomechanical module (GeoMech) to represent hydraulic fracturing from overpressure development. A new AOM module is fully coupled to T+H-GeoMech. - Domain and initial conditions: Mid-latitude conditions; seafloor depth ~520 m; initial bottom-water temperature 5°C; gas hydrate stability zone (GHSZ) from seafloor to ~20 mbsf; upper 5 m as initial sulfate reduction zone (SRZ) free of hydrate. Initial hydrate saturation 5% pore volume in the GHSZ (thermodynamic equilibrium). Porosity 0.6; sediment grain density 2700 kg/m3; heat flow 0.04 W/m2; wet/dry conductivities 1.21/0.34 W/mK; gas composition 100% CH4. Vertical 1-D model to 400 mbsf with fine grid (0–21 mbsf ~0.17 m cells; logarithmic increase below). - Permeability range: 10⁻¹⁷ to 10⁻¹⁴ m² (clay to clean sand), spanning regimes from fracture-dominated to porous flow. - Seafloor warming forcing: Base case linear warming 0.03°C yr⁻¹ (3°C over 100 years), then held constant to 200 years. Sensitivity forcings: 0.0025, 0.005, 0.01, 0.02°C yr⁻¹ for the first 100 years. - AOM representation: Sulfate-dependent AOM only. Maximum depth-integrated AOM capacity controlled by sulfate diffusive supply, parameterized via the empirical Egger relation linking AOM capacity to SMT depth. The SRZ/SMT depth dynamically adjusts toward an equilibrium depth set by the instantaneous CH4 flux into the SRZ. Adjustment is governed by an imposed timescale reflecting microbial biomass growth to new steady state (base 80 years; translates to adjustment speed ~6.23 cm/yr). The same rate is used for SRZ expansion or contraction. Gaseous CH4 is not directly oxidized; AOM acts as a sink on dissolved CH4, drawing from gas via dissolution. - Base parameters for AOM capacity: Minimum SMT depth (SMT_min) 2 cm, giving maximum depth-integrated AOM ~8 mmol m⁻² d⁻¹ from the Egger relation. - Sensitivity analyses: • SMT_min varied from 0.1 to 3.5 cm (Cases B1–B5), corresponding to maximum AOM capacities ~109 to 5 mmol m⁻² d⁻¹. • AOM microbial adjustment time 50 and 110 years (Cases C1–C2). • Grid resolution coarser/finer tests (Cases D1–D2) at k=10⁻¹⁷ m². • No-AOM scenario (Case E). • Seafloor warming rates (Cases F1–F4) as above. - Outputs evaluated: CH4 gas saturation distribution, seafloor CH4 fluxes (gas + dissolved), depth-integrated AOM, SMT depth (equilibrium vs actual), and AOM filter efficiency defined as 100 × F_AOM / (F_AOM + F_CH4), computed instantaneously and cumulatively at 100 and 200 years. - Key physical parameters used (Table 2): permeability 10⁻¹⁷–10⁻¹⁴ m²; porosity 0.6; initial/boundary salinity 3.5%; fracture permeability 10⁻¹⁰ m²; normalized overpressure threshold 1.0. Methanogenesis omitted due to negligible rates compared to hydrate-sourced CH4.

- In low-permeability sediments (clay; k ≲ 10⁻¹⁵.⁵ m²), CH4 migration is dominated by hydraulic fracturing, producing rapid, intense seafloor emissions during hydrate dissociation. Under a 0.03°C yr⁻¹ warming, most seafloor methane escape occurs within ~75–100 years and ceases when hydrates fully dissociate in the GHSZ. - A transient “window of opportunity” arises as the SMT shoals and AOM capacity increases too slowly to match rising CH4 supply; during this period, seafloor CH4 fluxes can exceed depth-integrated AOM rates by >100×. Instantaneous AOM filter efficiency is near zero during peak emissions in low-k sediments. - Cumulative AOM filter efficiency after 100 years: • Clay (k ~10⁻¹⁷ m²): <1%; more than 99% of total CH4 escape occurs during the period of minimal AOM efficiency. • Silt to clean sand (k ~10⁻¹⁴ m²): ~2%. • Clayey silt (k ~10⁻¹⁵.⁵ m²): higher cumulative efficiency (up to ~50% by 200 years), but absolute CH4 emissions are relatively small; net mitigating effect on total emissions remains minor. - Overall, the mitigating effect of AOM on seafloor methane emissions is negligible over the first 100 years across permeabilities; emission curves with and without AOM are nearly indistinguishable at 100 years. - Flux magnitudes: Seafloor CH4 fluxes can exceed 100 mmol m⁻² d⁻¹ during dissociation in fracture-dominated regimes, much larger than in models that prescribe only dissolved-phase, constant advective inputs. - Sensitivity to SMT_min and microbial adjustment time: Results at 100 years are largely insensitive to SMT_min (5–109 mmol m⁻² d⁻¹ maximum AOM capacity) and adjustment times (50–110 years). At 200 years, clayey silt shows some sensitivity in filter efficiency, but absolute emissions remain low; cumulative seafloor CH4 release is minimally affected. - Warming-rate threshold behavior: For seafloor warming ≥0.01°C yr⁻¹ (>1°C per century), AOM efficiency remains low (<5% in clay). For warming <0.01°C yr⁻¹, seafloor CH4 discharge is drastically reduced on the 100–200 year timescale (e.g., zero discharge after 100 years at 0.005°C yr⁻¹). A threshold near 0.005°C yr⁻¹ is indicated, above which seafloor emissions increase strongly. At low forcing, the AOM filter may become more effective on millennial scales due to slower, prolonged release. - Mechanistic insights: Hydraulic fracturing both bypasses the AOM filter and accelerates gas migration, ensuring that much emission occurs before the AOM community attains full capacity.

The study resolves whether sulfate-dependent AOM can substantially mitigate methane emissions from hydrate dissociation during seafloor warming. The fully coupled multiphase, geomechanical model with a dynamic AOM module demonstrates that the biological response (AOM capacity increase via SMT shoaling and biomass growth) lags the mechanical and hydraulic response (fracturing and gas migration). Consequently, a large fraction of methane escapes during a transient adjustment period when AOM efficiency is minimal. This effect is strongest in low-permeability sediments—typical of many slope hydrate settings—where fracturing dominates transport. Even when AOM capacity is increased by more than an order of magnitude in sensitivity tests, the mitigation remains minor in fracture-dominated regimes because peak fluxes greatly exceed plausible AOM capacities during the critical window. Only in clayey silt with modest fluxes and slower migration does cumulative AOM efficiency increase substantially over 200 years, but because total emissions are low in that regime, the net mitigation of seafloor fluxes remains small. These findings imply that sedimentary AOM cannot be relied upon to offset warming-driven seafloor methane release for realistic to high warming rates (≥0.01°C yr⁻¹) on centennial timescales. While water-column oxidation and bubble stripping likely limit the fraction of seafloor methane that reaches the atmosphere—especially at slope depths of ~300–500 m—sedimentary AOM alone provides limited mitigation during the periods of highest methane escape. The results reconcile previously noted discrepancies between low advective/diffusive flux models and higher fluxes seen when multiphase free-gas transport and hydraulic fracturing are included, emphasizing the importance of these physical processes for realistic projections.

By integrating a fully coupled, dynamically constrained AOM module into a thermo-hydraulic-geomechanical hydrate model, this study shows that anaerobic oxidation of methane provides only a minor mitigation of seafloor methane emissions from warming-induced hydrate dissociation. For seafloor warming rates ≥0.01°C yr⁻¹, especially in low-permeability, fracture-dominated sediments, cumulative AOM efficiencies are typically <5% over 100–200 years, and seafloor emissions with and without AOM are nearly identical over the first century. A threshold response to seafloor warming is identified near 0.005°C yr⁻¹, below which seafloor methane discharge is greatly reduced on centennial scales and AOM may become more effective over longer times. The principal mechanism limiting mitigation is the mismatch in response times: rapid gas mobilization and migration via fracturing occur faster than AOM capacity growth, creating a window during which most methane bypasses the microbial filter. Future work should incorporate methane dissolution kinetics and saturation effects near fractures, explicitly evaluate alternative electron acceptors and aerobic oxidation in the water column, and extend to millennial timescales and to other settings (e.g., shallow Arctic shelves) where atmospheric transfer pathways may differ.

- The model neglects methane dissolution kinetics and potential pore-water CH4 saturation near fractures, which could reduce dissolution and hence AOM consumption relative to simulations; this likely overestimates AOM efficiency. - Only sulfate-dependent AOM is explicitly modeled; other electron acceptors may enhance AOM capacity, though sensitivity tests boosting capacity by >13× did not qualitatively change outcomes in low-k regimes. - Aerobic methane oxidation in surficial sediments and the water column is not included; the fate of emitted methane in the ocean and the fraction reaching the atmosphere are not addressed. - Microbial adjustment time is prescribed (50–110 years tested) and uniform; true dynamics may vary with initial conditions and organic matter reactivity. - Methanogenesis is omitted, assumed negligible compared to hydrate-sourced methane during dissociation. - One-dimensional vertical modeling with parameterized geomechanics may not capture all 3-D heterogeneities in permeability, fracture networks, and focused flow pathways. - Results apply primarily to slope hydrate systems at ~300–500 m water depth; extrapolation to other environments (e.g., very shallow shelves with permafrost) requires caution.