Reaction-transport modelling of methane cycling beneath the Greenland Ice Sheet

Abstract. Glacial and ice sheet advances have buried large amounts of organic matter (OM), which under anoxic subglacial conditions can be microbially converted into methane (CH₄). Although CH₄ emissions have been observed at glacier margins, the capacity of subglacial environments to sustain such fluxes remains uncertain. To address this, we developed a reaction–transport model (RTM) to simulate CH₄ production, transformation, and transport in sediments beneath warm-based regions of the Greenland Ice Sheet (GrIS) margin. The model explores a wide range of environmental conditions, including sediment thickness, OM quantity and reactivity, O₂ availability, and methanotrophic activity.

Model simulations show that subglacial sediments are largely anoxic. Oxygen (O₂) penetration into subglacial sediments is generally restricted to the upper few tens of centimetres, with an average penetration depth of 22.8 cm. Microbial OM degradation and aerobic CH₄ oxidation (AeOM) represent the main O₂ sinks. Their relative contributions vary with CH₄ availability. AeOM dominates in methane-rich sediments, whereas OM degradation prevails in methane-poor environments. Modeled depth-integrated methanogenesis rates range from 0.1 to 1600 mmol-CH₄ m^-2 yr^-1 (mean 73 mmol-CH₄ m^-2 yr^-1) and are primarily controlled by OM reactivity, with sediment depth and OM concentration exerting only a small secondary influence. This sensitivity of CH₄ production rates to OM reactivity can produce sharp thresholds, where small decreases in reactivity strongly suppress CH₄ fluxes. A highly variable fraction of the generated CH₄ is consumed by AeOM within the shallow oxygenated zone, and is controlled by OM reactivity and the AeOM rate constant. Resulting net diffusive CH₄ fluxes can range between 0–234.7 mmol m^-2 yr^-1. Results show that even shallow sediments (<1 m) can sustain a significant CH₄ release into the subglacial environment when highly reactive OM is available, while oxidation efficiency tends to decline in thick, OM-rich deposits.

Comparison with field measurements of CH₄ export data from southwest GrIS catchments suggests that observed fluxes could be already sustained by subglacial sediments that contain as little as 0.6 wt% of relatively unreactive OM assuming a catchment sediment cover of 10 % with sediment depths of 9 m.