The potential for methane production and release from subglacial sediments underneath the Greenland Ice Sheet – a model sensitivity study

Recent studies have shown the release of methane (CH4) from the melting Greenland Ice Sheet (GrIS) and identified it as having an additional potential positive climate feedback. This methane originates mainly from acetoclastic methanogenesis in subglacial sediments, accumulates over time, and subsequently diffuses into the subglacial hydrologic network which transports it to the ice sheet margin. The rates of methane production and emission from GrIS subglacial sediments likely depend on a number of factors, including sediment depth and distribution, organic matter content in the sediment and its reactivity, the redox conditions, and downstream methanotrophic activity; however, their relative significance remains unquantified. Here, we use a reaction-transport model that accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport to quantitatively assess the potential for biogenic methane production and emissions from subglacial sediments underneath the Greenland Ice Sheet. The model is run over a large environmental condition model ensemble (n=3840) covering the entire range of plausible subglacial sediment thickness, subglacial organic matter availability and reactivity, oxygen concentration and methanotrophic activity as constrained by available field observations from subglacial and/or similar environments and/or laboratory experiments. Model results are discussed in the context of available field observations. Results show that methanogenic activity in subglacial sediments can produce large quantities of methane (10-5 -7.9⋅101 mmol m-2 yr-1). Subglacial methane production rates compare well with observations from laboratory studies. They are strongly controlled by organic matter availability and subglacial sediment depth, but are less sensitive to the availability of oxygen in overlying waters. Only for low organic carbon contents, low methanotrophic rate constants and/or high oxygen concentrations does methane production become more sensitive to oxygen concentration in overlying waters. Simulated methane effluxes vary four orders of magnitude and again strongly depend on organic matter availability and subglacial sediment depths. However, in contrast to methane production, methane efflux is also sensitive to oxygen concentration and methanotrophic activity. Methane effluxes generally decrease with increasing oxygen concentration and their sensitivity to oxygen concentration increases with increasing methanotrophic activity. Model results show that subglacial sediments can support methane effluxes that are up to 100 times higher than the flux required to sustain observed subglacial methane fluxes at the outflow (0.653 mmol m-2 yr-1 Lamarche-Gagnon et al., 2019) for realistic organic carbon contents (0.06 - 0.5 wt%), reactivity (0.013-1.1 yr-1), subglacial sediment depths (100-500 cm) and methanotrophic rate constants (1010-1012 mol cm-3 yr-1) under both anoxic and partly oxic conditions (<100 µM).