The potential for methane production and accumulation in Greenland’s subglacial environment– Steps Towards A New Mechanistic Hydrological-Biogeochemical Model For Subglacial Methane Cycling

Abstract

 Recent studies have shown the release of methane (CH4) through the melting Greenland Ice Sheet, and have thus identified it to have an additional potential positive climate feedback. This CH4 is thought to originate from biologically active methanogenic ecosystems in subglacial sediments, where microbes produce it by converting overridden organic carbon to CH4, which then accumulates over time. Subsequent CH4 diffusion into the subglacial hydrologic network transports it then to the ice sheet margin, where it is directly emitted to the atmosphere from supersaturated proglacial streams. Methanogenesis is highly dependent on anoxic conditions, which are in turn determined by the seasonally evolving subglacial environment subject to episodic flooding and thereby recharging oxygenated waters from surface melting. The main biogeochemical and hydrological drivers influencing the rate of CH4 production, as well as the magnitude and timing of these subglacial CH4 fluxes remain largely unknown and therefore unconstrained. Addressing these unknowns is essential because CH4 is not only a powerful greenhouse gas, but also because its unaccounted release exacerbates the ongoing climate amplification in the Arctic. The lack of observational data is primarily due to the challenging conditions for accessing the subglacial environment and the shortage of direct measurements of CH4 production, consumption, and export from the Greenland Ice Sheet and the complex nature of the subglacial system. This invites the application of reaction-transport modelling tools in combination with observational data to fill these knowledge gaps by disentangling the complex processes and drivers, and eventually quantifying CH4 cycling processes in Greenland’s subglacial sediments and their impacts on the global CH4 cycle and climate change. However, such modelling tools do not currently exist. Here, we develop a coupled subglacial sediment-cavity-stream model to  explore the potential of subglacial environments to produce and accumulate methane beneath the Greenland Ice shield. The model accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport. Current field data observations are used to initialize the model, but it will also be forced over a wide range of plausible conditions (i.e. organic matter availability and reactivity, sediment thickness, terminal electron acceptor availability) that have could be found  beneath the Greenland Ice shield. The results of this large model ensemble does not only help identify the most important biogeochemical and hydrological drivers on methane production and accumulation in subglacial environments, but also allows to identify areas beneath the ice sheet that could produce and accumulate important quantities of methane.These new developments present the first step in the development of a new fully coupled hydrological-biogeochemical model for subglacial environments, which will inform upscaling efforts and guide future field work.