Methane Pools and Dynamics within the Subglacial "Wetlands" of West Antarctica

Subglacial environments are seldom considered as integral parts of the global methane cycle, largely due to a dearth of in situ observations. Yet, they represent potentially enormous reservoirs of methane that can exchange with the atmosphere via subglacial water outflows or following retreat of the overlying ice. The sediments beneath the West Antarctic Ice Sheet are saturated with liquid water and connected by a hydrologically-active network of streams and lakes that eventually flow to the sea. This vast "dark wetland" habitat, akin to the "critical zone" of subaerial continental environments, hosts active microbial populations that include known methane-oxidizing bacteria. The role of subglacial methane in past and future climate depends crucially upon the size of the gas reservoirs and the extent to which these bacteria utilize methane as a carbon and energy source. We used clean-access hot water drilling technology during Dec 2018 - Jan 2019 to penetrate and sample the water column and sediments of 15 m deep Mercer Subglacial Lake, which lies 1092 m beneath the Mercer Ice Stream, West Antarctica. Here, we report measurements of water column and sediment porewater methane and oxygen concentrations, methane stable isotopes, C1:C2 hydrocarbon ratios, methane oxidation rates, and molecular characterization of microbial taxa involved in methane cycling. We compare and contrast these results with published data obtained from the Jan 2013 sampling of Whillans Subglacial Lake, a 2.2 m deep water body 800 m beneath the Whillans Ice Stream some 70 km to the northwest. These results allow us to produce a conceptual model of methane dynamics beneath West Antarctica that differs from models of subglacial systems elsewhere on Earth to date.