Microbial processes that mineralize organic carbon and enhance solute production at the bed of polar ice sheets could be of a magnitude sufficient to affect global elemental cycles. To investigate the biogeochemistry of a polar subglacial microbial ecosystem, we analyzed water discharged during the summer of 2012 and 2013 from Russell Glacier, a land-terminating outlet glacier at the western margin of the Greenland Ice Sheet. The molecular data implied that the most abundant and active component of the subglacial microbial community at these marginal locations were bacteria within the order Methylococcales (59-100% of reverse transcribed (RT)-rRNA sequences). mRNA transcripts of the particulate methane monooxygenase (pmoA) from these taxa were also detected, confirming that methanotrophic bacteria were functional members of this subglacial ecosystem. Dissolved methane ranged between 2.7 and 83 lM in the subglacial waters analyzed, and the concentration was inversely correlated with dissolved oxygen while positively correlated with electrical conductivity. Subglacial microbial methane production was supported by d 13 C-CH 4 values between À 64% and À 62% together with the recovery of RT-rRNA sequences that classified within the Methanosarcinales and Methanomicrobiales. Under aerobic conditions, 498% of the methane in the subglacial water was consumed over B30 days incubation at B4 1C and rates of methane oxidation were estimated at 0.32 lM per day. Our results support the occurrence of active methane cycling beneath this region of the Greenland Ice Sheet, where microbial communities poised in oxygenated subglacial drainage channels could serve as significant methane sinks.

[1] Most soils of the Dry Valleys of Antarctica are ice-cemented within a few decimeters of the ground surface despite the hyperarid conditions. This fact brings into question current sublimation models since they indicate that water vapor in soils is being lost at rates that would rid them of ice to at least several meters in less than a few thousand years, and yet most ice-rich soils in the Dry Valleys are much older. In this paper, we explore mechanisms that slow or may reverse ice loss from the soil to the atmosphere and incorporate them into a sublimation model that uses high-resolution climate and soil temperature data from 2002 to 2005 in Victoria Valley, where the surface is $10 ka old and the soil is ice-cemented 0.22 m below the surface. According to this model, ice currently sublimates 0.22 mm a À1 , which corresponds to a descent of the ice cement boundary of $1.2 mm a À1 . Water vapor condenses in the upper dry soil during the winter but is lost completely to the atmosphere during the austral summer. Some water vapor diffuses downward into the frozen soil, condensing at rates of 0.02-0.09 mm a À1 . Snow cover in the summer temporarily reverses the vapor transport and reduces the annual ice loss. Hence while snow slows long-term sublimation, the dearth of data on the duration and timing of snow cover prevent us from quantifying this effect and from assessing the potential of snowmelt to offset water loss from the soil.

Improved understanding of Greenland ice sheet hydrology is critically important for assessing its impact on current and future ice sheet dynamics and global sea level rise. This has motivated the collection and integration of in situ observations, model development, and remote sensing efforts to quantify meltwater production, as well as its phase changes, transport, and export. Particularly urgent is a better understanding of albedo feedbacks leading to enhanced surface melt, potential positive feedbacks between ice sheet hydrology and dynamics, and meltwater retention in firn. These processes are not isolated, but must be understood as part of a continuum of processes within an integrated system. This letter describes a systems approach to the study of Greenland ice sheet hydrology, emphasizing component interconnections and feedbacks, and highlighting research and observational needs.

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