Revealing unknown subglacial carbon processes using high frequency gas measurements and stable isotopes

Ice sheets and glaciers play an important role for the global carbon cycle through the exchange of their subglacial carbon with the proglacial aquatic environment and the atmosphere in the form of CH₄ and CO₂. However, the subglacial environment below ice sheets and glaciers is largely inaccessible from the surface and hence we know very little about the carbon turnover processes in these extreme habitats that lead to this carbon export.

Biological CH₄ production and oxidation has been found in subglacial sediments across Canada, Antarctica, west Greenland and at the center of the Greenland Ice sheet. This points at a common glacial process for gaseous CH₄ and CO₂ emissions, but this knowledge is backed by very few direct field observations from two locations in Greenland and one in Iceland. The lack of field based studies is the single most-limiting factor for increasing our understanding of the magnitude and extent of subglacial carbon emission to the atmosphere and its relevance for the global carbon budget.

We present new field measurements suggesting that it is possible to quantify the carbon turnover processes in the subglacial environment using high frequency concentration measurements and stable isotope composition of CH₄ and CO₂ in gaseous and dissolved form sampled at a subglacial meltwater outlet.

During three field campaigns in the early, mid and late melt season in 2018 and 2019 we measured significantly elevated CH₄ and CO₂ concentrations in the air and water exiting a subglacial cave system. We devised a field sampling program for retrieval of discrete gas and water samples that allow identification of the original (common) source of the gaseous and dissolved CH₄ and CO₂ by quantifying the d¹³C and dH signature of the source.

Our field measurements are amongst the first to directly quantify the emission of CH₄ and CO₂ to the atmosphere and our isotopic investigations clearly show a biological source of CH₄ and its oxidation to CO₂ in the subglacial environment and point to a hydrological control on the release of both _CH4 and CO₂.

These types of data are instrumental to improve the understanding of subglacial carbon processes and design future field investigations to assess its climatic relevance and to narrow the uncertainty of emission estimates.