CARBON GAS CYCLING IN SUPRAGLACIAL DEBRIS COVERS

Supraglacial debris covers are significant and expanding features of the World’s glacierized mountain ranges, particularly in the Arctic, subarctic, high-mountain Asia and southern Andes, and provide ideal environments for carbon exchange with the atmosphere via chemical weathering and microbial activity. Debris-covered glaciers could therefore play an important role in regional and global cycling of major greenhouse gases. This study aims to measure rates, variability and environmental controls on surface CO₂ fluxes on an alpine debris-covered glacier during the summer ablation season. Direct measurements of the near-surface vertical CO₂ flux were made using an eddy covariance and gas analyser (EC) system installed on Miage glacier, Italian Alps, at a thick debris site (>0.2 m debris) and a thin debris site (<0.06 m debris) in the June to September periods of 2013 and 2016, respectively. Upon analysis, it is clear there is a strong daily cycle in downwardly directed carbon flux, closely linked to variation in energy input from incomming shortwave radaition and debris surface temperature, contrasting with a low-magnitude upwardly directed flux during hours of darkness. In common with previously published findings, these data indicate that thick supraglacial debris covers are a strong summer melt season sink of CO₂. At the thick debris site, the average net flux is nearly -0.5 g m^-2 carbon per day, more than 2 orders of magnitude higher than reported fluxes over cryoconite. In contrast, the thin debris site has an average net flux of 0.024 g m^-2 per day, indicating a small net release of CO₂ from debris to the atmosphere. Infrequent, high-magnitude ‘pulse events’ of up to -5 g m^-2 per 0.5 hour were also observed following rainfall, although their veracity is difficult to assess due to interference of rain water with the gas analyser sensors. Future work will aim to uncover controlling processes and determine the relative roles of chemical weathering and microbial activity in carbon gas cycling, using portable greenhouse gas analysers in conjuction with the EC system on glaciers in different climatic and geological settings.