| Summary: | Approximately, 7.3% of all mountain glacier area outside the Greenland and Antarctic Ice Sheets is covered by supraglacial debris. With abundant fresh sediment, water and energy, these environments provide ideal conditions for carbon gas exchange with the atmosphere driven by chemical rock weathering. Direct measurements of the near-surface vertical CO 2 flux were made using an eddy covariance-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 ablation periods of 2013 and 2016, respectively. A net downward CO 2 flux is observed at the thick debris site (mean rate = 1.57 g m -2 d -1 ), with flux magnitude increasing with increasing debris temperature and incoming shortwave radiation. During the night, the flux reverses with CO 2 released to the atmosphere, but at a much lower rate than during the daytime. The thin debris site displays a similar daily CO 2 flux cycle to the thick debris site, but the daytime downward flux magnitude is much smaller, and the daily net flux direction is upwards and close to zero (mean = 0.06 g d -1 ). Daytime fluxes increase markedly on days following overnight freezing of debris, implying mechanical shattering of rocks by frost action augments CO 2 drawdown through the supply of fresh reactive sediment. These results support previous findings that thick debris covers are important CO 2 sinks during ablation seasons and further work is needed to assess regional and global significance.
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