Carbon cycling in supraglacial debris covers

Brown, Grace L. (2024) Carbon cycling in supraglacial debris covers. Doctoral thesis, Northumbria University.

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Abstract

The cryosphere plays an important role in the global carbon cycle, but few studies have examined carbon fluxes specifically over debris-covered ice. To address this limitation, this thesis investigates spatial and temporal variations in the surface CO2 flux, and its relationship to environmental variables, at two debris-covered glaciers in the Italian Alps. Near-surface CO2 fluxes were monitored over thick (0.23 m) and thin (0.06 m) debris sites over two ablation seasons, using an eddy covariance system. The CO2 flux alternates between downward and upward orientation in the day and night, respectively, and is dominated by uptake of CO2 in thick debris (mean flux = 1.58 g m-2 d-1) whereas flux magnitude is smaller and near parity over thin debris (mean flux = -0.06 g m-2 d-1). The strong correlation of daytime CO2 flux magnitude with debris temperature, and elevated ion levels recorded in debris meltwater streams, suggests atmospheric CO2 is consumed in sulphide oxidation and silicate weathering reactions at sediment-water interfaces in the debris. Night-time release of CO2 is likely to be due to microbial respiration, when melt and chemical weathering rates are low. Additional point CO2 flux measurements were made at the surface and subsurface of supraglacial debris at six sites using a portable Li-Cor gas analyser to examine CO2 flux variations across the glacier. These measurements were supplemented by water and atmospheric samples providing the first assessment of CO2 flux origin considering δ13C, as previous studies have relied on bulk chemistry alone. Interestingly, subsurface flux measurements show a net CO2 release supporting the notion of microbial presence within supraglacial debris. A future outlook for this work is to estimate the global CO2 flux attributable to debris-covered glaciers using existing glacier inventories in combination with these results.

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