Evolution of the sparse debris cover during the ablation season at two small Alpine glaciers (Gran Zebrù and Sforzellina, Ortles-Cevedale group)

Despite the increase of sparse debris on many glaciers in the world and its undisputed impact on glacier ablation, both its evolution and its detailed effect have been poorly investigated yet. In this study we evaluated the evolution of the sparse debris cover during the ablation season at two small glaciers of the Ortles-Cevedale group (Central Italian Alps, Italy), the eastern tongue of Gran Zebra glacier and Sforzellina glacier. We investigated the different effects of the sparse debris on albedo, on the glacier surface temperatures (using ground-based thermal imaging) and on ablation. The supraglacial debris cover increased three times faster at Sforzellina glacier, both in terms of mass and percentage debris cover, with a mean debris coverage rate of 621 g/m2 per day and a percentage of 2.07% per day, with respect to Gran Zebra glacier. As debris accumulates on the surface mainly by melt out of englacial debris, the debris coverage rates depend both on the ablation and on the englacial debris concentration, which was found to be higher at Sforzellina glacier (mean 10,810 g/m3) as compared to Gran Zebra glacier (mean 6131 g/m3). Debris is mainly composed by clasts coarser than 25 mm at both glaciers (ca 90%), with subangular sedimentary rocks prevailing at Gran Zebra and angular metamorphic rocks at Sforzellina glacier, suggesting a debris supply from the rockwalls surrounding the glaciers, with a longer englacial transport at Gran Zebra. Albedo decreases at increasing percentage of debris covering the surface, linearly at Sforzellina and logarithmically at Gran Zebra and surface temperatures are positively correlated with the mass of debris present on the glacier surface. The influence of the percentage of debris on the mean ablation rate is explained by a quadratic function, with the ablation rate increasing at increasing debris cover for moderate debris covers, due to the decreasing albedo, and decreasing ablation rates with high percentages of debris cover (>80%), due to the insulation effect of clasts.