The formation and evolution of the supraglacial weathering crust on the Greenland ice sheet

Abstract

The near-surface weathering crust is a thin (<0.5 m), low density ice layer that develops on glacier surfaces during the ablation season and is formed by internal melting driven by the penetration of shortwave radiation (SWR) into polycrystalline glacier ice. This ‘photic zone’ hosts microbial communities, mediates biogeochemical processes and routes meltwater to the channelised supraglacial drainage network. Despite these critical roles, direct field measurements of weathering crust formation and evolution are scarce—rather, current understanding is largely derived from modelling approaches. Here, we present in situ measurements of weathering crust evolution at five sites on the western Greenland ice sheet, each over a 19–25 hour period. Shallow ice cores revealed weathering crust ice densities of 420–910 kg m−3, demonstrating dynamic evolution of the weathering crust linked to diurnal SWR receipt. We compare our empirical data with two existing weathering crust models, neither of which fully reproduce the observed ice density or its temporal variability. Additionally, we reveal that the density of the uppermost 0.1 m of the weathering crust is a key control upon bare-ice albedo. Our findings highlight the need for improved process-level-understanding and parameterisations of weathering crust dynamics in surface energy balance models.