Surface processes darkening the southwestern ice sheet of Kalaallit Nunaat (Greenland)

Each melt season, a "Dark Zone" develops on the southwestern margin of the ice sheet of Kalaallit Nunaat, reducing surface albedo and enhancing surface melt rates. While several processes are known to locally influence albedo, their respective contributions to the regional-scale darkening remain unknown. Here, we combine laboratory measurements, in situ observations, and physical and probabilistic modeling to quantify the albedo-reducing mechanisms across the Dark Zone. Our results indicate that the darkening is primarily driven by a combination of cryoconite material accumulation and densification of the ice column, followed by the development of microalgal blooms, while mineral dust particles do not directly contribute to the darkening. Our study provides an empirically constrained physical albedo model to represent bare ice darkening processes at play in the Dark Zone and beyond, which can guide modeling efforts to include surface darkening in ice sheet mass loss predictions.