G.: The ablation zone in northeast Greenland: Ice types, albedos, and impurities

ABSTRACT. Ice types, albedos and impurity content are characterized for the ablation zone of the Greenland ice sheet in Kronprinz Christians Land (808N, 248W). Along this ice margin the width of the ablation zone is only about 8 km. The emergence and melting of old ice in the ablation zone creates a surface layer of dust that was originally deposited with snowfall high on the ice sheet. This debris cover is augmented by locally derived wind-blown sediment. Subsequently, the surface dust particles often aggregate together to form centimetre-scale clumps that melt into the ice, creating cryoconite holes. The debris in the cryoconite holes becomes hidden from sunlight, raising the area-averaged albedo relative to surfaces with uniform debris cover. Spectral and broadband albedos were obtained for snow, ice hummocks, debris-covered ice, cryoconite-studded ice and barren tundra surfaces. Broadband ice albedos varied from 0.2 (for ice with heavy loading of uniform debris) to 0.6 (for ice hummocks with cryoconite holes). The cryoconite material itself has albedo 0.1 when wet. Areal distribution of the major surface types was estimated visually from a transect video as a function of distance from the ice edge (330ma.s.l.). Ablation rates were measured along a transect from the ice margin to the slush zone 8 km from the margin (550ma.s.l.), traversing both Pleistocene and Holocene ice. Ablation rates in early August averaged 2 cmd–1. Impurity concentrations were typically 4.3mg L–1 in the subsurface ice