| Summary: | The western ablation zone of the Greenland ice sheet contains a region that is darker than the surrounding ice. This region is several tens of kilometres wide and stretched parallel to the margin of the ice sheet for more than 350 kilometres. The dark appearance implies low radiance and therefore low spectral albedos, leading to enhanced melting. An estimation of the influence of this dark region with a simple model shows that it can increase the local melt rate in this area by several tens per cent. Therefore, this dark region can significantly affect the total mass balance of the Greenland ice sheet. Satellite images reveal that the dark region is caused by outcropping layers of ice that contain more dust than the brighter surrounding ice. This dust was initially deposited in the accumulation zone of the Greenland ice sheet, transported through the ice sheet towards the margin and released in the ablation zone. Geochemical analyses of dust from the dark region and dust from brighter reference ice confirm this hypothesis and indicate a local source for the dust, probably the nearby tundra. In addition, abundant microorganisms were observed in the dark region. Part of these organisms formed granules together with the mineral dust. As the organic matter in the dust is known to have a high light absorbency, the dark region is not only caused by dust from the outcropping ice, but biological processes also contribute to the darkening of the surface. Finally, carbonaceous particles in the ice from the dark region reveal that the material is not modern, and settled on the accumulation zone during the Holocene, during periods of enhanced eolian activity. Therefore, dust fluxes towards the ice sheet in the past contribute to albedo variations in the ablation zone of the Greenland ice sheet at present and can enhance the melting of the Greenland ice sheet without external forcing.
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