Dust from the dark region in the western ablation zone of the Greenland ice sheet

Abstract. A dark region tens of kilometres wide is located in the western ablation zone of the Greenland ice sheet. The dark appearance is caused by higher amounts of dust relative to the brighter surroundings. This dust has either been deposited recently or was brought to the surface by melting of outcropping ice. Because the resulting lower albedos may have a significant effect on melt rates, we analysed surface dust on the ice, also called cryoconite, from locations in the dark region as well as locations from the brighter surrounding reference ice with microscopic and geochemical techniques to unravel its composition and origin. We find that (part of) the material is derived from the outcropping ice, and that there is little difference between dust from the dark region and from the reference ice. The dust from the dark region seems enriched in trace and minor elements that are mainly present in the current atmosphere because of anthropogenic activity. This enrichment is probably caused by higher precipitation and lower melt rates in the dark region relative to the ice marginal zone. The rare earth elemental ratios of the investigated material are approximately the same for all sites and resemble Earth's average crust composition. Therefore, the cryoconite probably does not contain volcanic material. The mineralogical composition of the dust excludes Asian deserts, which are often found as provenance for glacial dust in ice cores, as source regions. Consequently, the outcropping dust likely has a more local origin. Finally, we find cyanobacteria and algae in the cryoconite. Total Organic Carbon accounts for up to 5 weight per cent of the cryoconite from the dark region, whereas dust samples from the reference ice contain only 1 % or less. This organic material is likely formed in situ. Because of their high light absorbency, cyanobacteria and the organic material they produce contribute significantly to the low albedo of the dark region.