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

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Author(s): I. G. M. Wientjes | R. S. W. Van de Wal | G. J. Reichart | A. Sluijs | J. Oerlemans

Journal: The Cryosphere Discussions
ISSN 1994-0432

Volume: 4;
Issue: 4;
Start page: 2557;
Date: 2010;
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ABSTRACT
A dark region of tens of kilometres width is present on the western ablation zone of the Greenland ice sheet. The dark appearance is caused by higher amounts of dust. This dust has either been deposited recently or was brought to the surface by outcropping ice. Because the resulting lower albedos may have a significant effect on melt rates, we analysed surface dust, 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 the composition and origin. We find that (part of) the material indeed crops out from the ice, and that there is little difference between dust from the dark region and from the reference ice. Although, 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 Earths average crust composition. Therefore, the cryoconite does probably 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 regional origin. Finally, we find cyanobacteria and algae in the cryoconite. Total Organic Carbon accounts for up to 5 weight percentage 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.
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