What causes the dark region in the melt zone of the Greenland ice sheet?

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wientjes, Irene; Oerlemans, Johannes
Holding Date 04 October 2008

In the ablation zone of the western part of the Greenland ice sheet a region exists that is darker than the surrounding surface. It is localized between 65 and 71 N at 49 W. This feature implies a lower ice albedo and therefore increases melt rates. If we would know what causes the dark region, we may predict how it will develop and how it might affect the mass balance of the Greenland ice sheet in the future.
With the help of satellite images from MODIS (Moderate-Resolution Imaging Spectroradiometer), we have analyzed the dark region. We constructed time series of radiance images from 2000 to 2007. The dark region becomes visible when the melting starts and becomes more pronounced during the summer season. Comparison of different years reveals that the dark region appears always at the same location, although its intensity varies from year to year. We also studied the relative darkening of different spectral bands. The lowering of radiances in the dark zone with respect to the brighter ice is strongest in the visible part of the spectrum and decrease with increasing wavelengths.
This behaviour is typical for dirty glacier ice, indicating that the dark region contains more dust than the surrounding ice. Aerial photographs near the dark region reveal patterns typical for surfacing of internal layers by upward motion.
We speculate that the dust originates from the last glacial maximum, when dust fluxes were much larger than today. This dust was buried in the accumulation zone of the ice sheet and, after travelling through the ice sheet, is now surfacing in the ablation zone.