Color of Greenland: Tracing the Dark Ice Exposed at the Ice Sheet Margin

Abstract:

The color and albedo of the surface of a large ice sheet is critical to its response to a changing climate. Decreasing the ice surface albedo enhances surface melt and has been suggested as a mechanism to trigger rapid collapse. Each summer, dark bands of ice 20-40 km wide are exposed along the margin of the Greenland ice sheet. These dark bands are clearly visible in satellite imagery and best developed along the west coast.

We use airborne radar data in both northeast and western Greenland to demonstrate that the dark bands are the result of outcropping stratigraphy. Where these dark bands are exposed at the ice surface correlate with locations where the well defined stratigraphy imaged with airborne radar is truncated at the surface. Surface work in the northeast by Boogild and coworkers (2010) has constrained the age of the three major intervals of exposed strata. Pink or brown Pleistocene ice lies closest to the ice margin, and is overlain by white pre-Boreal ice. The impurity-rich dark strata, are dated as early Holocene. The dark strata are likely a result of either periods of elevated dust during the Holocene or excess melt during the Holocene Climatic Optimum.

We use satellite data to map the extent of the exposed dark ice in Greenland using imagery from Landsat 8, Landsat 7, ASTER VNIR, EO1 Ali, and Quickbird with spatial resolutions ranging from 0.65m to 30m. Image acquisition focused on the months of July and August when the stratigraphy is best exposed. Little dark ice is presently exposed in the southeastern margin of the Greenland ice sheet as this region experiences higher surface accumulation. By examining satellite images from multiple years we have identified areas where the patterns of the dark ice are changing. Both movement of the strata towards the margin due to ice flow and inland retreat due to increased erosion are documented.

An outstanding question is what will be color of the strata exposed as the bare ice region expands in Greenland. The next step in this work will be to determine the thickness of the dark ice and what fraction of it is presently exposed. Dating the dark ice could be completed with linking the stratigraphy to the major ice cores.