Greenland's ice sheet has a mysterious dark zone that has been getting darker in recent years. Researchers now reveal what's behind this anomaly.

Greenland's Dark Zone

The dark zone is a stripe of fast-melting ice on the western flank of Greenland's ice sheet. It spans about 248 miles wide in length and 62 miles at its widest point.

The area, referred to as the ablation zone, absorbs more energy. Hence, it melts faster compared to other sections of the ice sheet.

Earlier theories attributed the darkening to the presence of water on top of the ice sheet. However, a new study published in the journal Nature Communications proposed that this is due to impurities such as carbon and ice-dwelling algae.

Study author Alun Hubbard, from Norway's Center for Arctic Gas Hydrate, Environment and Climate, and colleagues used a drone to capture images of a section of the dark zone in Greenland's ice sheet.

While the crevasses and the pools of meltwater can help explain some of the dark spots, Hubbard and colleagues found that a uniform coating of impurities, such as trapped soot and dust that have collected over the years from faraway factories and fires, was present on most of the shadowy surfaces.

These impurities also include dark-colored algal blooms from microbes that were recently found to be capable of thriving in Greenland's harsh environment.

Ice Algae

Algae require nutrients and food, particularly, organic carbon, dust, and water. A finely distributed layer of dust, as well as black carbon that gives nutrition to dark-colored algae, cover the dark zone.

Likewise, a slight increase in atmospheric temperature and liquid water production seem to promote algae colonization across the ice sheet.

"In summer, these are plentiful, and the algal bloom takes off. Because algae are dark in color, they reinforce the dark zone," Hubbard said.

The researchers think that the surface impurities consisting of dust, black carbon and ice algae are the primary contributors to the dark patch that threatens to speed up melting.

"Distributed surface impurities-an admixture of dust, black carbon and pigmented algae-explain 73% of the observed spatial variability in albedo and are responsible for the dark zone itself," the researchers wrote in their study, which was published online in March.

"Crevassing and supraglacial water also drive albedo reduction but due to their limited extent, explain just 12 and 15% of the observed variability respectively."