Washington, April 18 : Scientists have discovered that meltwater can penetrate the thick Greenland ice all the way to the bed and can accelerate some of the large-scale summer movements of the ice sheet.
This was uncovered by researchers from the Woods Hole Oceanographic Institution (WHOI) and the University of Washington (UW).
The researchers made the dsicovery by documenting the sudden and complete drainage of a lake of meltwater from the top of the Greenland ice sheet to its base.
According to research by glaciologists Sarah Das of WHOI and Ian Joughin of UW, the lubricating effect of the meltwater can accelerate ice flow 50- to 100 percent in some of the broad, slow-moving areas of the ice sheet.
"We found clear evidence that supraglacial lakes-the pools of meltwater that form on the surface in summer-can actually drive a crack through the ice sheet in a process called hydrofracture," said Das, an assistant scientist in the WHOI Department of Geology and Geophysics.
Thousands of lakes form on top of Greenland's glaciers every summer, as sunlight and warm air melt ice on the surface.
Past satellite observations have shown that these supraglacial lakes can disappear in as little as a day, but scientists did not know where the water was going or how quickly, nor the impact on ice flow.
Researchers have hypothesized that meltwater from the surface of Greenland's ice sheet might be lubricating the base.
"To influence flow, you have to change the conditions underneath the ice sheet, because what's going on beneath the ice dictates how quickly the ice is flowing," said Das.
"If the ice sheet is frozen to the bedrock or has very little water available, then it will flow much more slowly than if it has a lubricating and pressurized layer of water underneath to reduce friction," he added.
In July 2006, the research team used their instruments to capture the sudden, complete draining of a lake that had once covered 5.6 square kilometers of the surface and held 0.044 cubic kilometers of water.
"It's hard to envision how a trickle or a pool of meltwater from the surface could cut through thick, cold ice all the way to the bed," said Das.
The seismic signature of the fractures, the rapid drainage, and the uplift and movement of the ice all showed that water had flowed all the way down to the bed. As cracks and crevasses form and become filled with water, the greater weight and density of the water forces the ice to crack open.
As water pours down through these cracks, it forms moulins (cylindrical, vertical conduits) through the ice sheet that allow rapid drainage and likely remain open for the rest of the melt season.