July 17, 2013
Understanding The Mechanisms Of Supraglacial Lake Drainage
Brett Smith for redOrbit.com - Your Universe Online
Lakes often form on top of a glacier and drain onto the bedrock below, lubricating a glacier's movements as a result. A new study from an international group of researchers has found that the way these lakes drain can affect the velocity, extent and direction of a glacier's progress.
Previous research has focused on the lubrication action of supraglacial lakes after they drain, but not on the draining mechanism itself. According to the authors of the new study, which was recently published in Environmental Research Letters, these lakes can drain via two different mechanisms: a vertical shaft within the glacier called a moulin, or a crack that opens up at the base of the lake.
Using observations made in 2011 from five GPS stations, the team analyzed the drainage of two different supraglacial lakes in West Greenland -- each one by a different mechanism. The smaller Lake Half Moon overflowed its banks on one side to a moulin, which emptied the lake in about 45 hours. The larger Lake Ponting drained through a crack that formed just below the lake in two hours.
"At first, a crack in the ice beneath the lake may be small, but it deepens as water enters it because the pressure of the water overcomes the compressive action of the ice, which is trying to close the crack," explained study co-author Marco Tedesco, a professor of Earth and Atmospheric sciences at the City College of New York. "When the crack reaches the bed beneath the glacier, which could be [3,300 feet] or more below the surface, the lake empties rapidly, like a bathtub after its plug is pulled."
The team found the drainage of both lakes accelerated glacial movement. However, the drainage from Lake Ponting caused the glacier to move more rapidly and farther. The moulin drainage from Lake Half Moon caused the glacier to accelerate from a base of around 300 feet per year to a maximum of around 4,600 feet a year.
Meanwhile, Lake Ponting's drainage accelerated the glacier to a maximum speed of 5000 feet per year. The different drainage methods also affected the glacier's trajectory differently. The moulin drainage did not shift the glacier's direction, but Lake Ponting's drainage caused a slight southerly shift in the glacier's direction. "Because the different draining mechanisms affect ice velocity, they could also affect the amount of ice lost through the calving of glaciers, which results in icebergs," Tedesco said. "Because what happens on a glacier's surface impacts what is going on below, researchers are trying to look at glaciers as a system instead of as independent components.
"The surface is like the skin of a tissue and the subglacial and englacial channels that develop because of the surface water act like arteries or veins that redistribute this water internally," Tedesco added.
Tedesco said his team's study could also have implications for climate change research surrounding glacial melting.
"Knowledge of the draining mechanisms allows us to improve our understanding of how surface melting can impact sea-level rise, not only through the direct contribution of meltwater from the surface, but also through the indirect contribution on the mass loss through ice dynamics," Tedesco said.