Subglacial lakes flood, glaciers speed up

Scientists link increase in Antarctic ice flow rate to water movements deep below

1:09pm, November 16, 2008
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Floods that occasionally surge from immense lakes trapped beneath the Antarctic ice sheet can significantly speed the flow of ice in overlying glaciers, a new study shows.

Scientists have known about the existence of large subglacial lakes in Antarctica — some of them larger than North America’s Lake Ontario — for decades. Only in the last few years, however, have satellite observations of shifts in the ice cap hinted that large volumes of water sometimes spill from one lake to another (SN: 6/17/06, p. 382).

Now, scientists’ observations, published online in Nature Geoscience on November 16, link one such flood to a 14-month–long acceleration of one of Antarctica’s largest glaciers.

The findings provide “the piece in the water-iceflow puzzle that had been missing so far: direct evidence for glacier acceleration as a result of subglacial floods,” says Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography in La Jolla, Calif. “This paper really nails it.”

“This is the smoking gun we thought was out there,” says Robert Bindschadler, a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “This [finding] will bring smiles to a lot of scientists’ faces.”

Each year, Byrd Glacier drains about 20 billion metric tons of ice from an area of Antarctica that is almost twice the size of Texas. The main trunk of the glacier, which dumps that ice into the Ross Ice Shelf, is about 75 kilometers long and 20 kilometers wide, says Leigh Stearns, a glaciologist at the University of Maine in Orono. Data suggest that between November 1960 and December 2005, ice at the glacier’s grounding line — the point where the glacier flows into the floating ice shelf — moved seaward at a steady rate of about 825 meters per year.

Then, between December 2005 and February 2007, satellite observations indicate that the lowermost 100-kilometer stretch of Byrd Glacier sped up significantly. During this time, flow rates at the glacier’s grounding line reached about 900 meters per year, a 9 percent increase. That boosted ice discharge from the glacier in 2006 by 1.7 billion metric tons, Stearns and her colleagues estimate.

Before and during the same time period, NASA’s Earth-orbiting ICESat probe detected rapid changes in the elevation of the ice sheet nearby. Between March 2004 and November 2005, two broad areas about 200 kilometers upstream of Byrd Glacier’s grounding line rose several meters — a sign that the subglacial lakes inferred to be located there gained about 2.1 cubic kilometers of water. Then, the surface of the ice sheet over those lakes began to fall, signifying drainage of the freshly accumulated water. Between June 2006 and April 2007, about 1.7 cubic kilometers of that water flowed downstream — right beneath the trunk of the glacier, which at that time was speeding along much faster than normal. The precise dates that these events began and ended are uncertain, Stearns notes, because satellite observations of the region are sporadic.

The 14-month–long acceleration of the Byrd Glacier probably isn’t related to climate change, the researchers note. Computer models used to estimate the amount of ice flowing off Antarctica should be updated in the future to account for the occasional episode of subglacial flooding, Stearns suggests. At present, however, satellites capable of detecting subglacial flooding haven’t been observing Antarctica long enough to know how often these episodes occur.

Previous studies have noted an increase in the speed of Antarctic ice streams after the ice flowed across subglacial lakes, a hint that the bedrock downstream was better lubricated by water flowing out of those lakes (SN: 3/31/07, p. 202).  And although none of the occasional floods from Antarctica’s subglacial lakes have previously been linked to changes in the behavior of overlying glaciers, scientists have suspected the lubricating effect of such floods could be substantial (SN: 3/3/07, p. 142).

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