Glaciers may seem to be all ice, but it turns out they can be soggy with water, a finding that should help researchers understand how glaciers slide toward the sea, and improve their predictions about rising sea levels in the face of climate change.
"Adding water to the base of glaciers and ice sheets can make them speed up," said glacialogist Joel Harper of the University of Montana at Missoula, who with his colleagues discovered an unexpected amount of water near the bottom of a glacier in Alaska.
Scientists had long known that water will pool in the crevasses at the bottom of a glacier, the huge cracks that extend upward into the ice from the bedrock. However, precisely how much water is in these "basal crevasses" was uncertain because it was hard to peer into these giant mounds of ice.
Now, after using radar and seismic imaging of Bench Glacier in southeastern Alaska, as well as by directly drilling into it, researchers learned that glaciers can possess extensive networks of interconnected, water-loaded basal crevasses.
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Harper and his colleagues focused on the temperate valley glacier, about 4 miles (7 kilometers) long and up to 650 feet (200 meters) thick, located in the Chugach Mountains east of Anchorage. Their experiments discovered numerous water-filled basal crevasses that regularly extended as much as 260 feet (80 meters) up from the bedrock into the ice.
Based on their data, the researchers estimated that the basal crevasses in Bench Glacier hold enough water to cover the glacier's bed with water 4 inches deep (10 cm). Although that may not sound like much, even a roughly 1.5-inch (4-cm) increase in the amount of water caused by spring melting can trigger a fivefold increase in the sliding speed of the glacier, Harper explained.
Harper and his colleagues actually discovered these crevasses by accident. They were originally investigating the pressure that water experiences under glaciers — this pressure and the sliding speed of glaciers can sometimes be closely tied. To observe the effects of the pressure, they aimed to melt holes in the glacier 590 feet (180 meters) deep, all the way down to the base , using jets of hot water to bore into the ice. When all the water from this drilling hit the water pooled under the glacier, they predicted, a big pressure spike would result and they could detect it at other boreholes, giving clues as to how water pressure under glaciers changes over time and space.
"Much to our surprise, the instruments showed a huge draining event long before the drill hole hit bottom," Harper recalled. "We lowered a video camera down the boreholes and found we had intersected a basal crevasse."
The researchers then began drilling to look for other water-filled crevasses and started imaging them with radar and seismic tools to learn more about them.
Glacier speed and sea level change
Given the discovery of numerous water-loaded crevasses in Bench Glacier, "there is reason to believe that they can sometimes exist in other glaciers, but now we need to figure out how common they are in other places," Harper said. "We are now drilling in Greenland."
A better understanding of how water flows under glaciers and influences their speed is key to improving our projections of sea level change, Harper said.
"One of the major barriers to generating reasonable projections of sea level rise is our limited understanding of how meltwater influences the sliding speed of glaciers and ice sheets," Harper explained. "Glacier speed influences sea level through calving of icebergs. Faster speeds result in more icebergs generated at the calving front."
The scientists detail their findings online today (Sept. 29) in the journal Nature.
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