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Sediment wedge key to glacial environmental stability

A wedge of sediment, pushed up by glacial movement, may be a buffer against moderate sea level rise, pointing to ocean temperature rise as the key factor in glacial retreat, according to two papers published today (March 1) in Science Express.

"Sediment beneath ice shelves helps stabilize ice sheets against retreat in response to rise in relative sea level of at least several meters," says Richard Alley, the Evan Pugh professor of geosciences, Penn State. "Large sea level rise, such as the more than 325 feet at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to do the same."

The researchers identified a sediment wedge beneath the Whillans Ice Stream in Antarctica using snowmobile-towed radar where ice from the Whillans Ice Stream in West Antarctica begins to float in the Ross Sea forming the Ross Ice Shelf. They report this research in Science Express in the article "Discovery of Till Deposition at the Grounding Line of Whillans Ice Stream."

The radar imaged a miles-long pile of sediments as thick as 100 feet deposited beneath the Ross Ice Shelf over the last 1000 years. The sediments are eroded out of the ground by the moving ice sheet that then drags them along and deposits them in a wedge-shaped delta.

"We found this miles-long pile of deposited sediment just where the ice stream goes afloat," says Sridhar Anandakrishnan, associate professor of geosciences, Penn State. "This showed us that sediment transport beneath the ice plays an important role in determining the size of this ice stream."

Antarctic glaciers form over the Antarctic land mass and glacial ice streams flow toward the oceans. When the edge of the glacier flows past the edge of land, that portion of the glacier begins to float and forms an ice shelf. Portions of ice shelves occasionally calve off and float into the oceans. Previous research suggested that rising sea level would push back the grounding line -- the line where grounded glacier and ice shelf meet -- shrinking the glaciers.

"Our results suggest that the grounding line is well above the point at which the ice floats and will tend to remain in the same location even though sea level changes, until sea level rises sufficiently to overcome the effect of the sediment wedge," says Anandakrishnan. "We determined the grounding line location from the drop in ice surface elevation, which was 33 feet over only about 2 miles."

According to the researchers, "the grounding-line position has probably been stable near the present position for a millennium."

Anandakrishnan and colleagues note that the wedge depicted by radar imaging closely matches wedges found beyond the floating Ross Sea on the ocean bottom. These wedges are those left at the glacial maximum and as the glaciers retreated to their present day location, indicating that this wedge formation is a natural part of ice stream movement. "The modern grounding line occurs where the bed falls away rather than where the ice thins," says Anandakrishnan.

The Science Express paper, "Effect of Sedimentation on Ice-Sheet Grounding-Line Stability," suggests reasons why the sediment wedge provides stability against the increase or decrease of a few meters or more of sea-level change.

The researchers used three different ice-flow models to first model the configuration approximating the Whillans Ice Stream and the adjacent Ross Ice Shelf assuming a flat glacial bed. After the ice streams stabilized, they instantaneously added a wedge of sediment similar to that located by the radar.

The response of these models to instantaneous sea-level rise, both with and without the sediment wedge, showed that without the sediment wedge, the ice shelf forms at the point where the ice thins; however, with the sediment wedge, the ice shelf forms where the bed falls away.

"In all three models, sea-level rise without a wedge causes grounding-line retreat," says Alley. "With the wedge, the ice over the wedge thickens to above flotation mass so that small increases in sea-level cause only small grounding line retreat which never reaches the point where the ice over the wedge floats."

However, large sea-level increase could push the grounding-line much farther back, allowing the ice above to float and the glacier as a whole to retreat. Further calculations indicate that a sea-level rise of more than 33 feet may be required to force the ice to retreat from the wedge.

"Our results, together with recent evidence that ice shelves respond sensitively to ocean-temperature changes and quickly propagate the response inland, point to greater importance of other environmental variables, and especially sub-ice-shelf temperatures," says Alley.

The researchers caution that sea level may be the primary control on the ice sheet if other variables that affect ice sheets more quickly, such as water temperature under ice shelves, remain stable.

"Common climatic forcing, including an increase in ocean temperatures, which can have very large and very rapid effects on ice sheets, is more likely to cause Antarctic glacial retreat," says Alley.

Floating ice shelves around Antarctica run aground on submerged islands. Friction from the islands helps hold back the ice behind. Warming of the water beneath the ice shelf of only one degree Fahrenheit increases the melt rate of the floating ice by almost 20 feet per year. The melting reduces friction with the islands, letting the ice flow faster. The resulting decrease in ice may be enough to allow the ice to float free of the sediment wedge, shrinking the ice sheet and raising sea level.

"Recent discoveries, including the changing lakes beneath the ice that flows into the Ross Ice Shelf, show that the great ice sheet still has many mysteries," says Alley. "Understanding these mysteries will be necessary to predict the behavior of the great ice sheet in a warming world."

Vicki Fong | EurekAlert!
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