AUSTIN, Texas—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 drawing on data gathered by Ginny Catania, a research associate at the Institute for Geophysics at The University of Texas at Austin’s Jackson School of Geosciences.
"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, co-author on both papers and the Evan Pugh Professor of Geosciences at Pennsylvania State University. "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."
Catania used a snowmobile-towed radar to gather data in the region where ice from the Whillans Ice Stream in West Antarctica begins to float in the Ross Sea, forming the Ross Ice Shelf. She and her colleagues, including first author Sridhar Anandakrishnan, associate professor of geosciences at Pennsylvania State University, then identified a sediment wedge beneath the ice, as reported in the Science Express article "Discovery of Till Deposition at the Grounding Line of Whillans Ice Stream."
The grounding line is where ice sheet transitions from resting on the Antarctic land mass to floating over water on the ice shelf.
"In the past there has been very little information about grounding lines and how they control ice flow," says Catania. "New studies reveal that grounding lines have greater control over ice flow than previously realized."
The discovery may help scientists predict how Antarctic ice responds to rising ocean temperatures and contributes to rises in sea level. The subject was a major area of uncertainty in the recent assessment from the Intergovernmental Panel on Climate Change. The geological record shows that in the past ice sheets have collapsed causing rapid sea-level rise. Such natural variability needs to be better understood in context with increased climate warming.
"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.
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 imaged by radar 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 forms as 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. 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.
Researchers on the "Discovery of Till Deposition" paper include Anandakrishnan; Alley; Catania; and Huw Horgan, graduate student in geosciences, Pennsylvania State University. Researchers on the "Effect of Sedimentation" paper include Alley; Anandakrishnan; Todd K. Dupont, assistant professor of earth and environmental sciences, University of Illinois at Chicago; Byron Parizek, assistant professor of physics, College of New Jersey; and David Pollard, senior research associate, geosciences, Pennsylvania State University. The National Science Foundation and the Gary Comer Science and Education Foundation supported portions of this work.