Radar mapping could yield new clues to past Antarctic ice stream activity

December 12, 1999

SAN FRANCISCO - A new technique using ice-penetrating radar is allowing scientists for the first time to reveal long-ago changes in West Antarctic ice streams, rivers of ice believed to be linked to the stability of the massive West Antarctic Ice Sheet. Disintegration of the ice sheet could gradually increase global sea levels 15 to 20 feet.

In the past, changes in the ice streams were determined from observing surface features, which only provided information for a few hundred years. Evidence of changes from a more distant past was carried away in the streams' relatively fast flow. Ice streams move at 0.5 to 1 kilometer a year, while the rest of the ice in the sheet moves a few meters or less per year.

Now, radar measurements of internal layers in the islands of slow-moving ice between the streams allow scientists to look at ice stream changes much farther back in time, said Nadine Nereson, a geophysics research associate at the University of Washington who worked with other scientists to develop the technique.

The five primary ice streams within the 360,000-square-mile West Antarctic Ice Sheet are labeled A through E, and the three most prominent ridges are called Siple Dome (between streams C and D) and Ridges BC and DE (named for the streams between which they lie).

The shape of layers beneath Siple Dome indicates its high point has been moving to the north. Other evidence suggests the movement likely was caused by thinning of Ice Stream C, at the southern edge of Siple Dome, before it stopped flowing rapidly about 150 years ago. Radar data from Siple Dome and ridges BC and DE in 1998 have enabled Nereson and her colleagues to analyze the pattern of change in the thickness of the ice stream system all along the Siple Coast during the past several thousand years.

"The most exciting finding is that Ice Stream B may have thinned by nearly 200 meters relative to Ice Stream C in the 1,000 years," she said.

Nereson presented the findings during a news conference at the fall meeting of the American Geophysical Union in San Francisco today (Dec. 13). She and Charles Raymond, a UW geophysics professor and principal investigator for the project, found evidence that the divide position on the ridge between ice streams B and C is the least stable of all the ridges, suggesting significant changes in the ice streams that border the ridge.

The technique of using radar to map the annual layers of glacial ice was described earlier this year in the journal Nature by UW geophysicist Edwin Waddington and colleagues from the British Antarctic Survey. Their radar images showed an arch in the layers beneath an ice divide. They called such arches "Raymond Bumps" because Nereson's co-author first postulated their existence in 1983. He showed that ice deep under an ice divide should be very hard and slow to flow, so the upper layers would tend to drape themselves over it.

The new technique looks at how an ice divide is affected by the changing depth of nearby ice streams. Radar measurements made at Siple Dome in 1994 and 1996 showed a significant Raymond bump, indicating the ice streams at its boundaries are relatively stable, Nereson said.

Preliminary evidence from data gathered in 1998 indicates Ridge BC has almost no Raymond bump, indicating that streams B and C fluctuated greatly causing the divide to move frequently. The data shows Ridge DE showed a slight Raymond bump, suggesting slightly more stability. It is not yet certain, however, whether the evidence from the other two ridges can be interpreted as clearly as that from Siple Dome, Nereson said.

The work, funded by the National Science Foundation, could help determine whether the West Antarctic Ice Sheet is disintegrating, as some have suggested, or is instead becoming more stable.

Evidence that a natural cycle has thinned the ice sheet for thousands of years and could bring its complete demise about 7,000 years from now was presented recently in the journal Science by a team led by UW geophysicist Howard Conway.
For more information, contact Nereson at (206) 685-2887 or nadine@geophys.washington.edu. During the AGU meeting, reporters can leave a message for her at the Renaissance Parc 55 Hotel, (800) 650-7272 or (415) 392-8000.

University of Washington

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