Hitchhiking rocks provide details of glacial melting in West Antarctic

Rocks deposited by glaciers on mountain ranges in West Antarctica have given scientists the most direct evidence yet that parts of the ice sheet are on a long-term, natural trajectory of melting.

The West Antarctic Ice Sheet has been melting and contributing water continuously to the ocean for the last 10,000 years and is likely to keep doing so, says John Stone, University of Washington associate professor of Earth and space sciences.

Measuring and understanding changes in the Earth?s ice sheets over the past few decades, and predicting their future behavior are major challenges of modern glaciology. But it is important to view these changes in the context of what?s been happening naturally over centuries and millennia. This work establishes a background pattern of steady decline in the West Antarctic ice sheet, Stone says. If melting continues at the same pace in future, the West Antarctic Ice Sheet ? about 360,000 square miles, or about the size of Texas and Colorado combined ? would melt away in another 7,000 years.

It is still unknown if that process is being speeded by human-caused warming of the oceans and atmosphere, Stone says, but because much of the bedrock beneath the ice is below sea level, the ice sheet could be particularly susceptible to any future thinning and warming of the oceans around its edges.

The ice sheet contains enough water to raise global sea level by about 5 meters, or 16 to 17 feet, but says Stone, lead author of a paper in the Jan. 3 issue of the journal Science. ?A rapid melting event that released even a small faction of this amount could have disastrous consequences for coastal regions?.

Previous research inferred the history of the ice sheet indirectly, from such things as changing beach levels or volcanic debris. In this study, the scientists gathered rocks deposited by glaciers on mountain peaks and dated them using a new technique that allowed them to track the thinning of the ice sheet over the last few thousand years. The scientists believe they have documented the retreating margins of the ice like never before.

A research grant and logistic support from the National Science Foundation made it possible for researchers to visit seven peaks in the Ford Ranges, a series of mountain ranges near the Ross Sea. The Ford Ranges are one of only a handful of places in West Antarctica where mountains protrude through the ice sheet.

Even the peaks of the Ford Ranges ? some that now jut nearly half a mile above the ice surface ? were buried by ice 10,000 years ago, only emerging after glaciers scraped down their flanks. In the process, the glaciers left behind time capsules of a sort: rocks ranging in size from bricks to boulders that hitched rides inside glaciers until the ice melted away, leaving the rocks stranded high and dry on the mountainsides.

As the covering layer of ice thinned and disappeared, the rocks were exposed to bombardment by cosmic rays, altering their isotopic makeup. Using a particle accelerator to count the cosmic ray-produced atoms in a rock allows scientists to determine its age and, as a result, the time the glacier and rock parted ways.

?In all cases we got very tight, consistent correlations of age with altitude, so we are able to track the margins of the ice sheet coming down the mountain sides with this approach,? Stone says. The most surprising aspect is how recently the ice has thinned in West Antarctica. Ice sheets which once covered huge areas of North America and Europe had all but disappeared by 10,000 years ago. Deglaciation in West Antarctica had only just begun by that time. Hundreds of meters of ice have since disappeared, under climatic conditions very similar to the present day.

?The Ice Age never really came to an end in that part of the world,? Stone says.

Co-authors on the Science paper are Gregory Balco and Seth Cowdery, University of Washington graduate students in earth and space sciences (Cowdery was an undergraduate at Colorado College when the work was done); David Sugden, professor of geography, University of Edinburgh, Scotland; Marc Caffee, associate professor of physics, Purdue University (he was with Lawrence Livermore National Laboratory when the work was done); Louis Sass, National Outdoor Leadership School (an undergraduate student at Colorado College when the work was done) and Christine Siddoway, professor of geology, Colorado College.