By Lee Anna Sherman
Why should the residents of Seattle, San Francisco, New York City and Boston worry about warming in Greenland, an ice-laden island in the North Atlantic? Because if all the water locked in the massive Greenland Ice Sheet flowed into the oceans, low-lying coastal cities worldwide would be inundated.
“The Greenland Ice Sheet could contribute up to seven meters of global sea-level rise if it were to melt,” says OSU marine geologist Joseph Stoner. “We don’t know if it’s going to melt, but that’s how much water is in the ice sheet. Therefore, we need to better understand the processes at work.”
In search of that understanding, Stoner and researchers at the University of Wisconsin-Madison are studying sediments flowing seaward in streams and rivers on the island’s southern tip. Those sediments — remnants of bedrock pulverized over eons by grinding glaciers and rushing rivers — hold clues to the ice sheet’s history across geologic time, he explains. Scientists know that the 680,000-cubic-mile chunk of snow, compressed from white to crystalline blue over many millennia, is receding. Satellite images from the past several decades show significant shrinkage. What isn’t known is the speed of melting or the extent that melting might take in coming years. By studying Greenland’s past with support from the National Science Foundation through the American Recovery and Reinvestment Act, Stoner and his colleagues hope to bring its future into clearer focus.
“The key to understanding the Greenland Ice Sheet is to use the natural record of past variability as a sort of manual to what it could do in the future,” says Stoner, an associate professor in the College of Oceanic and Atmospheric Sciences. “We’re trying to use the natural geological archive to test how the ice sheet works.”
To recreate the ice sheet’s prehistoric behavior, he and his graduate students will collect sediment samples this summer, some dating back to Earth’s infancy when the atmosphere was a soup of greenhouse gases. Tracing the origins of these silts and sands should tell the researchers where the island was exposed during “interglacial” periods — warm stretches between ice ages — and where it lay buried beneath tons of frozen snow during colder periods.
The “markers” that will reveal these ancient patterns are both chemical and magnetic, Stoner says. He explains that isotopes of lead, strontium and neodymium serve as chemical hieroglyphics, telling stories about the ages and origins of the sediments that contain them. And the magnetic properties of those sediments lend additional details to the geologic record.
To read the magnetic profiles of marine and terrestrial sediments, Stoner’s lab recently acquired a new-generation instrument: a super-conducting magnetometer for measuring the magnetic properties and composition of rocks. Instead of using liquid helium as a coolant like old-style cryogenic magnetometers do, this one compresses helium gas till it reaches 3.5 degrees Kelvin, “just a little above absolute zero,” Stoner says. “It works through super-conductivity, which only happens at extremely cold temperatures.”
Stoner’s findings could cause scientists to rethink Greenland’s role in climate-change scenarios.
“When I first got into this field, people thought ice sheets behaved really slowly,” he says. “But the geologic evidence is telling us ‘no.’ We just didn’t understand the process by which ice sheets behave quickly. It’s a reminder that just because you don’t understand the process, it doesn’t mean something’s not happening.”
