Sci-News.com
written by Staff
Thursday April 17, 2014
U.S. geologists have discovered what they say is a Pleistocene landscape preserved about 3 km beneath the Greenland Ice Sheet.
“We found organic soil that has been frozen to the bottom of the ice sheet for 2.7 million years,” said Dr Paul Bierman, a geologist with the University of Vermont and the lead author of the paper appearing online in the journal Science.
“The ancient soil under the Greenland Ice Sheet helps to unravel an important mystery surrounding climate change – how did big ice sheets melt and grow in response to changes in temperature?” said co-author Dr Dylan Rood from the Scottish Universities Environmental Research Center and the University of California, Santa Barbara.
The discovery indicates that even during the warmest periods since the ice sheet formed, the center of Greenland remained stable. This allowed a tundra landscape to be locked away, unmodified, under ice through millions of years of global warming and cooling.
“The traditional knowledge about glaciers is that they are very powerful agents of erosion and can effectively strip a landscape clean. Instead, we demonstrate that the Greenland Ice Sheet is not acting as an agent of erosion; in fact, at it’s center, it has performed incredibly little erosion since its inception almost three million years ago,” said co-author Lee Corbett, a graduate student at the University of Vermont.
Dr Bierman added: “rather than scraping and sculpting the landscape, the ice sheet has been frozen to the ground, a refrigerator that’s preserved this antique landscape.”
The scientists tested 17 dirty ice samples from the bottommost forty feet of the 3,053-meter GISP2 ice core extracted from Summit, Greenland, in 1993.
From this sediment, they extracted a rare form of the element beryllium, an isotope called beryllium-10. Formed by cosmic rays, it falls from the sky and sticks to rock and soil. The longer soil is exposed at Earth’s surface, the more beryllium-10 it accumulates. Measuring how much is in soil or a rock gives geologists a kind of exposure clock.
The researchers expected to only find soil eroded from glacier-scoured bedrock in the sediment at the bottom of the ice core.
They planned to work diligently to find vanishingly small amounts of the beryllium – since the landscape under the ice sheet would have not been exposed to the sky.
“On a global basis, we only find these sorts of beryllium concentrations in soils that have developed over hundreds of thousands to millions of years,” said co-author Joseph Graly from the University of Wyoming.
The findings show that the soil had been stable and exposed at the surface for somewhere between 200,000 and 1 million years before being covered by ice. To help interpret them, the scientists also measured nitrogen and carbon that could have been left by plant material in the core sample.
“The fact that measurable amounts of organic material were found in the silty ice indicates that soil must have been present under the ice,” said co-author Dr Andrea Lini from the University of Vermont.
“Greenland really was green! However, it was millions of years ago. Greenland looked like the green Alaskan tundra, before it was covered by the second largest body of ice on Earth,” Dr Rood said.
To confirm their findings about this ancient landscape, the researchers also measured beryllium levels in a modern permafrost tundra soil on the North Slope of Alaska.
“The values were very similar, which made us more confident that what we found under Greenland was tundra soil,” Dr Bierman concluded.
**************************
NASA Science
Production editor, Dr. Tony Phillips
August 29, 2014
Data from a NASA airborne science mission has revealed an immense and previously unknown canyon hidden under a mile of Greenland ice.
"One might assume that the landscape of the Earth has been fully explored and mapped," said Jonathan Bamber, professor of physical geography at the University of Bristol in the United Kingdom, and lead author of the study published in today's issue of Science. "Our research shows there's still a lot left to discover."
The canyon has the characteristics of a winding river channel and is at least 460 miles (750 kilometers) long, making it longer than the Grand Canyon. In some places, it is as deep as 2,600 feet (800 meters), on scale with segments of the Grand Canyon. This immense feature is thought to predate the ice sheet that has covered Greenland for the last few million years.
The scientists used thousands of miles of airborne radar data, collected by NASA and researchers from the United Kingdom and Germany over several decades, to piece together the landscape lying beneath the Greenland ice sheet.
A large portion of this data was collected from 2009 through 2012 by NASA's Operation IceBridge, an airborne science campaign that studies polar ice. One of IceBridge's scientific instruments, the Multichannel Coherent Radar Depth Sounder, can see through vast layers of ice to measure its thickness and the shape of bedrock below.
In their analysis of the radar data, the team discovered a continuous bedrock canyon that extends from almost the center of the island and ends beneath the Petermann Glacier fjord in northern Greenland.
At certain frequencies, radio waves can travel through the ice and bounce off the bedrock underneath. The amount of times the radio waves took to bounce back helped researchers determine the depth of the canyon. The longer it took, the deeper the bedrock feature.
"Two things helped lead to this discovery," said Michael Studinger, IceBridge project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "It was the enormous amount of data collected by IceBridge and the work of combining it with other datasets into a Greenland-wide compilation of all existing data that makes this feature appear in front of our eyes."
The researchers believe the canyon plays an important role in transporting sub-glacial meltwater from the interior of Greenland to the edge of the ice sheet into the ocean. Evidence suggests that before the presence of the ice sheet, as much as 4 million years ago, water flowed in the canyon from the interior to the coast and was a major river system.
"It is quite remarkable that a channel the size of the Grand Canyon is discovered in the 21st century below the Greenland ice sheet," said Studinger. "It shows how little we still know about the bedrock below large continental ice sheets."
The IceBridge campaign will return to Greenland in March 2014 to continue collecting data on land and sea ice in the Arctic using a suite of instruments that includes ice-penetrating radar.
No comments:
Post a Comment