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Hidden valleys buried beneath the seafloor in the North Sea were rapidly eroded during the “death spasms” of an ancient ice sheet towards the end of the last Ice Age about 20,000 years ago, a new study shows. The surprising subsurface structures could provide clues as to how modern ice sheets will respond to the rapid warming caused by climate changesay researchers.
The buried structures, known as tunnel valleys, are massive subterranean canyons etched into the ancient seafloor by meltwater draining into channels beneath the ice sheets. The enormous weight of the rapidly melting ice sheets forced the flowing water to carve deep canyons in the seabed; These channels have since been covered by hundreds of meters of sedimentary deposits. According to a, tunnel valleys can be up to 150 kilometers long, 6 km wide and 500 meters deep statement of researchers (opens in new tab).
In 2021 researchers from the British Antarctic Survey (BAS) mapped the network of tunnel valleys in the North Sea, which was once covered by a massive ice sheet that also covered parts of continental Europe and Britain during the last ice age between 126,000 and 12,000 years ago. Using 3D seismic reflection technology, which emits sound waves to search for structures beneath the seafloor, the team uncovered thousands of buried canyons, some dating back around 2 million years. These results were published in the journal in September 2021 geology (opens in new tab).
In the new study, published Oct. 5 in the journal Reviews of the Quaternary Sciences (opens in new tab), the same researchers used the canyon maps in combination with computer models to determine exactly how some of the tunnel valleys formed. The results indicated that the tunnels were likely carved over a period of a few centuries, which is much faster than the team initially anticipated.
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“This is an exciting discovery. We know these spectacular valleys are being sculpted during the death throes of the ice sheets,” the study’s lead author James Kirkham, a graduate student at BAS, said in the statement. “We learned that tunnel valleys under ice sheets exposed to extreme heat can erode rapidly.”
Scientists have known of similar tunnel valleys for decades, but until now the formation of these channels has been a mystery.
“We’ve observed these giant meltwater channels in areas covered by ice sheets for more than a century in the past, but we didn’t really understand how they formed,” said study co-author Kelly Hogan, a marine geophysicist at BAS, into the statement.
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Tunnel valleys form when meltwater drains through vertical cracks in the ice into a meltwater river beneath the ice sheet, which channels fluid like a massive “piping system,” the researchers wrote in the paper. As a result, valley formation is highly seasonal, with increased summer melt leading to more meltwater temporarily accelerating valley growth.
Although the tunnel valleys form towards the end of an ice sheet’s life, the study’s authors surmise that this drainage system may actually have reduced ice melt and extended the lifespan of the ancient North Sea ice sheet. This hypothesis proposes that by diverting meltwater away from the ice sheets, the channels prevented liquid from pooling on or under the ice, thereby preventing more ice from melting.
However, researchers are not sure how fast the ice sheet was melting at that point. Some tunnel valleys showed signs of limited ice movement, suggesting that the valleys slowed ice loss. But others showed evidence of rapid ice retreat, which could mean the valleys actually had the opposite effect of increasing the rate of ice loss, according to the statement.
Scientists will therefore continue to study the tunnel valleys to see if they can figure out how meltwater channels can affect ice loss rates. “The key question now is, will this additional meltwater flow in channels cause our ice sheets to flow out to sea faster or slower?” Hogan said.
Answering this question could be crucial to accurately predicting how modern ice sheets, such as those in Antarctic and Greenland, will be affected by climate change, the researchers said.
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Current models predicting the rate of ice loss in these regions do not account for tunnel valleys, meaning researchers are missing an important piece of the puzzle. If new tunnel valleys form or “turn on” (assuming they haven’t already) under modern ice sheets, it could drastically change the rate of ice sheet melting, especially since these structures take only a few hundred years to form. the researchers wrote.
“The rate at which these giant channels can form means they’re an important but currently ignored mechanism,” Kirkham said.
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