Like a bowl of spaghetti noodles spilled across the floor of the North Sea, a vast array of hidden tunnel valleys wind and meander across what was once an ice-covered landscape.
These valleys are remnants of ancient rivers that once drained water from melting ice sheets.
Now, scientists have achieved the clearest view yet of these channels. They're buried hundreds of feet beneath the seafloor, and they are enormous, ranging from about 0.6 to 3.7 miles (1 to 6 kilometers) wide.
The new imaging reveals fine-grained details within these expansive features: small, delicate ridges of sediment, larger walls of sediment that can be miles long and craters called kettle holes left behind by melting chunks of ice.
Related: See photos of Earth's vanishing ice
"We didn't expect to find these kinds of footprints of the ice sheet within the channels themselves," said study lead author James Kirkham, a marine geophysicist at the British Antarctic Survey and the University of Cambridge. "And that tells us, actually, that the ice was interacting with the channels a lot more than previously assumed."(opens in new tab)
Footprints of glaciers
These channels are the footprints of glaciers left behind from between 700,000 and 100,000 years ago, when most of the North Sea, as well as the northern two-thirds of the United Kingdom and all of Ireland were often buried under huge ice sheets. (The ice advanced and retreated seven or eight times within this period, Kirkham told Live Science.)
During periods when the climate warmed and the ice retreated, these ice sheets discharged water through hidden glacial channels beneath the ice. These channels left their imprint on the sediments below. More sediments then piled on top as the ice vanished, entombing the imprints deep beneath the seafloor.
To see these ancient impressions, geophysicists use a technology called 3D seismic reflection. In this process, scientists shoot bursts of compressed air toward the seafloor. The resulting sound waves travel through the rock and sediment layers beneath the seafloor and bounce back, where they're picked up by a shipboard receiver. Because sound travels at different speeds through different types of rock and sediment, the data can be reconstructed into a picture of the subsurface.
A map of the undersea tunnel valleys looks like a vast array of squiggles, like a scattering of spilled noodles. But zoomed in, the channels are visible in stunning detail. They meander and wind like rivers (which they once were), bounded by shear cliffs and rugged slopes. Some plummet 310 miles (500 km) deep into the sediment and are dozens of miles long.
Water and ice
The landforms within the tunnel valleys paint a complicated picture of ice retreat, Kirkham said. At times, there are signs of fairly slow and steady retreat. For example, eskers are ridges of sediment around 16 feet (5 meters) high that can stretch for many miles. They're also seen in modern-day glaciers that are moving gradually.
In other spots, the channels are punctuated by small, delicate ridges that indicate fast, dynamic ice flow, Kirkham said. Another sign of fast-moving surges of ice and water are kettle holes, which are spots where a large iceberg that has detached from the main ice sheet and moved to a new location finally gets stuck and melts.
The channels seem to have been carved by both water and ice. In some spots, there are braided channels meandering through the bottom of the canyons, Kirkham said. These were formed by flowing water, which seems to have eroded the sediment beneath the ice sheet. Once that void formed, however, the underside of the ice sagged into that gap, carving out a broader path. There are also places where valley walls seem to have collapsed, probably after the ice that was filling the valley melted away, enabling the sediment to slump in its stead.
These undersea tunnel valleys are an interesting snapshot of the past, but their real value may be in helping to predict the future. As the climate warms, the ice caps are again on the retreat. If the climate gets hot enough, West Antarctica might someday look a lot like the North Sea did 100,000 years ago, Kirkham said. The Greenland ice sheet, too, is melting rapidly. Studying the remnants of the North Sea channels and how they formed could reveal more about the dynamics governing the loss of today's ice sheets. In particular, the geological record could hint at how small-scale factors like moving water affect how much ice ends up melting into the sea, and how quickly, which could lead to better models of sea-level rise.
"In the future, we'd like to explore that idea a bit further by continued mapping, and also some computer modeling to work out how we've produced these landforms and what would need to happen at the base of an ice sheet to generate them," Kirkham said.
The findings appear today (Sept. 8) in the journal Geology.
Originally published on Live Science