The valleys of the ancient ice age provide clues about future changes in the ice sheet

Deep valleys buried beneath the seabed of the North Sea record how ancient ice sheets that once covered Britain and Europe pushed out water to stop them from collapsing.

A new study published this week surprised a research team by finding that the valleys took just hundreds of years to form as they carried huge amounts of meltwater from beneath the ice to the sea.

This new understanding of when huge ice sheets melted 20,000 years ago has implications for how glaciers may respond to a warming climate today. The study is published in the journal Quaternary Science Reviews.

The tunnel valleys are huge channels, sometimes up to 150 km long, 6 km wide and 500 m deep (each several times the size of Loch Ness), which drain water from under the melting ice. Thousands buried under the seabed of the North Sea record the melting of the ice sheet that has covered Great Britain and Western Europe for the past two million years.

Lead author James Kirkham of the British Antarctic Survey (BAS) and the University of Cambridge says: “This is an exciting discovery. We know that these spectacular valleys were carved out during the dying hours of the ice sheet. Using a combination of state-of-the-art subsurface imaging techniques and a computer modelwe learned that tunnel valleys can erode rapidly under ice sheets that experience extreme temperatures.’

The team analyzed the “staggeringly detailed” seismic images which provide a 3D scan of the Earth’s buried layers. Based on the subtle clues found in the valleys, the authors conducted a series of computer simulation experiments to model valley development, and test how quickly they formed when the last ice sheet to cover Great Britain melted at the end of the last ice age, around 20,000 years ago.

Research shows that this process occurs rapidly on geological timescales, with melting ice forming giant tunnel valleys over hundreds of years, releasing water that would otherwise accelerate the rate of ice loss.

Drainage of water from beneath the ice sheet is traditionally believed to stabilize ice flow, a process that could potentially protect modern-day ice sheets from collapse under a warming climate. But when examining detailed seismic scans, the authors began to find distinctive signatures of both stagnant and rapid ice movement in the valleys, complicating the picture of how these rapidly forming channels might affect the ice sheet’s future behavior.

What’s certain is that the astonishingly fast speed at which these tunnels form means that scientists need to start factoring their effects into models of how modern ice sheets will evolve over the coming decades or centuries.

There are no modern analogues to this rapid process, but these ancient valleys, now buried hundreds of meters under the silt of the North Sea seafloor, capture a mechanism for how ice sheets respond to extreme heat that is not present in modern ice sheet models. . Such models do not currently address small-scale water drainage processes, despite being an important control on future rates of ice loss and ultimately sea-level rise.

“The speed with which these giant channels can form means that they are an important but so far overlooked mechanism that could potentially help stabilize the ice sheet under warming conditions. climate change continues to drive the retreat of the present-day Greenland and Antarctic ice sheets at an ever-increasing rate, our results call for new research into how tunnel valleys may help stabilize present-day ice loss and thus sea-level rise if they are subsumed under the Earth’s ice sheets in the future.” says James Kirkham.

Dr Kelly Hogan, co-author and BAS geophysicist, says: ‘We’ve been observing these huge channels of meltwater from areas covered by ice sheets in the past for over a century, but we didn’t understand how they formed. the results show for the first time that the most important mechanism is likely to be summer melt at the surface of the ice, which makes its way to the bed through cracks or chimney-like pipes and then flows under ice sheet pressure to cut channels.

“Surface melting is already critical to the Greenland ice sheet today, and this process of transporting water through the system will only increase as our climate warms. The crucial question now is whether this ‘extra’ flow of meltwater into the channels will cause our ice sheets to flow faster or slower into the sea.”

The work highlights a process that is currently forgotten, and which could quickly turn on under the melting ice sheet. Whether these channels will act to stabilize or destabilize Earth’s current ice sheets under warming conditions remains an important and open question.