Ancient Ice Age valleys offer clues to future ice sheet change

The deep valleys buried beneath the seabed of the North Sea bear witness to how the ancient ice sheets that covered the UK and Europe expelled water to keep from collapsing.

A new study published this week surprised the research team, who found that the valleys only took hundreds of years to form because they carried large amounts of meltwater away from under the ice. and towards the sea.

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

Tunnel valleys are huge channels, sometimes up to 150km long, 6km wide and 500m deep (each several times larger than Loch Ness), which drain water under melting ice caps . There are thousands buried beneath the seabed of the North Sea that record the melting of the ice sheets that have covered the UK and Western Europe over the past two million years.

Lead author James Kirkham, from the British Antarctic Survey (BAS) and the University of Cambridge, says: “This is an exciting find. We know that these spectacular valleys are carved out during the agony of the ice caps. Using state-of-the-art subterranean imaging techniques and a computer model, we learned that tunnel valleys can erode rapidly under ice caps experiencing extreme heat.”

The team analyzed “breathtaking” seismic images that provide a 3D scan of the Earth’s buried layers. Informed by tricky clues discovered in the valleys, the authors carried out a series of computer modeling experiments to simulate the development of the valleys and test how quickly they formed when the last ice cap covering the UK melted. at the end of the last ice age. 20,000 years ago.

Research suggests this process is rapid on geologic timescales, with melting ice forming giant tunnels over a few hundred years, expelling water that could otherwise accelerate rates of ice loss.

Traditionally, the drainage of water under the ice sheets is thought to stabilize the flow of ice, a process that could potentially protect modern ice sheets from collapse in a warming climate. But by inspecting the detailed seismic scans, the authors began to find telltale signatures of fast, stagnant ice movement in the valleys, complicating the picture of how these rapidly forming channels could affect the ice sheet’s future behavior. glacial.

What is certain is that the surprisingly rapid rate at which these tunnels are forming means that scientists must begin to consider their effects in models of the evolution of today’s ice sheets in the decades to come.

There are no modern analogues for this rapid process, but these ancient valleys, now buried hundreds of meters below the sludge of the North Sea seabed, record a mechanism for how ice sheets react to the extreme heat that current ice sheet models lack. . Such models do not currently resolve small-scale water drainage processes, although they appear to be an important control on future rates of ice loss and, ultimately, sea level rise.

“The rate at which these giant channels can form means they are an important, but currently overlooked, mechanism that could potentially help stabilize ice sheets in a warming world. As climate change continues to push Greenland and Antarctic ice sheets at ever-increasing rates, our findings call for renewed investigation into how tunnel valleys may help stabilize contemporary ice loss, and thus sea level rise, if they light up under the Earth’s ice caps in the future,” says James Kirkham.

Dr Kelly Hogan, co-author and geophysicist at BAS, says: “We have been observing these huge meltwater channels from areas covered by ice caps in the past for more than a century, but we don’t understand really how they formed.Our results show, for the first time, that the most important mechanism is probably summer melting on the ice surface that makes its way to the bed through fissures or ducts in the shape of a chimneys, then flows under the pressure of the ice sheet to cut the channels.

“Surface melting is already extremely important 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 is Now to know if this “extra” flow of meltwater in the channels will cause our ice sheets to flow faster, or slower, into the sea.”

The work highlights a currently overlooked process that can rapidly activate under melting ice caps. Whether these channels will act to stabilize or destabilize Earth’s contemporary ice sheets in a warming world remains an important and open question.