Ancient Ice Age valleys provide clues to future ice sheet changes

Newswise – Deep valleys buried beneath the seabed of the North Sea document how the ancient ice sheets that used to cover Britain and Europe threw up water to protect themselves from collapsing.

A new study published this week surprised the research team, who discovered the valleys took only hundreds of years to form as they transported vast amounts of meltwater away from beneath the ice and out to sea.

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

Tunnel valleys are huge channels, sometimes up to 150km long, 6km wide and 500m deep (each many times larger than Loch Ness), draining water beneath melting ice sheets. Thousands are buried beneath the seabed of the North Sea, documenting the melting of the ice sheets that have covered Britain 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 discovery. We know these spectacular valleys were formed in the death throes of the ice sheets. Using a combination of state-of-the-art subsurface imaging techniques and a computer model, we have learned that tunnel valleys under ice sheets exposed to extreme heat can erode rapidly.”

The team analyzed stunningly detailed seismic images that provide a 3D scan of Earth’s buried layers. Based on subtle clues discovered in the valleys, the authors ran a series of computer modeling experiments to simulate the evolution of the valleys and test how quickly they formed when the last ice sheet to cover Britain ended the last ice age melted 20,000 years ago.

Research suggests that this process is rapid on geological timescales, with melting ice forming huge tunnel valleys over hundreds of years and ejecting water that could otherwise accelerate ice loss.

Traditionally, the runoff of water beneath ice sheets has been thought to stabilize ice flow, a process that could potentially protect modern ice sheets from collapsing in a warming climate. But while examining the detailed seismic scans, the authors began to find telltale signatures of both stagnant and fast-moving ice in the valleys, complicating the picture of how these rapidly-forming channels might affect future ice sheet behavior.

What is certain is that the surprisingly rapid rate at which these tunnels are forming means scientists need to start incorporating their effects in models of how today’s ice sheets will evolve over the coming decades to centuries.

There are no modern analogues for this rapid process, but these ancient valleys, now buried hundreds of meters below the mud of the North Sea seafloor, record a mechanism of how ice sheets respond to extreme heat that is absent from today’s ice sheet models. Such models do not currently solve fine-scale water drainage processes, although they appear to be an important control over future rates of ice loss and ultimately sea level rise.

James Kirkham continues: “The speed at which these giant channels can form means they are an important but currently ignored mechanism that can potentially help stabilize ice sheets in a warming world. As climate change drives the retreat of today’s Greenland and Antarctic ice sheets at ever-increasing rates, our findings warrant a re-examination of how tunnel valleys may help stabilize today’s ice losses, and hence sea level rise, when they transition under the ice sheets of the future Earth.”

dr Kelly Hogan, co-author and geophysicist at BAS says:

“We’ve observed these giant meltwater channels from areas covered by ice sheets for more than a century in the past, but we didn’t really understand how they formed. Our results show for the first time that the most important mechanism is probably summer melting at the ice surface, which works its way through fissures or vent-like channels to the bottom and then flows under ice sheet pressure to sever the channels. The melting of the surface is already of enormous importance to the Greenland ice sheet today, and this process of water transport through the system will only increase as our climate warms. The key question now is whether this “additional” meltwater flow in channels will cause our ice sheets to flow faster or slower into the sea.”

The work highlights a currently overlooked process that can quickly kick in under melting ice sheets. Whether these channels will stabilize or destabilize Earth’s present-day ice sheets in a warming world remains an important and open question.

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