Ancient melting ice sheets carved tunnel valleys on the sea floor, says study

Are we overlooking an important mechanism to understand the impact of climate change?
Ameya Paleja
A 3D map of valley tunnels
A 3D map of valley tunnels

James Kirkham 

Around 20,000 years ago, as a giant ice sheet over the North Sea melted, it carved out tunnel valleys on the ocean floor that have remained buried for millennia. Now, researchers are using 3D seismic reflection to uncover these hidden canyons, hoping they will help us understand how climate change will impact ice sheets melting today, Live Science reported.

Tunnel valleys are giant underground ravines carved out by meltwater from ice sheets that drain into channels below them. The immense weight of the water cut deep canyons into the sea floor, which were then covered up by sediments over thousands of years.

According to a statement released by the British Antarctic Survey (BAS), these valleys have been measured to be up to 93 miles (150 km) long, 3.7 miles (6 km) wide, and 1,640 feet (500 m) deep. It is likely that the collapse of ice shelves in Antarctica is also experiencing similar changes, except that we are not looking for them.

Tunnel valleys in the North Sea

Last year, the researchers at BAS mapped out the tunnel valleys that exist in the North Sea, which was covered by a giant ice sheet that covered parts of modern-day Europe as well as the U.K. during the last Ice Age. While the ice sheet is estimated to have formed 126,000 years ago and lasted till about 12,000 years ago, some of the buried canyons discovered by the researchers date back to two million years, the BAS statement said.

The researchers used 3D seismic reflection technology that emits sound waves under the seafloor to reveal the structures below. Now, combining the maps created using this technology with computer models, the researchers tried to probe the origin of these valleys and found them to have been carved out in a few centuries.

They also found that the formation of the valleys varies by season, with summer melting leading to more meltwater which can temporarily accelerate the valley's growth.

Ancient melting ice sheets carved tunnel valleys on the sea floor, says study
Seasonal fluctuations in the meltwater

Did tunnel valleys prolong the lifespan of ice sheets?

Researchers know those tunnel valleys formed at the end of the life of an ice sheet. However, they also suspect that by draining the meltwater away from the ice, these channels also stopped water from pooling on top of or below the ice, thereby preventing it from melting.

The researchers aren't sure how quickly ice was melting since some tunnel valleys show limited ice movement while others showed rapid ice loss, the statement said. By getting to the bottom of this question, the team is hopeful that they will be able to predict how ice sheets in Antarctica or Greenland will be impacted by climate change.

Currently, used prediction methods do not take into account the contribution of tunnel valleys. Since these structures form very rapidly, we could see drastic changes in how ice caps melt in the future, and it is important to include them in predictions.

The research findings were published in the journal Quarternary Science Reviews.

Abstract

The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming.

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