In 2014, a lake hidden beneath the Greenland Ice Sheet suddenly began to drain. The event occurred with such force that a surge of water was sent directly upwards, splitting through the ice to create a crater almost one kilometre deep.
In the 10 days that followed, 90 million cubic metres of water were released from the lake – roughly equivalent to nine hours of water crashing over Niagara Falls during peak flow. It was one of the largest subglacial floods ever recorded in Greenland.
Despite the drama of the event itself, researchers were even more surprised by what they discovered downstream: 385,000 square metres (54 football pitches) of fractured and distorted ice – a chaotic scene that comprised enormous blocks 25 metres high, along with six square kilometres of scoured ice. None of this was there before the flood.
Now, in a new study published in the journal Nature Geoscience, researchers from Lancaster University reveal further insights not only on the discovery of the great flood but also how this little-understood process may affect the way the ice sheet responds to future climate change.
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The flood
The study explains that “one of the most recently discovered, yet poorly understood, components of Greenland’s subglacial hydrological system is its network of active subglacial lakes.” The 2014 lake drainage event therefore presented an opportunity to find out more.
Using high-resolution surface models and multiple satellite sources from NASA and the European Space Agency, the team were able to study the lake – and the flood event – in striking detail. They were surprised at what they found.
“When we first saw this, because it was so unexpected, we thought there was an issue with our data,” explains lead author Dr Jade Bowling. “However, as we went deeper into our analysis, it became clear that what we were observing was the aftermath of a huge flood of water escaping from underneath the ice.”
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Previously, it was thought that meltwater flowed from the surface down to the bed and out to the ocean. This event, however, shows that water can also be forced in the opposite direction, tearing upwards through the ice under pressure.
Approximately one kilometre downstream of the collapsed basin, a newly formed zone of fractures appeared in the ice surface, “consisting of crevassing and uprooted ice blocks with a combined height (crevasse depth plus ice block height) of 40 metres,” the study explains.
“Downslope of the fracture zone, an ~6-km2region of the ice surface had been scoured clean. Together, these observations indicate that a substantial volume of water broke up through the ice at this location and flooded across the surface.”
The scientists were also surprised to find that the flood occurred in a region where models previously predicted that the ice was frozen at the bed. Given the existence of the lake, this can't be the case.
The researchers suggest perhaps the pressure-driven fracturing of ice along the ice bed created a pathway for the water to flow.
Lessons from the flood
“What we have found in this study surprised us in many ways,” says Dr Amber Leeson, an expert in ice sheet hydrology. “It has taught us new and unexpected things about the way that ice sheets can respond to extreme inputs of surface meltwater.”
The findings raise questions about whether current models accurately capture the behaviour of the Greenland Ice Sheet under a warming climate, say the researchers, who explain that as global temperatures rise, more meltwater events like this could happen – yet we still have much to learn about how they affect the future of our ice sheets.
Find out more about the study: Outburst of a subglacial flood from the surface of the Greenland Ice Sheet
Top image: the outburst fracture zone, based on satellite imagery acquired on 28 April 2015. Credit: CPOM, Lancaster University | DigitalGlobe, Inc. (2015), provided by European Space Imaging
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