Can liquid water persist within an ice sheet?

Scientists have discovered a large aquifer, the size of Ireland, near the surface of the Greenland Ice Sheet. “This was a big surprise,” said Jason Box, a researcher for the Geological Survey of Denmark and Greenland, “because we were drilling before melt had begun.” So liquid water had to survive since at least the previous year. Such water storage within the ice had not been previously considered, not on this massive scale. How can a giant reservoir of water exist inside a frozen ice sheet?

Scientist Jason Box drills a long hole to extract ice core samples. These enable him to map precipitation and melt levels for the South Eastern tip of Greenland. Credit: Nick Cobbing/Greenpeace.

Scientist Jason Box drills a long hole to extract ice core samples. These enable him to map precipitation and melt levels for the South Eastern tip of Greenland. Credit: Nick Cobbing/Greenpeace.

A new chapter

“We’re adding a chapter to the textbooks and that chapter is not yet written,” Box said. Scientists know how glaciers and ice sheets form. New layers of snow bury old layers, forcing snow to recrystallize into firn, an intermediary stage between snow and ice before finally becoming glacial ice. The compression traps pockets of air, critical in ice coring and analyzing Earth’s past atmosphere. “But now we have a completely different storage mechanism,” said Richard Forster, a researcher at the University of Utah, “where liquid water is stored as a reservoir in the firn year round.” This is new.

The discovery was an accident. Southeast Greenland, known for its high snow accumulation rates, has been understudied; but Box and Forster wanted to know just how much. In drilling for ice cores, they struck water. Traversing the rough terrain with snowmobiles, they moved the operation several kilometers away, and hit water again.

Water drains from an extracted core, pulled out 12 meters (40 feet) below the surface of the Greenland Ice Sheet. Just below the snow, water persists within the firn layer before the summer surface melt, with air temperatures of minus 15 degrees Celsius (5 degrees Fahrenheit). Credit: Ludovic Brucke, NASA

Water drains from an extracted core, pulled out 12 meters (40 feet) below the surface of the Greenland Ice Sheet. Just below the snow, water persists within the firn layer before the summer surface melt, with air temperatures of minus 15 degrees Celsius (5 degrees Fahrenheit). Credit: Ludovic Brucke, NASA

This was not a fluke. It was big. Consulting radar instruments, the team was able to map the aquifer’s size. Radar waves propagate through the ice sheet and as they hit different media—from air to ice or snow to water—the wavelengths reflect. Electrical properties also shift with density, and water is a great conductor. “When you hit a liquid layer, you get a really big reflection,” Forster said. This shimmering layer stretches 70,000 square kilometers (27,000 square miles). The ice sheet here is about 800 meters (2600 feet) thick, while the liquid is between 5 and 50 meters (16 to 160 feet) deep. This aquifer then rests in the upper skin of the ice sheet beneath the snow layer, within the firn. But why doesn’t this water refreeze?

Feathers and air bubbles

“New snow acts like a thermal insulator,” said Forster. “Like a down jacket, the air space between the feathers—that’s what’s keeping you warm.” The air space between snow grains insulates liquid water. But another process is also at work. “Certainly some of that liquid water is feeling the cold atmosphere,” said Forster. As some of the water refreezes, it releases latent heat. Surrounding water absorbs this heat, and along with the snow insulation, provides enough warmth to maintain water in liquid state.

Relying on modeling, scientists determined the aquifer has existed since the 1970s. So far, fieldwork confirms the models. “No matter how sophisticated the satellite, aircraft or computer model is, you always need to gauge its accuracy,” Box said. “Even very low tech observations on ground tell us a lot.” Since April 2011, when the team first identified the aquifer, they have returned twice. Models show the aquifer extent has only marginally increased, but fieldwork is the only way to determine its thickness beneath the surface. For now, only one in situ measurement exists, recording a thickness of 22 meters (72 feet), between 12 and 34 meters (39 to 112 feet) deep.

(Left) Scientist Jason Box extracts an ice core sample, which enables him to map precipitation and melt levels to build a map of weather and climate changes on the south eastern tip of Greenland. (Right) Scientist Jason Box scrutinizes an ice core extract. Credit: Nick Cobbing/Greenpeace

(Left) Scientist Jason Box extracts an ice core sample, which enables him to map precipitation and melt levels to build a map of weather and climate changes on the south eastern tip of Greenland. (Right) Scientist Jason Box scrutinizes an ice core extract. Credit: Nick Cobbing/Greenpeace

The two extremes

How does this water factor into global sea level rise? “Now we have to really figure out how the water is moving through this aquifer system,” said Forster. Glaciers and ice sheets are dynamic. How much water eventually enters the ocean from melting ice sheets has always been a key question in calculating sea level rise. The two extremes of this aquifer system depend on where water goes. It either doesn’t go anywhere or it migrates through the system, connects to crevasses or moulins, vertical pipelines, and gets to the oceans, a process that may take a few years or decades. “In reality,” Forster said, “it’s probably some of each, depending on the location.”

The next step is to do more fieldwork and to date the water using chemical analysis. Figuring out the depth of liquid water is essential to knowing just how much water there is. Only then can contribution to sea level rise be accurately calculated. “You get this aquifer in areas where there is a lot of melting and a lot of snowfall,” Box said. “I think it’s natural, not climate driven, but we need to figure out the climate change response for this and that could have really fundamental implications for the ice properties and ice flow.”

Video

Water drains from a core out of the Greenland perennial firn aquifer. The extraction is lifted from 33 feet below the surface of the ice sheet. 

Reference

Forster, Richard F. et. all. 2013. Extensive liquid meltwater storage in firn within the Greenland ice sheet. Nature Geoscience. doi:10.1038/ngeo2043.

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