Huge ancient lake bed discovered deep beneath the Greenland ice sheet

Inaccessible for now, a single site can hold secrets from the past.

Scientists have detected what they say to be sediments from a huge sealed ancient lake bed more than a mile below the ice of northwest Greenland – the first ever discovery of such a subglacial feature anywhere in the world. Apparently formed at a time when the area was ice-free but now completely frozen, the lake bed may be hundreds of thousands or millions of years old and contain unique fossil and chemical traces of past climates and life. Scientists consider such data vital to understanding what the Greenland ice sheet could do in the coming years as the climate warms, and so the site represents an attractive target for drilling. A document describing the discovery is in press at the magazine Science letters of the earth and planets.

“This could be an important repository of information, in a landscape that is totally hidden and inaccessible right now,” said Guy Paxman, postdoctoral researcher at Columbia Universityof Lamont-Doherty Earth Observatory and lead author of the report. “We are working to try to understand how the Greenland ice sheet has behaved in the past. It’s important if we want to understand how it will behave in the decades to come. ”The ice sheet, which has been melting at an accelerating rate in recent years, contains enough water to raise global sea levels by about 24 feet.

The researchers mapped the lake floor by analyzing data from airborne geophysical instruments that can read ice-penetrating signals and provide images of underlying geological structures. Most of the data came from aircraft flying low over the ice sheet as part of NASAOperation IceBridge.

The team says the basin once housed a lake that covers about 7,100 square kilometers (2,700 square miles), about the size of the US states of Delaware and Rhode Island combined. The sediments in the basin, loosely shaped like a cleaver, appear to be 1.2 kilometers (three-quarters of a mile) thick. The geophysical images show a network of at least 18 apparent watercourse beds carved into the adjacent bedrock in a north-sloping escarpment that must have fed the lake. The image also shows at least one apparent outlet stream to the south. Researchers calculate that the water depth in the lake once ranged from about 50 meters to 250 meters (a maximum of about 800 feet).

In recent years, scientists have discovered existing subglacial lakes in both Greenland and Antarctica, containing liquid water embedded in ice or between bedrock and ice. This is the first time anyone has identified a fossilized lake bed, apparently formed when there was no ice, and then later covered and frozen. There is no evidence that the Greenland basin today contains liquid water.

Paxman says there’s no way of knowing how old the lake bottom is. The researchers say ice is likely to have periodically advanced and retreated over much of Greenland over the past 10 million years, and possibly as far back as 30 million years. A 2016 study by Lamont-Doherty geochemist Joerg Schaefer suggested that most of Greenland’s ice may have melted for one or more extended periods in the last few million years or so, but the details are inaccurate. This particular area could have been repeatedly covered and uncovered, Paxman said, leaving a wide range of possibilities for the lake’s history. In any case, Paxman says, the considerable depth of the sediments in the basin suggests they must have accumulated during ice-free periods for hundreds of thousands or millions of years.

“If we could get to those sediments, they could tell us when the ice was present or absent,” he said.

The researchers assembled a detailed picture of the lake basin and its surroundings by analyzing radar, gravitational and magnetic data collected by NASA. Ice penetrating radar provided a basic topographical map of the Earth’s surface beneath the ice. This revealed the contours of the smooth, low basin nestled among the taller rocks. Gravity measurements showed that the material in the basin is less dense than the surrounding hard and metamorphic rocks – evidence that it is composed of side-washed sediments. Measurements of magnetism (sediments are less magnetic than solid rock) helped the team map sediment depths.

The researchers say the basin may have formed along a long dormant fault line as the bedrock elongated and formed a low point. Alternatively, but less likely, earlier glaciations may have carved out the depression, leaving it to fill with water as the ice receded.

What the sediments might contain is a mystery. The washed-out material from the edges of the ice sheet has been found to contain remnants of pollen and other materials, suggesting that Greenland may have experienced warm spells during the last million years, allowing plants and perhaps even forests to take hold. But the evidence isn’t conclusive, in part because it’s difficult to date such loose materials. The newly discovered lake bed, by contrast, could provide an intact archive of fossils and chemical cues from a distant, hitherto unknown past.

The basin “could therefore be an important site for future sub-ice drilling and recovery of sediment records that could provide valuable insights into the region’s glacial, climatological and environmental history,” the researchers write. With the top of the sediment located 1.8 kilometers below the current ice surface (1.1 miles), such drilling would be daunting, but not impossible. In the 1990s, researchers penetrated nearly 2 miles into the summit of the Greenland ice sheet and recovered several feet of rock, at the time the deepest core of ice ever drilled. The five-year feat has never been repeated in Greenland, but a new project is planned for the next few years to reach a shallower bedrock in another part of northwestern Greenland.

Reference: Science letters of the earth and planets.

The study was co-authored by Jacqueline Austermann and Kirsty Tinto, both also based at the Lamont-Doherty Earth Observatory. The research was supported by the US National Science Foundation.