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The largely featureless surface of the Greenland Ice Sheet, seen from the window of a P3 aircraft carrying geophysical instruments intended to detect geological features below. Credit: Kirsty Tinto / Lamont-Doherty Earth Observatory
Inaccessible for now, a unique site may contain secrets from the past.
Scientists have detected what they say are sediments from a huge ancient lake bed sealed more than a mile under the ice of northwestern Greenland, the first 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 chemical and fossil traces from past climates and lives. Scientists consider this data vital to understanding what the Greenland Ice Sheet can do in the coming years as the climate warms, and therefore the site is a tempting target for drilling. An article describing the discovery is in press in the journal. Earth and planetary science charts.
“This could be an important repository of information, in a landscape that is totally hidden and inaccessible right now,” said Guy Paxman, a postdoctoral researcher at University of ColombiaLamont-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 is important if we want to understand how it will behave in the coming decades ”. The ice sheet, which has been melting at a rapid rate in recent years, contains enough water to raise global sea levels by about 24 feet.
The researchers mapped the lake bed by analyzing data from airborne geophysical instruments that can read signals penetrating the ice and provide images of the geological structures below. Most of the data comes from airplanes flying low over the ice sheet as part of POTOperation IceBridge.
A newly formed lake on the edge of the Greenland Ice Sheet, exposing sediment released by the ice. These lake beds are becoming common as the ice recedes. Credit: Kevin Krajick / Earth Institute
The team says the basin once housed a lake that covered 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, which are vaguely shaped like a meat cleaver, appear to be up to 1.2 kilometers (three-quarters of a mile) thick. The geophysical images show a network of at least 18 apparent stream beds carved out of the adjoining bedrock into a north-sloping escarpment that must have fed the lake. The image also shows at least one apparent outflow to the south. The researchers estimate that the depth of the water in the ancient lake ranged from 50 to 250 meters (a maximum of about 800 feet).
In recent years, scientists have found subglacial lakes in both Greenland and Antarctica, which contain liquid water sandwiched in ice or between bedrock and ice. This is the first time anyone has seen a fossil lake bed, apparently formed when there was no ice, and then covered and frozen in place. There is no evidence that the Greenland Basin contains liquid water at present.
Paxman says there is no way of knowing how old the lake bed is. The researchers say that the ice has likely moved back and forth periodically over much of Greenland over the past 10 million years, and perhaps dates back 30 million years. A 2016 study led by Lamont-Doherty geochemist Joerg Schaefer has suggested that most of Greenland’s ice may have melted over one or more extended periods at some point in the last few million years, but the details are vague. This particular area could have been repeatedly covered and discovered, Paxman said, leaving a wide range of possibilities for the lake’s history. In any case, Paxman says, the substantial depth of the sediments in the basin suggests that they must have accumulated during ice-free times for hundreds of thousands to millions of years.
“If we could get to those sediments, they could tell us when the ice was present or absent,” he said.
Using geophysical instruments, scientists have mapped a huge ancient lake basin (outlined here in red) beneath the Greenland ice, covering approximately 2,700 square miles. Redder colors mean higher elevations, lower greens. A stream system etched into the bedrock that once fed the lake is shown in blue. Credit: Adapted from Paxman et al., EPSL, 2020
The researchers assembled a detailed image of the lake basin and its surroundings by analyzing radar, gravity and magnetic data collected by NASA. The ice-penetrating radar provided a basic topographic map of the land surface underlying the ice. This revealed the contours of the smooth, low basin, nestled between higher elevation rocks. Gravity measurements showed that the material in the basin is less dense than the surrounding hard metamorphic rocks, evidence that it is composed of sediment washed away from the sides. Magnetism measurements – sediments are less magnetic than solid rock – helped the team map the depths of the sediments.
The researchers say the basin may have formed along a now-dormant fault line, when the bedrock spread out and formed a low point. Alternatively, but less likely, previous ice ages may have excavated the depression, letting it fill with water as the ice receded.
What the sediments might contain is a mystery. The material washed from the edges of the ice sheet has been found to contain traces of pollen and other materials, suggesting that Greenland may have gone through warm periods over the last million years, allowing plants and perhaps even the forests will take hold. But the evidence is not 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 signals dating to a previously unknown distant past.
Therefore, the basin “may be an important site for future drilling under the ice and retrieval of sediment records that can provide valuable information on the glacial, climatological and environmental history” of the region, the researchers write. With the top of the sediment 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 to the top of the Greenland ice sheet and recovered several feet of bedrock – at the time, the deepest ice core ever drilled. The feat, which took five years, has not been repeated in Greenland since, but a new project aimed at reaching shallower bedrock in another part of northwestern Greenland is being planned for the next few years.
Reference: Earth and planetary science charts.
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.
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