Inaccessible for the moment, a unique site can hold secrets from the past.
Scientists have figured out what they are saying is the sediments of a huge ancient seabed sealed more than a mile under the ice of northwest 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 fossil and chemical traces of past climates and life. Scientists believe such data is important to understand what the Greenland ice sheet can do in the coming years if the climate warms. Hence, the site is an attractive target for drilling. An article describing the discovery is in press in the journal Earth and Planetary Science Letters.
“This could be an important store of information in a landscape that is currently completely hidden and inaccessible,” said Guy Paxman, postdoctoral fellow at Columbia UniversityLamont-Doherty Earth Observatory and lead author of the report. “We are working to understand how the Greenland ice sheet has behaved in the past. It is important if we are to understand how it will behave in the decades to come. “The ice sheet, which has been melting faster and faster in recent years, contains enough water to raise global sea levels by about 24 feet.
Researchers mapped the lake floor by analyzing data from airborne geophysical instruments that can read signals entering the ice and provide images of the geological structures below. Most of the data came from aircraft flying over the ice sheet as part of low altitude NASAOperation IceBridge.
The team says the basin was once home to a lake that covered approximately 7,100 square kilometers (2,700 square miles), roughly the size of the U.S. states of Delaware and Rhode Island combined. Sediments in the basin, vaguely shaped like a cleaver, appear to be up to 1.2 kilometers thick. The geophysical images show a network of at least 18 seemingly unique stream beds that were carved into the adjacent bedrock in a sloping bank to the north that must have fed the lake. The picture also shows at least one apparent outlet flow to the south. The researchers calculate that the water depth in the former lake was between 50 and 250 meters (maximum 800 feet).
In recent years, scientists in both Greenland and Antarctica have found subglacial lakes that contain liquid water in the ice or between bedrock and ice. This is the first time anyone has discovered a fossil sea bed that appeared to have formed when there was no ice and that was later covered and frozen. There is no evidence that the Greenland Basin now contains liquid water.
Paxman says there is no way of telling how old the lake bed is. Researchers say it is likely that the ice has evolved and retreated regularly over much of Greenland over the past 10 million years, possibly dating back as far as 30 million years. A 2016 study by Lamont Doherty geochemist Jörg Schäfer showed that most of the Greenland ice could have melted for one or more extended periods of time over the past million years, but the details are sketchy. 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, says Paxman, the considerable depth of the sediments in the basin suggests that they must have accumulated over hundreds of thousands or millions of years in ice-free times.
“If we could get to these sediments, they could tell us when the ice was there or not,” he said.
The researchers compiled a detailed picture of the lake basin and its surroundings by analyzing the radar, gravity, 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 outline of the smooth, low-lying basin, nestled between higher rocks. Gravity measurements showed that the material in the basin is less dense than the surrounding hard, metamorphic rocks – evidence that it is made up of sediments washed in from the sides. Measurements of magnetism (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 long dormant fault line when the bedrock expanded and formed a low point. Alternatively, but less likely, previous icings cut out the depression and filled it with water when the ice receded.
What the sediments might contain is a mystery. Material washed away from the edges of the ice sheet was found to contain pollen and other materials, suggesting that Greenland may have been exposed to warm periods over the past million years, allowing plants and possibly even forests to move in. However, the evidence is inconclusive, partly because it is difficult to date such loose material. In contrast, the newly discovered lake floor could provide an intact archive of fossils and chemical signals from a previously unknown distant past.
The basin “could therefore be an important location for future sub-ice drilling and the restoration of sediment records, which could provide valuable insight into the region’s glacial, climatological and environmental history,” the researchers write. If the top of the sediment is 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 3 km to the summit of the Greenland ice sheet and retrieved several feet of bedrock – at the time the deepest ice core ever drilled. The feat, which lasted five years, has not been repeated in Greenland since, but a new project is planned over the next few years that aims to reach flatter bedrock in another part of northwest Greenland.
Reference: Earth and Planetary Science Letters.
The study was written by Jacqueline Austermann and Kirsty Tinto, both also at the Lamont-Doherty Earth Observatory. The research was supported by the US National Science Foundation.