History Stories

In the latest issue of the Geological Society of America Bulletin, the new study’s authors shared findings collected during their recent work in the rocky highlands of central northwestern Greenland, near Baffin Bay and the town of Upernavik. In order to determine the age and history of the landscape, they used a method known as cosmogenic nuclide dating to measure the concentration of two rare isotopes, beryllium-10 and aluminum-26, in minerals of quartz found in the surface rocks. These two isotopes are produced when cosmic rays from space hit oxygen and argon, respectively, inside exposed rocks. The more isotopes there are, the longer those rocks have been at the surface; while fewer isotopes indicate that erosion has stripped away older rocks from the surface and left younger ones exposed.

Through this process, the researchers concluded that the landscape near Upernavik is extremely old and dates back at least 800,000 years in some locations. The relative age of these rocky highlands stands in stark contrast with other areas of Greenland’s landscape such as its deep fjords, which were carved by glaciers during the last glacial period (or Ice Age) barely 11,000 years ago. Their findings led the Upernavik researchers to conclude that the land surfaces they examined have been preserved under layers of non-erosive glacial ice – so-called “ghost glaciers” – during many glacial periods over the course of geologic time.

According to lead study author Lee Corbett, who conducted the Upernavik research as a master’s student at the University of Vermont at Burlington and is now a doctoral student at Dartmouth College: “These ghost glaciers come and go, and leave very little evidence of their presence.” Despite their tremendous erosive power, glaciers need water in order to facilitate their grinding motion across mountains. In this case, Corbett and her colleagues suspect that a cold climate and high elevation combined to freeze the glaciers to the bedrock below, preventing them from eroding or shaping the landscape. Such glaciers covered the landscape, but were incapable of eroding it, so they receded without leaving any geologic evidence of their presence.

Scientists have seen similar effects in mountain ranges such as the Alps and the Andes. A 2010 study focusing on the Andes in Patagonia, the southernmost region of South America, was the first to identify the effect of such ghost glaciers, which was dubbed “glacial armoring.” They learned that instead of scraping away the mountain surfaces, glaciers in such a cold, high-latitude climate froze to the bedrock, shielding the sides and tops of mountains from erosion. As a result of this process, the peaks of the southern Patagonian Andes tower some 1,000 meters (3,300 feet) higher than similar mountains in the more temperate parts of the Andes further north.

The Upernavik study forms part of a larger project that aims to understand past changes in Greenland’s ice sheet in order to predict future ice loss. According to Corbett, “Trying to understand times when the ice sheet was bigger or smaller will be really helpful for scientists to figure out where we’re going in the future with Greenland ice loss.” Greenland’s massive ice sheet has been melting at accelerating rates for more than a decade, a phenomenon that scientists believe is due to warmer ocean water and rising atmospheric temperatures.

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