November 3, 2023
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A preserved river landscape from the time before Antarctica was icebound persists more than a mile below the East Antarctic Ice Sheet
An ancient river system from the era when the bottom of the world was ice-free is buried more than a mile deep below the East Antarctic Ice Sheet.
The system consists of two deeply incised fjords and a series of stream paths that are completely invisible from the surface but for a few subtle elevation changes detectable by satellite. Researchers have previously discovered other landmarks beneath the East Antarctic Ice Sheet, including a 26-mile-long lake and buried mountains.
The icebound river landscape may be at least 34 million years old, meaning that it would date to right before ice encrusted Antarctica. At that time, atmospheric carbon dioxide dropped, and the climate started cooling. Based on the topography of the landscape, glaciers crept down and carved into the river valleys, widening them slightly, says study co-author Stewart Jamieson, a professor of geography at Durham University in England. Then the climate got colder, and ice subsumed the entire region.
“It’s like putting that landscape in a freezer,” Jamieson says.
The pristine preservation of the landscape is a clue that these climate changes happened quickly, says Robert Larter, a marine geophysicist at the British Antarctic Survey, who was not involved in the study but reviewed the research for publication in the journal Nature Communications. Ice with a “wet” base that undergoes frequent freezing and thawing, like that of modern-day glaciers in the Alps, tends to gnaw away at landscapes via erosion. Ice sheets with “dry” bases that don’t undergo thawing, on the other hand, preserve landscapes as is. The under-ice river system is still sharp and uneroded.
“This is an indication that it really was rather quick,” Larter says. “We didn’t have hundreds of thousands or millions of years of this intermediate state where you had wet-based ice flowing.”
The newfound landscape sits near the edge of the East Antarctic Ice Sheet in a region called Wilkes Land. Jamieson and his colleagues first noticed something odd in data from the RADARSAT Constellation Mission, a trio of satellites launched by the Canadian Space Agency. These satellites send out microwave signals that hit Earth’s surface; the properties of the signals that are reflected back to the satellites can show tiny variations in the planet’s topography. Even very deeply buried landscapes can subtly affect the surface of the ice sheet. In this case, a few rough patches—which were too subtle to notice from ground level—hinted at something interesting beneath, says study co-author Shuai Yan, a postdoctoral researcher in geophysics at the University of Texas at Austin.
These differences wouldn’t be apparent if you were standing on the ice surface, Jamieson says, but they “popped out” on the satellite data. To confirm the presence of the landscape, the researchers used data from the International Collaborative Exploration of the Cryosphere through Airborne Profiling (ICECAP) project, which uses ice-penetrating radar instruments mounted on aircraft to map subglacial topography.
The mapping revealed three separate blocks of a highland that are each between about 2,700 and 3,860 square miles in size, which at the upper end is slightly smaller than the state of Connecticut. The blocks are separated by two fjords up to 25 miles wide. The fjords plunge 4,856 feet and 2,034 feet, respectively, below the global mean sea level. Tectonics has pulled the blocks apart over the millennia, but it’s possible to trace stream paths throughout the landscape and “join the dots” to see how the blocks once joined up, Jamieson says.
The landscape is in a region where the ice has advanced and retreated, Jamieson says, so it’s possible the highland emerged during an ice-free period as recently as 14 million years ago. Confirming the age would require drilling through 1.2 miles of ice with a water drill to get a sample of sediment beneath, something that might happen in the future, he says.
“If we could get some sediment from that area, there’s a chance that could contain some fossil pollen that might tell us something about the vegetation that also used to grow on that landscape,” Jamieson says.
Figuring out when the landscape was last exposed could help reveal how much the ice sheet has expanded and retreated in response to past climates, Jamieson says, which could help improve models of how the ice will behave in the future.
“There have been a few warm periods in Antarctica where we know we’ve got some ice loss,” he says, “but there are big questions about where and how much.”
