Antarctica’s Thwaites Glacier suffered a period of fast retreat, doubling its current rate of shrinking, during the past couple hundred years. This is the conclusion reached by an international group of researchers who acquired high-resolution imagery off the front of Thwaites. The group used state-of-the-art autonomous submersibles, which revealed unusual marks left on the seafloor by the retreating ice.
Thwaites is one of the main concerns of scientists studying Antarctic ice. As large as Florida and several kilometers thick, the melting of this mass of ice is responsible for 4% of present-day sea level rise worldwide. And warming waters and a seabed that deepens toward the ice sheet’s interior have primed the glacier for a rapid collapse that could raise sea levels by more than half a meter in the next century.
Scientists, however, don’t know enough about the glacier’s recent history to confidently forecast its future behavior. That’s why a large British-American research initiative, the International Thwaites Glacier Collaboration (ITGC), was launched in 2017 to reveal the glacier’s past and predict its future.
In 2019, an expedition on board the Nathaniel B. Palmer icebreaker approached the front of the glacier and released a remotely operated submersible that mapped an area of 13 square kilometers of the seabed with specialized sonar and other instruments. As soon as the researchers recovered the submersible and looked at the images, they realized they had made an extraordinary finding. “None of us could explain what we were seeing,” said Alastair Graham, an associate professor of geological oceanography at the University of South Florida and first author of the new study. “It was like putting on your glasses for the first time and being able to see.”
The images showed hundreds of parallel ridges covering an underwater plateau at depths ranging from 630 to 670 meters. The researchers think this plateau was a pinning point at a former grounding line, a region where the land-based glacier ends and the floating ice shelf begins. The ridges, ranging between 10 and 70 centimeters tall, were likely created by the glacier’s front as it bobbed up and down with the tides. When the tide fell, the glacier pressed the sediments to produce one rib. The distance between ribs reveals how much the glacier receded during the daily tidal cycle—typically between 6 and 7 meters every day, but reaching up to 10 meters in some cases.
“In general, [the ridges are in] quite deep water, so they are below the reach of the main tidal currents and wave action,” said Robert Larter, a marine geophysicist with the British Antarctic Survey who was the lead scientist on board the Nathaniel B. Palmer during the expedition. “And in many areas of the polar continental shelf there are very low rates of sediment accumulation so [the ridges] don’t get buried, either.”
By looking at the ribs, the researchers realized they had a daily record of Thwaites Glacier’s retreat over a period of 5.5 months. During that time, the glacier moved at a rate of 2.1 kilometers per year, twice the current rate as measured by satellite imagery.
Coming up with the mechanism that produced the ribs wasn’t straightforward, though. The marks are so regularly spaced that they looked “made by humans,” Graham said. “It took a long time for us to settle on an idea for what they might be and why they are forming.” His analysis showed that the amplitude and height of the ridges follow a pattern that matches the region’s natural tidal cycles, reaching a maximum in amplitude and height every 14 days. The findings were reported in Nature Geoscience.
The researchers don’t know when exactly the ridges formed, but on the basis of Thwaites’s current rate of retreat, they think the ridges aren’t older than 200 years. Most likely, they formed around the 1940s, when the neighboring Pine Island Glacier started retreating. A direct sample of the seafloor sediments could have allowed dating the ridges more precisely, but the scientists had to hastily leave the area when a mélange of icebergs and sea ice moved into the area in February 2019. This was the last time a vessel has gotten so close to the glacier, which has been encased by floating sea ice ever since.
Unbeknownst to the scientists on board the Nathaniel B. Palmer, as they explored the seafloor in front of Thwaites, another group of researchers, led by Julian Dowdeswell with the University of Cambridge, was making a similar finding in the Larsen Inlet on the other side of Antarctica. Using an identical submersible, this team found landforms that closely resembled those reported by Graham and his colleagues. However, these ridges were much older—about 10,000 years—and formed during the deglaciation of the Larsen shelf after the end of the last ice age. Dowdeswell and his team documented even faster retreat rates, reaching 40 to 50 meters per day, or more than 10 kilometers per year.
“It was really nice to see that a completely different science team had seen similar features in another area of Antarctica,” said Christine Batchelor, a physical geographer at Newcastle University who coauthored the Larsen Inlet study, “especially as they had the same interpretation as to how the features are formed.”
In both cases, the ridges are made of the material that accumulates underneath ice sheets—a mixture of gravel, sand, and mud that researchers call diamicton. Because it’s compressed by the weight of the glacier as it settles into the seafloor, diamicton can remain stiff, Graham explained.
Possible New Pattern of Retreat
For Thwaites, the finding provides two important clues about the glacier’s future. First, the glacier has the potential to retreat much more rapidly than its current rate. Second, the shape of the seafloor seems to play a key role in controlling the rate of retreat of the glacier, particularly when it retreats from flat-topped ridges, which is the present situation. For this reason, the researchers expect that a new pulse of rapid retreat might occur if Thwaites migrates behind its current grounding line.
In the future, the team hopes to be able to obtain direct samples of the sediments forming the newfound ridges in front of Thwaites, whenever they can return to the area. In the meantime, they will turn to mathematical simulations to try to replicate how the process might occur.
—Javier Barbuzano (@javibarbuzano), Science Writer
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