On Greenland’s coast, glaciers meet the sea in narrow fjords that have been carved over hundreds of thousands of years. Ice cliffs tower hundreds of meters high.
At a glacier’s terminus, where those cliffs crash into the waters of the Atlantic, small (bus-sized) chunks of ice slough off all the time. Occasionally, a stadium-sized iceberg plunks into the water.
All this glacial calving impacts sea level rise and global climate, but there’s a lot that researchers don’t yet know about how calving happens. Now, scientists have gotten a detailed look at the whole process using a fiber-optic cable on the seafloor 500 meters from a glacier’s calving front. The findings were published last month in Nature.
Maneuvering Through the Mélange
Physical processes at the calving front control a glacier’s stability, said Dominik Gräff, a glaciologist at the University of Washington in Seattle who led the new work.
But gaining access to a glacier’s front can be difficult, and remote sensing methods are able to visualize only the tiny fraction of the ice mass that isn’t submerged. “We don’t have much idea what’s actually going on below the water,” Gräff said.
“It’s always impressive for people to get any observations near the glacier front,” agreed David Sutherland, a physical oceanographer at the University of Oregon in Eugene who did not contribute to the new paper. Researchers working at the front, he explained, risk losing expensive equipment and have to navigate the mélange, a closely packed mix of sea ice and icebergs.
This was the first time fiber-optic sensing was deployed at a calving front. Unlike other methods, such as remote sensing and the use of submerged seismometers, fiber-optic sensing can capture myriad events across a range of times. “It can just sense everything,” Sutherland said.
Gräff and his team dropped a 10-kilometer (6.2-mile) cable on the ocean bottom across the fjord of the Eqalorutsit Kangilliit Sermiat (EKaS) glacier in South Greenland. The maneuver was somewhat tricky. “If you go too slow, the ice mélange that you push open with your vessel [will close] quickly,” Gräff said. “And that prevents your cable from sinking down.”
Once the cable was in place, researchers were able to collect a wealth of data.
Waves, Wakes, and Cracking
Laser light pulsing through the fiber-optic cable allowed it to function like an entire network of sensors snaking across the fjord.
Acoustic vibrations associated with calving, for instance, stretched and compressed the cable and changed backscattered light signals. Measuring these changes is the basis for distributed acoustic sensing, or DAS.
In addition to measuring acoustics, fiber optics also allowed researchers to measure how light signals change because of temperature, a technique called distributed temperature sensing, or DTS. DAS and DTS allowed researchers to capture calving events that lasted mere milliseconds.
During the 3-week experiment at EKaS, the glass fiber captured 56,000 iceberg detachments.
That volume of observations meant researchers could trace the calving process from start to finish. It began as cracks formed in glacial ice. Sounds associated with the cracking traveled through the fjord and were picked up by the cable. Then icebergs detached from the glacier, creating underwater waves that traveled between the ice and the sediment below. Iceberg detachments also caused small, local tsunamis that could be identified by pressure changes on the cable at the bottom of the fjord.
In addition to tsunamis and surface waves, the fiber-optic cable was also able to detect internal gravity waves, which travel at the interface between an iceberg’s upper, cold layer of fresh water and the warmer layer of salty seawater below. The EKaS icebergs created wakes as they drifted from the glacier, dragging internal gravity waves behind them and causing circulation in the water. Researchers measured the resulting temperature changes using DTS.
Finally, the fiber-optic cable captured the sounds of icebergs disintegrating. These signals were similar to the initial sound of cracking in the glacier but instead came from the fjord.
Wealth of Data
“There are very few seismological datasets where, within such a short amount of time, you record so many different phenomena,” said Andreas Fichtner, a seismologist at ETH Zürich in Switzerland who was not part of the work but collaborates with one of the study’s authors. It takes detective work to decode all those signals and assign them to physical processes, he said. “It’s pretty remarkable.”
Gräff and the other researchers hope their rich datasets can improve glacial calving models, which often underestimate the melt that occurs below the surface. Sutherland said it’s not yet clear how to incorporate details from the study into such models, however. Researchers will need to connect the observed processes and the amount of ice lost to factors they can easily measure or estimate, such as ocean temperature and ice thickness, he explained. And they’ll need to study the calving process of different glaciers. EKaS sits on bedrock where it meets the sea, for instance, while other glaciers have a floating terminus.
Still, having a huge set of observations along with information about ocean conditions, which the researchers collected using a suite of other tools, “is pretty powerful,” Sutherland said. “Maybe we can start using this dataset to try to make predictions of when icebergs are going to calve.”
This article originally appeared in EOS Magazine.
