Meltwater drainage, break-away icebergs linked at shrinking Helheim Glacier

"Greenland is losing a lot of ice, and it drains from the interior of the ice sheet to the ocean through outlet glaciers like Helheim," said Sierra Melton, a doctoral candidate in geosciences at Penn State. "Understanding what's happening at these glaciers is important."

During warm periods, enough meltwater drains from underneath Helheim that plumes of buoyant fresh water rise to the surface of the sea in front of the glacier and are visible as patches of open water, the scientists said.

Tracking these plumes using satellite and time-lapse images, the scientists found when the plumes were visible on the surface that large icebergs stopped breaking away, or calving, from the glacier near the plumes.

"We'd see a lot of calving happening, and then it would stop when the plume was visible and start again after the plume disappeared," Melton said. "And when calving did occur, it happened away from the plume. They were always separated by space and time."

Calving at Helheim involves large chunks of ice breaking off from behind the cliff at the front of the glacier, which is up to 300-feet tall in some locations. Helheim once ended in a floating extension called an ice shelf or ice tongue, like larger Antarctic glaciers, but that ice has already broken off and melted, exposing the cliff. Calving accounts for about half of the ice loss from the Greenland Ice Sheet and is a significant contributor to sea level rise, the scientists said.

"Sierra's work, including this paper, is an important contribution to the larger effort to understand how iceberg calving really works and what controls its speed, so we can do a better job of projecting what will happen in Greenland, as well as Antarctica, and what that will mean for sea-level rise and costal people," said Richard Alley, Evan Pugh University Professor of Geosciences at Penn State, Melton's adviser and a co-author on the paper.

While the relationship between the plumes and calving was previously observed at Helheim, making direct observations is difficult because of impassable terrane on the glacier and ice in the sea. The scientists conducted a more comprehensive study using high-resolution satellite images and thousands of time-lapse photos from cameras stationed around the glacier from 2011 to 2019.

The findings, reported in the Journal of Glaciology, suggest that changes in hydrology and pressure beneath the glacier are responsible for the relationship between meltwater discharge and calving.

During melt season, water begins pooling in crevasses and forms lakes on the glacier surface. Some meltwater drains to the glacier bed, where it begins to fill up cavities and form a network between them, the scientists said.

"The way a subglacial drainage system evolves is if there's not very much water under the glacier, then there is low water pressure," Melton said. "As the water increases under the glacier, the pressure starts to increase with it."

As more water flows to the bottom and the water pressure rises, the speed of the glacier's march toward the sea increases and cracks can form in the ice, making it more vulnerable for calving, the scientists said.

But eventually under this pressure, and if enough water is present at the glacier bed, the water can carve channels in the bottom of the ice that direct meltwater into the sea, acting as a kind of relief valve that reduces the water pressure under the glacier ice, the scientists said. These channels can release enough fresh water for plumes to be visible at the surface of the sea.

"We think this lower pressure configuration inhibits the large calving because the fractures in the bottom of the ice can't form," Melton said. "So basically, the system that supports the plume existence should suppress the calving."

Also contributing from Penn State were Sridhar Anandakrishnan, professor of geosciences and Melton's co-adviser, and Byron Parizek, professor of mathematics and geosciences.

Leigh Stearns, associate professor and Michael Shahin, doctoral candidate, at the University of Kansas, and Adam LeWinter, physical scientist, and David Finnegan, director of remote sensing, at the Cold Regions Research and Engineering Laboratory, also contributed.

The National Science Foundation, U.K. Natural Environment Research Council and Heising-Simons Foundation supported this research.