The contours of the seafloor affect how ocean water circulates and also the future of sea level rise. That’s because hill-like features known as sills can block warm water from reaching the ends of ocean-terminating glaciers, a process that can trigger melting. But traditional methods of surveying the ocean bottom’s topography require expensive and time-consuming measurements with ship-mounted instruments. Researchers now have added an unlikely tool to their arsenal for locating submerged sills: stranded icebergs.
Icebergs as Depth Sounders
A team of scientists led by Jessica Scheick, a glaciologist working at the University of Maine when this research was done, used high-resolution satellite imagery of stationary icebergs to compute their drafts. Because stationary icebergs are likely grounded—that is, resting on the seafloor—these calculations provide an estimate of water depth and therefore seafloor topography. Scheick and her colleagues confirmed that their estimates were largely consistent with sonar-derived measurements of water depth, a finding that paves the way for using iceberg data to infer the bathymetry of previously unstudied regions.
Scheick and her team focused on icebergs that had broken off glaciers on the Greenland ice sheet. This ice sheet—Earth’s second largest—has the ability to powerfully shape the world’s future coastlines: Sea levels would rise by several meters globally if it were to melt completely. Determining whether, and how quickly, the Greenland ice sheet is melting depends on knowing if warm water is eroding the undersides of its outlet glaciers. These glaciers are the gatekeepers holding back the ice from flowing into the sea, Scheick said. If they were to rapidly melt, it’d be like “removing a dam.”
A Three-Dimensional View
Scheick and her colleagues analyzed 37 stationary icebergs in Ilulissat Icefjord and Naajarsuit Fjord off the western coast of Greenland using stereo images from the WorldView satellites, a suite of privately managed Earth-imaging satellites. These stereo images, with a spatial resolution of about 2 meters, allowed the researchers to reconstruct a three-dimensional view of each iceberg and estimate its height above the water. By applying the principle of hydrostatic equilibrium—that is, that roughly 90% of an iceberg is below the water—and assumptions about iceberg shape, Scheick and her team estimated the draft of each iceberg and therefore the local water depth. The scientists also applied scaling relations inferred from these icebergs to analyze an additional 84 icebergs lacking WorldView data but observed by instruments on board the Landsat or Sentinel-2 satellites.
The researchers calculated water depths ranging roughly from 100 to 600 meters. The uncertainties on these measurements were roughly 90 meters, the researchers estimated. Most of that error comes from not knowing the shape of an iceberg’s submerged portion, said Scheick. “We don’t have a great sense of what icebergs look like under the water.”
Mapping Unstudied Regions
Despite the uncertainties, the researchers showed that their iceberg-inferred water depths generally agreed with sonar-derived depths of the same areas of the seafloor in Ilulissat Icefjord and Naajarsuit Fjord. “[Icebergs] did a really great job finding the actual water depth,” said Scheick. These results were presented in December at AGU’s Fall Meeting 2018 in Washington, D. C.
The next step will be to use this technique in places like northern Greenland where the bathymetry hasn’t been mapped, said Scheick. There’s a drawback to working in that region, however, she said: Icebergs can sometimes become stuck in one place because they’re encased in sea ice rather than truly being grounded. If they can’t be sure the iceberg is stranded, they can’t employ the method.
This work is very smart, said Mathieu Morlighem, a glaciologist at the University of California, Irvine, not involved in the research. “[It] has the potential to improve current maps of fjord bathymetry.”