Researchers for the first time have used seismic sensors to track meltwater flowing through glaciers and into the ocean, an essential step to understanding the future of the world’s largest glaciers as climate changes.
Meltwater moving through a glacier into the ocean is critically important because it can increase melting and destabilize the glacier in a number of ways: The water can speed the glacier’s flow downhill toward the sea; it can move rocks, boulders, and other sediments toward the terminus of the glacier along its base; and it can churn and stir warm ocean water, bringing it in contact with the glacier.
“It’s like when you drop an ice cube into a pot of warm water. It will eventually melt, but it will melt a lot faster if you stir that water,” says Timothy Bartholomaus, a postdoctoral fellow at the University of Texas at Austin Jackson School of Geosciences. “Subglacial discharge provides that stirring.”
The new technique, described in the journal Geophysical Journal Letters, offers scientists a tool for tracking meltwater at glaciers that end in the ocean, called tidewater glaciers.
Unlike landlocked glaciers, where scientists can simply measure the meltwater flowing in glacial rivers, there previously had not been a method available to track what’s occurring within tidewater glaciers.
“All of the biggest glaciers in Greenland, all of the biggest glaciers in Antarctica, they end in the ocean,” says Bartholomaus, the study’s lead author.
“We need to understand how these glaciers are moving and how they are melting at their front. If we want to answer those questions, we need to know what’s occurring with the meltwater being discharged from the glacier.”
Researchers discovered the new method while trying to study earthquakes caused by iceberg calving—when large chunks of ice break off glaciers. The ability to identify these earthquakes, known as icequakes, varied over the season, and they were much more difficult to detect during summer because seismic background noise was obscuring the icequake signals.
The team set about trying to determine what was causing the background noise, investigating potential causes such as rainfall, iceberg calving, and the movement of the glacier over the ground.
Eventually, as the researchers discounted these theories, they discovered that the seismic vibrations being detected by the equipment was caused by meltwater percolating down through the glacier and weaving its way through the complicated plumbing system in the interior of the ice.
Researchers tested the theory on glaciers with meltwater rivers and found that the timing of the meltwater and the seismic signals synced perfectly. The method is very good at identifying when the glacial discharge is flowing into the ocean, but it will take more research to determine exactly how much water is flowing out, Bartholomaus says.
“Now that we know when subglacial discharge is faster or slower, we can make better measurements of glacier change. My hope is that this method will really help us understand how the glaciers and the oceans are coupled, and how the ocean might be affecting the behavior of tidewater glaciers.”
Researchers from the University of Alaska Southeast, the University of Alaska Fairbanks, and the US Geological Survey contributed to the study.