When scientists placed seismometers on the McMurdo Ice Shelf, they recorded hundreds of thousands of tiny “ice quakes” that pools of partially melted ice expanding and freezing at night appear to cause.
The phenomenon may help scientists track glacier melting—and explain the breakup of large ice shelves.
“In these areas we would record tens, hundreds, up to thousands of these per night,” says study coauthor Douglas MacAyeal, a professor of geophysical sciences at the University of Chicago and a glaciologist who has been traveling to the Antarctic to study the behavior of ice and snow for decades. “It’s possible that seismometers may be a practical way for us to remotely monitor glacier melting.”
Climate change is causing the Antarctic to melt, but glaciologists are still mapping how, where, and why. They still don’t understand much about the process—as the massive Larsen B ice shelf collapse in 2002, which took glaciologists by surprise, suggests. They say they will need to understand these mechanisms to predict the ice’s future.
MacAyeal and the team were interested in the role of “quakes” on the floating ice shelves. (You may remember reports of ice or frost quakes around Chicago and the Midwest during the cold snap the polar vortex caused a few weeks ago, when residents reported booms or cracking sounds at night; this is the same mechanism.) But they wondered how often the phenomenon was occurring in ice in the Antarctic, and what role it might play in the melting and breakup of ice.
To find out about the quakes, researchers set up seismometers for 60 days during the melt season in two locations near seasonal meltwater lakes on the McMurdo Ice Shelf. One was drier; the other was slushier, with pools of melted water forming and refreezing. The wetter location, they found, was alive with seismic activity at night.
“In these ponds, there’s often a layer of ice on top of melted water below, like you see with a lake that’s only frozen on top,” MacAyeal says. “As the temperature cools at night, the ice on the top contracts, and the water below expands as it undergoes freezing. This warps the top lid, until it finally breaks with a snap.”
The energy vibrates out into the surroundings, where seismometers detect it. Some of the cracks re-heal, but some do not, MacAyeal says.
It may explain why icebergs actually break off more frequently during colder times of the year. “Perhaps this is happening at longer, slower scales,” MacAyeal says.
The discovery adds an important piece to our understanding of the physics and processes around melting ponds on ice shelves—especially if it helps researchers remotely keep track of Antarctic melting, MacAyeal says. “It may be very useful to add this to our other ways of monitoring ice.”
The National Science Foundation, NASA, and the Leverhulme Trust funded the work. Additional researchers from the University of Chicago, the University of Colorado Boulder, Northwestern University, and the University of Cambridge contributed to the study, which appears in the Annals of Glaciology.
Source: University of Chicago