A new study by NASA scientists has proven that a geothermal heat source called a mantle plume, deep underground in Marie Bird in Antarctica, explains the rapid melting that creates lakes and rivers beneath the ice sheet. While the heat source is not a new or increasing threat to the West Antarctic ice sheet, it may help explain why the ice sheet is unstable today.
The stability of an ice sheet is closely related to how much water flows from below, which makes glaciers easier to slide. Understanding the sources and future of melt water in West Antarctica is important in assessing the rate at which ice can melt and increase ocean water levels.
Antarctica's glaciers are unstable and filled with rivers and lakes, the largest of which is Lake Erie. Many lakes quickly fill and drain, causing the ice surface thousands of feet above them to rise and fall as much as 6 meters. The movement allows scientists to estimate where and how much water should exist.
About 30 years ago, a scientist at the University of Colorado Denver suggested that heat from a mantle plume underground by Marie Bird could explain regional volcanic activity and the topographic function of the dome. Very recent seismic imaging has supported this concept.
With the small direct measurements that exist along the ice, scientists at JPL have come up with a better way to study the mantle plume idea through numerical simulations. They used the Ice Sheet System Model (ISSM), a numerical description of the physics of ice sheets developed by scientists at JPL and the University of California, Irvine.
To ensure that the model was realistic, the scientists monitored changes in the surface of the ice sheet using data from NASA's IceSat satellite and the Operation IceBridge air campaign.
Since the location and size of the possible mantle plume were unknown, they tested the full range of what was physically possible for several parameters, producing dozens of different simulations.
They found that the flow of energy from the mantle plume should be no more than 150 milliwatts per square meter. For comparison, in regions of the United States without volcanic activity, the heat flow from the Earth's mantle ranges from 40 to 60 milliwatts.
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In Yellowstone National Park - a famous geothermal hotspot - heat from below is around 200 milliwatts per square meter averaged across the park, although individual geothermal properties, such as geysers, are much hotter.
Simulations by scientists Serucy and Ivins using heat flux above 150 milliwatts per square meter showed too much melting to be consistent with space-based data except in one place: an area deep in the Ross Sea known for intense water flows. This area required a heat flux of at least 150-180 milliwatts per square meter. However, seismic imaging has shown that the heat of the mantle in this region can reach the ice sheet through a rift, that is, a break in the earth's crust, such as appeared in the Great Rift Valley in Africa.
Mantle plumes are thought to be narrow streams of rock that rise through the Earth's mantle and spread like a mushroom cap beneath the earth's crust. The buoyancy of the material, some of which is melted, causes the crust to bulge upward. The mantle plume theory was proposed in the 1970s to explain geothermal activity that occurs far from the tectonic plate boundary such as Hawaii and Yellowstone.
Marie Bird's mantle plume formed 50 to 110 million years ago, long before the West Antarctic ice sheet appeared. At the end of the last ice age, about 11,000 years ago, the ice sheet went through a period of rapid, sustained ice loss, when changes in global weather conditions and rising sea levels pushed warm water closer to the ice sheet - just as it does today.
"The presence of this mantle plume is important because it suggests that Antarctica's ice is more vulnerable in this area: this additional heat heats up the ice, which suggests greater weakness in the face of future and past environmental changes," the researchers say.