March 19, 2013
Model Shows How Carbon Escapes From Deep Earth To Surface
redOrbit Staff & Wire Reports - Your Universe Online
Computer simulations of water under extreme pressure reveals how carbon might one day be recycled from hundreds of miles below the surface of the Earth, according to researchers at the University of California, Davis and Johns Hopkins University.
The cycling of carbon through the oceans, the atmosphere and the shallow crust of the Earth has been intensively studied, but relatively little is known about what happens to carbon deep in the Earth.
"We are trying to understand more about whether carbon can be transported in the deep Earth through water-rich fluids," said study co-author Dimitri Sverjensky, professor of earth and planetary sciences at Johns Hopkins University.
There is plenty of water in the Earth´s mantle, the layer of the planet extending hundreds of miles below its crust. However, scientists are not sure how water behaves under the extreme conditions there, where pressure can reach hundreds of tons per square inch and temperatures are over 2,500 F.
Experiments reproducing these conditions are challenging, said study co-author Giulia Galli, a professor of chemistry and physics at UC Davis.
Geochemists have models to understand the deep Earth but have lacked a vital parameter for water under these conditions: the dielectric constant, which determines how easily minerals will dissolve in water.
"When people use models to understand the Earth, they need to put in the dielectric constant of water — but there are no data at these depths," Galli said.
Researchers speculate carbon, trapped as carbonate in the shells of tiny marine creatures, sinks to the ocean floor and gets carried into the mantle on sinking crustal plates. It is then recycled and escapes through volcanoes, Sverjensky explained.
However, until now there has been no theoretical mechanism to explain how this might happen.
Ding Pan, a postdoctoral researcher at UC Davis, used computer simulations of water to predict how it behaves under extreme pressure and temperature, and found the dielectric constant changes significantly.
By introducing that new factor into the existing models of water in the mantle, the researchers were able to predict magnesium carbonate, which is insoluble on the Earth's surface, would at least partially dissolve in water at that depth.
"It has been thought that this remains solid, but we show that at least part of it can dissolve and could return to the surface, possibly through volcanoes," Sverjensky said.
"Over geologic timescales, a lot of material can move this way."
Sverjensky noted the new modeling work is only a "first step" toward understanding how carbon trapped deep in the Earth can return to the surface.
Their research results were published March 18 in the journal Proceedings of the National Academy of Sciences.
Galli and Sverjensky serve as leaders of the Deep Carbon Observatory, which seeks to understand the role of carbon in chemistry and biology deep inside the Earth.