Understanding the carbon cycle is a vital part of understanding how our planet will react to a continually warming planet, especially because that warming is caused by increasingly high levels of carbon dioxide in the atmosphere.
One tantalisingly elusive aspect of the carbon cycle is the carbon stored deep in the Earth’s mantle.
“We are trying to understand more about whether carbon can be transported in the deep Earth through water-rich fluids,” said coauthor Dimitri Sverjensky, professor of earth and planetary sciences at Johns Hopkins University.
Sverjensky, along with colleagues from the University of California Davis and the Shell Technology Centr in Bangalore, India, have worked on a study that was recently published in the journal Proceedings of the National Academy of Sciences and relates the results of computer simulations of water under extreme pressure, mimicking water found in the Earth’s mantle.
Unsurprisingly, given that we haven’t actually managed to create submersible vehicles that will also drill their way down into the Earth’s mantle, we don’t have any reliable data on how water reacts under such pressure. Water in the mantle will find itself subjected to pressures in the hundreds of tonnes per square inch and temperatures over 1300 degrees Celsius.
Giulia Galli, a professor of chemistry and physics at UC Davis and co-author on the paper, notes that reproducing these conditions through experiments is very hard to do. While geochemists have models to understand the deep Earth, they do not have any data for the dielectric constant, which determines how 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.
Enter Ding Pan, a postdoctoral researcher at UC Davis, who created a computer simulation to predict how water behaves under extreme pressures and temperatures which showed that the dielectric constant changes significantly in such conditions.
Combining this new piece of information with existing deep Earth models allowed the researchers to determine that carbon such as magnesium carbonate would at least partially dissolve in water at that depth, allowing it to return to the surface.
“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 believes that this discovery is the “first step” in understanding how deep Earth carbon returns to the surface.