DCO Collaboration Illuminates the Deep Carbon Cycle

DCO scientists Giulia Galli and Dimitri Sverjensky have turned to a novel combination of sophisticated theoretical modeling and theoretical aqueous geochemistry to study Earth's mantle.

 A particular challenge in deep carbon chemistry is modeling chemical changes under deep-Earth conditions. A chemical reaction that takes place in a lab, at room temperature, bears little relation to what is happening 400km beneath Earth’s surface.

At this depth, in the mantle, temperatures reach 1700K and pressures soar to 13GPa (around 100,000 times atmospheric pressure). Molten rock and aqueous fluids, and even the solid mantle itself, are constantly moving in response to changes in local temperatures, pressures, and chemical composition. Studying the mantle, naturally, presents several problems, however DCO scientists Giulia Galli and Dimitri Sverjensky have turned to a novel combination of sophisticated theoretical modeling and theoretical aqueous geochemistry to do just that.

Characterizing deep water

A number of important chemical reactions rely on water. But in the mantle, what does water even “look” like? An important characteristic to take into consideration is the dielectric constant. This quantitative, numerical value imparts useful information about the reactivity of water, and how it might interact with other compounds. For example, at ambient temperature and pressure the dielectric constant of water is approximately 78. As temperature increases, the dielectric constant of water decreases, thus changing how it behaves.

Water is an exceedingly influential compound. Indeed, without it there would be no life on our planet. One way in which it exerts this influence is by dissolving inorganic salts, such as sodium chloride - table salt. In this process, charged ions exert an electric field causing molecules of water to organize around them in a “shell”, thanks to the electrical properties of water itself. How efficiently this happens, or how soluble a salt is at defined temperatures and pressures, depends upon the dielectric constant of water.

In a paper published today (19 March 2013) in Proceedings of the National Academy of Sciences, the dielectric constant of water at mantle-like temperatures and pressures is derived. The result of a productive collaboration between Galli, Sverjensky, and the members of their labs, the study forms a fundamental basis for deep chemical research

"When people use models to understand Earth, they need to put in the dielectric constant of water - but there are no data at these depths," Galli said.

In the paper, the dielectric constant for water at 1000K, 300km below Earth's surface, is derived as approximately 38, a value that is about half of what is observed under ambient conditions. Unsurprisingly, this dramatic change has important consequences.

What does this result mean for deep carbon?

One of the primary interests of the DCO is how carbon might move from the surface, into the deep Earth, and then be liberated once more into the atmosphere. To address how the altered dielectric constant of water at high temperatures and pressures might affect carbon chemistry in the mantle, lead author Ding Pan and his colleagues asked how it might affect the solubility of a specific mineral, magnesite. They found that the normally insoluble magnesite is partially soluble at 1000 K and 10GPa.

"It has been thought that carbonate remains in the solid magnesite, but we show that at least part of it can dissolve and could possibly return to the surface, perhaps as CO2 through volcanic emissions," Sverjensky said. "Over geologic timescales, a lot of material can move this way."

This discovery has important implications for various aspects of carbon chemistry, but perhaps most importantly it provides evidence for carbon flow in aqueous solution. Entering the mantle in subduction zones, cycling through the mantle, and evacuating through explosive volcanoes, the presence of aqueous carbon at high temperatures and pressures represents a crucial data point in the big picture of deep carbon chemistry.

A commentary by Craig E. Manning, Extreme Physics and Chemistry Co-Chair, appeared in the April 9th PNAS Early Edition and can be accessed here.

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