New Study of Carbon and Water at the Bottom of Earth’s Upper Mantle Challenges Previous Models

The deep carbon cycle brings carbon from Earth’s interior to surface reservoirs and back again over millions to billions of years.

Within DCO’s Extreme Physics and Chemistry (EPC) Community, the question of how carbon reacts with aqueous fluids in the mantle looms large, with implications for understanding how carbon is stored in deep Earth and for how long, as well as how carbon is released into the atmosphere at subduction zones and during volcanic eruptions.

water in the mantle

In a paper published recently in Science Advances, DCO’s Ding Pan and Giulia Galli (University of Chicago, USA), conducted new calculations that challenge a key assumption prevalent in the field [1]. Models of fluids in Earth’s mantle have long assumed carbon bonds with oxygen in aqueous solution to form molecular CO2. Pan and Galli show, through a series of first-principles molecular dynamics simulations, that carbon in aqueous solutions at 11 GPa and 1000K is predominantly stored as carbonate and bicarbonate ions.

Water is a major transporter of important chemical species in the mantle, and therefore understanding its chemical behavior at high pressure and temperature (P/T) is critical. However, studying water at high P/T in the laboratory is challenging because it is difficult to obtain clear spectroscopic signals – the preferred method for generating clear data about chemical speciation.

Scientists therefore turn to sophisticated computer modeling simulations to address the physics and chemistry of deep Earth. These simulations are built using the basic laws of quantum mechanics, without any fit to experimental data, and require impressive computing power. When considering a major player like water, it is extremely important to conduct such simulations and study systems at the atomistic and molecular level.

Short video created by the DCO Engagment Team.

After performing a series of computer simulations to test and validate widely held assumptions, Pan and Galli began questioning central assumptions often made when studying water and carbon dioxide at extreme conditions. Namely, that carbon in aqueous solution in Earth’s mantle primarily exists as CO2. As they investigated further, their data showed something quite different.

“This work built on our previous experiences simulating water and carbonates under pressure, an investigation we carried out in close collaboration with experimental groups,” said Pan. “Water is a major component of geofluids, which help transport great amounts of carbon in Earth’s interior. However, we still do not fully know the forms of dissolved carbon in water at such extreme conditions. Our study shows that carbon-bearing fluids cannot be simply modeled as the mixtures of neutral gas molecules. Rather, they may exhibit complicated ionic interactions in water at these high pressure and high temperature conditions.”

At 11 GPa and 1000K, the P/T conditions at the bottom of Earth’s upper mantle (about 400km below Earth’s surface), the authors found that carbon is far more likely to exist as CO32- or HCO3-. They therefore carefully investigated why this might be the case. Pan and Galli traced the origins of these unique results to the fact that water at high pressure behaves differently when compared to ambient conditions and hence interacts in a very different manner with carbon dioxide. This study found that CO2 dissociation in water happens much faster at depth than at Earth’s surface, making HCO3- and H3O+ much more readily available. The authors therefore conclude that it is unlikely much CO2 could exist in aqueous solution in Earth’s interior.

“This is an exciting and important finding,” said EPC Community Chair Craig Manning (University of California Los Angeles, USA). “It challenges long held assumptions about the nature of carbon in crustal and mantle fluids. Abundant carbonate anions will cause carbon-bearing fluids to be substantially more chemically active than if carbon were present as CO2 molecules. It is likely that geochemists, petrologists and geophysicists need to think about carbon and fluids in fundamentally new ways.”

“Our study shows the importance of of accounting for the changes of water properties at the atomistic scale, under extreme conditions,” added Galli. “Only by doing so, one can understand chemical reactions in aqueous media at high pressure and temperature.“

Image: The chemical reactions of dissolved carbon in water in Earth's interior. Credit: Peter Allen and Emmanuel Gygi.

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