When studying the deep carbon cycle, scientists can’t afford to discount the actions of water, which is an important vehicle moving carbon between the surface and subsurface. But what form does carbon dioxide take when dissolved in water? At the surface, this mixture is just sparkling water, but recent research has shown that these compounds behave very differently under the high temperatures and pressures found in the mantle.
A new paper  by DCO Extreme Physics and Chemistry Community member Ding Pan (The Hong Kong University of Science and Technology, China) and his graduate student Nore Stolte, suggests that carbon dioxide (CO2) turns into carbonic acid (H2CO3) under upper mantle conditions. The researchers used complex computer simulations to predict how the molecules would behave under a range of elevated pressures and temperatures. If large amounts of carbonic acid exist in watery mantle fluids, that could impact the pH of the upper mantle and how fluids dissolve minerals in deep Earth. The researchers report their findings in The Journal of Physical Chemistry Letters.
“We have ignored carbonic acid for at least half a century.” said Pan. “At ambient conditions, if you dissolve carbon dioxide in water, like in a cola, the majority of dissolved carbon is still carbon dioxide.” Based on these surface observations, scientists left carbonic acid out of geochemical models that predict how carbon behaves in subsurface fluids.
Then, in a 2016 paper, Pan and Giulia Galli (University of Chicago, USA), a DCO Extreme Physics and Chemistry Scientific Steering Committee member, made the surprising discovery that in dilute solutions of carbon dioxide and water, at temperatures and pressures found in the mantle, carbon is most likely stored as carbonate (CO32-) and bicarbonate (HCO3-) ions. They made the prediction using the same type of computer simulation used in the current paper, called first-principles molecular dynamics. These simulations require significant computer power and take months to run.
Under the recommendation of Craig Manning (University of California Los Angeles, USA) and Dimitri Sverjensky (Johns Hopkins University, USA), who are both members of the Extreme Physics and Chemistry Community Steering Committee, Pan decided to perform the simulations using higher concentrations of carbon dioxide. The more carbon dioxide they added to the water in the model, the more carbonic acid resulted. It peaked only when they added so much carbon dioxide that it overwhelmed the available water and stopped turning into the acid.
The presence of carbonic acid in watery fluids would lower the pH in the upper mantle and likely impact which minerals dissolve and mobilize in the subsurface. Those dissolved minerals might erupt out of volcanoes or migrate into the crust.
Also, unlike carbon dioxide, which has no charge, carbonic acid is very chemically active. The researchers even observed “proton hopping,” where a proton moves directly from one atom to an atom on another molecule. “Normally we expect a lot of proton hopping between water molecules, and also between carbon and water,” said Pan. “Now we see protons hop between carbon species – carbonic acid and bicarbonate ions. To the best of my knowledge, this is the first example of carbon-to-carbon proton hopping in water ever observed.”
The discovery may also explain a puzzling observation. Previously, scientists calculated that if subsurface water only carries dissolved carbon dioxide, then it can’t contain enough carbon to mesh with estimates of the total amount of carbon dissolved in deep fluids, based on geochemical models. It’s possible that carbonic acid might be an unaccounted form of carbon in deep fluids.
The next step will be to confirm these findings by heating and pressurizing mixtures of carbon dioxide and water in the lab, to see the resulting compounds. Currently Pan’s team is using their computer simulations to help DCO collaborators identify the different carbon species they detect in this type of experiment. “Even at ambient conditions it’s very hard to measure carbonic acid in water,” said Pan, and detecting the acid under mantle conditions is even more challenging. “We hope that we can study more fluid-rock interactions in the presence of carbonic acid, and the more complicated carbon reactions that occur in these solutions.”