The fate of carbon dioxide in water-rich fluids at extreme conditions

TL;DR

This study uses first-principles simulations to show that in Earth's upper mantle conditions, carbon primarily exists as solvated carbonate and bicarbonate ions, not as molecular CO2, affecting our understanding of deep carbon transport.

Contribution

It provides new insights into the chemical speciation of carbon in deep Earth fluids, challenging previous models that emphasized molecular CO2 as the dominant species.

Findings

Carbon exists mainly as carbonate and bicarbonate ions at high P-T conditions.

Molecular CO2 is less abundant than previously thought in deep Earth fluids.

Ion pairing between Na+ and carbonate/bicarbonate decreases with increasing pressure.

Abstract

Investigating the fate of dissolved carbon dioxide under extreme conditions is critical to understanding the deep carbon cycle in the Earth, a process that ultimately influences global climate change. We used first-principles molecular dynamics simulations to study carbonates and carbon dioxide dissolved in water at pressures (P) and temperatures (T) approximating the conditions of the Earth's upper mantle. Contrary to popular geochemical models assuming that molecular CO$_2$(aq) is the major carbon species present in water under deep earth conditions, we found that at 11 GPa and 1000 K carbon exists almost entirely in the forms of solvated carbonate (CO$_3^{2-}$) and bicarbonate (HCO$_3^-$) ions, and that even carbonic acid (H$_2$CO$_3$(aq)) is more abundant than CO$_2$(aq). Furthermore, our simulations revealed that ion pairing between Na$^+$ and CO$_3^{2-}$/HCO$_3^-$ is greatly affected by P-T conditions, decreasing with increasing pressure at 800$\sim$1000 K. Our results suggest that in the Earth's upper mantle, water-rich geo-fluids transport a majority of carbon in the form of rapidly interconverting CO$_3^{2-}$ and HCO$_3^-$ ions, not solvated CO$_2$(aq) molecules.