title: The fate of carbon dioxide in water-rich fluids under extreme conditions.
abstract: Investigating the fate of dissolved carbon dioxide under extreme conditions is critical to understanding the deep carbon cycle in 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 Earth's upper mantle. Contrary to popular geochemical models assuming that molecular CO2(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 ([Formula: see text]) and bicarbonate ([Formula: see text]) ions and that even carbonic acid [H2CO3(aq)] is more abundant than CO2(aq). Furthermore, our simulations revealed that ion pairing between Na+and [Formula: see text]/[Formula: see text] is greatly affected byP-Tconditions, decreasing with increasing pressure at 800 to 1000 K. Our results suggest that in Earth's upper mantle, water-rich geofluids transport a majority of carbon in the form of rapidly interconverting [Formula: see text] and [Formula: see text] ions, not solvated CO2(aq) molecules.
authors.affiliation: The Institute for Molecular Engineering, the University of Chicago, Chicago, IL 60637, USA.
authors.forename: Ding
authors.identifier: 0000-0002-2353-0130
authors.initials: D
authors.lastname: Pan
authors.affiliation: The Institute for Molecular Engineering, the University of Chicago, Chicago, IL 60637, USA.; Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: Giulia
authors.initials: G
authors.lastname: Galli
keywords: CO2; First-principles molecular dynamics; deep carbon cycle; supercritical water