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

Science Advances  12 Oct 2016:
Vol. 2, no. 10, e1601278
DOI: 10.1126/sciadv.1601278

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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 (Embedded Image) and bicarbonate (Embedded Image) ions and that even carbonic acid [H2CO3(aq)] is more abundant than CO2(aq). Furthermore, our simulations revealed that ion pairing between Na+ and Embedded Image/Embedded Image is greatly affected by P-T conditions, 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 Embedded Image and Embedded Image ions, not solvated CO2(aq) molecules.

  • First-principles molecular dynamics
  • deep carbon cycle
  • supercritical water
  • CO2

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