  The reactions of CO$_2$ in water under extreme pressure-temperature
conditions are of great importance to the carbon storage and transport below
Earth's surface, which substantially affect the carbon budget in the
atmosphere. We applied ab initio molecular dynamics simulations to study
aqueous carbon solutions nanoconfined by graphene and stishovite (SiO$_2$) at
10 GPa and 1000$\sim$1400 K. We found that CO$_2$(aq) reacts more in
nanoconfinement than in bulk. The stishovite-water interface makes the
solutions more acidic, which shifts the chemical equilibria, and the interface
chemistry also affects the reaction mechanisms. Our findings suggest that
CO$_2$(aq) in deep Earth may be more active than previously thought, and
confining CO$_2$ and water in nanopores may enhance the efficiency of mineral
carbonation.
