  We investigate the effect of pressure, temperature and acidity on the
composition of water-rich carbon-bearing fluids at thermodynamic conditions
that correspond to the Earth's deep Crust and Upper Mantle. Our
first-principles molecular dynamics simulations provide mechanistic insight
into the hydration shell of carbon dioxide, bicarbonate and carbonate ions, and
on the pathways of the acid/base reactions that convert these carbon species
into one another in aqueous solutions. At temperature of 1000 K and higher our
simulations can sample the chemical equilibrium of these acid/base reactions,
thus allowing us to estimate the chemical composition of diluted carbon dioxide
and (bi)carbonate ions as a function of acidity and thermodynamic conditions.
We find that, especially at the highest temperature, the acidity of the
solution is essential to determine the stability domain of carbon dioxide,
bicarbonate and carbonate ions.
