  The chemistry of carbon in aqueous fluids at extreme pressure and temperature
conditions is of great importance to Earth's deep carbon cycle, which
substantially affects the carbon budget at Earth's surface and global climate
change. At ambient conditions, the concentration of carbonic acid in water is
negligible, so aqueous carbonic acid was simply ignored in previous geochemical
models. However, by applying extensive ab initio molecular dynamics simulations
at pressure and temperature conditions similar to those in Earth's upper
mantle, we found that carbonic acid can be the most abundant carbon species in
aqueous CO$_2$ solutions at ~10 GPa and 1000 K. The mole percent of carbonic
acid in total dissolved carbon species increases with increasing pressure along
an isotherm, while its mole percent decreases with increasing temperature along
an isobar. In CO$_2$-rich solutions, we found significant proton transfer
between carbonic acid molecules and bicarbonate ions, which may enhance the
conductivity of the solutions. The effects of pH buffering by carbonic acid may
play an important role in water-rock interactions in Earth's interior. Our
findings suggest that carbonic acid is an important carbon carrier in the deep
carbon cycle.
