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 themajor 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 ($CO^{2-}_3$) and bicarbonate ($HCO^{-}_3$) ions and that even carbonic acid [H₂CO₃(aq)] is more abundant than CO₂(aq). Furthermore, our simulations revealed that ion pairing between Na⁺ and $CO^{2-}_3$ /$HCO^{-}_3$ is greatly affected by P-T conditions, decreasing with increasing pressure at 800 to 1000 K. Our results suggest that in Earth's uppermantle,water-rich geofluids transport a majority of carbon in the form of rapidly interconverting $CO^{2-}_3$ and $HCO^{-}_3$ ions, not solvated CO₂(aq) molecules.