The solubility of carbonate minerals and aqueous carbonate speciation in deep fluids is of critical importance to the long-term carbon cycle. However, no experimental data exist in the calcium carbonate–water–salt system at pressures greater than 10 kbar. Here we present an integrated experimental and theoretical study of carbon speciation and solubility during equilibration of aragonite with NaCl solutions at 300 °C from 20 to 70 kbar. Using in situ Raman spectroscopy in a diamond anvil cell with NaCl concentrations up to 1.0 m (5.5 wt.%), bicarbonate and carbonate ions were the only C-bearing aqueous species spectroscopically detected in the fluids. The proportion of total dissolved bicarbonate to carbonate increased with NaCl concentration and was interpreted using a thermodynamic model to retrieve the dissociation constant of the NaHCO30 complex. The results were extended to higher temperatures using the Deep Earth Water (DEW) model to predict calcite solubility and speciation at 700 °C and 10 kbar in chloride solutions. With a one parameter extension of the Debye–Hückel model, the calculated solubilities agree with data from Newton and Manning (2002) up to 10 m NaCl where the solubility is about 250 millimolal. Such solubilities might contribute substantial transport of carbon in fluids at depth in subduction zones.