The ocean has played a major role in controlling the variation of the CO2 content in the atmosphere. The partial pressure of CO2 (Pco2) is moderated by the CO2 concentration in oceans. The pH of seawater is governed by carbonate equilibration, so for a given pH value, it is possible to calculate the aqueous CO2 concentration and, thus, to make quantitative estimates of atmospheric Pco2. We were able to estimate pH of ancient seawater by measuring the boron isotope composition in the calcium carbonate (δ11Bcc) that was precipitated from it. This is because boron in seawater occurs as two species, B(OH)4‾ and B(OH)3, and their relative proportions are a function of pH. Boron isotopes are fractionated between these two species and the fractionation is highly pH dependent over the natural acidity range of seawater, such that δ11B in marine carbonates record the pH of the seawater from which they precipitated.