The abundance of boron isotopes in ancient marine carbonates can be used to estimate oceanic pH that reflects atmospheric CO2 levels. This proxy requires that the boron isotopic composition of seawater at the time the carbonate has formed is known, and thus the past changes in seawater chemistry. Here we report the boron isotopic composition of selected ancient marine halites and modern sea salts. The signal, interpreted as marine, reveals a clear evolution of the boron isotopic composition of seawater (to 8‰ variations over the Cenozoic). Comparison between this reconstructed curve and the Mg/Ca ratio reveals a high level of consistency that will help to better define oceanic geochemical cycles.