Understanding the boron isotopic composition of seawater (δ11
) is key to calculating absolute estimates of CO2
using the boron isotope pH proxy. Here we use the boron isotope gradient, along with an estimate of pH gradient, between the surface and deep ocean to show that the δ11
varies by ~2 ‰ over the past 23 million years. This new record has implications for both δ11
records and understanding changes in the ocean isotopic composition of a number of ions through time.