Author name: Thomas B. Chalk

The atmospheric partial pressure of CO₂ (pCO₂) is the key driver of climate variability. Boron isotopic compositions (δ¹¹B) of marine calcium carbonates reveal pCO₂ of the geologic past because boron isotope incorporation is sensitive to seawater pH, which closely reflects atmospheric pCO₂. Biocarbonate δ¹¹B values record environmental pH through a metabolic prism (so-called “vital effects”), sometimes complicating interpretations. However, biocarbonate boron isotopes, coupled with boron concentrations (B/Ca), can also reveal the processes of calcification. Here, we review the link between seawater pH and the effective pH recorded by marine organisms via biomineralisation and summarise pCO₂ reconstructions from boron isotopes for the Cenozoic (≈70 Ma to modern times), arguably the most significant contribution of this proxy system to date.