Boron Isotopes: A "Paleo-pH Meter" for Tracking Ancient Atmospheric CO₂

The boron isotope composition of calcium carbonate shells of marine organisms has the unique potential to record surface ocean pH, allowing the calculation of atmospheric pCO₂ due to the established relationship between pH and the partial pressure of (atmospheric) CO₂ (pCO₂). This "paleo-pH meter" allows scientists to produce a record of the natural fluctuations of atmospheric pCO₂ over geologic time, which will help us better understand the impacts of the recent anthropogenic addition of CO₂ to Earth's atmosphere. Towards this end, a tremendous effort to understand the systematics of boron uptake in marine carbonates is underway. Here, we review the potential of boron isotopes to constrain ocean pH and, thus, atmospheric pCO₂.