Towards a new, experimentally-grounded model of the evolution of the boron isotope composition of seawater

The boron isotope-pH proxy, measured in foraminiferal shells, is demonstrably our most promising tool for reconstructing past atmospheric CO2 pressure beyond the ice core record (>800,000 years ago). Precise boron-based CO2 estimates, however, rely on knowledge of the bulk stable Boron (B) isotope composition of seawater (11B/10B(sw)). 11B/10B(sw) is increasingly poorly constrained as one goes further back in geological time (> 10 Ma). This introduces large uncertainty on Cenozoic CO2 reconstructions, and thus is currently a major barrier in using past greenhouse climates and warming events to constrain Earth’s climate and cryosphere sensitivity to CO2. To reduce this uncertainty, we must better quantify boron fluxes and isotopic fractionations in the Earth system, and how they change. The overarching aim of this project, therefore, is to provide a new, experimentally- grounded model estimate for how 11B/10B(sw) has changed over the past 150 million years, unlocking a whole range of new applications for this exciting CO2 proxy. To do this, we will: 1) carry out new experiments quantifying B sorption onto and desorption from clay minerals, and the associated isotope fractionation, under differing solution chemistries (major ion concentration and carbonate chemistry), using state-of-the-art mass spectrometry; 2) couple B and Li isotope measurements in a subset of these experiments to compare sorption behavior under similar conditions, towards perhaps using carbonate Li isotopes to further constrain 11B/10B(sw) in the future; 3) use our experimental data to build a new model for the evolution of 11B/10B(sw) over geological time. This model will incorporate the influence of seawater chemistry (defined by our experiments) and the most up-to-date estimates of crustal spreading rates and changing sediment fluxes and mineralogies from land over geological time. Ultimately, in doing so, we aim to greatly improve our record of how CO2 levels have shaped Earth’s climate over the geological past.