Late Cenozoic Rise in Seawater δ11B Not Driven by Increasing Boron Adsorption

The boron isotopic value of ancient seawater (δ¹¹B_sw) is a prerequisite for the reconstruction of seawater pH and atmospheric CO₂ concentrations. Available models and some proxy records suggest that δ¹¹B_sw underwent a large increase during the last 45 million years. This increase has been attributed to an acceleration in sediment discharge into the ocean and the enhanced adsorption of boron on particle surfaces. However, whether global sedimentation rates have increased in the late Cenozoic is contested. Additionally, adsorption efficiency was likely modulated by secondary factors related to seawater chemistry and the sedimentary mineral content, that could have counteracted changes in sedimentation rates. Here we revisit the controls on boron adsorption over the last 100 million years. We found that changes in the seawater concentration of dissolved inorganic carbon (HCO₃⁻, CO₃²⁻) and major ions (Ca²⁺, Mg²⁺, SO₄²⁻) had a negligible impact on boron adsorption. Instead, the sedimentary mineral assemblage and the acidity of seawater were important subordinate factors. By considering several possible sediment production scenarios, we propose that the ability of sediment to adsorb boron was lower in the Cretaceous but has remained similar to the present-day since the Eocene. When these results are incorporated into a seawater model, δ¹¹B_sw exhibits a step-wise enrichment over the Cenozoic, that is, at times, 2‰ above previous model results. Our analysis precludes a dominant role of adsorption in the boron isotope cycle of the late Cenozoic, but nevertheless supports the view that δ¹¹B_sw was lower than today for the last 60 million years.