A record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera

The boron isotope composition (delta B-11) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the delta B-11 of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater delta B-11 based on the delta B-11 difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their delta C-13 values. Carbon cycle model simulations underscore that the Delta pH-Delta delta C-13 relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that delta B-11(sw) was similar to 37.5% during the early and middle Miocene (roughly 23-12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61 parts per thousand. Strikingly, this pattern is similar to the evolution of the seawater isotope composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotope composition of riverine input to the ocean since 14 Ma.