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

Abstract. The boron isotope composition (δ¹¹B) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the δ¹¹B of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater δ¹¹B based on the δ¹¹B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their δ¹³C values. Carbon cycle model simulations underscore that the ΔpH–Δδ¹³C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that δ¹¹B_sw was  ∼  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 ‰. 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.