© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Climate of the Past 13 (2017): 149-170, doi:10.5194/cp-13-149-2017.
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 δ11Bsw 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.
was supported by NERC grants NE/I006176/1 (Gavin L. Foster and
Caroline H. Lear), NE/H006273/1 (Gavin L. Foster), NE/I006168/1
and NE/K014137/1 and a Royal Society Research Merit Award
(Paul A. Wilson), a NERC Independent Research Fellowship
NE/K00901X/1 (Mathis P. Hain) and a NERC studentship
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