Abstract. The boron isotope composition (δ11B) of planktic foraminiferal calcite, which reflects seawater pH, is a well-established proxy for reconstructing palaeo-atmospheric CO2 and seawater carbonate chemistry. However, to translate δ11B measurements determined in calcareous fossils into pH we need to know the boron isotope composition of the parent seawater (δ11Bsw). While a number of δ11Bsw reconstructions exist, the discrepancies between them reveals uncertainties and deficiencies that need to be addressed. Here we present a new δ11Bsw record based on the δ11B difference between planktic and benthic foraminifera and an estimate of the pH gradient between surface and deep water. We then calculate δ11Bsw two different ways. One variant of our method assumes that the pH gradient between surface and deep has remained the same as today over the past 23 Ma; the other uses the δ13C gradient between surface and deep to represent change in the pH gradient through time. The results of these two methods of calculating δ11Bsw are broadly consistency with each other, however, based on extensive carbon cycle modelling using CYCLOPS and GENIE we favour the δ13C gradient method. In our favoured δ11Bsw reconstruction, δ11Bsw is around 2 ‰ lower than today at ~37.5 ‰ during the early and middle Miocene and increases to the modern value (39.61 ‰) by ~5 Ma. A similar pattern of change is evident in the seawater composition of three other stable isotope systems, Mg, Li and Ca. Concurrent shifts in the seawater isotopic composition of all four of these elements during the late Miocene, suggest a common forcing mechanism. We hypothesise the most likely cause of these shifts is a change in the isotopic composition of the riverine input, potentially driven by an increase in secondary mineral formation since ~15 Ma.