The boron isotopic composition of seawater (δ11Bsw) is a prerequisite for reliably estimating past variation of seawater pH from boron isotopes in marine carbonates (δ11Bc). Here we compose a 50 m.y. stack of δ11Bc from Atlantic and Pacific Ocean deep-sea sediments, and discuss the potential effects of deep-sea pH and δ11Bsw on our record. Using model-based Cenozoic pH values, the inferred δ11Bsw values suggest an ∼3‰ increase since the Late Eocene, superimposed on oscillating variations with amplitudes of up to 2‰. Our estimates match existing reconstructions of δ11Bsw, but call into question published fluid δ11B in halite as a recorder of δ11Bsw.

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