The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH

Ocean pH is particularly sensitive to atmospheric carbon dioxide content1,2,3. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed4,5,6,7,8,9,10,11. This requires independent estimates of the δ11B of dissolved boron in sea water through time. The marine δ11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ11B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ11B in foraminiferal carbonates9,10,11, suggesting that foraminifera δ11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.