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

Ocean pH is particularly sensitive to atmospheric carbon dioxide content 1 , 2 , 3 . Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ 11 B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . This requires independent estimates of the δ 11 B of dissolved boron in sea water through time. The marine δ 11 B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ 11 B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ 11 B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ 11 B in foraminiferal carbonates 9 , 10 , 11 , suggesting that foraminifera δ 11 B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.