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- 3). Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (delta B-1 1) contained in the carbonate shells of marine organisms varies according t o pH, from which ocean pH can be reconstructed(4-11). This requires indepen dent estimates of the delta B-11 of dissolved boron in sea water through ti me. The marine delta B-11 budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as e stimated from delta B-11 measurements in 22 of the world's main rivers), th e marine boron isotope budget can be balanced. We also derive ocean delta B -11 budgets for the past 120 Myr. Estimated isotope compositions of boron i n sea water show a remarkable consistency with records of delta B-11 in for aminiferal carbonates(9-11), suggesting that foraminifera delta B-11 record s may in part reflect changes in the marine boron isotope budget rather tha n changes in ocean pH over the Cenozoic era.