1. Laboratoire de Géochimie et Cosmochimie, Institut de Physique du globe de Paris, 4 place Jussieu, 75252 Paris Cedex 05

Correspondence to: D. Lemarchand Correspondence and requests for materials should be addressed to D.L. (e-mail: Email: lemarcha@ipgp.jussieu.fr).

Abstract

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 (¹¹B) 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 ¹¹B of dissolved boron in sea water through time. The marine ¹¹B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from ¹¹B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean ¹¹B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of ¹¹B in foraminiferal carbonates^{9, 10, 11}, suggesting that foraminifera ¹¹B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.