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Letters to Nature

Nature 408, 951-954 (21 December 2000) | doi:10.1038/35050058; Received 20 June 2000; Accepted 6 November 2000

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

D. Lemarchand, J. Gaillardet, É. Lewin & C. J. Allègre

  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).

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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 (delta11B) 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 delta11B of dissolved boron in sea water through time. The marine delta11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from delta11B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean delta11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of delta11B in foraminiferal carbonates9, 10, 11, suggesting that foraminifera delta11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.