Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic

The chemical composition of the oceans and long-term climate changes are believed to be linked. Reconstruction of seawater pH evolution is desirable as pH may be related to atmospheric pCO₂, and hence to climate evolution. Boron isotopes in oceanic carbonates have been suggested to be a proxy for oceanic paleo-pH reconstruction. Nevertheless, the calculation of paleo-pH values over geological periods requires a precise knowledge of the boron isotopic composition of the oceans when calcite precipitated. We present the systematics of boron isotopic composition of the world's main rivers. We deduce a continental boron flux to the oceans of 38×10¹⁰ gB/year with a mean isotopic composition of +10‰. These results lead to a balanced boron budget in the oceans and allow the development of a model for the marine boron secular evolution over the past 100 Myr. It is shown that the oceanic boron cycle is mainly controlled by the boron continental discharge and the boron uptake from the oceans during low temperature alteration of oceanic crust. However, the recent important increase of the clastic sediment supply, linked to the Himalayan erosion, impacts the oceanic boron budget by enhancing significantly the boron uptake by adsorption on sediments. We predict a boron isotopic composition in the oceans lower during the Cenozoic and slightly higher during the Cretaceous than today. The modelled values for the marine boron isotopes follow the variations of boron isotopes in carbonates over the Cenozoic era provided by previous studies, suggesting that the variations of the seawater pH may not have been important on this time scale. If this is the case, it involves that buffering mechanisms occur in the oceans to maintain seawater pH at a roughly constant value against past atmospheric pCO₂ variations.