Balancing the Cenozoic carbon and alkalinity cycles: Constraints from isotopic records

Abstract

A simple model of the carbon, alkalinity and strontium cycles is built up and used to interpret the carbon and strontium isotopic evolution of seawater over the Cenozoic as recorded in marine limestones. The idea that weathering fluxes have increased globally over the Cenozoic in response to the Himalayan uplift is critically examined. It is shown that such an increase of global weathering, postulated to explain the sharp increase in the 87Sr/86Sr ratio, is not compatible with the carbon isotopic record, if the budgets of carbon, alkalinity and strontium are considered simultaneously. Another scenario of the Cenozoic evolution of global weathering consistent with the carbon isotopic record is proposed in which the global weathering flux varies only slightly over the Cenozoic with a small increase between 15 Ma and the present. This second scenario appears more consistent with the reconstructed carbonate sedimentation rate and the evolution of the carbon isotopic fractionation in continental and oceanic environments.