ISOTOPIC CONSTRAINTS ON THE CENOZOIC EVOLUTION OF THE CARBON-CYCLE

In the last few years, several models have been built to explore the C enozoic evolution of the carbon and strontium cycles. Of particular in terest is the study of the impact on the carbon cycle of major mountai n uplifts such as the Himalayan orogeny. To explain the Cenozoic incre ase in the measured seawater strontium isotopic ratio, it was recently proposed that the Himalyan uplift could be responsible for an enhance d consumption of atmospheric CO2 by continental silicate weathering. H ere, a new model of the carbon cycle evolution over Cenozoic times is presented. It calculates the various fluxes involved in the organic an d inorganic components of the carbon cycle from the seawater delta C-1 3, the biological isotopic fractionation in the ocean and the seafloor spreading rate. The model equilibrates the budgets of the carbon and alkalinity cycles on the million year timescale, assuming as many prev ious investigators that the system remains close to equilibrium. The v alidity of this equilibrium approximation is examined critically. Vari ous sensitivity experiments are performed in order to test the impact of the model parameters on the results. The calculated history of the carbonate deposition rate is consistent with the available reconstruct ion. The continental silicate weathering rate calculated by the model appears to be widely insensitive to the model parameters, showing thre e distinct evolutions over the Cenozoic. The model indeed suggests a t ime of relative constancy of the silicate weathering flux before 40 Ma , followed by a period of slow decrease until 15 Ma and finally a mark ed increase up to the present. In a progressively cooler world, this e volution may be interpreted as a change from a 'chemically' controlled to a 'physically' controlled weathering regime. The evolution of cont inental silicate weathering thus partly appears decoupled from the inc rease in the observed seawater strontium isotopic ratio. For this reas on, the evolution of the calculated riverine Sr-87/ Sr-86 ratio shows a strong increase over the Cenozoic, from about 0.710 to 0.712. Howeve r, this increase may largely be reduced by considering the recycling o f a pelagic carbonate reservoir increasing over the Cenozoic or by ass uming that seafloor basalt weathering is a CO2- or climate-dependent p rocess. (C) 1998 Elsevier Science B.V. All rights reserved.