CONTROLS ON THE CRETACEOUS AND CENOZOIC EVOLUTION OF SEAWATER COMPOSITION, ATMOSPHERIC CO2 AND CLIMATE

A new box model for the global carbon-calcium-strontium cycle is developed to simulate the evolution of Cretaceous and Cenozoic seawater and atmosphere. The model accounts for carbon masses in ocean and atmosphere, in carbonate, in particulate organic carbon, and in the mantle. Major processes considered in the model are mantle degassing and hydrothermal fluxes, alteration of oceanic crust, silicate weathering, carbonate weathering, metamorphism of carbonates, carbonate accumulation, carbonate turnover in subduction zones, and the turnover of particulate organic carbon. Model outputs are the partial pressure of CO2, seawater pH, concentrations of Ca, Sr, and HCO3 in seawater as well as the carbon and strontium isotopic composition of seawater and marine carbonates. A comprehensive review of current fluxes is given to define the flux equations and parameters. Secular trends recorded in marine carbonates (Sr/Ca ratio, 87Sr/86Sr ratio, d13C values) are used to constrain the remaining open model parameters. The model includes a new parameterization of oceanic crust alteration and silicate weathering to consider the consumption of CO2 during seafloor alteration and weathering of volcanic deposits. Sensitivity tests show that the enhanced input of volcanic CO2 during periods of intense volcanic/tectonic activity is largely compensated by weathering of volcanic deposits and alteration of oceanic crust. Due to this adjustment, only weak negative feed-backs are needed to maintain moderate CO2 levels throughout the model period. These feed-backs are provided by the temperature dependence of continental runoff. Removal of CO2 via silicate weathering and POC burial is enhanced by increased runoff so that excess CO2 is taken up during periods of strong volcanic activity. The model also considers the impact of physical denudation on silicate weathering. In accordance with paleoclimatic evidences, it predicts high atmospheric CO2 and surface temperatures for the mid-Creatceous and early Cenozoic and low values for the cool periods during the late Cretaceous and late Cenozoic.