CRETACEOUS CARBON CYCLING AND CLIMATE EVOLUTION AS CONSTRAINED FROM MARINE ISOTOPE DATA

A model is presented which simulates the composition of seawater (dissolved inorganic carbon, carbonate alkalinity, pH, Ca, Mg, Sr, sulfate) and atmosphere (pO2, pCO2) over the Cretaceous and Cenozoic. Isotope data measured in marine carbonates (d13C, d18O, 87Sr/86Sr) and evaporites (d34S) are used to constrain changes in biogeochemical cycling and climate. The various effects of enhanced volcanism during the mid-Cretaceous are presented and discussed. It is shown that rapid weathering of basaltic rocks and burial of organic carbon compensate effectively for enhanced volcanic CO2 emissions. Moreover, the atmospheric oxygen produced by enhanced organic carbon burial is consumed during the oxidation of reduced volcanic gases and hydrothermal solutions. Thus, the net effect of enhanced mid-Cretaceous volcanism on climate change and atmospheric composition is rather limited. Finally, major open questions are addressed including the controls on seawater Ca/Mg and Sr/Ca ratios and the effects of galactic cosmic radiation on Cretaceous climate change.