Carbon cycle and climate change during the Cretaceous inferred from a biogeochemical carbon cycle model

Abstract

The carbon cycle and climate change during the Cretaceous are reconstructed by using a carbon cycle model, and discussed. The model takes into account the effects of the enhanced magma eruption and organic carbon burial rates, both of which characterize the carbon cycle during the Cretaceous. The result for the CO2 variation is roughly consistent with the pattern of paleoclimate change inferred from the geological record. The CO2 level during the mid‐Cretaceous is estimated to be 4–5 times the present atmospheric level, corresponding to a surface temperature of 20–21°C. The warm, equable Cretaceous resulted from the effects of tectonic forcing such as enhanced CO2 degassing, although the enhanced organic carbon burial has a tendency to decrease the CO2 level. The organic carbon burial rate during the Cretaceous is generally larger than those for the Cenozoic, and is characterized by three major peaks (~ 1.5–1.8 times the present‐day value) corresponding to the major oceanic anoxic events. In the case for the extensive mantle plume degassing, although the CO2 levels are only 10% higher than those for the standard case during 120–100 Ma, the causes for the enhanced CO2 levels would be quite different. If the globally averaged surface temperature had increased due to paleogeographic forcing effects, the greenhouse effect of CO2 (and thus the CO2 level) should be lower than the values estimated for the standard case. If the CO2 levels are similar to, but the surface temperature is higher than, those for the standard case, either the parameter β (an influence of the Himalayas–Tibetan Plateau on the global weathering today) may be unreasonably large or the dependence of the silicate weathering rate on the CO2 partial pressure and the surface temperature should be much weaker than those previously proposed.