CLIMATE CHANGE DURING THE LAST 150 MILLION YEARS: RECONSTRUCTION FROM A CARBON CYCLE MODEL

Abstract

Variations of the atmospheric CO2 level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO2 degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO2 level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO2 level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO2 level (4-5 PAL) maintained by the enhanced CO2 degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO2 level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.