THE CLIMATIC AND GEOCHEMICAL AFTERMATHS OF THE PANGEA BREAKUP: A STUDY WITH THE GEOCLIM CLIMATE/GEOCHEMICAL COUPLED MODEL

Most of the numerical models calculating the CO2 partial pressure at the million year timescale are based on the balance existing between continental silicate weathering (CO2 sink) and solid Earth degassing (CO2 source). These models are up to now 0D (GEOCARB type models) numerical models. Since continental silicate weathering depends on climatic conditions through runoff and air temperature, these models assume a simple relationship between these two climatic parameters and atmospheric CO2: global mean air temperature is rising with CO2, and a warmer world always corresponds to wetter continents. Here we present the results of a new coupled numerical model of climate and global geochemical cycles (GEOCLIM, Donnadieu et al., 2004, Nature) to simulate the impact of the Pangea breakup on the atmospheric CO2 partial pressure and climatic evolution of the Earth surface. The climate module of the GEOCLIM model is the FOAM GCM (spatial resolution of 7.5°x4.5°) while the geochemical modules COMBINE (Godderis and Joachimski, 2004) now includes the calculation of the weathering rates of silicate rocks on the continents with the spatial resolution of FOAM . This coupling allows the realistic estimation of the runoff and temperature above continental surfaces, and hence of weathering rates. We calculate that the partial pressure of CO2 falls from 1465 ppmv (250 Ma) to 280 ppmv (65 Ma), assuming a constant present day solid Earth degassing. This general decrease is linked to the slow increase in weathering of continental silicates with increasing runoff, itself triggerred by the breakup of Pangea. Interestingly, calculated continental air temperature remains roughly constant from the Aptian to the Maastrichtian (within 1°C), while PCO2 declines sharply (from 828 to 280 ppmv).