A sink- or a source-driven carbon cycle at the geological timescale? Relative importance of palaeogeography versus solid Earth degassing rate in the Phanerozoic climatic evolution

The Phanerozoic evolution of the atmospheric CO2 level is controlled by the fluxes entering or leaving the exospheric system. In this contribution, we focus on the role played by the palaeogeographic configuration on the efficiency of the CO2 sink by continental silicate weathering, and on the impact of the magmatic degassing of CO2. We use the spatially resolved numerical model GEOCLIM to compute the response of the silicate weathering and atmospheric CO2 to continental drift for 22 time slices of the Phanerozoic. Regarding the CO2 released by the magmatic activity, we reconstruct several Phanerozoic histories of this flux, based on published indices. Again using the GEOCLIM model, we calculate the CO2 evolution for each degassing scenario. We show that the palaeogeographic setting is a main driver of the climate from 540 Ma to about the beginning of the Jurassic, with the noticeable exception of the Late Palaeozoic ice age. Regarding the role of the magmatic degassing, the various reconstructions do not converge towards a single signal, and thus introduce large uncertainties in the calculated CO2 level over time. Nevertheless, the continental dispersion, which prevails since the Jurassic, promotes CO2 consumption by weathering and forces atmospheric CO2 to stay low. Warm climates of the 'middle' Cretaceous and early Cenozoic require enhanced CO2 degassing by magmatic activity.