Shield effect on continental weathering:: Implication for climatic evolution of the Earth at the geological timescale

A model sensitivity study is performed in order to explore the impact of the development of thick weathering profiles in the equatorial area on the geological evolution of climate. Long term climatic evolution is driven by the balance existing between solid Earth degassing and continental silicate weathering. The CO2 consumption through continental silicate weathering is here calculated as a function of climate, based on the correlation existing with mean annual continental runoff for large drainage basins. These correlations appear to be dependent on the climatic regime: under mid-latitude temperate climate, the silicate weathering is 5 times more sensitive to continental runoff than in the warm humid equatorial belt. These correlations emphasize the role of the thick weathering profiles existing in equatorial area, shielding the bedrock from further weathering. We use this set of relationships between runoff and silicate weathering as a function of the geoclimatic zone, to explore the impact of this shield effect on the Mesozoic evolution of climate, using the GEOCLIM model. Compared to previous work, we suggest that atmospheric CO2 was generally underestimated in the past, evidencing a complex interplay between the paleogeographic setting, and the shield effect. Particularly, calculated atmospheric CO2 during the Early-Middle Jurassic rises up to 1600 ppmv, while it was limited to 700 ppmv when shield effect was neglected, corresponding to a global warming of more than 4 degrees C. This study emphasizes the need for the building up of numerical models describing the growth of the weathering profiles that can be coupled to vegetation and climate models to investigate biogeochemical cycle and climate evolution, even at the geological timescale. (C) 2008 Elsevier B.V. All rights reserved.