Modelling the snowball Earth: from its inception to its aftermath

International audienceThe relationship between CO2 variation and Neoproterozoic glaciation has deeply evolved these last years. Through the use of an innovative climate-carbon coupled model, the causes of the CO2 decrease that led to the onset of the global glaciation (Sturtian) has been shown to be strongly related to the dislocation of the Rodinia super continent, promoting CO2 consumption through silicate weathering1. Another important issue is the evolution of atmospheric CO2 during the Snowball episode itself. It appears not to be just a linear accumulation with time through the ongoing solid Earth degassing. Indeed, efficient CO2 diffusion in seawater might have promoted the oceanic crust dissolution, resulting in an asymptotic CO2 rise in the atmosphere2,3, stressing the question of the snowball melting threshold. Indeed, it has been shown that greenhouse climate induced by the storage of the CO2 in the atmosphere invoked to escape a snowball Earth was possibly not sufficient to melt the snowball Earth due to thermal inversion in vertical column4. Therefore, CO2 is may be not the only trigger for the deglaciation. Finally, the super greenhouse climate thought to have followed the snowball episode was explored. We demonstrate that, despite very high temperatures under 0.2 bars of CO2, the amount of rainfall might have been limited by the availability of latent heat which cannot be higher that the total energy provided by the sun. As a consequence of limited increase in the water cycling, CO2 consumption by continental weathering might not exceed 10 times its present day value. The return to normal climatic conditions after the snowball melting should thus have lasted several million of years, further increasing the biological perturbations linked to a snowball event5. The aim of this contribution is to revisit the issue of the role of atmospheric CO2 before, during and after a Snowball Earth and to deliver a new picture of its feedbacks with climate. 1. Donnadieu, Y., Y. Godderis, et al. (2004). "A 'snowball Earth' climate triggered by continental break-up through changes in runoff." Nature 428(6980): 303- 306. 2. Ramstein G., Donnadieu Y., Goddéris Y. Proterozoic glaciations. Comptes Rendus Geoscience 336 (7-8): 639-646 Jun 2004 3. Le Hir G., Goddéris Y., Donnadieu Y., Ramstein G. (2008). A geochemical modelling study of the evolution of the chemical composition of seawater linked to a "snowball" glaciation. Biogeosci. 5, 253-267. 4. Le Hir Guillaume , Goddéris Yves, Donnadieu Yannick , Ramstein Gilles (2008) A scenario for the evolution of the atmopsheric pCO2 during a Snowball Earth, Geology, 36 (1): 47-50 5. Pierrehumbert, R.T. (2004). High levels of atmospheric carbon dioxide necessary for the termination of global glaciation: Nature, v. 429, p. 646-649, doi: 10.1038/nature02640. 6. Le Hir Guillaume, Donnadieu Yannick, Goddéris Y, Pierrehumbert Raymond T., Halverson Galen P., Macouin Mélina, Nédélec Anne b, Ramstein Gilles. (2008).The snowball Earth aftermath: Exploring the limits of continental weathering processes, Earth and Planetary Science Letters, EPSL-09573; No of Pages 11