Modeling of continental weathering under high-CO2 atmospheres during Precambrian times

Batch experiments were conducted to simulate abiotic weathering of the continental crust under high-CO2 atmospheric conditions during Precambrian times, i.e., corresponding to the general Archaean conditions as well as to the immediate aftermath of the Neoproterozoic Snowball Earth ice ages. Three types of rock (basalt, granodiorite and tonalite) representative of the Archaean to Proterozoic continental crust were reacted in the form of powders for 1 year at 40 degrees C with pure water under various water/rock ratios, in oxic and anoxic atmospheres containing 10% CO2, that is, under conditions assumed to characterize the greenhouse effect which prevailed at that time. Chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps: (1) rapid leaching of the fresh surfaces, probably related to the proton-donor capacity of the CO2 and (2) a steady state reaction under near-neutral conditions. These observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature. Using a 1-D weathering model reproducing the two steps, we evaluate the atmospheric CO2 consumption rate at the Snowball Earth aftermath, focusing on the relative contribution of surface leaching versus steady state reaction. The results show that the process is dominated by surface exchange.