POSSIBILITY OF CHEMICAL-WEATHERING BEFORE THE ADVENT OF VASCULAR LANDPLANTS

CHEMICAL weathering today is generally assumed to occur primarily in s oils1,2. The rise of vascular plants during the Silurian and Devonian periods about 400 Myr ago brought about an increase in soil microbial activity and thus in soil CO2 generation, and it has therefore been wi dely believed that, as a result of these changes, soil CO2 replaced at mospheric CO2 as the primary agent of chemical weathering3-6. Here we show that the aerated region above the water table (the vadose zone) e xerts a strong influence on the CO2 concentration to which runoff is e xposed as it percolates beneath the soil, and we argue that this could have been the case before the Silurian. We present calculations which show that, for present-day atmospheric CO2 concentrations, a low leve l of microbial - respiration may be sufficient to support appreciable CO2 concentrations in the vadose zone because of the slow rate of CO2 loss to the surface. Despite the small amount of microbial respiration in pre-Silurian soilS, CO2 concentrations in subsoil vadose zones mig ht therefore have been sufficient to account for the apparent constanc y of chemical weathering since the mid-Proterozoic7, obviating the nee d to invoke high levels of atmospheric CO2 to explain the weathering r ecord.