POSSIBILITY OF CHEMICAL-WEATHERING BEFORE THE ADVENT OF VASCULAR LAND PLANTS

CHEMICAL weathering today is generally assumed to occur primarily in soils1,2. 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 widely believed that, as a result of these changes, soil CO2 replaced atmospheric CO2 as the primary agent of chemical weathering3-6. Here we show that the aerated region above the water table (the vadose zone) exerts a strong influence on the CO2 concentration to which runoff is exposed 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 level 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 might therefore have been sufficient to account for the apparent constancy of chemical weathering since the mid-Proterozoic7, obviating the need to invoke high levels of atmospheric CO2 to explain the weathering record.