Possibility of chemical weathering before the advent of vascular land plants

Abstract

CHEMICAL weathering today is generally assumed to occur primarily in soils^1,2. 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 weathering^3–6. 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.