THE CRITICAL ZONE: EARTH'S WEATHERING ENGINE

All terrestrial life on earth is supported by the fragile skin of the planet defined from the outer edges of vegetation down to the lower limits of groundwater. This zone has been named the Critical Zone (CZ) because of its essential role in natural and managed ecosystems. The CZ, present on earth and possibly on other planets, is a complex mixture of air, liquid water, solid earth materials, and biota. The reactions of water and gases with the rocky crust are catalyzed by enzymes to provide all of the nutrients and some of the energy that sustain terrestrial ecosystems. As rocks that have equilibrated with fluids at depth re-equilibrate with fluids at the surface under the influence of tectonic, orbital, and anthropogenic forcings, environmental gradients develop documenting the imprint of life on our planet. These environmental gradients in regolith and aquifers in turn nurture the ecosystems that provide the energy and resources that sustain humans. Given the importance of weathering rocks – soils -- to our species, can we predict how these biomaterials will evolve in the face of environmental change? At present, solute fluxes and regolith profiles can only be predicted for simplified lithologies under abiotic conditions. Many scientists are joining together across disciplines to produce quantitatively predictive models of weathering and to interpret the record of weathering that lies all around us.