CO sub(2) dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone

Natural unsaturated-zone gas profiles at the U.S. Geological Survey's Amargosa Desert Research Site, near Beatty, Nevada, reveal the presence of two physically and isotopically distinct CO sub(2) sources, one shallow and one deep. The shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone. Scanning electron micrograph results indicate that at least part of the deep CO sub(2) source is associated with calcite precipitation at the 110-m-deep water table. We use a geochemical gas-diffusion model to explore processes of CO sub(2) production and behavior in the unsaturated zone. The individual isotopic species super(12)CO sub(2), super(13)CO sub(2), and super(14)CO sub(2) are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured P sub(COd2), delta super(13)C (in CO sub(2)), and delta super(14)C (in CO sub(2)) profiles, indicate that the shallow CO sub(2) source from root and microbial respiration composes similar to 97% of the annual average total CO sub(2) production at this arid site. Despite the small contribution from deep CO sub(2) production amounting to similar to 0.1 mol m super(-2) yr super(-1), upward diffusion from depth strongly influences the distribution of CO sub(2) and carbon isotopes in the deep unsaturated zone. In addition to diffusion from deep CO sub(2) production, super(14)C exchange with a sorbed CO sub(2) phase is indicated by the modeled delta super(14)C profiles, confirming previous work. The new model of carbon- isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones.