Robert G. Striegl David E. Prudic David A. Stonestrom Michelle Ann Walvoord 2005 <p>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 CO₂ 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 CO₂ source is associated with calcite precipitation at the 110-m-deep water table. We use a geochemical gas-diffusion model to explore processes of CO₂ production and behavior in the unsaturated zone. The individual isotopic species ¹²CO₂, ¹³CO₂, and ¹⁴CO₂ are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured <img class="inlineGraphic" src="http://onlinelibrary.wiley.com/store/10.1029/2004WR003599/asset/equation/wrcr10149-math-0001.gif?v=1&amp;s=28e038537ad2664695a6f7358c9ddadb5339aea0" alt="image" data-mce-src="http://onlinelibrary.wiley.com/store/10.1029/2004WR003599/asset/equation/wrcr10149-math-0001.gif?v=1&amp;s=28e038537ad2664695a6f7358c9ddadb5339aea0"> δ¹³C (in CO₂), and δ¹⁴C (in CO₂) profiles, indicate that the shallow CO₂ source from root and microbial respiration composes ∼97% of the annual average total CO₂ production at this arid site. Despite the small contribution from deep CO₂ production amounting to ∼0.1 mol m⁻² yr⁻¹, upward diffusion from depth strongly influences the distribution of CO₂ and carbon isotopes in the deep unsaturated zone. In addition to diffusion from deep CO₂ production, ¹⁴C exchange with a sorbed CO₂ phase is indicated by the modeled δ¹⁴C 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.</p> application/pdf 10.1029/2004WR003599 en AGU Publications CO2 dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone article