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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 (CO2)-C-12, (CO2)-C-13, and (CO2)-C-14 are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured P-CO2, delta(13)C( in CO2), and delta(14)C( inCO(2)) profiles, indicate that the shallow CO2 source from root and microbial respiration composes similar to97% of the annual average total CO2 production at this arid site. Despite the small contribution from deep CO2 production amounting to similar to0.1 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, C-14 exchange with a sorbed CO2 phase is indicated by the modeled delta(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.