CO2 dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone

	Become an AGU Member Subscribe to AGU Journals



Read Full Article (file size: 1454837 bytes) Cited by WATER RESOURCES RESEARCH, VOL. 41, W02006, doi:10.1029/2004WR003599, 2005 CO₂ dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone Michelle A. Walvoord U.S. Geological Survey, Lakewood, Colorado, USA Robert G. Striegl U.S. Geological Survey, Lakewood, Colorado, USA David E. Prudic U.S. Geological Survey, Carson City, Nevada, USA David A. Stonestrom U.S. Geological Survey, Menlo Park, California, USA Abstract 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 P _CO₂, 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. Received 25 August 2004; accepted 30 November 2004; published 4 February 2005. Keywords: carbon 14 exchange; C isotopes; CO₂ transport; geochemical modeling; unsaturated zone. Index Terms: 1875 Hydrology: Vadose zone; 0414 Biogeosciences: Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912); 0428 Biogeosciences: Carbon cycling (4806); 0454 Biogeosciences: Isotopic composition and chemistry (1041, 4870). Read Full Article (file size: 1454837 bytes) Cited by Citation: Walvoord, M. A., R. G. Striegl, D. E. Prudic, and D. A. Stonestrom (2005), CO₂ dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone, Water Resour. Res., 41, W02006, doi:10.1029/2004WR003599. This paper is not subject to U.S. copyright. Published in 2005 by the American Geophysical Union.

Abstract