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ORIGINAL RESEARCH

A Transient Model of Vadose Zone Reaction Rates Using Oxygen Isotopes and Carbon Dioxide

^a Dep. of Geological Sciences, Univ. of Saskatchewan, Saskatoon, SK, Canada, S7N 5E2
^b National Water Research Inst., Environment Canada, 11 Innovation Blvd., Saskatoon, SK, Canada, S7N 3H5
^c Dep. of Earth and Atmospheric Sciences, 126 Earth Sciences Bldg., Univ. of Alberta, Edmonton, AB, Canada, T6G 2E3
^* Corresponding author (tkb130{at}mail.usask.ca)

Received 11 January 2006.

The importance of identifying and quantifying subsurface geochemical reaction rates and processes by monitoring and modeling CO₂ and O₂ concentrations is well established. These parameters, however, are typically studied independently under presumed steady-state conditions. Here we present models of seasonally variable vadose zone CO₂ and O₂ concentrations that use ¹⁸O of O₂ as a constraint to create a dynamic link between these three parameters under transient conditions. The gas transport modeling was used to quantify the controls of biogeochemical processes and parameters (i.e., temperature and moisture content) on vadose zone distributions of CO₂ and O₂ gas concentrations. The investigation was conducted on a 3-m-thick, unvegetated, fine-sand vadose zone located in northern Alberta, Canada (56°40'N, 111°07'W). Using the modeled molar ratio of surface fluxes for O₂ and CO₂, the change in reaction rate for a temperature change of 10°C (Q₁₀), moisture content at maximum reaction rates, and biogeochemical discrimination against consumption of ¹⁸O¹⁶O (_k), we determined that organic C oxidation by microbial respiration was the predominant mechanism consuming O₂ and producing CO₂. The mean _k was determined to be 0.973, suggesting that subsurface respiration was via the alternative oxidase pathway, which may be common in cold climates. Modeling revealed that the moisture content of a moist, surficial clayey sand layer (0.1–0.3 m thick) had a dramatic effect on pore-gas CO₂ and O₂ concentrations and on ¹⁸O_O2. The vadose zone in this study was at an unvegetated site to simplify the model application; however, it can be modified to include root respiration and applied to natural vadose zones to help quantify the role of subsurface respiration in global O₂ and C budgets.

Abbreviations: C_T, total carbon • TIC, total inorganic carbon • TOC, total organic carbon • VSMOW, Vienna Standard Mean Ocean Water