A Transient Model of Vadose Zone Reaction Rates Using Oxygen Isotopes and Carbon Dioxide
2007
Birkham, Tyler K. | Hendry, M Jim | Wassenaar, Leonard I. | Mendoza, Carl A.
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 δ¹⁸O 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 (Q ₁₀), moisture content at maximum reaction rates, and biogeochemical discrimination against consumption of ¹⁸O¹⁶O (αₖ), we determined that organic C oxidation by microbial respiration was the predominant mechanism consuming O₂ and producing CO₂ The mean αₖ 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 δ¹⁸OO₂ 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.
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