A transient model of vadose zone reaction rates using oxygen isotopes and carbon dioxide

The importance of identifying and quantifying subsurface geo-chemical reaction rates and processes by monitoring and modeling CO2 and O-2 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 O-2 concentrations that use delta O-18 of O-2 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 O-2 gas concentrations. The investigation was conducted on a 3-m-thick, unvegetated, fine-sand vadose zone located in northern Alberta, Canada ( 56 degrees 40'N, 111 degrees 07'W). Using the modeled molar ratio of surface fluxes for O-2 and CO2, the change in reaction rate for a temperature change of 10 degrees C (Q(10)), moisture content at maximum reaction rates, and biogeochemical discrimination against consumption of (OO)-O-18-O-16 (alpha(k)), we determined that organic C oxidation by microbial respiration was the predominant mechanism consuming O-2 and producing CO2. The mean alpha(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 O-2 concentrations and on delta O-18(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 O-2 and C budgets.