Isotopic composition of soil carbon dioxide in the Tucson basin

Abstract

Most models to correct ¹⁴C groundwater ages require the δ¹³C value of soil CO₂ which is related most of the time with the photosynthetic pathway of the vegetation cover. Samples of soil CO₂ were collected in three permanent sites in the Tucson area from July 1980 to June 1981 to determine the extent of seasonal variations and the influence of vegetation on the δ¹³C and CO₂ concentration of soil gases. The mean δ¹³C value, about -18°/₀₀, was similar to the isotopic composition of the vegetation cover. Seasonal variations were observed in both δ¹³C and CO₂ concentration. The δ¹³C variations, up to 4.5°/₀₀, seemed to be caused by exchange with atmospheric CO₂, by seasonality in plant activities and/or by changes in the δ¹³C of respired CO₂. The variations of soil CO₂ were caused by the changes in the activity of plants. Most of the CO₂ at these sites seemed to be produced by plant root respiration. Soil gas samples were also collected in the riverbeds which are the assumed recharge areas of the Tucson aquifer. Both CO₂ concentration and δ¹³C values showed changes with time and location. The mean δ¹³C for these samples was about -20°/₀₀. High concentrations of CO₂ of up to 5% were found. The source of CO₂ at these sites is most likely organic matter decomposition and plant root respiration.