Distribution of Gaseous ¹²CO₂, ¹³CO₂, and ¹⁴CO₂ in the Sub-Soil Unsaturated Zone of the Western US Great Plains

Abstract

Data on the depth distribution of the major atmospheric gases and the abundance of gaseous 12CO2, 13CO2, and 14CO2 in the subsoil unsaturated zone have been obtained from several sites in the western Great Plains of the United States. Sample profiles range from land surface to depths of 50m. Although each site must be considered on an individual basis, several general statements can be made regarding the profiles. 1) Diffusion of these gaseous molecules through the unsaturated zone is an important transport mechanism. 2) As predicted by diffusion theory, depth profiles of the various isotopic species of CO2 differ substantially from one another, depending on individual sources and sinks such as root respiration and oxidation of organic carbon at depth. 3) In general, post-bomb (> 100% modern) 14C activities are not observed in the deep unsaturated zone, in contrast to diffusion model predictions. 4) In spite of generally decreasing 14C activities with depth, absolute partial pressures of 14CO2 in the subsoil unsaturated zone are 1–2 orders of magnitude higher than the partial pressure of 14CO2 in the atmosphere.

A model for soil 14CO2 and its implications for using 14C to date pedogenic carbonate

“…Other assumed parameters used in our model calculation are listed in Table 2. In their study, THORSTENSON et al ( 1983) concluded that the measured 14C0, profiles cannot be readily explained with a diffusion model due to some as yet unexplained mechanisms. However, Fig.…”

“…In Fig. 4, the 14C data (reported as A 14C, percent modem carbon (pmc) = (6'4C/1000 + 1)(1 -2*(25 + 6 13C)/ lOOO)* 100) for soil CO2 and CO2 concentrations (data obtained in May) from site #6 in HAAS et al ( 1983) and THORSTENSON et al ( 1983) are plotted (different symbols represent different sampling dates) and compared to our model calculation (solid line). Since there were no "Co.m.…”

“…Moreover, the 614C value of organic matter changes with depth and time ( GOH et al, 1976;O'BRIEN and STOUT, 1978;TRUMBORE, 1993)) suggesting that the COZ produced at different depths should have different 14C contents depending on the 14C content of soil organic matter at that depth and also on the relative contribution of root respiration to the total CO1 production. HAAS et al ( 1983) and THORSTENSON et al (1983) observed lower 14C contents in respired CO2 during winter time due to oxidation of older soil organic matter in the absence of significant root respiration. However, their model failed to describe the "C02depth profile observed in their study.…”

“…The diffusion mechanism applies to its isotopic species '%JOz, 13C0,, and "COr as well THORSTENSON et al, 1983;CER-LING, 1984, 1991CERLING~~ al., 1989;QUADE et al, 1989). Different isotopic species of CO2 react and diffuse independently of each other according to their own concentration gradient and their own sources and sinks THORSTENSON et al, 1983). Since the concentration of 14C02 in a soil profile is controlled by the production and decay of "C02, and by diffusion through the soil to the atmosphere, the concentration of 14COz can be described by a diffusion- …”

Multiphase Carbon-14 Transport in a Near-Field-Scale Unsaturated Column of Natural Sediments

“…This simplification ignores partitioning among solid, liquid, and gas phases in the unsaturated subsurface (vadose zone). While solid and liquid phase partitioning retard 14 C migration, gas phase partitioning is expected to increase vadose zone 14 C mobility (Thorstenson et al 1983). Thus, a more realistic prediction of vadose zone 14 C migration would include multiphase partitioning and the effect of geochemistry on partitioning and transport.…”