A MODEL FOR SOIL (CO2)-C-14 AND ITS IMPLICATIONS FOR USING C-14 TO DATE PEDOGENIC CARBONATE

A diffusion-reaction model for soil (CO2)-C-14 is described that analyzes the (CO2)-C-14 distribution in soils. It shows that the C-14 content of soil CO2 is not the same as that of atmospheric CO2 and varies with depth depending on various factors. The most important factors affecting the C-14 C content of soil CO2 include the C-14 content of soil organic matter, the relative contribution of root respired CO2 to total CO2 production, soil respiration rate, atmospheric CO2 concentration and (CO2)-C-14 content, and soil properties such as temperature and moisture content etc. The C-14 content of soil CO2 not only can be a sensitive indicator of the residence time of decomposing organic matter in the soil, but also determines the C-14 content of pedogenic carbonate. Our model suggests that soil CO2 could be enriched or depleted in C-14 relative to atmospheric CO2, depending on the relative contribution of root respiration to total soil respiration and on the turnover rate of the soil organic matter contributing to the soil CO2. Therefore, the initial C-14 dates of soil carbonate could differ from the true ages of pedogenesis. The processes and factors considered by the model are a first step in determining whether the C-14 content of soil carbonate could lead to reliable dates of pedogenesis.