FACTORS AND PROCESSES GOVERNING THE C-14 CONTENT OF CARBONATE IN DESERT SOILS

A model is presented describing the factors and processes which determ ine the measured C-14 ages of soil calcium carbonate. Pedogenic carbon ate forms in isotopic equilium with soil CO2. Carbon dioxide in soils is a mixture of CO2 derived from two biological sources: respiration b y living plant roots and respiration of microorganisms decomposing soi l humus. The relative proportion of these two CO2 Sources can greatly affect the initial C-14 content of pedogenic carbonate: the greater th e contribution of humus-derived CO2, the greater the initial C-14 age of the carbonate mineral. For any given mixture Of CO2 Sources, the st eady-state (CO2)-C-14 distribution vs. soil depth can be described by a production/diffusion model. As a soil ages, the C-14 age of soil hum us increases, as does the steady-state C-14 age of soil CO2 and the in itial C-14 age of any pedogenic carbonate which forms. The mean C-14 a ge of a complete pedogenic carbonate coating or nodule will underestim ate the true age of the soil carbonate. This discrepancy increases the older a soil becomes. Partial removal of outer (and younger) carbonat e coatings greatly improves the relationship between measured C-14 age and true age. Although the production/diffusion model qualitatively e xplains the C-14 age of pedogenic carbonate vs. soil depth in many soi ls, other factors, such as climate change, may contribute to the obser ved trends, particularily in soils older than the Holocene.