Radiocarbon Dating of Soil Organic Matter

Abstract Radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the 14C content of soil organic matter. Measured 14C ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils. Differences in soil C dynamics due to climate or soil depth will result in significantly different 14C signatures of soil organic matter for soils of the same age. As a result, the deviation of the measured 14C age from the true age of soil formation could differ significantly among different soils or soil horizons. Our model calculations also suggest that 14C ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur. Once a soil or a soil horizon has reached a steady state, 14C dating of soil organic matter will provide no useful information regarding the age of the soil. However, for soils in which steady state has not been reached, it is possible to estimate the age of soil formation by modeling the measured 14C contents of soil organic matter. Radiocarbon dating of buried soils could, in general, overestimate the true age of the burial by as much as the steady-state age of the soil or soil horizon.