A diffusion-adsorption model of uranium uptake by archaeological bone

An argument for the possibility of uranium uptake by buried bone taking place through the adsorption of uranyl species on bone mineral is advanced. In the light of this a diffusion-adsorption model for uranium uptake by buried bone is developed, the necessary constants are evaluated from the literature and from laboratory measurements of the partition coefficient between solution and bone mineral. The geochemical and hydrological parameters which control uptake are discussed. The predictions of the model are shown to be in general accordance with the timescale, magnitude, and distribution of uranium uptake in archaeological bone. Using the model, specific predictions of the variation of apparent uranium-series ages in bone can be made, and bone is shown clearly not to conform to the closed system assumption. When the model is extended to tooth enamel it is found to be incompatible with the early uptake model used for ESR dating, but to fall between the early uptake and linear uptake models. Similarly, it suggests that uranium-series dates on enamel assuming a closed system are liable to underestimate the true age by at least one-third.