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.
Previous studies by a number of workers have shown that uranium is inhomogeneously distributed in excavated bones. It has been suggested that the higher concentrations of uranium found towards the outside of some bones may indicate that the uranium has been taken up by a diffusion process. This paper briefly outlines a quantitative model of uranium uptake by diffusion and chemical reaction, and the measurement of uranium distributions in a number of samples. The relationship of these distributions to environmental factors and to other diagenetic changes is explored, and comparisons made between them and the model. Finally the implications of these results for the uranium series dating of bones are considered, and the way forward in modelling uranium uptake suggested.
This paper develops a theory for describing those diagenetic changes in bone which involve its interaction with groundwater. Three main processes are considered, as examples of such changes; namely the uptake of uranium, the dissolution of bone, and the increase of crystallinity of the bone mineral (carbonate hydroxyapatite or dahllite). Here simple models of the chemistry involved are postulated (although how bone interacts with water on a molecular scale is not clearly known) in order to demonstrate the theory with explicit mechanisms and values. Greater emphasis is given to uranium uptake, since the model used is comparatively detailed, being based on the authors' previous work.
The basic assumption is that the rate-limiting process in diagenetic change is the movement of solutes to, from, or how the physical structure of the bone itself, together with the hydrology of the burial site, interact to determine how water and its solutes move into, within and from a bone during burial. This interaction can be of three kinds, defined by the site hydrology. These are termed here, diffusion, hydraulic flow and recharge. All three types may operate together, and their relative importance depends on the extent to which the pore structure of a bone has been altered by diagenesis, as well as the type of chemical change taking place. It is shown that diffusion is usually the most common and important process, but that it is possible to predict the hydrological regimes in which other mechanisms dominate. It is shown how knowledge of site hydrology (mainly the specification of soil structure and moisture variation), the physical state of the bone, and the chemistry of the diagenetic observation, suggesting this approach to be on the right lines. Qualitative predictions also result from the theory. The main value of this approach is to identify those situations where particular diagenetic changes are simplest (e.g. sites where the hydrology gives rise to a single and quantifiable hydraulic process) so that they may be decisively tested against the quantitative predictions of the theory.
Four diagenetic parameters have been chosen to represent the state of diagenesis of bone buried on archaeological sites. They are: histological preservation, protein content, crystallinity, and porosity. How these parameters are measured is described and results from populations of bones from three different sites are presented. The results show the extent and variation in the degree of change, both within a site and between sites. In particular the correlations between diagenetic parameters are examined, which give clues about the processes which cause alteration. The value of porosity determinations (both at the intercrystalline level, and at coarser levels) in revealing the degree of diagenetic change in bone, and in underlying the dynamics of the interaction between buried bone and the surrounding water is stressed. The data also indicate (but are too restricted to prove) the following: Microbiological attack is generally complete within less than 500 years; Substantial levels of protein may remain in bone after maximal micromorphological alteration; Loss of protein appears to be independent of other diagenetic change; The correlated loss of microporosity with increase of crystallinity suggests these changes may arise from the dissolution, perhaps with subsequent recrystallization, of the smallest hydroxyapatite crystallites.
Notes the rediscovery of a paved causeway crossing a stream, the only previous evidence for its existence being AS writings. There is discussion of the documentary and archaeological evidence, providing insight into AS use of the terms `ford' and `bridge'.
Last updated 08 July 2004.
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