Bones and Groundwater: Towards the Modelling of Diagenetic Processes

Abstract

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 amolecularscale 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 within the bone. Therefore the main thrust of the paper is to show 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 process can provide estimates for the rate and manner of the particular diagenetic process. Such estimates agree with 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.