Postmortem chemical changes in buried bone: An investigation of environmental formation processes at the Roonka archaeological site, South Australia

Over the last decade, the chemical analysis of archaeological bone has emerged as a new method to address the dietary composition of past human populations. Paleodietary studies provide one means to obtain invaluable information regarding prehistoric intra- and inter-group behavioral differentiation. Unlike other prehistoric archaeological data, materials recovered from the mortuary context provide access to individual behavior. The total elemental and isotopic composition of bone record quantitative signatures of long-term individual dietary intake. These dietary signatures provide one means to make inferences about differential access to resources in prehistoric societies. Archaeological bone chemistry data can be used to establish prehistoric access to subsistence resources from different marine and terrestrial ecosystems by various individuals and territorial groups. The most reliable quantitative dietary information is derived from stable carbon and nitrogen isotope ratios in bone collagen. In contrast, in viuo elemental dietary signatures in bone mineral are obscured by postmortem chemical changes in the burial environment. Hence, accurate dietary inferences from inorganic bone apatite will depend on controls for these postmortem alterations. Schiffer's recent treatise, Formation Processes of the Archaeological Record (1987), provides an explicit theoretical framework in which to address postmortem chemical changes in buried bone. Previous attempts to elucidate the nature of chemical interactions between buried bone and associated soils have been limited by their use of l) simple diffusion models, and 2) total elemental soil composition or acid soluble extracts rather than the soluble and exchangeable ions available to solution under field conditions. Models addressing the fossilization process in bone must consider both ionic substitution and mineralization mechanisms of diagenesis. Current solubility profile techniques for the differentiation of biogenic and diagenetic chemical phases in fossil bone neglect ionic substitution. Ionic soil solution data from an arid-land South Australian archaeological site are presented to demonstrate the importance of such information to the development and testing of these predictive models. The Roonka Flat Dune provides an excellent natural laboratory to address post-mortem diagenesis in buried bone. The chemically homogenous sand dune was used a s a n Aboriginal burial ground during a major portion of the last 10,000 years. The ionically depleted soil solutions in this siliceous dune should limit both secondary mineralization and ionic substitution mechanisms of diagenesis. Thus, if bone mineral is ever expected to escape the ravages of postmortem chemical reactions this is one burial environment in which to test the null hypothesis. In order to address the various diagenetic mechanisms, elemental ratios in the dune soil solutions were compared with those in rib bone from archaeological human skeletons and modern terrestrial mammal controls. Even though significant diagenetic changes were not expected, the Roonka archaeological bone was enriched with Si, Al, Mn, Ba, Fe, S, Sr, K, and Ti and depleted in Mg relative to the modern controls. A variety of processes including physical contamination with quartz and secondary carbonates and ionic substitution from the soil solution are responsible for these postmortem chemical alterations. The concentrations of the soluble alkaline earth metals Mg, Sr, and Ba increase relative to Ca with duration of burial, whereas the alkali metal K and the less soluble Ti, Si, Fe, Al, and Mn show no significant changes with time. The mobile Mg, Sr, and Ba occur in secondary minerals and substitute for Ca in the hydroxyapatite lattice. In contrast, there is a rapid equilibration between the less soluble soil elements and the buried rib bone. It appears that the bone has been physically contaminated by sediments containing Ti, Si, Fe, Al, and Mn. Thus, diagenetic mechanisms involving secondary mineralization and ionic substitution from the soil solution are not likely for these elements. These postmortem chemical changes will provide major obstacles to paleodietary inferences from the elemental composition of archaeological bone mineral. In the absence of extensive archaeologically associated fauna1 controls with known diets, dietary inferences from elements in the inorganic apatite component of interred human bone will remain problematic.