Postmortem preservation and alteration of in vivo bone collagen isotope ratios in relation to palaeodietary reconstruction

Abstract

Since its introduction in 1977¹, stable isotope analysis of bone collagen has been widely used to reconstruct aspects of prehistoric human and animal diets^2–11. This method of dietary analysis is based on two well-established observations, and on an assumption that has never been tested. The first observation is that bone collagen ¹³C/¹²C and ¹⁵N/¹⁴N ratios reflect the corresponding isotope ratio of an animal's diet^1–5,12. The second is that groups of foods have characteristically different ¹³C/¹²C and/or ¹⁵N/¹⁴N ratios^13,14. Taken together, the two observations indicate that the isotope ratios of collagen in the bones of a living animal reflect the amounts of these groups of foods that the animal ate. Thus, it has been possible to use fresh bone collagen ¹³C/¹²C ratios to determine the relative consumption of C₃ and C₄ plants^15–17, while ¹³C/¹²C and ¹⁵N/¹⁴N ratios have been used to distinguish between the use of marine and terrestrial foods¹⁴. The ¹⁵N/¹⁴N ratios of fresh bone collagen probably also reflect the use of leguminous and non-leguminous plants as food⁵, but this has not yet been demonstrated. Prehistoric consumption of these same groups of foods has been reconstructed from isotope ratios of collagen extracted from fossil bone^1–11. Implicit in the application of the isotopic method to prehistoric material is the assumption that bone collagen isotope ratios have not been modified by postmortem processes. Here I present the first examination of the validity of this assumption. The results show that postmortem alteration of bone collagen isotope ratios does occur, but that it is possible to identify prehistoric bones whose collagen has not undergone such alteration.