Climatic influence on the isotopic composition of bone nitrogen

Abstract

The ¹³C/¹²C isotope ratios in animal¹ and human² bone can be used as indicators of diet, more recently it was shown that the ¹⁵N/¹⁴N ratios of animals and humans are similarly determined by the food they eat^3–5. Specifically, the stable carbon isotope composition reflects the proportion of C₃ and C₄ plants at the base of the food chain^1,2, while both ¹⁵N and ¹³C reveal the difference between a marine and terrestrial diet in modern as well as archaeological contexts^5–7. Here we present data for human and animal bones from southern Africa which only partly conform to previously recognized patterns for ¹⁵N/¹⁴N ratios. Prehistoric human bones from a particular coastal region of South Africa show ¹⁵N/¹⁴N ratios consistent with the marine and terrestrial diets indicated by the ¹³C/¹²C ratios, but bones of both prehistoric humans and modern wild animals from a larger part of the subcontinent show variations in ¹⁵N/¹⁴N ratios which cannot be ascribed to known variations in diet. It appears that, in some environments, nitrogen isotope studies must also take into account the possible influence of the climate.