Carbon isotope ratios of apatite from fossil bone cannot be used to reconstruct diets of animals

Abstract

The reconstruction of animals' diets from measurements of stable isotope levels in fossils relies on the fact that the ¹³C/¹²C ratio of animal carbon reflects the ¹³C/¹²C ratio of dietary carbon^1,2. Two phases in fresh bone, collagen and the carbonate occurring in apatite, the predominant bone mineral, have isotopic ratios that are related to the ¹³C/¹²C ratio of the diet¹. The isotopic ratios of both phases have been used to study the diets of extant animals^3,4. Reconstruction of the diets of fossil animals using the isotopic method has been limited to analysis of collagen preserved in bone^5–8. It has not been possible to use the ¹³C/¹²C ratios of carbon in the inorganic phase of fossil bone for dietary reconstruction because most fossil bones contain significant amounts of calcium carbonate, deposited after the animal's death, that contribute to the CO₂ evolved from the bone during acid hydrolysis^3,4. However, Sullivan and Krueger⁹ recently presented data which led them to conclude that the ¹³C/¹²C ratio of CO₂ extracted by acid hydrolysis from the apatite phase of fossil bone records information about an animal's diet. We have now determined the ¹³C/¹²C ratios of both the apatite phase and the collagen of 24 fossil animal and human bones, and our results indicate that the ¹³C/¹²C ratio of fossil bone apatite cannot be used for dietary reconstruction.