STABLE CARBON ISOTOPE FRACTIONATION IN BONE COLLAGEN AND APATITE: INTRACLASS AND INTERCLASS VARIATIONS

The stable carbon isotope compositions of bone collagen and apatite are commonly employed for diet reconstruction and food web studies. Differences in biosynthetic pathways result in distinct isotopic compositions for these two materials. Members of a single species eating the same diet should exhibit a similar fractionation for apatite carbonate and collagen, although the magnitude of this fractionation can vary as diet changes, owing to dietary routing. It is likely that the range of fractionation for apatite carbonate and collagen will be greater for species within a class than for individuals of the same species. Similarly, it is hypothesized that organisms from different classes might exhibit even larger differences with respect to this fractionation, owing to differences in diet and metabolic pathways. In this study, bones from modern museum specimens of fish, reptiles, amphibians, birds and mammals were analyzed to assess intraclass and interclass variations for fractionations between collagen and apatite carbonate. Bone apatite carbonate was analyzed by digestion in 100% phosphoric acid. The lipid fraction was removed from most bones prior to analysis of collagen by sealed tube combustion. Intraclass variation for the fractionation between apatite carbonate and collagen was 4.5 to 7.5 per mil. However, the average fractionation observed for fish and amphibians was 3 to 5 per mil greater than that observed for mamals, reptiles and birds.