The Effects of Sample Treatment and Diagenesis on the Isotopic Integrity of Carbonate in Biogenic Hydroxylapatite

The study of the isotopic composition of structural carbonate in fossil vertebrate hydroxylapatite has been hampered by uncertainty concerning sample treatment and arguments about the susceptibility of this material to diagenetic alteration. Our investigation of sample treatment reveals that some methods commonly used to remove organic matter and secondary minerals induce significant isotopic offsets, particularly for oxygen isotopes in bone and dentin. Treatment with dilute sodium hypochlorite, to oxidize organic matter, followed by leaching with very dilute or acetatebuffered acetic acid, to dissolve secondary mineral contaminants, is recommended. Using this protocol, we explore the isotopic fidelity of different carbonand oxygen-bearing components from individual fossil skeletons of Holocene humans and late Pleistocene mastodons and mammoths. Enamel hydroxylapatite yields reliable results, with low within-group isotopic variability and values that match those expected from well-preserved collagen from the same individual. In contrast, isotopic values from bone and dentin hydroxylapatite are more variable within groups of samples, and many specimens do not produce isotopic values consistent with those expected from collagen or enamel from the same individual. These results suggest that bone and dentin are unreliable source materials for isotopic analysis of structural carbonate, even for relatively young, mid-Holocene to Pleistocene specimens. ? 1997 Academic Press Limited