In situ oxygen isotope analysis of fossil human teeth using a secondary ion micro-probe: a new tool for palaeoecology and archaeology

Maxime Aubert¹, Ian Williams¹, Rainer Grün¹, and Marie-Hélène Moncel²

¹ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
² Département de Préhistoire Muséum National d'Histoire Naturelle 1 Rue René Panhard, 75013 Paris, France

The oxygen isotopic composition (δ18O) in tooth enamel of freely drinking animals is closely associated to the isotopic composition of the drinking water at the time of tooth formation. While the isotopic composition of surface waters may depend on a large number of factors, in moderate climates, a strong seasonal change is observed, mainly driven by changes in temperature. As a result, compositional changes in the oxygen isotope signature in teeth give powerful insights into seasonal variability over time.

Traditionally, samples are obtained through micro-drilling. While this is adequate for the analysis of fast growing faunal teeth, any seasonal signatures in human teeth are averaged out.  Following major modifications to the RSES SHRIMP II ion microprobe, it is now possible to analyse oxygen isotopic compositions on polished sections of tooth enamel on a scale of about 30 µm, allowing the detailed analysis of human teeth with a weekly resolution. A 10 kV beam of caesium ions is focused onto the tooth, thereby sputtering oxygen ions from the enamel for real time isotopic analysis by a high resolution multiple collector mass spectrometer. Each analysis only consumes about 2 ng of enamel, with a precision of about 0.2 ‰(s.d.).

A series of experiments is currently under way to optimize measurement parameters, with a particular emphasis on the analysis of Neanderthal molars. Herbivore teeth recovered from the Neanderthal fossil sites have shown large seasonal signatures, indicating that the original oxygen isotopic compositions were relatively well preserved. So far, the apparent seasonality in a Neanderthal molar was much smaller. This may reflect a much more restricted range of diet, a much more uniform sources of drinking water during tooth formation, namely in the first five years of a Neanderthal child's life or diagenetic alteration.