Oxygen isotopes in tooth enamel: Strengthening the foundation of a foundational proxy method

Oxygen isotopes measured in the phosphate or carbonate groups of tooth enamel hydroxyapatite have been used as proxies for paleo-climate, seasonality, and migration throughout more than a half-century of research. Despite many successful applications, the fundamental relationships between enamel isotope ratios and environmental forcing factors remain relatively poorly documented and sometimes ambiguous. We revisit these relationships in the context of a new large-scale study of tooth enamel from modern humans. We reaffirm that geographic variation in the oxygen isotope ratio of environmental (drinking) water is the primary driver of variation in tooth enamel carbonate δ¹⁸O values and quantitatively predicts a large fraction (~70%) of the isotopic variance among individual USA residents. This is true regardless of whether individuals resided in a single or multiple locations during the period of tooth growth. We also highlight results from meta-analysis and new experimental work that demonstrate how inconsistencies in experimental and laboratory procedures can substantially obscure the relationship between tooth enamel and environmental δ¹⁸O. Collectively, these results suggest substantial promise for the future utility of enamel oxygen isotope data in modern and paleo- context, but emphasize that mindful sample selection and preparation are required to derive robust inferences from these data.