Recovering stable isotope time-series in tooth enamel: a new method

Reconstructing environmental and climatic changes on short time scales, such as seasonality, is important for understanding human evolution. However, estimating changes in seasonality in the fossil record remains difficult. Stable isotopes are non-radioactive variants of elements such as carbon or oxygen that differ by mass. Isotopes in tooth enamel represent an important tool for understanding the diets, habitats, and climates characterizing living and extinct animals. Conventional methods for sampling enamel rely on sampling the outer surface of the tooth. However, this enamel develops over a period of a few months to a few years, which obscures patterns of short-term isotope variability corresponding with, for example, seasonal changes in diet, habitat, or climate. The innermost enamel layer, an extremely thin layer about 0.01-0.02 mm across, developments more quickly. Secondary ion mass spectrometry (SIMS) is the only technique capable of sampling a spot within this layer. A rodent incisor from an animal included in a controlled drinking water switch experiment was sampled with SIMS. The record of oxygen isotope changes recovered from the innermost enamel layer corresponds closely with the pattern of switched water oxygen while the isotope profile from the enamel surface does not capture the full shift. This suggests that SIMS can be used to understand short-term isotope shifts associated with rapid changes in diet and environments in fossil animals. Future work will focus on understanding changes in seasonality characterizing the past six million years of human evolution in East Africa.