Isotopic records of climate seasonality in equid teeth

Abstract We investigate how oxygen isotopes in equid teeth can be used as a record of seasonality. First, we use in situ laser ablation and conventional microsampling techniques to understand time-averaging of environmental signals in intra-tooth isotope profiles in modern feral horse teeth (n = 5) from Mongolia, which today is characterized by a large seasonal oxygen isotopic gradient. We demonstrate that laser ablation can be used to sample inner, middle, and outer enamel layers in large mammalian herbivore teeth. The inner enamel layer records less attenuated isotopic signals than other layers, as predicted by the mineralization patterns, but intra-tooth signal amplitude is similar for laser and conventional sampling methods. Second, we use modern zebra teeth (n = 21) collected in eastern Africa to evaluate how intra-tooth oxygen isotope variation relates to rainfall. We show that intra-tooth isotope range increases with intra-annual precipitation isotopic range, which in turn relates to aridity in the equatorial bimodal rainfall region but is influenced by other hydroclimate processes in the region as a whole. Finally, we address past isotopic seasonality using oxygen isotopes in fossil equid teeth (n = 11) from the Homa Peninsula, Kenya and previously published isotope data from fossil equid teeth (n = 5) from northern Tanzania. We find variable isotopic seasonality, similar to modern climates in eastern Africa, consistent with the notion that hominins and other mammals were able to accomodate environmental variability on intra-annual scales in addition to well-documented orbital cycles.