How Teeth Record and Attenuate Seasonal Signals

Abstract

Variability of oxygen isotope ratios in environmental water is recorded in mammalian tooth enamel, which provides a record of seasonal change, dietary variability, and animal mobility. Physiological processes dampen this variability, however, as oxygen passes from environmental sources into the bloodstream and into forming teeth. Here, we showcase two new methods of high resolution, 2-dimensional enamel sampling, and conduct modeling to report the extent to which, and how, environmental oxygen isotope variability is reduced in animal bodies and teeth. First, using two modern experimental sheep, we introduce a new sampling method, die-saw dicing, that provides high-resolution physical samples (n = 109 and 111 sample locations per tooth) for use in conventional stable isotope and other molecular measurement protocols. Second, we use a Sensitive High-Resolution Ion Microprobe to sample oxygen isotopes from the near-innermost and innermost enamel in an experimental sheep (n = 156 measurements), and from the innermost enamel augmented by spatial grid sampling in a Pleistocene orangutan (n = 177 measurements). The experimental data show that compared to environmental drinking water, oxygen isotope variability in blood is reduced to 70–90%. Inner and innermost enamel retain between 36–48% of the likely drinking water stable isotope range, but this recovery declines to 28–34% in outer enamel. 2D isotope sampling across sectioned teeth suggests that declines in isotopic variability, and shifted isotopic oscillations throughout enamel, result from the angle of secretory hydroxyapatite deposition and its overprinting by maturation. We find that this overprinting occurs at all locations including innermost enamel, though it is greatest in outer enamel. These findings confirm that while sampling inner or innermost enamel preserves more environmental variability than other regions, even the smallest sampled increments integrate environmental signals over time. We further show how the resolution of stable isotope sampling — not only spatial resolution within teeth, but also temporal resolution of water in the environment — impacts our measurement and estimate of how much variation teeth recover from the environment. We suggest that researchers use inverse methods, or multiply observed variability by standard factors determined by ecology, taxon, and sampling strategy, to reconstruct the full scale of seasonal environmental variability. We advocate high-resolution sampling when possible, and sampling strategies informed by the spatiotemporal pattern of enamel formation, to recover seasonal records from the archive of animal dentitions.

Keywords: Dicing microsampling, serial sampling, enamel biomineralization, innermost enamel, oxygen isotopes, seasonality and paleoecology, palaeoseasonality