Reconstructing Oxygen Isotope Seasonality in Large Herbivores Through Mineralization Modeling, Experimentation and Optimization

Abstract

The seasonality of climate shapes behavior, adaptation and evolution, and figures in environmental theories of human origins. Because blood and tooth oxygen isotope (δ18O) values reflect landscape hydrology, and because teeth mineralize incrementally, tooth δ18O values preserve information about past seasonality. However, efforts to reconstruct seasonal patterns from teeth are constrained by uncertainty in the relationship between environmental and blood δ18O, and in the nature of tooth mineralization. This dissertation addresses these uncertainties and builds tools that can reconstruct past seasonality from isotopes in teeth, using sheep as representatives of large herbivores common in fossil assemblages. First, I characterize molar mineralization in a population of Dorset sheep using synchrotron x-ray density mapping. I employ Markov Chain Monte Carlo (MCMC) sampling to transform variation in mineralization timing and magnitude from the entire population into a dynamic model. Teeth mineralize primarily in two stages, each distinct in morphology and timing, and mineralization slows towards the end of formation. Next, I test models that link environmental and blood oxygen by raising a population of Dorset sheep, and by providing them with Massachusetts (δ18O enriched) and Montana water (δ18O depleted). Blood rapidly tracks environmental water, recovering discrete precipitation events, and is sensitive to animal evaporative water loss. Under controlled conditions, individual and population blood δ18O variation exceeds rain δ18O variation at some tropical sites relevant to human evolution. Lastly, I produce a method for reconstructing seasonal drinking water δ18O from tooth δ18O patterns. To do this I test the mineralization and blood models developed here by finely sampling δ18O from the molars of my experimental sheep. These tests broadly confirm mineralization patterns, but show mineralization appears to include resetting of hydroxyapatite (HAp) constituents. Seasonal drinking water δ18O histories are reconstructed from tooth δ18O values through iterative, computational techniques that draw upon mineralization and blood physiology models. I find that conventional serial sampling without modeling fails to reflect the timing and magnitude of drinking water δ18O seasonality. By contrast, approaches combining mineralization, blood models and optimization accurately reconstruct seasonality. Simulations show that higher resolution sampling is more important for seasonality reconstruction in the tropics. This work makes the reconstruction of seasonal climates relevant to human evolution more feasible, and will help elucidate the environmental context of our own origins.