ECOLOGY AND CLIMATE OF THE EARLY MIOCENE SANTA CRUZ FORMATION, ARGENTINA
We collected isotope compositions from ~175 individuals over ~150 m of stratigraphic section. Large herbivores – Nesodon, Adinotherium, and Astrapotherium – are best represented. Each sample was assigned an age and uncertainty based on its stratigraphic position using new isotope dilution U-Pb zircon dates of interbedded tuffs (range ~17.8-16.6 Ma) and a Bayesian age-depth model. We used a Monte Carlo approach to propagate the uncertainty in age to the isotope compositions to produce a continuous model of δ13C and δ18O values over an ~1.2 Ma interval. Modeled enamel δ13C values indicate C3 feeding with an increase from ~-13‰ to -10‰ over the interval. δ13C values increase sharply by ~2‰ from ~17.9-17.5 Ma followed by a gradual rise of ~1‰ to 16.6 Ma. Modeled δ18O gradually increases ~2‰ over the entire interval from ~23 to ~25‰. Based on δ13C of fossils (plants consumed) and the atmosphere, MAP decreased ~63% over the interval from ~1350 mm/y to ~500 mm/yr, while MAT rose from ~22° to 27°C. Isotopic zoning is low (~2‰) in several serial sampled molars, suggesting little seasonal variability in diet and drinking water composition. These data suggest aridification of the SCF and an ecological shift towards more open habitats at the onset of the MMCO. Modern atmospheric pCO2 reached MMCO concentrations in mid 2016, making studies of this interval newly relevant to both paleo and modern studies of climate and ecology.