ECOLOGY AND CLIMATE OF THE EARLY MIOCENE SANTA CRUZ FORMATION, ARGENTINA

High atmospheric CO₂ concentrations (pCO₂>400ppm), low ice volume, and high ocean temperature during the mid-Miocene Climatic Optimum (MMCO; ~17.0-14.5 Ma) are thought to have driven the expansion of warm, wet ecosystems to high latitudes. Faunas of fossil bearing strata of the Santa Cruz Formation (SCF), southern Argentina (47-52° south latitude) record this expansion. The SCF has produced a diverse assemblage of vertebrate fossils, with species richness similar to modern lowland tropical forests. Previous faunal analysis suggested high mean annual precipitation (MAP > 1000 mm/yr) and warm mean annual temperatures (MAT > 14°C, but did not investigate ecological changes through time. Here we present new stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions of fossil enamel and bone recovered from strata spanning the initiation of the MMCO to quantify changes to precipitation and temperature.

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.