USING GEOCHRONOLOGY AND ISOTOPE ECOLOGY TO RECONSTRUCT PAST CLIMATES: A CASE STUDY FROM THE MIOCENE SANTA CRUZ FORMATION, ARGENTINA (Invited Presentation)

Reconstructing past climates frequently requires combining quasi-continuous climate proxy records (e.g., geochemical data, fossils) with stratigraphically sparse geochronology (U-Pb, ⁴⁰Ar/³⁹Ar). Convolving these two data types often requires constructing continuous models that link age and stratigraphic position (depth). Classical statistical methods of constructing age-depth models lead to unrealistically small uncertainties and do not account for real-world geologic complexities. For this reason, Bayesian age-depth models have largely superseded classical methods. Existing Bayesian models can simultaneously account for absolute age, relative age (superposition), sedimentation rates and astrochronology. More recent efforts further account for geologic complexities such as magmatic crystal populations, detrital zircon ages, magnetostratigraphy and chemostratigraphy. Here we combine U-Pb geochronology and stable isotope data (δ¹³C, δ¹⁸O) of fossil teeth from the Santa Cruz Formation (SCF), Argentina, to investigate climatic change during the Miocene Climatic Optimum (MCO).

The MCO is the most recent period of warm, wet conditions in the Cenozoic with atmospheric CO₂ concentrations similar to projections of the next decades. SCF sediments span the initiation of the MCO (~17.4 - 16.4 Ma) and offer the opportunity to investigate the response of terrestrial climates to increasing CO₂ and global temperatures in a unique, southern high-latitude site. We applied a Bayesian model (modified from “Bchron”) to create an age-depth model for SCF sediments. We propagated model ages and uncertainties onto our stable isotope data using a Monte Carlo method to create a continuous record of mean annual precipitation (MAP) and temperature (MAT) with associated errors. δ¹³C-based estimates of MAP indicate aridification at the MCO onset before rebounding as global temperatures increased. δ¹⁸O-based MAT estimates are high (at least 20°C) and reach ~25°C by 16.4 Ma, about 12 - 17°C warmer than today. Taken together, these data show a more complex manifestation of the MCO in southern Patagonia than recognized elsewhere, and point to the importance of detailed regional studies for better understanding global warming events.