Stable Isotopes of Macrofossils and Bulk Carbonates from the Late Miocene to Pleistocene Santa Rosalia Basin, Baja California Sur, Mexico

Modeling of the response to future climate change predict that northwest North America will become more arid. By studying sedimentary deposits from the late Miocene and Pliocene time periods, when mean global air temperatures were ~3 ºC warmer than today and sea-surface temperatures were 3-8 °C warmer than today, scientists can further address the potential future impact of climate change. The late Miocene to Pleistocene Santa Rosalía Basin, located along the western margin of the Gulf of California in Baja California Sur, Mexico has a complex history of sedimentation but the paleoenvironments of fluvial, marginal-marine, and marine deposits in this area are poorly understood and the late Miocene to early Pliocene climate is relatively unknown. In this study, bulk carbonates from the late Miocene Boleo Fm. and bivalve and barnacle macrofossils from the late Miocene Boleo Fm., early to middle Pliocene Tirabuzón Fm., late Pliocene Infierno Fm., and Pleistocene Santa Rosalía Fm. were collected and processed for petrography, species identification, X-ray diffraction, and stable oxygen and carbon isotope analyses. X-ray diffraction analysis and the covariation trends between δ18O and δ13C suggest that the values recorded in these deposits are original. Comparison between the δ18O and δ13C values in this study and common values for Quaternary carbonates reveals a strong freshwater signal on deposits that were previously regarded as marine. In this study, δ18O and δ13C values from the late Miocene Boleo Fm. and the Pliocene Tirabuzón and Infierno Fms. suggest a substantial freshwater source into the basin not present today. δ18O and δ13C values from the Pleistocene Santa Rosalía Fm. no longer record a freshwater influence, as the climate transitioned to the modern arid regime. This suggests that during the late Miocene and Pliocene warmer temperatures did not lead to increased aridity in this region and instead this area was likely more humid than modern times. The Santa Rosalía Basin was likely influenced by a prolonged and intensified North American monsoon resulting from increased sea-surface temperatures and opening of the Gulf of California during the late Miocene and Pliocene.