NO MAJOR CHANGES ON PALEOCLIMATE AND PALEOENVIRONMENT ACROSS THE EOCENE–OLIGOCENE TRANSITION IN THE CENTRAL ROCKY MOUNTAINS

Abstract

Earth’s climate has experienced dramatic cooling during the Eocene-Oligocene Transition (EOT) based on well-constrained marine geological records, however, a consensus on the terrestrial responses to this event has not been reached. The White River Formation in the central Rocky Mountains, western U.S.A., is a well-dated stratigraphic unit covering the EOT. The formation contains tuffaceous mudrock and sandstone that were deposited in fluvial and eolian depositional environments. Here I study oxygen isotope ratios and clumped isotope temperatures of carbonate cements and bulk organic carbon isotope ratios in order to reconstruct the paleoenvironment and paleoclimate across the EOT. These carbonate cements were formed as low-Mg calcite in equilibrium with unevaporated surface water during early diagenesis based on XRD results, low Mg/Ca and Sr/Ca ratios, and petrographic observations, thus the isotope compositions reflect near-surface climate and environmental conditions. My results show that clumped isotope temperature and organic carbon 13C values remained stable across the EOT. The carbonate 18O and calculated water 18O values both increase 0.5 ‰ after the EOT, which I interpret as a result of gradual drying during the middle Cenozoic. My results show that the paleoclimate and paleoenvironment in the central Rocky Mountains did not experience major changes across the EOT. I suggest that high topography in the central Rocky Mountains prior to the global cooling event may have buffered local responses to global cooling.