LATE EOCENE-EARLY OLIGOCENE PALEOCLIMATE AND PALEOENVIRONMENT RECORDS FROM THE WHITE RIVER FORMATION IN EASTERN WYOMING

Earth’s climate underwent a dramatic shift from its “greenhouse” state to its “icehouse” state during the late Eocene to early Oligocene. This cooling event is relatively well constrained and studied in marine geological records, but a consensus on the terrestrial responses of this climate transition has not been reached. The White River Formation near Douglas, WY is a well-dated stratigraphic section covering the late Eocene-Oligocene transition (EOT). The formation consists predominantly of interbedded tuffaceous mudstones, siltstones, and sandstones that were deposited in fluvial and eolian environments. We studied the oxygen and carbon isotope compositions of the carbonate cement and bulk organic carbon isotope compositions in order to reconstruct the paleoenvironment and paleoclimate during the EOT. XRD results of selected carbonate samples show that the cement carbonates are low-magnesium calcite. Observations under polarizing and cathodoluminescence microscopes show that these carbonates have high intergranular volumes, suggesting the carbonate were formed during eodiagenesis before compaction. Microscopic observations also show that the carbonates are mostly micritic with minor amounts of spars of different luminescence. These carbonates were formed in vadose and phreatic zones near the surface and record primary climate and environmental information. Mg/Ca and Sr/Ca values of the carbonates remain low across the EOT suggesting the carbonates were formed in fresh waters without the influence of evaporation. Clumped isotope results of five selected samples show the carbonate formation temperature varies between 25 and 31 °C, and drop slightly, at most ~3 °C, across the EOT. The calculated water d¹⁸O values do not change across the EOT suggesting no obvious change in humidity or aridity. Data presented in this study improve understanding to the response of terrestrial paleoenvironment and paleoclimate during the EOT in western North America.