PALEOCLIMATE RECONSTRUCTION THROUGH STABLE ISOTOPE ANALYSIS OF SOIL CARBONATE NODULES FROM THE SOUTHERN WYOMING WASHAKIE BASIN

Global temperatures changed dramatically over the course of the Eocene, shifting from a peak during the Initial Eocene Thermal Maximum to much cooler levels by the close of this epoch. Extensive geochemical and paleobotanical study of the earliest part of the Eocene has created very detailed records of Earth's climate history during this period of extreme global temperatures, but less attention has been paid to developing similar climate records for the middle Eocene. To begin filling in this gap, a paleoclimate record for the middle Eocene of Wyoming was constructed from coupled carbon and oxygen isotope analysis of paleosol carbonates. The oxygen isotope composition of these carbonates can serve as a proxy for relative changes in temperature or hydrology in an area, whereas the carbon isotope composition provides information on the local flora. Carbonate nodules were collected from thirty paleosol horizons of the middle Eocene Washakie Formation in the Washakie Basin of southwestern Wyoming. Five nodules were collected from each of these horizons at a depth of 1-3 ft below the top of the horizon. Each nodule was cut in half, and replicate carbonate samples were drilled from micritic calcite at the center of the nodule. The carbon and oxygen isotope composition of the samples was measured at the University of Wyoming Light Stable Isotope Facility. By comparing isotopic differences both within and between horizons, we can describe the general local climate of the middle Eocene, and climatic trends over time. Preliminary data suggests that the climate of this area was overall cooler and drier than that of the early Eocene, but isotope results from the upper part of the section suggest that temperatures and humidity were increasing with time. Future work will include analysis of the carbon isotope composition of the organic matter present in these soil horizons to provide a supplementary record of climate change for the locality.