Paleoclimate and paleoenvironment reconstruction of the middle Eocene in the Washakie Basin, southwest Wyoming

The middle Eocene was a period of transition from the global greenhouse conditions of the early Eocene to the formation of ice sheets over Antarctica in the late Eocene. A nearly complete record of Eocene strata was preserved in the Washakie Basin in southwest Wyoming. Paleosols from these strata contained records of paleoclimate and paleoenvironment. By measuring stable carbon and oxygen isotope values and major oxide contents from paleosols in the early middle Eocene Cathedral Bluffs Tongue of the Wasatch Formation and the late middle Eocene Adobe Town Member of the Washakie Formation, climatic and environmental trends could be established and compared to other regional and global records. Nodular calcite and matrix samples were collected from beds in the Adobe Town Member, while only matrix could be collected from the Cathedral Bluffs Tongue. Major oxide contents were measured in rock samples from both localities. These measurements were used to quantify the extent of weathering in soils, and to estimate mean annual precipitation (MAP). Fossil mammal and turtle material was sampled to estimate the isotopic composition of local waters, and to assess diagenesis of paleosol carbonates. Analyses of Cathedral Bluffs Tongue samples revealed no organic matter, and only trace amounts of carbonate unusable for environmental reconstruction. Within Adobe Town Member samples, few contained enough carbonate to provide reliable estimates of environmental conditions, and there was evidence of diagenetic alteration of isotope values. Paleosols from the Cathedral Bluffs Tongue were well-drained and clayey, with deep-penetrating root traces, and showed evidence of cumulative development. No temperature estimate could be made, but MAP was estimated at 123.4 +/- 1.2 cm / yr. Paleosols from the Adobe Town Member were wetter, more variable in texture, and showed evidence of interruption of soil development likely due to proximity to a river channel. Mean annual temperature was estimated from nodular calcite to be 15.4 +/- 1.0°C, while MAP was estimated at 102.2 +/- 3.5 cm / yr. Vegetation as estimated from tooth enamel and carbonates had carbon isotope values comparable to the average for Eocene C3 plants. Climate parameters at both sites were similar to previous estimates based upon fossil leaf assemblages, and reflect trends in precipitation that agree with local and continental records.