PALEOECOLOGICAL RECONSTRUCTION OF THE CENOZOIC GLOBAL CLIMATE OPTIMUM USING PALEOSOL PROXIES FROM THE GREEN RIVER BASIN (SOUTH PASS, WYOMING)

Examining the margins of fluvial/lacustrine basins provides important information on terrestrial paleoclimatic, paleoecological, and paleoenvironmental factors and how they control and respond to global changes and spatio-temporal heterogeneity. Records of these dynamics are especially necessary at times of major global change, like the initiation and termination of the extreme warm/wet/high-CO₂ conditions of the Cenozoic Global Climate Optimum (CGCO), because they can serve as a basic model for understanding ecological responses to climatic events. The formation of the Green River Basin (southwestern Wyoming) has been studied for decades, but the climatic, environmental, and ecological dynamics of its evolution has been largely overlooked.

The margins of the Green River system during the CGCO are an extremely important location for understanding ecological composition and potential climatic drivers of North American floral diversification, because this area is widely considered the point of origin for many modern grass clades. To address these issues, we examined paleosols preserved in the fluvial, basin-margin Wasatch Formation preserved near South Pass, Wyoming. Field identification of the paleosols indicated a suite that includes Entisols, Inceptisols, and Alfisols, many with significant redoximorphic features and Bg horizons that indicate a ponded floodplain, and others with deep Bk horizons (>100 cm) consistent with more well-drained, but still sub-humid to humid conditions. Based on the identification of these features, paleosols could be robustly correlated and sampled across kilometer-scale transects along the basin margin.

Previous workers have also documented isotopically heavy C isotope values in the Wasatch Formation that could indicate either early C₄ photosynthesis by plants in the area or extremely water-stressed vegetation. To examine this question, pedogenic carbonates were analyzed isotopically, and samples were collected and extracted for phytoliths (taxonomically diagnostic plant silica bodies). By combining these paleobotanical proxies with quantitative climatic proxies (mean annual temperature and precipitation) based on whole rock geochemistry, we will present an integrated vegetation-climate history of the CGCO at the margins of the Green River Basin.