THE LATE MIOCENE CRITICAL ZONE AT COFFEE RANCH, TX: PALEOCLIMATE, PALEOVEGETATION AND PALEOSOLS

Long-term, dynamic changes in the ecophysiology of floral biomes in the North American midcontinent have occurred from the Neogene-Recent and include the well-studied C₃-C₄ transition. While most studies of the geologic history of C₄ vegetation focus on building long-term records using the isotopic composition of pedogenic carbonates or bioapatite, relatively few investigations have considered the role of soil properties on biome structure. In this study, paleosols and their archives of climate and vegetation were analyzed from strata at Coffee Ranch in the Texas Panhandle, a Late Miocene (6.6 Ma) vertebrate fossil locality that preserves the as-yet earliest evidence of C₄ herbivory in North America. Climatic parameters including mean annual temperature, precipitation, and hydroclimatic seasonality were reconstructed using a combination of paleosol B horizon classification, micromorphologic observation, and a range of quantitative pedotransfer functions. Vegetation types were reconstructed from the stable carbon isotopic composition of pedogenic carbonates and preserved soil organic matter. Finally, a unique suite of pedotransfer functions based on the elemental chemistry of Vertisols was used to reconstruct a number of ecologically-significant soil characterization properties that are not directly measureable in paleosols. Taken together, the results suggest that a range of floodplain paleosols formed under conditions of nearly twice the modern mean annual precipitation but similar mean annual temperature. Floral biomes were likely a mixed, seasonal woody savannah ecosystem that experienced seasonal water deficit. Soils with coarser texture may have hosted a greater proportion of C₄ vegetation, effectively mimicking drier, water-stressed environments.