NO CHANGE IN CLUMPED ISOTOPE TEMPERATURES DURING C4 GRASSLAND EXPANSION IN THE MEADE BASIN, KANSAS, DURING THE PLIOCENE (Invited Presentation)

The Meade Basin, southwest Kansas, USA, preserves terrestrial sedimentary rocks that span the Miocene to Pleistocene. Carbon isotope values from paleosol carbonate preserved in these deposits were used to infer the timing of the transition to C4-dominated grasslands, while the rich archive of small mammal fossils has provided a view of concomitant faunal changes. An outstanding question for this region, and other areas with C4-dominated grasslands, is what drove this transition, and what role that played in faunal change. To address the potential role of environmental factors such as temperature in driving C4 grassland expansion and faunal change, we generated carbonate clumped isotope values of the carbonates preserved in the Meade Basin to estimate paleotemperature change in the region. Overall, clumped isotope temperatures range between ~20°C and 35°C, consistent generally with carbonate formation during the summer. There are no significant trends in temperatures with time, but the record is characterized by high variability over small stratigraphic intervals. Meade Basin deposits have never been deeply buried, but we used optical petrography and cathodoluminescence to assess the role of early and late stage diagenesis. Results indicate a range of fabrics and multiple generations of fluids that resulted in secondary cement and altered carbonate, but after accounting for these effects, significant temperature variability remains. In addition to the role of diagenesis, we reinterpreted the sedimentology of the region and now recognize that preserved carbonates include both paleosol carbonates and palustrine carbonates that formed in shallow wetlands. This environmental distinction matters for interpretation of both paleotemperatures and carbon isotopes, and helps explain some of the variability in both temperatures and estimated oxygen isotopes of water that the carbonates formed from. These results suggest there was little role of temperature change in driving C4 grassland expansion in the region, but environmental changes that could affect the timing of carbonate formation could explain this result. Overall, these records highlight the importance of assessing diagenesis and interpreting single and clumped stable isotope records in conjunction with detailed carbonate facies determination.