UTILIZING STABLE CARBONATE ISOTOPE DATA TO RECONSTRUCT TERRESTRIAL BASIN ENVIRONMENTS DURING THE EOCENE-OLIGOCENE TRANSITION IN THE NORTHERN ROCKY MOUNTAINS

We are still learning much about the dynamic climate fluctuations that occurred throughout Earth’s history, and when considering our future on this planet, understanding these processes on a regional scale can allow us to better predict upcoming environmental conditions and prepare for future global-scale climatic changes. Sedimentary rocks preserve detailed records of climatic and topographic evolution. This data can be extracted and used to reconstruct topographic and atmospheric changes over time or space. The objective for this project was to interpret the evolution of regional Eocene to Oligocene basin environments in Wyoming and Montana through the analysis of carbon and oxygen stable isotope data (δ¹³C and δ¹⁸O) obtained from terrestrial carbonates. The carbonate material assessed in this study consisted of paleosol (ancient soil) nodules, ichnofossils, and calciferous shells collected from late Eocene to middle Oligocene strata. After collection, the carbonate samples were microscopically assessed for signs of diagenesis. Carbonate powder was obtained from viable samples through micro drilling and crushing; these powdered samples were then analyzed for δ¹³C and δ¹⁸O isotope concentrations via stable isotope ratio mass spectrometry. δ¹³C values indicate variations in atmospheric temperature and ecological conditions, and changes in δ¹⁸O values indicate variations in basin hydrology, precipitation levels, and elevation. The comparison of the data from this study to records of changes in δ¹³C and δ¹⁸O values between basins over time and to existing deep-sea records can then be used to help distinguish between global changes in atmospheric conditions, or local changes in elevations or basin hydrology. Preliminary results from the Wind River basin in central Wyoming and from the Muddy Creek and Jefferson basins in southwestern Montana indicate a positive trend in δ¹³C values and a negative trend in δ¹⁸O values. δ¹³C values range from -10.0‰ ± 1.4‰ (VPDB) in the late Paleocene to early Eocene to -5.4‰ ± 0.5‰ (VPDB) by the early Oligocene. δ¹⁸O values range from -16.5‰ ± 2.9‰ (VSMOW) to -19.1‰ ± 0.1‰ (VSMOW) over this time interval. These new data are consistent with the cooling and aridification trends that previous researchers documented in the Sage Creek Basin in southwestern Montana.