STABLE ISOTOPE COMPOSITIONS OF EARLY EOCENE CARBONATES ASSOCIATED WITH FLUVIAL SEDIMENTATION IN CENTRAL WYOMING

Characterizing extreme climate events in Earth’s history is important to understanding of how these events occurred. The Early Eocene Climatic Optimum (EECO) (53-50 Ma) is the warmest and wettest period in the western U.S.A. during the Cenozoic, and high-resolution stable isotope compositions of carbonate minerals may bring new understanding to climate characteristics of this time period. The lower Eocene Wind River Formation in the Wind River Basin, central Wyoming, is a sequence of sedimentary rocks formed in braided river depositional environment during the EECO. The rocks contain abundant authigenic groundwater carbonate cement and soil carbonate as well as recycled marine limestone from the Paleozoic and Mesozoic strata distributed in the nearby mountains. Here we study the stable isotope compositions of bulk carbonate in the Wind River Formation in order to reconstruct paleoclimate and document climate cyclicity. A previous study suggests that the authigenic carbonates in the Wind River Formation have not been diagenetically altered, and retain information of paleoclimate during deposition. Our preliminary isotopic study shows that the δ¹³C values of bulk carbonate vary from -8.3 ‰ to -1.2 ‰ and the δ¹⁸O values vary from -10.6 ‰ to -5.5 ‰. The δ¹⁸O and δ¹³C values are linearly correlated. The peak δ¹⁸O and δ¹³C values may reflect dry climate conditions in which surface water was evaporated and atmospheric CO₂ was the dominant carbon source when soil respiration was low. The trough δ¹⁸O and δ¹³C values may reflect wet climate conditions in which evaporation was low and soil respired CO₂ from C₃ plants was the dominant carbon source when soil respiration and vegetation density were high. The covariation of δ¹⁸O and δ¹³C values during the EECO could be equally explained by changing abundance of recycled marine limestone clasts through time. Climatic variations between dry and wet periods may influence the abundance of recycled limestone by enhancing or diminishing chemical weathering of recycled sediments. Future work aims at differentiating the two plausible mechanisms, and collecting high-resolution stable isotope data in order to document the cyclicity of climate variations during the EECO.