Calcrete clumped and stable isotopes reveal transient cooling and heterogeneous Eocene-Oligocene paleo-environments in SW Montana

Global cooling during the Eocene-Oligocene Transition (EOT; 33.9 Ma) drove pronounced environmental and biotic shifts across the globe. However, the paleo-climatic response on the North American continent remains debated, especially in the high-elevation Cordillera, which may have been cold and dry already before the EOT. To test the response of this high-elevation terrain to global climate forcing, we studied three sedimentary sections in SW Montana (Easter Lily, Black Butte and Lion Mountain) that span the Eocene-Oligocene boundary and contain calcretes for paleo-environmental reconstructions. Dual clumped isotope thermometry in the Easter Lily section shows cooling of ~2°C during the earliest Oligocene followed by warming to pre-EOT temperatures. This indicates that EOT cooling was only transient and that long-term temperatures during the early Oligocene were similar to the late Eocene. In addition, calcrete oxygen (δ¹⁸O) and carbon (δ¹³C) isotope ratios within the three sections do not record major changes across the EOT. Instead, large differences are observed among the studied sections in δ¹⁸O values (up to 2‰) and especially in δ¹³C values (up to 6‰). This suggests strong heterogeneity of the intermontane paleo-environments in SW Montana, with individual basins recording different temperatures and degrees of aridity. Such a topographically and climatologically complex landscape may have produced the diverse endemic mammal fauna observed in these fossil localities.