Oligocene cooling in the North American cordillera (SW Montana, USA) revealed by dual clumped isotope thermometry

Cooling during the Eocene-Oligocene Transition (EOT; 34 Ma) and a shift towards open habitats during the end of the Oligocene (∼26 Ma) are key characteristics of the paleoclimatic history of western North America. Yet, the paleo-temperature evolution during the rest of the Oligocene remains poorly constrained. Here, we present a new temperature record from the high-elevation North American Cordillera (Sage Creek Basin; SW Montana, USA) covering 34 to 27 Ma as revealed by zircon U-Pb geochronology of four volcanic tuffs. High-precision dual clumped isotope thermometry (Δ₄₇ and Δ₄₈) is used to identify calcretes unbiased by NO₂ contaminants and kinetic effects. The Δ₄₇ values of these calcretes show no major temperature change across the EOT, but instead gradual cooling of 10 ± 1 °C during the early Oligocene (32 ± 1 to 29 ± 2 Ma). Protracted cooling after, rather than abrupt temperature changes during the EOT, may explain the lack of mammalian turnover in North American fossil assemblages compared with other continents. Reconstructed water oxygen isotope compositions remain unchanged during the early Oligocene cooling, indicating no major surface uplift at this location. Furthermore, global climate reconstructions show only a minor decrease in atmospheric CO₂ concentrations at this time. The mechanisms driving this Oligocene cooling thus remain elusive, but may be related to land surface feedbacks operating in the high-elevation North American Cordillera. Given the large magnitude of the cooling that we observe in SW Montana, these mechanisms should be considered in climate model simulations and proxy reconstructions of high-elevation regions.