Warm High-Elevation Mid-Latitudes During the Miocene Climatic Optimum: Paleosol Clumped Isotope Temperatures From the Northern Rocky Mountains, USA

Interrupting a long-term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17-15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO(2) levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long-term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Delta(47)) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7-21.5 Ma) to Barstovian (16.9-14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Delta(47)-temperatures ranging from 17 degrees C to 24 degrees C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 +/- 2 degrees C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Delta(47)-temperatures (8 degrees C-10 degrees C) concomitantly with elevated Delta(47)-temperatures (ca. 22 degrees C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Delta(47)-temperature record, combined with carbon isotope (delta C-13) and reconstructed soil water oxygen isotope (delta O-18(sw)) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available records from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.