Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum

As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar / 39Ar ages from the middle and top of the Blue Rim escarpment constrain the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr−1) and temperature (10.4 to 12.0 ∘C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section and represented a highly preserved event later in time. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical-like forest (i.e., paratropical) throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene Climatic Optimum. The reconstructed atmospheric δ13C value (−5.3 ‰ to −5.8 ‰) closely matches the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.