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

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