Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum
Authors: Stein Rebekah A. , Sheldon Nathan D. , Allen Sarah E. , Smith Michael E. , Dzombak Rebecca M. , Jicha Brian R. 
Published: December 2021

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, 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.