29 Apr 2021

29 Apr 2021

Review status: this preprint is currently under review for the journal CP.

Climate & Ecology in the Rocky Mountain Interior After the Early Eocene Climatic Optimum

Rebekah A. Stein1,2, Nathan D. Sheldon1, Sarah E. Allen3, Michael E. Smith4, Rebecca M. Dzombak1, and Brian R. Jicha5 Rebekah A. Stein et al.
  • 1Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, 48109, USA
  • 2Department of Earth & Planetary Sciences, University of California, Berkeley, CA, 94720, USA
  • 3Department of Biology, Penn State Altoona, Altoona, PA, 16601
  • 4School of Earth & Sustainability, Northern Arizona University, Flagstaff, AZ, 86011, USA
  • 5Department of Geosciences, University of Wisconsin-Madison, Madison, WI 53706

Abstract. As increasing atmospheric carbon dioxide (CO2) and temperatures accompany 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 to today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwest 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. The Blue Rim escarpment exposes approximately 100 meters of the lower Bridger Formation, which includes plant and mammal fossils, paleosols 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 age of its strata to ~49.5–48.5 Ma ago, during the “falling limb” of the early Eocene climatic optimum. Using several geochemical tools, we reconstructed 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 at the time, including temperature and precipitation (from paleosols) and the isotopic composition of CO2 from plants found in the floral assemblages, before comparing them to reconstructions for the same time 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. 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 forest 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 both the independently reconstructed value from marine microfossils (−5.4 ‰), as well as the isotopic composition of the mantle (−5.4 ‰), suggesting that the warm conditions were maintained by volcanic outgassing.

Rebekah A. Stein et al.

Status: open (until 24 Jun 2021)

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Rebekah A. Stein et al.

Data sets

Blue Rim escarpment geochemical data Rebekah Stein

Rebekah A. Stein et al.


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Short summary
Modern climate change drives us to look to the past to understand how well prior life adapted to warm periods. During the early Eocene, a warm period approximately 50 million years ago, southwest Wyoming was covered by a giant lake. This lake and surrounding environments made for excellent preservation of ancient soils, plant fossils, and more. Using geochemical tools and plant fossils, we determine the region was a warm, wet forest and that elevated temperatures were maintained by volcanoes.