ENVIRONMENTAL CONDITIONS OF EOCENE LAKE GOSIUTE AND GEOCHEMICAL EVIDENCE FOR HIGH TOPOGRAPHY RUNOFF

The Rife Bed was deposited in the margins of paleolake Gosiute as part of the Eocene Green River Formation (GRF) and represents a record of environmental conditions within the lake. Present within the Rife Bed is a laterally extensive bench of stromatolites ( ~51.6 Ma) that were deposited during the Early Eocene Climatic Optimum (EECO), a period of warmth and high atmospheric carbon dioxide levels that is often used as an analog for future climate. Stratigraphically the Rife Bed stromatolite (RBS) is located near the transition from an open basin system in the Tipton Member to a closed-basin system in the Wilkins Peak Member. A sample of RBS was collected from within the Bridger Basin near Boar’s Tusk and was microdrilled for high-resolution stable isotope analysis. Overall, 24 separate layers were sampled, and 4 particularly continuous layers were chosen for lateral sampling at a half-centimeter spacing to examine depositional variability and the consistency of terrestrial archives. Using traditional stable oxygen and carbon isotopes coupled with clumped isotopes, triple oxygen isotopes, and strontium isotopes on a ~15cm thick slab of Rife Bed stromatolite, we were able to determine the temperature of the lake water, how evaporative the area was, and trace the source of water input into the lake. Statistically, analysis of the laterally sampled layers show a high degree of consistency, with low depositional variability. Clumped isotopes (Δ₄₇) reveal that the average temperature in this shallow part of the lake was around 38°C. Δ¹⁷O values for the stromatolite parent waters average -27 per meg, putting the lake in the range of modern closed-basin lakes. ⁸⁷Sr/⁸⁶Sr averages around 0.7156. This suggests that the primary source of water to the lake was likely runoff from the surrounding Precambrian-cored Laramide uplifts. δ¹⁸O_rucp from triple oxygen analysis following the method by Passey & Ji (2019), indicates that the unevaporated lake water had an isotopically negative source at -15‰. This suggests that the runoff to the lake was supplied from higher elevations of over 3000m, possibly originating as snowmelt.