A Paleoclimatic and Paleohydrologic Reconstruction of Pleistocene Fossil Lake, Oregon

Abstract

Fossil Lake, Oregon, is a Pleistocene lacustrine basin (~ 650-13 ka) in the northwestern part of the Great Basin best known for its abundant and diverse vertebrate assemblage. Multi-proxy studies using lithostratigraphy, fossil ostracode faunal assemblages, and ostracode stable isotope geochemistry from cores taken at Fossil Lake record changes in paleoenvironment, paleoclimate, and paleohydrochemistry. From lithostratigraphic analysis, the depositional sequence was subdivided into eight lithosomes composed of fining-upward sequences, bounded by unconformities, indicating that the lake underwent several lake-level excursions. The two oldest lithosomes, ~ 646-610 ka, record deep lake environments deposited during wet conditions and correspond to marine oxygen isotope stages (MIS) 16 and 15. A major unconformity from ~ 610 ka until ~ 71 ka interrupts the record. Lithosomes III and IV, ~ 71-47 ka, were deposited during wet conditions that produced cool to cold, deep, alkaline lakes that were fresh to slightly saline and corresponds to MIS 4 and 3, respectively. Lithosome IV also records a short period of drier conditions with decreased lake level and increased methanogenesis rates that produced highly enriched &delta13C values in ostracodes. Lithosome V, VI, and VII, ~ 47-28 ka, were deposited during dry conditions that produced cold, shallow, alkaline lakes and correspond to MIS 3. Salinities ranged from saline to slightly saline in Lithosomes V and VI to relatively fresh in Lithosome VII. Wet conditions return abruptly in Lithosome VIII (~ 15 ka) that records a deep, cold lake environment and corresponds to MIS 2. The repetitive cycles of flooding, lake stand, and desiccation indicates that Fossil Lake was highly susceptible to changes in precipitation and evaporation ratios, suggesting that climate forcing played a major role in the lake-level fluctuations. Over all, high and very high stands coincide with glacial cycles in MIS 16, 4, and 2.