A PALEOCLIMATIC AND PALEOHYDROLOGIC RECONSTRUCTION OF PLEISTOCENE FOSSIL LAKE, OREGON USING OSTRACODE FAUNAL ASSEMBLAGES AND STABLE ISOTOPE GEOCHEMISTRY

Fossil Lake in south-central Oregon is a Pleistocene lacustrine basin (~ 650 to 13 ka) in the northwestern part of the Great Basin. The basin lithostratigraphy is subdivided into nine lithosomes composed of fining-upward sequences, bounded by unconformities or paraconformities. Based on tephrochonology and radiocarbon dates of vertebrate fossils, the two oldest lithosomes, I and II, correspond to marine oxygen isotope stages (MIS) 16 and 15, while the younger lithosomes, III through VIII, correspond to MIS 4 (possibly 5) through 2. Interpretation of Fossil Lake paleoclimate and paleohydrology were made from lithostratigraphy and ostracode faunal assemblages, taphonomy, and stable isotope analyses. Ostracodes were picked from 1 g samples from cores extracted from the northern and southern part of the basin for Lithosomes III through VIII. Four species of ostracodes were recovered, Limnocythere ceriotuberosa, L. platyforma, Candona patzcuaro, and Fabaeformiscandona caudata. L. ceriotuberosa dominates throughout most of the record indicating an alkaline lake environment. Lithosomes III and IV were deposited during MIS 4 when wet conditions produced cool to cold, deep, alkaline lakes that were fresh to slightly saline. Lithosome IV also records a short period of drier conditions with decreased lake level and increased methanogenesis rates that is recorded by highly enriched δ¹³C values (up to 16.99 ‰) in ostracodes. Lithosome V, VI, and VII were deposited during MIS 3 when drier conditions produced cold, shallow, alkaline lakes. Salinities ranged from saline to slightly saline in Lithosomes V and VI to relatively fresh in Lithosome VII. Wet conditions return abruptly during deposition of Lithosome VIII that records a deep, cold lake environment and corresponds to MIS 2. Ostracodes were rare or absent in all samples from Lithosome VIII. 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 played a major role in the lake-level fluctuations. Overall, high and very high stands coincide with glacial cycles in MIS 4 and 2.