Thermodynamic evaporation and freshwater mixing models to test salinity proxies for late Pleistocene lake levels, Mono Lake, California

ABSTRACT Times of higher paleolake levels in Mono Lake basin correspond to higher abundances of authigenic minerals such as calcite and Mg-smectite in the Wilson Creek Formation, the lake sediments exposed around the modern lake that represent the persistent wetter conditions of the last glacial cycle. It has been suggested that precipitation of these minerals in Mono Lake is controlled by the flux of water (surface and ground), which replenishes Ca2+ and Mg2+ ions in the lake. This water is subsequently depleted due to the high rates of evaporation in the Mono Basin, resulting in precipitation of calcite and Mg-smectite mineral phases. Thermodynamic evaporation models starting with Sierra Nevada spring water can simulate the chemical composition of Mono Lake remarkably well. These models do not, however, consider the mixing of freshwaters in the lake that is hypothesized to result in precipitation of calcite and Mg-smectite. Here, we present the results of our empirical evaporation and mixing (E&M) model using simple thermodynamic approaches. Although this model is highly simplified, it provides a valuable test of the hypothesis.

14C Ages of Ostracodes from Pleistocene Lake Sediments of the Western Great Basin, Usa—Results of Progressive Acid Leaching

Progressive dissolution experiments were performed on samples of ostracode shells from lacustrine sediments from the western Great Basin to remove contamination of the surface by secondary calcite. The observed age differences between the external and residual fractions were as great as 2000 to 6000 yr. A “plateau” in ages of the last fractions was obtained only for 1 sample; however, results of repeated experiments resulted in very good agreement of the final ages. A comparison with previously published chronologies based on bulk radiocarbon ages of ostracodes from Wilson Creek (Benson et al. 1990) shows that leaching is imperative for dating samples older than 20 ka B P. This study focuses on the problem of contamination and its removal. However, the final chronology of the Wilson Creek Formation (and other late Pleistocene lacustrine sediments) will require additional dating of other sections as well as establishment of a reservoir effect correction.

Correlation of Late-Pleistocene Lake-Level Oscillations in Mono Lake, California, with North Atlantic Climate Events

Oxygen-18 (18O) values of sediment from the Wilson Creek Formation, Mono Basin, California, indicate three scales of temporal variation (Dansgaard–Oeschger, Heinrich, and Milankovitch) in the hydrologic balance of Mono Lake between 35,400 and 12,900 14C yr B.P. During this interval, Mono Lake experienced four lowstands each lasting from 1000 to 2000 yr. The youngest lowstand, which occurred between 15,500 and 14,000 14C yr B.P., was nearly synchronous with a desiccation of Owens Lake, California. Paleomagnetic secular variation (PSV) data indicate that three of four persistent lowstands occurred at the same times as Heinrich events H1, H2, and H4.18O data indicate the two highest lake levels occurred ∼18,000 and ∼13,100 14C yr B.P., corresponding to passages of the mean position of the polar jet stream over the Mono Basin. Extremely low values of total inorganic carbon between 26,000 and 14,000 14C yr B.P. indicate glacial activity, corresponding to a time when summer insolation was much reduced.