DO PLAYA SEDIMENTS TRACK PALEOENVIRONMENT?: SEDIMENTOLOGICAL ANALYSES OF QUATERNARY LAKE CORES FROM PILOT VALLEY, NV

This project aims to determine whether regional climate changes in semi-arid playa settings are faithfully preserved in playa sediments or if long-term climate signals are overprinted by seasonal wet/dry cycles. Pilot Valley, NV is a remnant playa from paleo-Lake Bonneville and records sediments from the lake's transgressions and regressions over the last 32,000 years. A 70 cm sediment core was manually extracted from the modern playa, and analyzed through X-ray diffraction for bulk mineralogy, SEM for microfossil identification, and standard sedimentological methods to determine grain size. Preliminary analysis of bulk sediments indicates three main mineral constituents throughout the core: quartz, halite, and calcite. These results are expected in an evaporative playa setting, however X-ray diffraction analysis indicates that some halite diffraction peaks are shifted towards lower 2-theta values, resembling the pattern for sylvite (potassium chloride). Within the first 10 cm of the core, there seems to be a shift from potassium-rich salts at the top to more sodium-rich salts at depth, which may indicate fractionation of the halides with increased evaporation towards the top of the core. Sylvite is more soluble than halite and therefore should be found stratigraphically above halite deposits. Thus, even within the first few centimeters, we see an environmental signal in the mineralogy due to evaporation and desiccation of the playa. It is unclear at this time whether this signal is primary (and therefore faithfully recording the long-term lake desiccation) or secondary (recording an annual overprint as spring runoff evaporates on the playa). Comparison of bulk mineralogical changes downcore with geochemical signatures of modern surface- and ground-waters will determine if the sedimentological trends can be caused by seasonal hydrologic variations (e.g., evaporation of modern waters), or if the sediments do indeed record a primary signal of long-term lake desiccation.