TESTING LEACHABLE LI AS A PROXY FOR PALEO-SALINITY AT MONO LAKE, CA, USA

The Great Basin of the western United States is known to have experienced large and abrupt changes in wetness during the last glaciation on orbital timescales, and evidence is growing that millennial-scale changes such as D/O and Heinrich events also impacted the region. The Mono Lake basin is unique in recording variations in both lake level and valley glacier activity, which represent independent responses to seasonal and annual temperature and precipitation. We have collected continuous high-resolution sediment samples from outcrops of the Wilson Creek Formation. Our new relative paleomagnetic intensity (RPI) age model allows us to confidently correlate to North Atlantic records. In nearby Owens Lake, Bischoff et al. (1997, in Quat. Res. v48, 313-325) measured acid-leachable Li, residing in an authigenic Mg-silicate phase, and found it to be a very sensitive indicator of salinity, when compared with diatom evidence. On a ternary diagram of Ca/Na-Al-Mg, preliminary samples from Mono Lake fall on a trend between a Sierran pluton-average shale mixture and the pure MgO apex, indicating an authigenic Mg-silicate phase, shown by XRD and SEM examination to be smectite. The acid-leachable Li proxy should thus be applicable to the Mono sediments as a proxy of lake level. However, our preliminary results are not consistent with this simple hypothesis, as the leachable Li concentration is high when the physical stratigraphic evidence indicates higher lake levels. We have two alternative hypotheses for this apparent contradiction: (1) The concentration of leachable Li is a good measure of the concentration of Mg-smectite, but may be diluted by another phase, for example, glacial flour. (2) Alternatively, if the lake water was concentrated enough that Mg concentrations were drawn down (e.g., modern concentration = 41mg/L; Tomascak et al. 2003, in Geochem. Cosmochem. Acta v67, 601-611), and then the lake became fresher, there would have been renewed precipitation of Mg-smectite as the Mg concentration increased. Further work is under way to test these alternate hypotheses.