Paper No. 6
Presentation Time: 9:55 AM


ALI, Guleed AH, Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, ZIMMERMAN, Susan H., Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, WANG, Xianfeng, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, HEMMING, Sidney R., Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 and STINE, Scott, California State University, Hayward, CA 94542,

Like all hydrographically closed water bodies of the North American Great Basin, Mono Lake in eastern California fluctuated widely through the late Quaternary in response to changes in inflow and evaporation. Mono's fluctuations are recorded both geomorphically (as stranded shorelines), and stratigraphically (as sedimentary sections exposed in the faces of wave-cut cliffs, and along the channel walls of deeply incised streams). Such exposures, up to several km in length, reveal up to 85 vertical meters of alternating glacio-fluvial and glacio-lacustrine sediments intercalated with 19-plus locally erupted volcanic ashes. This sequence of glacially- and volcanically-derived sediments deposited in a high standing, though widely fluctuating, lake constitutes the Wilson Creek Formation of Lajoie (1968, Berkeley Ph.D. thesis).

We are in the process of dating the Wilson Creek Formation based on U/Th analysis of clean, dense carbonate, and on C-14 analysis of terrestrial macrofossils. Our stratigraphic observations so far require that the global Last Glacial Maximum (~33-19 kyr), though punctuated by one substantial lake highstand, was broadly characterized by times when the lake was shrinking—namely, a lowstand of 1,975 meters sometime between ~23 and 20.6 kyr. Shortly after attaining that low level, the lake rose dramatically, reaching 2,155 meters—its highest level of Marine Isotope Stage 2. We tentatively conclude that throughout most of the period of regional (i.e. Sierra Nevada) glacier and global ice sheet growth, the Mono Basin’s climate was cold and relatively dry. By dating samples that we have already collected from critical elevations in the Mono Basin, we expect to further clarify the timing of persistently dry conditions that drove the low lake levels; and of the shift in the atmospheric circulation that forced the lake to its Marine Isotope Stage 2 highstand.