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Show 1 Kirsten M. Menking Vassar College Dept. of Geology and Geography Box 59 Poughkeepsie, NY 12604 OWENS RIVER SYSTEM: 800,000- YEAR OWENS LAKE HISTORY In 1992, the U. S. Geological Survey cored Owens Lake in southeastern California to obtain a continuous paleoclimate record for the Sierra Nevada region. Owens Lake heads a chain of closed- basin lakes which are separated by a series of bedrock sills, which received their water primarily from Sierra Nevadan precipitation, and which overflowed during wet periods. In coring the lake, the USGS hoped to determine what combinations of runoff and evaporation must have been necessary to allow the growth of large lakes during the late Pleistocene and to compare the timing of lake highstands to growth of mountain glaciers in the Sierra Nevada and to growth of the Laurentide ice sheet. The 323- m- long core OL- 92 records the histories of cyclic glaciation of the Sierra Nevada and water- balance of the chain of lakes over the past - 800 kyrs. A variety of paleoclimatic proxies have been studied, details of which may be found in Smith and Bischoff ( 1997) and Menking et al. ( 1997) and which will be discussed briefly here. To date, the core has been investigated at two resolutions. The initial phase of study ( Smith and Bischoff, 1997) involved 3.5- m- long channel samples, representing ~ 7 kyrs each and extending the full length of the core, which were analyzed for sediment mineralogy and geochemistry, grain size, and stable isotopic compositions of lacustrine carbonate. In addition, discrete point samples were taken for tephrochronology, palynology, identification of aquatic plant and animal species, pore fluid analysis, and magnetostratigraphy. The results of these studies indicate that Owens Lake was large and overflowing during times when glaciers expanded in the Sierra Nevada, and shallow and confined to its own drainage basin at times of minimum glacial extent. During glacial episodes, Owens Lake received large volumes of glacial rock flour, identified on the basis of its mineralogic and geochemical composition ( clay- size biotite, quartz, and feldspar). In addition, diatoms, ostracodes, and fish species, as well as a lack of primary carbonate, indicate that the lake was very fresh and, therefore, overflowing. Pollen investigations indicate an increase in cold- loving juniper species and a decline in pines at these times. Conversely, as glaciers retreated, Owens Lake appears to have diminished in size, becoming sufficiently evaporatively concentrated to deposit abundant carbonate ( locally up to 30 wt. % CO3 in some samples), but not concentrated enough to deposit salts. Furthermore, the lake water favored diatom and ostracode species assemblages more suited to saline water conditions. Abundant smectite in core sediments indicates either authigenic precipitation within a more saline lake or soil clays derived from the Owens River drainage basin. Rock flour input dropped markedly, and pines replaced juniper. |