A RADIOCARBON-BASED HYDROGRAPH FOR THE LATE-PLEISTOCENE REGRESSION OF PLUVIAL LAKE FRANKLIN

Lake Franklin was a large pluvial lake in northeastern Nevada during the late Pleistocene. At its MIS-2 highstand of ~1850 m the lake inundated 1100 km² with a mean depth of 22 m and a water volume of 24 km³. Previous work (Thompson, 1992: QR 37: 1-15) constrained the timing of this highstand to 21-18.5 ka BP. Here we report an updated post-highstand chronology of lake-level changes based on extensive field reconnaissance of Lake Franklin regressive shorelines. Six new radiocarbon dates, when calibrated and combined with existing unpublished data (Lillquist, 1994: U. Utah Ph.D. dissertation), reveal a history of dramatic water level oscillations superimposed on the overall regression from the highstand. Following construction of the highstand shoreline at 1850 m, the lake dropped to 1830 m by 19.5 ka BP, with a corresponding area loss of 41%. The lake transgressed back to 1843 m by 18.2 ka BP, at which point the surface area was just 9% smaller than during the lake highstand (1000 km²). A rapid regression followed, during which the lake dropped to 1823 m, with a surface area 54% smaller than the highstand lake (507 km²) by 17.8 ka BP. By 16.8 ka BP the lake was back at the 1843 m elevation again. Thus, the lake area varied by a factor of two through the interval from 18.2 to 16.8 ka BP. After this major oscillation the lake regressed again, pausing to construct prominent shorelines at 1836 m (16.3 ka BP), 1829 m (15.8 ka BP), and 1826 m (15.5 ka BP) before transgressing once more to the 1840 m level at 15.0 ka BP. The water level fell over the next 1500 years to <1820 m (246 km², 78% reduction from the highstand), before a minor transgression to 1820 m at 13.2 ka BP. After this time the lake either desiccated completely, or became too small and shallow for formation of robust shoreline landforms. The major oscillations of Lake Franklin centered on 19.5 ka BP (1830 m), 17.8 ka BP (1823 m), and 15.5 ka BP (1826 m) represent significant climatic variability during the overall transition from pluvial to arid conditions in this region.