ELEVATED YOUNGER DRYAS LAKE LEVELS IN THE GREAT BASIN, WESTERN U.S.A

Climates of the Younger Dryas (YD) chronozone (11,000-10,000 ¹⁴C yr BP, i.e., 13,000-11,500 cal yr BP) are of interest to many sectors of the paleoscience community. Empirical evidence of the magnitude and timing, including seasonality, of YD climatic change in oceanic, cryospheric, and land regions is essential to constrain models of YD general circulation. Unequivocal diagnostic records of YD climates are unavailable in many regions, but in western North America endorheic subbasins of the Great Basin contain direct and rapid response records in lake-edge deposits at numerous fluviolacustrine and littoral lacustrine sites. In general, the base levels of lake-edge processes are lake surface elevations, which fluctuate in response to basin-wide water budgets. Direct and rapid response of surface elevations of Great Basin lakes due to regional and hemispheric climate perturbations on intra-annual to decadal timescales has been well documented in historic times.

Deltas and beaches near the mouths of large Great Basin streams were well situated to record YD climatic change. These deposits not only were linked directly to large climate fields, their high rates of sedimentation locally produced distinctive stratigraphic packages, many of which were exposed by later downcutting. Moreover, high rates of fresh-water runoff locally supported molluscan faunas, and minimized hard-water and residence-time effects on radiocarbon ages. In the eastern Great Basin, the tributary river base levels were 10-20 m higher than historic levels about 10,900-10,300 ¹⁴C yr BP. In the western Great Basin, the Humboldt and Carson River base levels were 20-30 m higher about 10,380 ± 80 ¹⁴C yr BP. In the southern Great Basin, the Owens River base level at Searles Lake was 60 m higher about 10,630 ± 40 ¹⁴C yr BP, suggesting that mid-YD spilling of Owens Lake may have coincided with mid-YD highstands elsewhere in the Great Basin. In the Bonneville Basin, the YD highstand pluvial hydrologic index (lake area ÷ tributary area)=0.16 (historic Great Salt Lake=0.03-0.07 and Lake Bonneville highstand=0.60). Thus, the hydrologic response of the Bonneville Basin lake to excess moisture of the YD was about 27% of the last ice age highstand and about 320% of the historic average.