LATE QUATERNARY HISTORY OF LAKE MANIX: ENVIRONMENTAL RECONSTRUCTION USING LUMINESCENCE DATING ON SELECTED CORE INTERVALS

Lake Manix was the upstream precursor to Lake Mojave, and was located in a basin between present day Barstow and Baker, California. Detailed mapping and stratigraphic tracing of the best Lake Manix outcrops have been completed in order to reconstruct basin configuration and relate them to records obtained from a continuous 45 m long core. This core was taken from the margin of the Manix subbasin and recovery of the thick, partially dated sequence of Lake Manix muds was excellent. A unique stratigraphic marker exists in a tephra layer which is found around 19 m depth. This tephra is believed to have been deposited around 185 ka (K/Ar), but more precise dating is being pursued. In addition to the mapping and stratigraphic control, selected core samples are being studied for particle size, TIC and TC, ostracode distribution and assemblages, pollen abundance and type, and stable isotope ratios. Where possible and appropriate, the core is also being dated using a wide variety of geochronological tools. U-series, magnetic secular variation fluctuations, radiocarbon, amino acid racemization and optically stimulated luminescence (OSL) are being employed with varying degrees of success. Of particular concern is the preliminary OSL data on the quartz sand-sized grains. The OSL dating, when taken in the context of other ages obtained from radiocarbon and U-series, is returning underestimates. Since the OSL dating is being carried out on the upper core in selected samples from 4 m to just above the tephra layer, it is not supposed that the age of the core samples is contributing to the problem. Dose recovery tests are preliminary and are not complete for all the samples, but at the moment, do not appear to be contributing to the obvious equivalent dose underestimation. Tests for preheat treatments, a difference in sensitivity correction between natural and regenerative cycles (Tribolo and Stokes, 2006), comparisons with IRSL, replicate HF etching procedure and subsequent checks for purity of the quartz, thermal stability tests, use of a “double-SAR” (Stokes, et al., 2003) procedure and a modified SAR protocol (Roberts, 2006) will be presented. Dosimetry calculations and measurements will also be examined, along with an evaluation of a linearly-modulated OSL signal to establish the presence of a ‘fast component'.