CAN THE SNOWMASTODON LACUSTRINE SEDIMENT BE DATED USING OPTICALLY STIMULATED LUMINESCENCE? A CASE STUDY FROM SNOWMASS, ZIEGLER RESERVOIR, COLORADO

To better understand paleoclimatic conditions and to provide a chronological framework for the Snowmastodon, CO paleontological site, we undertook a study of the glacial basin stratigraphy using Optically Stimulated Luminescence (OSL) dating. Eleven samples were collected at various depths and locations throughout Ziegler Reservoir during the June 2011 excavation, and later, an additional four samples were collected from sediment encased in mammoth tusks or from large intact blocks of sediment that were collected for other analyses. The fine-grained quartz sand (90 to 63 microns) was extracted from the samples under dark room conditions and analyzed using single aliquot regeneration techniques (SAR). Environmental dose rates were estimated and compared using elemental concentrations from ICP-MS, gamma spectrometry, and neutron activation.

Samples were taken in a vertical profile from several sites and in a cross-section of the lake for Unit 10. OSL ages range from 69.5 ± 4.2 ka for the uppermost layer sampled , to 148 ± 7.69 ka for the lowermost unit sampled. Unit 10, nicknamed as "the yellow brick road" was sampled from three locations and produced OSL ages of 91.8 ± 4.79, 100 ± 6.72, and 105 ± 5.82 ka for the middle, south, and north sections of the basin. These ages are stratigraphically consistent and support the hypothesis that the creation of the damning moraine is a result of Bull Lake age glaciation.

OSL characteristics for the quartz displayed excellent decay curves with little to no slow component and no evidence for thermal transfer, suggesting a source of mature sedimentary material, possibly eolian in origin. Although the OSL ages are old (>100 ka in many cases), there is no compromise in the signal due to saturation of the luminescence trap sites within the mineral grains. When the equivalent doses are plotted in statistical programs they show very little scatter, indicative of eolian origin for the sand-sized grains. Where scatter is apparent the simplest explanation seems to be that there may be more of an alluvial component than an eolian component. Ages within the stratigraphy are often closely spaced and within error, suggesting that the sediment accumulated in the basin on timescales of 10's of ka thus contributing to the excellent preservation of the paleontology.