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Paper No. 121-6
Presentation Time: 9:00 AM-6:30 PM
EVALUATION OF OPTICALLY STIMULATED LUMINESCENCE PROPERTIES OF MASSIVE GLACIAL OUTWASH RIDGE DEPOSITS
Optically Stimulated Luminescence (OSL) is an established geologic dating technique, however, solar resetting conditions within glacial outwash deposits can be extremely variable. As a preliminary phase of a larger project on the deglaciation of the Central Michigan highlands, the OSL properties of massive outwash ridge deposits needed to be evaluated. The goal of this preliminary study was to determine which sand grain sizes would produce the optimum OSL dating results. Two sediment samples, taken from different locations within a glacial outwash ridge, were divided into four grain size fractions from each sample: very fine sand (90-150 microns), fine sand (150-250 microns), medium sand (250-355 microns), and coarse sand (355-425 microns). OSL equivalent dose data for each grain size fraction were collected using single aliquot regenerative (SAR) dose procedures from approximately 96 aliquots each. Kernel density estimation was used to examine and compare equivalent dose distributions for all data sets and to evaluate solar resetting. Additionally, quantitative parameters such as distribution asymmetry, data dispersion, and dose recovery fidelity were calculated and compared among the data sets. We initially hypothesized that the smaller grain sizes would be better reset, as they would have travelled higher in the water column during transport of the outwash, thereby, having a higher probability of solar exposure. The results were counter to this hypothesis. Although all grain size fractions of the outwash were partially reset, larger grain sizes exhibited lower summative equivalent dose data, suggesting that the larger grain size fraction may have been more completely reset. Based on this interpretation, and given consideration of the other calculated parameters, it was determined that the medium sand fraction (250-355 microns) would be most likely to produce the optimum OSL results for large outwash features like those in our project area.