PUSHING THE LIMITS OF OSL APPLICATION TO GREAT LAKES COSTAL DEPOSITS: HOW YOUNG AND HOW SMALL?

Optically Stimulated Luminescence (OSL) dating has become highly useful to geomorphologists and sedimentologists studying a range of coastal deposits throughout the Great Lakes region. OSL dating has been used to constrain the timing of eolian activity in numerous coastal areas of the Great Lakes as well as to produce paleohydrographic data for the upper Great Lakes with better temporal confidence than radiocarbon-based chronologies. As part of ongoing studies two distinctive deposits were encountered that challenged the limits of OSL applications. In one location, a beach ridge of Lake Huron, a very young and extremely well age-constrained sample was collected providing an opportunity to evaluate what OSL methodologies could be used to date modern (< 20 years) sediments with the greatest accuracy and precision. Young samples can be difficult to date via OSL due to the short time interval for signal accumulation. Additionally, experience indicates that the problem is exacerbated for shoreline deposits of the Great Lakes because they generally have weak signal response to applied radiation and often have low environmental dose rates. At another location, sub-sampling from a light-protected sediment core taken from a dune-dammed lake in western coastal Michigan, allowed exploration of minimum sample size limits for OSL dating. In addition to the OSL results, this presentation will discuss the context that made these deposits distinctive and that allowed the questions of, “How young?” and “How small?” to be addressed. Where “pushing the limits” of OSL methodologies could lead for modern process studies, land use and climate change studies in the Great Lakes region will also be highlighted.