A COMPARISON OF 14 C- AND OSL-BASED LAKE-LEVEL RECONSTRUCTIONS FOR THE PAST ~1200 YEARS DERIVED FROM BEACH RIDGES AT BAILEY'S HARBOR, WISCONSIN, LAKE MICHIGAN

The upper Great Lakes assumed their modern hydrologic configuration approximately 2000-1200 years ago when glacial isostatic adjustment (GIA) caused the separation of Lake Superior from Lake Michigan-Huron. The development of reliable records of lake-level variability during the past ~2000 years is critical, for each of the Laurentian Great Lakes, to understand the natural variability of this human-altered system and for effective water management planning in the 21st century. Beach ridges preserved in coastal embayments of the Great Lakes provide a geologic record of lake-level variability and GIA that can be used to extend historical instrumental water-level records. Thompson and Baedke created a ~4500-year record of lake-level in Lake Michigan using 14C-based age models of strandplain development derived from ages for basal peats in associated swales. However, the possible effects of the changing configuration of the basin were not yet considered.

Recent work has shown that optically stimulated luminescence (OSL) dating of quartz separates collected directly from the foreshore facies of individual ridges can improve the accuracy of strandplain chronologies. Questions remain as to whether OSL can provide the accuracy and precision necessary to generate a decadal-scale record of beach-ridge formation necessary for the past ~2000 years. This contribution compares strandplain chronologies, resultant age models, and lake-level reconstructions for the Bailey's Harbor strandplain, Lake Michigan, generated from OSL and existing 14C data used by Thompson and Baedke (1997). Bailey's Harbor preserves ~30 individual beach ridges formed during the past ~1200 years, as determined through previous 14C studies. Five discrete OSL ages provide decadal-scale resolution for the ~30 ridges preserved at Bailey's Harbor. In contrast, 14C ages for ten swales exhibit multiple age ranges, decadal-scale variability, and age reversals, highlighting issues of provenance and variability in the atmospheric production of 14C versus the decay rate of this radioisotope during the past ~1000 years. The results indicate that OSL provides the necessary resolution to reconstruct water-level change during the past ~2000 years, though there is still a need to evaluate differences in local rates of GIA that are produced by geologic, GPS, and tide-gauge approaches of measurement.