WHAT IS THE RATE OF GLACIAL ISOSTATIC ADJUSTMENT BETWEEN BASINS OF THE UPPER GREAT LAKES (SUPERIOR, MICHIGAN, HURON)?

Glacial isostatic adjustment (GIA) is a difficult process to characterize because of spatial and temporal data limitations and complexities associated with this ancient process. Models of GIA attempt to describe GIA by combining a multitude of datasets, but here we examine the idiosyncrasies of commonly used datasets in the Upper Great Lakes. These include measurements from global positioning systems (GPS), water level gauges, and ancient shorelines. Comparison of similar-age ancient shorelines around a common lake basin afford the first perspective of this long-term and commonly unobserved process. Geological investigations dating back for more than a century provide a multi-millennial view of this relatively slow process since Laurentian Ice Sheet deglaciation. However, the low resolution, especially towards modern times and uncertainty interpreting elevations from various sediment or organic material, in addition to age uncertainties has made it difficult to extrapolate GIA from geologic data to the present. Instrumental records attempt to fill this gap but have not always coincided, raising concern that these relatively short records do not adequately describe the long-term GIA process. We present new detailed analyses of specific beach ridges preserved adjacent to the upper Great Lakes that can help resolve GIA within and between lake basins.