EVALUATING TRENDS AND PATTERN OF GLACIAL ISOSTATIC ADJUSTMENT NEAR LAKE SUPERIOR

The motion of the ground beneath and adjacent to Lake Superior continues to be influenced by the long-gone Laurentian Ice Sheet. The rate and pattern of vertical ground movement, glacial isostatic adjustment (GIA), is related to many factors that include variations in ice thickness and duration during the oscillatory retreat of the Laurentian Ice Sheet. Many previous research projects have recorded data of, or associated with, GIA by various methods; however, considering the different time periods examined by different research projects, the accuracy and consistency of these data is unknown. Hence, these data need to be analyzed and compiled to provide one view of GIA near Lake Superior. Here we present data collected from three sources, global positioning system (GPS), lake level gauges and ancient shorelines (beach ridges). Published rates of GIA from GPS stations surrounding Lake Superior were selected from a dataset covering North America. These data were then plotted and contoured to derive a rate and pattern of GIA based upon GPS data spanning recent decades. Water level gauge data for Lake Superior was updated from 2006 and reanalyzed following methods used in the most recent International Upper Great Lakes Study. This provided a view of GIA based upon water level gauge data that extended many decades before GPS data. Comparison between ancient shorelines or strandplains of beach ridges adjacent to Lake Superior was analyzed to provide a rate and pattern of GIA over several millennia. After each source was analyzed independently, they were then compared between one another and with previously published data. A new compiled rate and pattern of GIA in Lake Superior is generated and presented here.