Radioisotope Geochronology of Lake Superior Sediments

This study uses radioisotope geochronology to quantify sediment deposition rates throughout Lake Superior, and to provide time control for a study of persistent, bioaccumulative, and toxic chemical pollutants. Lake Superior is the largest freshwater lake in the world by surface area, and sediment sampling efforts are infrequent. Sediment samples were collected from nine sites throughout Lake Superior onboard the U.S. Environmental Protection Agency’s Research Vessel Lake Guardian in 2011. These cores were sub-sectioned to enable analysis of chemical pollution with depth. 264 total subsamples were freeze-dried and then analyzed using twin high-efficiency germanium crystal gamma spectrometers to determine the amount of radioactive Pb-210, Ra-226, Cs-137, and Am-241 present in each sample. These results have been converted to activity quantities for each isotope, and the activities used to determine sedimentation rates. The sedimentation rates have been obtained using unsupported Pb-210 modeled in two ways: a constant initial concentration model and a constant rate of Pb-210 supply model. Comparison of the results from these models is commonly used to determine if the sediments have been mixed, either in-situ or during collection. The sedimentation rate at each site is compared to previously collected data from nearby sites. All known sedimentation rate measurements throughout the lake are used to create a sedimentation map for the lake using ArcGIS geographic information system (GIS) software. This is the first quantitative approach to development of such a map, and enables new analysis of lake wide sedimentation processes. Finally, sedimentation rates for each core are utilized to obtain dates of deposition for each section of each core, providing time control necessary to observe effects of new pollution sources and environmental regulation in the Great Lakes region in the last 300 years.