Variations in Sedimentation Rate and Sediment Focusing in Lake Michigan Using Radionuclide Profiles

Sediment cores from ten study sites were collected in 2010 and 2011 from Lake Michigan as part of the US-EPA funded Great Lakes Sediment Surveillance Program (GLSSP). New calibration methods were developed and existing gamma spectroscopy methods were updated for use with new well-format gamma spectrometers. Gamma spectroscopy was used to measure Pb-210, Ra-226, Cs-137, and Am-241 activities and sedimentation rates were developed using the Constant Rate of Supply, Constant Initial Concentration, and Cs-137 linear dating models for every site. The use of multiple dating models and multiple radionuclides improves the process for validation of the dating model results, and allows more accurate site by site interpretation of the radionuclide results. Pb-210 focus factors and Cs-137 focus factors were determined for each site. Sedimentation rates range from 0.011 to 0.066 g cm-2 year-1, decreasing from south to north in the lake, from south to north in the South Chippewa basin, and from north to south in the Chippewa Basin. Sedimentation rates in this study are similar to those found in earlier studies based on a comparison of 227 sites from 11 other published studies of Lake Michigan. Pb-210 focus factors range from 1.64 to 3.00, and Cs-137 focus factors range from 1.14 to 3.12. The Pb-210 focus factor is more consistent lake-wide than the Cs-137 focus factor, which is higher and more variable in the southern area of the lake, indicating focusing or importation of Cs-137 into the South Chippewa basin.