(Table, pages 50-80), Chemical analyses of sediments underlying manganese nodules in Lake Michigan

The investigation of the sediments at the bottom of a lake provides a record of past geologic and climatic events that have influenced the lake and its associated drainage basin. The most recent sediments of a lake are indicators of man's impact on the surrounding watershed and emphasize the complex nature of interaction between chemical, biological, and physical processes that affect the distribution of sediments and their associated minerals and chemical species. The conclusions of this report are based on samples and measurements obtained during an extensive research cruise conducted in Lake Michigan by the Canada Survey Ship Limnos in August, 1975. Grab samples were collected at the intersections of a 12-by-12-km Universal Transverse Mercator (UTM) grid over most of the lake bottom; a more detailed 7-by-7-km UTM grid was used in Green Bay and in the northeastern corner of the lake. Systematic chemical analysis was performed on the sediment samples over 23 elements. In Green Bay, elevated levels of a number of chemical elements in the sediments - notably arsenic, barium, manganese, and iron - suggest that a local geochemical process or source is important. One possible explanation is that ferromanganese nodules or concretions were observed in surface sediments from a number of locations in Green Bay and extreme northwestern Lake Michigan.