RECENT SEDIMENTATION IN LAKE-MICHIGAN

Sediment cores from 21 locations in Lake Michigan are evaluated to identify sedimentation patterns including accumulation rates, surface layer mixing, focusing, and processes responsible for discontinuous Pb-210 profiles. These patterns were identified based on two different sediment dating models. Accumulation rates vary consistently with basin topography in the north and central areas of the lake, while rates in the southern basin vary widely between sites. Mixing is apparent in all north basin sites but is seldom observed in the south basin and not in the central lake. Storm surges have influenced sediment accumulation, particularly in the south basin where sites are identified that have lost and gained sediment resulting from a storm. In particular, we have found evidence of the 1888, 1905 and, most recently, the 1975 storms. Markers for these storms were not previously reported for Lake Michigan. Storm related slumps and sand layer discontinuities were also identified. An extended period of dry and hot weather during 1913-52 may have caused periodic lower sedimentation rates in the north basin, but not in the south where sediment redistribution by storms has obliterated records. Sediment focusing is a minor process in the north basin, while in the south, the areas of highest sediment accumulation rates are also sites of highest measured focusing, indicating that redistribution is a major variable in south basin sedimentation processes.