The spatial distribution of biogenic silica (BSi) in the surficial sediments of Lake Michigan is described from Shipek grab samples collected in 1975 and gravity cores obtained in 1983. Significantly smaller surficial BSi concentrations in the 1975 samples were attributed primarily to the inability of a Shipek grab to collect intact surficial sediment samples. Lower concentrations o.f BSi were found in nondepositional and transitional areas of sediment accumulation than in depositional basins. Therefore, BSi accumulation is restricted primarily to the 40% of the lake bottom where sediments are presently accumulating. High concentrations found in Green Bay surficial sediments are related to high productivity in the bay coupled with inputs of new silica from the Fox River. In the open lake, BSi concentrations of surficial sediments in depositional basins appeared to vary inversely with sedimentation rate in that lower BSi concentrations were found in areas with higher sedimentation rates.

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