Sedimentation Rates and a Sediment Budget for Lake Superior

Present-day sedimentation rates were determined at 10 offshore locations, representative of the depositional basins in Lake Superior. Sedimentation rates were calculated by averaging the weight of sediment deposited above the Ambrosia pollen horizon, dated at 1890. The rates are variable ranging from a low of 25 g m −2 yr −l (0.1 mm yr −1) to a maximum value of 780 g m −2 yr −1 (2.0 mm yr −1). Sedimentation rates were related to the proximity of terrestrial source, circulation patterns and bottom topography. The highest rates were found at locations closest to the edges of the depositional basins and at the base of steep-sided troughs. The present-day sedimentation rates were proportional to the postglacial sedimentation rates at almost every locality and were half the value of the postglacial rates. A total of 6.029 million tons of fine-grained sediment is annually deposited in the lake. Erosion of the red clay bluffs of Wisconsin is the major sediment source accounting for up to 58% of the external input. Rivers contribute 30% of the external sediment load. It is calculated that the Duluth basin is the repository of 37% of the total present-day natural sediment loading to the lake.