The Keweenaw Current and ice rafting: Use of satellite imagery to investigate copper-rich particle dispersal

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The immense surface area and large volume of Lake Superior causes thermal characteristics to resemble marine waters, yet the completely bounded shoreline and low flushing rate introduce unique features. Previously, shoreline inputs were considered minor, as annual river discharges account for only 0.36% of the total hydrologic volume of the lake. However, thermal bar formation and wind shear from prevailing westerlies impound warm waters along the southern coastline, creating a coastal exposure corridor with strong counterclockwise circulation known as the Keweenaw Current. Discharges from rivers and industrial sources are confined, then entrained. Here infrared AVHRR (Advanced Very High Resolution Radiometer) satellite imagery was utilized, verified by NDBC (NOAA National Data Buoy Center) buoy surface data, to document thermal features of offshore waters and the coastal zone. Five stamp mills at Freda/Redridge discharged over 45 million metric tons of stamp sands between 1895 and 1922. The coarse fraction forms beach sands that now extend 23 kilometers north from their sources and that blanket shallow-water sandy sediments. The finer fractions disperse much farther than the coarse fractions, moving along the primary track of the Keweenaw Current. SPOT and TM (Thematic Mapper) imagery were used to document how Ontonagon clays and Freda/Redridge stamp sand particles are entrained by the Keweenaw Current. The two particle types have distinctive reflective spectra. An additional transport mechanism, revealed by RADARSAT ScanSAR (Synthetic Aperature Radar) imagery, is ice rafting. Nearshore ice incorporates large amounts of coastal sands and deeper-water sediments. Spring break-up of coastal ice results in large drifting ice packs that are pushed by prevailing westerlies and currents around the tip of the Keweenaw Peninsula into the Caribou Basin.

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Journal of Great Lakes Research