Paleolimnological Study of Copper Mining Around Lake Superior: Artificial Varves from Portage Lake Provide a High Resolution Record

On the Keweenaw Peninsula region of Lake Superior, $\sim0.5 Gt$ of copper mine tailings were dumped along rivers, lakes, and connecting waterways between 1850 and 1968-an era of active copper and silver mining. In the low-energy environment of Portage Lake, a part of the Keweenaw Waterway, clay particles from stamp sand discharges created artificially varved lake sediments that preserved a remarkably detailed record. Measurements of these varves, in conjunction with radiodating techniques, allow precise determinations of deposition rates, sediment, and Cu fluxes. Concentration and flux profiles produce fundamentally different patterns. Copper concentrations have remained high in sediments despite complete cessation of active tailings discharge. However, densitometer scans of X-rayed cores and flux calculations at sites closest to sluicing locations reveal a good correspondence with the historic record of stamp sand production. Sedimentation rates and Cu fluxes have declined steadily since cessation of native copper mining, although elevated Cu levels continue to circulate in the ecosystem. Comparisons of Cu profiles from sediment cores across eastern Lake Superior suggest wide-spread signatures of mining and major contribution to sediments.