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, approximately 0.5 G t of copper mine tailings were dumped along rivers, lakes, and connect ing 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 a rtificially varved lake sediments that preserved a remarkably detailed record. Measurements of these varves, in conjunction with radiodating techniques, allow precise determinations of deposition rates, sedimen t, and Cu fluxes. Concentration and flux profiles produce fundamentall y different patterns. Copper concentrations have remained high in sedi ments despite complete cessation of active tailings discharge. However , densitometer scans of X-rayed cores and flux calculations at sites c losest to sluicing locations reveal a good correspondence with the his toric record of stamp sand production. Sedimentation rates and Cu flux es have declined steadily since cessation of native copper mining, alt hough elevated Cu levels continue to circulate in the ecosystem. Compa risons of Cu profiles from sediment cores across eastern Lake Superior suggest wide-spread signatures of mining and major contribution to se diments.