Anthropogenic copper inventories and mercury profiles from Lake Superior: Evidence for mining impacts

During the past 150 years, the mining industry discharged more than a billi on tons of railings along Lake Superior shorelines and constructed numerous smelters in the watershed Given the vast size of Lake Superior were sedime nt profiles at locations far offshore impacted by nearshore activities? Did copper and associated precious metal mining modify regional fluxes for cop per and mercury? Samples from thirty sediment cores document that backgroun d concentrations of copper are high (mean 60.9 +/- 7.0 mu g/g), due to the proximity of natural ore sources. Anthropogenic inventories uncorrected for focusing also are high, ranging from 20 to 780 mu g/cm(2) (mean 187 +/- 54 mu g/cm(2)). Focusing factor corrections decrease the mean estimate and re duce variance (144 +/- 24 mu g/cm(2)). Several approaches to estimating inp uts suggest that only 6 to 10% of historic copper deposition originated dir ectly from atmospheric sources, emphasizing terrestrial sources. Moreover, coastal sediment cores often show synchronous early increases in copper and mercury with buried maxima. Around the Keweenaw Peninsula, twenty-two core s trace high copper and mercury inventories back to mill and smelting sourc es. Direct assays of ores from thirteen mine sites confirm a natural amalga m source of mercury in the stamp mill discharges. Core records from inland lakes (Michigamme Project) also reveal patterns of copper and mercury input s from a variety of mining sources: historic tailing inputs, amalgam assay releases, and atmospheric smelter plumes.