Copper mining pollution detected on Isle Royale in Lake Superior between 6500 and 5400 years ago using sediment geochemical studies

Abstract:

Isle Royale, in Lake Superior, was the site of indigenous copper mining, however the timing and geographical context of past mining activity is poorly understood. We analyzed sorbed metal concentrations, carbon, nitrogen, and organic matter to document past mining pollution in sediment cores recovered from McCargoe Cove; a long, narrow inlet of Lake Superior on Isle Royale adjacent to a high concentration of ancient copper mines. Concentrations of lead, copper, and potassium increase in the sediments at McCargoe Cove after 1860 AD and between 6500 and 5400 years before 1950 AD or present (yr BP). Metal pollution increases coincide with radiocarbon dates associated with prehistoric copper artifacts and mines. These results, in combination with other lake sediment reconstruction sites located on Michigan’s Keweenaw Peninsula, demonstrate that from 6500 to 5400 yr BP mining activities are detectable across the Lake Superior region. Interestingly a coherent cessation of lead emissions at multiple study sites after ~5400 yr BP coincides with the onset of dry conditions found in regional paleoclimate proxy records. After ~5000 yr BP, lead concentrations on both Isle Royale and the Keweenaw Peninsula remain at background levels until the onset of modern pollution increases ~1860 AD.