Geochemistry, Mineralogy, Volcanology News

Miners Left a Pollution Trail in the Great Lakes 6000 Years Ago

Scientists find evidence of ancient copper mining in polluted lake sediments from Isle Royale National Park.


Moose, wolves, and a few intrepid humans roam the wilds of Michigan’s Isle Royale National Park today, but thousands of years ago, the island boasted a thriving mining industry.

A glacially rounded copper nugget, as an example of the raw material worked by people of the Old Copper Complex. Photo by Rob Lavinsky (CC BY-SA 3.0).
A glacially rounded copper nugget, as an example of the raw material worked by people of the Old Copper Complex. Photo by Rob Lavinsky (CC BY-SA 3.0).

The veins of copper that ripple through its bedrock drew the attention of early Native Americans, who used the metal to make tools. However, many details of their activities—such as when they mined—remain hidden behind the thick haze of time.

Now, new research suggests that Isle Royale’s mining boom peaked about 6000 years ago and left a legacy of aquatic pollution. The high levels of copper, lead, and potassium in sediments from a cove on the island point to a long and intense period of indigenous mining. Researchers presented these results, published recently in the journal The Holocene, in a poster session on 16 December at the 2014 American Geophysical Union Fall Meeting.

Evidence of Ancient Mining

European explorers first noticed evidence for indigenous copper mines back in the 1800s. In some places, miners had dug down more than 20 meters into bedrock—an impressive feat considering their limited tools. However, without a way to date the pits directly, the timing of these mining activities could only be loosely constrained by the ages of copper artifacts found across the Great Lakes region. Archaeologists have dated many objects associated with the so-called Old Copper Complex, but the objects span thousands of years.

Copper artifacts created by people of the Old Copper Complex.
Copper artifacts left by people of the Old Copper Complex.

The mines caught the interest of David Pompeani, a Ph.D. student at the University of Pittsburgh, who went looking for clues beneath the chilly waters of Lake Superior. He hypothesized that chemicals released during the process of mining and annealing copper would leak into the lake and settle in its sediments. These sediments could then be dated using carbon-14 and other radioactive isotopes.

One such indicator of mining activity is lead, which would have leached from mine tailings and vaporized when miners heated copper to shape the metal, only to collect again in nearby waters. Pompeani and his colleagues previously found lead pollution in 8000- to 5000-year-old sediments along the south shore of Lake Superior. They interpreted this pollution as evidence of an extended era of widespread copper mining on Michigan’s Keweenaw Peninsula.

A Focus on Isle Royale

Several questions followed. How far did these ancient miners range? Did they migrate from one mine to another? To learn more, Pompeani’s team jumped across the lake to Isle Royale, a streak of rock that lies just off the Canadian shore. There, indigenous miners excavated the largest known pre-Columbian copper mine on top of Minong Ridge.

In nearby McCargoe Cove, a deep inlet that cuts diagonally across the island’s glacial striations, the researchers found elevated levels of lead and copper, along with potassium—a by-product of the fires used for mining and annealing. At their peak, lead and copper concentrations reached values an order of magnitude greater than background levels and about half as high as those associated with modern contamination.

The Fate of Ancient Miners

The spike in pollution began 6500 years ago and lasted for about a millennium. Then, abruptly, it ended, suggesting mining ended too. Pollution did not rise again until the mid-1800s, when mining resumed on Isle Royale, smelting began on the Keweenaw Peninsula, and leaded gasoline emissions grew.

The scientists do not know why mining screeched to a halt. They speculate that miners may have exhausted all the easily accessible veins and moved on. Climate changes may have also played a role—evidence from lake sediments around the Midwest suggests climate began to get dryer. In time, geologic clues may continue to provide more information.

—Julia Rosen, Writer

Citation: Rosen, J. (2014), Miners left a pollution trail in the Great Lakes 6000 years ago, Eos, 95, doi:10.1029/2014EO021147.

© 2014. The authors. CC BY-NC 3.0
  • Steve J.

    Considering that BC period mining was determined to have stopped about the same time as the bronze age in Europe stopped, there would be a good bet as to the reason why it stopped here. That and the older mines are dated to coincide directly with that same bronze age period. Unless that is we are still of the silly assumption that roughly 500,000 tons of copper was mined by the ancient Indians to make simple tools, jewelry and hooks with and there is no evidence that they ever used anywhere near even a fraction of that much copper in their culture. So where did all that copper go? Doesn’t matter right?

    We’ll forgo logical reasoning and let a remedial science that can’t prove anything it can’t dig up (unless it’s looking to prove something 😉 ) decide common sense. We’ll also dismiss that 10 tons of copper oxhide ingots recovered from the late Bronze Age (1300 BC) Uluburun shipwreck off the coast of Turkey was “extraordinarily pure” (more than 99.5% pure), and that it was not the product of smelting from ore. The oxhides are all brittle “blister copper”, with voids, slag bits, and oxides, created when the oxhides were made in multiple pourings outdoors over wood fires. Only Michigan Copper is of this purity.

    I know, it doesn’t fit the mold we’re trying to adhere to and raises to many eyebrows, so even though it’s fact, we’ll discard it in this pile of refuse over here with all the other stuff we were looking for. Yes, it was the primitive North American Indians who mined all that copper for their use, even though their ancient stories speak specifically of light skinned men who came and mined the copper…men who even had advanced ways of detecting it. We can’t believe that though as the were a stupid story making people and our stories are much better and believable.

    I guess we’re just getting used to the “planet is not flat” theory and Columbus WAS the first white man to step foot in North American too, so don’t expect this tortoise pace logic and science to move any faster. I just don’t understand who benefits and sleeps better from continued closed-minded and doggedly stubborn ideology. And people think religious zealots, as well as UFO and Bigfoot believers are nuts.

    Archaeology: The science of proving what we want to believe and already stubbornly believe, while occasionally finding a new wrinkle or fact along the way, but only if it fits out predetermined path of belief. Sound about right?