The fascination with copper has persisted for thousands of years. The Lake Superior region of North America is home to many ancient and modern mines of highly useful metals. Long before modern miners extracted ore from deep underground, local indigenous communities dug ore from shallow pit mines.
The age of these prehistoric mines has been a “long-standing mystery,” says geologist David Pompeani of the University of Hawaii at Manoa. Previous studies used archaeological artifacts to assess when mining sites were active, but subsequent mining at the same sites often obliterated ancient artifacts. says Pompeani. To get around this, he and his colleagues took a different approach. Instead of artifacts, they looked for signs of mining preserved in the environment.
In recent research Anthropocene, researchers examined sediments from two small inland lakes near an ancient mine at the isolated Isle Royale in Lake Superior, Michigan. It is affected and acts like the rings of a tree. Each layer is a snapshot of what happened in a particular year, such as weather events, wildfires, or pollution.
Even pre-industrial copper mining caused contamination mainly by lead impurities in copper deposits. “Lead is a good proxy for documenting human influences…it’s not a metal that we can get naturally,” said a geochemist at the Andalus de Ciencias de la Tierra Institute in Spain. says one Francisca Martínez-Ruiz. .
Before modern machinery, copper mining was labor intensive. Native Americans hammered it out of the rock, a hard, dusty job that held fine particles of stone or metal into the air. Pompeani says bonfires may have been used to heat the rocks. This softened the copper and liquefied the easily soluble lead. These fires volatilized lead, drifting it into surrounding areas and scattering particles onto land and lakes. Researchers who analyzed the lake’s sediments found evidence that lead pollution peaked during the Archaic period about 6,000 years ago. This suggested that large-scale copper mining peaked at the same time, and was consistent with archaeological evidence for the same period.
“The paper shows that reeds are a reliable surrogate that can be used to rebuild the area,” says Martínez-Ruiz. She adds that similar studies of pollution in small lakes could be used to study the effects on humans around the world.
Pompeani said the study confirms some of the world’s earliest known large-scale mining operations and puts a new twist on how indigenous communities operated. “There’s an idea that hunter-gatherers weren’t able to organize and do mining operations,” he says. “However, the lake’s sediments indicate that it was detectably mined in the environment during Archaic times.”
