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Archaeologist Michelle Bebber of Kent State University, Kent, made these replica copper arrowheads and knives made by people from the ancient copper culture of North America.
Michelle Bebber / Kent State University Experimental Archeology Laboratory
By David Malakoff
About 8,500 years ago, hunter-gatherers living on the edge of Eagle Lake in Wisconsin hammered a 10-centimeter-long conical projectile point made of pure copper. The finely crafted tip, used for hunting big game, highlights a New World technological triumph – and a puzzle. A new study of this artifact and other traces of prehistoric mining concludes that what is known as the ancient copper culture emerged, and then mysteriously vanished, much sooner than previously thought.
The dates show that the first Native Americans were among the first in the world to mine metal and turn it into tools. They also suggest that regional climate change could help explain why, after thousands of years, pioneer metallurgists abruptly stopped making most tools from copper and largely reverted to tools of stone and bone.
The largest and purest copper deposits on the planet are found around the Great Lakes of North America. At one point, Native Americans learned to harvest the ore and heat it, hammer it and grind it into tools. They left behind thousands of mines and countless copper artifacts, including deadly projectile points, heavy knives and axes, as well as small hooks and awls. Today it is not uncommon to come across locals’ who have buckets of copper artifacts [that they’ve found] tucked away in their basements, ”says David Pompeani, a geologist at Kansas State University, Manhattan, who studies ancient mining.
When researchers started dating the artifacts and mines, they saw a puzzling pattern: the dates suggested that the inhabitants of the ancient copper culture started producing metal tools around 6,000 years ago, and then, for reasons that were not clear, copper instruments abandoned around 3000 years ago. After that, the early Native Americans used copper primarily for smaller, less utilitarian items associated with adornments, such as beads and bracelets. “The story is so special,” in part because many other ancient cultures did not give up metal tools once they learned how to make them, Pompeani says.
About 10 years ago, Pompeani began doctoral research which cast doubt on the timeline of Old Copper. He extracted sediment cores from lakes adjacent to prehistoric mines on the Keweenaw Peninsula and Isle Royale in Michigan and measured trace metals in the cores, including lead and titanium, which had been released by the ore processing. Analyzes showed that copper mining began around 9,500 years ago in some areas – some 3,500 years earlier than previously thought. It also ended earlier, around 5,400 years ago, Pompeani reported in The Holocene in 2015.
In lab tests, replicas of Old Copper Culture arrowheads behaved in much the same way as stone arrowheads. Perhaps this is why the people of the ancient copper culture finally gave up on copper dots after using them for thousands of years.
Michelle Bebber / Kent State University Experimental Archeology Laboratory
Now a team led by Pompeani is presenting new evidence for the revised timeline. The researchers used modern methods to reanalyze 53 radiocarbon dates, including eight newly collected dates, associated with ancient copper culture. Some came from wood or ropes still attached to spear points; others came from charcoal, wood or bones found in mines and human burials. The oldest reliably dated artifact turned out to be the 8,500-year-old projectile tip found in Wisconsin.
This month Radiocarbon, the team reports that the most reliable dates, combined with sediment data, indicate that the ancient copper culture emerged at least 9,500 years ago and peaked between 7,000 and 5,000 years ago. This makes it at least as old, and possibly older, than the documented copper-working cultures of the Middle East, where archaeologists have documented an 8,700-year-old copper pendant.
The ancient window of Old Copper Peak comes as no surprise to archaeologist Michelle Bebber of Kent State University, Kent, who has studied the culture. The dates confirm “that the hunter-gatherers [were] very innovative, ”she says, and willing to“ regularly experiment with new materials ”.
But why did the old copper experiment abruptly end? Bebber’s work reproducing Old Copper-style arrowheads, knives, and punches suggests that they were not necessarily superior to alternatives, especially after taking into account the time and effort required to produce metal tools. In controlled lab tests, like shooting arrows at blocks of clay that simulate meat, she found that stone and bone instruments were mostly as effective as copper. This could be because Great Lakes copper is exceptionally pure, which makes it soft, unlike harder natural copper alloys found elsewhere in the world, she says. Only copper punches were found to be superior to bone punches.
Pompeani identified another potential contributor to the discoloration of old copper around 5,000 years ago. Sediment cores, tree rings data and other evidence suggest that a prolonged drought hit the area around this time, he says. This could have fueled social and ecological upheavals that made it difficult to devote time and resources to making copper tools. Over time, copper may have become a luxury item, used to signal social status.
However, copper punches resisted this trend: they required relatively little ore to make, Bebber notes, and the people of the Great Lakes continued to use them for thousands of years.
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