Ingots reveal early Saharan trade
Copper chemistry helps researcher tap into Africa's past.
Ancient copper jewellery and ingots in sub-Saharan Africa have been found to bear the unique signature of copper ore from Morocco. This strongly suggests that trans-Saharan trade began hundreds of years earlier than previously thought.
Caravans of thousands of camels laden with packages once crossed the desert, bringing gold north and taking goods, including copper, south.
Most historians say these trade caravans began around the eighth century. But work by Thomas Fenn now points to an earlier date.
Fenn, a graduate student in anthropology at the University of Arizona, Tucson, examined old copper items from cities south of the Sahara. They included chain mail, earrings, and ingots that were traditionally strapped to the ankle for transport, in much the same way that some people stash money in their socks.
The copper items contain trace amounts of four lead isotopes, three of which result from the slow decay of radioactive elements such as uranium. Fenn used the relative quantities of these to date the copper ore used to make the items, and distinguish one source of copper from another.
He began the work because he was interested in how the influx of northern copper into the southern region changed the metal-working that was already going on there. Sub-Saharan Africa has its own smaller sources of copper ore.
But to his surprise, Fenn began to find copper that was clearly from Morocco in items that archaeologists had dated to as far back at 400 AD. He presented his findings at the American Chemistry Society meeting on 26 March in Atlanta, Georgia.
"Saharan trade is one of the great mysteries," says Fenn. "We have absolute clear evidence of things being carried down there, but that doesn't indicate the scale of trade." Fenn is off to Africa in May to do more sampling.
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