for National Geographic News
Silver mining in the Bolivian Andes was a well-established industry 400 years earlier than commonly thought, according to a geological study published today. But what happened to thousands of tons of silver extracted from the Earth a thousand years ago remains a mystery.
Researchers were able to trace the mining history of Cerro Rico de Potosi, a major silver deposit in southern Bolivia, by tracking the metal residues that result from smelting in the sediments of a nearby lake, Laguna Lobato.
The sediments, which date from about A.D. 1000 to 1200, provide evidence of a major pre-Inca silver industry. The timing coincides with the waning years of the Tiwanaku Empire, which dominated the Bolivian Andes from about A.D. 400 to 1000. Legend holds that the silver deposit wasn't discovered until the mid-15th century and that the discovery was made by Inca ruler Huayna Capac.
"It's not completely unexpected that there was some smelting going on at that time," said Mark Abbott, a geologist at the University of Pittsburgh, and a co-author of the study. "What is unexpected, I think, is the magnitude; there was a lot going on, and the metal is not accounted for in the archaeological record."
Very little archaeological work has been done in the region, and so it is unclear whether the artifacts of the Tiwanaku have simply not been found yet, or whether they were looted and recycled by first the Inca and then the Spanish when they arrived in Potosi in 1545.
History in Lake Sediments
The oldest archaeological site in South America containing metal artifacts is located in coastal Peru; the artifacts are dated to between 1400 B.C. and 1100 B.C. From 200 B.C. to A.D. 1000, sheet metal working was pervasive throughout the Andes.
When the Spanish arrived in the area, Inca silver mining was a huge industry, said Abbott. But there is a curious gap in the archaeological record from about 1100 to 1450, a period known as the Altiplano Period, that separates the Tiwanaku and Inca empires.
To fill this gap, Abbott and Alexander Wolfe extracted a 74.5 centimeter (29.3-inch) sediment core from the deepest part of the lake nearest to Cerro Rico, the largest silver deposit in the Bolivian tin belt. By analyzing the accumulation of metals associated with smelting, a heating process used to separate metals, like silver, from ore, they were able to detect spikes in the metal concentrations that are indicative of heavy mining. The data suggest that several thousand tons of silver were produced in pre-Inca times.
The study is published in the September 26 issue of the journal Science.
"The findings are likely to generate controversy because the lake sediments hold a record of what was going on that's not supported by the archaeological record," said Abbott. "But analyzing sediment and pollution trails is a way to use geology and chemistry to look at human history in a different way. Even a thousand years ago that lake was pretty polluted."
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