A heavy-metal history
A millennium of metallurgy is recorded in Peruvian lake sediments.
A dirty little secret lies buried at the bottom of a lake in Peru. According to new research published today on ES&T's Research ASAP website (DOI: 10.1021/es062930+), the lake sediments contain evidence that pre-Incan cultures in the Morococha region of the Andes were smelting copper and its alloys from ore as early as 1000 A.D, leaving behind traces of metal pollution. The Andean people later switched to silver when Incas moved into the area around 1450 A.D.
The authors say villagers probably ramped up silver production to pay the heavy tribute taxes that Incas demanded in the form of silver objects. About 80 years later, Spaniards came with more advanced technologies. "Once the Spanish show up, atmospheric pollution goes up 10-fold," says Colin Cooke, the first author on the paper, who worked on the project as a graduate student at the University of Pittsburgh. Airborne metal pollution rains down on small lakes, ultimately leaving behind traces in the bottom sediments.
Archaeologists have known for some time that the Incas used silver extensively, but no one had found evidence of local smelting. That may be because archaeologists usually rely on finding artifacts, Cooke says. The team looked to sediments, knowing that any metal artifacts in Peru "may have been looted or melted down and sent to Spain" long ago. Two coauthors of the ES&T paper, Mark Abbott of the University of Pittsburgh and Alexander Wolfe of the University of Alberta (Canada), first found evidence of pre-Incan silver smelting in the Bolivian Andes and reported the findings in Science.
Now that the team has found evidence of early metal use in Bolivia and Peru, they plan a larger survey to learn more about how the technology was spread throughout the New World.
"These lakes are natural archives, a book no one has thought to open," says Earl Brooks, adjunct professor of geology at George Mason University. Brooks has been studying lead at a mining site in Bolivia and says that geological techniques could also help reveal historic use of other metals of current concern, such as mercury and arsenic.
The findings "raise some potentially serious questions about effects on these humans at the time," says the University of Florida's Mark Brenner, who uses sediment cores to study environmental change. The high Andes have a good environment for preserving biological specimens, he says, adding that "it might be worth looking at metal levels in human remains" to see how much people were exposed to.
