How Geology Becomes Archeology: Pitt Geology Professor Reveals Pre-Incan Silver Industry In Bolivian Andes

PITTSBURGH—Applying a technique never used in South America, University of Pittsburgh Assistant Professor of Geology Mark Abbott has found evidence that a major pre-Incan silver industry began in 11th century Bolivia, 400 years earlier and on a much larger scale than previously thought. Geologists Abbott and coinvestigator Alexander Wolfe of the University of Alberta, Canada, who study lake sediments, reported their findings Sept. 26 in Science.

"Because the goal of the project was to study the paleoclimate history of the area, the mining history was not part of the research design," said Abbott, whose work was funded by the National Science Foundation. "Sometimes you get lucky and find the perfect site to investigate more than one issue."

According to legend, the Inca discovered silver at Cerro Rico de Potosi, the largest silver deposit in the Bolivian tin belt, in the mid-15th century. Few known artifacts remain near Cerro Rico, because looting has been pervasive. It is believed that the silver was recycled and transported elsewhere in the Americas or exported by the Spanish.

Abbott and Wolfe analyzed layers of sediment from Laguna Lobato, a lake 6 kilometers upslope and downwind from Cerro Rico. They dated the samples from the last 10,000 years based on radiocarbon and lead-210, and analyzed the metal content of the mud.

Higher than normal concentrations of metal in Bolivian lake sediments indicate smelting, which separates pure silver from ore at high temperatures in a furnace. During smelting, some metal was volatized, or turned into aerosols, and carried by wind into the lake bed, where it settled to the bottom.

Abbott and Wolfe found that concentrations of silver began to rise in 1100 C.E., coinciding with the late stages of the Tiwanaku civilization, prior to the rise of the Incan empire. Before 1000 C.E., the metal concentration levels in the lake sediments of Laguna Lobato were low and stable, similar to levels found in samples from 2000 B.C.E. to 600 C.E.

Known events, such as tin mining at Cerra Rico during World War I, corresponded with increased levels of tin in sediment from that time. A peak in silver after the Spanish arrived in 1542 further supports Abbott's research.

Until Abbott's work in Bolivia, using lake sediment composition for archeological work rarely had been attempted, and had never been applied in South America. In Europe, researchers have attempted to look at levels of lead in pre-Roman time, with limited success. Accurate results depend heavily on the proximity to the source, and the signal must be very strong to show a difference.

"It was obvious to do," Abbott said, "but we didn't think of it until we were standing over it, looking at the lake, and I was kind of surprised that it worked as well as it did. It opens a door for new research because now we can do this in lakes close to ancient mine sites, and there are a lot of these in South America."

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9/25/03/tmw