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Testing their metal Climate-change researchers sample lake cores and find evidence of South American metallurgical activity as early as A.D. 1000.
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COLIN COOKE
lead deposition. “Once I saw the rehen Spaniard Francisco Pizarro of Alberta (Canada) had traveled to sults, I was shocked, frankly, because I and his retinue arrived in what is Potosí, Bolivia, to take core samples couldn’t believe how obvious the pollunow Peru, they were astonished by the from a nearby lake, Laguna Lobato, to tion signal was,” says Abbott. The core quantity of gold and silver that the Inca study climate change and the impact of samples revealed that lead concentrawantonly displayed in their homes and drought in that region. As they carried tions in the lake sediments first rose holy places. After the Spanish captured out their work, their eyes were inevitathe Incan leader, Atahuallpa, and imbly drawn to Cerro Rico, the main mine above background just after A.D. 1000, before the Inca were even on the scene. prisoned him as a means to control the site near Potosí, which belched a dust Later, more sensitive innative population, he is ductively coupled plasma said to have offered PizarMS (ICPMS) analysis of ro a large “ransom room” lead and other metals, full of gold and two such as silver, bismuth, equally grand rooms of antimony, and tin, consilver in exchange for his firmed that pre-Incan release. The Spanish accultures were responsible cepted the precious metals for the initial mining and but executed the Incan smelting of metal near leader anyway, believing Potosí (Science 2003, that he was too much of a 301, 1893–1895). threat to set free. But Pre-Incan cultures left where did this enormous very little archaeological quantity of Incan gold record, so researchers and silver come from? don’t know who conAnd where did it go? trolled the territory The second question around Potosí at the beis much more easily anginning of the last milswered than the first belennium. The area is on cause, unlike the Inca, the Laguna Pirhuacocha near the Morococha mining region in the Peruvian Andes. The team’s truck and coring equipment are in the foreground. the edge of the region Spanish kept impeccable controlled by the Tiwritten records. Only a wanaku, a people who thrived from that drifted onto their study lake. They small percentage of pre-Colombian pretheir base near Lake Titicaca from A.D. knew that the mine had been active cious metal objects survive today, be~500 to 1000. The rise of metallurgy since the Spanish arrived but wondered cause the conquerors—their appetites may have overlapped with the final for gold and silver no doubt whetted by whether it had a far older history, one phase of the Tiwanaku era, but the rethat could be read from the lake cores the ransom room—looted and melted searchers believe that the activity at Pothey were collecting. down most of the valuable artifacts for tosí might have been started by people Abbott compares lake cores to a shipment back to the Old World. So, arfrom the Tiwanaku culture who had mibook that goes back in time; the stratichaeologists know the fate of all that grated away from Lake Titicaca during fied “pages” nearest the surface correprecious metal, but so far its origin has the civilization’s decline. spond to modern times, and the deeper remained shrouded in mystery. After a 300-year interregnum that layers get progressively older. Cores Shrouded, that is, until a couple of was marked by the lack of a strong domfrom Andean lakes can be as deep as paleoclimatological researchers decided inant culture in the area, the Inca rose 4–6 m, corresponding to 10,000 years to use their expertise to address the to power around A.D. 1450, and the of deposition. But for this study of relaquestion. “We didn’t go there at all to lake core data again point to an increase tively recent history, the top meter told investigate metallurgy,” says Mark Abin metal pollution in Laguna Lobato. the researchers all they needed to know. bott of the University of Pittsburgh The researchers could even pinpoint the The investigators initially used atomabout the original intentions of his time when the Spanish appropriated ic absorption and radiometric dating 1997 trip to South America. Abbott Cerro Rico and began applying their methods to rough out a chronology of and Alexander Wolfe of the University © 2007 AMERICAN CHEMICAL SOCIETY
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Castilian stone furnaces to the smelting of extracted metals; these furnaces failed in this area because they overheated the ore and volatilized the metals, including the valuable silver the Spanish coveted. The lake core data show a sharp spike in all metals in A.D. ~1500, and then a decline as the Spanish abandoned their European technology and relinquished smelting duties to the native Inca and their wind-drafted huayra furnaces. Although that first lake was a “target of opportunity”, according to Abbott, the success of the study led the investigators to seek other appropriate sites. Abbott recruited graduate student Colin Cooke, who conducted an extensive literature search targeting archaeological evidence for pre-Spanish mining centers. Cooke had to make sure that the lakes had no surface-water or groundwater connection to the mine sites to avoid contamination from mine tailings. The lakes also had to be downwind from the mines and have clear stratigraphic layers. “These lakes are near the top of the world, so you really want to look for protected lakes that sit down below a headwall so they’re not mixed by wind all the time,” explains Abbott. The lakes also had to be accessible. Although a metal-rich belt runs through the Andes all the way across Bolivia and Peru, the team, which usually consists of three to five people in one or two pickup trucks, was limited by simple transportation issues. In some places, “the roads are terrible, or there aren’t roads” at all, says Abbott. One lake near the Morococha mining region in the Peruvian Andes, Laguna Pirhuacocha, met all the team’s criteria. In contrast to Potosí, this area of South America was dominated by different pre-Incan cultures, first the Wari from A.D. 500 to 1000 and then the Wanka from A.D. 1000 to 1450. Metal artifacts have been recovered from the Wanka era, but whether these objects were crafted locally or imported from an outside source had been unclear. The researchers used a combination of ICP atomic emission spectrometry and ICPMS to analyze lead, zinc, copper, silver, antimony, bismuth, and titanium in a core from Laguna Pirhua5508
cocha (Environ. Sci. Technol. 2007, 41, 3469–3474). The data showed that metal smelting commenced in this region between A.D. 1000 and 1200, concurrent with the rise of the Wanka. As opposed to the core from Laguna Lobato, however, not all metals appear at the same time in the historical record. During the Wanka era, copper and zinc show the first large, sustained increases in concentration over background by A.D. ~1200. The researchers attribute this to the Wanka society’s archaeologically recorded penchant for bronze, a copper-based alloy. Then, in A.D. ~1450 and concurrent with the rise of the Inca, antimony, bismuth, and lead suddenly begin to appear in large concentrations in the lake sediment. “It looks like there is a distinct change in technology at this time, where they go from producing copper and copper alloys to silver, and that’s pretty neat,” says Abbott. It has long been known that Incan smelting methods use lead as a flux during silver production. “It was a very dirty process to get pure silver,” says Abbott. “It resulted in the release of a lot of soot and volatized lead into the atmosphere.” Records indicate that they extracted a “tribute tax” of silver objects from their conquered populations, and the researchers suggest that the increase in lead deposition may be a result of a scramble to meet Incan silver demands. Despite their tantalizing findings, Abbott says that a lot of work still needs to be done before they can make concrete inferences about just how much silver the Inca really had in their possession at the time of the Spanish conquest. He believes that their work may suggest that a larger amount of silver was looted from the Inca than the Spanish historical records admit. But he also cautions that “with one or two sites, you can’t say a lot, other than it’s surprising how much activity appears to have been going on.” The team is currently investigating more sites to try to fill in the metallurgical record across a greater territory and to find further evidence of the Incan shift in preference away from copper alloys to silver. a — Jennifer Griffiths
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