By Will DunhamWASHINGTON (Reuters) - After vanquishing the Inca Empire with superior weapons and a touch of treachery, the Spanish conquistadors sought to satisfy their lust for riches by forcing multitudes of native people to toil in silver mines in dire conditions that claimed many lives.Scientists on Monday described evidence of this bitter chapter of South American history preserved deep in an ice cap in the Peruvian Andes in the form of residue from the relentless clouds of metallic dust spewed from the mines starting in the 16th century.The mountaintop mines of Potos in Bolivia were the world's richest silver source.While the Incas had long extracted silver, a new processing method introduced by the Spanish in 1572 greatly increased production even as it belched lead dust and other pollutants into the atmosphere. The pollution blew over the entire region, including the Quelccaya Ice Cap some 500 miles (800 km) northwest in southern Peru.The Spanish refining process involved pulverizing silver ore, containing both lead and silver, into powder, which sent metallic dust into the atmosphere. The powder was mixed with mercury. The silver was separated by heating the mixture to allow the mercury to evaporate.Writing in the Proceedings of the National Academy of Sciences, the scientists said they drilled into the glacier at an altitude of about 18,000 feet (5,600 meters) to learn about past air pollution.The age of the ice was determined with precision because it was laid down in discernible layers caused by the annual alternation between wet and dusty dry seasons.The pollutants spawned by the Spanish colonial-era silver operations from the 16th century through the 18th century consisted mostly of lead but also arsenic and others.The researchers called it the earliest evidence of large-scale, human-produced air pollution in South America, beginning more than two centuries before the industrial revolution.The pollutants were reminders of "the sad conditions and fate of tens of thousands locals exploited in the silver mining operations during the colonial period," Ohio State University environmental scientist Paolo Gabrielli said."Their work conditions must have been truly terrible. Many died because of the strenuous physical efforts but it was also not infrequent that underground mine galleries collapsed, burying and killing hundreds of people," Gabrielli said. Ohio State earth sciences professor Lonnie Thompson called the Quelccaya ice a "Rosetta Stone" for studying climate history, saying the samples also can reveal past temperatures, aridity and perhaps even the evolution of bacteria and viruses.(Reporting by Will Dunham; Editing by Sandra Maler)Join the conversation about this story Click here to read full news..