A group of scientists led by Dr Paolo Gabrielli of Ohio State University has discovered evidence of air pollution within the Andean ice that predates the Industrial Revolution by more than two centuries.

In the 16th century, during its conquest of South America, the Spanish Empire forced countless Incas to work extracting silver from the mountaintop mines of Potosi, in what is now Bolivia.

The Inca already knew how to refine silver, but in 1572 the Spanish introduced a new technology that boosted production many times over and sent thick clouds of lead dust rising over the Andes for the first time in history.

Winds carried some of that pollution 800 km northwest into Peru, where tiny remnants of it settled on the Quelccaya Ice Cap.

Dr Gabrielli and his colleagues discovered a layer within a Quelccaya ice core that dates to the Spanish conquest of the Inca, contains bits of lead and bears the chemical signature of the silver mines of Potosi.

The core provides the first detailed record of widespread human-produced air pollution in South America from before the industrial revolution, and makes Quelccaya one of only a few select sites on the planet where the pre-industrial human impact on air quality can be studied today.

“This evidence supports the idea that human impact on the environment was widespread even before the industrial revolution,” said Dr Gabrielli, a co-author of the paper published in the Proceedings of the National Academy of Sciences.

For their study, the scientists used a mass spectrometer to measure the amount and type of chemicals present in the Quelccaya ice dating back to 800 CE.

They looked for antimony, arsenic, bismuth, molybdenum and especially lead.

That’s because the refining process that the Spanish introduced to South America involved grinding silver ore – which contains much more lead than silver – into powder before mixing it with mercury in a process called amalgamation. So atmospheric pollution from silver production would chiefly contain traces of lead particulates.

The study revealed some spikes in the concentrations of these elements in the years before Spanish rule, but those layers all likely coincide with natural contamination sources, such as volcanic eruptions.

Starting just before 1600, however, the ice began capturing much larger quantities of these elements, and the high amounts persisted until the early 1800s, when South American countries declared independence from Spain.

To pin down where the pollution came from, the researchers compared their data with those from a peat bog in Tierra del Fuego, Chile, and from sedimentary lake records from regions including Potosi and other mines throughout Bolivia and Peru.

These latter sites would have captured the pollution generated in their local area during that time.

The chemical signatures in the Quelccaya ice meshed with what scientists knew from written records: most of the pollution likely came from Potosi, where the Spanish produced the vast majority of silver. Other mines throughout the region contributed to the Quelccaya pollution to a lesser extent.

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Chiara Uglietti et al. Widespread pollution of the South American atmosphere predates the industrial revolution by 240 y. PNAS, published online February 09, 2015; doi: 10.1073/pnas.1421119112