Hike nearly 5500 meters up in the Peruvian Andes and you’ll find the Quelccaya Ice Cap. The landscape is stark and seemingly pristine, with barely a shrub eking out a living on the rocks surrounding the tropical glacier. But drill into that ice and you’ll find a dirty history: the record of air pollution in South America. New research on an ice core taken from Quelccaya reveals that humans began polluting the region centuries before the industrial revolution arrived with its steam engines and coal plants. The results suggest that the Anthropocene, a new geological epoch defined by humans’ effect on the planet, began at different times around the world.

For as long as people have been releasing pollution into the atmosphere, ice in Earth’s glaciers has been trapping it. The Quelccaya ice core offers a particularly vivid record of atmospheres past, thanks to the tropics’ annual pattern of wet and dry seasons. The wet season brings snow to Quelccaya, and the dry season brings dust. Once everything gets packed down into the glacier, the alternating seasons show up as stripes of “clean and dusty ice,” explains Paolo Gabrielli, an earth scientist at Ohio State University, Columbus. Anything that’s in the air at the time the snow or dust lands will eventually be trapped inside the glacier.

That means scientists can use ice cores like the one from Quelccaya to reconstruct what Gabrielli calls pollution histories. For example, trace metals released into the atmosphere by ancient Greek and Roman mining operations have been discovered in ice from Greenland. Gabrielli thought South America’s glaciers might have recorded similar human activities, especially because the continent has a long history of mining and metallurgy.

People were mining and smelting copper in South America as early as 1400 B.C.E., and the Incas introduced the smelting of silver ore (which often contains lead) in the 15th century C.E., explains François De Vleeschouwer, an environmental geochemist at EcoLab in Toulouse, France, who was not involved in the study. Smelting these metals in open furnaces released particles into the atmosphere, where they became part of the precipitation and dust that eventually landed, among other places, on the Quelccaya Ice Cap. Gabrielli’s team found traces of metals, including copper and lead, dating to the pre-Columbian period in the Quelccaya ice core, supporting similar evidence of Incan and pre-Incan emissions from De Vleeschouwer’s study of peat records in Tierra del Fuego, Argentina. The new paper is the first to quantify precolonial air pollution in South America, showing that the emissions levels “are [high] enough to consider the Incas polluters,” De Vleeschouwer says.

But it wasn’t until the Spanish colonized South America in the 16th century that air pollution really took off, Gabrielli says. The main culprit was probably the gigantic silver mine in Potosí, Bolivia, which exploited the planet’s largest deposit of the precious metal throughout the colonial period—and released unprecedented levels of lead and other metals into the South American atmosphere. Between 1450 C.E. and 1900 C.E., lead levels in the Quelccaya ice core nearly doubled, and the amount of the metal antimony in the ice increased 3.5 times, Gabrielli and his colleagues report online today in the Proceedings of the National Academy of Sciences.

Air pollution tapered off a bit in the 19th century, when South America was rocked by wars of independence and its economy languished because of lasting damage to infrastructure and capital. But it roared back with a vengeance in the 20th century, as new copper mines opened; coal-burning trains and, later, cars fueled by leaded gasoline were introduced; and novel metals, such as molybdenum, began to be mined in the region. Between 1900 and 1989, the last year included in the Quelccaya ice core, silver levels in the atmosphere nearly tripled, copper and lead levels doubled, and molybdenum levels rose more than twofold. “It would be important to check whether atmospheric pollution of molybdenum persists today, because dust enriched in this element may be toxic to humans,” Gabrielli notes.

Although the vast majority of South America’s air pollution was released in the 20th century, Gabrielli and his colleagues believe that colonial mines like Potosí had such a dramatic impact on the environment that they should be considered the beginning of the Anthropocene in the region. That’s 240 years before the industrial revolution began, which suggests that “the timing for the onset of the Anthropocene may differ around the world,” says Alexandre Correia, an atmospheric physicist at the University of São Paulo in Brazil who has studied South American ice cores and was not involved in the current study. But whenever it started, the Anthropocene has left its mark on the present, Gabrielli says. “Today, there are no glaciers on Earth” in which traces of human-produced air pollution cannot be detected.