When the Spanish conquered South America in the 16th century they took over the Incas' mines and soon began to pump clouds of lead dust over the Andes. The silver the conquistadors sent back home made them wealthy. It also made them the world’s first industrial-scale toxic metal air polluters – perhaps causing us to rethink the timing of the moment when humans truly began to change the environment.

Formal recognition of the Anthropocene epoch, the “Age of Humans”, will acknowledge the occurrence of an unprecedented impact of human activities on Earth. As scientists, we’ve begun using the term informally, especially in regard to anthropogenic (“human-caused”) climate change. Officially, though, we all live in the Holocene, the epoch named by geologists to mark the end of the last ice age.

To officially say that we live in the Anthropocene – that is, declare the Holocene over and the Anthropocene already underway – we would have to draw an unequivocal line between the two. We’d have to agree on a point in time when human impacts on the environment became large enough to warrant an official change in scientific nomenclature. Some would assign it to the start of agriculture 11,000 years ago, while others tie it to the advent of the nuclear era in 1945, but most recognise the Anthropocene as beginning with the industrial revolution (1780s-1830s).

However we now have evidence, from an ice core of the Quelccaya Ice Cap in Peru, of anthropogenic pollution of the South American atmosphere that precedes the industrial revolution by around 240 years. The discovery by my colleagues and I, published in the Proceedings of the National Academy of Sciences, underscores the difficulty in defining the onset of the Anthropocene.

In search of the earliest pollution

While we have plenty of information from around the world about pollution during the industrial period, pre-industrial pollution records are very rare. We have to look to special places on Earth where atmospheric chemicals would have been preserved chronologically, such as lake sediments or the accumulated snow on an ice cap.

Quelccaya is one of those places. The largest ice sheet in the tropics is a fast-melting poster child for global warming. It’s also a perfect place to learn more about the past climate and environment – the ice core we drilled there in 2003 contained more than 1,200 years of accumulated atmospheric chemistry.

A millennia of climate history.Paolo Gabrielli, Author provided

South America has a rich history of mining and metallurgy. We wondered, would the ice record evidence of ancient metallurgical activity? Air pollution would have to have existed on a truly continental scale to drift on the air from the heart of South American metallurgy in Bolivia across the Andes and onto Quelccaya, some 800 km away.

It did. The story of South American metallurgy – from the rise of the Inca Empire to the Spanish conquest and even the industrial stagnation that followed the end of Spanish rule – is written in the ice.

An empire built on pollution

Like the native peoples before them, the Inca gathered metal ore from outcrops or exposed veins and smelted it in primitive wind-driven furnaces called huayra. The Quelccaya core first records evidence of pollution from Inca metallurgy around 1480 in the form of trace amounts of bismuth, likely released into the atmosphere during the creation of bismuth bronze, an alloy which has been recovered from the Inca citadel at Machu Picchu. Remarkably, no increases of other trace elements are apparent in the Quelccaya ice record during that period, indicating that the well-known metallurgic activities performed during the Inca reign had a negligible impact on the South American atmosphere.

Cerro Rico, near the Bolivian city of Potosi, was known as the ‘mountain that eats men’.Mining in Potosí (1596), by Theodoor de Bry

The Spanish conquistadors lead by Francisco Pizarro defeated the Incas in 1532, starting the colonial period of South America. Silver smelting quickly became the most important industrial activity on the continent, and the Spanish used imported and inefficient Castilian stone furnaces as well as thousands of local huayras as silver extraction spread across Bolivia and Peru. Increases in lead levels in the Quelccaya ice core date to approximately 1540 and document this initial phase of Spanish metallurgy.

In 1572, the Spanish introduced a new technique called amalgamation, which allowed them to process even low-quality ores that contained much more lead than silver. This cold technique involved grinding the ore into powder, which could easily have become airborne. We believe this accounts for the sudden and dramatic spike in lead concentrations in the ice core starting around that time.

Lead concentrations spike during Spanish rule (pink) and drop off after.Uglietti et al.

Even the independence war of 1833, which marked the end of Spanish rule, is recorded in the ice. Elsewhere in the world, the industrial revolution was booming – and air pollution growing. But at Quelccaya, lead levels fell and remained low for years after the war, likely due to army destruction of mines in Bolivia and Peru and the post-war lack of infrastructure.

The ice provides a detailed record of more than 1,000 years of South American history that can inform discussions of the Anthropocene timeline. Did it spread out through South America with the trace bits of pollution from the Incas' bismuth bronze? Or the lead concentrations from increased smelting upon the Spanish arrival? Or perhaps the more dramatic pollution created in the era of amalgamation marks the turning point.

This discovery suggests that our new epoch emerged sporadically through space and time, at different points during human history. Only as we connect the Quelccaya ice core to records elsewhere on Earth can we assemble a clearer picture of the dawn of the Anthropocene.

Paolo Gabrielli receives funding from National Science Foundation (USA) and Autonomous Province of Bolzano (Italy).