Ancient cultures like the Romans, Carthaginians, and Phoenicians sought to leave their marks on the world in the form of culture, architecture, and military conquest. Some relics of these great transformations remain, but they’re not nearly as well preserved as the chemical relics these cultures left in the ground. A new study in Proceedings of the National Academy of Sciences show that these cultures left behind chemical marks in the form of industrial pollution — not just locally, but in lands thousands of miles away.

In the study, published Monday, a team of researchers show evidence that mining and metallurgy in Europe from 1,100 BCE through 800 CE left their mark as far away as the Arctic, providing a remarkable 1,900-year record of the economies that produced the pollution. Traces of lead that they discovered in ice core samples from Greenland showed a clear correlation with economic and political happenings thousands of miles away in a rapidly developing Europe. The lead they found in Greenland’s ice was an indirect window into ancient Europe’s much more expensive metallurgic obsession: silver — and the coins they could make with it.

lead in greenland ice
Scientists melted ice cores from Greenland to measure the concentrations of lead. They found that the lead pollution very closely matched the political and economic activities happening in Europe.

Lead isn’t a precious metal, but ancient Europeans needed it to mine and process silver. The lead fumes from mining operations eventually got carried away by the wind thousands of miles to the west and then eventually fell as snow and rain, creating a record of European silver mining activity in Greenland’s ice. Since the Roman Empire used silver in its currency — the denarius — the researchers concluded that higher levels of lead emissions in the ice probably correlated with periods of economic growth, during which times the denarius contained higher concentrations of silver.

These traces of lead give us the best approximation to date of ancient Rome’s gross domestic product, a figure with which historians have long struggled. “I wouldn’t say the lead pollution graph is a close reflection of GDP but it’s probably the best overall proxy for economic health we’ve got,” Andrew Wilson, Ph.D., a professor of archaeology at the University of Oxford and one of the authors of the study, told The New York Times on Monday.

In the new study, Wilson and his colleagues correlated the concentrations of lead samples in the ice with over nearly 2,000 years of data on the historical events we know took place in ancient Rome, Phoenicia, and Carthage. They found that the two trends were a remarkable match.

The level of correlation is “astounding,” Dennis Kehoe, Ph.D., a scholar of Roman economic history and law at Tulane University, told Science Magazine on Monday. “It’s really the rise and fall of a monetary system based on silver,” said Kehoe, who wasn’t involved in the study. “Prices were reckoned in silver, so they had to have silver.”