Annual lead emissions in Europe closely varied with historical events, including imperial expansion, wars, and major plagues, according to new research published in the Proceedings of the National Academy of Sciences.
“Thousands of years ago, during the height of the ancient Greek and Roman empires, lead emissions from sources such as the mining and smelting of lead-silver ores in Europe drifted with the winds over the ocean to Greenland — a distance of more than 2,800 miles (4,600 km) — and settled onto the ice,” explained Desert Research Institute’s Professor Joe McConnell and co-authors.
“Year after year, as fallen snow added layers to the ice sheet, lead emissions were captured along with dust and other airborne particles and became part of the ice-core record that scientists use today to learn about conditions of the past.”
Professor McConnell and colleagues used ice samples from the North Greenland Ice Core Project to measure, date and analyze European lead emissions that were captured in Greenland ice between 1100 BC and 800 CE.
“Our record of sub-annually resolved, accurately dated measurements in the ice core starts in 1100 BC during the late Iron Age and extends through antiquity and late antiquity to the early Middle Ages in Europe — a period that included the rise and fall of the Greek and Roman civilizations,” Professor McConnell said.
“We found that lead pollution in Greenland very closely tracked known plagues, wars, social unrest and imperial expansions during European antiquity.”
The researchers made more than 21,000 precise lead and other chemical measurements to develop an accurately dated, continuous record for the 1900-year period.
To determine the magnitude of European emissions from the lead pollution levels measured in the Greenland ice, they used state-of-the-art atmospheric transport model simulations.
Most of the lead emissions from this time period are believed to have been linked to the production of silver, which was a key component of currency.
“Because most of the emissions during these periods resulted from mining and smelting of lead-silver ores, lead emissions can be seen as a proxy or indicator of overall economic activity,” Professor McConnell said.
Using their detailed ice-core chronology, the team looked for linkages between lead emissions and significant historical events.
Their results show that lead pollution emissions began to rise as early as 900 BC, as Phoenicians expanded their trading routes into the western Mediterranean.
Lead emissions accelerated during a period of increased mining activity by the Carthaginians and Romans primarily in the Iberian Peninsula and reached a maximum under the Roman Empire.
The highest sustained levels of lead pollution emissions coincided with the height of the Roman Empire during the 1st and 2nd centuries CE, a period of economic prosperity known as the Pax Romana.
The record also shows that lead emissions were very low during the last 80 years of the Roman Republic, a period known as the Crisis of the Roman Republic.
“The nearly four-fold higher lead emissions during the first two centuries of the Roman Empire compared to the last decades of the Roman Republic indicate substantial economic growth under Imperial rule,” said Professor Andrew Wilson, from the University of Oxford.
The scientists also found that lead emissions rose and fell along with wars and political instability, particularly during the Roman Republic, and took sharp dives when two major plagues struck the Roman Empire in the 2nd and 3rd centuries.
The first, called the Antonine Plague, was probably smallpox. The second, called the Plague of Cyprian, struck during a period of political instability called the third-century crisis.
“The great Antonine Plague struck the Roman Empire in CE 165 and lasted at least 15 years. The high lead emissions of the Pax Romana ended exactly at that time and didn’t recover until the early Middle Ages more than 500 years later,” Professor Wilson said.
Joseph R. McConnell et al. Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity. PNAS, published online May 14, 2018; doi: 10.1073/pnas.1721818115