The history of Europe's civilizations spanning 3,000 years is written in lead on the otherwise pristine Arctic ice. Our measurements are now so subtle we can track the timing of ancient wars and economic booms in the concentrations found in ice cores taken from central Greenland.
Lead had its uses in the ancient world, sometimes at the cost of societies made sick when it leached into their drinks. However, it was its association with silver deposits that left a mark in Greenland. High-temperature smelting of lead-silver ores, beginning around 3,000 years ago, released lead particles into the atmosphere. Despite their density, these particles were small enough to be blown in the wind, sometimes settling in the Arctic ice sheets where thousands of years later scientists have measured their concentrations.
Previous studies of Greenland ice revealed fluctuations, hinting at the capacity to use lead concentrations to track economic activity in ancient Europe. However, the small samples and imperfect dating involved have made these far from conclusive. Now, Dr Joseph McConnell of the Desert Research Institute has used a much more detailed approach and precisely dated ice core stretching over almost 2,000 years to expand our knowledge of the ancient world.
In the Proceedings of the National Academy of Sciences, McConnell and colleagues report that as the Phoenicians spread their civilization across the Eastern Mediterranean, they tapped new silver mines and left a legacy in ice. When Rome and Carthage became the great powers of their day, mining deposits in what is now Spain to fund their empires, even more lead fell in Greenland, interrupted by plagues and political instability. Atmospheric modeling shows lead from China, and other advanced civilizations of the era, was nearly 10 times less likely to reach Greenland, so did little to affect the clarity of the European record.
Although the greatest lead production occurred in the early years of the Roman Empire, this crashed when the era of great plagues began in 165 CE, and did not reach its former levels until more than 500 years later. Indeed, for all the might of Imperial Rome, less smelting was going on during the crises that beset the empire from 235-284 CE than had been going on 1,000 years before.
We can see from the ice core that each war affecting the Iberian Peninsula, where Europe’s major silver mines lay, caused an immediate dip in lead emissions, with a subsequent increase as the workers and equipment diverted to war returned.
Confirmation that lead emissions were mostly a by-product of silver production can be found from changes in the purity of silver coins of the era. Periods when Greenland's lead levels fell match up with when coins were debased because there wasn't enough silver to go round.