An Ice Core Reveals the Economic Health of the Roman Empire
Posted May 14, 2018 4:21 p.m. EDT
A year-by-year economic history of the Roman Empire might seem as impossible to reconstruct as the lost 107 books of Livy’s history of Rome. Yet something close to such a record has now been retrieved from the unlikeliest of places: a glacier in central Greenland.
The record is written not in Latin but in lead. Lead emissions generated by mining operations in northern Europe reached Greenland and were washed down in snowfall. The snow accumulated, turned into ice, and preserved a record that stretches back thousands of years.
Ice cores from Greenland have long been used to track global climate change, which is recorded in the frozen water’s oxygen isotopes. The project to measure ancient lead emissions in ice cores was initiated by Andrew I. Wilson, an archaeologist at the University of Oxford who studies the Roman economy. A French team tried this in the 1990s, but Wilson believed new technology might allow a more comprehensive approach and reached out to Joseph R. McConnell, a leading expert in ice core analysis at the Desert Research Institute in Reno, Nevada.
Deep Greenland ice cores are hard to obtain because it can take three or four years to drill to bedrock. But McConnell knew of a core that had to be abandoned when the drill got stuck at the 6,500-foot level. Still, the core recorded 40,000 years of annual snowfalls, and the Danish custodians of the core let McConnell’s lab use a section of some 1,400 feet from its upper portions, corresponding to the years 1235 B.C. to 1257 A.D.
In the lab, the ice core was cut into rods just over 3 feet long that were stood on a heating pad and melted from the bottom. Grooves in the pad directed water from the central and purest part of the core to instruments known as mass spectrometers that continuously measured the quantity of lead down to one hundredth of a picogram, which is one trillionth of a gram. When the ice core was set to melt at the rate of 2 inches per minute, McConnell’s team found they could take 12 measurements per year throughout the Roman era.
The dates of these ice years were verified by synchronizing them with other chronologies, such as those derived from tree rings and volcanic eruptions.
The continuous record of lead pollution is not as good as having figures for Roman gross domestic product, which no one knows, but it does seem to reflect the general economic health of the Roman state.
The results, published in Monday’s issue of the Proceedings of the National Academy of Sciences, show a fluctuating line whose peaks and troughs correspond to salient events in Roman history. Lead emissions rose in periods of peace and prosperity, such as the Pax Romana, which ran from 27 B.C. to A.D. 180, and dropped during the civil wars that preceded the Pax and the rise to power of the emperor Augustus. There were also dramatic drops that coincided with the Antonine plague of A.D. 165-180, thought to have been smallpox, and the Cyprian plague, cause uncertain, of A.D. 250-270.
Lead was widely used in the Roman economy for making water pipes and sheathing the hulls of boats. Its production was also a proxy for a central economic activity, the use of silver in the Romans’ standard silver coin, the denarius. Silver occurs in lead ores, and the process of separating the silver from lead at high temperatures was a major source of airborne lead. In the early Roman Empire the denarius was 100 percent silver. But under the emperor Nero, starting in A.D. 64, the proportion of silver was reduced to 80 percent and the state made a tidy profit by recycling the all-silver denarii into debased ones.
These changes coincided with, and perhaps were caused by, a drop in silver production, and just such a fall in lead emissions is recorded in the Greenland ice core shortly after A.D. 60. Under the emperor Trajan there was a brief return from A.D. 103 to 107 to making coins from newly mined silver and this historical event too is reflected in a brief spike of lead pollution that ends in A.D. 107.
Lead emissions, as reflected in the ice core, dropped to an absolute low during the Imperial Crisis of A.D. 235-284, when the empire nearly collapsed under the stresses of internal discord, barbarian invasions and the Cyprian plague. Thereafter the economy, to judge by lead levels, recovered a little but entered a final period of decline marked by the withdrawal of Roman legions from Britain in the early fifth century and the collapse of the western Roman Empire in A.D. 476. Economic historians have tried to reconstruct the Roman Empire’s gross domestic product but have to make too many assumptions, Wilson said. “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,” he said.
He and McConnell are now working to see if the lead isotopes (lead atoms of different weight) in the ice core will help identify the geographic sources of lead production and thus make allowance for the fact that nearby sources of lead, such as in Britain, would have contributed more heavily to the Greenland pollution than distant ones such as the Rio Tinto mines in Spain.