Researchers found that European lead pollution emissions closely tracked known plagues, wars, social unrest and imperial expansion during ancient civilizations.
Through the use of very detailed lead measurements in archived North Greenland Ice Core Project (NGRIP) samples and state-of-the-art atmospheric transport modeling, researchers accurately dated measurements in the ice core spans over 1,900 years.
The resulting analysis was published on Tuesday, May 15, in the journal Proceedings of the National Academy of Sciences.
This July 17, 2011 photo shows the melting front of a glacier at the edge of the Greenland ice sheet, near Kangerlussuaq, Greenland. Greenland is the focus of many researchers trying to determine how much its melting ice may raise sea levels. (AP Photo/Brennan Linsley)
Their measurements started in 1100 B.C. during the late Iron Age and extended through antiquity to the early Middle Ages in Europe, according to Study Lead Author Joe McConnell.
This period includes the rise and fall of Greek and Roman civilizations.
Most of the emissions during these periods resulted from mining and smelting of lead-silver ores.
Therefore, lead emissions can be seen as a proxy or indicator of overall economic activity, McConnell explained.
“Silver was very important to these ancient economies. It was one of the primary metals used in coinage and the production of new silver was linked directly to lead-silver ore mining and smelting,” McConnell said.
The resulting emissions drifted up into the atmosphere and settled onto Greenland’s frozen surface.
In a cyclical process, snow and ice covered this lead pollution creating numerous sedimentary layers. The process created a geological record extending for hundreds of feet into the ice.
The researchers focused on lead pollution for a number of reasons.
Lead was widely used in commercial and industrial applications in the ancient world and still is today.
Lead pollution has a close association with precious metals like silver, meaning “that lead pollution is a sensitive indicator of industrial activity and the overall economy.”
For example, ice-core records of recent lead pollution in Greenland show a pronounced drop during the economic downturn of the Great Depression, according to McConnell.
Thousands of frozen ice cores stored in canisters inside the minus-33 degree Fahrenheit environment of the archive warehouse at the National Ice Core Laboratory, in Lakewood, Colo, on Aug. 8, 2016. At this facility, ice pulled from the depths of Antarctica and Greenland is sliced up, photographed and tested. (AP Photo/Brennan Linsley)
This is not the first time that lead measurements were made on ancient ice cores.
In the mid-1990s, a pioneering study of lead measurements were made on selected sections of an earlier ice core from the Greenland Ice Core Project (GRIP).
However, only 18 measurements were made on discrete samples during this 1,900-year period. The record was very sparse and discontinuous, according to McConnell.
The recent research made over 21,000 measurements during this same 1,900-year period using a unique continuous ice-core analytical system.
“Our record essentially is continuous in time and these vastly more extensive measurements provide a much different picture of ancient economic activity,” McConnell said.
A critical issue in any sort of historical work is precise dating, particularly if comparisons are to be made between documented historical events.
Significant progress has been made in accurately dating ice cores and linking ice-core records to tree-ring and other well-dated proxies, according to McConnell.
These developments and the continuous, high-resolution nature of the new measurements allowed the researchers to develop a much more accurate age scale than was used in the earlier study.
“Evaluation of our new ice-core chronology against the well-dated composite tree-ring record suggests that our age scale is accurate to within 1 to 2 years during the Roman era,” McConnell said.
Ancient Roman copper, bronze and silver coins dated from the third century D.C. are on display in the San Giovanni underground station of Rome’s brand new third metro line, Thursday, April 6, 2017. (AP Photo/Andrew Medichini)
The study’s findings will be helpful for historians and economists.
“Historians and archaeologists are interested keenly in understanding how events, such as plagues and wars, impacted ancient societies. In addition, economic historians would like to know how ancient economies fared through time and in response to internal and external factors,” McConnell said.
Lead pollution emissions began to rise at least as early as about 900 B.C., coinciding with the expansion of Phoenician trading routes into the western Mediterranean.
That overall rise continued through the late second century B.C., with ups and downs corresponding with specific war periods, political instability and the opening of new geographic regions to mining.
Their record also shows that lead emissions were low during the last 80 years of the Roman Republic, a period referred to as the Crisis of the Roman Republic.
Emissions returned to their previous highs at the beginning of the Imperial Rome in the last few years of the first century B.C.
This rise coincided with the period of economic prosperity that is often referred to as the Pax Romana.
Two great pandemics, the Antonine Plague and the Plague of Cyprian, struck the Roman Empire in the second and third centuries, and both coincided with major declines in European lead emissions.
“Clearly plagues had a major and lasting impact on mining and smelting activities and probably on the supply of new silver for coinage as well,” McConnell said.
European lead emissions stayed low throughout the three to four centuries of the Migration Period. Lead emissions did not return to the high levels of the Pax Romana until the early Middle Ages, when significant mining started in other areas of Europe including France and England.