But humanity’s once-prolific use of lead might actually be a boon for historians. Lead pollution collected from Arctic ice cores can reveal, in year-to-year detail, the economic ups and downs of medieval Europe, according to the authors of a new study, published Monday in Proceedings of the National Academy of Sciences. “Lead is nice because the background levels are extremely low,” says Joe McConnell, a hydrologist at the Desert Research Institute and lead author of the new study. While volcanoes and desert dust naturally release some lead in the atmosphere, human activities are the source of the overwhelming bulk of lead that’s made its way into the sky. In ancient and medieval times, the main source of lead came from melting galena, a lead-silver ore, to make silver coins. “When you try to extract silver, you also extract lead, something like 100,000 times as much lead as silver,” says McConnell. When the ore is boiled to separate the two metals, lead bubbles off first, escaping into the atmosphere. Then, from the start of the Industrial Revolution until the 1970s, lead entered the air primarily through fossil fuel burning; coal and oil contain the metal. At every point in history, winds transported this pollution to the Arctic, where it settled, becoming encapsulated in ice with each coming snow. When a core of ice is bored from Arctic landscapes, it’s basically a tree ring of atmospheric chemistry. So scientists can scrape through the layers to reveal the past’s pollution. McConnell had been doing this for years, trying to analyze the lead pollution impacts of the Industrial Revolution. But historians approached him about going back further. Since lead pollution pre-1850 mostly came from making those silver coins, the medievalists thought it could potentially serve as a quantitative measure of the economy way into the past. (Previously, they’d been grain prices or dating trees used in medieval construction, but “lead is a very objective record and much more complete,” says McConnell.) To create this record, McConnell and his team used 13 dated ice cores from Greenland and the Russian Arctic. They focused on layers from the years 500 to 2010, intending to analyze the Middle Ages, Early Modern, and Modern periods. The scientists measured the lead concentrations from each and then used an atmospheric model to approximate the source of this pollution. From the early Middle Ages to the 1970s, lead pollution in the Arctic grew between 250 and 300 times. Around 1850, the pace at which that pollution was dumped jumped, reflecting all the coal humans started burning around then. But lead pollution was increasing before then, too. “I had always heard the Middle Ages was a flat period, not an economically vibrant period,” says McConnell. “[But] you see really vigorous growth… especially between 500 AD and 1300.” While lead pollution increases, on average, across the period the scientists looked at, there are lots of ups and downs year-to-year or decade-to-decade. Lead pollution increased in time with known historical events, like new mine discoveries, or when new technologies debuted. And the metal decreased during times of famine, war, plague, or climate disruptions. During the Little Ice Age, which started around 1300 and brought cold weather and crop failures, lead pollution plummeted. Emissions slowed even more during the Black Death, when a third of the European population died from the plague. “[The study] largely confirms what we knew (or suspected) about silver production in the medieval and early modern periods, though at many points it seems to bring the historical record into sharper focus,” says Lawrin Armstrong, medieval economic historian at the University of Toronto. But, he adds, he was surprised at the dramatic increase in lead pollution between 625 to 1000. “If this study is correct, silver coins (or ingots for the settlement of larger payments) played a more important role in the early medieval economy than I and most other economic historians imagine.” McConnell hopes his work will be of value uncovering such historical information. “The idea is to provide a proxy or indicator of industrial output that historians will get very excited about.”
