Earth's atmosphere was heavily polluted by lead more than 2000 years ago, according to a report in the December issue of Environmental Science and Technology. What's more, the lead's isotopic signature, revealed in ice layers in Greenland, indicates that it originated in a region of southern Spain that was heavily mined for silver and lead by the Romans. Their smelting of ore may have caused the first major assault on air quality.
The evidence comes from ice deposits, 350 to 500 meters deep, which retain trace elements in layers as they fall from the air, creating an ideal record of atmospheric contamination. And like trees, ice deposits have a built-in clock: The ratio of the isotopes oxygen-16 to oxygen-18 peaks in the summer and wanes in winter, creating regular cycles that can be used to index the passage of time. "For the Roman period we have an error margin of about 20 to 30 years," says Jean-Pierre Candelone of the CNRS Laboratory for Glaciology and Geophysics of the Environment in Saint-Martin-d'Hères, France, a co-author of the report on Greenland's ice.
The researchers found that the concentration of lead tripled during Roman times between 600 and 300 B.C. Although the levels are almost one-tenth of present-day lead concentrations in ice deposits, they are still quite substantial, says Candelone, who worked with colleagues from the Curtin University of Technology in Perth, Australia. "During Roman times, production techniques were primitive, and a good deal of the lead in the ores ended up in the atmosphere--up to 10% to 15%," he says. The purification of silver was particularly polluting, he adds; it was simply heated until all the lead evaporated.
The isotopic composition of lead in that period matched that of ores found in the Rio Tinto mines in Spain. These mines were first exploited by the Carthaginians and then by the Romans, who, in their heyday, extracted up to 100,000 tons of lead per year. The researchers also report that the lead concentration levels in the Greenland ice samples return to baseline values after the demise of the Roman Empire.
The findings emphasize that pollution is not a new problem, says Jerome Nriagu of the School of Public Health at the University of Michigan, Ann Arbor. And it sheds light on how severe ancient pollution was: "For us to see this signature so far away [in Greenland], the emission must have been much higher than what we believed up to now."