Two thousand years ago, the Romans smelted precious ores in clay furnaces, extracting silver and belching lead into the sky. Some of that lead settled on Greenland’s ice cap and mixed in with ever-accumulating layers of ice. Now, scientists studying annual deposits of those ice layers have found that spikes and dips in lead pollution during the Roman era mirror the timing of many historical events, including wars fought by Julius Caesar.
Scientists have known about the Roman-era spike in lead pollution since the 1990s, continues Katie Langin in today's Science. Back then, researchers measured lead levels at a few places along the length of cores extracted from Greenland’s ice cap—with each measurement representing a 2-year period. Later studies confirmed the same pattern in soil samples from peat bogs in Spain, Scotland, and the Faroe Islands. But those studies couldn’t show how lead pollution changed year by year.
So Andrew Wilson, an archaeologist at the University of Oxford in the United Kingdom and an expert on the Roman era, teamed up with ice core experts to get a more complete picture. The team measured lead levels along a roughly 400-meter cross-section of Greenland ice, representing layers that froze between 1100 B.C.E. and 800 C.E. They melted the ice bit by bit, from one end to the other, and siphoned off the ice melt for analysis—obtaining around 12 measurements per year during the Roman era. Not all the lead came from pollution related to ore smelting; some came from naturally occurring dust and volcanic emissions, which researchers estimated and subtracted from the total lead count.
The result: an incredibly detailed 1900-year timeline of Roman lead pollution, published today in the Proceedings of the National Academy of Sciences. Lead pollution was greatest at the height of the Roman Empire—during the first century C.E.—at levels roughly six times higher than during the 11th century B.C.E. But after the Antonine Plague hit in 165 C.E., likely killing millions, lead pollution suddenly dropped back down to pre-Roman levels and remained that way for 500 years. Dips in lead pollution also occurred in the middle of the Roman era, particularly when wars erupted in Spain—a hot spot for lead-silver smelting—during the last few centuries B.C.E.
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