Pollution of the Caesars: archeology

The ancient Romans, for all their knowledge, didn't know that lead was poisonous. They used it to preserve food and to halt the fermentation of wines. Lead glazes coated drinking vessels and cookware. Some historians have speculated that the brain-damaging effects of lead poisoning contributed to Rome's downfall. Rome's leaden legacy survives in pottery, pipes, and other artifacts. It also survives, a physicist has now found, in Greenland's ice sheet. Kevin Rosman of the Curtin University of Technology in Perth, Australia, has detected traces of lead in Greenland's ice, evidence of large-scale pollution of Earth's atmosphere two millennia ago. And some 70 percent of that pollution can be pinned to Roman mining in one region: Rio Tinto in southwestern Spain.

Rosman wanted to trace the source of the lead found in a Greenland ice core studied by two of his colleagues. The lead concentrations in that 3,300-yard ice core peaked between 150 B.C. and A.D. 50. This matched historical records of Roman mining activity at Rio Tinto. Lead pollution levels during the Roman era were about four times greater than natural background levels of lead but were still low by modern standards. Between the 1930s and 1970s, the lead concentration in the ice was 25 to 50 times higher than during Roman times, due in large part to leaded gasoline.

Rosman was able to trace the Greenland lead to the Spanish mine because of the unique ratios of lead isotopes in the Rio Tinto ore. Lead and its isotopes form over billions of years from the decay of uranium and thorium. The ratio of two isotopes in particular--of lead 206 to lead 207--generally increases in crustal rocks over time. (The numbers refer to the total of protons and neutrons in lead nuclei.)

Unlike lead in crustal rocks, lead at Rio Tinto was trapped in a vein of silver ore and was isolated from uranium and thorium. So Rio Tinto's lead still has the lower ratio of lead 206 to 207 that it had when it became trapped in the ore billions of years ago. When Rosman found this low ratio in pieces of the Greenland ice sample dating from 600 B.C. to A.D. 300, he consulted historical records to determine which mines were active during ancient times and compared lead isotope fingerprints taken from them in recent years to isotope ratios from the Greenland core. Rio Tinto captured much of the blame.

"Greenland is quite a way from Spain, so the lead that went into the atmosphere affected the entire Northern Hemisphere," Rosman says. He also notes that hard scientific facts show the history books were right. "It's a beautiful example of the application of science using sophisticated instruments to really understand the past."

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