By MALCOLM W. BROWNE
Published: December 09, 1997
SAMPLES extracted from Greenland’s two-mile-deep ice cap have yielded evidence that ancient Carthaginian and Roman silver miners working in southern Spain fouled the global atmosphere with lead for some 900 years.
The Greenland ice cap accumulates snow year after year, and substances from the atmosphere are entrapped in the permanent ice. From 1990 to 1992, a drill operated by the European Greenland Ice-Core Project recovered a cylindrical ice sample 9,938 feet long, pieces of which were distributed to participating laboratories. The ages of successive layers of the ice cap have been accurately determined, so the chemical makeup of the atmosphere at any given time in the past 9,000 years can be estimated by analyzing the corresponding part of the core sample.
Using exquisitely sensitive techniques to measure four different isotopes of lead in the Greenland ice, scientists in Australia and France determined that most of the man-made lead pollution of the atmosphere in ancient times had come from the Spanish provinces of Huelva, Seville, Almeria and Murcia. Isotopic analysis clearly pointed to the rich silver-mining and smelting district of Rio Tinto near the modern city of Nerva as the main polluter.
The results of this study were reported in the current issue of Environmental Science & Technology by Dr. Kevin J. R. Rosman of Curtin University in Perth, Australia, and his colleagues there and at the Laboratory of Glaciology and Geophysics of the Environment in Grenoble, France.
One of the problems in their analyses, the authors wrote, was the very low concentrations of lead remaining in ice dating from ancient times — only about one-hundredth the lead level found in Greenland ice deposited in the last 30 years. But the investigators used mass-spectrometric techniques that permitted them to sort out isotopic lead composition at lead levels of only about one part per trillion.
Dr. Rosman focused on the ratio of two stable isotopes, or forms, of lead: lead-206 and lead-207. His group found that the ratio of lead-206 to lead-207 in 8,000-year-old ice was 1.201. That was taken as the natural ratio that existed before people began smelting ores. But between 600 B.C. and A.D. 300, the scientists found, the ratio of lead-206 to lead-207 fell to 1.183. They called that ”unequivocal evidence of early large-scale atmospheric pollution by this toxic metal.”
All ore bodies containing lead have their own isotopic signatures, and the Rio Tinto lead ratio is 1.164. Calculations by the Australian-French collaboration based on their ice-core analysis showed that during the period 366 B.C. to at least A.D. 36, a period when the Roman Empire was at its peak, 70 percent of the global atmospheric lead pollution came from the Roman-operated Rio Tinto mines in what is now southwestern Spain.
The Rio Tinto mining region is known to archeologists as one of the richest sources of silver in the ancient world. Some 6.6 million tons of slag were left by Roman smelting operations there.
The global demand for silver increased dramatically after coinage was introduced in Greece around 650 B.C. But silver was only one of the treasures extracted from its ore. The sulfide ore smelted by the Romans also yielded an enormous harvest of lead.
Because it is easily shaped, melted and molded, lead was widely used by the Romans for plumbing, stapling masonry together, casting statues and manufacturing many kinds of utensils. All these uses presumably contributed to the chronic poisoning of Rome’s peoples.
Adding to the toxic hazard, Romans used lead vessels to boil and concentrate fruit juices and preserves. Fruits contain acetic acid, which reacts with metallic lead to form lead acetate, a compound once known as ”sugar of lead.” Lead acetate adds a pleasant sweet taste to food but causes lead poisoning — an ailment that is often fatal and, even in mild cases, causes debilitation and loss of cognitive ability.
Judging from the Greenland ice core, the smelting of lead-bearing ore declined sharply after the fall of the Roman Empire but gradually increased during the Renaissance. By 1523, the last year for which Dr. Rosman’s group conducted its Greenland ice analysis, atmospheric lead pollution had reached nearly the same level recorded for the year 79 B.C., at the peak of Roman mining pollution.