Record of anthropogenic lead emissions over the past 2,000 years in the Bolivian Altiplano. Shown are lead enrichment factors (EFs) compared to the regional background, reconstructed based on an ice core from the Illimani glacier. Before the use of leaded gasoline (period AD 0–1960), lead emissions from mining activities were dominant, especially during periods of the pre-Colombian cultures Tiwanaku/Wari and the Incas, the colonial era and with the increasing industrialization in the 20th century (brown, blue). Emissions from leaded gasoline were primarily responsible for the significant increase after 1960 (green). Picture: Paul Scherrer Institute. Click to enlarge.

Lead (Pb) is highly neurotoxic, and in contrast to many other heavy metals including cobalt, copper, and zinc, it has no beneficial effects in humans even at low concentrations. The introduction of leaded gasoline in the 1920s initiated a period of unabated growth in the global emissions of Pb. Before the onset of leaded gasoline phase-out in the 1970s, atmospheric Pb levels increased dramatically. Worldwide Pb emissions from gasoline in AD 1983 is estimated at about 248,000 tons, which was reduced to ~90,000 tons in AD 1995 after the implementation of environmental strategies through the introduction of low-leaded and unleaded gasoline.

Apart from combustion of leaded gasoline, major sources of anthropogenic Pb emissions include mining, metallurgical processing, and coal burning. Long-term histories of Pb pollution in Eastern and Western Europe, North America, and Asia show evidence that, in these regions, the highest Pb emissions of the past 2000 years occurred in the second half of the 20^th century. These studies suggest that emissions from metallurgy and coal combustion in the Northern Hemisphere, which significantly influenced atmospheric Pb concentrations [for example, during the Roman period and the first half of the 20th century], are minor in relation to emissions caused by the intensified use of alkyl-lead in gasoline. There exist, however, no equivalent data for South America.

—Eichler et al.

On the Altiplano, located between the western and eastern Andes, extractive metallurgy from mineral ores has been releasing large amounts of lead into the environment since the pre-colonial era.

The researchers used measurements from a 138m ice core, drilled out of the Nevado Illimani glacier in eastern Bolivia. Glacier ice is an invaluable archive of past air pollution. By drilling ice from deep below the glacier surface and analyzing it in the lab, scientists can reconstruct how high the concentrations of these air pollutants were in the past. The authors of the study have now succeeded in distinguishing local emissions from the Altiplano that can be attributed to mining from those originating from leaded gasoline that had been burnt mainly in more distant regions and carried along by the wind. Using a sensitive mass spectrometer, they determined the lead concentrations and the different composition of the isotopes in the lead from these two sources.

Isotopes are variants of a chemical element that differ from each other in their respective atomic weight. While chemically the various isotopes behave in the same way, due to their differing weights, they can be separated in the mass spectrometer. Lead naturally occurs in the form of eight different isotopes. The four lighter ones are stable, while their four heavier counterparts decay radioactively. The origin of the lead in an environmental sample can be determined based on the different proportions of these isotopes.

The researchers have now found the fingerprint for leaded gasoline revealed in the ratio of the two heaviest of the stable lead isotopes.

We detected a lower ratio of lead-208 to lead-207 after 1960. This isotope ratio deviates from that which is typical of lead from the Altiplano mines, but is in good agreement with the isotope ratio measured in the air in Chilean, Argentinean, and Brazilian cities in the 1990s. The majority of the lead in these air samples can clearly be traced back to leaded gasoline.

—Anja Eichler, first author

Record of the ratio of the lead isotopes 208Pb/207Pb in the Illimani ice core during the period AD 1850–2000. While the lead isotope ratios agree with those in the rock of local mines (brown) prior to 1960, they correspond to those of urban air samples in Chile, Argentina, and Brazil (green) after 1960. In the latter samples, the lead primarily originated from the consumption of leaded gasoline. Picture: Paul Scherrer Institute. Click to enlarge.

The researchers’ analyses also revealed that the anthropogenic lead emissions prior to 1960 primarily entered the atmosphere via mining activities. The pollution was particularly severe during periods of the pre-Colombian cultures Tiwanaku/Wari and the Incas, the colonial era and with the increasing industrialization of the 20^th century.

Lead was primarily released from silver mining and metallurgy until the end of the 19^th century, after which emissions from the production of tin, copper, and nickel dominated. The strongest increase in the last 2,000 years, however, can be attributed to the use of leaded gasoline after the 1960s, when the pollution level tripled compared to the historical values.

Leaded gasoline contributed twice as much to the anthropogenic lead emissions as the region’s mining. The researchers found a clear indication of the overwhelming contribution from road traffic after 1960 in the simultaneous rise in the ice core nitrate concentration. Nitrate is formed in the air from nitrogen oxides, which are emitted from combustion engines. Like lead, nitrate is “washed” out of the air with precipitation and deposited in the snow or glacier ice.

Resources

• A. Eichler, G. Gramlich, T. Kellerhals, L. Tobler, and Margit Schwikowski (2015) “Pb pollution from leaded gasoline in South America in the context of a 2000 year metallurgical history ” Science Advances Sci. Adv. 1, e1400196 doi: 10.1126/sciadv.1400196