Toxic metals taint ancient dust

From San Francisco, at the 2001 fall meeting of the American Geophysical Union

A new study of dust lofted to Antarctica suggests that significant amounts of trace metals coated dust grains long before industries began loading the atmosphere with such pollutants.

Blowing dust in the American Southwest often carries large amounts of lead, cadmium, and arsenic. Yet those elements are minor constituents of Earth’s crust, says Todd K. Hinkley, a geochemist with the U.S. Geological Survey in Denver. Some of these tainted dusts can be traced back to specific, polluted sources, such as California’s Owens Lake (SN: 10/6/01, p. 218: Ill Winds), but the origins of others can’t be pinned down.

Hinkley contends that in some cases, however, the apparent excess of trace metals in those dusts may not be a product of modernity. Dust that fell on Antarctica as many as 13,000 years ago, during the height of the last Ice Age, shows concentrations of toxic metals similar to those in modern dust. Grains dropped on the icy continent 13,000 and 6,100 years ago contain almost 400 times the concentration of cadmium found in the average hunk of Earth’s crust.

Those same dusts sport concentrations of lead more than 20 times that of the crust.

Fine grains of dust scavenge trace metals from the air during their travels, says Hinkley. In modern times, those pollutants have presumably come from smokestacks. In ancient dusts, however, those trace metals probably got into the atmosphere from the emissions of volcanoes, Hinkley says. Chemical analyses of such dusts could help scientists identify periods of increased volcanic emissions.

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