Ice cores, much like tree rings, can tell stories about the past. And in Antarctica, cores have revealed a 2,000-year record of heavy metal fallout from the Southern Hemisphere.
Levels of toxic metals such as lead and cadmium—produced by mining and metallurgy—dramatically increased after the Spanish conquest of the Inca in the 16th century, according to new research. And brief downturns coincided with events such as wars and the outbreak of disease.
These findings shed new light on how environmental changes are linked to history in the Southern Hemisphere.
The Faintest Whiffs
The water that trickles through Joseph McConnell’s laboratory is pure enough to drink. McConnell, an environmental scientist at the Desert Research Institute in Reno, Nev., and his colleagues analyze the chemical composition of ice cores by gently heating the cores and running the resulting meltwater through exquisitely sensitive instruments. The sensors in McConnell’s laboratory can detect whiffs of elements and compounds present in just a few parts per trillion or even quadrillion, said team member Nathan Chellman, a snow hydrologist also at the Desert Research Institute. “The concentrations are so, so low.”
McConnell and his collaborators recently analyzed a suite of ice cores drilled from five different sites in East Antarctica between 2013 and 2021. The researchers focused on the uppermost 100–150 meters of each ice core, which encompassed the snow that fell on the White Continent over about the past 2 millennia.
Snow is, of course, dominated by water molecules, but McConnell and his team had their eyes on something else: pollution.
Heavy metal particles, created by activities such as fossil fuel burning, mining, and metallurgy, show up in ice cores because the particles often hitch a ride aboard dust grains. Those grains can, in turn, be lofted into the troposphere and carried by currents thousands of kilometers before eventually raining out of the atmosphere when it snows.
McConnell and his colleagues used atmospheric modeling to show that dust grains found in East Antarctica could have easily traveled from the nearest inhabited landmasses—Australia and South America—in just a week or two.
The Timing of Mining
The researchers precisely measured the concentrations of various elements in their five ice cores. They also ascribed a time stamp to each of those measurements by correlating the timing of known volcanic eruptions with the signatures of volcanic fallout observed in the cores.
“For every ounce of silver that you produce, you might produce 10,000 ounces of lead.”
McConnell and his colleagues focused on three specific heavy metals: lead, cadmium, and bismuth. Those elements can be produced by mining and metallurgy. Mining for something like silver, for example, typically means sifting through a lot of lead-containing material, McConnell explained. “For every ounce of silver that you produce, you might produce 10,000 ounces of lead.”
But lead, cadmium, and bismuth are also released by both normal rock weathering and volcanic activity. “These natural sources can significantly contribute to the background levels of heavy metals in the atmosphere,” said Mariusz Potocki, a glaciochemist at the University of Maine not involved in this research.
To account for those sources of heavy metals, McConnell and his colleagues corrected their data on the basis of measurements of cerium and thallium in the same ice cores. Those elements trace rock weathering and volcanism, respectively. Such corrections were a critical aspect of this new analysis, McConnell said. “That’s what made all this possible.”
The Mad Rush of Bullion
The ice cores showed that concentrations of lead began ticking upward in the 1200s. And a consistent and pronounced increase—nearly 3 times the levels measured from 1 to 400 CE—persisted by the beginning of the 17th century.
That makes sense, the researchers suggested: The Inca Empire had fallen to the Spanish in the 16th century, and the colonizers wasted no time developing extensive mining operations in South America to extract precious metals such as silver to send back to Spain as bullion. One of those operations was the Potosí silver mine, which went on to produce a significant fraction of the world’s silver.
Cadmium and bismuth also showed smaller, but still consistent, increases by the beginning of the 17th century, the team noted. By the middle of the 19th century, the concentrations of all three heavy metals had risen by at least a factor of 10 above early Common Era levels. Much of the more recent heavy metal pollution could be ascribed to the onset of lead mining in Australia and tin mining in Bolivia, the team concluded.
When McConnell and his team looked closely at their measurements, however, they were surprised to discover various dips in the data. For starters, lead, cadmium, and bismuth concentrations all decreased in the late 1580s.
That timing, researchers realized, corresponded to the so-called Andean Epidemic, a disease outbreak of possibly smallpox or measles. Widespread disease among Indigenous Peoples—the labor force working in South American mines—would have likely affected silver production and therefore the emission of heavy metals, the team concluded.
“It’s pretty wild to think that a pandemic in the Andes in the early 1500s altered pollution in Antarctica.”
“It’s pretty wild to think that a pandemic in the Andes in the early 1500s altered pollution in Antarctica,” McConnell said.
In the early 19th century, lead and cadmium levels dropped again after South American wars of independence, the team found. Those battles damaged mining infrastructure and curbed international investments in South American mining, McConnell and his colleagues suggested. The study was published in Science of the Total Environment.
These results highlight the far-reaching impacts of anthropogenic activity, Potocki said. They “are not only credible but also emphasize the extent of human influence on the environment.” In the future, it’d be valuable to investigate how the biology and ecology of a place are affected by heavy metal deposition, Potocki added.
—Katherine Kornei (@KatherineKornei), Science Writer
