But a new analysis of what’s inside the
forest’s leaves and birds’ feathers tells a different story: The same canopy
that supports some of the richest biodiversity on the planet is also sucking up
alarming levels of toxic mercury, according to a study published Friday.
The mercury is released into the air by miners
searching for gold along nearby riverbanks. They use mercury to separate the
precious metal from surrounding sediment and then burn it off. Carried in the
air, particles catch on leaves like dust and are washed onto the forest floor
by rain. Other particles are sucked into the leaves’ tissue. From there,
mercury appears to have transferred up the food web to songbirds, which showed
levels of mercury 2-12 times as high as those in comparable areas farther from
“The patterns were so much more stark and so
much more devastating than we expected to find,” said Jacqueline Gerson, a
biogeochemist at the University of California, Berkeley, who led the research
as a doctoral student at Duke University. The study was published in the
journal Nature Communications.
The findings, from the Madre de Dios region of
Peru, provide new evidence of how people are altering ecosystems around the
world, as species extinction rates accelerate, with little understanding of the
Scientists have long known that mercury, which
is also released into the air by burning coal, is a dangerous neurotoxin to
humans and animals. In aquatic ecosystems, it can easily convert into a very
poisonous form called methylmercury. As big fish eat smaller ones, the mercury
sticks around, accumulating up the food web. For this reason, doctors advise
pregnant women around the world to avoid eating large, predatory fish like
shark, king mackerel and swordfish.
In the Madre de Dios region, where illegal
gold mining has surged in recent years along with the price of gold on global
markets, the government declared a health emergency in 2016 after 40% of people
tested in 97 villages had dangerously high levels of mercury in their systems.
Researchers have mostly focused on human
exposure to mercury in rivers, lakes and oceans. They have not been as worried
about it on land, since it is less likely to become methylmercury. But the
sheer load of mercury going into the forest, combined with rainy conditions and
soil, are leading to concerning levels of methylmercury there.
“It’s been assumed that people living in the
Peruvian Amazon have been getting all their methylmercury exposure from eating
fish,” Gerson said. “That may not be the case.”
The kind of gold mining that happens in the
Madre de Dios region, called artisanal and small-scale gold mining, occurs in
about 70 countries, often illegally or unofficially, and it is the largest
source of mercury pollution in the world. It also accounts for about 20% of
global gold production.
Julio Cusurichi Palacios, president of the
Native Federation of the Madre de Dios River and Tributaries, a group formed by
Indigenous communities in the region, said the government should combat illegal
mining with enforcement but also by strengthening alternative livelihoods for Indigenous
and other local people. They harvest fish, Brazil nuts, yucca and corn, he
said, but need help “improving their goods, selling their goods, so they don’t
fall into thinking, ‘I better go into mining, since my product doesn’t have a
For the research, Gerson and her team
collected soil, leaves, forest litter and other samples at three sites near
mining activity and two farther away. To collect certain leaves, they used a
giant slingshot to shoot a rope with a weight into the canopy and pull branches
When the mercury levels came back, it was the
protected old-growth site near gold mining activity that stood out. Those areas
had more than 15 times as much mercury as nearby clearings, presumably because
the thick canopy and vegetation caught and stored the mercury.
Shocked by the numbers, Gerson kept searching
the scientific literature for examples of forests with similar levels. The only
one she found was in an industrial area in Guizhou, China, polluted by mercury
mining and coal burning. Some levels in the healthy-looking old-growth Amazon
were even higher.
By capturing the mercury, the forests are
helping to keep it out of aquatic systems, said Emily Bernhardt, a professor of
biogeochemistry at Duke and co-author of the study.
“These are some of the most biodiverse forests
on Earth,” Bernhardt said. “We already knew they sequester tons of carbon in
their biomass and their soils, and we have now uncovered an additional,
incredibly important service.”
But the service is not without cost. Mercury
poisoning can affect birds’ ability to navigate and sing, and can cause them to
lay fewer eggs, she noted. It can also make their eggs less likely to hatch.
Previously, scientists had assumed that the
airborne mercury pollution from this kind of gold mining would have less impact
locally, said Daniel Obrist, an environmental science professor at the
University of Massachusetts Lowell who has studied mercury in forests in the
northeastern United States and the Arctic and was not involved with the Amazon
“It fills a very important gap in
understanding what happens there with small scale mining and what the
implications are,” Obrist said. “Not only for global processes, but also for
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