A patch of pristine rainforest has been found to contain the highest levels of atmospheric mercury poisoning ever discovered, new research has found.

Reporting in Nature Communications, an international research team studied mercury levels near small and artisanal gold mines in the Amazon rainforest. It's already known that these small-scale mines, largely informal or illegal, are a bigger source of mercury than burning coal. By studying samples of air, leaf litter, soil, and leaves from the top of trees of mining areas, pristine areas, and areas with low density of trees, the team hoped to find out how much damaging mercury the forests were capturing and storing.

“We found that mature Amazonian forests near gold mining are capturing huge volumes of atmospheric mercury," Jacqueline Gerson, who led the study as part of her PhD at Duke, said in a statement. "More than any other ecosystem previously studied in the entire world."

Illegal miners use mercury to separate gold particles from river sediment, as mercury binds the gold. The pellets are then big enough to be caught in the miners' sieves. It's then a matter of heating the pellets in open fire ovens. The gold melts and is saved, while the mercury burns off and becomes atmospheric mercury, before being captured by the leaves and soil of the nearby rainforest.

The team found that the thicker the canopy, the more mercury it clung onto.

"Intact forests in the Peruvian Amazon near gold mining receive extremely high inputs of mercury and experience elevated total mercury and methylmercury in the atmosphere, canopy foliage, and soils," the team wrote in their report. "Here we show for the first time that an intact forest canopy near artisanal gold mining intercepts large amounts of particulate and gaseous mercury."

The team looked at how the mercury was affecting wildlife by measuring levels in the wings of three different types of songbirds. On the birds in the Los Amigos Biological Station — where levels of atmospheric mercury pollution were exceptionally high — the team found 3-12 times more mercury in their feathers than on birds in areas of forest far from these gold mines.

The mining activities, which are often accompanied by deforesting, are already known to cause damage to biodiversity hotspots, decrease diversity, and have led to high levels of mercury in predators and people.

The demand for gold is pushing the hunt for it into new areas. However, the team says that they are not seeking to get the mines shut down, but to protect those involved or near to the mining, as well as wildlife and the forest. 

“There's a reason why people are mining,” Gerson said. “It's an important livelihood, so the goal is not to get rid of mining completely, nor is it for people like us coming in from the United States to be the ones imposing solutions or determining what should happen.

“The goal is to highlight that the issues are far vaster than water pollution, and that we need to work with local communities to come up with ways for miners to have a sustainable livelihood and protect indigenous communities from being poisoned through air and water." 

While mercury being captured by the canopy is no good thing, as contamination of wildlife has shown, it could be protecting Earth from a bigger problem.

“These forests are doing an enormous service by capturing a huge fraction of this mercury and preventing it from getting to the global atmospheric pool,” Emily Bernhardt, professor of Biology at Duke, said. “It makes it even more important that they not be burned or deforested, because that would release all that mercury back to the atmosphere.”