The protected old-growth forest in the Amazon of southeastern Peru is so thick that it sometimes feels like scientists are in the middle of the night.
The same canopy that supports some of the richest on the planet is also sucking up alarming levels of toxic mercury according to a study.
The miners are looking for gold along the riverbanks. They use mercury to separate the precious metal and burn it off. Dust and leaves are washed onto the forest floor by rain when particles are Carried in the air. Particles are sucked into the leaves. Songbirds in areas farther from mining activity have levels of mercury two to 12 times higher than those in similar areas.
The patterns were much more stark and much more devastating than we expected, according to a biogeochemicalist at the University of California Berkeley who led the research. The study was published in a journal.
The findings from the Madre de Dios region of Peru show how people are altering the environment, as species extinction rates accelerate with little understanding of the consequences.
Mercury, which is also released into the air by burning coal, is a dangerous neurotoxin to humans and animals. It can become a very poisonous form in the water. Mercury builds up in the food web as big fish eat smaller ones. Doctors advise pregnant women 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, the government declared a health emergency in 2016 after 40 percent of people tested had dangerously high levels of mercury in their systems.
Humans are exposed to mercury in rivers, lakes and oceans. They haven't been as worried about it on land because it's less likely to become a drug. There is a lot of mercury going into the forest and it is leading to a lot of levels of methylmercury.
It has been assumed that people living in the Peruvian Amazon have been exposed to all of their methylmercury from eating fish.
The largest source of mercury pollution in the world is caused by the kind of gold mining that happens in the Madre de Dios region. It accounts for 20 percent of global gold production.
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, but need help in selling their goods, so they don't fall into thinking.
Dr. Gerson and her team collected soil, leaves, forest litter and other samples at three sites near mining activity and two farther away. They used a giant slingshot to shoot a rope into the canopy and pull branches down.
The protected old-growth site near gold mining activity stood out when the mercury levels came back. The thick canopy and vegetation caught and stored the mercury in those areas.
Dr. Gerson kept looking for examples of forests with similar levels. The only place she could find that was polluted by mercury mining and coal burning was in China. The levels in the old-growth Amazon were higher.
Emily Bernhardt is a professor of biogeochemistry at Duke University and co-author of the study.
The forests are some of the most biodiverse on Earth.
The service is not free. She noted that Mercury poisoning can affect birds and cause them to lay fewer eggs. It can make their eggs less likely to hatch.
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, said that scientists had assumed that the pollution from gold mining would have less impact on the environment.
It fills an important gap in understanding what happens with small scale mining.