Mercury pollution in the Amazon is recorded in tree rings

For centuries, the Amazon has drawn treasure seekers in search of gold. These days, that pursuit continues, but what’s left is harder to find and comes with a heavy cost – toxic mercury pollution.

Small-scale and artisanal miners often use mercury to extract even the tiniest specks of gold. Unfortunately, this practice releases toxins that harm the natural environment and human health.

Tracking mercury pollution in tree rings

Scientists from Cornell University, working with an international research team, have found a surprising new way to track mercury pollution: tree rings.

By studying trees in the Peruvian Amazon, they’ve shown that mercury pollution can be traced by measuring the amount of heavy metals stored year by year inside tree trunks.

Dr. Jacqueline Gerson is an assistant professor in biological and environmental engineering at Cornell University and the lead author of the study.

“We show that Ficus insipda tree cores can be used as a biomonitor for characterizing the spatial and potentially the temporal footprint of mercury emissions from artisanal gold mining in the neotropics,” said Dr. Gerson.

“Trees can provide a widespread and fairly cheap network of biomonitoring, by archiving a record of mercury concentration within tree bole wood.”

How mercury ends up in the forest

In many artisanal mining operations, workers add mercury to gold-rich soil. The mercury binds with the gold particles and produces a substance called amalgam.

These amalgams have a low melting point, so miners can easily burn them to extract the gold. Consequently, this process releases mercury vapor into the atmosphere.

To test whether tree rings could record these emissions, the researchers collected core samples from three different tree species: wild figs (Ficus insipida), Brazil nuts (Bertholletia excelsa), and tornillos (Cedrelinga catenaformis).

Only the Ficus insipida trees produced visible rings that could be studied reliably year by year. These trees are common across the region and form annual rings even in tropical climates.

The researchers sampled five sites. Two were far from mining, and three were within five kilometers of gold mining towns. One of the mining sites bordered a protected forest, offering a unique comparison point.

Trees record heavy metal concentrations

“There are many variables that drive individual tree Hg concentrations, and it is difficult to determine the specific drivers,” explained Dr. Gerson.

“The trees in the study were all the same species and from the same sites, exposed to the same atmospheric Hg concentration. That is why we sample multiple trees and then use average values.”

The findings revealed clear patterns. For example, trees closer to mining towns contained significantly higher mercury pollution levels in their bole wood – the dense inner wood of the trunk.

The protected forest site had slightly lower levels. The most distant sites had the lowest concentrations.

Mercury levels surged after mining boom

“Higher atmospheric Hg concentrations are generally associated with nearby mining locations,” noted Dr. Gerson.

“In the Peruvian Amazon, where mining is the main source of Hg, the association between higher Hg concentrations and proximity to a mining site can readily be drawn.”

Notably, mercury levels spiked after the year 2000, especially in trees near the towns. This trend tracks with the region’s mining boom.

“This is likely due to the expansion of gold mining activities around this time,” said Gerson.

What tree rings can tell us

While the study supports the use of trees as a mercury-tracking tool, it also points out a few challenges. One limitation is that researchers didn’t always know the exact distance of the trees from active mining sites. That’s because many of these operations are illegal and not well documented.

This uncertainty could have affected the mercury pollution found in each tree. Even so, Ficus insipida shows strong promise for tracking mercury emissions over large areas.

“Ficus insipida can be used as a cheap and powerful tool to examine large spatial trends in Hg emissions in the neotropics,” said Dr. Gerson. “Using bole wood could allow for regional monitoring efforts.”

This type of monitoring could help international efforts to reduce mercury pollution under agreements like the UN’s Minamata Convention on Mercury. Organizations like limit mercury emissions worldwide and protect people and ecosystems.

By looking inside trees, researchers have found a quiet but reliable witness to decades of pollution. This important data may help protect the future of the Amazon rainforest.

The full study was published in the journal Frontiers in Environmental Science.

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