Richard Musgrove
Dr Richard Musgrove is a zoologist and science journalist. He writes for Cosmos and Double Helix.
Wild fig trees may provide a biomonitoring tool for illegal gold mining in the Amazon.
Gold, sought after and fought over for millennia, is found in the Amazon rainforest although artisanal and small-scale gold mines are often illegal. Mercury, (Hg) used for separating gold, is a major health and environmental problem.
Mercury is added to a sediment slurry to bind gold particles. This amalgam is then collected, and the mercury, which has a lower melting point than gold, is burnt away, leaving the yellow metal.
Tailings from these mines contaminate land and water, and burning releases mercury gas into the atmosphere, where it can travel long distances, eventually settling via rain or dust or getting into trees through stomata in leaves.
Such mining practices are the main sources of anthropogenic atmospheric mercury says geochemist Dr Jacqueline Gerson of Cornell University. But estimates Hg emissions from artisimal and small gold mining sites are poorly constrained due to a lack of monitoring data and the informal, generally unregulated nature of this industry. Trees accumulate atmospheric gaseous elemental mercury (GEM) in bolewood following stomatal uptake and have the potential to be used as biomonitors
Mercury taken up by Wild fig (Ficus insipida) trees is revealed in annual tree rings so it’s been suggested that these could be used to monitor illegal gold mining in the Peruvian Amazon.
Researchers led by Gerson travelled to the Peruvian Amazon and surveyed Wild Fig trees, which were the only species found to have obvious growth rings. Tree core samples were collected well away from mining towns and compared with those taken within 5 km of the mining sites.
“There are many variables that drive individual tree mercury concentrations, and it is difficult to determine the specific drivers,” Gerson says in her yet to be published paper.
“The trees in the study were all the same species and from the same sites, exposed to the same atmospheric concentration. That is why we sample multiple trees and then use average values.”
Tree-ring concentrations were almost an order of magnitude greater near gold mining sites, says Gerson.
“We show that Wild Fig tree cores can be used as a bio-monitor for characterizing the spatial and potentially the temporal footprint of mercury emissions from artisanal gold mining in the neotropics,” says Gerson.
“The fact that they managed to find a suitable tropical species is remarkable,” says Dr Larissa Schneider of the ANU, who was not involved in the research. Schneider is also from Brazil so is very familiar with the landscape.
It has also not escaped notice that scientific research into illegal mining projects has some personal risk.
Schneider told Cosmos that counting the rings would be a challenge — many tropical trees have false rings brought on by drought or other stressors. So, there is more work involved as each ring might also have to be radiocarbon dated to ensure precision. “It’s not a problem — you can get around it if you do more analysis.”
The paper has been provisionally accepted in Frontiers of Environmental Science.
Peru is a signatory to the UN Minimata Convention. The convention, which came into force in 2017 says that ratifying countries are bound by international law to put measures in place that control and reduce mercury emissions; phase out the use of mercury in certain products and industrial processes; restrict its trade; and eliminate mercury mining.
Mercury in plants
