Monday, October 20, 2025

Invisible poison: Airborne mercury from gold mining is contaminating African food crops, new study warns

European Geosciences Union

image:
*Figure 1: Using mercury passive air samplers (MerPAS) to measure gaseous elemental mercury concentrations in and around both ASGM site and farming areas*
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Credit: Authors of the publication

In a recent study published today in the European Geosciences Union (EGU) journal Biogeosciences, scientists have confirmed that mercury pollution from artisanal and small-scale gold mining (ASGM) is contaminating food crops not through the soil, as previously believed, but directly from the air. Driven by the surging price of gold, which has increased by more than tenfold since 2000, the rapid expansion of unregulated mining in these regions raises urgent questions about food security, human health, and environmental justice

The study, conducted by an international team of scientists led by Excellent O. Eboigbe and David McLagan at Queens University, and Abiodun Odukoya Mary at the University of Lagos, focused on a farming community in Nigeria situated near an artisanal and small-scale gold mining site. The researchers compared crops from a field located 500 meters from the ASGM site with those grown 8 kilometres away. The contrast was striking mercury concentrations in leaves and grains were approximately 10-50 times higher in the farm closer to the mining site.

For decades, scientists have assumed that mercury enters food crops primarily via the roots, after leaching into the soil or water. But this new research, using sophisticated mercury stable isotope analyses, reveals a very different mechanism at work. Most of the mercury found in plant tissues came from the atmosphere, taken up through leaves during photosynthesis. In short: plants are breathing in mercury. David McLagan states that:

“Mercury uptake by plants from air represents the largest sink of mercury from air to terrestrial [land and freshwater] systems. While this critical ecosystem service helps reduce the amount of mercury being globally redistributed through the atmosphere, it raises human health concerns when it is staple crops that are the mechanism stripping the air of mercury.”

The research team found that leafy plant parts, which are often consumed by humans and livestock, retained the highest mercury concentrations. Edible, non-leafy parts of the plants, like cassava roots or maize kernels, had lower concentrations yet still showed significant contamination. While levels remained below international mercury consumption thresholds, the authors warn that there could still be health concerns when consuming mercury contaminated crops near ASGM sites, as international standards employed conservative crop consumption rates, and that even greater contamination of air, soils, and crops have been observed in other studies. This is especially relevant in communities dependent on local agriculture for survival.

Used to extract gold from raw ore, Mercury is a powerful neurotoxin. Long-term exposure, even at low levels, can damage the nervous system, impair cognitive development in children, and cause serious cardiovascular and reproductive problems. Due to its frequent use in artisanal and small-scale mining operations, vulnerable populations in low-income rural areas are at higher risk.

" Miners will not stop using mercury for gold extraction unless they get a readily available alternative that is also cost-effective.” Said Odukoya Abiodun Mary

ASGM is now the largest source of mercury emissions globally, according to the UN Environment Programme. Yet regulation and monitoring are limited in many parts of the Global South, where ASGM is often an economic lifeline for communities facing poverty and displacement. This study pushes a critical but overlooked consequence of that boom to the forefront: food systems are being contaminated, quietly and invisibly, by elevated levels of mercury in the air.

The research is also a call to action for governments and international organizations tasked with enforcing the ‘Minamata Convention on Mercury’. Current monitoring strategies focus largely on water bodies, sediment, and seafood, not crops. This study shows those efforts are missing a key vector of exposure.

“Due to the toxicity of bioaccumulation and biomagnification potential of methylmercury, fish consumption in ASGM areas has been a major focus of epidemiological research in ASGM areas. Yet this work demonstrates that there are other dietary sources of mercury, and mercury from these different sources can have cumulative effects.”

The study concludes that new policies are urgently needed to monitor and mitigate airborne mercury exposure in agricultural regions near mining activities. Considering the rapid growth of artisanal and small-scale gold mining, millions of people across Africa, South America, and Asia may be facing long-term health risks from something as simple and essential as growing and consuming local foods.

Press contact:
Media and Engagement Manager Asmae Ourkiya
European Geosciences Union
Email: media@egu.eu

About the EGU

The European Geosciences Union (EGU) is Europe’s premier geosciences union, dedicated to the pursuit of excellence in the Earth, planetary, and space sciences for the benefit of humanity, worldwide. It is a non-profit interdisciplinary learned association of scientists founded in 2002 with headquarters in Munich, Germany. The EGU publishes a number of diverse scientific journals that use an innovative open access format and organises topical meetings plus education and outreach activities. Its annual General Assembly is the largest and most prominent European geosciences event, attracting more than 20,000 scientists from all over the world. The meeting’s sessions cover a wide range of topics, including volcanology, planetary exploration, the Earth’s internal structure and atmosphere, climate, energy, and resources.

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Figure 2: Rudimentary gold extraction method used at this ASGM site and many others across the globe (left) and groundnuts (peanuts), one of the studied crops, collected at a farm adjacent to the ASGM site (right).

Credit

Authors of the publication

DOI

10.5194/bg-22-5591-2025

Subject of Research

Not applicable

Article Title

Mercury contamination in staple crops impacted by Artisanal Small-scale Gold Mining (ASGM): Stable Hg isotopes demonstrate dominance of atmospheric uptake pathway for Hg in crops

Article Publication Date

20-Oct-2025

MIT scientists uncover traces of Earth's lost ancestor deep beneath surface

Deep rock samples from Greenland, Canada, Hawaii reveal remnants of planet’s 4.5-billion-year-old precursor, offering new insights into Earth’s earliest chemistry

Gizem Nisa Demir | 20.10.2025

ISTANBUL

Scientists from the Massachusetts Institute of Technology (MIT) and international collaborators have discovered rare remnants of “proto Earth,” the planet’s ancient precursor that existed 4.5 billion years ago.

They offer a glimpse into the material that predated the massive impact that shaped our modern world.

The findings were published Tuesday in Nature Geosciences, according to ScienceDaily.

“This is maybe the first direct evidence that we've preserved the proto Earth materials,” said Nicole Nie, the Paul M. Cook career development assistant professor of Earth and planetary sciences at MIT.

“We see a piece of the very ancient Earth, even before the giant impact. This is amazing because we would expect this very early signature to be slowly erased through Earth's evolution,” he said.

The team identified an unusual chemical signature in deep rock samples from Greenland, Canada and the US state of Hawaii, showing a deficit in the potassium-40 isotope compared to most modern Earth materials.

The anomaly suggests the rocks may contain tiny portions of the proto Earth, surviving billions of years of planetary upheaval, according to the findings released.

Scientists have been trying to understand Earth's original chemical composition by combining the compositions of different groups of meteorites. But our study shows that the current meteorite inventory is not complete, and there is much more to learn about where our planet came from,” Nie explained.

Did marine life in the palaeocene use a compass?

Based on a sophisticated method, an international team has succeeded in mapping magnetic domains in giant fossilised magnetic microparticles: these may have served as compasses for ancient organisms

Helmholtz-Zentrum Berlin für Materialien und Energie

Some ancient marine organisms produced mysterious magnetic particles of unusually large size, which can now be found as fossils in marine sediments. An international team has succeeded in mapping the magnetic domains on one of such ‘giant magnetofossils’ using a sophisticated method at the Diamond X-ray source. Their analysis shows that these particles could have allowed these organisms to sense tiny variations in both the direction and intensity of the Earth’s magnetic field, enabling them to geolocate themselves and navigate across the ocean. The method offers a powerful tool for magnetically testing whether putative biological iron oxide particles in Mars samples have a biogenic origin.

A few years ago, mysterious particles of magnetite were discovered in marine sediments. These magnetite particles were exceptionally large – about 10-20 times larger than the ‘conventional magnetite magnetofossils’ that are made by magnetotactic bacteria for the purposes of passive orientation in the Earth’s field. Giant magnetofossils are observed in a variety of shapes, including needles, spindles, bullets and spearheads. To date, it is unknown which organisms were able to form these gigantic magnetite particles and for what purpose they were used. Although some giant magnetofossils resemble conventional magnetofossils in shape, their unusually large size was though to make them poorly optimised for the purpose of magnetic alignment alone. Instead, an accepted option is that some living beings used such magnetite particles primarily as a protective shield against predators due to their mechanical hardness, meaning that their magnetic properties did not play a major role. However, not all researchers are convinced by this theory.

A compass for sea creatures?

Sergio Valencia, a physicist at HZB, and palaeomagnetism researcher Richard J. Harrison from the University of Cambridge, UK, have now investigated an alternative hypothesis. They suspect that these creatures did indeed use the magnetic properties of these particles to help them navigate using the Earth's magnetic field by detecting small variations in intensity and direction of the field – a kind of in-built magnetic GPS system. To test this idea, it was necessary to map the three dimensional magnetic structure of the magnetofossils, enabling the magnetic energy and associated forces on the particle in the local Earth's magnetic field to be estimated.

Non-destructive investigation

Harrison and Valencia examined a particle shaped like a spearhead, with a diameter of 1.1 µm and a length of 2.25 µm. It came from the team of Liao Chang, University Beijing, and was found in a marine sediment in the North Atlantic that is around 56 million years old. A major challenge was to examine the internal magnetic structure of this rather thick sample without slicing and destroying it as this modifies the magnetic domain structure. This could be achieved at the Diamond X-ray source in Oxford, UK, using a newly developed technique devised by Claire Donnelly at the Max Planck Institute for the Chemical Physics of Solids (MPI CPfS) in Dresden, Germany. The so-called pre-edge phase X-ray magnetic circular dichroism (XMCD) ptychography allowed them to visualise the magnetic domain structure within the full volume of the sample in a non-destructive way. "This was a truly international collaboration involving experts from different fields, all working together to shed light on the possible functionality of these magnetofossils," says Sergio Valencia, the principal investigator who initiated and coordinated the effort.

3D mapping of the sample

The team was able to map the entire sample volume in three dimensions and at high resolution. ‘With the help of magnetic vector tomography, all three components of the magnetisation could be reconstructed and spatially resolved throughout the entire volume of the grain with a resolution of a few 10 nm,’ says Valencia, emphasising: ‘Once we have set up the successor source BESSY III, such measurements could also be carried out in Berlin.’

Magnetic vortex detected

The results show that the magnetite particle contains a single magnetic vortex that reacts to spatial fluctuations in the Earth's magnetic field with strong forces and could thus provide an organism with an accurate way to map the Earth’s magnetic field intensity, enabling magnetic navigation.

‘Marine organisms, for example a fish, may have used this property for magnetic navigation,’ says Harrison. Even if the particles studied here originally developed as protective armour, it is possible that, in the course of evolution, its descendants also used these particles as a navigational tool.

Use of the Earth's magnetic field to navigate is a widespread phenomenon today, observed in molluscs, amphibians, fish, reptiles, birds and mammals. Giant magnetofossils have been found in sediments dating back as far as 97 Million years, providing fossil evidence that magnetoreceptive navigation developed at least that long ago.

Outlook on particles from Mars

‘Iron-oxide particles resembling those made by some bacteria on Earth have been discovered on the Martian meteorite ALH84001, although their biological origin is strongly disputed. As we continue the search for evidence of life on Mars through sample return missions, we now have a method to investigate any new potential magnetofossils found and provide evidence to support or refute their biological origin,’ says Richard Harrison. ‘It would be very exciting to use this experimental technique to evaluate the morphological and magnetic fingerprints of those iron oxide particles. This could help in the search for evidence of past life,’ says Valencia.

Journal

Communications Earth & Environment

Cuts of the 3D magnetic volume of the fossil in three different heights: The blue/red colour indicates the in-plane component of the magnetisation, with magnetic moments swirling around a central line, forming a vortex pattern much like a tornado (see projections at the bottom). The 3D reconstruction has been obtained based on 140 individual images at different angles.