Mercury in nature comes overwhelmingly from deep inside the Earth, delivered to the surface and atmosphere through volcanic eruptions, hydrothermal vents, and the slow weathering of mercury-rich rock. Volcanism alone sends an estimated 170 to 336 tonnes of mercury into the atmosphere each year. But the element’s story gets more interesting once it reaches the surface, because mercury doesn’t just arrive and stay put. It cycles continuously between air, water, soil, and living organisms, so that the mercury a fish accumulates in its tissue today may have first entered the atmosphere from a volcano millions of years ago, been deposited into the ocean, re-emitted, and deposited again many times over.
The Mantle and the Making of Mercury Ore
Mercury exists throughout Earth’s crust, but the richest concentrations trace back to the upper mantle. The element hitches a ride upward in hot, mineral-laden fluids that form when tectonic plates collide or pull apart. When those fluids cool near the surface, mercury combines with sulfur to crystallize as cinnabar, the bright red mineral that has been the primary mercury ore for millennia. A 2025 study of cinnabar from giant ore deposits worldwide found that roughly 80% of the samples had an isotopic fingerprint matching that of continental flood basalts, consistent with an upper mantle origin for the mercury they contain.1American Journal of Science. From Source Rock to Cinnabar – How the Giant Mercury Deposits in Earth’s Crust Formed
Not all mercury ore is a simple mantle delivery, though. Work on deposits in South China shows that cinnabar there drew mercury from two sources: the deep mantle and the ancient Precambrian basement rock underneath the region. Tectonic events spanning hundreds of millions of years triggered extension and upwelling that remobilized mercury from both reservoirs, mixing them into the hydrothermal fluids that eventually formed ore deposits.2Geology. Unique mercury isotopic signature of mercury-bearing hydrothermal systems in South China and its geological and environmental implications The upshot is that mercury concentrates in the crust through repeated tectonic recycling, not a single event. Earth has been cooking up mercury deposits for billions of years.
Volcanoes Are the Largest Natural Source
Active volcanoes release mercury in two ways: explosive eruptions that blast rock and gas into the upper atmosphere, and the quieter, continuous degassing that happens through fumaroles and vents between eruptions. A widely cited early estimate put the combined volcanic mercury flux at around 112 tonnes per year, with eruptions contributing roughly 57 tonnes and degassing about 38 tonnes, before corrections for unmeasured sulfur dioxide emissions brought the total higher.3PubMed. Volcanic emissions of mercury to the atmosphere: global and regional inventories
More recent work using satellite observations of sulfur dioxide as a proxy has revised that number upward considerably. A 2024 study produced a central estimate of 232 tonnes per year, with an interquartile range of 170 to 336 tonnes, making volcanism by far the largest single natural pathway for mercury entering the biosphere.4Geophysical Research Letters. Impacts of Volcanic Emissions on the Global Biogeochemical Mercury Cycle: Insights From Satellite Observations and Chemical Transport Modeling The difference between the older and newer estimates reflects improvements in satellite detection and a better understanding of the mercury-to-sulfur-dioxide ratio in volcanic plumes. Even the lower bound dwarfs any other purely natural source.
Hydrothermal Vents on the Ocean Floor
Volcanoes on land get most of the attention, but the mid-ocean ridges where tectonic plates spread apart are volcanically active too. Superheated water circulates through fresh basalt at these ridges, dissolving metals and spewing them into the deep ocean through hydrothermal vents. Mercury is among those metals. Sediment cores collected near the Central Indian Ridge show extremely high mercury concentrations associated with iron-manganese oxides and other hydrothermal trace metals, confirmed by isotopic ratios pointing to a vent origin.5PubMed. Mercury proxy for hydrothermal and submarine volcanic activities in the sediment cores of Central Indian Ridge
These submarine vents are a meaningful source of mercury to the deep ocean, where it becomes part of the marine mercury pool that eventually works its way into sediments, organisms, and back into the atmosphere. Because the global mid-ocean ridge system stretches tens of thousands of kilometers, the cumulative contribution of seafloor venting is not trivial, even if it is harder to measure than the plume from a land-based eruption.
What Happens to Mercury in the Atmosphere
Once mercury enters the atmosphere, it mostly takes the form of gaseous elemental mercury, a vapor that is remarkably stable. That stability gives it a long atmospheric lifetime, on the order of several months to over a year, which means it gets carried by winds across entire continents and oceans before it is removed.6Atmospheric Chemistry and Physics. Long range transport of mercury to the Arctic and across Canada This is why mercury is considered a global pollutant rather than a local one: a volcano in Indonesia or a coal plant in China can send mercury that eventually deposits in the Arctic.
Removal happens through chemical oxidation. In the atmosphere, elemental mercury is oxidized by bromine atoms and hydroxyl radicals, with ozone playing a supporting role. These reactions convert the vapor into oxidized mercury compounds that are water-soluble and sticky, so they latch onto aerosol particles or dissolve into rain and snow, falling to the surface.7PubMed. Improved Mechanistic Model of the Atmospheric Redox Chemistry of Mercury The balance between how fast mercury is oxidized out of the air and how fast surfaces re-emit it back determines how much mercury accumulates in any given ecosystem.
The Ocean as Both Sink and Source
Oceans absorb enormous quantities of mercury from the atmosphere through wet and dry deposition, but they also give some of it back. Dissolved mercury in surface waters can be reduced to its elemental form by sunlight and microbial activity, at which point it evaporates back into the air. Measurements in the low-latitude Pacific and Atlantic have found that both regions are net sources of elemental mercury to the atmosphere, meaning more mercury evaporates from the sea surface than deposits onto it.8Deep Sea Research Part I: Oceanographic Research Papers. The air-sea exchange of mercury in the low latitude Pacific and Atlantic Oceans
This ocean-to-air pathway is especially dramatic in polar regions during summer. In the Arctic, reduced sea-ice cover and warmer conditions promote the evasion of elemental mercury from surface waters. Modeling work has found that oceanic evasion contributes about 64% of the elevated summertime atmospheric mercury concentrations over the Arctic, far outpacing mercury delivered by atmospheric transport from lower latitudes.9PubMed Central. Oceanic evasion fuels Arctic summertime rebound of atmospheric mercury and drives transport to Arctic terrestrial ecosystems As sea ice declines with warming, this pathway is expected to strengthen.
Forests, Soils, and Falling Leaves
On land, forests are powerful mercury scavengers. Tree leaves absorb gaseous elemental mercury from the air through their stomata, the same pores used for gas exchange during photosynthesis. When those leaves fall, they carry their accumulated mercury to the forest floor. Research in boreal forests of northwestern Ontario found that the combined mercury flux from throughfall (rain dripping through the canopy) and litterfall was roughly three times greater than the mercury arriving in direct wet deposition alone.10PubMed. Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems Crucially, the mercury in that leaf litter was new input, not recycled from the soil below. Forests, in other words, act as a kind of atmospheric filter, pulling mercury out of the air and concentrating it in the topsoil.
Over centuries and millennia, this process builds up substantial mercury stores in forest soils and peat. That stored mercury stays put as long as the soil stays intact. But when something disturbs it, the mercury enters the cycle again.
Wildfires and Permafrost Release Mercury Back Into Play
Wildfires are one of the most dramatic ways that stored mercury re-enters the atmosphere. When vegetation burns, essentially all of the mercury in the plant material is released as gaseous elemental mercury. But the bigger concern is what happens when fire heats the soil beneath: compacted organic matter in forest floor layers and mineral soil releases additional mercury that has been accumulating for decades or longer.11Atmospheric Environment. Mercury emissions from burning of biomass from temperate North American forests: laboratory and airborne measurements As wildfires grow more frequent and intense with climate change, especially in high-latitude boreal forests where soils hold large mercury reservoirs, these emissions are accelerating.12Earth’s Future. Initial Estimates of Soil Mercury Emissions Induced by Soil Heating During Global Wildfires
Permafrost is an even larger concern. Mercury deposited in northern soils over thousands of years has been trapped by organic matter and preserved by frozen ground. By some estimates, more mercury is locked in permafrost than exists in the global atmosphere, oceans, and non-frozen soils combined.13PubMed Central. Metal Mobilization from Thawing Permafrost Is an Emergent Risk to Water Resources As permafrost thaws, that mercury is released through multiple pathways: erosion of thaw slumps, groundwater transport of dissolved mercury, and gaseous emissions from waterlogged thermokarst landscapes. Perhaps most worryingly, the warm, oxygen-poor wetlands that form in thawing permafrost zones are ideal habitats for microbes that convert inorganic mercury into methylmercury, the far more toxic form that accumulates in food webs. Modeling of future emissions under moderate warming scenarios suggests permafrost regions could release on the order of 100,000 tonnes of mercury into the global cycle by 2300, and several times that under high-warming scenarios.14Nature Communications. Potential impacts of mercury released from thawing permafrost
How Mercury Climbs the Food Chain
Mercury in water and soil is concerning on its own, but the real health risk comes from what happens next. Certain anaerobic bacteria and archaea, particularly sulfate reducers and iron reducers, possess a gene pair called hgcAB that allows them to convert inorganic mercury into methylmercury.15PubMed Central. Kinetics of Enzymatic Mercury Methylation at Nanomolar Concentrations Catalyzed by HgcAB This methylated form is easily absorbed by organisms and is excreted very slowly, which means it accumulates in tissue over a lifetime and concentrates as it moves up the food chain, a process called biomagnification.
In marine food webs, the effect is striking. Small organisms at the base absorb methylmercury from the water, zooplankton eat those organisms and concentrate it further, and so on up through fish to top predators. In the Gulf of Maine, biomagnification factors greater than ten were measured in swordfish, bluefin tuna, harbor porpoises, and sharks relative to their prey.16PubMed Central. Bioaccumulation of methylmercury within the marine food web of the outer Bay of Fundy, Gulf of Maine A worldwide meta-analysis found that biomagnification is strongest in cold, low-productivity waters, which helps explain why Arctic and subarctic marine life tends to carry higher mercury burdens per unit of food-web length than tropical species.17Environmental Science & Technology. Biomagnification of Mercury in Aquatic Food Webs: A Worldwide Meta-Analysis
This is not just an aquatic problem. Research along a mercury-contaminated river in Virginia found that terrestrial songbirds living near the river had blood mercury levels as high as birds feeding directly from the water. The main delivery vehicle was spiders, which feed on aquatic insects that emerge from the river carrying methylmercury, and are then eaten by birds.18PubMed. The movement of aquatic mercury through terrestrial food webs In montane forests far from obvious contamination, mercury still biomagnifies from plants through herbivorous insects to predatory arthropods and on to songbirds and raptors, with blood mercury levels in carnivorous birds consistently higher than in their prey.19PubMed. Mercury bioaccumulation and trophic transfer in the terrestrial food web of a montane forest Even in subtropical urban forests, both total mercury and methylmercury increase through the food chain from grasses and pine needles up through insects and spiders to nestling songbirds.20PubMed. Terrestrial mercury and methylmercury bioaccumulation and trophic transfer in subtropical urban forest food webs
The Global Budget and the Human Fingerprint
On an annual basis, natural sources account for roughly 5,200 tonnes of mercury emitted to the atmosphere, while anthropogenic sources contribute about 2,300 tonnes.21Atmospheric Chemistry and Physics. Global mercury emissions to the atmosphere from anthropogenic and natural sources That natural figure sounds dominant until you realize it includes a large share of re-emission: mercury that was originally released by human activity, deposited into soils and oceans, and then volatilized back into the air through natural processes. Teasing apart “truly natural” mercury from “previously anthropogenic mercury that nature is recycling” is one of the hardest problems in mercury science. The distinction matters because even if every coal plant and gold mine shut down tomorrow, the mercury already deposited in soils, sediments, and ocean water would continue cycling through the environment for centuries.
Ice-core and sediment records give us a sense of scale for the human impact. A glacial ice-core record from Wyoming showed a roughly 20-fold increase in atmospheric mercury deposition from preindustrial times to the mid-1980s.22Environmental Science & Technology. Atmospheric mercury deposition during the last 270 years: A glacial ice core record of natural and anthropogenic sources A global compilation of peat, lake, ice, and marine sediment records found a five- to nine-fold increase in mercury accumulation over the period 1700 to 2012, with the trends in lake and peat deposits closely tracking modeled atmospheric deposition.23National Science Review. Mercury records from natural archives reveal ecosystem responses to changing atmospheric deposition The good news embedded in these records is that mercury deposition has declined in some regions since regulations began phasing out major sources in the 1980s and 1990s. The bad news is that the legacy mercury already in the system will keep circulating.
Mercury Spikes in Deep Time
Earth’s mercury cycle has been disrupted on a grand scale before, long before humans existed. Geologists have learned to use mercury anomalies in ancient rock layers as a fingerprint for massive volcanic episodes. The most dramatic example is the end-Permian extinction about 252 million years ago, the worst mass extinction in Earth’s history. At the exact horizon where the extinction is recorded in South Chinese rock sections, mercury concentrations spiked to roughly nine times background levels, and isotopic signatures pointed to an atmospheric volcanic origin for the excess mercury.24Earth and Planetary Science Letters. Mercury anomalies across the end Permian mass extinction in South China from shallow and deep water depositional environments
Recent work has gone further, using coupled mercury isotope systems to resolve individual eruption pulses from the Siberian Traps volcanic province across three distinct biotic crises during the Permian-Triassic transition. The isotopic patterns shift in a characteristic way during each crisis interval that is absent before and after, suggesting repeated injections of volcanic mercury into the atmosphere at exactly the times when life was collapsing.25PubMed Central. Coupled Hg isotope dynamics reveal eruption pulses across the Permian–Triassic mass extinction Mercury toxicity alone probably did not cause these extinctions, but the mercury spikes serve as powerful tracers for the volcanic outgassing that disrupted climate and ocean chemistry on a catastrophic scale.
A Greenlandic ice core spanning the entire Holocene, the roughly 11,700 years since the last ice age, shows that even in more recent geologic time, volcanic eruptions and climate shifts leave clear mercury fingerprints.26PubMed Central. Mercury accumulation over the Holocene revealed from a Greenlandic ice core The record reveals a natural baseline that fluctuated with eruptions and climate excursions well before any human influence, eventually giving way to the anthropogenic signal that dominates the most recent centuries.
Microbes That Learned to Live With It
Mercury has been part of Earth’s chemistry for as long as life has existed, and some organisms have evolved sophisticated defenses. Many bacteria carry a cluster of genes called the mer operon, which encodes proteins that detect mercury ions, transport them into the cell, and then reduce toxic ionic mercury to the far less harmful elemental form, which simply evaporates away. In nitrogen-fixing soil bacteria, acquiring the mer operon through horizontal gene transfer was shown to confer a tenfold increase in mercury tolerance.27PubMed Central. Horizontal gene transfer of the Mer operon is associated with large effects on the transcriptome and increased tolerance to mercury in nitrogen-fixing bacteria The fact that this resistance system is shared across distantly related microbial lineages through gene swapping, rather than inherited vertically, suggests that mercury exposure has been a persistent selective pressure throughout microbial evolution. These mercury-resistant microbes also play a role in the global mercury cycle: by reducing ionic mercury back to elemental mercury, they contribute to the re-emission of mercury from soils and water back into the atmosphere, completing yet another loop in a cycle that has no real beginning or end.