Mercury pollution rides ocean currents north, threatening Arctic wildlife

Mercury levels in Arctic animals keep climbing even though the world has reduced new emissions. A new study shows that mercury already in the ocean is drifting northward on century-long currents and silently refueling pollution.

Researchers from Aarhus University and the University of Copenhagen analyzed more than 700 samples collected across Greenland.

The experts compared modern tissues from polar bears, seals, fish, peat, and even 40-year-old archives, then matched each sample’s six mercury isotopes – slight atomic variants that act like barcodes – to known ocean pathways.

“These isotope signatures act like fingerprints, revealing the sources and transport pathways of mercury,” explained lead author Jens Søndergaard, a senior researcher at Aarhus.

Old emissions ride ocean loops

When smokestacks or gold mines release mercury, the vapor can circle the globe for a year before it falls out of the air. Once it sinks into seawater, however, the metal can linger for three hundred years.

The new study shows that this “legacy mercury” is now the main supplier to the Arctic. Water entering central West Greenland rides the North Atlantic’s Irminger Current and arrives with a different isotopic stamp than water feeding the north and east coasts, which drifts down from the Arctic Ocean.

In regions swept by the Atlantic branch, animals carry less total mercury but show higher values of the heavy isotope δ202Hg. Farther north and east, concentrations are higher and the isotope mix points to older Arctic stores.

Mercury climbs in Arctic predators

Scientists have tracked Arctic mercury since the late 1970s, hoping to see a decline after many countries ratcheted down pollution.

Instead, concentrations in polar bears and toothed whales are now twenty- to thirtyfold above pre-industrial levels.

“We’ve monitored mercury in Arctic animals for over 40 years. Despite declining global emissions since the 1970s, we see no corresponding decrease in Arctic concentrations – on the contrary,” said senior author Rune Dietz, an ecologist at Aarhus.

The new isotope data show why: mercury carried by currents can take up to a century and a half to reach Greenland.

Isotopes trace mercury’s path

Mercury atoms come in several naturally occurring forms that differ slightly in mass. Industrial smokestacks, volcanic plumes, rivers, and ocean chemistry each leave a characteristic mix.

By scanning these mixes in muscle, fur, and ancient peat layers, the team drew a map of hidden routes. In central West Greenland the isotope blend matches Atlantic inflow; in north-eastern sites it matches the high-mercury Arctic Basin.

The peat cores also tell a time-lapse story: layers laid down in recent centuries hold heavier isotopes than layers deeper in the permafrost, confirming that modern pollution is still piling up.

Health impacts of mercury

“Mercury affects the immune system, reproduction, and possibly sensory functions in animals, which can impact their survival,” said co-author Christian Sonne, a professor at Aarhus.

Seals, polar bears, and seabirds already harbor levels linked to suppressed immunity and hormonal disruption.

Indigenous communities that rely on marine mammals for food face the same toxic threat. With mercury reservoirs still drifting north, even the toughest emission cuts may not deliver quick relief.

Arctic slow to reflect progress

The United Nations Minamata Convention aims to curb new mercury emissions. The Danish team’s work explains why Arctic numbers refuse to fall despite progress elsewhere.

Ocean inventories of legacy mercury – much of it released during the 19th- and 20th-century coal age – are simply too vast and too slow-moving. Any evaluation of the treaty’s success, the authors warn, must account for that time lag.

The group is expanding its isotope surveys through the GreenPaths project and collaborating with international programs such as WhaleAdapt to follow mercury’s course through other Arctic food webs.

By stitching together isotope records from peat, glaciers, rivers, and marine life, they hope to predict hotspots years before the pollutant arrives.

What can be done now?

Reducing modern mercury still matters; every gram kept out of the water is one less gram entering the conveyor belt.

But policymakers must pair emission goals with long-term monitoring, dietary guidance for northern communities, and studies on how climate-driven shifts in currents could speed or slow the toxic flow.

The lesson is stark: once mercury reaches the sea, it becomes a traveler with no passport expiry. And as the new isotope “fingerprints” show, the Arctic’s wildlife – and the people who depend on it – will keep feeling the aftershocks of 19th- and 20th-century pollution well into the 22nd.

The study is published in the journal Nature Communications.

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