Atmospheric mercury is declining—but Arctic wildlife is more contaminated than ever. The culprit? Ocean currents loaded with legacy pollution.
Key Points at a Glance
- Ocean currents transport legacy mercury into Arctic food webs
- Mercury levels are rising in polar bears, seals, and fish despite falling air pollution
- Stable isotope analysis reveals regional and trophic-level differences in mercury uptake
- Findings challenge assumptions behind global mercury reduction policies
In a sweeping 40-year study across Greenland, scientists have uncovered a disturbing paradox: while mercury emissions from the atmosphere have decreased in recent decades, Arctic marine life is accumulating more mercury than ever before. Published in Nature Communications, this landmark research pinpoints ocean currents—not the air—as the dominant source of mercury now poisoning Arctic ecosystems.
Using sophisticated mercury stable isotope analysis, researchers tracked how different forms of mercury moved through marine and freshwater food webs, peat deposits, and predator muscle tissue. They found that areas influenced by North Atlantic currents—like central-western Greenland—carried significantly different mercury isotope signatures than areas exposed to currents from the Arctic Ocean. These differences persisted across fish, seals, peat cores, and even polar bears.
“This shows that legacy mercury, already deposited decades ago, is now making its way back into ecosystems through the ocean,” said lead author Jens Søndergaard. “It explains why Arctic animals show rising mercury loads even as atmospheric pollution declines.”
The implications are enormous. Mercury is a potent neurotoxin that bioaccumulates up the food chain, particularly affecting top predators like polar bears and the Inuit populations who rely on marine mammals for sustenance. The research found that mercury levels in marine mammal livers have increased significantly since the 1980s, even exceeding known toxic thresholds.
The isotope data also revealed that mercury is being transformed through biological and environmental processes. In freshwater species like Arctic char, intense photochemical demethylation—a process driven by sunlight—creates distinctive mercury fingerprints. In higher predators, isotope differences between liver and muscle tissues reflect internal metabolic transformations as the animals try to detoxify their food.
But perhaps most striking is the geographic fingerprint. While air currents distribute mercury evenly across vast distances, ocean currents show stark regional differences. Central West Greenland, bathed by the Irminger Current and Atlantic waters, had higher δ202Hg values and lower total mercury concentrations than Arctic Ocean-fed regions like Qaanaaq and Ittoqqortoormiit.
This suggests that long-term mercury storage in the ocean—some of it centuries old—is being slowly re-released and funneled northward, where it re-enters the food chain. Sea spray and evaporation also contribute to reintroducing mercury into Arctic atmospheres, further blurring the lines between past and present contamination.
“The ocean acts like a time capsule for pollution,” explained co-author Rune Dietz. “What was emitted a hundred years ago is still poisoning wildlife today.”
The findings raise critical questions about the global fight against mercury pollution. The UN’s Minamata Convention, which aims to reduce mercury emissions, may need to account for these delayed effects if Arctic biota continues to suffer from long-buried contaminants.
With ocean mercury turnover times exceeding 300 years, the Arctic may be trapped in a toxic loop for decades or even centuries—regardless of how quickly we clean up our skies.
Source: Nature Communications
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