Mercury isotopes uncover climate-driven exposure pathways in Arctic food webs

The Arctic is warming faster than anywhere else on Earth, a shift that is melting sea ice, thawing frozen soil, and altering ocean currents. These physical changes are not just reshaping the landscape; they are also remobilizing ancient pollutants trapped in the ice and soil for centuries. One of the most concerning of these pollutants is mercury, a toxic metal that enters the food chain and accumulates in the bodies of animals and people. While global efforts have successfully reduced the amount of mercury released into the atmosphere from factories and mines, the levels of this toxin in Arctic wildlife are not dropping as expected. In some places, they are actually rising. This discrepancy has left scientists puzzled: if the air is getting cleaner, why are the animals getting sicker? The answer lies in understanding how mercury moves through the environment, how it changes form, and how the changing climate is opening new doors for this poison to enter the food web.

To solve this mystery, researchers turned to a unique tool: the chemical fingerprints left behind by mercury atoms. Mercury exists as several slightly different versions, called isotopes, which act like distinct tags. Some of these tags are altered when mercury is exposed to sunlight, while others change when it is processed by plants or bacteria. By measuring these subtle differences in the muscle tissue of ringed seals, a key species found throughout the Arctic, scientists can trace exactly where the mercury came from and what path it took before it reached the animal. This study, the first of its kind to compare these isotopic signatures across the entire Arctic region, reveals that the source of the mercury is not uniform. Instead, it depends heavily on whether the seal lives in an area dominated by land runoff or ocean currents.

The researchers analyzed muscle samples from 232 ringed seals collected between 2014 and 2024 from eight different locations, ranging from Alaska and Canada to Greenland and Svalbard. They found a striking divide in the data. Seals living in the Canadian High Arctic and northern Greenland carried the highest levels of mercury. These animals also showed a specific chemical signature indicating that the mercury had spent time on land. It had been deposited from the air onto the tundra, absorbed by plants and frozen into the soil, and then washed into the ocean by rivers as the permafrost thawed. Because this mercury had been processed by the land and had not been exposed to much sunlight in the water, its chemical tags remained distinct. In contrast, seals from West Svalbard and parts of Greenland had much lower mercury levels. Their chemical signatures told a different story: this mercury had traveled long distances through the ocean, carried by warm currents flowing north from the Atlantic. This oceanic mercury had been exposed to more sunlight and processed differently before entering the food web.

The study explicitly rules out the idea that these differences are caused by what the seals eat. While diet can influence chemical signatures, the patterns observed were too large and too consistent with ocean geography to be explained by food choices alone. The researchers also found that the seals in the high Arctic were not simply eating more mercury because they were higher up the food chain; the source of the mercury itself was different. The high levels in the Canadian and Greenlandic seals are driven by the release of legacy mercury stored in the soil, a process accelerated by climate change. As the ground thaws and rivers flow more strongly, this stored poison is flushed into the coastal waters, where it is quickly taken up by tiny organisms and passed up to the seals.

Looking ahead, the researchers used computer models to simulate how the Arctic Ocean might change by the end of the century. These simulations suggest that the situation could worsen. As the climate continues to warm, the thawing of permafrost will likely release even more mercury from the land into the rivers. At the same time, changes in ocean circulation, such as a weakening of the great currents that currently keep some areas fresh, could alter how this mercury is distributed. If the ocean currents shift, areas that are currently safe might begin to receive higher loads of mercury from the land, while other regions might see different patterns of exposure. The study suggests that even if humanity stops all new mercury emissions today, the climate-driven release of old, stored mercury will continue to threaten wildlife and the Indigenous communities that rely on them for food.

This research highlights a critical gap in our understanding of environmental health. It shows that reducing emissions is only half the battle; the climate itself is now acting as a secondary source of pollution, unlocking reservoirs of toxins that have been locked away for generations. The ringed seal, a species that has long served as a sentinel for the health of the Arctic, is now telling a story of a changing world where the past is catching up to the present. By tracking these chemical fingerprints, scientists can now predict where the next waves of contamination might hit, offering a vital tool for protecting both the fragile Arctic ecosystem and the people who call it home.