Toxic metal on the rise in the Baltic Sea

Thallium, one of the most toxic heavy metals on Earth, is more abundant in the Baltic Sea than in waters with otherwise similar chemistry. A A new study Inside Environmental Science and Technology It has found that human activity—especially since the 1940s—may be behind these elevated levels.

The Baltic Sea is euxinic, meaning the water lacks oxygen and contains high levels of hydrogen sulfide. Such conditions, more common in the ancient sea, exist in the Baltic Sea today, partly because of its isolated geography. But anthropology Nutritional pollution Sources including wastewater treatment facilities and agricultural and urban runoff play an increasingly large role in creating euxinic conditions: these runoff nutrients cause excessive growth of algae and plants, which use up oxygen in the water and other marine life die. , creating what is known as the dead zone.

Geologists use waters with euxinic conditions as a proxy for the chemistry of ancient oceans. But when Chad OstranderThen a postdoctoral researcher with the Woods Hole Oceanographic Institution, taking samples of Baltic waters, his project took an unexpected turn in modern history.

„Some human functions are altered by thallium levels.”

„The thallium isotope fingerprint of the water column is off,” said Ostrander, now a geologist at the University of Utah. „It’s not what we expected.”

Thallium has two stable isotopes. Under similar euxinic conditions in the Cariago Trench in the Baltic and Black Seas and the Caribbean, the ratio of the two isotopes is the same in the water column and in the bottom sediments. But when Ostrander compared the Baltic samples with those from other locations, he found more thallium than expected, a result he found „with interest.”

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Ostrander and his colleagues next looked at a Baltic Sea sediment core that was reliably dated to around 1850 and found that the high thallium isotope ratio appeared in 1940-1947.

„What they concluded, I think correctly, is that it indicates an increased influence from humans,” he said Tim Lyons, a biogeochemist at the University of California, Riverside, who studied the Black Sea and California’s Salton Sea but was not involved in the current work. „Some human functions are altered by thallium levels.”

Concrete contributions

Industrial processes including coal combustion, pyrite roasting and concrete production are all sources of thallium, the authors note. In the late 1940s, massive reconstruction efforts in Europe following the devastation of World War II included a huge increase in demand for concrete.

„There is some evidence that seems to have increased concrete production in the region in the 1940s, and a lot of thallium was released, which could be a strong culprit,” Ostrander said.

A model collects water from the Baltic Sea during the Rosette 2021 research mission. Credit: Colin Hansel

Thallium is highly toxic to mammals and can accumulate in fish such as lake trout. From there, it enters the human food chain and poisons people. But Ostrander notes a „silver lining”: Although thallium levels are higher than expected, thanks to the rare euxinic conditions of the Baltic Sea, they are relatively low.

„The Baltic Sea really does a good job of removing thallium from the water column and storing it in the sediment,” Ostrander said. Although thallium is highly soluble and mobile, the Baltic sulfidic conditions render the metal insoluble. „So the euxinic nature of the Baltic Sea actually helps in this regard.”

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A subtle presence

„The worst human effect of fertilizing is actually removing thallium from the water column.”

This geochemistry presents a challenge to scientists and policymakers considering ways to address nutrient pollution in the Baltic Sea, a driving force behind these euxinic conditions. While pollution sources, including fertilizer runoff, contribute to the aquatic dead zone of increasing concern to scientists, „the worst human effect of fertilizer runoff is actually removing thallium from the water column,” Lyons said.

These are well-intentioned efforts to prevent the spread of this dead zone Injects oxygen into the Baltic SeaCan have disastrous consequences.

„If this is an approach to oxygenation in the Baltic Sea in the future, it could bring a lot of thallium into the water column,” Ostrander said. Copper is another element concentrated in sulfide minerals. Reducing the anoxia of the Baltic Sea may eventually release it as well. As governments and other organizations working in the Baltic consider how to improve water quality, Ostrander said he believes thallium is high on their list of concerns.

The new paper makes strong use of the Baltic as a barometer for human impact on the oceans, Lyons said, noting that the Baltic’s semi-isolated conditions make it easier to track local inputs than the open ocean. But the delicacy of how to clean up agricultural water without unleashing another problem could make the Baltic Sea „a canary in the coal mine,” he added.

— Amy Meyer (@AmyHMayer), science writer

Quote: Mayer, A. (2024), Increasing toxic metal concentrations in the Baltic Sea, EOS, 105, https://doi.org/10.1029/2024EO240263. Published on 17 June 2024.
Text © 2024. Authors. CC BY-NC-ND 3.0
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