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 new study in Environmental Science and Technology finds that human activity—particularly since the 1940s—is likely behind these elevated levels.
The Baltic Sea is euxinic, meaning the water both lacks oxygen and has high levels of hydrogen sulfide. Such conditions, more common in the ancient ocean, are present in the Baltic Sea today, partly because of its isolated geography. But anthropogenic nutrient pollution from sources including wastewater treatment facilities and agricultural and urban runoff is playing an increasingly large role in creating euxinic conditions: These runoff nutrients cause excess growth of algae and plants, which use up the oxygen in the water and can cause other marine life to die off, creating what’s known as a dead zone.
Geologists often use waters with euxinic conditions as a proxy for the chemistry of ancient oceans. But when Chad Ostrander, then a postdoctoral researcher with the Woods Hole Oceanographic Institution, took water samples from the Baltic, his project took an unexpected turn into modern history.
“Some human activity is changing fundamentally the amount of thallium that’s coming in.”
“The thallium isotope fingerprint of the water column was off,” said Ostrander, now a geologist at the University of Utah. “It wasn’t what we expected it to be.”
Thallium has two stable isotopes. In the Baltic and in similarly euxinic conditions in the Black Sea and the Cariaco Trench in the Caribbean, the ratio of the two isotopes is the same in the water column and the sediments below. But when Ostrander compared the Baltic samples to those other places, he found more thallium than expected, a result he found “curious.”
Ostrander and his colleagues next looked at a Baltic Sea sediment core that had been reliably dated to about 1850 and found that the high thallium isotope ratio emerged sometime around 1940–1947.
“What they decided, and I think correctly, is that this represents increased influence from humans,” said Tim Lyons, a biogeochemist at the University of California, Riverside, who has studied the Black Sea and California’s Salton Sea but was not involved in the current work. “Some human activity is changing fundamentally the amount of thallium that’s coming in.”
Concrete Contributions
Industrial processes, including coal combustion, pyrite roasting, and concrete production, are all sources of thallium, the authors note. During the late 1940s, massive rebuilding efforts in Europe following the destruction of World War II included a huge increase in the demand for concrete.
“There’s a couple of lines of evidence that suggest that concrete production in the area, which seemed to increase in the 1940s and we know releases a lot of thallium, could be a strong culprit” behind today’s thallium levels, Ostrander said.
Thallium is highly toxic to mammals and has the potential to accumulate in fish such as lake trout. From there, it could enter the human food chain and poison people. But Ostrander notes a “silver lining” of sorts: Though thallium levels are higher than anticipated, they are still relatively low, thanks to the rare euxinic conditions of the Baltic Sea.
“The Baltic Sea is actually doing a very good job of stripping thallium from the water column” and storing it in sediment, Ostrander said. Though thallium can be highly soluble and mobile, the sulfidic conditions of the Baltic change that by making the metal insoluble. “So the euxinic nature of the Baltic Sea actually helps in this case.”
A Delicate Balance
“That nasty human consequence of fertilizer runoff is actually stripping thallium from the water column.”
This geochemistry presents a challenge to scientists and policymakers who are considering ways to address Baltic Sea nutrient pollution, which is a driving force behind these euxinic conditions. Although pollution sources, including fertilizer runoff, are contributing to an aquatic dead zone of increasing concern to scientists, “that nasty human consequence of fertilizer runoff is actually stripping thallium from the water column,” Lyons said.
This means well-intentioned efforts to stop the spread of this dead zone, such as pumping oxygen into the Baltic Sea, could have catastrophic consequences.
“If that’s an approach that is taken in the future, to better oxygenate the Baltic Sea, that could bring a lot of thallium into the water column,” Ostrander said. Copper is another element that concentrates in sulfide minerals. Reducing the Baltic Sea’s anoxia could ultimately release it, too. Ostrander said he hopes governments and other entities working on the Baltic have thallium on their list of concerns as they consider how to improve water quality.
Lyons said the new paper makes strong use of the Baltic as a barometer for human impact on the oceans, given that the Baltic’s semi-isolated conditions make tracking local inputs easier than in the open ocean. But, he added, the delicacy of how to clean up agricultural runoff without unleashing a different problem could make the Baltic Sea a “a canary in a coal mine.”
—Amy Mayer (@AmyHMayer), Science Writer
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