EOS - 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.
“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 a higher isotope ratio 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.”