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Human Activity Is Causing Toxic Thallium to Enter the Baltic Sea, According to New Study

Sampling Rosette The sampling rosette is deployed from the deck of the RV Elisabeth Mann Borgese into a deep basin of the Baltic Sea. (Photo credit: Colleen Hansel, ©Woods Hole Oceanographic Institution)

May 2, 2024

Woods Hole, Mass. -- Human activities account for a substantial amount - anywhere from 20% to more than 60% - of toxic thallium that has entered the Baltic Sea over the past 80 years, according to new research by scientists affiliated with the Woods Hole Oceanographic Institution (WHOI) and other institutions.

Currently, the amount of thallium (element symbol TI), which is considered the most toxic metal for mammals, remains low in Baltic seawater. However, the research suggests that the amount of thallium could increase due to further anthropogenic, or human induced, activities, or due to natural or human re-oxygenation of the Baltic that could make the sea less sulfide rich. Much of the thallium in the Baltic Sea, the largest human-induced hypoxic area on Earth, accumulates in the sediment thanks to abundant sulfide minerals.

“Anthropogenic activities release considerable amounts of toxic thallium annually. This study evidences an increase in the amount of thallium delivered by anthropogenic sources to the Baltic Sea since approximately 1947,” according to the journal article, “Anthropogenic forcing of the Baltic Sea thallium cycle,” published in Environmental Science & Technology.

Chad Ostrander

Chad Ostrander, lead author of the study, preparing a short sediment core collected from the East Gotland Basin during the investigation. (Photo credit: Colleen Hansel, ©Woods Hole Oceanographic Institution)

“Humans are releasing a lot of thallium into the Baltic Sea, and people should be made aware of that. If this continues –  or if we further change the chemistry of the Baltic Sea in the future or if it naturally changes – then more thallium could accumulate. That would be of concern because of its toxicity,” said Chadlin Ostrander lead author of the article, which he conducted as a postdoctoral investigator in WHOI’s Department of Marine Chemistry and Geochemistry. Currently, he is an assistant professor in the Department of Geology & Geophysics at the University of Utah.

For the study, the researchers set out to better understand how thallium and its two stable isotopes 203Tl and 205Tl are cycled in the Baltic Sea. To discern modern thallium cycling, concentration and isotope ratio data were collected from seawater and shallow sediment core samples. To reconstruct thallium cycling further back in time, the researchers supplemented their short core samples with a longer sediment core that had been collected earlier near one of the deepest parts of the sea. They found Baltic seawater to be considerably more enriched in 205Tl than predicted. This enrichment started around 1940 to 1947 according to the longer sediment core.

It would be “highly coincidental” if the 205Tl increase was not associated with the “nearly contemporaneous trends linked to anthropogenic activities,” the article states. Though the exact sources of the thallium increase are not yet known, the article indicates that regional cement production, which was enhanced after the end of World War II, may play an important role, with other possible sources including coal combustion and the roasting of pyrite, an iron sulfide.

Early morning cast

An early-morning cast, to collect water samples and make in-situ measurements, during the Baltic Sea investigation. (Photo credit: Colleen Hansel, ©Woods Hole Oceanographic Institution)

“For me, the most important aspect of the study is that we basically discovered that large portions – if not most – of the Baltic Sea are contaminated with the toxic metal thallium from human activities surrounding the basin,” said co-author Sune Nielsen, an adjunct scientist in WHOI’s Department of Geology & Geophysics. “As far as I am aware, this constitutes the geographically most extensive area of thallium contamination ever documented. It has long been known that the Baltic Sea has been strongly affected by anthropogenic activity, not least via the increasingly persistent loss of oxygen that has led to big losses for the fishing industry over the last several decades. As a Danish national, I follow the (bad) news about the Baltic in the Danish media, and our finding just adds another dimension to the already poor conditions in the basin for marine life. While thallium contamination may not be the most immediate concern for the Baltic Sea ecosystem, there is no doubt in my mind that it adds to the urgency of needing to do something to bring the Baltic Sea back to a state where humans and marine life can co-exist naturally.”

“Our data strengthens evidence that the removal of thallium from seawater and storage within sediments is tightly controlled by the absence of oxygen and presence of sulfide,” said co-author Colleen Hansel, a senior scientist in WHOI’s Department of Marine Chemistry and Geochemistry. “It is therefore concerning that recent movements to ‘solve the anoxia problem’ in the Baltic Sea involve pumping oxygen into the bottom waters. This oxygenation of the Baltic will likely lead to the release of thallium, as well as other sulfide-hosted metals like mercury, into the overlying seawater where they could bioaccumulate to toxic levels in fish. We predict, based on activities in the region, that the source of the thallium pollution is historic cement production in the region. As cement production continues to rise globally, this research could serve to caution manufacturers about the need to mitigate potential downstream effects of cement kiln dust on surrounding aquatic and marine ecosystems. This study highlights the utility of isotopes in identifying sources of pollutants to marine ecosystems, which is difficult to disentangle with concentration data alone.”

Funding for this research was provided by the U.S. National Science Foundation and by the Leibniz Association.


Authors:  Chadlin M. Ostrander1,2,†,*, Yunchao Shu2,3,††, Sune G. Nielsen2,3,†††, Olaf Dellwig4, Jerzy Blusztajn2,3, Heide N. Schulz-Vogt5, Vera Hübner6, Colleen M. Hansel2


1Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

2NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

3Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

4Department of Marine Geology, Leibniz Institute for Baltic Sea Research Warnemünde, IOW, Rostock, Germany

5Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research Warnemünde, IOW, Rostock, Germany

6Microsensor group, Max Planck Institute for Marine Microbiology, Bremen, Germany

Now at Department of Geology & Geophysics, University of Utah, Salt Lake City, Utah, USA

††Now at Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong

†††Now at CRPG, CNRS, Université de Lorraine, 54500 Nancy, France

* Corresponding author

About Woods Hole Oceanographic Institution

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