changing chemistry of the Baltic Sea

May 6, 2024

Above: Assistant Professor of Geology & Geophysics Chad Ostrander stands in front of the Elisabeth Mann Borgese research vessel.

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.

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

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, using stable isotope analysis, 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.

“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 Tl than predicted. This enrichment started around 1940 to 1947 according to the longer sediment core.

Read the full press release from Woods Hole Oceanographic Institution here.

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