Mercury pollution at Earth’s surface is leaving a mark on the deepest parts of the ocean. A new study suggests that sinking fish carcasses transport the element to seafloor ecosystems.
“Mercury is not limited to the upper thousand meters of the ocean, as we once thought,” said Joel Blum, a biogeochemist at the University of Michigan in Ann Arbor. Blum and his colleagues scoped out the mercury content of organisms collected from roughly 6,000–10,000 meters below the ocean surface. That mercury contained chemical clues pointing to fish carcasses from shallower waters as its source, researchers reported in Proceedings of the National Academy of Sciences of the United States of America. The carrion provides a mercury “fast track” to the deep ocean, Blum said.
As it moves through the environment, both naturally and released through human activities such as burning fossil fuels and gold mining, mercury can take on different chemical forms. Methylmercury, a potent neurotoxin, can form when inorganic mercury encounters aquatic microbes in lakes, rivers, and the ocean.
Methylmercury tends to accumulate in fish muscle and can lead to high mercury levels in fish that humans consume. Although the deep-sea creatures studied in the new research don’t end up on dinner plates, scientists hope this work will help develop better models of how mercury moves around in the ocean. The research may also help scientists predict and track changes associated with the Minamata Convention on Mercury, an international treaty meant to curb mercury pollution, said Marc Amyot, a biogeochemist at the University of Montreal in Canada who was not involved with the study. “It’s a first step that is very impressive, because it’s so difficult to get these types of samples from the bottom of the ocean,” Amyot said.
Glimpsing Mercury’s Past
To snag the animals for study, submersible vessels equipped with bait and traps traveled to the Pacific’s Mariana and Kermedec Trenches. They caught 25 amphipods and six snailfish. The team measured the total amount of mercury in the animals as well as the distribution of mercury isotopes.
Mercury has seven isotopes, which “do some very particular chemistry that allows you to figure out where the mercury…has recently spent some time,” said Carl Lamborg, a chemical oceanographer at the University of California, Santa Cruz who was not involved with the study. The ratios of different mercury isotopes in a sample can reveal a history of where that mercury has been and the chemical reactions it has experienced.
In the deep-sea amphipods and snailfish, Blum’s research group found an isotopic signature of reactions requiring light that would have had to occur in the sunlit parts of the ocean. “These are critters living at the opposite end of the ocean than where this signal should be coming from,” Lamborg said. The mercury “had to have been mercury that used to be at the surface ocean and now found its way into the deep ocean.”
Because this signal from light-driven reactions matched well with values from deep-sea creatures and upper ocean fish, Blum and his team suggested that those fish may become carrion carrying mercury to the deep sea. Their proposal is an alternative to the idea that mercury sinks to the trenches predominantly in particles, as reported in a July Nature Communications study. Whereas there are enough particles, called marine snow, to account for the amount of mercury dropped into the deep sea, the isotopic signature of mercury in those particles didn’t match those in the trench dwellers, Blum said.
However, the authors of the earlier study said they included carrion along with other organic particles, geochemists Yi Liu and Ruoyu Sun of Tianjin University in China wrote to Eos in an email. Together, the researchers said, the pair of studies point to how pervasive anthropogenic mercury is across the world’s oceans. “This is a very important and timely work with solid data,” Liu and Sun wrote.
Blum, too, had expected that deep in the ocean, his team would find less mercury. Instead, it seems “there is no corner of the Earth or the oceans that has escaped human-derived pollution.”
—Carolyn Wilke (@CarolynMWilke), Science Writer
Correction, 22 December 2020: This article was updated to indicate that methylmercury tends to accumulate in fish muscle, not fish fat.