Mercury fingerprint of Pacific fish points to Asian coal plants
Sunlight breaks down the kind of mercury that’s dangerous on the sushi platter: monomethylmercury, according to a new study. That makes species in shallower waters, such as yellowfin tuna and mahi mahi, a safer dining bet, researchers say.
Los Angeles Times
LOS ANGELES — Mercury found in high levels in deep Pacific Ocean fish such as swordfish has a chemical fingerprint, and it implicates coal-burning power plants in Asia, according to a new study.
A research team from the universities of Hawaii and Michigan looked at mercury in the flesh of nine species common to the North Pacific Subtropical Gyre, the largest ecosystem on the planet, at 7 million square miles.
Four years ago, the team found that mercury levels in such fish as tuna increased with the depth of the fish’s habitat. This time, the researchers set out to find out why, and what forces were at play. On the way, however, they found evidence implicating coal. The results were published online in Nature Geoscience.
Sunlight breaks down the kind of mercury that’s dangerous on the sushi platter: monomethylmercury, according to the study. That makes shallower species, such as yellowfin tuna and mahi mahi, a safer dining bet, suggested Brian Popp, a University of Hawaii geochemist who was part of the research team.
“If you are concerned about high levels of mercury in fish, consume those that are feeding shallowest in the water,” Popp said.
But different processes also change the ratio of isotopes of mercury; chemists call it fractionation. The sunlight-driven (photochemical) processes in shallower water result in one isotope ratio, while processes driven by living things — microbes, for instance — leave another fingerprint. Comparing those ratios and doing a bit of calculation helped the team figure out the “fingerprint” of the mercury before these processes came into play. And that mercury fingerprint looked suspiciously like the mercury that comes from the atmosphere.
“People have speculated that the main source of mercury to the ocean is through atmospheric deposition, but people have also argued that it comes from sediments, hydrothermal activity, coastal sediment that’s moved from the coast to the open ocean. Our work suggests the main source is from the atmosphere,” Popp said.
Prevailing winds carry atmospheric mercury from Asia, where countries such as China have greatly increased use of coal to generate electricity.
Further bolstering the role of Asian coal burning are previous studies that have shown drops in organic mercury levels in deep Atlantic fish — in areas adjacent to countries where mercury emissions have been reduced.