Hydrothermal vents on the ocean floor release mercury into the sea, geologists have found. The finding raises questions about whether this "natural pollution" contaminates fish.
Mark Hannington of the Geological Survey of Canada, Ottawa, and his colleagues were studying water and rock samples gathered last year from hydrothermal vents in the Bay of Plenty off the coast of New Zealand. Analysis of the samples showed that the vents release mercury from sediments containing pure mercury and mercury sulphides (Geology, vol 27, p 931). The team calculates that each hot spring releases up to a kilogram of mercury each year.
Hannington's colleague Bob Garrett points out that these hot springs are home to bacteria that turn mercury into its organic form, methyl mercury, which is easily taken up by fish. "It starts moving up the food chain, eventually reaching the top predators," says Garrett. In humans, methyl mercury damages the nervous system and in extreme cases is lethal (New Scientist, 6 February, p 4).
It is still unclear how much mercury from the New Zealand vents enters the food chain. But in the 1970s, Japanese fishermen abandoned tuna fishing in the region when fish there turned out to contain inexplicably high levels of mercury. The mercury probably came from the seafloor vents.
It is also unclear how many such springs there are worldwide and how much mercury lies on the seafloor. "But it could be locally significant," warns Garrett.
Author: Jeff Hecht
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