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Scientists from California have linked rising levels of methylmercury
in their state's coastal fog to upwelling from deep ocean water.
Lead researcher Peter Weiss-Penzias
, an environmental toxicologist at the University of California, Santa Cruz, was quick to point out that the amount of the toxic mercury compound detected in coastal fog is not a direct public health concern.
"These are parts-per-trillion levels, so when we say elevated, that's relative to what was expected in atmospheric water," he said. "The levels measured in rain have always been fairly low, so the results from our first measurements in fog were surprising."
The team first recorded the elevated mercury levels in coastal fog back in 2011 and published their findings in the February 2012 edition of the journal Geophysical Research Letters
. The paper noted that the mercury levels detected in coastal fog were higher than those found in rain water and the upwelling of ocean water could be responsible.
In the summer of 2012, the team collected more samples of coastal fog along with water samples at different depths in Monterey Bay.
Weiss-Penzias said the new results, which were presented earlier this week at the American Geophysical Union
meeting in San Francisco, suggest an upwelling mechanism that facilitates the passage of methylmercury into coastal fog through evaporation.
While understanding the methylmercury mechanism is a priority, Weiss-Penzias said that the presence of a highly volatile compound called dimethylmercury is also cause for concern.
"Dimethylmercury is more stable in the deep ocean, but we're not quite sure how it forms or where it's coming from," he said. "We found elevated levels in the surface water during upwelling, and it readily evaporates from the surface into the atmosphere, where it decomposes into monomethylmercury and gets into fog droplets."
The team's results so far have relied heavily on the efforts of undergraduate researchers like Cruz Ortiz, who is continuing the group's work by looking at mercury concentrations in coastal-dwelling insects that typically encounter fog. His early data sets show higher concentrations of mercury during the summer, when coastal fog is more common than in late winter or early spring.
"There is so little data on this now, we're just trying to fill in the map to improve our understanding of the cycling of mercury in the environment," Weiss-Penzias said. "We want to know its sources and sinks and transformations."
Mercury is generally released into the environment through human activities, including the burning of fossil fuels and gold mining operations. Once the metal is in the soil, bacteria there transforms it into methylmercury compounds that are particularly toxic and eagerly absorbed by organisms.
While environmental methylmercury is not much of a threat to humans, it becomes increasingly concentrated in organisms as it moves up the food chain, resulting in some predatory fish accumulating more toxic amounts.
The forces driving the contamination of ocean fish have been accumulating in the oceans since the industrial revolution began in the 19th century. The California researchers theorize that the mercury that moves from ocean waters into coastal fog is probably the result of the historical mercury pollution coming to the surface via upwelling.