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 esearch scientists at Scripps Institution of Oceanography
at the University of California, San Diego, have identified salt
marshes as a major natural source of the environmentally- and
economically-important compound methyl bromide. The study, which
also implicates salt marshes as a source of methyl chloride,
is published in the Jan. 20 issue of the journal Nature.
 Methyl
bromide is produced naturally from oceans and plants on land.
But it is also widely manufactured around the world because of
its effectiveness as a pesticide against insects, nematodes,
weeds, pathogens and rodents. Methyl bromide also is generated
as a byproduct of leaded fuel combustion and vegetation burning.
The US Environmental Protection Agency (EPA) estimates that 72,000
tons of methyl bromide are used around the world each year. Scientists
estimate that about half escapes into the atmosphere. In the
United States, the EPA estimates about 21,000 tons are used annually
in agriculture, commodity and quarantine treatment, and structural
fumigation.
Because
it is considered a significant ozone-depleting substance, governments
have developed controls that limit methyl bromide production.
Scientists
have estimated that 20 percent of the methyl bromide that reaches
the atmosphere can be attributed to fumigation, about 10 percent
to vegetation burning, and roughly 30 percent to production from
the oceans. But the balance of this methyl bromide "budget,"
a significant 40 percent, was missing. The new study uncovers
about 10 percent of the absent budget.
Working
in this area as environmental "accountants," Scripps
geochemists Robert Rhew, Benjamin Miller, and Ray Weiss looked
to salt marshes for part of the missing portion. Conducting a
yearlong study at Mission Bay Marsh in San Diego and San Dieguito
Lagoon in Del Mar, Calif., the researchers documented significant
amounts of methyl bromide and methyl chloride released to the
atmosphere.
Although
they make up only 0.1 percent of the earth's surface, salt marshes
may be responsible for producing approximately a surprising 10
percent of the total methyl bromide and methyl chloride budget,
the study indicates. As a result, salt marshes may constitute
Earth's largest natural terrestrial source of methyl bromide
and methyl chloride. "Scientists suspected that there was
a large natural terrestrial source, based on evidence from ocean
cruises and computer models, but the source proved to be somewhat
elusive," said Rhew, a graduate student in the Geosciences
Research Division at Scripps. "We found that salt marshes
emit methyl bromide at rates greater than any other natural environment,
on a per area basis. We're getting much closer to nailing down
the methyl bromide budget, and this study adds a significant
piece to the puzzle."
Future
studies will seek to expand these findings and correspond the
data to other salt marshes and different environments, such as
mangrove forests. Research for the study was funded by the University
of California Natural Reserve System, the National Science Foundation
Graduate Research Fellowship Program, the Methyl Bromide Global
Coalition, and NASA's Upper Atmosphere Research Program. 
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Scripps
Institution of Oceanography, at the University of California,
San Diego, is one of the oldest, largest, and most important
centers for global science research and graduate training in
the world. The National Research Council has ranked Scripps first
in faculty quality among oceanography programs nationwide. The
scientific scope of the institution has grown since its founding
in 1903 to include biological, physical, chemical, geological,
geophysical, and atmospheric studies of the earth as a system.
More than 300 research programs are under way today in a wide
range of scientific areas. The institution has a staff of about
1,300, and annual expenditures of approximately $100 million,
from federal, state, and private sources. Scripps operates the
largest academic fleet, with four oceanographic research ships |
