This satellite image shows an
aerosol cloud that presumably developed over the Asian
continent and is moving over the North Pacific. A survey
of North Pacific waters conducted a year after this image
was taken showed the highest levels of silver contamination
ever measured in the open ocean.
March 14, 2005
Survey finds silver contamination in North
Pacific waters, probably from industrial emissions in Asia
By Tim Stephens
The highest levels of silver contamination ever observed in
the open ocean turned up in samples collected during a survey
of the North Pacific in 2002. UCSC researchers measured silver
concentrations 50 times greater than the natural background
Researchers prepare a sampling bottle to collect water samples
at a specific depth during a survey of the North Pacific
conducted in the summer of 2002.
Photo: M. Ranville
Though still well below levels that would be toxic to marine
life, this contamination of what had been considered relatively
pristine waters highlights the increasingly global impact of
industrial emissions from Asia, the researchers said.
"The most likely source of the silver contamination is
atmospheric emissions from coal burning in Asia," said
Russell Flegal, professor of environmental toxicology. "Silver
concentrations in the North Pacific trace the atmospheric depositions
of industrial aerosols from Asia, with the highest concentrations
in those waters closest to the Asian mainland."
Mara Ranville, a researcher in Flegal's lab who earned her
Ph.D. in December, collected the samples on a 35-day cruise
in the summer of 2002 as part of the Global Investigation of
Pollution in the Marine Environment (GIPME), a program of the
UNESCO Intergovernmental Oceanographic Commission. Ranville
and Flegal reported their findings in the March 9 issue of Geochemistry,
Geophysics, Geosystems, an electronic journal published
by the American Geophysical Union and the Geochemical Society.
Ranville found silver concentrations as high as 1.2 parts per
trillion in samples of North Pacific surface waters taken during
the cruise. This is about 50 times higher than baseline levels
of silver in uncontaminated waters sampled by UCSC researchers
on a previous cruise in the Atlantic Ocean. Because the natural
background level is so low, scientists may be able to use silver
as a tracer element for tracking the fate of industrial emissions
from Asia, Ranville said.
"Atmospheric pollution from Asia is becoming a serious
problem for the western United States, and this may be a valuable
tool for tracing those emissions, both in the atmosphere and
in the water," she said.
Prevailing westerly winds carry atmospheric pollution across
the North Pacific ocean from China, where coal is the primary
fuel and the use of emissions controls is limited. Major pollutants
from the burning of coal are nitrogen and sulfur compounds that
contribute to acid rain and smog. Previous studies by Flegal's
group have also found high mercury levels in rainfall on the
U.S. West Coast, which they linked to contamination from coal
burning in Asia.
"Unlike mercury, silver is not a human health concern.
But silver is second only to mercury in its toxicity to marine
invertebrates," Flegal said.
Sam Luoma, a U.S. Geological Survey scientist who has worked
with Flegal to study heavy metal contamination in San Francisco
Bay, said the elevated silver concentrations in the open ocean
suggest there may be "hotspots" in Asian coastal waters
where the concentrations reach toxic levels.
"The open ocean is vastly diluted, so there may be some
massive hotspots of silver contamination around the Asian continent.
That's where effects on marine organisms would occur,"
In San Francisco Bay, silver and copper from industrial discharges
reached such high levels in the 1970s and early 1980s that clams
stopped reproducing and some species of invertebrates disappeared
from mudflats, he said. At that time, silver concentrations
in the bay were 100 times higher than the levels Ranville and
Flegal found in the open ocean. With stricter pollution controls,
however, silver has now declined to around 6 parts per trillion
in bay waters, and the affected invertebrates have recovered,
The North Pacific survey completes a preliminary survey of
silver throughout the world's oceans. The goal was to establish
current levels of contamination as a baseline for future studies,
"We found by far the highest levels of silver in the North
Pacific. Even though the Atlantic Ocean is subject to industrial
emissions from North America and Europe, the amounts are small
compared to what goes into the Pacific from Asia," he said.
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