(Washington,
D.C. – May 1, 2009) A new landmark study published today
documents for the first time the process in which increased mercury emissions
from human sources across the globe, and in particular from Asia, make their way
into the North Pacific Ocean and as a result
contaminate tuna and other seafood. Because much of the mercury that enters the
North Pacific comes from the atmosphere, scientists have predicted an additional
50 percent increase in mercury in the Pacific by 2050 if mercury emission rates
continue as projected.
“This unprecedented USGS study is critically
important to the health and safety of the American people and our wildlife
because it helps us understand the relationship between atmospheric emissions of
mercury and concentrations of mercury in marine fish,” said Secretary of the
Interior Ken Salazar. “We have
always known that mercury can pose a risk, now we need to reduce the mercury
emissions so that we can reduce the ocean mercury levels.”
“This study
gives us a better understanding of how dangerous levels of mercury move into our
air, our water, and the food we eat, and shines new light on a major health
threat to Americans and people all across the world,” said EPA Administrator
Lisa P. Jackson. “With this
information in hand, plus our own
mercury efforts, we have an even
greater opportunity to
continue working with our
international partners to significantly cut mercury pollution in the years ahead
and protect the health of millions of people.”
Water sampling cited in
the study shows that mercury levels in 2006 were approximately 30 percent higher
than those measured in the mid-1990s.
This study documents for the first time the formation of methylmercury in
the North Pacific Ocean. It shows that methylmercury is produced
in mid-depth ocean waters by processes linked to the “ocean rain.” Algae, which
are produced in sunlit waters near the surface, die quickly and “rain” downward
to greater water depths. At depth,
the settling algae are decomposed by bacteria and the interaction of this
decomposition process in the presence of mercury results in the formation of
methylmercury. Many steps up the
food chain later, predators like tuna receive methylmercury from the fish they
consume.
One unexpected
finding from this study is the significance of long-range transport of mercury
within the ocean that originates in the western Pacific Ocean, off the coast of
Asia.
“Mercury researchers typically
look skyward to find a mercury source from the atmosphere due to emissions from
land-based combustion facilities. In this study, however, the pathway of the mercury was a little
different. Instead, it appears the recent mercury enrichment of the
sampled Pacific Ocean waters is caused by
emissions originating from fallout near the Asian coasts. The mercury-enriched
waters then enter a long-range eastward transport by large ocean circulation
currents,” said USGS scientist and coauthor David Krabbenhoft.
Scientists sampled Pacific
Ocean water from 16 different sites between Honolulu, Hawaii and
Kodiak, Alaska.
In addition, the scientists constructed a computer simulation that links
atmospheric emissions, transport and deposition of mercury, and an ocean
circulation model.
In the United
States, about 40 percent of all human exposure to mercury
is from tuna harvested in the Pacific Ocean,
according to Elsie Sunderland, a
coauthor of the study. Methylmercury is a highly toxic form of mercury
that rapidly accumulates in the food chain to levels that can cause serious
health concerns for those who consume the seafood. Pregnant women who consume
mercury can pass on life-long developmental effects to their children. That is
why in 2004 EPA and FDA issued the landmark Joint Guidance on the Consumption of
Fish specifically targeted towards pregnant women and nursing
mothers. Previous studies show that 75 percent of
human exposure worldwide to mercury is from the consumption of marine fish and
shell fish.
Scientists have known for some time that mercury deposited
from the atmosphere to freshwater ecosystems can be transformed (methylated)
into methylmercury, but identifying the analogous cycles in marine systems has
remained elusive. As a result of this study we now know more about how the
process which leads to the transformation of mercury into methylmercury.
The paper, “Mercury
sources, distribution and bioavailability in the North Pacific Ocean--Insights
from data and models and information on other USGS mercury research,” is
available at:
http://toxics.usgs.gov/highlights/pacific_mercury.htmlThe
study appeared today in Global Biogeochemical Cycles, which is published by the
American Geophysical Union. In addition to USGS mercury expert David
Krabbenhoft, the authors include Elsie Sunderland, Harvard University; John Moreau, University of Melbourne, Australia (until recently a USGS, NRC
Post Doctoral Candidate); William Landing, Florida State University; and Sarah Strode, Harvard University.