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GLIN==> UM Research on Tracking Mercury Pollution by Isotope Analysis

Sept. 13, 2007
Contact: Nancy Ross-Flanigan, (734) 647-1853, rossflan@umich.edu


New fingerprinting method tracks mercury in environment

ANN ARBOR, Mich.---With mercury polluting our air, soil and water and 
becoming concentrated in fish and wildlife as it is passed up the 
food chain, understanding how the potent nerve toxin travels through 
the environment is crucial.

A new method developed at the University of Michigan uses natural 
"fingerprints" to track mercury and the chemical transformations it 
undergoes. A report on the work is published today in Science Express.

Mercury is a naturally occurring element, but some 150 tons of it 
enter the environment each year from human-generated sources in the 
United States, such as incinerators, chlorine-producing plants and 
coal-fired power plants. Mercury is deposited onto land or into 
water, where microorganisms convert some of it to methylmercury, a 
highly toxic form that builds up in fish and the animals that eat 
them. In wildlife, exposure to methylmercury can interfere with 
reproduction, growth, development and behavior and may even cause death.

Effects on humans include damage to the central nervous system, heart 
and immune system. The developing brains of young and unborn children 
are especially vulnerable.

Because of such profound and irreversible effects on health and the 
environment, "it's very important to understand how and where mercury 
transforms into its most toxic forms and how it moves around in the 
environment, leading to human and animal exposure," said research 
fellow Bridget Bergquist, who is first author on the paper.

"I have often dreamed of how useful it would be if we could mark 
individual atoms of mercury with an indelible fingerprint of key 
chemical reactions and use this fingerprint to follow them around in 
the environment," said co-author Joel D. Blum, who has been working 
on the problem for more than a decade. "This is precisely what we 
have been able to achieve with the experiments that we're reporting. 
Our work opens the door to an entirely new method for tracing mercury 
pollution and for investigating mercury behavior in the environment 
and in the food chains of humans and other animals."

Bergquist and Blum based their new tracking method on a natural 
phenomenon called isotopic fractionation, in which different isotopes 
(forms) of mercury react to form new compounds at slightly different 
rates, something like bicycle racers in the Tour de France. Some 
riders perform better in the mountainous stages of the race and are 
separated from the pack due to their strength; others distinguish 
themselves on the flat stages of race due to their superior speed. 
With mercury isotopes, it's mass, not athletic ability, that dictates 
their behavior---in one type of isotopic fractionation, at least. In 
this mass-dependent fractionation (MDF), different mercury isotopes 
participate differently in chemical reactions, based on their masses.

"While mass-dependent fractionation is a well-known phenomenon in 
lighter elements and forms the basis for how we determine such things 
as past climates on the Earth and dietary food chains of animals, 
mercury was thought to be too heavy for the signal to show up," said 
Blum, who is the John D. MacArthur Professor of Geological Sciences. 
But in this work, Bergquist and Blum show that mass-dependent 
fractionation can be used to track mercury. Because the process is 
observed naturally in fish as they grow, the mercury the fish excrete 
must have a different isotopic composition than the mercury they take 
in, so MDF may reveal how much mercury fish consume, how much they 
excrete and how it changes during the fishes' lifetimes.

In the current work, the researchers exploited both MDF and another 
type of isotopic fractionation called mass-independent fractionation 
(MIF), in which isotopes segregate based not on absolute mass but on 
whether their masses are odd or even. Bergquist and Blum discovered 
that this type of fractionation occurs only in reactions involving 
sunlight, such as those that take place in surface waters and result 
in methylmercury being detoxified and released to the atmosphere. 
Mass-independent fractionation of mercury and other heavy elements 
had been predicted but never carefully documented in nature.

By combining two methods that provide distinct isotope signatures, 
Bergquist and Blum came up with a tracking tool that is more powerful 
than either one alone.

"We found that fish from a wide range of lakes and from the ocean all 
have large degrees of both mass-independent and mass-dependent 
isotope fractionation," Bergquist said. "So now we're able to use the 
mass-independent isotope signatures to estimate the proportion of 
toxic methylmercury at each location that was detoxified and released 
to the atmosphere by photochemical reactions, and we're also able to 
use the mass-dependent isotope signatures to study the accumulation 
of mercury in fish as they age and grow larger. Together the two 
signatures provide a label that allows us to understand the sources 
of methylmercury to fish and to differentiate fish from different localities."

Using the method in this way illustrates its potential for much wider 
application, Blum said. "One example is a complementary study that we 
reported at a recent scientific meeting." In that study research 
fellow Abir Biswas, working with Bergquist and Blum, found that 
mercury in coals from various coal-producing regions around the world 
vary in their mass-dependent and mass-independent isotopic 
composition. "This suggests that we may be able to use the mercury 
isotope studies to distinguish different sources of mercury to the 
atmosphere, which has far-reaching practical applications," Blum 
said. "In short, this entirely new approach to studying mercury 
sources, mobility and toxicity in the environment paves the way for a 
wide range of studies that should enhance our understanding of this 
important toxin in the environment."

The researchers received funding from the Division of Earth Sciences 
of the National Science Foundation and the Turner Postdoctoral 
Fellowship from the U-M Department of Geological Sciences.

For more information:

Bridget Bergquist: bbergqui@umich.edu

Joel Blum: 

Science Express: http://www.sciencemag.org/sciencexpress/recent.dtl

Basic information on mercury from the EPA: http://eap.gov/mercury/about.htm

National Science Foundation: http://www.nsf.gov/

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