New Study Offers Novel Approach to Mercury Tracking in the Arctic
15.02.2010 - Atmosphere & Space, Water & Oceans, Ice & Snow, Flora & Fauna, Arctic
The results of a study by University of Michigan researchers and published in Nature Geoscience offers a new approach to tracking the movement of mercury through Arctic ecosystems.
While mercury occurs naturally, some 2000 tons of it enter the global environment each year as a result of human activities. When released into the atmosphere in its reduced form, mercury is not very reactive is not a problem in the concentrations at which it naturally occurs. However when it becomes oxidized through reactions with sunlight and often bromine, the mercury becomes very reactive.
Mercury remains in a benign gaseous form during the dark Arctic winter due to the lack of sunlight and little available bromine. However in spring when the sea ice starts to break up, water vapour rises through openings in the ice, carrying bromine from the sea water. Once it enters the atmosphere, the bromine mixed with the oxidizing power of sunlight converts mercury into a reactive form, which then sticks to snowflakes and ice crystals in the air and eventually ends up deposited on the surface of the snow. During this phenomenon, which is known as mercury depletion, the normally steady levels of mercury in the atmosphere quickly drop to near zero, while concentrations of mercury on the surface of the snow rise to extremely high levels.
However the researchers were able to learn that a significant portion of mercury deposited on snowpack returned to the atmosphere as a gas as the sun shone on the snow and reduced the mercury. Any mercury left behind retained a unique isotopic fingerprint, a result of a natural phenomenon called isotopic fractionation, in which different mercury isotopes react to form new compounds at slightly different rates.
In one type of isotopic fractionation, mass-dependent fractionation (MDF), the masses of the isotopes determines the rate of fractionation. In mass-independent fractionation (MIF), the behaviour of the isotopes is determined by whether their masses are odd or even. So by collecting and testing samples, researchers found that MIF occurs during the sunlight-driven reactions in snow, which gives a characteristic MIF fingerprint absent in atmospheric mercury.
This new method makes it possible for scientists to estimate how much mercury is lost from the snowpack to the atmosphere and how much remained behind to contaminate Arctic ecosystems.
