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Biogeochemistry

Methylated Mercury Cycling in the Canadian Arctic Marine Boundary Layer

Article: Pascale A. Baya, Michel Gosselin, Igor Lehnherr, Vincent L. St.Louis, and Holger Hintelmann. Determination of Monomethylmercury and Dimethylmercury in the Arctic Marine Boundary Layer. Environ. Sci. Technol. Publication Date (Web): December 1, 2014 (Article) DOI: 10.1021/es502601z.

 

Introduction to Methylmercury

Monomethylmercury (MMHg) is the most toxic form of mercury (Hg) to humans and wildlife. Indeed Methylmercury from industrial wastewater deposited by Chisso Corporation gave rise to an outbreak of Minamata disease in Japan in 1956. Patients experienced symptoms such as ataxia, numbness in hands and feet, damage to hearing and speech, and, in some cases, paralysis and death. Methylmercury in the environment concentrates (or “bioaccumulates”) in fish and shellfish. This increase in methylmercury concentration is further amplified up the food chain when, for example, people consume seafood.

Nowadays, there are few anthropogenic sources of methylmercury. Indirect sources such as burning of waste containing mercury or fossil fuels contribute to most of the mercury in the atmosphere.  U.S. industry introduces an average of 158 tons per year of mercury to the atmosphere.  Gaseous elemental mercury (Hg0) is less toxic and bioaccumulative than methylmercury. However, after oxidation (removal of electrons due to UV radiation) to reactive mercury (Hg2+), mercury can reach the surface ocean by atmospheric deposition (for example Hg2+ can bind to particles in the air and fall to the ocean with rain and snow). Anaerobic organisms (those that grow in low-oxygen or no-oxygen environments) living in the ocean have the ability to add methyl (a carbon atom attached to three hydrogen atoms, also known as “CH3”)  group to Hg2+. Addition of one methyl group results in MMHg and, while addition of two methyl groups results in dimethylmercury (DMHg).

Mercury found in Arctic marine mammals and fish are on the rise, raising concerns over the potential impacts on the environment and human health.  It remains unclear if atmosphere is another source of methylmercury to open waters other than methylation at local water columns. There are three hypotheses: (1) Hg transformation to methylmercury in presence of acetate in wet air, (2) MMHg evaporation from surface water, (3) DMHg degradation to MMHg after its evaporation from surface water.

MMHg and DMHg Detecting Experiment

In order to verify the existence of MMHg and DMHg in the atmosphere, researches from several Canadian Universities sent out two cruises over two years to the Canadian Arctic in search of these compounds. (Air and water sampling locations are shown in Figure 1.) Air samples were collected by pumping 200 liter air above the sea surface through insulated PTFE(a kind of plastic that is very stable and inactive; one famous name for it is Teflon) tubing.  Water samples were collected using Niskin bottles at the surface and the subsurface chlorophyll maximum (where phytoplankton blooms). MMHg and DMHg samples were then sent to laboratories, extracted and quantified for masses.

Figure 1. Cruise map.

Figure 1. Cruise map.

Results

The measured MMHg and DMHg in the Arctic marine boundary layer (MBL) (part of the atmosphere that has direct contact and, hence, is directly influenced by the ocean) have an average of 2.9 and 3.8 pg/m3 respectively.  These numbers suggest that methylated Hg species represent only a minor (~1%) fraction of the global atmospheric gaseous Hg pool, whereas MMHg may be of greater significance in combination of gaseous mercury in the Arctic MBL.

Water concentrations of methylated Hg species were closely related to concentrations in the air. This trend suggests that the two pools are interconnected through gas evaporation and deposition.  However it is difficult to judge whether one compartment is the driver of the other. Higher volatility (tendency to get into gaseous phase) and lower solubility (ability to dissolve in water) of DMHg suggested a greater contribution from marine sources of organic mercury to the atmosphere. DMHg is unstable and is expected to decompose into MMHg once it enters the atmosphere.  Therefore, DMHg in the air is one potential source to MMHg in the air.

Sea-ice cover was studied as one factor controlling MMHg and DMHg air-sea exchange flux (the rate of transfer of MMHg and DMHg across a unit area of air-sea surface). As sea-ice inhibits methylated mercury evaporate into the air as well as their photo degradation, MMHg and DMHg gradually accumulate under the ice barrier. As soon as the ice melts, these compounds rapidly escape to the air.  Another controlling factor studied was primary production rate. It has been reported that Hg methylation occurring in oxic seawater is linked to microorganism abundance and activity. Such phenomenon is observed as in Figure 3b.

Considering atmospheric input into the ocean, MMHg in the air is believed to be a direct source to marine surface oceans. Wet deposition and dry deposition are the two pathways for MMHg falling to the surface (Figure 3d). Wet deposition occurs with rainfall and snow. Methylmercury chloride (which is ionized form of MMHg, consisting of methylmercury cation and chloride anion) has a significant solubility indicating the possibility of wet deposition.  MMHg detected in mid Arctic precipitation and snow further demonstrated this pathway. Dry deposition occurs by chemicals binding to aerosols, but is assumed to be of less importance, and is thus not considered by this study.

 

Figure 2. Distribution of MMHg and DMHg (pg/m3).

Figure 2. Distribution of MMHg and DMHg (pg/m3).

 

Figure 3. Relationships between (a) dimethylmercury (DMHg) concentrations in the Arctic marine boundary layer (MBL) and % ice cover at time of sampling in HB and the Canadian Arctic Archipelago in 2010, (b) DMHg concentrations in the MBL and primary production in the water column, (c) MMHg concentrations in the MBL and % cloud cover and (d) DMHg concentrations in the MBL and MMHg concentrations in surface seawater.

Figure 3. Relationships between (a) dimethylmercury (DMHg) concentrations in the Arctic marine boundary layer (MBL) and % ice cover at time of sampling in HB and the Canadian Arctic Archipelago in 2010, (b) DMHg concentrations in the MBL and primary production in the water column, (c) MMHg concentrations in the MBL and % cloud cover and (d) DMHg concentrations in the MBL and MMHg concentrations in surface seawater.

 

Conclusions and significance

As methylated forms of mercury are extremely toxic, it is important to know where they are and how much are there. This study suggests that DMHg from the open ocean is an important mechanism controlling the cycling of mercury in the Arctic. Sea-ice cover and primary production rate were shown to be two major factors controlling methylated mercury in Canadian Arctic MBL. This study contributes to our overall understanding of the transport and transformation between different compartments in the Arctic, which helps to build up a complete biogeochemical cycling map of mercury.

Figure 4. Cycling of MMHg and DMHg in the Arctic Marine Boundary Layer.

Figure 4. Cycling of MMHg and DMHg in the Arctic Marine Boundary Layer.

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