Professor Robert Mason – Uncovering the Mysteries of Marine Mercury

Professor Mason describes how mercury moves around the biosphere: ‘Mercury travels through the atmosphere mostly as a gas in its elemental form (Hg(0)), and is transported globally from its sources, both natural and anthropogenic, prior to being deposited back to the terrestrial and ocean surface mostly as ionic mercury (Hg(II)). Chemical and biological processes in soils, vegetation and the ocean can either convert the Hg(II) back to Hg(0), so it can be released again to the atmosphere and transported further, or transform it to methylmercury (MeHg).’ The conversion of Hg(II) to Hg(0) therefore impacts the amount transformed into the more toxic and bioaccumulative MeHg. A current focus of Professor Mason’s work is on the mercury water–air exchange fluxes in the Arctic Ocean, where the effects of climate change are most severe. ‘Mercury enters the Arctic Ocean surface waters from the atmosphere, rivers, erosion, ice melt etc., mostly as Hg(II),’ Professor Mason tells us, when asked about the mercury cycle. ‘In surface waters it is reduced to Hg(0) and this form of mercury is a dissolved gas in water. It is volatile, so can be lost to the atmosphere. In the atmosphere, chemistry converts the Hg(0) to Hg(II), and the cycling repeats. Ice cover mediates the extent to which this exchange occurs.’

US Geotraces cruise on the Healy

Mercury Bioaccumulation

Professor Mason elaborates on why the build-up of mercury, in the oceans and subsequently as MeHg in organisms, is of concern: ‘Some of the mercury entering the ocean is converted to MeHg, whose elevated concentrations in seafood and freshwater fish are a global health concern.’ Most human exposure to the MeHg neurotoxin, comes from our consumption of large predatory fish, such as tuna. He adds: ‘About 90% of the mercury entering the ocean from the atmosphere – with precipitation, and particle and gas deposition – and via rivers, point source inputs and groundwater, is lost back to the atmosphere due to the conversion of the Hg(II) back to Hg(0) and subsequent gas evasion. If this was not the case, the contamination of the ocean by human activity would be much greater than it actually is. Understanding this exchange at the sea surface – how much is released back to the atmosphere – is fundamental to understanding the impact of human activity on MeHg concentrations in ocean fish and seafood consumed by humans and wildlife.’

Atlantic Adventure

Between 2008 and 2010, Professor Mason and his team participated in six research cruises in the West Atlantic Ocean, which were funded by the U.S. National Science Foundation (NSF), Chemical Oceanography division. Two of these departed from the east coast of the United States, and four were conducted in the waters surrounding Bermuda. During the expeditions, the team simultaneously measured the concentration of both sea surface and atmospheric mercury at high resolution, together with the total mercury distribution. They found that Hg(0) levels were lowest near coastlines and regions influenced by river inputs, while higher levels were found in the open ocean. They also found that levels of Hg(0) varied by more than a factor of three between cruises. The team concluded that they needed more information on the role of dissolved organic carbon in the ocean, and how this affects the redox kinetics of Hg(0) and Hg(II) in the marine environment. With this, they would then be able to improve their estimates of mercury exchange between the sea and the air.

Pacific Pursuits

Then in October 2011, Professor Mason’s group was involved in an additional NSFfunded expedition into the Pacific Ocean, in the waters between Hawaii and Samoa. The team set out to further understand the variability of mercury quantities in both the ocean and atmosphere. To do this, they again collected high-resolution measurements of mercury spanning large gradients in seawater temperature, salinity, and productivity. Their measurements were then input into an ocean general circulation model coupled with an atmospheric chemical transport model, in order to create a model for the mercury inputs and losses in the surface ocean.

High Mercury Levels at the Intertropical Convergence Zone

The research group found greater variability in the amount of Hg(0) in the sea surface water than in the surrounding atmosphere. Additionally, concentrations of Hg(0) at the intertropical convergence zone were three times higher than in surrounding regions. This agreed well with observations from the Atlantic Ocean expeditions. The models created by Mason’s collaborators at Harvard University revealed that the high levels of surface ocean Hg(0) in this zone are due to a combination of high precipitation levels and a shallow surface ocean mixed layer. However, the model underestimated the amount of mercury in the surface ocean, likely due to the model’s incomplete parameterisation of scavenging of reactive mercury ions in the upper atmosphere. While the highest concentrations of Hg(0) were found in the ITCZ, the highest MeHg concentrations in zooplankton were near the equator, indicating differences in the locations of Hg(0) formation and that of MeHg bioaccumulation. These differences point to the complexity of how changes in Hg inputs and Hg(0) evasion impact the accumulation of MeHg in the food chain.

Arctic Exploration

These expeditions to the Atlantic and Pacific Oceans led Professor Mason and his team to propose studies in the Arctic Ocean as part of the international GEOTRACES Program. This program is an international effort to examine the distributions of trace elements and isotopes in all the ocean’s waters. This latest cruise departed from Alaska aboard the US Coast Guard Research Vessel Healy, taking a route north through the ocean and sea ice, to the North Pole and back, between August and October in 2015. ‘The Arctic Ocean is an ocean that is responding most dramatically to temperature rise and climate change. Therefore, studies in the Arctic Ocean can help us understand the combined potential impacts of changes in human emissions of mercury, which are now dominated by anthropogenic emissions from Asia, and how climate change will affect levels of methylmercury in seafood and therefore human exposure,’ Professor Mason tells us. The measurement approach throughout the cruise was similar to those taken in the previous studies and were made continuously throughout the trip. The ship initially sailed through open water, and then through increasing amounts of ice cover, from marginal to total ice cover. During their return, the opposite trend was seen.

Arctic Ice and Its Effects on Mercury Levels

Although many of the results from the expedition are still under analysis, some interesting findings have already been released. Professor Mason talks about his observations so far: ‘Based on the measured concentrations of Hg(0) in the surface waters and in the atmosphere, it is possible to estimate the potential rate at which Hg(0) could be lost to the atmosphere. Of course, under ice this is a potential flux, and not a real flux. As the cruise travelled to the pole and back, we saw increasing potential fluxes as ice increased, and then it decreased in the latter part of the cruise as ice cover was again reduced. In open waters there was little flux.’ He adds: ‘We can use differences in the concentrations in the water and typical conditions of wind and temperature to estimate how long it would take the built-up Hg(0) to be lost, once ice is removed. Our initial findings suggest it would take several months. Also, we can compare these potential flux rates to other ocean regions.’

Sofi ice sampling in the Arctic. CREDIT: Michelle Nerentorp Mastromonaco

The Mercury Cycle in the Arctic

Prior measurements and models suggest that approximately 60% of the Hg(II) added to the surface waters of the Arctic is converted into Hg(0), and 90% of that is then lost to the atmosphere, and the group’s recent results agree with these estimates. More Hg(0) appears to return to the atmosphere than is deposited from the atmosphere to the sea. In addition, given the team’s findings, the extent of loss of Hg(0) to the atmosphere will depend on the amount of ice and its persistence in summer. Hg(0) builds up to much higher concentrations under the ice than in the open water. When the ice melts, this will cause a large, short-term release of Hg(0) to the atmosphere over several months. Thus, the extent and duration of ice cover effects the amount, and rate of release, of Hg(0) into the atmosphere. When more Hg(0) is lost to the atmosphere, through a complex set of interactions, this leads to less Hg(II) being transformed into methylmercury and therefore lower amounts of its subsequent accumulation in fish and other sea creatures.

Methylmercury and Dimethylmercury in the Arctic

The remaining Hg(II) added to the sea is transported downward and mixes into the deeper waters of the ocean. Here, it is methylated into toxic and bioaccumulative methylmercury. It is then also converted into toxic dimethylmercury, which can evade back into the atmosphere. Sofi Jonsson, a post-doc with Mason, recently returned from another Arctic expedition on the Swedish Research Vessel Oden in collaboration with Swedish colleagues, where she collected samples for MeHg and total Hg analysis, measured dimethylmercury on board, and did experiments examining the formation and degradation of MeHg in Arctic waters. These results build on studies of photochemical degradation of MeHg being done by Mason’s graduate student, Brian DiMento, and will aid in understanding the extent to which dimethylmercury formed is lost to the atmosphere, and where its major formation occurs. Recent studies completed by Sofi and graduate student, Nash Mazrui have suggested that dimethylmercury formation could occur through the reactions of MeHg with sulfide minerals and organic matter in ocean waters. As the ice and permafrost is predicted to melt significantly as a consequence of climate change in the coming years, the team express concerns that this may lead to potentially higher levels of methylmercury in the Arctic food chain. However, the current results suggest that this could be offset by a potential decline in mercury concentrations driven by increased loss to the atmosphere in ice-free surface waters. Results of samples currently being analysed will lead to a better understanding of the rate of loss of mercury from the ocean as Hg(0) or dimethylmercury, and what then happens to it. Finally, global and regional regulatory actions, such as those proposed under the United Nations Minamata Convention on Mercury, that decrease Hg inputs to the ocean should also rapidly affect surface water aquatic mercury concentrations, even in the Arctic which is remote from these anthropogenic inputs.

Further Studies into the Role of Microorganisms on Methylmercury Levels

When asked what lies ahead for his team’s research, Professor Mason concludes by saying: ‘The chemistry of mercury in the ocean and the role of microorganisms in transformations of Hg(II) into MeHg, and also the processes of formation of dimethylmercury, are all focuses of my currently funded research, along with the studies examining the inputs and outputs of mercury from ocean systems, both the open ocean and coastal environments.’

Meet the researcher

Professor Robert Mason
Professor of Marine Sciences and Chemistry
Department of Marine Sciences
University of Connecticut , USA

Robert Mason is Professor of both Marine Sciences and Chemistry at the University of Connecticut in the USA. Professor Mason completed his BS and MS degrees in chemistry in his native South Africa, and then went on to work at the federal South African Sea Fisheries Research Institute, before moving to the USA to study for his PhD in Marine Science at the University of Connecticut. Following its completion in 1991, he went on to a post-doctoral position at MIT and then to the University of Maryland, Center for Environmental Sciences prior to returning to the University of Connecticut to take up his current position.

CONTACT

E: robert.mason@uconn.edu
T: (+1) 860 405 9129
W: http://marinesciences.uconn.edu/faculty/mason/
W: http://www.uscg.mil/pacarea/cgchealy/
W: http://polarforskningsportalen.se/en/arctic/expeditions/arcticocean-2016

KEY COLLABORATORS

Brian DiMento, graduate student, University of Connecticut
Kati Gosnell, graduate student, University of Connecticut
Sofi Jonsson, Post-doc, University of Connecticut
Nash Mazrui, graduate student, University of Connecticut
Prentiss Balcom, Harvard University
Steve Brooks, University of Tennessee Space Institute
Chris Moore, Gas Technology Institute,
Reno Elsie Sunderland, Harvard University
Anne Soerensen, Stockholm University
Katarina Gardfeldt, Chalmers University, Sweden
Carl Lamborg, University of California – Santa Cruz
Katlin Bowman, University of California – Santa Cruz
Chad Hammerschmidt, Wright State University
Celia Chen, Dartmouth College

FUNDING

NSF/Chemical Oceanography Division
NSF/CHEM
NIH/NIEHS
NOAA/Sea Grant

REFERENCES

S. Jonsson, NM Mazrui and RP Mason, Dimethylmercury formation mediated by inorganic and organic reduced sulfur surfaces, Scientific Reports 2016, 6, #27958. DOI: 10.1038/srep27958

A Gosnell and RP Mason, Mercury and methylmercury incidence and bioaccumulation in plankton from the central Pacific Ocean, Mar. Chem., 2015, 177, 772–780.

AL Soerensen, RP Mason, PH Balcom, DJ Jacob, Y Zhang, J Kuss and EM Sunderland, Elemental Mercury Concentrations and Fluxes in the Tropical Atmosphere and Ocean, Environ. Sci. Technol., 2014, 48, 11312−11319.

AL Soerensen, RP Mason, PH Balcom and EM Sunderland, Drivers of Surface Ocean Mercury Concentrations and Air−Sea Exchange in the West Atlantic Ocean, Environ. Sci. Technol., 2013, 47, 7757−7765.