Project: Constraining the air-sea exchange of inorganic and methylated mercury with high resolution spatial and temporal measurements in the Sargasso Sea

Mercury (Hg), a global pollutant, is released to the biosphere predominantly by human activity, through intentional use or as a trace element in coal, ores, and other products. These releases are exacerbated by climate change. Atmospheric Hg is predominantly elemental Hg (Hg0) - a gas with a relatively long residence time that can be transported globally to remote regions. After its transformation by atmospheric chemical reactions to ionic Hg, it is deposited to the ocean through precipitation, aerosol, and gaseous Hg deposition. In surface waters, ionic Hg is biologically and abiotically transformed into Hg0, which is then returned to the atmosphere via gas exchange. Overall, atmospheric deposition is the primary source, and Hg0 gas evasion is the primary loss mechanism for ocean Hg. The extent of air-sea exchange therefore alters the ocean Hg burden and impacts the global mercury cycle. Furthermore, the oceanic ionic Hg can be transformed into the more toxic and bioaccumulative methylmercury (MeHg), which concentrates in seafood sufficiently to be a global human health concern and impact wildlife. Understanding the factors that mediate the air-sea exchange of Hg is of vital importance for assessing the impact of humans and climate change on MeHg levels in seafood, and human health.

The project will evaluate the concentrations of Hg in the surface ocean and atmosphere to examine the factors controlling their concentration and chemistry, and the magnitude of the Hg sources and sinks. There is limited data examining the seasonal and temporal variability of Hg speciation in the surface ocean at a specific location, and its air-sea exchange, and this is the study’s primary focus. Measurements will be made in the North Atlantic Ocean near Bermuda, during both monthly and specific Hg-focused cruises. Atmospheric samples will be collected as well at the atmospheric sampling station in Bermuda.  During cruises, high resolution Hg speciation data in the atmosphere (gaseous and aerosol) and surface waters (gaseous elemental Hg, dissolved and particulate ionic Hg, MeHg and dimethylmercury) will be gathered to examine the sources of Hg and its transformations, and how this impacts the formation of MeHg. Additionally, measurements of Hg stable isotopes in atmospheric and water samples will be used to identify sources (e.g., geogenic or anthropogenic inputs) and delivery mechanisms of Hg to this region. This proposed research will further assess the importance of natural variability, human activity, and climate change on altering Hg and MeHg levels in the ocean, and in marine organisms consumed by humans.