Mercury cycling in the global marine environment

Doctoral thesis, 2016

Mercury is a globally distributed contaminant that exists in the atmosphere in its elemental form as a stable monoatomic gas. Having a residence time of around one year in air allows it to be transported far from emission sources and end up in polar ecosystems. Gaseous elemental mercury (GEM) can in air be oxidized by photo-induced processes which produce water soluble oxidized forms of mercury which are more easily deposited. Deposited mercury can in the environment be transformed to organic and bio-accumulating compounds which are neurotoxic, making mercury a global concern. Deposited oxidized mercury into the sea can be reduced back to the elemental form (GEM) and be re-emitted to air. This re-evasion constitutes of around 30% of the total emissions of mercury to air and originates from both natural and anthropogenic sources. Models have estimated that the yearly mercury emission from global sea surfaces is between 2000 and 3000 tonnes. The mercury flux rate at the interphase between air and water depends on the Henry´s law constant, the concentration gradient and the gas transfer velocity. How to properly account for weather parameters such as wind speed, and how to accurately adjust the flux model to mercury (originally developed for CO2) has been debated in the literature and have resulted in diverse results of mercury flux rates.  In this work, mercury has been measured in air and in seawater during several campaigns in Antarctica, the Mediterranean Sea, the west coast of Sweden, Northern Finland and in the Arctic. From measured concentrations of mercury, the mercury flux rates from the studied areas were calculated using the gas exchange model described in Johnson (2010). Large spatial and seasonal variations of measured mercury concentrations were found which resulted in similar variations in calculated flux rates. In Antarctica and the Arctic, high concentrations of mercury were also measured in the sea ice environment. Seasonal variations in mercury concentrations were found and a correlation between solar radiation and the photo-production of elemental mercury in sea ice was discovered. The sea ice was suggested to affect the global marine cycling of mercury in several ways: acting as a cap preventing elemental mercury to evade from sea surfaces in Polar Regions, acting as barrier against direct atmospheric deposition and being a significant reservoir of mercury. Climate change will likely affect the cycling of mercury in global marine environments due to an increase in temperature, leading to enhanced mercury evasion, and diminishing and melting sea ice causing an increased input of mercury into polar oceans. Results presented in this thesis bring new insights about how mercury is cycling in the global marine environment and the new collected mercury data from remote and inaccessible areas are valuable for future modeling. However, more research is needed to further understand and quantify the accumulation of mercury in vulnerable marine ecosystems.