Georg, Bastian

Monomethylmercury (MMHg), the most bioavailable form of mercury (Hg) and a potent neurotoxin, is present at elevated concentrations in Arctic marine mammals posing serious health threats to the local populations relying on marine food for their subsistence living. The sources of MMHg in the Arctic Ocean surface water and the role of dimethylmercury (DMHg) as a source of MMHg remain unclear. The objective of this research was to determine the sources and fate of methylated Hg species (MMHg and DMHg) in the marine ecosystem by investigating processes controlling the presence of methylated Hg species in the Arctic Ocean marine boundary layer (MBL) and surface waters. A method based on solid phase adsorption on Bond Elut ENV was developed and successfully used for unprecedented measurement of methylated Hg species in the MBL in Hudson Bay (HB) and the Canadian Arctic Archipelago (CAA). MMHg and DMHg concentrations averaged 2.9 ± 3.6 (mean ± SD) and 3.8 ± 3.1 pg m-3, respectively, and varied significantly among sampling sites. MMHg in the MBL is suspected to be the product of marine DMHg degradation in the atmosphere. MMHg summer (June to September) atmospheric wet deposition rates were estimated to be 188 ± 117.5 ng m-2 and 37 ± 21.7 ng m-2 for HB and CAA, respectively, sustaining MMHg concentrations available for bio-magnification in the pelagic food web. The production and loss of methylated Hg species in surface waters was assessed using enriched stable isotope tracers. MMHg production in surface water was observed from methylation of inorganic Hg (Hg(II)) and, for the first time, from DMHg demethylation with experimentally derived rate constants of 0.92 ± 0.82 x 10-3 d-1 and 0.04 ± 0.02 d-1 respectively. DMHg demethyation rate constant (0.98 ± 0.51 d-1) was higher than that of MMHg (0.35 ± 0.25 d-1). Furthermore, relationships with environmental parameters suggest that methylated Hg species transformations in surface water are mainly biologically driven. We propose that in addition to Hg(II) methylation, the main processes controlling MMHg production in the Arctic Ocean surface waters are DMHg demethylation and deposition of atmospheric MMHg. These results are valuable for a better understanding of the cycle of methylated Hg in the Arctic marine environment.

Author Keywords: Arctic Ocean, Atmosphere, Demethylation, Dimethylmercury, Methylation, Monomethylmercury

Soluble uranium (U) has been observed continuously in the porewaters of Bentley Lake,

a lake with semi-permanent anoxic sediments, despite the fact that reduced U(IV) is known to be

insoluble. To be able to predict the fate and mobility of U that has been deposited in lake

sediments, it is very important to understand the factors that determine soluble uranium in anoxic

environments. Understanding soluble U species is crucial for predicting its behavior in natural

systems as well as for the development of U remediation schemes.

To explore the factors affecting soluble U in natural environments, anoxic lake sediments

and porewaters were tested using two analytic methods, ICP-MS and ESI-HR-MS. Reduced

uranium (U(IV)) can be precipitated as U(IV)-NdF3. Using this method revealed that most of the

uranium in porewater is not able to be co-precipitated with NdF3. In addition, UO2+ was found

using ESI-HR-MS, showing uranyl ions exist in reduced porewater. However, the UO2+ might be

attached to some organic groups rather than present as free ions.

Seasonal variation and air exposure experiments on the mobility of U between sediments

and porewater were observed to test for changes of the redox state of U as a function of sample

collection and storage. The results of this study will contribute to better remediation strategies for

U tailings and will help U mining operations in the future.