On the origin of seawater methylmercury in the Canadian Arctic: in-situ production vs. long-range advection

A subsurface enrichment of methylmercury (MeHg) has been observed at shallow depths (~100–300 m) in several regions of the Arctic Ocean. The spatial distribution of this subsurface seawater MeHg has been suggested to be responsible for the spatial variability in mercury concentrations of marine animals in the Canadian Arctic. The origin of the sub-surface MeHg in seawater, however, remains a subject of debate. In most other ocean basins, seawater MeHg typically peaks at deeper depths with a much lower dissolved oxygen content, and is thought to be produced in situ and in association with organic matter remineralization. In contrast, our water mass analysis suggests that MeHg enrichment in the shallow, well-oxygenated waters in the Canadian Arctic bears a signature that is most consistent with long-range transport of the Upper Halocline Pacific Water (UHW). Our results further show that seawater MeHg concentrations exhibit a significant correlation with a denitrification tracer, N*, and that the MeHg-to-N* slope decreases progressively from west to east across the Canadian Arctic. The negative N* values in the Canadian Arctic are known to have originated from denitrification in the productive Chukchi Sea shelf sediments, where anaerobic mercury methylation could also be favored. As MeHg- and N*-carrying UHW advects eastwards toward the Canadian Arctic, MeHg is progressively lost through demethylation, resulting in the observed decreasing trend in the MeHg-to-N* slope. The long-distance transport implies that the half-life of MeHg in Arctic seawater below the euphotic zone must be much longer than previously reported. This is supported by a critical literature review, which casts doubt on mercury methylation and demethylation rates previously determined from a seawater incubation approach due to unexplainable methylation and demethylation at time zero and poor fitting of the experimental data to first-order kinetics. Our results thus call for a better understanding of productive shelf sediments as potential MeHg “hotspots” in the Arctic and more reliable measurements of mercury methylation and demethlyation rates in the ocean.