Mercury in the Arctic: Are We Overlooking the Ocean

摘要

Two properties of mercury (Hg) assure a continued scientific interest in its cycle in the Arctic Ocean—Hg accumulates to potentially harmful concentrations in many aquatic predators, and there remain puzzles and controversy as to whether or not there are unique processes that make the Arctic Ocean and its biota a global sink for atmospheric Hg contamination. There is absolutely no doubt that the global Hg cycle has been enhanced through human activities (Sunderland and Mason 2007), and this is apparent everywhere in the upper global ocean. Furthermore, there is unimpeachable evidence that a process unique to polar regions after polar sunrise, atmospheric mercury-depletion events (AMDEs), enhances the deposition of atmospheric Hg (as Hg[II]) onto snow and ice surfaces, sometimes stripping much of the approximately 1.5 ng/m3 Hg out of the bottom kilometer of the troposphere (Steffen et al. 2008). Based on models, it is estimated that this deposition across the entire Arctic Ocean accounts for approximately 90 to 450 t/y of Hg, an estimate that is said to be generally well accepted (Poissant et al. 2008). The potential importance of AMDEs to contamination of Arctic ecosystems has not lacked supporters, and most of the research effort on Arctic Hg pathways during the past decade has been focused on unraveling the complex halogen chemistry behind AMDEs and subsequent reactions in snow. One major problem with this view, however, is that we do not really know the actual net deposition of Hg into the ocean nor what proportion of the deposited Hg recycles back to the atmosphere because of photoreduction. Furthermore, the subsequent processes and pathways between deposited Hg and its ultimate uptake by aquatic biota are poorly understood.