Role of the ocean in the global mercury cycle

Abstract

Air-sea exchange of mercury (Hg) is a critical part of the global Hg cycle as it determines, to a large degree, the response time of the biosphere to changes in mercury inputs. Recent measurements have demonstrated that the cycling of Hg between the ocean and atmosphere is complex, principally because of the enhanced oxidation of elemental Hg (Hg°), and the formation of reactive gaseous Hg (RGHg) in the marine boundary layer. We estimate that the dry deposition of RGHg to the ocean, which has not been previously considered in global budgets, is 35% of the total Hg input to the ocean. A further reevaluation of the global Hg cycle suggests that there is a net transfer of Hg from the terrestrial environment to the ocean and that the deep ocean Hg concentration is increasing by a few percent per year. Similarly, anthropogenic inputs on land have increased Hg on the Earth's surface layer with accumulation in the terrestrial environment accounting for nearly 80% of the net input from man's activities. Dry deposition of RGHg is important for the terrestrial realm but because of its relatively short residence time in the atmosphere, it is the oxidation of Hg° over the ocean, rather than RGHg transport offshore, which is primarily contributing to oceanic RGHg deposition.