Sources, sinks and biogeochemical cycling of mercury in the ocean

A review of the available information on the sources and sinks for oceanic mercury (Hg) illustrates the importance of the ocean in the global Hg cycle. The principal source of oceanic Hg is atmospheric deposition with riverine sources contributing about 10% of the total inputs. The primary loss term is gas evasion at the sea surface. Burial of Hg in ocean sediments is a minor sink (10% or less of the total flux). Mass balance estimates suggest that hydrothermal sources do not contribute significantly to the oceanic Hg pool. Overall, about 11 Mmol/yr is currently being added to (and lost from) the ocean reservoir. Much of this Hg is of anthropogenic origin. Mercury deposited to the ocean is effectively reduced, principally by biologically-mediated processes, to elemental Hg and this leads to a rapid recycling to the atmosphere of much of the deposited Hg. Reactive Hg is converted in deeper ocean waters to methylated Hg species. Methylated Hg species and elemental Hg are important components of the Hg pool and the interconversion between the different Hg forms determines, to a large extent, the fate and transport of Hg in deep ocean waters,and the extent of Hg accumulation in marine food chains.