A model of mercury cycling and isotopic fractionation in the ocean

Abstract

Abstract. Mercury speciation and isotopic fractionation processes have been incorporated into the HAMOCC offline ocean tracer advection code. The model is fast enough to allow a wide exploration of the sensitivity of the Hg cycle in the oceans, and of human exposure to Hg via monomethyl-Hg incorporation into fish. Vertical particle transport of Hg appears to play a discernable role in setting present-day Hg distributions, which we surmise by the fact that in simulations without particle 10 transport, the high present-day Hg deposition rate leads to an Hg maximum at the sea surface, rather than a subsurface maximum as observed. Hg particle transport has only a relatively small impact on anthropogenic Hg uptake, but it sequesters Hg deeper in the water column, so that excess Hg is retained in the model ocean for longer after anthropogenic Hg deposition is stopped. The concentration of monomethyl Hg is sensitive to its production rate, with model experiments suggesting that human impacts on ocean oxygen concentrations could have as significant an impact on oceanic MMHg 15 concentration as the anthropogenic Hg emission itself. Eight different isotopic fractionation mechanisms are simulated, independently and combined together, to predict their expression in the spatial distributions of isotopic signatures of Hg species in the ocean.