Modelling the mercury stable isotope distribution of Earth surface reservoirs: Implications for global Hg cycling

Mercury (Hg) stable isotopes are useful to understand Hg biogeochemical cycling because physical, chemical and biological processes cause characteristic Hg isotope mass-dependent (MDF) and mass-independent (MIF) fractionation. Here, source Hg isotope signatures and process-based isotope fractionation factors are integrated into a fully coupled, global atmospheric-terrestrial-oceanic box model of MDF (delta Hg-202), odd-MIF (Delta Hg-199) and even-MIF (Delta Hg-200). Using this bottom-up approach, we find that the simulated Hg isotope compositions are inconsistent with the observations. We then fit the Hg isotope enrichment factors for MDF, odd-MIF and even-MIF to observational Hg isotope constraints. The MDF model suggests that atmospheric Hg-0 photo-oxidation should enrich heavy Hg isotopes in the reactant Hg-0, in contrast to the experimental observations of Hg-0 photo-oxidation by Br. The fitted enrichment factor of terrestrial Hg-0 emission in the odd-MIF model (5 parts per thousand) is likely biased high, suggesting that the terrestrial Hg-0 emission flux (160 Mg yr(-1)) used in our standard model is underestimated. In the even-MIF model, we find that a small positive atmospheric Hg-0 photo-oxidation enrichment factor (0.22 parts per thousand) along with enhanced atmospheric Hg-II photo-reduction and atmospheric Hg-0 dry deposition (foliar uptake) fluxes to the terrestrial reservoir are needed to match Delta Hg-200 observations. Marine Hg isotope measurements are needed to further expand the use of Hg isotopes in understanding global Hg cycling. (C) 2018 Elsevier Ltd. All rights reserved.