Modelling the mercury stable isotope distribution of Earth surface reservoirs: Implications for global Hg cycling
Résumé
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 (δ202Hg), odd-MIF (Δ199Hg) and even-MIF (Δ200Hg). 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 Hg0 photo-oxidation should enrich heavy Hg isotopes in the reactant Hg0, in contrast to the experimental observations of Hg0 photo-oxidation by Br. The fitted enrichment factor of terrestrial Hg0 emission in the odd-MIF model (5‰) is likely biased high, suggesting that the terrestrial Hg0 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 Hg0 photo-oxidation enrichment factor (0.22‰) along with enhanced atmospheric HgII photo-reduction and atmospheric Hg0 dry deposition (foliar uptake) fluxes to the terrestrial reservoir are needed to match Δ200Hg observations. Marine Hg isotope measurements are needed to further expand the use of Hg isotopes in understanding global Hg cycling.