ReviewEarth SciencesMass-independent fractionation of even mercury isotopes

Practically all physical, chemical, and biological processes can induce mass-dependent fractionation of mercury (Hg) isotopes. A few special processes such as photochemical reduction of Hg(II) and photochemical degradation of methylmercury (MeHg) can produce mass-independent fractionation (MIF) of odd Hg isotopes (odd-MIF), which had been largely reported in variable natural samples and laboratory experiments, and was thought to be caused by either nuclear volume effect or magnetic isotope effect. Recently, intriguing MIF of even Hg isotopes (even-MIF) had been determined in natural samples mainly related to the atmosphere. Though photo-oxidation in the tropopause (inter-layer between the stratosphere and the troposphere) and neutron capture in space were thought to be the possible processes causing even-MIF, the exact mechanism triggering significant even Hg isotope anomaly is still unclear. Even-MIF could provide useful information about the atmospheric chemistry and related climate changes, and the biogeochemical cycle of Hg.