Onshore mud volcanoes as a geological source of mercury: Case study from the Kerch Peninsula, Caucasus continental collision zone

Three mud volcanoes (MVs) in the Kerch Peninsula were studied as a geological source of mercury. The study focused on total mercury (THg) concentrations in MV waters, mud masses and plants colonizing MV areas; gaseous elemental mercury (GEM) in the atmosphere above MVs; and sulfide mercury (HgS) and HgCl₂ species in representative samples of mud masses. THg concentrations in the illite-smectite mud masses ranged from 38 to 920 ng/g. They contained up to 70% of total mercury in sulfide form (in pyrite and cinnabar), but lacked HgCl₂. THg values in MV waters of HCO₃-Cl/Na- and/or Cl-HCO₃/Na-types with рН = 7.4-9.5 mostly fell in a range of 79-440 ng/L, but rarely exceeded 600 ng/L, being comparable with those for geothermal systems. Another issue of interest was the distribution of THg in below- and above-ground parts of halophyte plant Limonium caspium. THg was incorporated into the plant roots, leaves and flowers; the roots exhibited higher concentrations of THg relative to the other organs. The Hg bioaccumulation factor ranged from 0.06 to 0.76. GEM concentrations measured over large bubbling MV pools and newly formed cracks showed values (50 to 520 ng·m^-3) higher than background values (≤3 ng·m^-3) associated with pristine test sites and background values measured within three MV areas of the Kerch peninsula that is slightly higher than background concentration for the Northern Hemisphere. Maximum GEM contents were comparable with the values found in geothermal and magmatic volcanic provinces.