Mercury cycle



The mercury cycle is a biogeochemical cycle influenced by natural and anthropogenic processes that transform mercury through multiple chemical forms and environments.

Mercury is present in the Earth's crust and in various forms on the Earth's surface. It can be elemental, inorganic, or organic. Mercury exists in three oxidation states: 0 (elemental mercury), I (mercurous mercury), and II (mercuric mercury).

Mercury emissions to the atmosphere can be primary sources, which release mercury from the lithosphere, or secondary sources, which exchange mercury between surface reservoirs. Annually, over 5000 metric tons of mercury is released to the atmosphere by primary emissions and secondary re-emissions.

Processes
Mercury is transported and distributed by atmospheric circulation, which moves elemental mercury from the land to the ocean. Elemental mercury in the atmosphere is returned to the Earth's surface by several routes. A major sink of elemental mercury (Hg(0)) in the atmosphere is through dry deposition. Some of elemental mercury, on the other hand, is photooxidized to gaseous mercury(II), and is returned to the Earth's surface by both dry and wet deposition. Because photooxidation is very slow, elemental mercury can circulate over the entire globe before being oxidized and deposited. Wet and dry deposition is responsible for 90% of the mercury of surface waters, including open ocean.

A fraction of deposited mercury instantaneously re-volatilize back to the atmosphere.

Inorganic mercury can be converted by bacteria and archaea into methylmercury ([CH3 Hg] +), which bioaccumulates in marine species such as tuna and swordfish and biomagnifies further up the food chain.

Certain xenophyophores have been found to have abnormally high concentrations of mercury within their bodies.