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, I, and II. 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 Mg of mercury is released to the atmosphere by primary emissions and secondary re-emissions.
Primary sources of mercury emissions can be natural or anthropogenic. Most natural mercury occurs as the mercury sulfide mineral, cinnabar, which is one of the only significant ores of mercury. Organic-rich sedimentary rocks can also contain elevated mercury. Weathering of minerals and geothermal activity release mercury to the environment. Active volcanoes are another significant primary source of natural mercury. Anthropogenic primary sources of mercury include gold mining, burning coal, and production of non-iron metals, such as copper or lead.
Secondary sources
Secondary natural sources, which re-emit previously deposited mercury, include vegetation, evasion from oceans and lakes, and biomass burning, including forest fires. Primary anthropogenic emissions are leading to increased sizes of mercury in surface reservoirs.
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 in the atmosphere is through dry deposition. Some of elemental mercury, on the other hand, is photooxidized to gaseous mercury, 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, which bioaccumulates in marine species such as tuna and swordfish and biomagnifies further up the food chain.
