The Mercury Cycle

Mercury (Hg) is an element that occurs in a fixed amount on earth and can neither be created nor destroyed. The most abundant natural source of mercury occurs as the solid mineral cinnabar, HgS, in mercury deposits. In this form, inorganic mercury, Hg(II), is bound tightly to sulfur, and is only slowly released to the environment by weathering. Trace amounts of mercury are also tied up in coal. When humans mine cinnabar, or combust coal, mercury is released as the pure element, Hg0, and as reactive mercury, or Hg(II). Volcanoes and forest fires also release mercury.

Elemental mercury is the only heavy metal that is liquid at room temperature. It is also very volatile meaning it evaporates, and forms a gas which enters the atmosphere and circulates around the globe. While some mercury is released naturally, scientists estimate that our combustion, mining and extraction of mercury from the earth has at least doubled, and possibly quadrupled the amount of mercury circulating the globe.

Elemental mercury released to the air from chlorine factories or other sources is deposited back on land and water as reactive mercury. This inorganic form of mercury can be converted to the more toxic organic form, methylmercury, by naturally occurring bacteria.

It is the methylmercury that bioaccumulates in aquatic food chains and contaminates the fish we eat. Deposited reactive mercury can also change back to elemental mercury and enter the global pool again. Over long periods of time, a small amount of reactive mercury combines with sulfur again, and falls out of the water and gets buried in sediments.



Questions? Email mercury@oceana.org or call 877-7OCEANA.