Natural sources of mercury

Mercury is an element that occurs naturally throughout our solar system. On Earth, geological deposits are most often found in the form of , a mercury sulfide mineral. The mercury content in this mineral, the most important ore of mercury, can reach 86%. Granite can contain 0.2 ppm (parts per million) of mercury, while other crustal rocks tend to contain less (approximately 0.1 ppm). Various natural processes, including volcanic eruptions, the weathering of rocks, and under sea vents can release mercury from the Earth's crust into water bodies, soils, and the atmosphere. In fact, atmospheric measurements of mercury above Hawaiian and Icelandic volcanoes are orders of magnitude above normal background atmospheric levels.

Once mercury is exposed to the biosphere it can enter the biogeochemical cycle or it can be transported long distances through the atmosphere. As mercury becomes incorporated into the biosphere, measurable releases can also be attributed to vegetation, forest fires, water bodies, sea salt spray, and soils. Although natural emissions occur mainly as elemental mercury vapour (Hg⁰), particulate and vaporous oxides, sulphides and halides as well as methylmercury vapour may also be released.

It is estimated that annual, natural emissions from continental sources are approximately 1 000 tonnes. In pre-industrial times, evasion from the oceans is thought to have been in the area of 600 tonnes. Today, however, evasion from the oceans has increased to approximately 2 000 tonnes due to the re-emission of mercury deposited as a result of human activities.