Iodine rarely occurs naturally in
mineral form, so it comprises a very small portion of rocks by mass.
Sedimentary rocks have higher concentrations of iodine compared to
metamorphic and
igneous rocks. Due to the low concentration of iodine in rocks,
weathering is a minor flux of iodine to
soils and the freshwater hydrosphere. Soils contain a much higher concentration of iodine compared to their parent rock, though most of it is bound to
organic and inorganic matter, potentially due to microbial activity. The major source of iodine to soils is through dry and wet
deposition of
aerosolized iodine in the atmosphere. Due to the high production of atmospheric iodine from the oceans, both the concentration of iodine and the flux of iodine to soils is greatest near coastal regions. Plants uptake iodine from the soil through their roots and return the iodine when they decompose. Fauna that consume plants may uptake this iodine but similarly return it to soils upon decomposition. Some iodine may also be cycled into the freshwater hydrosphere through
leaching and
runoff, where it may return to the oceans. Similar to oceanic iodine, the majority of iodine cycled out of soil is volatilized through conversion to methyl forms of iodine by bacteria. Unlike ocean volatilization, however, bacteria are thought to be the only organisms responsible for volatilization in soils. == Anthropogenic influences ==