Iodine cycle

The iodine cycle is a biogeochemical cycle that primarily consists of natural^[1] and biological processes^[3] that exchange iodine through the lithosphere, hydrosphere, and atmosphere.^[3][2] Iodine exists in many forms, but in the environment, it generally has an oxidation state of -1, 0, or +5.^[1]

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Related topics Biogeochemistry geochemical cycle chemical cycling environmental chemistry Biosequestration Deep biosphere Ocean acidification acid rain Biogeochemical planetary boundaries
Research groups DAAC GEOTRACES IMBER NOBM SOLAS
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Iodine in the ocean exists mostly in oceanic sediments and seawater.^[4] During subduction of oceanic crust and seawater, most of the iodine cycles into seawater through brine, while a minor amount is cycled into the mantle.^[4] Marine biota, including seaweed and fish, accumulate iodine from the seawater and return it during decomposition.^[2] Sedimentation of oceanic iodine replenishes the ocean sediment sink. ^[1]

The losses of iodine from the oceanic sink are to the atmospheric sink.^[1] Sea spray aerosolization accounts for a portion of this loss.^[2] However, the majority of the iodine cycled into the atmosphere occurs through biological conversion of iodide and iodate to methyl forms, primarily methyl iodide.^[3] Algae, phytoplankton, and bacteria are involved in reducing the stable Iodate ion to iodide,^[5] and different species produce volatile methyl iodide which leaves the oceans and forms aerosols in the atmosphere.^[3]

Iodine rarely occurs naturally in mineral form, so it comprises a very small portion of rocks by mass.^[2] Sedimentary rocks have higher concentrations of iodine compared to metamorphic and igneous rocks.^[4] Due to the low concentration of iodine in rocks, weathering is a minor flux of iodine to soils and the freshwater hydrosphere.^[1]

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.^[4] The major source of iodine to soils is through dry and wet deposition of aerosolized iodine in the atmosphere.^[1] 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.^[1] Plants uptake iodine from the soil through their roots and return the iodine when they decompose.^[2] Fauna that consume plants may uptake this iodine but similarly return it to soils upon decomposition.^[2] Some iodine may also be cycled into the freshwater hydrosphere through leaching and runoff, where it may return to the oceans.^[1]

Similar to oceanic iodine, the majority of iodine cycled out of soil is volatilized through conversion to methyl forms of iodine by bacteria.^[3] Unlike ocean volatilization, however, bacteria are thought to be the only organisms responsible for volatilization in soils.^[4]

Iodine is a necessary trace nutrient for human health and is used as a product for various industries.^[3] Iodine intended for human use and consumption is taken from brines, which accounts for a minor perturbation to the global iodine cycle.^[1] A much larger anthropogenic impact is through the burning of fossil fuels, which releases iodine into the atmosphere.^[1]

Iodine-129, a radioisotope of iodine, is a waste product of nuclear power generation and weapons testing.^[3] Unless present in high concentrations, I-129 likely does not present danger to human health.^[6] Early research has attempted to use the I-129/I-127 ratio as a tracer for the iodine cycle.^[6]