Iron circulates through the atmosphere, lithosphere, and oceans. Labeled arrows show flux in Tg of iron per year.[1][2][3][4] Iron in the ocean cycles between plankton, aggregated particulates (non-bioavailable iron), and dissolved (bioavailable iron), and becomes sediments through burial.[1][5][6] Hydrothermal vents release ferrous iron to the ocean[7] in addition to oceanic iron inputs from land sources. Iron reaches the atmosphere through volcanism,[8] aeolian activity ,[9] and some via combustion by humans. In the Anthropocene, iron is removed from mines in the crust and a portion re-deposited in waste repositories.[4][6]
The iron cycle (Fe) is the biogeochemical cycle of iron through the atmosphere, hydrosphere, biosphere and lithosphere. While Fe is highly abundant in the Earth's crust,[10] it is less common in oxygenated surface waters. Iron is a key micronutrient in primary productivity,[11] and a limiting nutrient in the Southern ocean, eastern equatorial Pacific, and the subarctic Pacific referred to as High-Nutrient, Low-Chlorophyll (HNLC) regions of the ocean.[12]
Iron exists in a range of oxidation states from -2 to +7; however, on Earth it is predominantly in its +2 or +3 redox state and is a primary redox-active metal on Earth.[13] The cycling of iron between its +2 and +3 oxidation states is referred to as the iron cycle. This process can be entirely abiotic or facilitated by microorganisms, especially iron-oxidizing bacteria. The abiotic processes include the rusting of iron-bearing metals, where Fe2+ is abiotically oxidized to Fe3+ in the presence of oxygen, and the reduction of Fe3+ to Fe2+ by iron-sulfide minerals. The biological cycling of Fe2+ is done by iron oxidizing and reducing microbes.[14][15]
Iron is an essential micronutrient for almost every life form. It is a key component of hemoglobin, important to nitrogen fixation as part of the Nitrogenase enzyme family, and as part of the iron-sulfur core of ferredoxin it facilitates electron transport in chloroplasts, eukaryotic mitochondria, and bacteria. Due to the high reactivity of Fe2+ with oxygen and low solubility of Fe3+, iron is a limiting nutrient in most regions of the world.
^ abNickelsen L, Keller D, Oschlies A (2015-05-12). "A dynamic marine iron cycle module coupled to the University of Victoria Earth System Model: the Kiel Marine Biogeochemical Model 2 for UVic 2.9". Geoscientific Model Development. 8 (5): 1357–1381. Bibcode:2015GMD.....8.1357N. doi:10.5194/gmd-8-1357-2015.
^Jickells TD, An ZS, Andersen KK, Baker AR, Bergametti G, Brooks N, et al. (April 2005). "Global iron connections between desert dust, ocean biogeochemistry, and climate". Science. 308 (5718): 67–71. Bibcode:2005Sci...308...67J. doi:10.1126/science.1105959. PMID 15802595. S2CID 16985005.
^Raiswell R, Canfield DE (2012). "The iron biogeochemical cycle past and present" (PDF). Geochemical Perspectives. 1 (1): 1–232. Bibcode:2012GChP....1....1R. doi:10.7185/geochempersp.1.1.
^ abWang T, Müller DB, Graedel TE (2007-07-01). "Forging the Anthropogenic Iron Cycle". Environmental Science & Technology. 41 (14): 5120–5129. Bibcode:2007EnST...41.5120W. doi:10.1021/es062761t. PMID 17711233.
^Völker C, Tagliabue A (July 2015). "Modeling organic iron-binding ligands in a three-dimensional biogeochemical ocean model" (PDF). Marine Chemistry. 173: 67–77. Bibcode:2015MarCh.173...67V. doi:10.1016/j.marchem.2014.11.008.
^ abMatsui H, Mahowald NM, Moteki N, Hamilton DS, Ohata S, Yoshida A, Koike M, Scanza RA, Flanner MG (April 2018). "Anthropogenic combustion iron as a complex climate forcer". Nature Communications. 9 (1): 1593. Bibcode:2018NatCo...9.1593M. doi:10.1038/s41467-018-03997-0. PMC 5913250. PMID 29686300.
^Emerson D (2016). "The Irony of Iron - Biogenic Iron Oxides as an Iron Source to the Ocean". Frontiers in Microbiology. 6: 1502. doi:10.3389/fmicb.2015.01502. PMC 4701967. PMID 26779157.
^Olgun N, Duggen S, Croot PL, Delmelle P, Dietze H, Schacht U, et al. (2011). "Surface ocean iron fertilization: The role of airborne volcanic ash from subduction zone and hot spot volcanoes and related iron fluxes into the Pacific Ocean" (PDF). Global Biogeochemical Cycles. 25 (4): n/a. Bibcode:2011GBioC..25.4001O. doi:10.1029/2009GB003761.
^Gao Y, Kaufman YJ, Tanre D, Kolber D, Falkowski PG (2001-01-01). "Seasonal distributions of aeolian iron fluxes to the global ocean". Geophysical Research Letters. 28 (1): 29–32. Bibcode:2001GeoRL..28...29G. doi:10.1029/2000GL011926.
^Taylor SR (1964). "Abundance of chemical elements in the continental crust: a new table". Geochimica et Cosmochimica Acta. 28 (8): 1273–1285. Bibcode:1964GeCoA..28.1273T. doi:10.1016/0016-7037(64)90129-2.
^Tagliabue A, Bowie AR, Boyd PW, Buck KN, Johnson KS, Saito MA (March 2017). "The integral role of iron in ocean biogeochemistry" (PDF). Nature. 543 (7643): 51–59. Bibcode:2017Natur.543...51T. doi:10.1038/nature21058. PMID 28252066. S2CID 2897283.
^Martin JH, Fitzwater SE (1988). "Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic". Nature. 331 (6154): 341–343. Bibcode:1988Natur.331..341M. doi:10.1038/331341a0. S2CID 4325562.
^Melton ED, Swanner ED, Behrens S, Schmidt C, Kappler A (December 2014). "The interplay of microbially mediated and abiotic reactions in the biogeochemical Fe cycle". Nature Reviews. Microbiology. 12 (12): 797–808. doi:10.1038/nrmicro3347. PMID 25329406. S2CID 24058676.
^Schmidt C, Behrens S, Kappler A (2010). "Ecosystem functioning from a geomicrobiological perspective – a conceptual framework for biogeochemical iron cycling". Environmental Chemistry. 7 (5): 399. doi:10.1071/EN10040.
^Kappler, Andreas; Straub, Kristina L. (2005-01-01). "Geomicrobiological Cycling of Iron". Reviews in Mineralogy and Geochemistry. 59 (1): 85–108. doi:10.2138/rmg.2005.59.5. ISSN 1529-6466.
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