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      Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean

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          Abstract

          <p id="d1919800e182">Iron is an essential micronutrient for life. However, its scarcity limits algae growth in about one-half of the ocean. Its cycle is therefore linked to the global carbon cycle and climate. We present an iron isotope section from the Southern Ocean. In contrast to the common but oversimplified view, according to which organic matter remineralization is the major pathway releasing dissolved iron below the surface layers, these data reveal other dominant processes at depth, likely abiotic desorption/dissolution from lithogenic particles. This suggests that the iron cycle, and therefore primary production and climate, may be more sensitive than previously thought to continental erosion, dissolved/particle interactions, and deep water upwelling. These processes likely impact other elements in the ocean. </p><p class="first" id="d1919800e185">As an essential micronutrient, iron plays a key role in oceanic biogeochemistry. It is therefore linked to the global carbon cycle and climate. Here, we report a dissolved iron (DFe) isotope section in the South Atlantic and Southern Ocean. Throughout the section, a striking DFe isotope minimum (light iron) is observed at intermediate depths (200–1,300 m), contrasting with heavier isotopic composition in deep waters. This unambiguously demonstrates distinct DFe sources and processes dominating the iron cycle in the intermediate and deep layers, a feature impossible to see with only iron concentration data largely used thus far in chemical oceanography. At intermediate depths, the data suggest that the dominant DFe sources are linked to organic matter remineralization, either in the water column or at continental margins. In deeper layers, however, abiotic non-reductive release of Fe (desorption, dissolution) from particulate iron—notably lithogenic—likely dominates. These results go against the common but oversimplified view that remineralization of organic matter is the major pathway releasing DFe throughout the water column in the open ocean. They suggest that the oceanic iron cycle, and therefore oceanic primary production and climate, could be more sensitive than previously thought to continental erosion (providing lithogenic particles to the ocean), particle transport within the ocean, dissolved/particle interactions, and deep water upwelling. These processes could also impact the cycles of other elements, including nutrients. </p>

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          Iron deficiency limits phytoplankton growth in the north-east Pacific subarctic

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            Distribution of thorium isotopes between dissolved and particulate forms in the deep sea

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              The biogeochemical cycle of iron in the ocean

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                Author and article information

                Journal
                Proceedings of the National Academy of Sciences
                Proc Natl Acad Sci USA
                Proceedings of the National Academy of Sciences
                0027-8424
                1091-6490
                January 31 2017
                January 31 2017
                January 31 2017
                January 17 2017
                : 114
                : 5
                : 858-863
                Article
                10.1073/pnas.1603107114
                5293069
                28096366
                adb053df-8a3b-462c-8bc6-1c63e885b0ef
                © 2017

                Free to read

                http://www.pnas.org/site/misc/userlicense.xhtml

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