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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2018-439
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-439
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 26 Oct 2018

Research article | 26 Oct 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Biogeosciences (BG) and is expected to appear here in due course.

Fe(II) stability in seawater

Mark J. Hopwood1, Carolina Santana-González2, Julian Gallego-Urrea3, Nicolas Sanchez4, Eric P. Achterberg1, Murat V. Ardelan4, Martha Gledhill1, Melchor González-Dávila2, Linn Hoffmann5, Øystein Leiknes4, Juana Magdalena Santana-Casiano2, Tatiana M. Tsagaraki6, and David Turner3 Mark J. Hopwood et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany
  • 2Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canaria, ULPGC, Las Palmas, Spain
  • 3University of Gothenburg, Sweden
  • 4Norwegian University of Science and Technology, Trondheim, Norway
  • 5University of Otago, Dunedin, New Zealand
  • 6Department of Biological Sciences, University of Bergen, Norway

Abstract. The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II). Here we deploy flow injection analysis to measure in-situ Fe(II) concentrations during a series of mesocosm/microcosm experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During 5 mesocosm/microcosm experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24–65 % of DFe suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were retarded relative to calculated constants derived from ambient temperature, salinity, pH and dissolved O2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations < 2 nM and attributed to a stabilising effect of cellular exudates upon Fe(II). This enhanced stability of Fe(II) under post-bloom conditions, and the existence of such a high fraction of DFe as Fe(II), challenges the assumption that DFe speciation is dominated by ligand bound-Fe(III) species.

Mark J. Hopwood et al.
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Mark J. Hopwood et al.
Mark J. Hopwood et al.
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Short summary
Fe(III)-organic species are thought to account for > 99 % of dissolved Fe in seawater. Here we quantified Fe(II) during experiments in Svalbard, Gran Canaria and Patagonia. Fe(II) was always a measurable fraction of dissolved Fe- up to 65 %. Furthermore, when Fe(II) was allowed to decay in the dark it remained present longer than predicted by kinetic equations suggesting that Fe(II) is an more important fraction of dissolved Fe in seawater than widely recognized.
Fe(III)-organic species are thought to account for  99 % of dissolved Fe in seawater. Here we...
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