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Fe(II) stability in seawater

Journal article
Authors Mark J. Hopwood
Carolina Santana-González
Julian A. Gallego-Urrea
Nicolas Sanchez
Eric P. Achterberg
Murat V. Ardelan
Martha Gledhill
Melchor González-Dávila
Linn Hoffmann
Øystein Leiknes
Juana Magdalena Santana-Casiano
Tatiana M. Tsagaraki
David R. Turner
Published in Biogeosciences Discussions
ISSN 1810-6277
Publication year 2018
Published at Department of marine sciences
Language en
Links https://www.biogeosciences-discuss....
Subject categories Climate Research, Water in nature and society, Spectroscopy, Environmental chemistry, Heavy metals and other metals

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 (0.2µm) 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 <2nM 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.

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