The composition of dissolved iron in the dusty surface ocean: An exploration using size-fractionated iron-binding ligands

TitleThe composition of dissolved iron in the dusty surface ocean: An exploration using size-fractionated iron-binding ligands
Publication TypeJournal Article
Year of Publication2015
AuthorsFitzsimmons J.N, Bundy R.M, Al-Subiai S.N, Barbeau K.A, Boyle E.A
JournalMarine Chemistry
Volume173
Pagination125-135
Date Published2015/07
Type of ArticleArticle
ISBN Number0304-4203
Accession NumberWOS:000355044800011
Keywordscathodic stripping voltammetry; Chemical oceanography; CLE-ACSV; colloidal iron; Colloids; diatom blooms; GEOTRACES; humic substances; Iron; Iron ligands; natural organic-ligands; North Atlantic Ocean; north-atlantic ocean; pacific-ocean; seawater; southern-ocean; speciation; Trace metals; trace-metal; Ultrafiltration
Abstract

The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1-0.65 nM dissolved iron (<0.2 mu m) was partitioned into the colloidal iron (cFe) size fraction (10 kDa < cFe <0.2 gm), as expected for areas of the ocean underlying a dust plume. The 1.3-2.0 nM strong organic iron-binding ligands, however, overwhelmingly (75-77%) fell into the soluble size fraction (<10 kDa). As a result, modeling the dissolved iron size fractionation at equilibrium using the observed ligand partitioning did not accurately predict the iron partitioning into colloidal and soluble pools. This suggests that either a portion of colloidal ligands is missed by current electrochemical methods because they react with iron more slowly than the equilibration time of our CLE-ACSV method, or part of the observed colloidal iron is actually inorganic in composition and thus cannot be predicted by our model of unbound iron-binding ligands. This potentially contradicts the prevailing view that greater than >99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton. (C) 2014 Elsevier B.V. All rights reserved.

DOI10.1016/j.marchem.2014.09.002
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