The composition of dissolved iron in the dusty surface ocean : an exploration using size-fractionated iron-binding ligands
The composition of dissolved iron in the dusty surface ocean : an exploration using size-fractionated iron-binding ligands
Date
2014-09
Authors
Fitzsimmons, Jessica N.
Bundy, Randelle M.
Al-Subiai, Sherain N.
Barbeau, Katherine A.
Boyle, Edward A.
Bundy, Randelle M.
Al-Subiai, Sherain N.
Barbeau, Katherine A.
Boyle, Edward A.
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Keywords
Iron
Iron ligands
CLE-ACSV
Colloids
Ultrafiltration
Trace metals
GEOTRACES
North Atlantic Ocean
Chemical oceanography
Iron ligands
CLE-ACSV
Colloids
Ultrafiltration
Trace metals
GEOTRACES
North Atlantic Ocean
Chemical oceanography
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 μm) was partitioned into the colloidal iron (cFe) size fraction (10 kDa <
cFe < 0.2 μm), 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 are 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. Untangling the chemical form of iron in the upper ocean has important
implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton.
Description
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Marine Chemistry 173 (2015): 125-135, doi:10.1016/j.marchem.2014.09.002.