Journal
MARINE CHEMISTRY
Volume 173, Issue -, Pages 125-135Publisher
ELSEVIER
DOI: 10.1016/j.marchem.2014.09.002
Keywords
Iron; Iron ligands; CLE-ACSV; Colloids; Ultrafiltration; Trace metals; GEOTRACES; North Atlantic Ocean; Chemical oceanography
Categories
Funding
- National Science Foundation Graduate Research Fellowship (NSF) [0645960]
- National Science Foundation (OCE) [0926204, 0926197]
- Center for Microbial Oceanography: Research and Education (NSF-OIA Award) [EF-0424599]
- NSF [OCE-0550302, OCE-1233733]
- NSF-GK12 graduate fellowship
- Directorate For Geosciences
- Division Of Ocean Sciences [1260164, 0926204] Funding Source: National Science Foundation
- Division Of Ocean Sciences
- Directorate For Geosciences [0926197] Funding Source: National Science Foundation
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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.
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