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Hopkinson, BM, Barbeau KA.  2007.  Organic and redox speciation of iron in the eastern tropical North Pacific suboxic zone. Marine Chemistry. 106:2-17.   10.1016/j.marchem.2006.02.008   AbstractWebsite

The organic and redox speciation of iron was examined in the strongly layered upper water column of the eastern tropical North Pacific, including oxic and sub oxic waters, in a region 100- 1300 km offshore. Suboxic conditions ([O-2] < 5 mu M) were found to affect the organic speciation of iron, and reduced dissolved iron, Fc(II), was present in the suboxic zone, but conditions were not sufficiently reducing to convert all iron to Fe(II). Dissolved iron concentrations in the suboxic zone were similar to concentrations found in oxic regions. Using a competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) method, natural ligands were found to have distinct characteristics in the oxic and suboxic waters with stronger ligands found in the suboxic zone. It is unusual to find stronger ligands below the euphotic zone, but their strength, logK(Fe'L) = 12.1-12.8, is within the range determined for surface ligands in other regions. These strong ligands may be the result of the unique chemistry of the suboxic zone stabilizing reduced or labile compounds, or they may be actively produced by microbes to enhance iron uptake. No onshore-offshore trends in ligand strength or concentration were detected suggesting the ligands may result from the inherent chemistry of the suboxic zone or production from denitrifiers, rather than the resident suboxic zone population of Prochlorococcus which were more abundant nearshore. A luminol-chemiluminescence based flow injection analysis (FIA) technique capable of detecting picomolar concentrations of Fe(II) was used to assess the redox state of iron in the suboxic zone and overlying oxic waters at a station 1300 km offshore. An elevated signal equivalent to 0.12-0.15 nM Fe(II), 21-24% of dissolved iron, was found only in the suboxic waters. Oxidation kinetics suggest that this Fe(II) is most likely produced by an in-situ process, as opposed to being transported from shelf sediment. The luminol-chemiluminescence Fe(II) method was systematically tested for inferences from reduced species potentially present in the suboxic zone to validate our Fe(II) results. Several species, V(IV) and V(111), produced significant signals, but considerations of the reducing state of the suboxic zone make it unlikely that reduced V is present. With additional information on the identity of the suboxic zone species provided by analysis of signal decay rate, it was determined that Fe(II) was the most reasonable source of the signal, and at minimum the chemiluminescence data allows us to set limits on the Fe(II) concentration in the offshore suboxic water column. (C) 2006 Elsevier B.V. All rights reserved.

Buck, KN, Moffett J, Barbeau KA, Bundy RM, Kondo Y, Wu JF.  2012.  The organic complexation of iron and copper: an intercomparison of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques. Limnology and Oceanography-Methods. 10:496-515.   10.4319/lom.2012.10.496   AbstractWebsite

Characterization of the speciation of iron and copper is an important objective of the GEOTRACES Science Plan. To incorporate speciation measurements into such a multinational program, standard practices must be adopted that allow data from multiple labs to be synthesized. Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) is the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater. The determination of concentrations and conditional stability constants of metal-binding ligands is particularly challenging, as results can be influenced both by experimental conditions and interpretation of titration data. Here, we report an investigation between four laboratories to study the speciation of iron and copper using CLE-ACSV. Samples were collected on the GEOTRACES II intercomparison cruise in the North Pacific Ocean in May 2009 at 30 degrees N, 140 degrees W. This intercomparison was carried out shipboard and included an assessment of the viability of sample preservation by freezing. Results showed that consensus values could be obtained between different labs, but that some existing practices were problematic and require further attention in future work. A series of recommendations emerged from this study that will be useful in implementing multi-investigator programs like GEOTRACES.