Dissolved iron(II) in the Pacific Ocean: Measurements from the PO2 and P16N CLIVAR/CO2 repeat hydrography expeditions

Abstract

The redox speciation of dissolved iron in open ocean seawater was evaluated during two Pacific Ocean research cruises. Using a highly sensitive flow injection method based on luminol chemiluminescence, vertical profiles of reduced iron concentration (Fe(II)) were obtained at 134 stations. In this paper, sampling and analytical methods are discussed and values obtained for Fe(II) are compared to shipboard measurements of total dissolved iron (Fe DISS). Concentration profiles are evaluated within the context of various proposed source mechanisms and experimental models of Fe(II) oxidation kinetics. Samples were collected from rosette-mounted GO-FLO bottles using trace metal clean equipment and techniques. While this allowed sample collection to depths of 1000 m, the length of time required for rosette retrieval coupled with the potential for rapid oxidative loss of Fe(II) complicates the detection of photochemical production processes that are expected to be operative in the upper water column. Acidification of seawater samples retards oxidation until sample analyses can be completed, but for undetermined reasons it contributes both to the blank response, and to minor instabilities in system response over time that are depth-specific, effects which must be considered and corrected for. Analysis by luminol chemiluminescence is fast and inexpensive, and typically yielded detection limits of 10-15 pM. A requirement of the method is that Fe(II) present in samples be uniquely oxidized in the reaction cell at the same rate as the Fe(II) used for standard preparation. By changing reaction kinetics, strong organic complexes like EDTA can produce false negatives. Also, reduced species other than iron can be oxidized at the high system pH, leading to positive interferences. Vanadium (IV) and vanadium (V) were found to significantly interfere. Because Fe(II) is believed to form weak complexes in seawater, and reduced vanadium has not been detected in oxic seawater, interferences were believed to be minimal. The results from the two cruises suggest a relatively consistent pattern of Fe(II) occurrence and distribution in the Pacific Ocean. Surface water maxima are present in most profiles, with median concentrations of 25-30 pM, accounting for approximately 12% of the total dissolved iron. Concentrations decline with depth to undetectable levels in the upper 100 meters. Below this depth, Fe(II) remains low. However, deepest samples (700-1000 m) frequently contained detectable Fe(II), though at a very low percentage of the total dissolved iron. The concentration profile for the upper water column is consistent with a photochemical production xii mechanism. However, this production should cease upon sample collection, and the rapid oxidative loss predicted for surface waters should remove these higher Fe(II) concentrations prior to sub-sample collection from the GO-FLO bottles. Fe(II) in deep samples was found in association with the oxygen minimum of the profile, possibly due to the remineralization of sinking biogenic particles...

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…Increases in the apparent solubility when considering the longer-term processing of lithogenic particles, similar to that for Th, have been reported for Fe (Boyd et al, 2010;Frew et al, 2006;Hansard et al, 2009). On this basis, we infer a general similarity in the dissolution of relatively insoluble lithogenic elements such as Fe, Nd, etc.…”

Iron biogeochemistry across marine systems – progress from the past decade

“…Calculations of the inorganic iron concentration based on measurements carried out by competitive equilibration cathodic stripping voltammetry show that inorganic iron concentrations in the ocean are of the order of 10 −14 -10 −11 M (Morel et al, 2008), although these calculations neglect the contribution of Fe(II), which may also be present at concentrations of the order of 10 −11 M in surface waters (e.g. Hansard et al, 2009;Roy et al, 2008b;Croot et al, 2001). It is not clear how much of the organically complexed iron is available to marine phyto-and bacterioplankton, and parameters controlling Fe bioavailability to primary producers are still poorly understood.…”

The regeneration of highly bioavailable iron by meso‐ and microzooplankton

“…However, others have found that protoporphyrin IX is a good chelator for Fe(II) but hardly binds Fe(III) in seawater at pH 8 (Rijkenberg et al 2006), calling into question the use of protoporphyrin IX as a model compound for regenerated Fe(III). Nevertheless, measurements of Fe(II) in certain regimes (Gledhill and Van Den Berg 1995;Blain et al 2008;Hansard et al 2009) have stimulated the discussion of Fe(II)-binding ligands (Breitbarth et al 2009;Statham et al 2012). If present, such organic ligands could inhibit the oxidation of Fe(II) (Theis and Singer 1974;Santana-Casiano et al 2000).…”

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