Both soluble and colloidal iron phases control dissolved iron variability in the tropical North Atlantic Ocean

The size partitioning of dissolved iron (dFe, <0.4 mu m) into soluble (sFe, <0.02 mu m) and colloidal (0.02 mu m < cFe < 0.4 mu m) phases was investigated at seven stations in the tropical North Atlantic Ocean, and the results are compared to the dFe size fractionation study of Bergquist et al. (2007) in the same region. Downwind of the North African dust plumes, cFe comprised 80 + 7% of the surface dFe pool at six stations, supporting the hypothesis that atmospherically-derived Fe is maintained in the colloidal size fraction. At the deep chlorophyll maximum, colloidal Fe had minimum concentrations or was completely absent, suggesting that cFe was either preferentially taken up by microbes and/or scavenged/aggregated at these depths. At remineralization depths, sFe was the dominant fraction both in the subtropical gyre-like stations (76% sFe; [ sFe] = 0.42 +/- 0.03 nmol/kg) and in the oxygen minimum zone (56% sFe; [ sFe] = 0.65 +/- 0.03 nmol/kg). Only at remineralization depths of stations with intermediate oxygen concentrations (100-110 mu mol/kg) did colloidal Fe dominate (contributing 58% of dFe), indicating that cFe may be serving as a conduit of dFe loss during mixing of high-Fe OMZ and low-Fe gyre waters. North Atlantic Deep Water (NADW) had a typical sFe concentration of 0.34 +/- 0.05 nmol/kg. In the deepest samples composed of a NADW/Antarctic Bottom Water mixture where the bottom water may have attained a similar to 0.1 nmol/kg hydrothermal Fe input during transit past the Mid-Atlantic Ridge, sFe did not increase coincidentally with dFe, indicating that any potential hydrothermal Fe contribution was colloidal. In general, the results of this study counter the previous hypothesis of Bergquist et al. (2007) that the colloidal Fe fraction predominately controls dFe variability, instead suggesting that both soluble and colloidal Fe are variable, and both contribute to the observed dFe variability throughout the North Atlantic. The nearly 50-50% dFe partitioning into soluble and colloidal phases below the DCM suggest one of two partitioning mechanisms persists: (1) soluble and colloidal Fe exchange rates reach a "steady state," over which regional, uniquely-partitioned Fe sources can be overlain, or (2) the partitioning of Fe-binding ligands between the two size fractions is variable in the open ocean and directly controls dFe partitioning. (C) 2013 Elsevier Ltd. All rights reserved.