A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES North Atlantic transect cruise (GEOTRACES GA03)

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A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES north Atlantic transect cruise (GEOTRACES GA03) P.N Sedwick, B.M. Sohst, S.J. Ussher, A.R. Bowie

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Cite this article as: P.N Sedwick, B.M. Sohst, S.J. Ussher, A.R. Bowie, A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES north Atlantic transect cruise (GEOTRACES GA03), Deep-Sea Research II, http://dx.doi.org/10.1016/j.dsr2.2014.11.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Manuscript revised for Deep-Sea Research II, August 8, 2014

A zonal picture of the water column distribution of dissolved iron(II) during the U.S. GEOTRACES North Atlantic Transect cruise (GEOTRACES GA03) P. N Sedwicka, B. M. Sohsta, S. J. Ussherb, A. R. Bowiec,d a

Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA 23529 USA

b

School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth

PL4 8AA, UK c

Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS 7001, Australia

d

Antarctic Climate and Ecosystems Cooperative Research Centre, Hobart, TAS 7001, Australia

ABSTRACT We report measurements of the transient species iron(II) in filtered water column samples collected during the U.S. GEOTRACES North Atlantic Transect cruise (GEOTRACES GA03), which was comprised of legs from Lisbon to Cape Verde in October-November 2010, and from Woods Hole to Cape Verde in November-December 2011. Dissolved iron(II) (dFe(II)) was determined at sea in 0.2-µm filtered samples as soon as possible after collection and filtration, using flow-injection analysis, with mean detection limits of 0.06 nM and 0.01 nM during the 2010 and 2011 cruise legs, respectively. Water column concentrations along the cruise transects were generally low (<0.2 nM), with the exception of deep water samples collected over the Trans Atlantic Geotraverse hydrothermal field, in which dFe(II) was as high as 70 nM and accounted for more than 80% of the dissolved iron pool in the near-field hydrothermal plume. Smaller local concentration maxima were observed near 1000 m depth in the low-oxygen, iron-rich waters to the east of Cape Verde, where dFe(II) is correlated with apparent oxygen utilization, and in the upper water column at several stations in the Subtropical Gyre, where dFe(II) can account for greater than 50% of the dissolved iron pool in the lower euphotic zone. Elevated dFe(II) concentrations were also observed over much of the water column on the Bermuda platform, although the source of this enrichment remains uncertain, in the absence of data between Woods Hole and Bermuda.

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Manuscript revised for Deep-Sea Research II, August 8, 2014 1. Introduction Among the bioactive trace elements present in seawater, the essential micronutrient iron (Fe) arguably holds the greatest importance for oceanic biology and carbon cycling, owing to its demonstrated role in regulating primary production and phytoplankton community structure in much of the surface ocean (Martin et al., 1991; Boyd et al., 2007). As such, the past few decades have seen much effort directed at improving our understanding of the distribution and biogeochemical cycling of Fe in the ocean (Johnson et al., 1997; Moore and Braucher 2008), with Fe being deemed a 'key parameter' to be measured on all ocean section cruises of the GEOTRACES program (Anderson et al., 2007; Sohrin and Bruland, 2011). In this context, the physicochemical speciation of dissolved Fe (dFe) is of interest for two reasons. First, because inorganic Fe(III) has low solubility in oxygenated seawater, whereas organically-complexed Fe(III) and inorganic Fe(II) are relatively more soluble; and second, because the biological availability of dFe is thought to vary according to its physicochemical speciation (Liu and Millero, 2002; Boyd and Ellwood, 2010). Dissolved Fe in the +2 oxidation state (dFe(II)) is thermodynamically unstable in oxic seawater (Millero et al., 1987; Santana-Casiano et al., 2006), having an oxidation half life ranging from minutes to days, depending on the ambient physical and chemical conditions (King et al, 1995; Ussher et al., 2007; Hansard et al., 2009). However, numerous studies have documented measurable concentrations of dFe(II) in the oceanic water column, which are thought to reflect sustained production or input of dFe(II), via photochemical or biological reduction of dissolved, colloidal or particulate Fe(III); release of dFe(II) via microbial remineralization, lysis and grazing; and external inputs from atmospheric deposition, seafloor sediments, submarine groundwaters, and hydrothermal emissions (Hansard et al., 2009; Sarthou et al. 2011). In addition, significant gradients in dFe(II) concentration, most likely due to in situ processes, have been documented in coastal hypoxic waters (Lohan and Bruland, 2008) and in the open ocean (Kondo and Moffett, 2013; Roy et al., 2008). Although the relative biological availability of the various physical and chemical species

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Manuscript revised for Deep-Sea Research II, August 8, 2014 of iron in the marine environment remains to be clearly established, reductive iron uptake is thought to be widely employed by both eukaryotic and prokaryotic phytoplankton (Shaked and Lis, 2012). Thus Fe(II) is likely to be more readily available to phytoplankton than inorganic and organic Fe(III) species (Sunda, 2000). Of importance in this context are the results of studies of iron redox cycling, which have shown that the presence of organic matter is a critical variable that can inhibit the precipitation of iron oxyhydroxides, and that the photoreductive dissociation of organic-Fe(III) complexes can release the more labile and bioavailable Fe(II) into seawater (Barbeau et al., 2001; Steigenberger et al., 2010). However, if this Fe(II) is not utilized by phytoplankton in situ, then high rates of photoreduction of organically complexed Fe(III) may enhance the precipitation and scavenging losses of dFe from surface waters (Rose and Waite, 2003). The apparent geochemical and biological importance of Fe(II) demands an understanding of the distribution and cycling of this transient chemical species at the ocean basin scale. Although there have been a number of recent studies of Fe(II) in specific oceanic regions (e.g., Boye et al., 2003; Hopkinson and Barbeau, 2007; Moffett et al., 2007; Ussher et al., 2007; Lohan and Bruland, 2008; Roy et al., 2008; Kondo and Moffett, 2013), the only Fe(II) data reported for basin-scale transects are by Hansard et al. (2009), who present dissolved Fe(II) data for two Pacific CLIVAR sections, and by Sarthou et al. (2011), who report 'labile Fe(II)' determined in unfiltered samples along a South Atlantic transect from the Cape Basin into the Weddell Sea Gyre. Additional basin-scale Fe(II) data are expected to emerge from the ocean section cruises conducted for the GEOTRACES program. Here we report shipboard measurements of dFe(II) in 0.2 µm-filtered watercolumn and surface-water samples that were collected during the U.S. GEOTRACES North Atlantic Transect cruise (GEOTRACES GA03), which included legs from Lisbon to Cape Verde in October-November 2010, and from Woods Hole to Cape Verde in November-December 2011. In addition, we present data for total dissolved iron (dFe), which was determined in duplicate 0.2 µm-filtered, acidified samples after the cruises by flow injection analysis. Dissolved iron was determined in samples from all major stations occupied during the GEOTRACES GA03 cruise by independent laboratories: at the University of Hawaii

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Manuscript revised for Deep-Sea Research II, August 8, 2014 using shipboard flow injection analysis (Measures et al., 1995); at the University of Miami using shore-based plasma source mass spectrometry (Wu and Boyle, 1998); and at Old Dominion University using shore-based flow injection analysis (Sedwick et al., 2008). There are small but discernible differences between these three dFe data sets, however they all display the same general features in terms of the water column distribution of dFe. More detailed discussions of the GA03 dFe data are presented by Hatta et al. (this issue), who provide an overview of the basin-scale distribution, sources and sinks of dFe, and by Wu et al. (this issue), who focus on the processes responsible for the dFe distribution in the oxygen minimum zone of the eastern tropical North Atlantic. In this paper we present the dFe data that were generated at Old Dominion University, primarily for the purpose of interpreting our dFe(II) data. These dFe data have been evaluated and approved by the GEOTRACES Standards and Intercalibration Committee for inclusion in the GEOTRACES Intermediate Data Product. 2. Methods 2.1. Sample Collection Water-column and surface-water samples were collected during the U.S. GEOTRACES North Atlantic Transect cruise (GEOTRACES cruise GA03), which was completed in two legs aboard R/V Knorr, from October 15-November 4, 2010 (Lisbon to Cape Verde, hereafter termed cruise leg USGT10) and November 6-December 11, 2011 (Woods Hole to Cape Verde, hereafter termed cruise leg USGT11). The water column sampling stations that were occupied are shown in Figure 1. Shipboard determinations of dissolved Fe(II) were made using water-column samples collected from 9 stations occupied during leg USGT10 and 8 stations occupied during leg USGT11. Due to a detector failure during cruise leg USGT11, dFe(II) was not measured in samples collected from stations between Woods Hole and Bermuda. Dissolved Fe was determined after the cruises in samples from 9 stations occupied during leg USGT10 and 13 stations occupied during leg USGT11. All water column samples were collected in 12 L General Oceanics GO-FLO bottles modified for trace metal sampling, which were deployed using the U.S. GEOTRACES

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Manuscript revised for Deep-Sea Research II, August 8, 2014 trace-metal clean CTD carousel system (Cutter and Bruland, 2012). Upon recovery, the GO-FLO samplers were brought into a shipboard Class-100 clean laboratory container for processing and sub-sampling. The samplers were then pressurized to 10 psi using filtered compressed air, and the water samples were filtered through pre-cleaned 0.2 µm Acropak Supor capsule filters (Pall) using rigorous trace-metal clean protocols (Cutter and Bruland, 2012). Subsamples for shipboard dFe(II) analysis were immediately collected in acid-cleaned 60 mL Nalgene fluorinated ethylene propylene bottles, which were rinsed once with sample then filled and capped with no headspace. In an effort to minimize oxidation of Fe(II), these subsamples were kept on ice in darkness until analysis, which was typically performed within one hour of filtration. Duplicate subsamples for shore-based dFe analysis were collected in acid-cleaned 125 mL Nalgene low density polyethylene bottles, and subsequently acidified at sea to pH 1.7 using ultrapure 6 N hydrochloric acid solution (Fisher Optima). In addition to the water-column samples, surface-water (~2 m depth) samples were collected using an underway towed-fish pumped seawater system developed by Geoffrey Smith and Ken Bruland (Johnson et al., 2007), with samples sequentially filtered in-line through acid-cleaned 0.45 µm and 0.2 µm pleated polycarbonate cartridge filters (Osmonics). Filtered subsamples for dFe(II) and dFe analysis were collected in the same manner as for the GO-FLO samples, using trace-metal clean protocols in a shipboard Class-100 container laboratory. 2.2. Sample Analyses Dissolved iron(II) (dFe(II)) analysis Iron(II) was determined by flow injection analysis with in-line preconcentration and chemiluminescence detection, modified after the method of Bowie et al. (2002, 2005). Standards used for the shipboard analyses included a primary stock standard of 0.02 M dFe(II), prepared at the beginning of each cruise leg by dissolving ferrous ammonium sulfate hexahydrate (Fluka BioUltra grade) in ultrapure 0.1 N hydrochloric acid solution (Fisher Optima); this solution was stabilized by the addition of 100 µM sodium sulfite (Sigma Aldrich BioExtra grade). Working standards with concentrations of 200 µM and

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Manuscript revised for Deep-Sea Research II, August 8, 2014 200 nM dFe(II) were prepared daily by serial dilution of the primary standard with ultrapure 0.01 N and 0.001 N hydrochloric acid solution (Fisher Optima), respectively. Aged, low-Fe seawater adjusted to pH 6.1 with 0.4 M ammonium acetate buffer was spiked with dFe(II) standard to obtain calibration standards in the typical concentration range of 0-0.8 nM dFe(II). The iron-rich hydrothermal plume samples were analyzed after dilution with aged, low-Fe(II) seawater, in order to bring them within the range of our dFe(II) calibration standards. The analytical limit of detection was estimated daily as the dFe(II) concentration corresponding to a signal equal to three times the standard deviation of triplicate analyses of the blank, following the definitions used by Bowie et al. (2004) and Sarthou et al. (2011). For the blank solution, we used filtered, aged, lowiron seawater that was stored in the dark, for which the chemiluminescence signal was analytically indistinguishable from reagents injected without seawater (see Bowie et al., 2005). The thus-defined limit of detection averaged <0.06 nM for all of the daily shipboard analyses (n = 16) during the USGT10 cruise leg, and <0.01 nM for all of the daily shipboard analyses (n = 31) on the USGT11 cruise leg (the lower detection limits achieved on leg USGT11 reflect the newer, more sensitive detector that was used on that cruise). There is no standard reference material for the determination of dissolved iron(II) in seawater, thus we are unable to provide rigorous estimates of the accuracy and precision of the dFe(II) determinations. However, some indication of our analytical uncertainty is provided by repeat measurements of standards prepared in low-Fe(II) seawater, which yielded average relative standard deviations of <17% and <11% for triplicate injections of 0.2 nM and 0.4 nM dFe(II) standards during the USGT10 cruise, and <13% and <11% for triplicate injections of 0.2 nM and 0.4 nM dFe(II) standards during the USGT11 cruise leg. Dissolved iron (dFe) analysis Dissolved iron was determined after the cruise at Old Dominion University by flow injection analysis with in-line preconcentration on resin-immobilized 8-hydroxyquinoline and colorimetric detection (Sedwick et al., 2005, 2008), using a method modified from Measures et al. (1995). The efficacy of our analytical method for dFe was verified through the analysis of the SAFe seawater reference materials. Over the period during

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Manuscript revised for Deep-Sea Research II, August 8, 2014 which these cruise samples were analyzed, we determined mean dFe concentrations of 0.122 ± 0.021 nM (n = 7) for SAFe seawater reference material S and 1.13 ± 0.19 nM (n = 2) for SAFe seawater reference material D2, which lie within one standard deviation of the community consensus concentrations of 0.095 ± 0.008 nM and 0.955 ± 0.024 nM, respectively (http://es.ucsc.edu/~kbruland/GeotracesSaFe/2012GeotracesSAFeValues). The analytical limit of detection is estimated as the dFe concentration equivalent to a peak area that is three times the standard deviation on the ‘zero-loading blank’ (or ‘manifold blank’), from which we estimate a detection limit of less than 0.04 nM (Bowie et al., 2004; Sedwick et al., 2005). Blank contributions from the ammonium acetate sample buffer solution (added on-line during analysis) and hydrochloric acid (added after collection) are typically negligible (i.e., too low to quantify). Robust estimates of our analytical precision are derived from multiple (separate-day) determinations of the SAFe seawater reference materials, which yield analytical uncertainties (expressed as ± one relative sample standard deviation on the mean) of ±14.9% (n = 33) at the concentration level of SAFe S seawater (0.095 nM), and ±9.2% (n = 16) at the concentration level of SAFe D2 seawater (0.955 nM). Data processing Following subtraction of appropriate blank values, water sample concentrations were calculated using least-square linear regressions fit to daily calibration curves, which were obtained by additions of dFe and dFe(II) standard solutions to low-iron seawater. Sample solutions were analyzed on a volumetric basis, thus dFe(II) and dFe concentrations are reported in units of nmol L-1 at laboratory temperature (~20°C). 3. Results and Discussion Contoured vertical concentration sections were prepared using Ocean Data View (Schlitzer, 2002), and are shown in three panels corresponding to the sections labeled 'zonal', 'Mauritanian', and 'meridional' in Figure 1. The 'zonal section' corresponds to the quasi-zonal transect from Woods Hole to Cape Verde during cruise leg USGT11 (stations 1-21); the 'Mauritanian section' corresponds to the quasi-zonal transect from the continental shelf of Mauritania to Cape Verde during cruise leg USGT10 (stations 9-12);

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Manuscript revised for Deep-Sea Research II, August 8, 2014 and the 'meridional section' corresponds to the quasi-meridional transect from Lisbon to the latitude of Cape Verde during cruise leg USGT10 (stations 1-10). 3.1. Dissolved Iron Distribution Detailed discussions of the distributions of dFe during the GEOTRACES GA03 cruise are presented by Hatta et al. (this issue) and Wu et al. (this issue), with additional data on the size distribution, organic complexation and isotopic composition of dissolved iron presented by Fitzsimmons et al. (this issue), Buck et al. (this issue), and Conway and John (2014), respectively. Here we summarize the main features of the dFe distribution during this cruise, which may provide primary controls on the distribution of dFe(II). The dFe determinations made at Old Dominion University are presented as contoured vertical sections in Figure 2. Briefly, along the USGT11 zonal section, dFe concentrations are enriched at the surface and depleted in the immediate subsurface into the thermocline, reflecting aeolian input to the sea surface, and subsurface removal via biological uptake and particle scavenging. Enrichment from continental shelf iron sources were observed along the western edge of the section, apparently associated with Upper Labrador Sea Water (ULSW), with elevated concentrations (~1 nM) below the thermocline that extend east beyond the Bermuda platform (Hatta et al., this issue). On the eastern edge of the USGT11 zonal section, and along the entire USGT10 Mauritanian section, elevated dFe concentrations extend from below the surface mixed layer into deep waters (Fig. 2), in a clear association with apparent oxygen utilization (AOU) down to ~1500 m depth (Fig. 3). The core of this subsurface dFe maximum is thought to reflect regeneration from Fe-rich particles exported from overlying surface waters, which are characterized by high levels of primary production and mineral dust deposition (Wu et al., this issue). However, the elevated dFe concentrations that extend to the seafloor at the eastern edge of the Mauritanian section, along with enrichment of other tracers of shelf input, suggest that there are significant sedimentary iron inputs from the continental slope as well (Hatta et al., this issue). Along the USGT10 meridional section starting from Lisbon, elevated dFe concentrations (~0.5-1 nM) were observed adjacent to the coast of Portugal, south of which lower

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Manuscript revised for Deep-Sea Research II, August 8, 2014 surface concentrations (<0.5 nM) extend into the main thermocline until ~30°N. Further south, relatively high sub-surface dFe concentrations (>1 nM) shoal toward the surface and extend toward the seafloor, as the Mauritanian section is approached. The most striking dFe signature of the GA03 cruise was observed in deep waters over the Trans Atlantic Geotraverse (TAG) hydrothermal site on the Mid Atlantic Ridge (MAR). Here, at station USGT11-16, deep water column dFe concentrations as high as 78 nM (Fig. 2) along with other chemical and physical anomalies indicate the presence of a hydrothermal plume, which appears to carry elevated dFe concentrations at least as far east as 40°W, and as far west as 50°W (Fig. 2; Hatta et al., this issue). 3.2. Dissolved Iron(II) Distribution An issue that is germane to the interpretation of our data concerns the chemical speciation of the dFe(II) that is determined by our analytical method. Using flow injection analysis, the species that we measure are operationally defined as any Fe(II) species that pass through a 0.2 µm pore-size filter, are retained by and then eluted from a resinimmobilized 8-hydroxyquinoline preconcentration column, and then catalyze the oxidation of luminol. Using a similar analytical method, Ussher et al. (2005) have examined a range of organic-Fe(II) complexes, and found that most were quantified by this analytical technique, with the exception of Fe(II) complexed by strong organic ligands. Little is known concerning the chemical speciation of Fe(II) in the oceanic water column. Many organic and inorganic complexes of Fe(II) are likely to be unstable in seawater under oxic conditions (e.g., Boukhalfa and Crumbliss, 2002), although Fe(II) that is complexed by stronger organic ligands or associated with proteins, as might result from bacterial remineralization (Boyd and Ellwood, 2010), may be relatively stable in seawater. In the absence of specific information on the chemical speciation of dissolved Fe(II) in the oceanic water column, we assume that our flow analysis method quantifies the majority of dissolved Fe(II) that is present in seawater. We used no preservative (i.e., reductant) to slow the oxidation of Fe(II) prior to shipboard analysis, with the rationale that the addition of a reducing agent to the samples might result in artefacts associated with the reduction of Fe(III) species. However, our choice

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Manuscript revised for Deep-Sea Research II, August 8, 2014 of sample treatment means that there was likely some oxidative loss of Fe(II) between the time of sample filtration and the dFe(II) measurement, despite keeping the filtered samples on ice and in darkness prior to analysis. In addition, the seawater samples had spent a considerable time inside the GO-FLO bottles prior to filtration, including up to several hours in the water column for the deepest samples, followed by as much as 2-3 hours inside the clean van before sub-samples were drawn. Over this period, however, the water samples remained close to their in-situ temperature, in darkness, without exposure to air, which likely minimizes oxidative loss of dFe(II) in the GO-FLO bottles. Previous laboratory and field studies allow estimates of the oxidative loss of dFe(II) in seawater. The temperature-dependent oxidation kinetics of Millero et al. (1987) and Millero and Sotolongo (1989) permit estimates of the half life of Fe(II) in the water column for given conditions of pressure, temperature, salinity, dissolved oxygen concentration, hydrogen peroxide concentration and pH. We have performed these calculations for stations GT11-12 (central subtropical gyre) and GT11-24 (core of oxygen minimum zone), using field data from these stations together with pH values estimated with total CO2 and alkalinity at nearby WOCE and CLIVAR stations (data from http://cdiac.ornl.gov/oceans/) and a likely range of hydrogen peroxide concentrations based on the data of Yuan and Shiller (2002). These calculations (results not shown) indicate oxidation half lives on the order of 1 minute in surface waters, and 100 minutes in deep waters. However, these kinetic calculations apply to dFe(II) present as inorganic hydroxyl and carbonate species, whereas dFe(II) may exist in the seawater as a variety of organic complexes (as discussed above), which may retard or accelerate the oxidation of Fe(II) in water column samples, depending on the nature of the organic species (SantanaCasiano et al. 2000). There are also field-based estimates of the rate of Fe(II) oxidation in seawater. For example, Ussher et al. (2005) examined the loss of Fe(II) from seawater samples collected over the European continental margin, which were filtered in-line prior to flow injection analysis. Over a period of 80 minutes, these workers observed a significant Fe(II) loss (~50%) from a surface sample, although no significant loss was observed for a benthic sample. Somewhat faster Fe(II) loss is reported by Sarthou et al. (2011), who

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Manuscript revised for Deep-Sea Research II, August 8, 2014 estimated average half lives of ~7 minutes and ~37 minutes for Fe(II) oxidation in surface and deep seawater samples, respectively. However, these results might not be directly applicable to our study, given that their analyses were performed on unfiltered samples maintained at 4°C, whereas we analyzed filtered samples maintained near 0°C prior to analysis. Although we did not conduct a rigorous assessment of Fe(II) oxidative loss during the cruise, some indication of the loss rate is provided by analysis of a filtered surface seawater sample (collected the previous day) after spiking with 0.6 nM of Fe(II) standard solution. Repeated analysis of this sample over a period of ~30 minutes, during which it was maintained at room temperature, showed a signal decrease of ~35%. This likely represents an upper limit on the extent of Fe(II) oxidation (and hence the analytical uncertainty for Fe(II)) in our filtered seawater samples, which were kept on ice and in darkness prior to analysis. Another caveat that must be considered for our dFe(II) data is that the concentrations measured in many of the samples were relatively close to the analytical limit of detection (0.01-0.06 nM). Thus, caution must be exercised in interpreting features in the distribution of Fe(II) where concentrations are relatively low (~0.1 nM or less). The results of the shipboard dFe(II) measurements are presented as contoured vertical sections in Figure 4. As noted above, over much of the three transects, water-column dFe(II) concentrations were relatively low (<0.2 nM), and accounted for less than half of the total dFe pool. The proportion of dFe present as Fe(II) is shown explicitly in Figure 5, which presents dFe(II) as a percentage of dFe for the three cruise transects. Similarly low concentrations of ‘labile’ (unfiltered) Fe(II) were reported by Sarthou et al. (2011) for water column samples collected along a transect from the far South Atlantic into the Southern Ocean, suggesting that dFe(II) concentrations on the order of 0.1 nM or less are typical oceanic ‘background’ levels in the absence of significant Fe(II) sources. However, the concentration sections shown in Figure 4 reveal three areas where dFe(II) concentrations are elevated relative to these background levels; namely (1) below the dissolved oxygen minimum near 1000 m depth at station USGT10-10 on the Mauritanian transect; (2) in and above the TAG hydrothermal plume, as defined by elevated deepwater dFe concentrations on the zonal transect; and (3) over most of the sub-thermocline

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Manuscript revised for Deep-Sea Research II, August 8, 2014 water column at station USGT11-10 on the zonal transect, in the Bermuda Atlantic Timeseries Study (BATS) area. On the Mauritanian transect, the elevated mid-depth dFe(II) concentrations (~0.3 nM) at station USGT10-10 (Fig. 4) occur within a larger area of elevated dFe concentrations (Fig. 2) and maximum upper-ocean AOU values (Fig. 3), where remineralization of organic matter is thought to provide a source of dFe to subsurface waters (Hatta et al., this issue; Wu et al., this issue). Indeed, this process also appears to provide a source of dFe(II): for the subsurface samples from leg USGT10, which traversed the oxygen minimum zone, there is a significant correlation (r2 = 0.32) between dFe(II) and AOU at the 99% confidence level (Fig. 6). This would seem to suggest that elevated dFe(II) concentrations in subsurface waters of the oxygen minimum zone reflect regeneration from sinking particles or microbial reduction of Fe(III), and the relative stability of Fe(II) at these in-situ conditions. The lowest dissolved oxygen concentrations measured in the water column (~60 µM) would not be expected to result in dissimilatory bacterial reduction of Fe(III), although this process might occur within reducing microenvironments, in association with the remineralization of particulate organic matter. Curiously, the dFe(II) maximum at station USGT10-10 occurs well below the core of the oxygen minimum, and is not observed at station USGT10-09 over the continental slope (Fig. 3), where elevated dFe concentrations are observed over the entire water column (Fig. 2). This perhaps reflects two sources of dFe to the water column in this region; namely, the remineralization of organic matter and lateral input from the continental margin, with the former being a more important source of dFe(II) in the vicinity of station USGT10-10. Indeed, other geochemical tracers measured as part of the cruise program (dissolved manganese, radium-228 and δ56Fe) provide evidence for the latter process (Hatta et al., this issue; M. Charette, pers. comm., Conway and John, 2014). By far the highest dFe(II) concentrations observed during the two cruise legs, up to 70 nM, were measured in samples collected from the 3,000-4,000 m depth range at station USGT11-16, over the TAG hydrothermal field on the MAR (Fig. 4). Here the large enrichments in dFe (up to 78 nM) and other hydrothermal tracers such as dissolved manganese (Hatta et al., this issue) and helium-3 (Jenkins et al., this issue) indicate that

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Manuscript revised for Deep-Sea Research II, August 8, 2014 the water column sampling intersected a plume of hydrothermal effluent from vents at or near the TAG field. The data from these well-oxygenated samples suggest that Fe(II) accounted for more than 80% of the dissolved iron in the core of the plume at station USGT11-16 (Figs. 5, 7), and that most of this Fe was present in the colloidal size fraction (0.02-0.2 µm; Fitzsimmons et al., this issue) and complexed by ligands (Buck et al., this issue). This colloidal-sized hydrothermal Fe may represent hydrothermal Fe(II) that is complexed by colloidal-sized organic moieties (Bennett et al., 2008; Toner et al., 2009), or, alternately, it may be present as pyrite nanoparticles (Yucel et al., 2011; Gartman et al., 2014). Significantly elevated dFe concentrations were also observed in deep waters at stations USGT11-14 and USGT11-18, (Fig. 2), suggesting that the hydrothermal plume was carried some 500 km to the west and east of the MAR axis. However, it should be noted that the apparent deep dFe(II) maxima at these stations shown in Figure 4 are artefacts of the interpolation performed by Ocean Data View; in fact, deep-water dFe(II) concentrations were low (<0.2 nM) at station USGT11-14, and dFe(II) was not measured in the deep samples collected at station USGT11-18. Importantly, a comparison of our dFe and dFe(II) data from stations USGT11-16 (over the MAR axis) and USGT11-14 (west of the MAR axis) reveals that Fe(II) accounts for a relatively small proportion (<10%) of the total dFe pool in the plume to the west of the ridge axis (Fig. 7). Assuming that the elevated deep water dFe concentrations represent only the TAG hydrothermal plume, this observation is consistent with previous estimates of Fe(II) oxidation rates in hydrothermal plumes (Rudnicki and Elderfield, 1993: Field and Sherrell, 2000; Statham et al., 2005), and compatible with the hypothesis that the colloidal-sized dFe(II) measured in the near field plume at station USGT11-16 was composed of pyrite nanoparticles, which may be slowly oxidized to colloidal sized Fe(II)/Fe(III) oxides during transport in the water column (Gartman and Luther, 2014). As mentioned above, no dFe(II) analyses were made for the deep water samples collected from station USGT11-18. Slightly elevated dFe(II) concentrations (~0.3 nM) were measured in samples near 1,200 m depth over the MAR axis, and near 2,000 m depth well east of the MAR at station USGT11-20 (Fig. 3). It seems unlikely that these anomalies were derived from the TAG hydrothermal field, although they may reflect inputs from shallower hydrothermal fields located elsewhere along the MAR axis. 13

Manuscript revised for Deep-Sea Research II, August 8, 2014 In the zonal transect across the Subtropical Gyre, several stations (USGT11-10, USCT1114, USGT11-18) show dFe(II) concentrations that are slightly elevated (~0.2 nM) in the upper water column, perhaps due to recent atmospheric inputs, which in this region are known to include readily soluble Fe(II) species derived from natural and anthropogenic sources (Kieber et al., 2001, 2005; Schroth et al. 2009; Measures et al., this issue). At these stations, there is a notable contrast in the vertical distributions of dFe(II) and dFe, with the latter displaying pronounced concentration minima in the lower euphotic zone, around the depth of subsurface chlorophyll maxima. Such subsurface dFe minima have been previously reported in the subtropical and tropical North Atlantic (Sedwick et al., 2005; Bergquist and Boyle, 2006; Fitzsimmons et al., 2013), and presumably reflect some combination of removal via biological uptake and particle scavenging. Importantly, our dFe(II) profiles show no corresponding minima in the lower euphotic zone (Fig. 8), such that Fe(II) apparently accounts for a large fraction of the dFe pool over this depth range (Fig. 4). Given that dissolved Fe(II) is thought to be more readily available for biological uptake than Fe(III) (Shaked and Lis, 2012), this might suggest that scavenging removal of the less soluble Fe(III) is primarily responsible for these subsurface dFe minima. Further, the near uniform distribution of dFe(II) implies rapid regeneration of dFe(II) at these depths via microbial and/or photochemical processes, or that dFe(II) is somehow stabilized with respect to biological removal. Also of interest along this section is an apparent similarity between the vertical distributions of dFe(II) and so-called soluble Fe (Fitzsimmons et al., this issue), which is defined as Fe that passes through a 0.02 µm pore-size filter, and includes truly dissolved Fe as well as Fe associated with small nanoparticles. This observation suggests that much of the soluble Fe size fraction in the upper water column exists in the +2 oxidation state, or is readily reduced to Fe(II). The vertical distribution of dFe(II) at station USGT11-10, in the area of the Bermuda Atlantic Time-series Study (BATS), is notable for the elevated concentrations (~0.2-0.3 nM) over most of the water column, as well as elevated dFe concentrations in and below the base of the thermocline (Figs. 2, 4). Between ~800 m and ~2,000 m depth, and possibly deeper, the dFe section shown in Figure 2 suggests that elevated concentrations of dFe over this depth range may be advected from continental margin sources in the ULSW (Hatta et al., this issue). These continental margin inputs sources might also be 14

Manuscript revised for Deep-Sea Research II, August 8, 2014 the source of the elevated dFe(II) concentrations observed in mid-depth and deep waters at this station. Unfortunately, in the absence of dFe(II) data during the Woods-HoleBermuda portion of the zonal transect (due to failure of our detector), we are unable to assess this possibility. The highest concentrations of dFe(II) at this station (~0.3 nM) are within 500 m of the seafloor, where dFe(II) accounts for almost half of the dFe pool (Fig. 5), suggesting that sediments on the Bermuda platform may provide significant inputs of dFe(II) and dFe to the deep ocean in this region. 4. Summary and Conclusions The U.S. GEOTRACES North Atlantic transect cruise has provided a 'snapshot' of the zonal distribution of dissolved iron(II) across the North Atlantic basin. Our results indicate that water column concentrations are typically low (<0.2 nM), with dFe(II) typically accounting for less than half of the dFe pool. Significantly elevated dFe(II) concentrations were observed in three areas: (1) below the depth of the oxygen minimum zone in the iron-rich waters west of Mauritania, possibly as a result of the remineralization of reduced forms of Fe from sinking particles, or microbial reduction of Fe(III); (2) within a near-field hydrothermal plume over the Mid Atlantic Ridge, where colloidal-sized dFe(II), possibly present as pyrite nanoparticles, dominates the dFe pool; and (3) over most of the water column in the BATS region, which may reflect advection of sedimentary iron from the North American continental shelf and, in deep waters, from sediments on the Bermuda platform. In addition, slightly elevated dFe(II) concentrations were observed in the upper water at some stations in the Subtropical Gyre, possibly derived from aeolian sources. Our dFe(II) data are generally in accord with the limited published data for Fe(II) in the Atlantic Ocean, with the somewhat lower range of concentrations reported by Sarthou et al. (2011) in the far South Atlantic perhaps reflecting the generally lower concentrations of dissolved iron in that ocean region. Further process-based studies will be required to constrain the mechanisms that allow Fe(II) to accumulate in the water column, and to establish the biological and geochemical fate of this transient iron species. Acknowledgements

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Manuscript revised for Deep-Sea Research II, August 8, 2014 We thank the cruise co-chief scientists Ed Boyle, Greg Cutter and Bill Jenkins, and are grateful for the sampling assistance that was provided by Ana Aguilar-Islas, Randelle Bundy, Jessica Fitzsimmons, Peter Morton, Rachel Shelley and Geoffrey Smith. We also thank the Oceanographic Data Facility team of Susan Becker, Mary Johnson, Rob Palomares and Courtney Schatzman for collection and processing of the ancillary cruise data. We owe a debt of gratitude to Angela Milne and Paul Worsfold for assisting with the loan and mid-cruise delivery of a replacement detector. Bill Jenkins very kindly made the results of his water mass analyses and AOU calculations available to us. Funding for ship time, sampling operations, and hydrographic data were provided by NSF grants to the U.S. GEOTRACES North Atlantic Transect Management team of W. Jenkins (OCE-0926423), E. Boyle (OCE-0926204), and G. Cutter (OCE-0926092). The primary funding for the research described here was provided by NSF award OCE0927285 to P. Sedwick. The manuscript benefitted from the thoughtful reviews that were provided by two anonymous referees and special issue editor Ed Boyle. References Anderson, R.F., Henderson, G.M., Adkins, J., Andersson, P., Eisenhauer, A., Frank, M., Oschlies, A., 2007. GEOTRACES - An international study of the global marine biogeochemical cycles of trace elements and their isotopes. Chemie der ErdeGeochemistry 67, 85-131. Barbeau, K., Rue, E.L., Bruland, K.W. and Butler, A., 2001. Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands. Nature, 413(6854): 409-413. Bennett, S.A., Achterberg, E.P., Connelly, D.P., Statham, P.J., Fones, G.R., German, C.R., 2008. The distribution and stabilisation of dissolved Fe in deep-sea hydrothermal plumes. Earth and Planetary Science Letters, 270, 157-167. Bergquist, B.A., Boyle, E.A., 2006. Dissolved iron in the tropical and subtropical Atlantic Ocean. Global Biogeochemical Cycles 20, GB1015. Bowie, A.R., Achterberg, E.P., Sedwick, P.N., Ussher, S., and Worsfold, P.J., 2002. Real-time monitoring of picomolar concentrations of iron (II) in marine waters

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Manuscript revised for Deep-Sea Research II, August 8, 2014 using automated flow injection-chemiluminescence instrumentation. Environmental Science and Technology, 36, 4600-4607. Bowie, A.R., Sedwick, P.N., Worsfold, P.J., 2004. Analytical intercomparison between flow injection-chemiluminescence and flow injection-spectrophotometry for the determination of picomolar concentrations of iron in seawater. Limnology and Oceanography: Methods 2, 42-54. Bowie, A.R., Achterberg, E.P., Ussher, S., Worsfold, P.J., 2005. Design of an automated flow injection-chemiluminescence instrument incorporating a miniature photomultiplier tube for monitoring picomolar concentrations of iron in seawater. Journal of Analytical Methods in Chemistry 2, 37-43. Boyd, P.W., Ellwood, M.J., 2010. The biogeochemical cycle of iron in the ocean. Nature Geoscience 3, 675-682. Boyd, P.W., et al. 2007. Mesoscale iron enrichment experiments 1993-2005: Synthesis and future directions. Science 315, 612-617. Boye, M., Aldrich, A.P., van den Berg, C.M., de Jong, J., Veldhuis, M., de Baar, H.J., 2003. Horizontal gradient of the chemical speciation of iron in surface waters of the northeast Atlantic Ocean. Marine Chemistry 80, 129-143. Buck, K.N., Sohst, B., Sedwick, P.N., 2013. The organic complexation of dissolved iron along the U.S. GEOTRACES North Atlantic section. Deep Sea Research II, this issue. Conway, T.M., John, S.G., 2014. Quantification of dissolved iron sources to the North Atlantic Ocean. Nature, doi:10.1038/nature13482. Cutter, G.A., Bruland, K.W., 2012. Rapid and noncontaminating sampling system for trace elements in global ocean surveys. Limnology and Oceanography Methods 10, 425-436. Field, M.P., Sherrell, R.M., 2000. Dissolved and particulate Fe in a hydrothermal plume at 9 45′ N, East Pacific Rise:: Slow Fe (II) oxidation kinetics in Pacific plumes. Geochimica et Cosmochimica Acta 64, 619-628.

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Manuscript revised for Deep-Sea Research II, August 8, 2014 Moore, J.K., Braucher, O., 2008. Sedimentary and mineral dust sources of dissolved iron to the world ocean. Biogeosciences 5, 631-656. Rose, A.L., Waite, T.D., 2003. Predicting iron speciation in coastal waters from the kinetics of sunlight-mediated iron redox cycling. Aquatic Sciences 65, 375-383. Roy, E.G., Wells, M.L., King, D.W., 2008. Persistence of iron(II) in surface waters of the western subarctic Pacific. Limnology and Oceanography 53, 89-98. Rudnicki, M.D., Elderfield, H., 1993. A chemical model of the buoyant and neutrally buoyant plume above the TAG vent field, 26 degrees N, Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta 57, 2939-2957. Santana-Casiano, J.M., Gonzalez-Davila, M., Millero, F.J., 2006. The role of Fe (II) species on the oxidation of Fe (II) in natural waters in the presence of O2 and H2O2. Marine Chemistry 99, 70-82. Sarthou, G., Bucciarelli, E., Chever, F., Hansard, S.P., González-Dávila, M., SantanaCasiano, J.M., Planchon, F., Speich, S., 2011. Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre. Biogeosciences 8, 2461-2479. Schlitzer, R., 2002. Interactive analysis and visualization of geoscience data with Ocean Data View. Computers and Geosciences 28, 1211-1218. Schroth, A.W., Crusius, J., Sholkovitz, E.R., Bostick, B.C., 2009. Iron solubility driven by speciation in dust sources to the ocean. Nature Geoscience 2, 337-340. Sedwick, P.N., Church, T.M., Bowie, A.R., Marsay, C.M., Ussher, S.J., Achilles, K.M., McGillicuddy, D.J., 2005. Iron in the Sargasso Sea (Bermuda Atlantic Time‐ series Study region) during summer: Eolian imprint, spatiotemporal variability, and ecological implications. Global Biogeochemical Cycles, 19, GB4006. Sedwick, P.N., Bowie, A.R., Trull, T.W., 2008. Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR SR3 section): Meridional and seasonal trends. Deep Sea Research I 55, 911-925. Shaked, Y., Lis, H., 2012. Disassembling iron availability to phytoplankton. Frontiers in

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Manuscript revised for Deep-Sea Research II, August 8, 2014 Figure Captions Figure 1. Map showing the cruise tracks and water-column sampling stations for the U.S. GEOTRACES North Atlantic Transect cruise GA03 (completed as legs USGT10 in fall 2010 and USGT11 in fall 2011). Figure 2. Contoured vertical sections of total dissolved iron (dFe) concentration along the three transects of GEOTRACES cruise GA03 (refer to Fig. 1). Note that maximum value of color scale is 1.8 nM, whereas concentrations as high as 78 nM were measured in deep samples from station 16 on the zonal section (refer to Fig. 5). Figure 3. Contoured vertical sections of dissolved oxygen (upper panels) and apparent oxygen utilization (AOU, lower panels) along the three transects of GEOTRACES cruise GA03 (refer to Fig. 1); AOU values provided by W. Jenkins. Figure 4. Contoured vertical sections of dissolved iron(II) (dFe(II)) concentration along the three transects of GEOTRACES cruise GA03 (refer to Fig. 1). Note that maximum value of color scale is 0.9 nM, whereas concentrations as high as 70 nM were measured in deep samples from station 16 on the zonal section of cruise leg USGT11 (refer to Fig. 5). No dFe(II) measurements were made at stations 1-8 on this section, nor in deep water samples at station USGT11-18. Note that the elevated dFe(II) concentrations indicated in deep waters at stations USGT11-14 and USGT11-18 are an artefact of the interpolation performed by the ODV software. Figure 5. Contoured vertical sections of dFe(II) concentration as a percentage proportion of dFe along the three transects of GEOTRACES cruise GA03 (refer to Fig. 1). Figure 6. Plot of dFe(II) versus AOU for all subsurface (>40 m depth) water-column samples collected during leg USGT10, showing best-fit linear regression. Figure 7. Vertical concentration profiles of dFe(II) and dFe from stations USGT11-16 (left) and USGT11-14 (right), which sampled a hydrothermal plume between 2,000 m and 4,000 m depth over the Mid Atlantic Ridge. Station USGT11-16 was located over

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Manuscript revised for Deep-Sea Research II, August 8, 2014 the ridge axis, and station USGT11-14 was located to the west of the ridge axis. Note different concentration scales used on the two plots. Figure 8. Vertical concentration profiles of dFe(II) and dFe from Subtropical Gyre stations 10, 14 and 18 on the zonal section of cruise USGT11, with gray band indicating depth range of the subsurface dFe minima.

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