# Dissolved iron distribution and budget in the Solomon Sea

Géraldine Sarthou, Eva Bucciarelli, Fabien Quéroué, François Lacan, Hélène Planquette, Viet Q Pham, Maxime Grand, Catherine Pradoux, Martine Rodier, Sophie Bonnet, et al.

► To cite this version:

Géraldine Sarthou, Eva Bucciarelli, Fabien Quéroué, François Lacan, Hélène Planquette, et al. Dis-solved iron distribution and budget in the Solomon Sea. Marine Chemistry, 2025, 273, pp.104567. 10.1016/j.marchem.2025.104567. hal-05299238

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# Dissolved iron distribution and budget in the Solomon Sea

Géraldine Sarthou<sup>a,*</sup>, Eva Bucciarelli<sup>a</sup>, Fabien Quéroué<sup>a</sup>, François Lacan<sup>b</sup>, Hélène Planquette<sup>a</sup>, Viet Q. Pham<sup>b</sup>, Maxime Grand<sup>c</sup>, Catherine Pradoux<sup>b</sup>, Martine Rodier<sup>d</sup>, Sophie Bonnet<sup>d</sup>, Gérard Eldin<sup>b</sup>, Sophie Cravatte<sup>b</sup>, Catherine Jeandel<sup>b</sup>, Alexandre Ganachaud<sup>b</sup>

## ARTICLE INFO

### Keywords:

Dissolved iron

Solomon Sea

Western tropical Pacific

Equatorial undercurrent

GEOTRACES

## ABSTRACT

As part of the PANDORA cruise (GEOTRACES GP12), concentrations of dissolved iron (dFe) were measured at 11 stations inside and outside the Solomon Sea, a semi-enclosed sea in the western tropical Pacific, with complex topography and straits, and strong western boundary currents supplying the equatorial current system. These measurements aimed to better assess the various sources of dFe in our study area and the Solomon Sea's potential as a source of dFe for the Equatorial Undercurrent (EUC). A simple box model allows calculating and discussing the fate of the dFe in the different water layers flowing through the Solomon Sea and suggests that the amount of dFe enrichment within the enclosed sea was not significant for the lower thermocline and intermediate waters, indicating that most of the dFe was acquired prior to reaching the Solomon Sea at the entrance and/or that inputs are approximately balanced by scavenging within the basin for these two layers. In contrast, dFe enrichment was significant for the upper thermocline layer and the deep waters, highlighting enrichments from external sources, as well as combination of internal processes, such as scavenging and/or organic complexation. The relative dFe contribution of the Solomon Sea to EUC was 20 %, on average. Other sources might thus provide dFe to the EUC, along the water transport downstream of the Solomon Sea (e.g. Bismarck Sea) or from the northern hemisphere. Diazotrophs such as *Trichodesmium* might also contribute to the dFe enrichment of the EUC after export and remineralization at depth outside the Solomon Sea.

## 1. Introduction

Iron (Fe) is a crucial micronutrient that plays a pivotal role in regulating marine biological productivity and metabolism (Boyd and Ellwood, 2010; Fitzsimmons and Conway, 2023; Moore et al., 2013; Tagliabue et al., 2017). This is particularly evident in large High Nutrient Low Chlorophyll (HNLC) regions, such as the Eastern Equatorial Pacific, where Fe limitation significantly impacts primary productivity (Behrenfeld et al., 1996; Blain et al., 2008; Boyd, 2007; Cavender-Bares et al., 1999; Coale et al., 1996; Martin et al., 1991). The equatorial Pacific Ocean, the world's largest continuous region of oceanic upwelling, is a key player in Earth's biogeochemical cycles (Chavez and Messié, 2009; Rafter, 2024).

The Equatorial Undercurrent (EUC) serves as the primary source of the water masses that are upwelled in the eastern equatorial Pacific,

feeding into the overlying South Equatorial Current (SEC) and then diverging from the equator (Fine et al., 1994; Grenier et al., 2011; Tsuchiya et al., 1989). The EUC exhibits elevated concentrations of dissolved iron (dFe), especially in its core (Coale et al., 1996; Gordon et al., 1997; Kaupp et al., 2011; Labatut et al., 2014; Landry et al., 1997; Mackey et al., 2002; Slemons et al., 2012; Slemons et al., 2010). This discovery has underscored the vital role this large-scale current plays in transporting iron to the eastern Pacific (Ryan et al., 2006). This delivery of Fe is critical, as it enhances primary productivity in the otherwise HNLC eastern Pacific region, which accounts for approximately 20 % of the world's primary production (Coale et al., 1996; Gorgues et al., 2010; Ryan et al., 2006). This Fe enrichment originates from a combination of inputs from continental margins, rivers, atmosphere, and hydrothermalism (Bonnet et al., 2023b; Guieu et al., 2018; Labatut et al., 2014; Michael et al., 2021; Radic et al., 2011; Resing et al., 2015;

<footnote><sup>a</sup> CNRS, Univ Brest, IRD, Ifremer, LEMAR, F-29280 Plouzane, France</footnote>

<footnote><sup>b</sup> Université de Toulouse, LEGOS (CNRS/CNES/IRD/UT3), Toulouse, France</footnote>

<footnote><sup>c</sup> Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039, USA</footnote>

<footnote><sup>d</sup> Aix Marseille Université, Université de Toulon, CNRS, IRD, Mediterranean Institute of Oceanography (MIO), Marseille 13288, France</footnote>

<footnote>* Corresponding author.
E-mail address: geraldine.sarthou@univ-brest.fr (G. Sarthou).</footnote>
Slemons et al., 2012; Slemons et al., 2010; Tilliette et al., 2022).

The Solomon Sea, located in the western tropical Pacific north of the Coral Sea, plays a crucial role in a significant ocean circulation pathway (Lindstrom et al., 1987). The northern segment of the westward-flowing SEC is joined by the low-latitude western boundary currents (LLWBCs), which pass through this partially enclosed sea. These currents are key components of the subtropical cells, transporting waters from mid to low latitudes into the western equatorial Pacific, making the Solomon Sea a vital link between the subtropical Pacific and the Equator (Fine et al., 1994; Ganachaud et al., 2017; Grenier et al., 2011; Johnson and McPhaden, 1999; Kessler et al., 2019). The Solomon Sea is particularly important for understanding how water properties change as they flow from the subtropical region to the Equator. Encircled by high, heavily weathered islands (Milliman et al., 1999) and characterized by complex topography and hydrography (Cravatte et al., 2011; Ganachaud et al., 2017), this region is suspected to be a significant source of trace elements (Behrens et al., 2020; Michael et al., 2021; Kaupp et al., 2011; Lacan and Jeandel, 2001, 2005; Mackey et al., 2002; Pham et al., 2019, 2022; Qin et al., 2016; Slemons et al., 2010). Michael et al. (2021) demonstrated that approximately 70 % of dissolved aluminium and 50 % of dissolved manganese in the EUC originate from waters that flow out of the Solomon Sea. On a more local scale, enrichments were observed near the coast and within the straits, likely due to sediment scouring (Behrens et al., 2020; Michael et al., 2021; Pham et al., 2019). However, it remains unclear whether the Solomon Sea serves as a net source of dFe for the waters passing through it and a more precise quantification of dFe out of the Solomon Sea, relative to the EUC is still needed.

In this context, based on dFe data collected within and outside the Solomon Sea (PANDORA cruise, GEOTRACES GP-12), this study aims to better quantify the various sources of dFe in this area, to establish a dFe budget for potential enrichment within the Solomon Sea and to assess its contribution to the EUC, thereby enhancing our understanding of iron

dynamics in the equatorial Pacific.

## 2. Sampling and methods

### 2.1. Seawater sampling

Samples were collected during the PANDORA cruise (GEOTRACES GP12, 28 June–6 August 2012, Ganachaud et al., 2017), onboard R/V L'Atalante. Sample locations are shown in Fig. 1. A total of 11 stations were sampled and analysed onboard for dFe. Three types of stations were considered. From south to north, stations 4, 10, 13 and 43 were located outside the Solomon Sea; stations 34, 39, 71 and 60 were sampled within the Solomon Sea; and stations 21, 42 and 77 were located at the straits. For seawater sample collection and in-situ measurements, two SBE-911Plus CTD/rosette systems were used. The primary one, hereafter called standard rosette, was outfitted with duplicated temperature-conductivity-oxygen SBE sensors, a Chelsea Aqua3 fluorometer, a Wetlab C-star transmissometer and a pair of RDI built Lowered-Acoustic Doppler Current Profilers (L-ADCP) attached to the CTD frame (150 kHz and 300 kHz, downward- and upward-looking, respectively). The rosette was fitted with 22 × 12 L Niskin bottles to obtain water samples for ancillary parameters (see below). The secondary rosette was a trace metal clean polyurethane epoxy-coated aluminium frame rosette (hereafter referred to as TMR) equipped with twelve 12L, externally closing, Teflon-lined GO-FLO bottles (General Oceanics) and attached to a 1350 m long conducting Kevlar® line. Details of the clean sampling system and protocol can be found in Measures et al. (2008). The TMR was fitted with titanium SBE 911Plus temperature-conductivity-oxygen sensors and a Wetlab Westar fluorometer. Sample bottles were closed by electrical signal from the ship, only while the rosette frame was moving upward through the water column at 5–10 m min⁻¹ into water that had not been in contact with the frame.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** PANDORA cruise stations (GEOTRACES GP12), modified from Pham et al. (2019) and Michael et al. (2021). Stations outside the Solomon Sea are indicated with red dots, at the entrance or inside the Solomon Sea with green ones, and at the straits with blue ones. Hashtags denote inflow stations, and stars outflow stations within the box model (see text for details). The main currents are depicted by arrows (Grenier et al., 2011). SEC: South Equatorial Current, NVJ: North Vanuatu Jet, NCJ: New Caledonia Jet, GPC: eastward Gulf of Papua Current, NQC: North Queensland Current, NGCU: New Guinea Coastal Undercurrent, NBCU: New Britain Coastal Undercurrent, NICU: New Ireland Coastal Undercurrent, SGU: Saint Georges Undercurrent, SSCC: South Subsurface Countercurrent, EUC: Equatorial Undercurrent. The main archipelago and islands are also reported in the map. Straits are identified as follows: Indispensable (I), Vitiaz (V), St. George (G) and Solomon Strait (S). A larger scale of the research area is given in the inset map. Stations indicated with triangles are stations sampled during the SEC cruise (Labatut et al., 2014; Slemons et al., 2012) and used for comparison of dFe profiles (see Fig. S4). Initial plot was generated by Ocean Data View (Schlitzer, 2022). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
The cleaning protocols for sampling bottles and equipment followed the guidelines of the GEOTRACES Cookbook (Cutter et al., 2017). Before taking the first samples, the GO-FLO bottles were filled with surface seawater for 48 h. After TMR recovery, GO-FLO bottles were transferred into a clean container pressurized with filtered air (ISO class 8) and equipped with an ISO class 5 laminar flow hood. The GO-FLO bottles were pressurized with 0.5 bar high quality dinitrogen filtered through 0.2 µm pore size PTFE Acrovent capsule filters at the entrance of each bottle. The samples were filtered through 0.2 µm pore size cellulose acetate Sartobran 300 capsule filters connected to the GO-FLO bottles with an acid cleaned silicone tubing and collected in acid-cleaned 60mL LDPE bottles, after rinsing three times with about 20mL of seawater. Before first use, the Sartobran capsules were rinsed with 10 L of surface seawater; then before each new sample with 240 mL of that sample. Filtered samples were acidified to pH ~ 1.7 with hydrochloric acid (HCl, Ultrapure® Merck, 2‰ v/v) under an ISO class 5 laminar flow hood inside the clean container and stored at room temperature until analysis.

## 2.2. Dissolved Fe (dFe) analyses

The dFe samples were collected from the TMR and concentrations measured onboard at least 24 h after collection using Flow Injection Analysis (FIA) with chemiluminescence detection, following a method adapted from Obata et al. (1993). The pH of the acidified samples was adjusted to 5 using ammonia (NH₃, Suprapur®, Merck) and an ammonium acetate buffer purified 3 times on a 8-hydroxyquinoline resin immobilised on hydrophilic vinyl polymer (TSK-8-HQ after Dierssen et al., 2001). Samples were loaded during 120 s on a TSK-8-HQ. All reagents, standards, samples, and blanks were prepared in acid-cleaned low-density polyethylene (LDPE) bottles. The purification of the luminol solution through a TSK-8-HQ column improved both the blank and detection limit of the method (Sarthou et al., 2003). The blank was determined each day as the mean of five measurements of a low iron concentration sample with only 5 s of preconcentration. It was equal to 0.07 ± 0.02 nmol L⁻¹ (1 standard deviation, SD, n = 12). The detection limit (DL), equal to three times the standard deviation of the blank, was 0.06 nmol L⁻¹. The individual contributions to the total blank of HCl, NH₃, and ammonium acetate buffer were determined by the addition of increasing amounts of these reagents to the sample and were lower than our detection limit. Each sample was analysed in triplicate, with the average value of the repeatability of each triplicate, quantified as ±1 SD, equal to 6 ± 3% (±1 SD, n = 125). Accuracy was determined from the analysis of consensus seawater samples (SAFe S and SAFe D2). The measured values were in excellent agreement with the consensus values (Table 1). In addition, at four stations (St 10–500 m, St 13–180 m, St 39–227 m, and St 43–300 m), duplicate samples were taken and the difference between two duplicates was always lower than the DL, confirming the accuracy of our measurements, including sampling and filtration steps (Table S1).

## 2.3. Ancillary parameters

Potential temperature (θ), salinity (S) and fluorescence were retrieved from the CTD sensors of the TMR and/or standard rosette. Dioxygen concentrations (O₂) from the standard rosette were measured

<table><caption>**Table 1**

values of consensus seawaters (SAFe S and SAFe D2) in samples collected during the PANDORA cruise.</caption><thead><tr><th rowspan="3">Seawater used</th><th colspan="6">dFe values (nmol L<sup>-1</sup>)</th></tr><tr><th colspan="3">On board FIA</th><th colspan="3">Reference or certified</th></tr><tr><th>Average</th><th>SD</th><th>n</th><th>Average</th><th>SD</th><th></th></tr></thead><tbody><tr><td>SAFe S</td><td>0.094</td><td>±</td><td>0.003</td><td>3</td><td>0.096</td><td>±</td><td>0.008</td></tr><tr><td>SAFe D2</td><td>0.95</td><td>±</td><td>0.05</td><td>3</td><td>0.96</td><td>±</td><td>0.02</td></tr></tbody></table>

by the Winkler method with potentiometric endpoint detection (Langdon, 2010). Samples for nitrate (NO₃⁻) concentration measurements were collected from both rosette systems and analysed using the GO-SHIP guidelines (Hydes et al., 2010), following pasteurization (Aminot and Kérouel, 2007; Daniel et al., 2012). Pasteurized samples were analysed on a SEAL-AA3 automated Segmented Flow Analysis (Aminot and Kérouel, 2007; Hydes et al., 2010). Samples for chlorophyll-α (Chl-α) concentrations were collected from the standard rosette in 0.55 L flasks, filtered onto GF/F Whatman filters, frozen (−80 °C) and analysed on board. Chl-α concentrations were determined after extraction with methanol (Herbland et al., 1985) using a Turner Design fluorometer equipped with the Chl-α extracted acidification module (part number 7200–040) and calibrated with pure Chl-α standard (Sigma).

## 2.4. NO₃⁻ as a proxy of remineralization

The apparent oxygen utilization (AOU) is often used as a proxy for remineralization (Aristegui, and Duarte, C.M., Agustí, S., Doval, M., Alvarez-Salgado, X.A., Hansell, D.A, 2002; Garcia and Gordon, 1992; Jenkins, 1998). Since NO₃⁻ concentrations were measured on samples from both TMR and standard rosettes, but O₂ concentrations were not measured on the TMR samples, we first examined whether NO₃⁻ could also serve as a proxy for remineralization. NO₃⁻ concentrations showed a significant correlation with AOU (Fig. 2a; *p* < 0.01, *n* = 59), indicating that NO₃⁻ can indeed be used as a tracer for remineralization and dFe originating from the remineralization can be estimated using the following equations:

$$[NO_3^-]_{\text{measured}} = [NO_3^-]_{\text{remineralized}} + [NO_3^-]_{\text{preformed}} \quad (1)$$

$$[NO_3^-]_{\text{remineralized}} = r_{N:O2} \times AOU \quad (2)$$

$$[dFe]_{\text{measured}} = [dFe]_{\text{remineralized}} + [dFe]_{\text{preformed}} + [dFe]_{\text{external-inputs}} - [dFe]_{\text{scavenging}} \quad (3)$$

$$[dFe]_{\text{remineralized}} = r_{Fe:N} \times [NO_3^-]_{\text{remineralized}} \quad (4)$$

Where [NO₃⁻]remineralized and [dFe]remineralized originate from the remineralization of the organic matter, and [NO₃⁻]preformed and [dFe]-preformed are physically transported into the ocean interior from subduction of surface waters (Broecker and Peng, 1982; Ito and Follows, 2005). [dFe]-external-inputs originates from local external inputs and [dFe]-scavenging corresponds to dFe loss by scavenging. rN:O2 and rFe:N are the remineralization ratios.

## 3. Hydrological context

The main currents encountered during the PANDORA cruise are shown in Fig. 1, briefly described below and in more detail in other studies (Ganachaud et al., 2017; Germineaud et al., 2016; Pham et al., 2019). At the thermocline level, the westward zonal South Equatorial Current (SEC) splits into two zonal currents when entering into the Coral Sea: the North Vanuatu Jet (NVJ) and the North Caledonian Jet (NCJ). Inflow currents enter the southern part of the Solomon Sea, on one hand with one stream of the NVJ, which feeds the Solomon Sea through an interior pathway and through the Indispensable strait, and on the other hand through a western boundary pathway. When the Gulf of Papua Current (GPC) fed by the NCJ joins the NVJ, they form the New Guinea Coastal Undercurrent (NGCU) (Kessler et al., 2019). The NGCU then flows equatorward, splitting south of Woodlark Island, rejoining north of the island before bifurcating again near the coast of New Britain. The eastward branch travels along the coast of New Britain as the New Britain Coastal Undercurrent (NBCU), exiting through both Solomon Strait and St. Georges Channel, the majority passing through the former (Alberty et al., 2019). The westward branch flows equatorward and exits through Vitiaz Strait.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** For all stations of the PANDORA cruise (a) nitrate (NO₃⁻) vs. Apparent Oxygen utilization (AOU), and (b) dFe vs. remineralized NO₃⁻ (see text for more detail) for the thermocline and intermediate waters. Stations outside the Solomon Sea (4, 10, 13, 43) are highlighted in red, inside and at the entrance of the Solomon Sea (34, 39, 60, and 71) in green, and at the straits (21, 42, 77) in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

A detailed description of the water masses can be found elsewhere (Ganachaud et al., 2017; Germineaud et al., 2016; Pham et al., 2022; Pham et al., 2019). Here, we briefly describe them focusing on the sampled stations. The different water masses encountered during the PANDORA cruise were identified using θ–S diagrams (Fig. S1) and previous literature (Ganachaud et al., 2017; Germineaud et al., 2016; Pham et al., 2019). They are also reported in Table S1, corresponding to the collected seawater samples. Below the tropical surface waters (TSW), in the upper thermocline the South Pacific Tropical Water (SPTW) is found at ~ σ₀ = 24.3–25.3 kg m⁻³. This water mass divides into two main branches. The southern branch enters the Coral Sea through the NCJ (σ₀ = 24.8 kg m⁻³, S ~ 35.6–35.7, and O₂ ~ 140–165 µmol kg⁻¹), while the northern branch enters the Coral Sea through the NVJ and is called South Pacific Equatorial Water (SPEW, σ₀ ~ 24.5 kg m⁻³). It is slightly more saline (S ~ 36.0) and less oxygenated (O₂ ~ 135 µmol kg⁻¹) than the NJC waters. The SPEW is only observed at stations 10, 13 and 43. Just below this water mass, the Western South Pacific Central Water (WSPCW) is found, characterized by high salinity and oxygen. Deeper is encountered the Subantarctic Mode Water (SAMW), centered around σ₀ = 27.0 kg m⁻³. The Antarctic Intermediate Water (AAIW) is found at σ₀ = 27.2 kg m⁻³ and characterized by a salinity minimum and oxygen maximum. The combination of AAIW and upwelled Pacific Deep Water forms the Equatorial Pacific Intermediate Water (EqPIW; ~σ₀ = 27.2 kg m⁻³), with a higher salinity and a lower

oxygen content than the AAIW. The EqPIW is only found at stations 13 and 43. The deep and bottom waters are not well constrained in our studied area. The Upper Circumpolar Deep Water (UCDW) is observed ~σ₀ = 27.65 kg m⁻³ and is characterized by a relatively high salinity and a low oxygen content. Just below is the Lower Circumpolar Deep Water (LCDW) defined by a salinity maximum reaching 34.72–34.73 (Germineaud et al., 2021).

## 4. Dissolved Fe box model in the Solomon Sea

Similarly to Pham et al. (2019) for dissolved REEs (dREEs), a simple box model was built for the Solomon Sea in order (1) to estimate the dFe budget for different isopycnal layers between its southern entrance (southern section) and its northern exits (Vitiaz Strait, St George's Channel and Solomon Strait) (Fig. 3), (2) to identify whether dFe concentrations get enriched or not while crossing the Solomon Sea, and (3) to identify the associated net fluxes (positive if inputs dominate, negative if outputs dominate, null if inputs and outputs are equivalent).

The Solomon Sea was considered as a closed area at steady state and a one-box model was applied to four isopycnal layers: the upper thermocline layer (UTW, σ₀ = 24.0–25.3 kg m⁻³), the lower thermocline layer (LTW, σ₀ = 25.3–26.9 kg m⁻³), the intermediate waters (IW, σ₀ = 26.9–27.4 kg m⁻³), and the deeper waters (DW, σ₀ > 27.4 kg m⁻³), as in Pham et al. (2019).
<figure><img src="image_6.png" /><figcaption>Fig. 3. Adapted from Pham et al. (2019). Schematic description of the Pandora box (doted white rectangular). Hashtags denote inflow stations, and stars outflow stations within the box model (see text for details). Yellow arrows represent southern inflows, while white arrows indicate outflows through the northern straits. Numbers and letters indicate different stations together with their transport. E: NGCU transport, N: direct NVJ transport, V, and G+S: Vitiaz Strait, St George's Channel plus Solomon Strait transports, respectively. Example is given for σ₀ 24–25.3 km m⁻³ (Table 2). Initial plot was generated by Ocean Data View (Schlitzer, 2022). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

In the model developed by Pham et al. (2019), waters flowing equatorward into the Solomon Sea include the transports of the NGCU equatorward (W_E) and of the direct NVJ (W_N), and three downstream water fluxes (NGCU, SGU, and NICU) exit through Vitiaz strait (W_V), St George's Channel (W_G), and Solomon Strait (W_S). The original box model had to be slightly modified since dFe and dREEs were not sampled systematically at the same stations. The initial model considered stations 29 (11° 3' 39.6" S; 156° 12' 54" E) and 34 as inflow stations ("N" and "E", with water flows W_E and W_N, respectively), and stations 42, 57 (4° 34' 12" S; 152° 31' 22.8" E), and 77 as outflow stations ("S", "G", and "V", with water flows W_S W_G, and W_V, respectively). Stations 29 and 57 were not sampled for dFe and station 34 was only sampled in the upper 350 m (σ₀ < 26.3 kg m⁻³). Instead of station 29, we thus considered station 10 as an inflow station to the Solomon Sea, since both stations were influenced by direct NVJ and station 10 was an upstream station of station 29. For the inflow through the NGCU, for UTW and LTW, both stations 10 and 34 were considered. However, for IW and DW, station 39 was considered, and W_E and W_N were summed. This was justified by the fact that this station was located at the southern entrance of the Solomon Sea, downstream of both stations 10 and 34, and that for IW and DW, θ-S diagrams were very similar for the three stations (Fig. S1). In addition, glider data showed that at the entrance of the Solomon Sea, close to station 34, the western boundary current and mid-basin inflows merge into a single current on the New Guinea side (Kessler et al., 2019). For the outflow stations, since stations 42 and 57 had similar water properties (Fig. S2), station 42 was considered as outflow station for both St George's Channel and Solomon Strait, and W_G, and W_S were summed.

Although diapycnal mixing is known to be important within the Solomon Sea (Germineaud et al., 2016), it was not accounted for in constructing the box model, as in Pham et al. (2019). The following equations were thus applied:

For the UTW and LTW:

$$W_N [dFe]_N + W_E [dFe]_E + F_{tot} = (W_G + W_S) [dFe]_{G+S} + W_V [dFe]_V \quad (5)$$

And for IW and DW:

$$(W_E + W_N) [dFe]_{E+N} + F_{tot} = (W_G + W_S) [dFe]_{G+S} + W_V [dFe]_V \quad (6)$$

Where F_{tot} is the total net dFe flux into seawater from the surrounding coasts. F_{tot} is positive when external inputs of dFe dominate while F_{tot} is negative when scavenging and/or uptake processes do.

The water mass transports (W_{E,N,V,G,S}) were computed from the

cruise data (Ganachaud et al., 2017; Germineaud et al., 2016; see Pham et al., 2019 for more details).

[dFe]_N, [dFe]_E, [dFe]_{E+N}, [dFe]_{G+S}, [dFe]_V, are the depth-weighted average concentrations of dissolved Fe in the specific isopycnal layers at stations 10, 34, 39, 42, and 77, respectively (Table 2).

A Monte Carlo statistical method (normal distribution, n = 1000) was used to estimate dFe flux uncertainties. Results are reported in Table 2.

## 5. Results

### 5.1. Dissolved Fe concentrations

Dissolved Fe (dFe) concentrations ranged from 0.11 ± 0.01 nmol L⁻¹ (St 10, 175 m, ±1 SD) to 1.29 ± 0.12 nmol L⁻¹ (St 42, 1300 m, ±1 SD). Depth profiles are shown in Fig. 4 and Fig. S3. They exhibited low concentrations in the (sub)-surface waters increasing with depth. Profiles are oceanographically consistent (Johnson et al., 1997; Schlitzer et al., 2018) and similar to literature values in the South Pacific (Table S2). When comparing our data with data from the EUCFe cruise at four stations close to our studied area (Fig. 1 & S4), dFe concentrations were generally lower than the ones measured by Labatut et al. (2014) and Slemons et al. (2012), highlighting strong spatial and temporal variabilities in this region and heterogeneities in the dFe sources and sinks.

Considering stations outside the Solomon Sea, station 4 was slightly higher in dFe in the surface waters (σ₀ < 24 kg m⁻³, depth above ~150 m) compared to the three other stations (10, 13 and 43) (0.26–0.35 nmol L⁻¹ and 0.13–0.24 nmol L⁻¹ respectively). In the thermocline and intermediate layers (24.0 kg m⁻³ < σ₀ < 27.4 kg m⁻³, depth between ~150 m and ~1000 m), dFe concentrations were higher at station 4 (averaged value of 0.68 ± 0.24 nmol L⁻¹, ±1 SD, n = 7) than at the other stations (0.37 ± 0.26 nmol L⁻¹, ±1 SD, n = 7, for station 10, 0.39 ± 0.28 nmol L⁻¹, ±1 SD, n = 7, for station 13, and 0.34 ± 0.17 nmol L⁻¹, ±1 SD, n = 9, for station 43). In these layers, the lowest concentrations were found in the SPEW (Fig. S5), only encountered at stations 10, 13, and 43, with an average value of 0.17 ± 0.05 nmol L⁻¹ (±1 SD, n = 4). At the density around 27.2 kg m⁻³ (700–1000 m), corresponding to the AAIW (stations 4 and 10) and EqPIW (stations 13 and 43), dFe concentrations were depleted at station 43 (0.53 nmol L⁻¹) compared to the other stations (0.84–0.94 nmol L⁻¹). Finally, below ~1000 m (σ₀ > 27.4
<caption>**Table 2**</caption>

Water masses of different density layers during Pandora. Average dFe concentrations in the different layers of the box model, with standard deviation (STD), and associated volume transports (VT in Sv) with STD. The total dFe net fluxes were estimated from eqs. 5 and 6 (see text for more detail).

<table><thead><tr><th rowspan="2">Water masses</th><th colspan="3">St 10</th><th colspan="3">St 34</th><th colspan="3">St 39</th><th colspan="3">St 42</th><th colspan="3">St 77</th></tr><tr><th>σ<sub>0</sub></th><th>dFe</th><th>nmol L<sup>-1</sup></th><th>STD</th><th>VT</th><th>STD</th><th>dFe</th><th>STD</th><th>VT</th><th>STD</th><th>dFe</th><th>STD</th><th>VT</th><th>STD</th><th>F<sub>tot</sub></th><th>Error</th></tr></thead><tbody><tr><td>UTW</td><td>24-25.3</td><td>0.12</td><td>±</td><td>0.01</td><td>3.5</td><td>±</td><td>0.39</td><td>±</td><td>0.04</td><td>1.5</td><td>±</td><td>0.5</td><td>0.38</td><td>±</td><td>0.05</td><td>3.5</td><td>±</td><td>1.0</td><td>0.36</td><td>±</td><td>0.04</td><td>1.5</td><td>±</td><td>0.5</td><td>75</td><td>±</td><td>45</td></tr><tr><td>LTW</td><td>25.3-26.9</td><td>0.24</td><td>±</td><td>0.11</td><td>7.8</td><td>±</td><td>0.5</td><td>0.57</td><td>±</td><td>0.18</td><td>9.2</td><td>±</td><td>0.5</td><td>0.43</td><td>±</td><td>0.10</td><td>9.0</td><td>±</td><td>0.5</td><td>0.48</td><td>±</td><td>0.07</td><td>8.0</td><td>±</td><td>0.5</td><td>54</td><td>±</td><td>260</td></tr><tr><td>IW</td><td>26.9-27.4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.72</td><td>±</td><td>0.24</td><td>3.9</td><td>±</td><td>0.5</td><td>0.62</td><td>±</td><td>0.12</td><td>4.5</td><td>±</td><td>0.5</td><td>93</td><td>±</td><td>114</td></tr><tr><td>DW</td><td>> 27.4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.54</td><td>±</td><td>0.05</td><td>8.4</td><td>±</td><td>0.6</td><td>1.29</td><td>±</td><td>0.13</td><td>5.5</td><td>±</td><td>1.0</td><td>330</td><td>±</td><td>152</td></tr></tbody></table>

kg m⁻³), the four stations had similar dFe concentrations with values between 0.71 and 0.83 nmol L⁻¹.

Among the stations at the entrance or inside the Solomon Sea, station 34 showed the highest concentrations in the surface layer (σ₀ < 24 kg m⁻³, 0.45 ± 0.15 nmol L⁻¹, ±1 SD, n = 2), followed by station 71 (0.29 ± 0.08 nmol L⁻¹, ±1 SD, n = 3). Stations 39 and 60 had similar dFe concentrations (0.15 ± 0.01 nmol L⁻¹, ±1 SD, n = 1 and 0.17 ± 0.03 nmol L⁻¹, ±1 SD, n = 3, respectively). In the thermocline and intermediate layers (24.0 kg m⁻³ < σ₀ < 27.4 kg m⁻³, depth between ~150 m and ~1000 m), while stations 34, 60, and 71 showed similar dFe concentrations, ranging from 0.34 to 0.42 nmol L⁻¹ just below 150 m to 1.02–1.24 at ~1000 m, station 39 exhibited lower dFe concentrations (from 0.26 ± 0.01 nmol L⁻¹ just below 150 m to only 0.57 ± 0.01 nmol L⁻¹ at ~1000 m). Below 1000 m, the same trend was observed with lower value at station 39 (0.57 ± 0.07 nmol L⁻¹) than at stations 60 and 71 (1.24 ± 0.06 nmol L⁻¹ and 1.02 ± 0.07 nmol L⁻¹, respectively).

At the straits, profiles are quite similar, except at the surface (σ₀ < 24 kg m⁻³), where station 42 showed the highest concentration (0.44 ± 0.03 nmol L⁻¹) and station 77 the lowest one (0.17 ± 0.01 nmol L⁻¹). In the thermocline and intermediate layers (24.0 kg m⁻³ < σ₀ < 27.4 kg m⁻³, depth between ~150 m and ~1000 m), dFe concentrations for stations 42 and 77 were similar. The highest difference was equal to 0.06 nmol L⁻¹ at 295–297 m, σ₀ 26.2–26.3 kg m⁻³ (0.42 ± 0.02 nmol L⁻¹ and 0.48 ± 0.04 nmol L⁻¹, respectively). At depth around 200 m (σ₀ 25.1–25.2 kg m⁻³), station 21 exhibited low dFe concentrations (0.22–0.27 nmol L⁻¹). Below 1000 m, station 42 displayed the highest concentrations (1.29 ± 0.12 nmol L⁻¹).

## 5.2. Ancillary parameters

Nitrate profiles are shown in Fig. S6 and concentrations are reported in Table S1 for TMR data. Concentrations were lower than 0.2 µmol L⁻¹ at all stations in the TSW (Fig. S6). Except for station 34 which showed a sub-surface maximum at 130 m (7.7 µmol L⁻¹), NO₃⁻ concentrations increased with depth at all stations and reached values between 35.0 µmol L⁻¹ (St 04) and 38.5 µmol L⁻¹ (St 43) at 1300 m.

Chl-α maximum concentrations ranged from 0.17 µg L⁻¹ (St 04 and 43) to 0.84 µg L⁻¹ (St 39) and was equal to 0.45 µg L⁻¹ at the surface at station 77 (Table S1). Fluorescence data show deep maximums at all stations between 40 m and 100 m except at stations 21, 34, and 77, where maximum Chl-α concentrations were observed in the upper 40–50 m (not shown).

# 6. Discussion

In the following, we discuss the potential sources, sinks, and processes occurring in the water column. We also assess the potential Fe enrichment within the Solomon Sea using a simple box model and estimate the relative contribution of dFe fluxes out of the Solomon Sea to the Equatorial Undercurrent (EUC).

## 6.1. dFe distribution

### 6.1.1. Surface waters – 0 – ~ 150 m (surf-24 σ₀)

Most of the surface concentrations observed at our stations inside and outside the Solomon Sea were among the lowest reported globally to date (< 0.1–0.2 nmol L⁻¹), but they were still higher than the lowest values observed in the South Pacific (0.02–0.03 nmol L⁻¹, Table S2). This suggests that external inputs, such as atmospheric, riverine and/or coastal/margin inputs, influence dFe distribution in our study area.

Using a reverse MADCOW model approach (Measurement of Aluminium for Dust Calculation in Oceanic Water, Measures and Brown, 1996), Michael et al. (2021) estimated that the atmospheric dissolved aluminium (dAl) and dissolved manganese (dMn) inputs over the Solomon Sea only represented ~ an average of 5 % and ~ 2 % of the total surface dAl and dMn pools, respectively. Using the same approach as in
<figure><img src="image_7.png" /><figcaption>Fig. 4. Dissolved Fe concentrations (nmol L⁻¹) vs. depth (m) from the PANDORA cruise for stations (a) outside the Solomon Sea (4, 10, 13, 43, red), (b) at the entrance or inside the Solomon Sea (34, 39, 60, and 71, green), and (c) at the straits (21, 42, 77, blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Michael et al. (2021), with the same dust flux of 1.0 g m⁻² yr⁻¹ to the Solomon Sea (Shank and Johansen, 2008), a residence time of surface waters in the Solomon Sea of 0.6–2 months (Hristova and Kessler, 2012; Alberty et al., 2019), a mixed layer depth of 100 m, a fractional solubility of Fe in dust of 5–20 % (Buck et al., 2013), and a crustal abundance of Fe of 3.5 % (Taylor and McLennan, 1995), the estimated atmospheric dFe inputs ranged from 31 to 125 µmol m⁻² yr⁻¹. These estimates are similar to estimates from a recent study in the South Pacific based on the thorium tracer (GEOTRACES process study GPpr09, 3.5–370 µmol m⁻² yr⁻¹, Pavia et al., 2020) or from direct aerosol and rainwater measurements in the Open Pacific Ocean (88 µmol m⁻² yr⁻¹; Martino et al., 2014). Considering the measured dFe at the shallowest depth at each station, these atmospheric inputs represented between 4 % and up to 138 % of the measured dFe pool (stations 42 and 39, respectively), suggesting that the dFe atmospheric inputs could be significant in our study area relative to the other dFe inputs and/or that scavenging processes are more important for dFe than for both dAl and dMn.

As pointed out by Michael et al. (2021), based on salinity data, there was no large riverine input to the Solomon Sea during our cruise. Three stations were particularly enriched compared to the median value (0.23 nmol L⁻¹): stations 34, 21, and 42 (0.34 ± 0.02, 0.36 ± 0.02, and 0.44 ± 0.03 nmol L⁻¹, respectively, ±1 SD, Fig. 4 and S3). Station 34 was located off the coast of Rossel Island (Fig. 1), while stations 21 and 42 were located at the Indispensable and Solomon Straits, respectively (Fig. 1). These high concentrations close to the coast and at the straits would suggest coastal/margin inputs. At the Indispensable Strait, the two stations 13 and 21 (Fig. 1), sampled just before and just after the strait, were positioned in order to identify possible changes in water mass properties across the strait. The dFe enrichment between the two stations increased by nearly a factor of 2 (0.21 ± 0.04 nmol L⁻¹ and 0.35 ± 0.02 nmol L⁻¹, respectively, ±1 SD, n = 3) and was consistent with enrichments observed in Al, Mn, and Rare Earth Elements (REEs, Michael et al., 2021; Pham et al., 2019), suggesting inputs from the islands and coastal sediment resuspension.

Station 77, located in the Vitiaz Strait, contained lower dFe concentrations than expected at the surface (0.17 ± 0.01 nmol L⁻¹, ±1 SD, Fig. S7), more than half that of the three stations mentioned above. In contrast, station 77 presented the highest values of REEs, Al, and Mn (Michael et al., 2021; Pham et al., 2019), which were attributed to the coupling of resuspension and entrainment of local sediments into the water column due to high current velocities in the narrow straits

(Alberty et al., 2019; Hristova and Kessler, 2012). Both Al and Mn were also enriched in the particulate phase, suggesting large particle inputs combined with rapid removal by scavenging (Lee et al., 2021; Michael et al., 2021; Tebo et al., 2004). This could also be the case for dFe, and because Fe is preferentially scavenged compared to Al and Mn (Landing and Bruland, 1987; Noble et al., 2012; Zheng and Sohrin, 2019), it could explain the low dFe concentrations at this station (Borrione et al., 2014; Cheize et al., 2019; Johnson et al., 1997; Slemons et al., 2012). Moreover, this station did not exhibit a deep chlorophyll maximum (DCM). Instead, it showed Chl-α concentrations in the upper layer (0.45 µg L⁻¹), which were slightly higher than the median Chl-α value (0.43 µg L⁻¹). This station was also the station where the highest dinitrogen fixation rates and *Trichodesmium* sp. abundance were measured among the stations sampled for dFe (Bonnet et al., 2015). *Trichodesmium* have high cellular Fe requirements and uptake rates (Berman-Frank et al., 2001; Lory et al., 2022; Rueter et al., 1992), and biological uptake might thus have contributed to decrease dFe concentrations at this station.

All stations that displayed a DCM (stations 4, 10, 13, 39, 42, 43, 60, and 71) also showed a local dFe minimum in this layer or just below (Table S1). These (sub)surface dFe minima might be due to a combination of biological uptake and scavenging onto biogenic and/or sinking particles (Croot et al., 2004; Sarthou et al., 2007; Sedwick et al., 2005), with a potential more elevated Fe requirement for phytoplankton at low light (Hopkinson and Barbeau, 2008). Moreover, the DCMs are generally isolated in stratified oligotrophic regions, thus reducing Fe supply from atmospheric sources as well as from remineralised sources from below (Hawco et al., 2021).

### 6.1.2. Thermocline and intermediate waters – ~150 – ~1000 m (24–27.4 $\sigma_0$)

In these water masses, the general increase in dFe concentrations with depth (0.1–1.0 nmol L⁻¹) might be due to remineralization of organic matter (OM), which may represent a significant contributor of both macro and micro-nutrients (Ploug and Grossart, 2000; Simon et al., 2002). In order to assess this hypothesis, we used NO₃⁻ as a proxy of remineralization (see method section).

When considering all stations, the linear relationship between dFe and NO₃⁻-remineralized (Fig. 2b) was significant ($p < 0.01$, $n = 74$). However, a station by station analysis revealed that two stations did not exhibit a significant relationship between dFe and NO₃⁻ (stations 34 and 39, p-values of 0.36 and 0.06, respectively, Fig. S8. These two stations,
influenced by a strong NGCU and positioned near the coast, might have been enriched by local coastal or margin inputs. Additionally, enrichment could have occurred when the GPC flowed along the Papua New Guinea coast. For all the other stations, remineralization represented a significant source of dFe in the intermediate waters.

When considering stations outside the Solomon Sea (St 04, 10, 13, and 43), station 04 was the only one showing significantly positive value of the intercept of the relationship between dFe and NO<sub>3</sub><sup>-</sup> (Fig. S8). This positive value might explain the higher values of dFe at intermediate depth compared to the other outside stations, since the r<sub>Fe:N</sub> value at this station was within the range of variation for the other stations (0.019 ± 0.02 mmol:mol, ±1 SD, compared to 0.012 ± 0.03–0.024 ± 0.03 mmol:mol, ±1 SD, for the other stations). The r<sub>Fe:C</sub> values for all the other stations were within this range of variation. When considering a Redfield ratio for C:N (r<sub>C:N</sub> = 6.6), the Fe:C ratios (r<sub>Fe:C</sub>) ranged from 1.9 to 3.6 µmol:mol, which were in the lowest range of the ratios found in the literature (2–1770 µmol:mol; Moore et al., 2004; Rijkenberg et al., 2014; Sarthou et al., 2005; Sunda and Huntsman, 1995; Twining and Baines, 2013), in particular for *Trichodesmium* which have high Fe requirements (Berman-Frank et al., 2001; Rueter et al., 1992). Although gene sequences affiliated to *Trichodesmium* sp. have been measured between 200 m and 1000 m depth in the Solomon Sea (Benavides et al., 2015), this would suggest that *Trichodesmium*, which dominated the phytoplankton biomass during PANDORA (Bonnet et al., 2015), did not significantly contribute to remineralised dFe during our study in the Solomon Sea. They might, however, have significantly contributed to export, as well as to dFe remineralisation and enrichment at depth outside the Solomon Sea (see § 5.3).

### 6.1.3. Deep waters > ~ 1000 m (> 27.4 σ₀)

For the deepest samples (~ 1000–1300 m), when excluding Station 77 (Vitiaz Strait) and using dMn and dAl concentrations measured by Michael et al. (2021), dFe showed a positive significant relationship with dMn (Fig. 5, *p* value <0.01) and no significant relationship with dAl (not shown, *p* values = 0.98). This suggested that release from shelf sediments and/or hydrothermal inputs influenced dFe and dMn distributions at depth. This release, if reductive, would impact Fe and Mn distributions but not Al distributions, since Al oxidation state is stable in natural waters (Michael et al., 2021; Resing et al., 2015). Unfortunately, Fe isotopic composition data are not yet available so far for the PANDORA cruise to assess this hypothesis.

For station 77, as stated above for surface waters, strong scavenging

of dFe might occur. Stations 77 and 42 (Solomon Strait) were the two stations with the highest concentrations of particulate Al, particulate Mn, and dissolved Mn, close to the bottom (Michael et al., 2021). Low dFe concentrations at St 77 might thus reflect a combination of scouring, sediment resuspension, inputs of sinking particles from shallow sources together with strong scavenging associated to high particle loading (Gourain et al., 2019; Klunder et al., 2011; Wu et al., 2001).

## 6.2. Potential enrichment in the Solomon Sea

The dFe budget results of the box model showed significant enrichments within the Solomon Sea, but only within the UTW and the DW (Table 2). In the UTW, a slight dFe enrichment was observed (75 ± 45 10³ mol d⁻¹, ±1 SD, Table 2), on the contrary to dissolved neodymium (dNd), where no enrichment was observed with a similar box model (Pham et al., 2019). Pham et al. (2019) hypothesized that external inputs of dNd were balanced by removal by scavenging. For dFe, the scavenging processes might be reduced by the presence of organic ligands which helped maintain Fe in solution (Gledhill and Buck, 2012). However, organic ligand measurements were not performed to assess this hypothesis. The difference between dFe and dNd budgets can also be explained by the much lower dFe concentrations in this layer at station 10 compared to the other stations, which was not observed for dNd concentrations. In the LTW, high dNd enrichment was observed while no enrichment nor removal was observed for dFe. The strong dNd enrichment was mainly driven by the high dNd concentrations at station 77 in this density layer, which was not observed for dFe. In the IW, both dFe and dNd budgets showed values which were not significantly different from zero. Finally, in the DW, while dNd did not show any significant enrichment, dFe was strongly enriched (330 ± 152 10³ mol d⁻¹, ±1 SD, Table 2). For dNd, in this layer as in the UTW, scavenging might balance external local inputs (Pham et al., 2019). It has notably been shown that Nd is strongly removed by hydrothermal particle scavenging (Waltermathe et al., 2024). On the contrary, for dFe, as seen above with the strong correlation with dMn at depth in our study area, release from shelf sediments and/or hydrothermal inputs increased both dFe and dMn concentrations in the deep waters, especially in the Solomon Strait (station 42). The differences observed between the Fe and Nd budgets highlighted different processes between the cycles of these two elements, such as redox processes, scavenging and organic complexation. Regarding the dFe budget, while a modest but significant increase was detected in the upper thermocline layer, this enrichment is particularly

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** dFe (nmol L⁻¹) vs. dissolved Mn concentrations (dMn, nmol L⁻¹, Michael et al., 2021) for all stations at depths below 1000 m. Station 77 was depicted in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
noteworthy because these waters are transported to the equatorial eastern Pacific via the lower portion of the Equatorial Undercurrent (EUC).

### 6.3. Contribution to the Equatorial Undercurrent (EUC) flux

As in Michael et al. (2021) for dAl and dMn, we estimated the contribution of dFe from the Solomon Sea to the EUC. This current is located at density layer 24–26.9 kg m⁻³ (UTW and LTW) and is fed by low-latitude western boundary currents (LLWBC), including the Mindanao Current from the north (Wang et al., 2024), and the New Guinea Coastal Undercurrent (NGCU), New Ireland Coastal Undercurrent (NICU), and Saint Georges Undercurrent (SGU) from the south (Butt and Lindstrom, 1994; Fine et al., 1994; Grenier et al., 2011; Tsuchiya et al., 1989). The NGCU flows along the North East coast of PNG, the SGU enters the Bismarck Sea through the St George channel and the NICU flows along the North East coast of New Ireland (Fig. 1). For the estimation of the outflow flux from the Solomon Sea, we summed the fluxes of the outflow stations (42 and 77) in UTW and LTW and found a value of dFe flux of 829 ± 195 10³ mol d⁻¹ (±1 SD). The flux in the EUC was estimated using the dFe value observed by Slemons et al. (2012) at a station located at 0°S-156°E during the EUCFe cruise (1.52 ± 0.45 nmol L⁻¹, ±1 SD) and the water flux corresponding to the sum of the outflow water fluxes (22.0 ± 2.5 Sv, ±1 SD). This gave a dFe flux of 2892 ± 917 10³ mol d⁻¹ (±1 SD). Using the same estimates of northern versus southern water contribution to the EUC as in Michael et al. (2021), with values varying between 52% and 100% (as an upper limit, Table 3), we estimated that between 15% and 29% (average value 20%) of the dFe in the EUC could come from waters flowing out of the Solomon Sea (Table 3). These percentages are lower than values estimated by Michael et al. (2021) for dissolved aluminium (62%–119%, average value 74%) and manganese (33–64%, average value 40%), suggesting the existence of other sources to the EUC, both enriched in Fe and with higher Fe:Al and Fe:Mn ratios. Dissolved Fe concentrations measured along the PNG coast within the Bismarck Sea by Slemons et al. (2010, 2012) and Labatut et al. (2014) (Fig. 1 and S5) were high compared to our measurements at the exit of the Solomon Sea. If we use results from a station located at 3.4°S, 143.9°E, just downstream of the Sepik river, during the EUCFe cruise (Station 28, Slemons et al., 2010, 2012), instead of station 77, the relative contributions to EUC of the Solomon and Bismarck Seas was increased by nearly a factor of 2, with dFe average values of 36%

<caption>**Table 3**</caption>

Contributions of the Solomon and Bismarck Seas to the EUC dFe pool based on variation in source waters.

<table><thead><tr><th>Reference</th><th>% of EUC derived from Southern Hemisphere</th><th>Method of estimate</th><th>%dFe contribution of the SS to EUC based on magnitude of source</th><th>%dFe contribution of the Solomon and Bismarck Seas to EUC based on magnitude of source</th></tr></thead><tbody><tr><td>Grenier et al. (2011)</td><td>100%</td><td>Upper bound</td><td>29</td><td>52</td></tr><tr><td>Tsuchiya et al. (1989)</td><td>59%</td><td>Model</td><td>17</td><td>31</td></tr><tr><td>Izumo et al. (2002)</td><td>67%</td><td>WEPOCS</td><td>19</td><td>35</td></tr><tr><td>Lehmann et al. (2018)</td><td>52%</td><td>Model</td><td>15</td><td>27</td></tr><tr><td>Qin et al. (2015)</td><td>70%</td><td>O₂, Si, N isotopes</td><td>20</td><td>36</td></tr><tr><td>Average</td><td>63%</td><td>Model</td><td>18</td><td>33</td></tr><tr><td></td><td></td><td></td><td>20</td><td>36</td></tr></tbody></table>

(27–52%, Table 3). For dAl and dMn, considering the Bismarck Sea also increased the relative contribution to the EUC but to a lesser extent (63–120% and 66–105%, respectively, Table S3). This is explained by the higher Fe:Al and Fe:Mn ratios at the station just downstream the Sepik river (station 28, Slemons et al., 2010, 2012) than at the station located at the Vitiaz Strait (station 77) (respectively, 0.14 mol:mol versus 0.05 mol:mol for Fe:Al, and 2.5 mol:mol versus 1.7 mol:mol for Fe:Mn). This suggests that the Bismarck Sea could significantly contribute to the EUC dFe flux in addition to the Solomon Sea. This is consistent with previous studies in this region (Behrens et al., 2020; Grenier et al., 2011; Radic et al., 2011; Labatut et al., 2014).

Remineralisation of organic matter produced within the Solomon Sea, but exported at depth outside the Solomon Sea, may also have contributed to dFe enrichment of the EUC. *Trichodesmium* dominated the plankton biomass during the PANDORA cruise (Bonnet et al., 2015), but deep Fe:N ratios revealed that they likely did not significantly contribute to remineralization in the Solomon Sea (see section 5.1.2). However, *Trichodesmium* have recently been shown to be significant contributors to organic matter export in mesopelagic waters, as evidenced in the subtropical South Pacific waters (~20°S170°E, ~22°S165W, Ababou et al., 2024; Bonnet et al., 2023a). As they were present in mesopelagic waters of the Solomon Sea (Benavides et al., 2015), we assume that some portion of sinking of *Trichodesmium* might have been remineralised outside the Solomon Sea, characterized by strong surface currents and a residence time of only few weeks (0.6–2 months, Michael et al., 2021). Dinitrogen fixation in the Solomon Sea was on average 624 µmol Nfixed m⁻² d⁻¹ during PANDORA (Bonnet et al., 2015). Considering a photic water layer of 150 m (Bonnet et al., 2015), it accounts for 4.2 µmol Nfixed m⁻³ d⁻¹. Within 0.6–2 months, it amounts to 76–252 nmol Nfixed L⁻¹. Taking into account a mean Fe:N ratio of 750 µmol mol⁻¹ in diazotrophs, as used in previous studies of the Pacific Ocean (Dutkiewicz et al., 2012; Martino et al., 2014), this would lead to an estimated value of 0.06–0.19 nmol L⁻¹ of Fe taken up by *Trichodesmium*. Given an outflow flux of 22 Sv (see above), and assuming that once entirely remineralised this Fe pool remains in the dissolved fraction, we calculated a dFe flux of 114–361 10³ mol d⁻¹, corresponding to an additional 4–12% of the EUC flux. Although non-negligible, this estimate indicates that additional sources are required to fully account for the dFe enrichment observed in the EUC.

## 7. Conclusion

This study presents vertical profiles of dissolved iron (dFe) collected at 11 stations during the PANDORA cruise (GEOTRACES GP12). Overall, the dFe profiles exhibited nutrient-like behavior, with some surface enrichments and dFe minima coinciding with deep chlorophyll maxima, likely resulting from biological uptake and reflecting higher Fe requirements in low-light conditions. Atmospheric deposition estimated using the reverse MADCOW model (Measures and Brown, 1996) could significantly contribute to the dFe pool, alongside other external sources such as coastal and margin inputs. However, biological uptake and scavenging processes may counterbalance these dFe sources, particularly in straits where strong enrichments were not consistently observed. Using apparent oxygen utilization (AOU) and nitrate concentrations as proxies for remineralization, we demonstrated that the recycling of organic matter is also a significant source of dFe at intermediate depths. Below 1000 m, a strong correlation between dFe and dMn (Michael et al., 2021) suggested that release from shelf sediments and/or hydrothermal inputs influenced the distribution of dFe. A box model modified from Pham et al. (2019) allowed us to quantify the net dFe fluxes of waters flowing equatorward through the Solomon Sea. While a modest but significant enrichment was observed in the upper intermediate layer, this enrichment was particularly noteworthy since these waters are advected to the equatorial eastern Pacific via the lower part of the Equatorial Undercurrent (EUC). Our estimates indicate that 20% on average (15%–29%) of the dFe in the EUC originated from waters
exiting the Solomon Sea, implying that other sources (including remi-
neralisation outside the Solomon Sea) contributed to dFe in the EUC,
either along its downstream transport or from the Northern Hemisphere.
A substantial enrichment in dFe was observed in the deepest layer,
highlighting the strong interactions between dFe and the bathymetry
within the Solomon Sea.

CRediT authorship contribution statement

Géraldine Sarthou: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Methodology, Investiga- tion, Funding acquisition, Formal analysis, Data curation, Conceptuali- zation. **Eva Bucciarelli:** Writing – review & editing, Writing – original draft, Supervision, Formal analysis, Data curation. **Fabien Quéroué:** Methodology, Formal analysis, Data curation. **François Lacan:** Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization. **Hélène Planquette:** Writing – review & editing, Writing – original draft, Supervision. **Viet Q. Pham:** Writing – original draft, Validation. **Maxime Grand:** Writing – review & editing, Writing – original draft. **Catherine Pradoux:** Writing – review & editing, Writing – original draft. **Martine Rodier:** Writing – review & editing, Writing – original draft, Data curation. **Sophie Bonnet:** Writing – review & edit- ing, Writing – original draft. **Gérard Eldin:** Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization. **Sophie Cravatte:** Writing – review & editing, Writing – original draft, Valida- tion, Methodology, Funding acquisition, Conceptualization. **Catherine Jeandel:** Writing – review & editing, Writing – original draft, Valida- tion, Funding acquisition, Conceptualization. **Alexandre Ganachaud:** Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization.

Funding sources

This work was supported by CNRS LEFE/CYBER and LEFE/IMAGO Solwara program and the LabexMER (ANR-10-LABX-19). The Interna- tional GEOTRACES Programme is possible in part thanks to the support from the U.S. National Science Foundation (Grant OCE-1840868) to the Scientific Committee on Oceanic Research (SCOR).

Declaration of competing interest

None.

Acknowledgements

This work is a contribution to the GEOTRACES International pro-
gramme. We deeply acknowledge Chris Measures, who lent the entire
sampling device (rosette, bottles, sensors and winch) from University of
Hawaii (USA). We are grateful to the ship captain and crew (RV l'Ata-
lante) who made possible the cruise with the various on-board opera-
tions. We thank the technical division of INSU (DT-INSU, O. Desprez de
Gesincourt, L. Fichen, F. Perault, E. de Saint-Leger, C. Brachet and L.
Souarnec); US-IMAGO (D. Lopez, L. Jamet), IRD Center Noumea (J.
Carso, V. Perrin, C. Hartmann, J.-L. Fuda, D. Varillon). We also greatly
thank Sébastien Hervé for infography.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.marchem.2025.104567.

Data availability

The dFe dataset is included in the GEOTRACES and LEFE-CYBER
databases:
https://www.bodc.ac.uk/data/published_data_library/catalogue/

10.5285/ff46f034-f47c-05f9-e053-6c86abc0dc7e/
https://explore.webod.awi.de/ocean/tracer/geotraces/idp2021/
seawater/geotraces_idp2021_seawater_discrete_sample_data_v2/
http://wwwobs-vlfr.fr/proof/php/pandora/x_datalist_1.php?
xxop=pandora&xxcamp=pandora

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