# Particulate iron seasonality between winter and spring in the open Southern Atlantic Ocean is primarily driven by non-biological processes

Antoine Ringard, Hélène Planquette, Thato Mtshali, Alakendra

Roychoudhury, Eva Bucciarelli

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Antoine Ringard, Hélène Planquette, Thato Mtshali, Alakendra Roychoudhury, Eva Bucciarelli. Particulate iron seasonality between winter and spring in the open Southern Atlantic Ocean is primarily driven by non-biological processes. Marine Chemistry, 2025, 272, pp.104552. 10.1016/j.marchem.2025.104552. hal-05233383

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# Particulate iron seasonality between winter and spring in the open Southern Atlantic Ocean is primarily driven by non-biological processes

Antoine Ringard<sup>a, *,</sup>, Hélène Planquette<sup>a, *</sup>, Thato Mtshali<sup>b</sup>, Alakendra Roychoudhury<sup>c</sup>, Eva Bucciarelli<sup>a, *</sup>

## ARTICLE INFO

### Keywords:

GEOTRACES

Particulate iron seasonality

Southern Ocean

Biogenic Lithogenic and Authigenic contributions

## ABSTRACT

In the open Southern Ocean, primary productivity is mainly driven by limiting concentrations of iron. However, limited available iron data during winter leave a gap in our understanding of how seasonality affects the iron cycle, and primary productivity. In this context, two cruises followed the same transect in the Atlantic sector of the Southern Ocean, from the SubTropical zone to the Antarctic zone, during spring and winter. This study aims to determine whether particulate iron concentrations showed seasonal changes, and to identify the processes driving these variations. Particulate iron ranged between 0.04–0.99 nM and 0.04–1.17 nM during spring and winter, respectively, with the lowest concentrations measured in surface waters. Overall, seasonality did not consistently affect total pFe distributions, and when differences existed, total pFe concentrations were higher in winter than in spring. Our findings indicate that biogenic particulate iron is higher in spring than in winter, and represents only a minor component of total pFe in surface waters during both seasons. Therefore, biological processes do not account for the observed seasonal changes in total pFe concentrations. At certain stations, higher pFe concentrations in winter relative to spring appear to be related to episodic external inputs of lithogenic particles, such as iron-rich dust deposition and inputs from the ACC. The authigenic fraction is the primary contributor to total pFe. These particles seem to be formed through the precipitation of dissolved iron derived from the dissolution of lithogenic particles or hydrothermal activity.

## 1. Introduction

The global ocean plays a crucial role in regulating the carbon biogeochemical cycle (Gruber et al., 2019; Sarmiento et al., 2004). The global ocean absorbs approximately 25 % of anthropogenic CO₂, with the Southern Ocean (SO) responsible for 30 % to 40 % of this absorption (DeVries, 2014), facilitated by both the solubility and biological carbon pumps. Biological absorption, considered as a minor driver of CO₂ uptake in the Southern Ocean (Gruber et al., 2019), is driven by ocean biogeochemistry which influences the phytoplankton community structure and primary production. In addition to the control exerted by the macro nutrients (nitrogen, phosphorus and silicon), availability of light (Deppeler and Davidson, 2017) and grazing pressure (Smetacek et al., 2004), the productivity and phytoplankton community structure are also regulated by the availability of trace metals such as iron (Fe), manganese (Mn), zinc (Zn), cobalt (Co), cadmium (Cd) and copper

(Cu) (Sunda, 2012). These bioactive trace metals are often present in very low concentrations (~ 0.1 nM) in the open ocean (Klunder et al., 2011; Middag et al., 2011), with Fe being the most studied as the primary limiting nutrient in the SO (Blain et al., 2007; Boyd et al., 2000; Boyd and Ellwood, 2010; Planquette et al., 2007; Shaked et al., 2020). Iron is involved in many phytoplankton cellular metabolisms, e.g., nitrate and nitrite reductase activities and electron transport chains in respiration and photosynthesis (Twining and Baines, 2013). Since the pioneering works of John Martin in the 1990's (Martin, 1990), many studies have highlighted the fact that low concentrations of dissolved iron (dFe) limit phytoplankton growth in many oceanic provinces (Bazzani et al., 2023; Boyd et al., 2000; Sedwick et al., 2000). In these areas, classified as High Nutrient Low Chlorophyll (HNLC), the availability of Fe becomes a crucial limiting factor for primary production. The Southern Ocean is recognized as the largest HNLC zone, where the highest rates of primary production occur around the Antarctic

<footnote><sup>a</sup> Univ Brest, CNRS, IRD, Ifremer, UMR 6539, LEMAR, F-29280 Plouzané, France</footnote>

<footnote><sup>b</sup> Department of Forestry Fisheries and the Environment, 2nd Floor, Foretrust Building, Cape Town, South Africa</footnote>

<footnote><sup>c</sup> Center for Trace Metal and Experimental Biogeochemistry, Department of Earth Sciences, Stellenbosch University, Stellenbosch 7602, South Africa</footnote>

<footnote>* Corresponding authors at: UMR 6539/LEMAR/IUEM, Technopôle Brest Iroise, Place Nicolas Copernic, 29280 Plouzané, France.</footnote>

<footnote>E-mail addresses: antoine.ringard@univ-brest.fr (A. Ringard), helene.planquette@univ-brest.fr (H. Planquette), eva.bucciarelli@univ-brest.fr (E. Bucciarelli).</footnote>
continental margin, and subantarctic islands, contrasting sharply with the low productivity observed in open ocean waters (Arrigo et al., 2008).

Iron can be introduced into Southern open ocean waters through several mechanisms, including resuspended sediments, advection, and diffusion on plateaus such as Kerguelen (Blain et al., 2007) and Crozet (Planquette et al., 2007), dust deposition (Tang et al., 2021), hydrothermal activity (Ardyna et al., 2019), vertical mixing (Tagliabue et al., 2014), and melting of icebergs and sea-ice (Deppeler and Davidson, 2017; Raiswell, 2011). Studies focusing on these processes were mainly achieved during summer season. Only a handful of studies on winter dFe distributions have been published so far (Ellwood et al., 2008; Mtshali et al., 2019), largely due to the extreme conditions present during this season (Ansorge et al., 2017). Consequently, seasonal cycles of biological processes governing primary production in the Southern Ocean remain poorly constrained (Kessler and Tjiputra, 2016).

Most studies conducted in the Southern Ocean have focused on the dissolved size fraction (dFe, <0.2 µm) due to its higher bioavailability for phytoplankton uptake (Shaked et al., 2020). In contrast, particulate iron (pFe, >0.45 µm) is often more abundant than the dissolved pool (Hurst et al., 2010) and was considered refractory. However, this view has been challenged by recent studies, highlighting the lability of authigenic pFe phases, which bioavailability likely varies depending on the mineral structure (Sofen et al., 2023; Tagliabue et al., 2023). Still, data on pFe remain scarce, with only a few localized studies available (Barrett et al., 2018; Marsay et al., 2017; Planquette et al., 2013; van der Merwe et al., 2019) and no winter values published to date. Nonetheless, dFe and pFe are in constant interaction (Boyd and Ellwood, 2010), notably through biological processes such as uptake and remineralization (Sunda, 2012), scavenging (Tagliabue et al., 2019) or desorption processes (Abadie et al., 2017). When taken up by phytoplankton cells, dFe contributes to the biogenic pFe pool, as well as iron contained in small heterotrophic organisms. Conversely, iron-containing particles can re-enter the dissolved pool through dissolution or remineralization, followed by stabilization by organic ligands (Boyd and Ellwood, 2010). In oxygenated waters, authigenic pFe arises from oxidation processes of dFe if not stabilized by ligands (Gledhill and Buck, 2012), leading to oxyhydroxide formation (Collier and Edmond, 1984), colloidal aggregation (Gourain et al., 2019) or adsorption and sedimentation (scavenging) onto particle surfaces (Marsay et al., 2017). Finally pFe is also composed by a lithogenic pool, with particles originating from terrestrial inputs (Blain et al., 2022; Latimer and Filippelli, 2001). Depending on these processes and the types of external sources, different fractions of pFe can be identified: lithogenic, biogenic, and authigenic.

Tagliabue et al. (2014) proposed that seasonal changes in dFe are influenced by mixing and biological activity. In our study, we further develop this framework by hypothesizing that biological processes are a dominant driver of the seasonal dynamics of both dFe and pFe, under the assumption that external sources such as dust deposition and sediment resuspension remain relatively stable throughout the year. We propose the following seasonal pattern: in winter, vertical mixing replenishes the dFe pool in the surface mixed layer (Tagliabue et al., 2014). As spring arrives, increased biological productivity results in greater biological uptake of dFe, leading to an increase in the pFe pool through intracellular incorporation and aggregation into particulate forms. In summer and autumn, the continuing biological demand and reduced supply lead to lower surface dFe concentrations, while remineralization processes at depth begin to recycle biogenic iron back into the dissolved phase. Therefore, we expect increased surface pFe concentrations in spring compared to winter, due to a rise in biogenic pFe following dFe uptake by phytoplankton. This seasonal dynamic would be consistent with observations along this transect, where phytoplankton biomass is higher during the productive season and associated blooms (chlorophyll *a* > 0.25 mg.m⁻³), with a pronounced seasonal cycle (Thomalla et al., 2011). At intermediate depths, we anticipate that dFe will be replenished during winter via remineralization, a process that should be concluded by winter, with all biogenic pFe already remineralized.

Consequently, we expect higher pFe concentrations at these depths in spring. However, this assumption should be approached with caution, as the only winter study of Baₓₓ, a proxy for remineralization, indicated that remineralization processes were still ongoing during early winter in the Indian sector of the Southern Ocean (van Horsten et al., 2022). While our interpretation assumes a predominantly vertical particle flux, lateral advection associated with mesoscale processes within the ACC could also have an impact on our expectations.

Based on these hypotheses, the present study aims to determine whether there are seasonal differences in pFe concentrations, using samples collected during the SCALE cruises (Southern Ocean seAsonal Experiment) in the Atlantic sector of the Southern Ocean (from 35°S to 59°S) in both spring and winter. The concentrations of total pFe from both seasons will be presented, followed by a discussion of the variances of the biogenic, lithogenic, and authigenic fractions. This will help elucidate the processes, sources, and sinks that influence total pFe distributions between the two seasons in surface and intermediate waters.

## 2. Material and methods

### 2.1. Study area

The SCALE oceanographic cruises were conducted aboard the South African (SA) Agulhas II research vessel in the south Atlantic sector of the Southern Ocean alongside the Good Hope line transect extending from 35°S to 59°S (Fig. 1). The first expedition spanned a duration of three weeks during austral winter (07-18-2019-08-12-2019; Chief Scientist: Dr. M. Vichi). The second cruise occurred during spring and lasted 5 weeks (10-12-2019-11-20-2019; Chief Scientist: Dr. T. Ryan-Keogh). At each station along the transect, samples were collected from near the surface (25 m) down to 2000 m depth during spring and to the ocean floor during winter. This study focuses on seven stations where particulate samples were collected (Fig. 1), located in diverse hydrographic regimes that allow for sampling across various water masses. Progressing from north to south along the transect, stations were located across key hydrographic zones of the Southern Ocean. Station GT09 was situated in the SubTropical Zone (STZ), north of the SubTropical Front (STF). Southward, station GT07 lay within the SubAntarctic Zone (SAZ), between the STF and the SubAntarctic Front (SAF). Stations GT06 and GT05 were positioned in the Polar Frontal Zone (PFZ), which spans the region between the SAF and Polar Front (PF). Further south, stations GT03, GT02 and GT01, were located in the Antarctic Zone (AZ), south of the PF.

### 2.2. Cleaning protocol

All materials used in this study were cleaned following trace metal clean protocols to avoid any contamination (Cutter et al., 2017). GO-FLO bottles were thoroughly acid-cleaned prior to the cruise and kept in PVC plastic wrap until deployment at sea. Similarly, filters (Supor®), filters holders (Swinnex®), tweezers and petri dishes used particle sampling at sea were cleaned according to the GEOTRACES Cookbook. Prior to use, 15 mL flat bottom vials (Savillex®) were filled with 6 mol L⁻¹ HCl (Fisher Scientific—Merck Suprapur grade) and heated at 130 °C for a minimum of 4 h, then rinsed three times with Milli-Q water. Vials were then cleaned with 3 mL of a solution made of 1.45 mol L⁻¹ HF (Fisher Scientific—Merck Suprapur Grade) and 8 mol L⁻¹ HNO₃ (Fisher Scientific—Suprapur Grade) for at least 4 h at 130 °C, then rinsed five times with Milli-Q water and dried under a laminar flow hood class ISO 3 (Planquette and Sherrell, 2012). Polypropylene tubes (Corning®) and cryotubes (Trucool®) were cleaned by filling them with 1.6 mol L⁻¹ HNO₃ (Fisher Scientific—Suprapur Grade), leaving them overnight at 60 °C, and rinsing them five times with Milli-Q water.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Ocean colour maps from multi satellite observations of the surface chlorophyll *a* (Chl *a*; mg·m⁻³ in a log scale) climatological mean in spring 2019 and winter 2019, showing the location of the stations sampled for particulate trace metals (white dots) during the SCALE cruises. The white lines represent the positions of the hydrographic fronts determined by maps of absolute dynamic topography (ADT, m) from the CLS-AVISO product using the boundary definitions from Swart et al. (2010). STF, SubTropical Front; SAF, SubAntarctic Front; PF, Polar Front; SACCF, Southern Antarctic Circumpolar Current Front; SBDY, Southern Boundary.</figcaption></figure>

## 2.3. Sample collection

A Sea-Bird CTD (SBE 911+) was deployed at each station to acquire temperature, salinity, pressure and fluorescence. All these data were post-calibrated, with in-situ measurements and by using the Seabird SBD 9/11 data processing software. Seawater for particulate samples were collected with 24 acid-cleaned GO-FLO bottles of 12 L mounted on epoxy coated aluminum frame carrousel and deployed with a Kevlar™ conducting cable. The trace metal clean rosette was deployed with a 24 depths resolution per station and bottles were closed during the CTD ascent. Immediately after recovery, GO-FLO bottles were covered in a PVC plastic wrap, and were transferred into a class ISO 5 clean containerized laboratory for subsequent sub-sampling.

Particle samples were collected on acid cleaned 25 mm diameter 0.45 µm polyethersulfone filters (Supor®) fitted on a single inline filter holder (Swinnex®). Between 5 and 10 L of seawater were filtered per sample. After the filtration, filters were removed from their filter holder using clean plastic tweezers under a class ISO 4 laminar flow hood. Then, filters were stored in an acid-washed Petri dishes at -20 °C until analysis back in the trace-clean laboratory at LEMAR.

## 2.4. Trace elements analyses (Fe, Al, P, Ba)

### 2.4.1. Digestion procedure

All filters were digested with 3 mL of an acid mix of 8 mol L⁻¹ of HNO₃ and 2.9 mol L⁻¹ of HF (Fisher Scientific—Optima grade concentrated acids) following the protocol outlined in Planquette and Sherrell (2012) for 4 h at 130 °C. To ensure an optimal reflux, filters were placed vertically against the wall of the vial while avoiding submersion in the digestion solution. Afterwards, vial lids were removed and vials were placed on a hot plate at 110 °C until near dryness under an ISO 5 HEPA fume hood. Once the solution was evaporated, the residue was redissolved with 200 µL of concentrated HNO₃ (Fisher Scientific—Optima grade) and evaporated again at 110 °C. Finally, the remaining residue was dissolved in 3 mL of 0.48 mol L⁻¹ HNO₃ (Fisher Scientific—Optima grade) before being archived into acid pre-cleaned 15 mL polypropylene tubes (Corning®) or 5 mL cryotubes (Trucool®) until SF-ICP-MS analysis (Planquette and Sherrell, 2012).

### 2.4.2. ICP-MS analyses

All the analyses were performed using a magnetic sector inductively coupled plasma mass spectrometer (SF-ICP-MS, Element XR - Pôle Spectrométrie Ocean, Brest) using low and medium resolution settings. Gravimetrically prepared in a 3 % (v/v) HNO₃ solution (spiked with 1 ppb of Indium), calibration solutions were analyzed at the beginning, middle and end of each analytical session. A number of 21 process blank

filters, i.e., 1 L of filtered seawater passing through a clean filter, were collected at all stations at several depths. Values for Fe, Al, P and Ba (ng) obtained for these blank filters are presented in Table 1 and were used to correct for blank contribution to the total concentration of the respective particulate element. The few particulate data below the detection limit (Table 1) were flagged accordingly. To assess the reproducibility of our analyses, many samples were measured in duplicate on the day following their first analysis. When the percentage relative standard deviation (%RSD) between duplicate measurements exceeded 10 %, the potential outliers were identified by assessing their consistency with data points immediately above and below them in the dataset. The method's accuracy presents a challenge as no certified reference material (CRM) exists for suspended marine particles. We thus relied on

<caption>**Table 1**</caption>

Process blank measured (ng) per filter and detection limit (nmol L⁻¹) for pFe, pAl, pP and pBa. Blank value represents the median of all process blanks realized and detection limit is defined as 3 times the standard deviation of process blank values assuming that 10 L of seawater were filtered. Iron, aluminum, phosphorus and barium concentrations determined for three CRMs. Concentrations are expressed in µg of element per g of mineralized material (µg·g⁻¹). Indicative (a), measured (b) and certified (c) values are indicated in italic below the determined value in our study. Mean recoveries are indicated in bold below the certified values. Note that no certified, indicative or measured values for Ba in PACS-3. Measured values can be found in GEOREM database ("GeoReM - Query by reference samples or materials (published values) (2024)").

<table><thead><tr><th></th><th>pFe<br>(n = 21)</th><th>pAl<br>(n = 21)</th><th>pP<br>(n = 21)</th><th>pBa<br>(n = 21)</th></tr></thead><tbody><tr><td>Median value ± sd<br>(ng per filter)</td><td>14.4 ± 7.2</td><td>31.8 ± 13.9</td><td>62 ± 65.9</td><td>1.64 ± 1</td></tr><tr><td>Detection limit<br>(nmol L<sup>-1</sup>)</td><td>0.038</td><td>0.15</td><td>0.64</td><td>0.002</td></tr><tr><td>Reference material</td><td>pFe<br>(n = 4)</td><td>pAl<br>(n = 4)</td><td>pP<br>(n = 4)</td><td>pBa<br>(n = 4)</td></tr><tr><td>BCR-414</td><td>1709 ± 100<br>(1850 ±<br>19)<sup>a</sup></td><td>13,650 ±<br>2094<br>(13,386 ±<br>97 ± 7 %</td><td>2408 ± 140<br>(2406 ±<br>3207)<sup>b</sup></td><td>31 ± 1<br>(31 ±<br>102 ± 15 %</td></tr><tr><td>MESS-4</td><td>40,100 ±<br>6375<br>(37,900 ±<br>1600)<sup>c</sup></td><td>866 ± 64<br>(1040 ±<br>90 ± 24 %</td><td>56,199 ±<br>9887<br>(79,100 ±<br>2000)<sup>c</sup></td><td>708 ±<br>74<br>(920)<sup>a</sup><br>98 ± 23<br>%</td></tr><tr><td>PACS-3</td><td>39,920 ±<br>5580<br>(41,060 ±<br>640)<sup>c</sup></td><td>733 ± 61<br>(937 ± 44)<sup>c</sup></td><td>44,236 ±<br>7098<br>(65,800 ±<br>96 ± 10 %</td><td>698 ±<br>39<br>(920)<sup>a</sup><br>91 ± 20 %</td></tr></tbody></table>
CRMs of freeze-dried plankton (BCR-414, European Commission), and two marine sediments: MESS-4 (Canada, G. of C.N.R.C, 2024a) and PACS-3 (Canada, G. of C.N.R.C, 2024b). These three CRMs underwent the same acid digestion protocol as the suspended marine particle samples, for a mass of CRM digested comprised between 15 and 30 mg. Concentrations obtained for these three used CRMs are reported in Table 1 and fell within the intervals provided by the analysis certificates. Recoveries for Fe were on average 97 % (BCR-414), 105 % (MESS-4) and 96 % (PACS-3), while P recoveries were on average 102 % (BCR-414), 90 % (MESS-4) and 94 % (PACS-3) and Ba recoveries were on average 100 % (BCR-414) and 98 % (MESS-4). Regarding Al, recoveries were a little bit lower than the certified values for MESS-4 and PACS-3. This could be explained by, firstly the weight of CRM used (between 15 and 30 mg) that was below the recommended amount (minimum of 250 mg) (NRC Canada), and by the low digestion solution volume added (6 mL). Here we only report Fe, Al, P and Ba concentrations (pFe, pAl, pP and pBa).

## 2.5. Estimation of pFe contributions

Elemental ratios were used to estimate the different contributions to total particulate iron. For each sample, the lithogenic (pFe$_{litho}$), biogenic (pFe$_{bio}$) and authigenic (pFe$_{authi}$) fractions were determined as described hereafter.

The lithogenic (crustal) contribution to total pFe can be estimated from measured pAl and the upper crustal Fe/Al ratio of 0.21 mol mol$^{-1}$ (Taylor and McLennan, 1985), assuming that all Al is of lithogenic origin:

$$pFe_{litho} = pAl \times 0.21 \left( \frac{Fe}{Al} \right) \text{ UCC} \quad (1)$$

This method for estimating lithogenic input is well established and has been applied in previous studies (Gourain et al., 2019; Lam et al., 2015; Milne et al., 2017; Planquette et al., 2013; Sun et al., 2016). The non-lithogenic iron that contributes to the total pFe is called excess particulate iron (pFe$_{excess}$). This iron pool may include iron associated to biogenic particles (i.e plankton) and authigenic iron.

$$pFe_{excess} = pFe_{bio} + pFe_{authi} \quad (2)$$

Biogenic pFe can be estimated from measured pP and Fe/P ratio determined in plankton cells (Twining and Baines, 2013).

$$pFe_{bio} = pP \times \left( \frac{Fe}{p} \right) \text{ phytoplankton} \quad (3)$$

The different Fe/P ratios determined in plankton cells reported in the literature exhibit large variations depending on the oceanic basins, plankton species, iron conditions and the methodologies employed (Ho et al., 2003; Rauschenberg and Twining, 2015; Strzepek et al., 2011; Tovar-Sanchez et al., 2003; Twining et al., 2004, 2014, 2015; Twining and Baines, 2013).

Along all the transect except in the STZ, the phytoplanktonic biomass in surface was dominated by nanoplankton such as diatoms and haptophytes (Flynn et al., 2023, T. Ryan-Keogh, pers.com.) during both seasons. This composition is similar to that observed in the SOFeX studies investigating the impact of artificial iron fertilization in the Southern Ocean (Twining et al., 2004), where pFe/pP ratios were determined outside and inside the fertilized patches for diatoms, autotrophic and heterotrophic flagellates. Accordingly, we applied the mean pFe/pP ratios of 0.51 to 1.66 mmol mol$^{-1}$ corresponding to Fe-limited waters outside and within the southern Fe-fertilized patch, respectively (Twining et al., 2004). This approach provides a range of pFe$_{bio}$ values in the SAZ, PFZ and AZ surface waters along our transect. In the STZ, prochlorophytes were the dominant phytoplankton group along with diatoms and haptophytes during both seasons (T. Ryan-Keogh, pers.com.), as generally observed in this zone (Fouilland et al., 1999).

Therefore, we applied a ratio of 1.41 ± 0.13 mmol mol$^{-1}$ which was determined in picoplankton cells from non-limiting iron conditions (Twining and Baines, 2013), to estimate pFe$_{bio}$ concentrations in the STZ. This ratio is similar to the ratio for diatoms and flagellates in non-iron limiting environments (1.66 mmol mol$^{-1}$). At all stations, we excluded samples where the phosphorus concentrations were lower than 10 % of its maximum value (GT03 winter, 200 m and GT09 winter 150 m) to keep only samples with enough biological matter to estimate pFe$_{bio}$.

As the authigenic fraction is calculated by subtracting the lithogenic and biogenic fractions to the total pFe, it is influenced by the ratios used to estimate these two fractions. The crustal ratio used with the aluminum proxy is considered robust. However, the Fe/P ratio from phytoplanktonic cells is expected to vary due to several factors, including water depth. In intermediate waters, it is preferable to discuss the “excess fraction” rather than the “authigenic fraction”. The limitations and uncertainties regarding the Fe/P ratio are further discussed in Section 3.5. For the presentation and discussion of these fractions, the mean ± standard deviation was calculated for each water mass at each station.

## 2.6. Ba$_{xs}$ estimations

Barium excess (Ba$_{xs}$) is used as a remineralization proxy, corresponding to barite crystals formed during the microbial degradation of organic matter (Cardinal et al., 2005; Lemaitre et al., 2018; Planchon et al., 2013). It is calculated by subtracting the lithogenic fraction of particulate barium (pBa) to total pBa. To estimate the lithogenic contribution to pBa, we used a similar approach as for iron. Particulate Al concentration was multiplied by the Ba/Al molar ratio in the UCC (0.00135, Taylor and McLennan, 1985) assuming that all pAl is of lithogenic origin:

$$Ba_{xs} = pBa - pAl \times 0.00135 \left( \frac{Ba}{Al} \right) \text{ UCC} \quad (4)$$

## 2.7. Statistical analyses and figures

Statistical analyses were performed using the software Rstudio version 4.3 (R: The R Project for Statistical Computing, 2023). Seasonal differences in element concentrations within the different water masses were evaluated using the Wilcoxon-Mann-Whitney test with a 95 % confidence interval considering the low number of samples per group (Bauer, 1972). Figures and plots were prepared using either Ocean Data View (Schlitzer, Reiner, Ocean Data View) or ggplot2 package available for Rstudio software.

# 3. Results and discussion

## 3.1. Hydrography

The transect crossed several boundaries, delimiting distinct biogeochemical and physical systems (Fig. 1). Water masses encountered during both seasons were identified using potential temperature, practical salinity and oxygen concentrations (Anilkumar et al., 2006; Chever et al., 2010; Cloete et al., 2021; Orsi et al., 1995) (Fig. 2). For both seasons, station GT09 was located in the subtropical zone (STZ), where surface water masses were characterized by the presence of SubTropical Surface Waters (STSW; θ > 12 °C, S > 35) (Fig. 2, Fig. S1). Moving southward, the cooling process and the drop in salinity indicated the transition towards the SubAntarctic Surface Waters (SASW; θ: 5–12 °C, S < 34.5) at stations GT07, GT06 and GT05. Further south, Antarctic Surface Waters (AASW; θ < 5 °C, S < 34.5) dominated the surface of the three stations located south of the Polar Front: GT03, GT02 and GT01. Below the surface, Antarctic Intermediate Waters (AAIW) were identified in the subtropical zone for both seasons between 500 and 1500 m of
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Vertical distribution of practical salinity (PSU) along SCALE transects during spring (A) and winter (B). Front positions are indicated in grey font: SACCF, Southern Antarctic Circumpolar Current Front; PF, Polar Front; SAF, Sub Antarctic Front; STF, Sub Tropical Front. The different water masses encountered are indicated in black font: AABW, Antarctic Bottom Water; AAIW, Antarctic Intermediate Water; AASW, Antarctic Surface Water; LCDW, Low Circumpolar Deep Water; NADW, North Atlantic Deep Water; STSW, Subtropical Surface Water; UCDW, Upper Circumpolar Deep Water.</figcaption></figure>

depth. AAIW were also present in the subantarctic zone (SAZ), albeit shallower than in the STZ (ranging from 300 m to 1200 m), characterized by a minimum in salinity (~34.2) and a northward inclination. Upper and Lower Circumpolar Deep Waters (UCDW and LCDW) surrounded the North Atlantic Deep Waters (NADW) with a salinity maximum (n > 34.7). The depths of both LCDW and UCDW were shallower southwards (Fig. 2). During winter, Antarctic Bottom Waters (AABW) were sampled at each station of the transect. However, during spring, AABW were only detected at stations GT02 and GT01, as the CTD did not go deeper than 2000 m. During spring, the shallowest mixed layers were observed at stations GT07, GT09 and GT05 with depths of 19, 24 and 31 m respectively. Station GT02 showed the deepest mixed layer during spring (112 m). During winter, MLDs ranged from 70 m (GT01) to 187 m (GT07). Stations located within the subantarctic and polar front zones (SAZ and PFZ) demonstrated significant vertical mixing with MLDs 4 to 10 times deeper during winter compared to spring. Conversely, stations located south of the PF displayed minor variations in MLD between our seasonal sampling. For instance, at GT03, the MLD was only 6 m deeper during our sampling in winter than during spring (Table S1). In the following sections, surface waters refer to AASW, SASW, and STSW, while intermediate waters refer to AAIW and CDW. These water masses contained a sufficient number of samples in each season to enable robust comparisons (with a minimum number of 3 samples for each season) (Table S1).

## 3.2. Total particulate iron (pFe) distribution

### 3.2.1. Spring

During spring, total pFe concentrations determined among all samples ranged from 0.04 (GT09, 75 m) to 0.99 nM (GT07, 25 m). The lowest concentrations were observed in surface waters, with values increasing until a depth of 500 m, decreasing between 500 m and 1000 m, and increasing again below 1000 m depth (Fig. 3A, Fig. S2). This vertical pattern of pFe distribution is consistent with other profiles observed during spring and summer along the same transect (Group, G.I. D.P., 2023) (GIPY04 F.Lacan et C.Pradoux), and in other sectors of the Southern Ocean (Barrett et al., 2018; Planquette et al., 2013; van der Merwe et al., 2019; van Manen et al., 2022; Bowie et al., 2015). SASW (i.e., the mean ± standard deviation of the n samples within this water mass) exhibited the highest pFe content, with concentrations of 0.43 ± 0.33 nM (n = 7) and 0.23 ± 0.11 nM (n = 4) at stations GT07 and GT06, respectively. The lowest pFe surface concentrations were measured in the Antarctic zone at stations GT03 (0.15 ± 0.03 nM; n = 4) and GT02 (0.16 ± 0.08 nM; n = 5). Values obtained during spring in the upper 500 m of the water column fall within the same concentration range (< 0.5 nM) as data collected during the same period along the same transect (Table 2). Our values are also comparable to those measured at other open ocean stations in other sectors of the Southern Ocean (Table 2) and they are up to 10 times lower in surface waters compared to those reported in previous coastal studies largely influenced by coastal and sediment inputs (Table 2). In the intermediate waters between 500 and 2000 m, pFe concentrations ranged from 0.32 ± 0.15 nM (n = 3, GT05) to 0.53 ± 0.27 nM (n = 4, GT07) in AAIW, and from 0.28 ± 0.08 nM (n = 5).
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Vertical distribution of pFe concentrations in nM during spring (A) and winter (B) along the SCALE transect down to 2000 m. Bathymetry is represented in black background. Front positions are indicated in grey font.</figcaption></figure>

<table><caption>**Table 2**
Existing values of pFe concentrations measured in other sectors of the Southern Ocean.</caption><thead><tr><th>Area</th><th>Depth range (m)</th><th>pFe values (nM)</th><th>Reference</th></tr></thead><tbody><tr><td>BONUS-GoodHope</td><td>0–500</td><td>&lt; 0.5</td><td>Group, G.I.D.P, 2023</td></tr><tr><td>Heard Island (ISPO2)</td><td>0–100</td><td>&lt; 0.5</td><td>van der Merwe et al., 2019</td></tr><tr><td>Indian Sector (between STF and SACCF)</td><td>0–300</td><td>&lt; 0.5</td><td>Barrett et al., 2018</td></tr><tr><td>Indian Sector (between STF and SACCF)</td><td>200–1000</td><td>0.1–0.4</td><td>Barrett et al., 2018</td></tr><tr><td>Indian Sector (South of SACCF)</td><td>0–500</td><td>&gt; 1</td><td>Barrett et al., 2018</td></tr><tr><td>Amundsen Sea (shelf waters)</td><td>0–500</td><td>&gt; 0.5</td><td>Planquette et al., 2013</td></tr><tr><td>Amundsen Sea (shelf waters)</td><td>0–500</td><td>&gt; 1</td><td>van Manen et al., 2022</td></tr><tr><td>Heard Island (plateau)</td><td>0–150</td><td>&gt; 10</td><td>van der Merwe et al., 2019</td></tr><tr><td>Kerguelen plateau (A3)</td><td>0–300</td><td>&lt; 10</td><td>Bowie et al., 2015</td></tr></tbody></table>

= 7, GT02) to 0.52 ± 0.20 nM (n = 4, GT01) in circumpolar deep waters (UCDW and LCDW).

### 3.2.2. Winter

During winter, total particulate iron concentrations ranged from 0.04 nM (GT02, 100 m) to 1.17 nM (GT09, 1750 m). Generally, concentrations of particulate iron were low in surface waters and increased with depth (Fig. 3, Fig. S2). In surface waters, pFe concentrations

remained consistently below 0.6 nM, except for one value of 0.75 nM measured at 200 m at GT05. The maximum surface concentrations of pFe were measured in the PFZ at GT05 (0.52 nM ± 0.16 nM; n = 5), while the lowest surface concentrations were found in the AZ at GT02 (0.22 ± 0.17 nM; n = 4). In circumpolar deep waters (UCDW and LCDW), the pFe concentration varies from 0.29 ± 0.19 nM (n = 7) at GT03 to 0.50 ± 0.15 nM (n = 3) at GT02. Antarctic intermediate waters (AAIW) showed different pFe concentrations depending on the oceanic region considered. For example, in the PFZ, pFe concentrations were 0.52 ± 0.13 nM (n = 8) and 0.64 ± 0.23 nM (n = 6) at stations GT05 and GT06 respectively. In contrast, lower concentrations were observed in the STZ at GT09 (0.38 ± 0.19 nM, n = 6) and in the SAZ at GT07 (0.42 ± 0.19 nM, n = 6). Our winter pFe data represent the first reported measurements of pFe in the Southern Ocean in winter, which precludes direct comparison with existing literature.

### 3.2.3. Seasonal variations

We compared the total pFe concentrations between the two seasons (Fig. 4). There was no significant difference between spring and winter when considering the entire dataset or the whole water column at each station, suggesting that seasonality did not systematically affect the overall pFe distribution between our two cruises. This is not unexpected as many different biogeochemical processes occur at different depths within the water column (e.g., Fe uptake, lithogenic inputs in surface waters and remineralization in intermediate waters). When comparing spring values to winter values within surface waters or within intermediate waters, however, significant differences (*p*-value < 0.05) were observed at some stations (Table S1, Fig. 4A & B). In all cases, these significant differences revealed higher total pFe concentrations in winter
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Mean total pFe concentrations (nM) at each station of the transect in surface waters (A) and intermediate waters (B) (based on water mass classification). Error bars correspond to the standard deviation associated to the mean. Black stars indicate a significant difference (Wilcoxon Mann Whitney test p-value <0.05) between winter (blue) and spring (green). Note that the vertical scale varies depending on the different water masses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

than in spring. Despite the anticipated decrease in phytoplanktonic dFe uptake during winter, surface pFe concentrations in winter were 1.7-, 1.6- and 2.3-fold higher than in spring at GT09, GT05 and GT03, respectively (Fig. 4A). Additionally, intermediate waters showed increased total pFe concentrations in winter, which contradicts the assumptions that remineralization is the dominant process driving the intermediate pFe concentrations and/or that it has been fully completed by the beginning of this season. Indeed, in AAIW, winter pFe concentrations increased by almost 1.6-fold at GT05 and GT06. South of the PF, CDW at GT02 showed a 1.8-fold higher winter pFe concentration. Since these differences were not systematic and did not align with our conceptual hypotheses, it appears that iron uptake and remineralization are not the primary drivers influencing total pFe distributions in our studies. To further investigate the factors influencing the observed seasonal differences, we separately considered the three pFe fractions: biogenic, lithogenic, and authigenic. We first examined the impact of seasonality on the biogenic fraction in surface waters, then we assessed the lithogenic and authigenic fractions in both surface and intermediate waters.

### 3.3. The biogenic fraction in surface waters

Phytoplankton cells consume dissolved iron as it is an essential micronutrient for their metabolism. Upon incorporation into biomass, this iron becomes part of the particulate iron pool (Boyd et al., 2017). During spring, elevated uptake of iron is expected to happen as phytoplankton blooms in the SAZ and PFZ (Arteaga et al., 2020), with abrupt transition between autumn and spring bloom initiations along the SCALE transect (Thomalla et al., 2011). Thus, we expected higher pFe<sub>bio</sub> during spring compared to winter. Fig. 5A shows that this is the case at all stations, with significant differences for stations GT06, GT05 and GT02 (p-value < 0.05). Using the biogenic fraction as a percentage, the differences are significant for stations GT09, GT06, GT05, GT03 and GT02. This result is expected as winter light conditions limit phytoplankton growth (Arteaga et al., 2020; Deppeler and Davidson, 2017). However, biogenic particulate iron represents only a minor fraction of total pFe (Table 3). During winter, this contribution ranges from 0.9% to 10.2% and the maximum contribution of 33% (AASW at GT03) was determined during spring in surface waters. The lowest biogenic contribution in spring was about 5.6% in the PFZ at GT05 where surface mean chlorophyll *a* (Chl *a*) and P concentrations are approximately 0.1 µg L<sup>-1</sup> and 16.3 ± 4.5 nM (Fig. S3), respectively. As sampling took place during early spring, it is plausible that biogenic contributions might have increased later in the season, with light levels increasing, and assuming sufficient dFe supply (Tagliabue et al., 2014). For instance, in the PFZ, other studies have reported that phytoplankton blooms peaked in January (Arteaga et al., 2020; Thomalla et al., 2023).

To account for more than 50% of total pFe, and assuming a constant Chl *a*/pP ratio, Chl *a* concentrations would have to exceed 0.5 µg L<sup>-1</sup>. However, neither measurements along this transect (Fig. S4), even during summer, nor the regional climatological mean Chl *a* from 1998 to 2022 (Thomalla et al., 2023) reach this Chl *a* threshold. Therefore, biological activity has played a minor role in the seasonal distribution of total pFe during our study. Contributions from the other pFe fractions and external sources have to be considered to explain the higher winter total pFe concentrations compared to spring.

### 3.4. The lithogenic fraction: external sources involved in pFe seasonality

In surface waters, lithogenic particulate iron (pFe<sub>litho</sub>) concentrations decreased from north to south in both seasons (Fig. 6A). Across the entire transect, pFe<sub>litho</sub> concentrations were higher in surface waters during winter than in spring. Additionally, significant differences (p-value < 0.05) were observed at stations GT09, GT05, and GT03 (Fig. 6A), consistent with the patterns of total pFe. This suggests that the lithogenic fraction is likely a key driver of total pFe seasonality at these stations. In intermediate waters, the seasonal trend is generally less pronounced, except in the PFZ. Here, pFe<sub>litho</sub> in AAIW at GT05 and GT06 was higher in winter than in spring, contributing to the elevated winter pFe concentrations in AAIW at GT05 (Fig. 4B). The following subsections will detail separately the different processes likely involved in the seasonal differences of pFe<sub>litho</sub> and total pFe along the transect.

#### 3.4.1. Atmospheric deposition and iceberg melting

The Southern ocean, among all oceanic basins, has the lowest atmospheric input of continental dust due to its considerable distance from land masses (Duce and Tindale, 1991; Jickells et al., 2005). Nevertheless, episodic aeolian deposition can be a significant source of iron in surface waters, enhancing primary production and carbon export (Boyd and Ellwood, 2010; Cassar et al., 2007). These iron enrichments from dust can be of natural origin or result from anthropogenic perturbation (Weis et al., 2022). To determine the potential origin of air masses during SCALE over the whole transect, we used the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (http://www.arl.noaa.gov/ready.html) which was run for all stations at heights of 10, 500, and 1000 m above the geoid level over a 5-
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Mean biogenic pFe concentrations (nM) at each station of the transect in surface waters. Squares and dashed-line correspond to the upper estimation of biogenic pFe, while dots and solid line correspond to the lower estimation. Filled area correspond to the range between the lower and upper estimations. Error bars correspond to the standard deviation associated to the mean. Black stars indicate a significant difference (Wilcoxon Mann Whitney test p-value <0.05) between winter (blue) and spring (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

day period (Fig. S5).

At Stations GT03 and GT09 when surface pFe<sub>litho</sub> was higher in winter than in spring, air mass backward trajectories show that an air mass, passing through Patagonia, was present during winter. Even if the lithogenic pFe fraction is not dominant in AASW at GT03 (Table 3), aeolian supply may still have contributed to the higher surface pFe<sub>litho</sub> and total pFe concentrations observed at this station during winter compared to spring. During both SCALE cruises, Kangueehi (2021) investigated dust compositions and also indicated aeolian inputs, likely from Patagonia, at station GT09 in winter. Their analyses of collected dust particles revealed higher iron concentrations during winter compared to spring (Kangueehi, 2021). Dust deposition is known to contribute to the dFe pool in oceanic surface waters, however an important fraction remains as pFe (Jickells et al., 2005). Given the residence time of pFe in surface waters, estimated to range between 10 and 100 days (Black et al., 2020) and the recent deposition of dust at GT09 (occurring only 10 days before the water column sampling), it is likely that these particles contributed to the pFe<sub>litho</sub> pool. Therefore, seasonal variations in pFe<sub>litho</sub> (as shown in Fig. 6A) within STSW at GT09 may be attributed to dust deposition at the ocean surface. Given the high proportion of pFe<sub>litho</sub> in surface waters at this station during both seasons (Table 3), this seasonal pattern is also reflected in total pFe concentrations (Fig. 4A).

While dust deposition was detected at two stations during winter, a consistent seasonal trend in dust inputs is not expected. Although such events can occur unpredictably throughout the year, has been shown that emissions of Patagonian dust peak during spring and summer days (Cosentino et al., 2021). However, despite this seasonality in dust emissions, the atmospheric transport itself exhibits limited seasonality (Neff and Bertler, 2015). These dust plumes are transported predominantly southward and eastward, reaching the region of our transect throughout the year (Neff and Bertler, 2015). Our dataset, showing a detectable signal only at specific locations during winter, underscores the complex and localized nature of dust deposition in the Southern Ocean.

Besides aeolian input, surface waters south of the PF may be influenced by drifting icebergs (Deppeler and Davidson, 2017) that can contain nanoparticles of lithogenic origin (Lin et al., 2011; Raiswell, 2011; Raiswell et al., 2008). During their melting, this potentially bioavailable pFe (Raiswell, 2011; Raiswell et al., 2008) can be released through dissolution processes at the ocean's surface, fueling phytoplankton blooms. This release process was shown for both sea-ice and glacier melting around Antarctica (Alderkamp et al., 2019; Vernet et al., 2011). While icebergs can be present in the region between the marginal sea ice zone and the polar front (Deppeler and Davidson, 2017), stations GT01, GT02 and GT03 are not in close proximity to the 15 % of ice cover during winter, and they are even further away during spring (Scale Cruise Report; https://zenodo.org/records/5906324). Therefore, it is more plausible that the higher pFe and pFe<sub>litho</sub> concentrations in AASW of GT03 during winter are primarily attributable to atmospheric deposition rather than to iceberg release.

### 3.4.2. Inputs from the Antarctic Circumpolar Current (ACC)

Station GT05 located in the PFZ displayed an intriguing seasonal pattern (Fig. 3, Fig. 4). In both surface and intermediate waters, pFe and pFe<sub>litho</sub> concentrations are significantly higher (p-value < 0.05) during winter than during spring (Fig. 4, Fig. 6). Interestingly, the composition of iron in the particles remained consistent across seasons, with the lithogenic fraction accounting for approximately 15.4 ± 4.9 % in SASW during spring and 17.2 ± 10.1 % during winter. Thus, the seasonal differences in pFe concentrations cannot be solely attributed to an influx of lithogenic particles, as might occur through dust deposition. Instead, it suggests that during winter, a more diverse array of particles (authigenic, biogenic and lithogenic) may be present throughout the water column, rather than a dominant lithogenic input at the surface. Several hypotheses could explain this winter increase in total pFe without significant changes in the relative contributions of different iron fractions. Previous studies have indicated that deep vertical mixing during winter may supply iron to surface and intermediate waters in the Southern Ocean (Mtshali et al., 2019; Tagliabue et al., 2014). In the PFZ
<caption>**Table 3**</caption>

Percentage contributions of the lithogenic and excess pFe (mean ± sd). The range of percentage for the biogenic contribution are only indicated for surface waters in italic below the excess pFe percentage. These values were determined for each water mass at each station for both seasons. Specific water masses where no values were available are indicated by NA. Number of samples per water mass is indicated by n.

<table><thead><tr><th rowspan="2">Station</th><th rowspan="2">Water mass</th><th colspan="2">Lithogenic pFe (%)</th><th colspan="2">Excess pFe (Biogenic + Authigenic) (%)</th></tr><tr><th>Spring</th><th>Winter</th><th>Spring</th><th>Winter</th></tr></thead><tbody><tr><td rowspan="3">GT01</td><td>AASW</td><td>15.1 ± 7.3 (n = 5)</td><td>9.6 ± 8.7 (n = 3)</td><td>84.9 ± 7.3 (n = 5)<br>6.7-19.8 (n = 5)</td><td>90.4 ± 8.7 (n = 3)<br>1.8-5.4 (n = 3)</td></tr><tr><td>CDW</td><td>39.3 ± 6.7 (n = 5)</td><td>37.7 ± 18.3 (n = 9)</td><td>60.7 ± 6.7 (n = 5)</td><td>62.3 ± 18.3 (n = 9)</td></tr><tr><td>AABW</td><td>44.5 (n = 1)</td><td>NA</td><td>55.5 (n = 1)</td><td>NA</td></tr><tr><td rowspan="3">GT02</td><td>AASW</td><td>27.8 ± 40.9 (n = 5)</td><td>32.6 ± 46.7 (n = 4)</td><td>72.2 ± 40.9 (n = 5)<br>11-32.8 (n = 5)</td><td>67.4 ± 46.7 (n = 4)<br>3.2-9.6 (n = 4)</td></tr><tr><td>CDW</td><td>23 ± 4.7 (n = 7)</td><td>19.7 ± 7.2 (n = 10)</td><td>77 ± 4.7 (n = 7)</td><td>80.3 ± 7.2 (n = 10)</td></tr><tr><td>AABW</td><td>33.2 (n = 1)</td><td>23.5 ± 3 (n = 2)</td><td>66.8 (n = 1)</td><td>76.6 ± 3 (n = 2)</td></tr><tr><td rowspan="3">GT03</td><td>AASW</td><td>18.1 ± 5.3 (n = 4)</td><td>41.6 ± 19 (n = 4)</td><td>81.9 ± 5.3 (n = 4)<br>11.1-33 (n = 4)</td><td>58.4 ± 19 (n = 4)<br>2.0-6.0 (n = 3)</td></tr><tr><td>CDW</td><td>39.5 ± 10 (n = 10)</td><td>19.7 ± 7.2 (n = 10)</td><td>60.5 ± 10 (n = 10)</td><td>80.3 ± 7.2 (n = 10)</td></tr><tr><td>SASW</td><td>15.4 ± 4.9 (n = 6)</td><td>17.2 ± 10.1 (n = 5)</td><td>84.5 ± 4.9 (n = 6)<br>5.6-16.5 (n = 6)</td><td>82.8 ± 10.1 (n = 5)<br>0.9-2.6 (n = 5)</td></tr><tr><td rowspan="3">GT05</td><td>AAIW</td><td>37 ± 4.4 (n = 3)</td><td>45.8 ± 7.1 (n = 9)</td><td>63 ± 4.4 (n = 3)</td><td>54.2 ± 7.1 (n = 9)</td></tr><tr><td>CDW</td><td>45.8 ± 3 (n = 2)</td><td>NA</td><td>54.2 ± 3 (n = 2)</td><td>NA</td></tr><tr><td>SASW</td><td>23.3 ± 12 (n = 4)</td><td>28.9 ± 5 (n = 3)</td><td>76.7 ± 12 (n = 4)<br>11-32.5 (n = 4)</td><td>71.1 ± 5 (n = 3)<br>1.6-4.9 (n = 3)</td></tr><tr><td rowspan="3">GT06</td><td>AAIW</td><td>43.4 ± 10.7 (n = 9)</td><td>50.4 ± 9.4 (n = 6)</td><td>56.6 ± 10.7 (n = 9)</td><td>49.6 ± 9.4 (n = 6)</td></tr><tr><td>NADW</td><td>50.3 ± 0.7 (n = 2)</td><td>NA</td><td>49.7 ± 0.7 (n = 2)</td><td>NA</td></tr><tr><td>SASW</td><td>37.1 ± 20.8 (n = 7)</td><td>54.8 ± 27.6 (n = 6)</td><td>62.9 ± 20.8 (n = 7)<br>6.3-18.6 (n = 7)</td><td>45.2 ± 27.6 (n = 6)<br>3.4-10.2 (n = 6)</td></tr><tr><td rowspan="3">GT07</td><td>AAIW</td><td>37.2 ± 20.3 (n = 6)</td><td>48 ± 27.9 (n = 8)</td><td>62.8 ± 20.3 (n = 6)</td><td>52 ± 27.9 (n = 8)</td></tr><tr><td>NADW</td><td>74.7 ± 28.9 (n = 2)</td><td>91 (n = 1)</td><td>25.3 ± 28.9 (n = 2)</td><td>9 (n = 1)</td></tr><tr><td>STSW</td><td>73.4 ± 27 (n = 7)</td><td>81.1 ± 25.2 (n = 7)</td><td>26.6 ± 27 (n = 7)<br>18.8-22.7 (n = 7)</td><td>18.9 ± 25.2 (n = 7)<br>5.5-6.6 (n = 6)</td></tr><tr><td rowspan="3">GT09</td><td>AAIW</td><td>90.4 ± 12.5 (n = 6)</td><td>96.8 ± 4.4 (n = 5)</td><td>9.6 ± 12.5 (n = 6)</td><td>3.2 ± 4.4 (n = 5)</td></tr><tr><td>NADW</td><td>100 ± 0 (n = 2)</td><td>17 (n = 1)</td><td>0 ± 0 (n = 2)</td><td>83 (n = 1)</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Mean lithogenic pFe concentrations (nM) at each station of the transect in surface waters (A) and intermediate waters (B). Error bars correspond to the standard deviation associated to the mean. Black stars indicate a significant difference (Wilcoxon Mann Whitney test p-value <0.05) between winter (blue) and spring (green). Note that the vertical scale varies depending on the different water masses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

at GT05 the spring MLD is about 24 m compared to 114 m during winter (Table S1). This winter depth falls within the range of previous MLD measurements in this region of the Southern Ocean (Sallée et al., 2010), suggesting that at the time of sampling, vertical mixing alone is unlikely to fully account for the elevated total pFe concentrations in winter. However, it is possible that intense vertical mixing may have occurred shortly before sampling due to a storm event (Carranza et al., 2018). Besides, this region of the Southern Ocean is known to be influenced by Antarctic Circumpolar Current and its associated fronts (Chapman et al., 2020). Hatta et al. (2013) and Measures et al. (2013) demonstrated that large amounts of iron (both dissolved and particulate), originating from enriched coastal waters of the Western Antarctic Peninsula (WAP), can be transported over long distances through the ACC flux which intensifies during winter (Hatta et al., 2013; Measures et al., 2013). Assuming a mean ACC speed current in the Scotia Sea of 0.8 Sv, a section of 1 km × 1 km and an approximative distance of 4150 km between the WAP and GT05, it would take around 60 days for pFe to be transported from the WAP to GT05. Although this estimate does not account for mixing, remineralization, dissolution, or aggregation processes, the timeframe aligns with the residence time of pFe in the upper dynamic
ocean, which ranges between 10 and 100 days (Black et al., 2020). This provides a plausible explanation for the observed increase in both total and lithogenic pFe concentrations during winter.

### 3.5. High prevalence of authigenic particulate iron in surface and intermediate waters

Among the different fractions contributing to the total particulate iron pool, the authigenic fraction is the most dominant throughout the transect at both seasons (Table 3). Except in the STZ, in surface waters pFeauthi (pFeexcess - pFebio) consistently accounts for more than one-third of the total pFe. This significant pFeauthi has also been observed in other parts of the Southern Ocean, such as the Ross Sea (Tagliabue et al., 2023). Authigenic iron, formed in-situ, can originate from the oxidation of dissolved iron (if not stabilized by ligands; Gledhill and Buck, 2012) into oxyhydroxides precipitates in oxygenated waters (Lam et al., 2015). It can also be derived from dissolved iron scavenging onto particle surfaces, whether lithogenic or biogenic (Boyd and Ellwood, 2010). The

mean pFeauthi values were generally higher in winter compared to spring in the PFZ and AZ, with a significant difference at GT05 (Fig. 7A). Given the very low concentration of biogenic pFe at GT05 (less than 0.02 nM in both spring and winter), it is likely that the authigenic value of 0.4 nM is primarily driven by interactions with the lithogenic fraction, involving processes such as dissolution, oxyhydroxide formation, and adsorption (Boyd and Ellwood, 2010; Tagliabue et al., 2023).

In intermediate waters, aside from lithogenic and authigenic particles, sinking particles can also be present at these depths being partly remineralized or on their way to the deep ocean. However, Fe and P have different remineralization timescales, with P being remineralized faster than Fe (Twining et al., 2014), which implies increasing particulate Fe/P ratios with depth. This raises the uncertainties linked to the estimation of the biogenic fraction using phytoplanktonic Fe/P ratios at these intermediate depths, and consequently to the estimation of the authigenic fraction. However, due to the extremely low particulate P concentrations in intermediate waters (consistently <6 nM, Fig. S3), even applying an Fe/P ratio of 6 mmol mol⁻¹ for remineralizing particles (Twining et al., 2014) would result in a contribution of no more than 21.90 % of total pFe (GT05 Spring, 150 m). Overall, this contribution was on average of 3.83 % (0.32–21.90) and thus, the excess iron fraction can reasonably be mainly attributed to the authigenic fraction in intermediate waters. This authigenic-like fraction contributes between 50 % and 80 % (Table 3), except at GT09 (AAIW) and GT07 (NADW), where it remains below 29 % at both seasons (Table 3). There is no consistent trend in mean intermediate total pFe values, nor a significant seasonal difference in pFeauthi (Fig. 7B), except at station GT02 in CDW. There, authigenic pFe is the only fraction significantly higher in winter compared to spring. This increase of ~0.2 nM accounts for the significant difference in total pFe at this station (Fig. 4B).

During spring in intermediate waters, the remineralization of the exported production from the surface occurs (Boyd et al., 2017). Peaks of Baxs in the mesopelagic zone during both seasons tend to return to a background level of 180 pmol L⁻¹ at depths greater than 1000 m, which is a typical value of the deep ocean (Dehairs et al., 1987). These peaks of Baxs in the mesopelagic zone during both seasons indicate that the remineralization process already occurs during early spring and is still ongoing during mid-winter (Fig. S6), which is in line with the only existing study in winter in the Indian sector of the Southern Ocean (van Horsten et al., 2022). This process contributes to the reduction of pFebio concentrations as phytoplankton cells degrade. If the concentration of dFe released into the water column exceeds that of Fe-complexing organic ligands, it may scavenge onto particle surfaces or precipitate as iron oxyhydroxides, thereby increasing authigenic iron concentrations (Boyd and Ellwood, 2010). However, given the low concentrations of biogenic pFe in surface waters, even in spring, and that remineralization is ongoing in winter and spring (as suggested by Baxs profiles), it is unlikely that the elevated winter pFeauthi concentrations result solely from the remineralization of biogenic pFe. Instead, it is more likely that these high pFeauthi concentrations are linked to the dissolution of lithogenic pFe during both seasons.

At station GT02, however, there is no corresponding increase in pFe litho (Fig. 6B) that could explain the rise in authigenic pFe (Fig. 7B). A plausible explanation could be that GT02 is influenced by hydrothermal activity, as it is located near the St. Paul and Conrad fracture zones, where hydrothermal activity has been reported previously (D'orazio et al., 2004). The presence of authigenic pFe, alongside the absence of lithogenic pFe, suggests that unique geochemical processes are occurring in this area. This hypothesis is further supported by the observed peaks in pMn (Fig. S7) at 1200 m, as a deep increase in pMn can indicate an hydrothermal influence (Fang and Wang, 2021; Fitzsimmons et al., 2017). Hydrothermal sources do not remain consistently active and often vary in intensity due to underlying volcanic and tectonic processes (Rudnicki and German, 2002). This variability supports our observations of variable pFe and pMn concentrations at depth.

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Mean authigenic pFe concentrations (nM) at each station of the transect in surface waters (A) and intermediate waters (B). Squares and dashed-line correspond to the upper estimation of authigenic pFe, while dots and solid line correspond to the lower estimation. Filled area correspond to the range between the upper and lower estimations. Error bars correspond to the standard deviation associated to the mean. Black stars indicate a significant difference (Wilcoxon Mann Whitney test p-value <0.05) between winter (blue) and spring (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
## 4. Conclusion

This first investigation into seasonal variability of particulate iron concentrations in the Southern Ocean revealed that, overall, seasonality did not systematically affect the total pFe distribution, and that, when differences existed, pFe concentrations were higher in winter than in spring. Throughout the transect, contribution from biogenic iron remained low, even during spring. Consequently, biogenic processes do not significantly influence the seasonal distributions of total pFe in winter and spring. External sources such as dust deposition and inputs from the Antarctic Circumpolar Current can explain higher lithogenic contributions and higher total pFe concentrations in winter. The authigenic fraction, which dominated the total pool, likely originated from the dissolution of lithogenic particles or hydrothermal inputs.

This study serves as a baseline for future research on the seasonality of iron in the Southern Ocean. As climate change continues, it will be important to observe how iron seasonality and the different iron pools and internal processes may be affected.

### CRediT authorship contribution statement

**Antoine Ringard:** Writing – review & editing, Writing – original draft, Investigation, Data curation, Conceptualization. **Hélène Planquette:** Writing – review & editing, Validation, Supervision, Methodology, Funding acquisition, Conceptualization. **Thato Mtshali:** Writing – review & editing. **Alakendra Roychoudhury:** Writing – review & editing. **Eva Bucciarelli:** Writing – review & editing, Validation, Supervision, Methodology, Funding acquisition, Conceptualization.

### Acknowledgements

We would like to thank the South African National Antarctic Program (SANAP), the captain and the crew of the RV SA Agulhas II, Thomas Ryan-Keogh and Marcello Vichi the chief scientists, and all the TracEx and CSIR-SOCCO teams for deployment and collecting samples during both winter and spring cruises. This work was supported by ISblue project, Interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) and co-funded by a grant from the French Government under the Program "Investissements d'Avenir" Embedded in France 2030 (SeaTraM project, LEMAR). Antoine Ringard is funded through the CNRS Joint Research Programme "Seasonality of iron and manganese in the Southern Ocean" led by H. Planquette. International collaboration was made possible by funding received by the French-South African National Research Foundation (NRF) Collaboration (PROTEA; FSTR180418322331, SeaTraM-PROTEA project). AR was funded by an anonymous charitable donor via Whales and Climate Change Program (whalesandclimate.org).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marchem.2025.104552.

### Data availability

Data will be made available on request.

### References

Abadie, C., Lacan, F., Radic, A., Pradoux, C., Poitrasson, F., 2017. Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean. Proc. Natl. Acad. Sci. 114, 858–863. https://doi.org/10.1073/pnas.1603107114.

Alderkamp, A.-C., van Dijken, G.L., Lowry, K.E., Lewis, K.M., Joy-Warren, H.L., van de Poll, W., Laan, P., Gerringa, L., Delmont, T.O., Jenkins, B.D., Arrigo, K.R., 2019. Effects of iron and light availability on phytoplankton photosynthetic properties in the Ross Sea. Mar. Ecol. Prog. Ser. 621, 33–50. https://doi.org/10.3354/meps13000.

Anilkumar, N., Luis, A.J., Somayajulu, Y.K., Ramesh Babu, V., Dash, M.K., Pednekar, S. M., Babu, K.N., Sudhakar, M., Pandey, P.C., 2006. Fronts, water masses and heat content variability in the Western Indian sector of the Southern Ocean during austral summer 2004. J. Mar. Syst. 63, 20–34. https://doi.org/10.1016/j.jmarsys.2006.04.009.

Ansorge, I.J., Skelton, P., Bekker, A., de Bruyn, P.J.N., Butterworth, D., Gilliers, P., Cooper, J., Cowan, D.A., Dorrington, R., Fawcett, S., Fietz, S., Findlay, K.P., Froneman, W.P., Grantham, G.H., Greve, M., Hedding, D., Hofmeyr, G.G.J., Kosch, M., le Roux, P.C., Lucas, M., MacHutchon, K., Meiklejohn, I., Nel, W., Pistorius, P., Ryan, P.G., Stander, J., Swart, S., Treasure, A., Vichi, M., van Vuuren, B. J., 2017. Exploring South Africa's southern frontier: a 20-year vision for polar research through the South African National Antarctic Programme. S. Afr. J. Sci. 113, 7. https://doi.org/10.17159/sajs.2017.a0205.

Ardyna, M., Lacour, L., Sergi, S., d'Ovidio, F., Sallée, J.-B., Rembauville, M., Blain, S., Tagliabue, A., Schlitzer, R., Jeandel, C., Arrigo, K.R.,CLAUSTRE, H., 2019. Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nat. Commun. 10, 2451. https://doi.org/10.1038/s41467-019-09973-6.

Arrigo, K.R., van Dijken, G., Long, M., 2008. Coastal Southern Ocean: a strong anthropogenic CO2 sink. Geophys. Res. Lett. 35. https://doi.org/10.1029/2008GL035624.

Arteaga, L.A., Boss, E., Behrenfeld, M.J., Westberry, T.K., Sarmiento, J.L., 2020. Seasonal modulation of phytoplankton biomass in the Southern Ocean. Nat. Commun. 11, 5364. https://doi.org/10.1038/s41467-020-19157-2.

Barrett, P.M., Resing, J.A., Grand, M.M., Measures, C.I., Landing, W.M., 2018. Trace element composition of suspended particulate matter along three meridional CLIVAR sections in the Indian and southern oceans: impact of scavenging on Al distributions. Chem. Geol. 502, 15–28. https://doi.org/10.1016/j.chemgeo.2018.06.015.

Bauer, D.F., 1972. Constructing confidence sets using rank statistics. J. Am. Stat. Assoc. 67, 687–690. https://doi.org/10.1080/01621459.1972.10481279.

Bazzani, E., Lauritano, C., Saggiomo, M., 2023. Southern Ocean iron limitation of primary production between past knowledge and future projections. J. Mar. Sci. Eng. 11, 272. https://doi.org/10.3390/jmse11020272.

Black, E.E., Kienast, S.S., Lemaitre, N., Lam, P.J., Anderson, R.F., Planquette, H., Planchon, F., Buesseler, K.O., 2020. Ironing out Fe residence time in the dynamic Upper Ocean. Global Biogeochem. Cycles 34, e2020GB006592. https://doi.org/10.1029/2020GB006592.

Blain, S., Quéguiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Brussaard, C., Carlotti, F., Christaki, U., Corbière, A., Durand, I., Ebersbach, F., Fuda, J.-L., Garcia, N., Gerringa, L., Griffiths, B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, C., Laan, P., Lefèvre, D., Lo Monaco, C., Malits, A., Mosseri, J., Obernosterer, I., Park, Y.-H., Picheral, M., Pondaven, P., Remenyi, T., Sandroni, V., Sarthou, G., Savoye, N., Scouarnec, L., Souhaut, M., Thuiller, D., Timmermans, K., Trull, T., Uitz, J., van Beek, P., Veldhuis, M., Vincent, D., Viollier, E., Vong, L., Wagener, T., 2007. Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446, 1070–1074. https://doi.org/10.1038/nature05700.

Blain, S., Planquette, H., Obernosterer, I., Guéneugues, A., 2022. Vertical flux of trace elements associated with lithogenic and biogenic carrier phases in the Southern Ocean. Glob. Biogeochem. Cycles 36, e2022GB007371. https://doi.org/10.1029/2022GB007371.

Bowie, A.R., van der Merwe, P., Quéroué, F., Trull, T., Fourquez, M., Planchon, F., Sarthou, G., Chever, F., Townsend, A.T., Obernosterer, I., Sallée, J.-B., Blain, S., 2015. Iron budgets for three distinct biogeochemical sites around the Kerguelen archipelago (Southern Ocean) during the natural fertilisation study, KEOPS-2. Biogeosciences 12, 4421–4445. https://doi.org/10.5194/bg-12-4421-2015.

Boyd, P.W., Ellwood, M.J., 2010. The biogeochemical cycle of iron in the ocean. Nature Geosci 3, 675–682. https://doi.org/10.1038/ngeo964.

Boyd, P.W., Watson, A.J., Law, C.S., Abraham, E.R., Trull, T., Murdoch, R., Bakker, D.C. E., Bowie, A.R., Buesseler, K.O., Chang, H., Charette, M., Groot, P., Downing, K., Frew, R., Gall, M., Hadfield, M., Hall, J., Harvey, M., Jameson, G., LaRoche, J., Liddicoat, M., Ling, R., Maldonado, M.T., McKay, R.M., Nodder, S., Pickmere, S., Pridmore, R., Rintoul, S., Safi, K., Sutton, P., Strzepek, R., Tanneberger, K., Turner, S., Waite, A., Zeldis, J., 2000. A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization. Nature 407, 695–702. https://doi.org/10.1038/35037500.

Boyd, P.W., Ellwood, M.J., Tagliabue, A., Twining, B.S., 2017. Biotic and abiotic retention, recycling and remineralization of metals in the ocean. Nat. Geosci. 10, 167–173. https://doi.org/10.1038/ngeo2876.

Canada, G. of C.N.R.C, 2024a. MESS-4: Marine Sediment Certified Reference Material for Total and Extractable Metal Content - NRC Digital Repository [WWW Document]. https://nrc-digital-repository.canada.ca/eng/view/object/?id=8a3fd39a-c068-4ce0-820c-d08cf742a20a.

Canada, G. of C.N.R.C, 2024b. PACS-3: Marine Sediment Certified Reference Material for Total and Extractable Metal Content - NRC Digital Repository [WWW Document]. https://nrc-digital-repository.canada.ca/eng/view/object/?id=6616d284-f101-4b7a-b3c1-a5280e4bd355.

Cardinal, D., Savoye, N., Trull, T.W., André, L., Kopczynska, E.E., Dehairs, F., 2005. Variations of carbon mineralisation in the Southern Ocean illustrated by the Baxs proxy. Deep-Sea Res. I Oceanogr. Res. Pap. 52, 355–370. https://doi.org/10.1016/j.dsr.2004.10.002.

Carranza, M.M., Gille, S.T., Franks, P.J.S., Johnson, K.S., Pinkel, R., Girton, J.B., 2018. When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean. J. Geophys. Res. Oceans 123, 7264–7289. https://doi.org/10.1029/2018JC014416.
Cassar, N., Bender, M.L., Barnett, B.A., Fan, S., Moxim, W.J., Levy, H., Tilbrook, B., 2007. The Southern Ocean biological response to Aeolian iron deposition. Science 317, 1067-1070. https://doi.org/10.1126/science.1144602.

Chapman, C., Lea, M.-A., Meyer, A., Sallée, J.-B., Hindell, M., 2020. Defining Southern Ocean fronts and their influence on biological and physical processes in a changing climate. Nat. Clim. Chang. 10, 209. https://doi.org/10.1038/s41558-020-0705-4.

Chever, F., Bucciarelli, E., Sarthou, G., Speich, S., Arhan, M., Penven, P., Tagliabue, A., 2010. Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea gyre. J. Geophys. Res. Oceans 115. https://doi.org/10.1029/2009JC005880.

Cloete, R., Loock, J.C., van Horsten, N.R., Fietz, S., Mtshali, T.N., Planquette, H., Roychoudhury, A.N., 2021. Winter biogeochemical cycling of dissolved and particulate cadmium in the Indian sector of the Southern Ocean (GEOTRACES Glpr07 transect). Front. Mar. Sci. 8.

Collier, R., Edmond, J., 1984. The trace element geochemistry of marine biogenic particulate matter. Prog. Oceanogr. 13, 113-199. https://doi.org/10.1016/0079-6611(84)90008-9.

Cosentino, N.J., Gaiero, D.M., Lambert, F., 2021. Present-day Patagonian dust emissions: combining surface visibility, mass flux, and reanalysis data. J. Geophys. Res. Atmos. 126, e2020JD034459. https://doi.org/10.1029/2020JD034459.

Cutter, G., Casciotti, K., Croot, P., Geibert, W., Heimbürger, L.-E., Lohan, M., Planquette, H., van de Flierdt, T., 2017. In: GEOTRACES International Project Office (Ed.), Sampling and Sample-handling Protocols for GEOTRACES Cruises. Version 3, August 2017. (Report).

Dehairs, F., Lambert, C., Chesselet, R., Risler, N., 1987. The biological production of marine suspended barite and the barium cycle in the Western Mediterranean Sea. Biogeochemistry 4, 119-140. https://doi.org/10.1007/BF02180151.

Deppeler, S.L., Davidson, A.T., 2017. Southern Ocean phytoplankton in a changing climate. Front. Mar. Sci. 4.

DeVries, T., 2014. The oceanic anthropogenic CO2 sink: storage, air-sea fluxes, and transports over the industrial era. Glob. Biogeochem. Cycles 28, 631-647. https://doi.org/10.1002/2013GB004739.

D'orazio, M., Boschi, C., Brunelli, D., 2004. Talc-rich hydrothermal rocks from the St. Paul and Conrad fracture zones in the Atlantic Ocean. Eur. J. Mineral. 16, 73-83. https://doi.org/10.1127/0935-1221/2004/0016-0073.

Duce, R.A., Tindale, N.W., 1991. Atmospheric transport of iron and its deposition in the ocean. Limnol. Oceanogr. 36, 1715-1726. https://doi.org/10.4319/lo.1991.36.8.1715.

Ellwood, M.J., Boyd, P.W., Sutton, P., 2008. Winter-time dissolved iron and nutrient distributions in the Subantarctic zone from 40-52S; 155-160E. Geophys. Res. Lett. 35. https://doi.org/10.1029/2008GL033699.

Fang, Z., Wang, W.-X., 2021. Size speciation of dissolved trace metals in hydrothermal plumes in the southwest Indian ridge. Sci. Total Environ. 771, 145367. https://doi.org/10.1016/j.scitotenv.2021.145367.

Fitzsimmons, J.N., John, S.G., Marsay, C.M., Hoffman, C.L., Nicholas, S.L., Toner, B.M., German, C.R., Sherrell, R.M., 2017. Iron persistence in a distal hydrothermal plume supported by dissolved-particulate exchange. Nature Geosci 10, 195-201. https://doi.org/10.1038/ngeo2900.

Flynn, R.F., Haraguchi, L., McQuaid, J., Burger, J.M., Mutseka Lunga, L., Stirnmann, L., Samanta, S., Roychoudhury, A.N., Fawcett, S.E., 2023. Nanoplankton: the dominant vector for carbon export across the Atlantic Southern Ocean in spring. Sci. Adv. 9, eadi3059. https://doi.org/10.1126/sciadv.adi3059.

Fouilland, E., Descolas-Gros, C., Courties, C., Pons, V., 1999. Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across a subtropical frontal zone (Indian Ocean). Polar Biol. 21, 90-96. https://doi.org/10.1007/s003000050339.

GeoReM - Query by reference samples or materials (published values), 2024 [WWW Document], n.d. URL. http://georem.mpch-mainz.gwdg.de/sample_query.asp (accessed 4.29.24).

Gledhill, M., Buck, K., 2012. The organic complexation of Iron in the marine environment: a review. Front. Microbiol. 3.

Gourain, A., Planquette, H., Cheize, M., Lemaitre, N., Menzel Barraqueta, J.-L., Shelley, R., Lherminier, P., Sarthou, G., 2019. Inputs and processes affecting the distribution of particulate iron in the North Atlantic along the GEOVEDE (GEOTRACES GA01) section. Biogeosciences 16, 1563-1582. https://doi.org/10.5194/bg-16-1563-2019.

Group, G.I.D.P., 2023. GEOTRACES IDP2021v2 [WWW Document]. British Oceanographic Data Centre. https://www.bodc.ac.uk/data/published_data_library/catalogue/10.5285/f4f6034-47c7-05f9-e053-6c86abc0dc7e/ (accessed 4.12.24).

Gruber, N., Landschützer, P., Lovenduski, N.S., 2019. The variable Southern Ocean carbon sink. Ann. Rev. Mar. Sci. 11, 159-186. https://doi.org/10.1146/annurev-marine-121916-063407.

Hatta, M., Measures, C.I., Selph, K.E., Zhou, M., Hiscock, W.T., 2013. Iron fluxes from the shelf regions near the South Shetland Islands in the Drake Passage during the austral-winter 2006. Deep Sea Res. II 90, 89-101. https://doi.org/10.1016/j.dsr2.2012.11.003.

Ho, T.-Y., Quigg, A., Finkel, Z.V., Milligan, A.J., Wyman, K., Falkowski, P.G., Morel, F.M., 2003. The elemental composition of some marine Phytoplankton1. J. Phycol. 39, 1145-1159. https://doi.org/10.1111/j.0022-3646.2003.03090.x.

Hurst, M.P., Aguilar-Islas, A.M., Bruland, K.W., 2010. Iron in the southeastern Bering Sea: elevated leachable particulate Fe in shelf bottom waters as an important source for surface waters. Cont. Shelf Res. 30, 467-480. https://doi.org/10.1016/j.csr.2010.01.001.

Jickells, T.D., An, Z.S., Andersen, K.K., Baker, A.R., Bergametti, G., Brooks, N., Cao, J.J., Boyd, P.W., Duce, R.A., Hunter, K.A., Kawahata, H., Kubilay, N., IaRoche, J., Liss, P. S., Mahowald, N., Prospero, J.M., Ridgwell, A.J., Tegen, I., Torres, R., 2005. Global

Iron connections between desert dust, ocean biogeochemistry, and climate. Science 308, 67-71. https://doi.org/10.1126/science.1105959.

Kanguechi, K.I., 2021. Southern African Dust Characteristics and Potential Impacts on the Surrounding Oceans (Thesis). Stellenbosch University, Stellenbosch.

Kessler, A., Tjiputra, J., 2016. The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks. Earth Syst. Dynam. 7, 295-312. https://doi.org/10.5194/esd-7-295-2016.

Klunder, M.B., Laan, P., Middag, R., De Baar, H.J.W., van Ooijen, J.C., 2011. Dissolved iron in the Southern Ocean (Atlantic sector). Deep Sea Res. Part II Top. Stud. Oceanogr. 58, 2678-2694. https://doi.org/10.1016/j.dsr2.2010.10.042.

Lam, P.J., Ohnemus, D.C., Auro, M.E., 2015. Size-fractionated major particle composition and concentrations from the US GEOTRACES North Atlantic zonal transect. Deep Sea Res. Part II Top. Stud. Oceanogr. 116, 303-320. https://doi.org/10.1016/j.dsr2.2014.11.020. GEOTRACES GA-03 - the U.S. GEOTRACES North Atlantic Transect.

Latimer, J.C., Filippelli, G.M., 2001. Terrigenous input and paleoproducibility in the Southern Ocean. Paleogeography 16, 627-643. https://doi.org/10.1029/2000PA000586.

Lemaitre, N., Planquette, H., Planchon, F., Sarthou, G., Jacquet, S., García-Ibáñez, M.I., Gourain, A., Cheize, M., Monin, L., André, L., Laha, P., Terryn, H., Dehairs, F., 2018. Particulate barium tracing of significant mesopelagic carbon remineralisation in the North Atlantic. Biogeosciences 15, 2289-2307. https://doi.org/10.5194/bg-15-2289-2018.

Lin, H., Rauschenberg, S., Hexel, C.R., Shaw, T.J., Twining, B.S., 2011. Free-drifting icebergs as sources of iron to the Weddell Sea. Deep Sea Res. II 58, 1392-1406. https://doi.org/10.1016/j.dsr2.2010.11.020.

Marsy, C.M., Barrett, P.M., McGillicuddy Jr., D.J., Sedwick, P.N., 2017. Distributions, sources, and transformations of dissolved and particulate iron on the Ross Sea continental shelf during summer. J. Geophys. Res. Oceans 122, 6371-6393. https://doi.org/10.1002/2017JC013068.

Martin, J.H., 1990. Glacial-interglacial CO2 change: the Iron hypothesis. Paleogeogr. Palaeoecol. 5, 1-13. https://doi.org/10.1029/Pa005i001p00001.

Measures, C.I., Brown, M.T., Selph, K.E., Apprill, A., Zhou, M., Hatta, M., Hiscock, W.T., 2013. The influence of shelf processes in delivering dissolved iron to the HNLC waters of the Drake Passage, Antarctica. Deep Sea Res. II 90, 77-88. https://doi.org/10.1016/j.dsr2.2012.11.004.

Middag, R., de Baar, H.J.W., Laan, P., Cai, P.H., van Ooijen, J.C., 2011. Dissolved manganese in the Atlantic sector of the Southern Ocean. Deep Sea Res. II 58, 2661-2677. https://doi.org/10.1016/j.dsr2.2010.10.043. 2006.

Milne, A., Schlosser, C., Wake, B.D., Achterberg, E.P., Chance, R., Baker, A.R., Forryan, A., Lohan, M.C., 2017. Particulate phases are key in controlling dissolved iron concentrations in the (sub)tropical North Atlantic. Geophys. Res. Lett. 44, 2377-2387. https://doi.org/10.1002/2016GL072314.

Mtshali, T.N., van Horsten, N.R., Thomalla, S.J., Ryan-Keogh, T.J., Nicholson, S.-A., Roychoudhury, A.N., Bucciarelli, E., Sarthou, G., Tagliabue, A., Monteiro, P.M.S., 2019. Seasonal depletion of the dissolved Iron reservoirs in the sub-Antarctic zone of the Southern Atlantic Ocean. Geophys. Res. Lett. 46, 4386-4395. https://doi.org/10.1029/2018GL081355.

Neff, P.D., Bertler, N.A.N., 2015. Trajectory modeling of modern dust transport to the Southern Ocean and Antarctica. J. Geophys. Res. Atmos. 120, 9303-9322. https://doi.org/10.1002/2015JD023304.

Orsi, A.H., Whitworth, T., Nowlin, W.D., 1995. On the meridional extent and fronts of the Antarctic circumpolar current. Deep Sea Res. Part I Oceanogr. Res. Pap. 42, 641-673. https://doi.org/10.1016/0967-0637(95)00021-W.

Planchon, F., Cavagna, A.-J., Cardinal, D., André, L., Dehairs, F., 2013. Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean. Biogeosciences 10, 803-820. https://doi.org/10.5194/bg-10-803-2013.

Planquette, H., Sherrell, R.M., 2012. Sampling for particulate trace element determination using water sampling bottles: methodology and comparison to in situ pumps. Limnol. Oceanogr. Methods 10, 367-388. https://doi.org/10.4319/limo.2012.10.367.

Planquette, H., Statham, P.J., Fones, G.R., Charette, M.A., Moore, C.M., Salter, I., Nédélec, F.H., Taylor, S.L., French, M., Baker, A.R., Mahowald, N., Jickells, T.D., 2007. Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean. Deep Sea Res. II 54, 1999-2019. https://doi.org/10.1016/j.dsr2.2007.06.019.

Planquette, H., Sherrell, R.M., Stammerjohn, S., Field, M.P., 2013. Particulate iron delivery to the water column of the Amundsen Sea, Antarctica. Mar. Chem. 153, 15-30. https://doi.org/10.1016/j.marchem.2013.04.006.

R: The R Project for Statistical Computing [WWW Document]. 2023. https://www.r-project.org/?fbclid=IwAR0wOZOCxL2iuQUn-oz7axM-k4uyZevKRBkAnVD7F17NDzuJG9L-pvRfQ (accessed 6.1.23).

Raiswell, R., 2011. Iceberg-hosted nanoparticulate Fe in the Southern Ocean: mineralogy, origin, dissolution kinetics and source of bioavailable Fe. Deep Sea Res. II 58, 1364-1375. https://doi.org/10.1016/j.dsr2.2010.11.011.

Raiswell, R., Benning, L.G., Tranter, M., Tulaczyk, S., 2008. Bioavailable iron in the Southern Ocean: the significance of the iceberg conveyor belt. Geochem. Trans. 9 (7). https://doi.org/10.1186/1467-4866-9-7.

Rauschenberg, S., Twining, B.S., 2015. Evaluation of approaches to estimate biogenic particulate trace metals in the ocean. Mar. Chem. 171, 67-77. https://doi.org/10.1016/j.marchem.2015.01.004.

Rudnicki, M.D., German, C.R., 2002. Temporal variability of the hydrothermal plume above the Kairei vent field, 25°S, central Indian ridge. Geochem. Geophys. Geosyst. 3. https://doi.org/10.1029/2001GC000240. XXX-XXX.
Sallée, J.-B., Speer, K., Rintoul, S., Wijffels, S., 2010. Southern Ocean thermocline ventilation. J. Phys. Oceanogr. 40, 509–529. https://doi.org/10.1175/2009JPO4291.1.

Sarmiento, J.L., Gruber, N., Brzezinski, M.A., Dunne, J.P., 2004. High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature 427, 56–60. https://doi.org/10.1038/nature02127.

Sedwick, P.N., DiTullio, G.R., Mackey, D.J., 2000. Iron and manganese in the Ross Sea, Antarctica: seasonal iron limitation in Antarctic shelf waters. J. Geophys. Res. Oceans 105, 11321–11336. https://doi.org/10.1029/2000JC000256.

Shaked, Y., Buck, K.N., Mellett, T., Maldonado, M.T., 2020. Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics. ISME J. 14, 1182–1193. https://doi.org/10.1038/s41396-020-0597-3.

Smetacek, V., Assmy, P., Henjes, J., 2004. The role of grazing in structuring Southern Ocean pelagic ecosystems and biogeochemical cycles. Antarct. Sci. 16, 541–558. https://doi.org/10.1017/S0954102004002317.

Sofen, L.E., Antipova, O.A., Buck, K.N., Caprara, S., Chacho, L., Johnson, R.J., Kim, G., Morton, P., Ohnemus, D.C., Rauschenberg, S., Sedwick, P.N., Tagliabue, A., Twining, B.S., 2023. authigenic iron is a significant component of oceanic labile particulate iron inventories. Global Biogeochem. Cycles 37, e2023GB007837. https://doi.org/10.1029/2023GB007837.

Strzepek, R.F., Maldonado, M.T., Hunter, K.A., Frew, R.D., Boyd, P.W., 2011. Adaptive strategies by Southern Ocean phytoplankton to lessen iron limitation: uptake of organically complexed iron and reduced cellular iron requirements. Limnol. Oceanogr. 56, 1983–2002. https://doi.org/10.4319/lo.2011.56.6.1983.

Sun, W.-P., Han, Z.-B., Hu, C.-Y., Pan, J.-M., 2016. Source composition and seasonal variation of particulate trace element fluxes in Prydz Bay, East Antarctica. Chemosphere 147, 318–327. https://doi.org/10.1016/j.chemosphere.2015.12.105.

Sunda, W., 2012. Feedback interactions between trace metal nutrients and phytoplankton in the ocean. Front. Microbiol. 3.

Swart, N.C., Lutjeharms, J.R.E., Ridderinkhof, H., de Ruijter, W.P.M., 2010. Observed characteristics of Mozambique Channel eddies. J. Geophys. Res. Oceans 115 (C9). https://doi.org/10.1029/2009JC005875.

Tagliabue, A., Sallée, J.-B., Bowie, A.R., Lévy, M., Swart, S., Boyd, P.W., 2014. Surface-water iron supplies in the Southern Ocean sustained by deep winter mixing. Nature Geosci 7, 314–320. https://doi.org/10.1038/ngeo2101.

Tagliabue, A., Bowie, A.R., DeVries, T., Ellwood, M.J., Landing, W.M., Milne, A., Ohnemus, D.C., Twining, B.S., Boyd, P.W., 2019. The interplay between regeneration and scavenging fluxes drives ocean iron cycling. Nat. Commun. 10, 4960. https://doi.org/10.1038/s41467-019-12775-5.

Tagliabue, A., Buck, K.N., Sofen, L.E., Twining, B.S., Aumont, O., Boyd, P.W., Caprara, S., Homoky, W.B., Johnson, R., König, D., Ohnemus, D.C., Sohst, B., Sedwick, P., 2023. Authigenic mineral phases as a driver of the upper-ocean iron cycle. Nature 620, 104–109. https://doi.org/10.1038/s41586-023-06210-5.

Tang, W., Llort, J., Weis, J., Perron, M.M.G., Basart, S., Li, Z., Sathyendranath, S., Jackson, T., Sanz Rodriguez, E., Proemse, B.C., Bowie, A.R., Schallenberg, C., Strutton, P.G., Matear, R., Cassar, N., 2021. Widespread phytoplankton blooms triggered by 2019–2020 Australian wildfires. Nature 597, 370–375. https://doi.org/10.1038/s41586-021-03805-8.

Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution.

Thomalla, S.J., Fauchereau, N., Swart, S., Monteiro, P.M.S., 2011. Regional scale characteristics of the seasonal cycle of chlorophyll in the Southern Ocean. Biogeosciences 8, 2849–2866. https://doi.org/10.5194/bg-8-2849-2011.

Thomalla, S.J., Nicholson, S.-A., Ryan-Keogh, T.J., Smith, M.E., 2023. Widespread changes in Southern Ocean phytoplankton blooms linked to climate drivers. Nat. Clim. Chang. 13, 975–984. https://doi.org/10.1038/s41558-023-01768-4.

Tovar-Sanchez, A., Sañudo-Wilhelmy, S.A., Garcia-Vargas, M., Weaver, R.S., Popels, L.C., Hutchins, D.A., 2003. A trace metal clean reagent to remove surface-bound iron from marine phytoplankton. Mar. Chem. 82, 91–99. https://doi.org/10.1016/S0304-4203 (03)00054-9.

Twining, B.S., Baines, S.B., 2013. The trace metal composition of marine phytoplankton. Ann. Rev. Mar. Sci. 5, 191–215. https://doi.org/10.1146/annurev-marine-121211-172322.

Twining, B.S., Baines, S.B., Fisher, N.S., Landry, M.R., 2004. Cellular iron contents of plankton during the Southern Ocean Iron experiment (SOFeX). Deep-Sea Res. I Oceanogr. Res. Pap. 51, 1827–1850. https://doi.org/10.1016/j.dsr.2004.08.007.

Twining, B.S., Nodder, S.D., King, A.L., Hutchins, D.A., LeCleir, G.R., DeBruyn, J.M., Maas, E.W., Vogt, S., Wilhelm, S.W., Boyd, P.W., 2014. Differential remineralization of major and trace elements in sinking diatoms. Limnol. Oceanogr. 59, 689–704. https://doi.org/10.4319/lo.2014.59.3.0689.

Twining, B.S., Rauschenberg, S., Morton, P.L., Vogt, S., 2015. Metal contents of phytoplankton and labile particulate material in the North Atlantic Ocean. Prog. Oceanogr. 137, 261–283. https://doi.org/10.1016/j.pocean.2015.07.001.

van der Merwe, P., Wuttig, K., Holmes, T., Trull, T.W., Chase, Z., Townsend, A.T., Goemann, K., Bowie, A.R., 2019. High lability Fe particles sourced from glacial erosion can meet previously unaccounted biological demand: Heard Island, Southern Ocean. Front. Mar. Sci. 6.

van Horsten, N.R., Planquette, H., Sarthou, G., Ryan-Keogh, T.J., Lemaitre, N., Mtshali, T.N., Roychoudhury, A., Bucciarelli, E., 2022. Early winter barium excess in the southern Indian Ocean as an annual remineralisation proxy (GEOTRACES GIPr07 cruise). Biogeosciences 19, 3209–3224. https://doi.org/10.5194/bg-19-3209-2022.

van Manen, M., Aoki, S., Brussaard, C.P.D., Conway, T.M., Eich, C., Gerringa, L.J.A., Jung, J., Kim, T.-W., Lee, S., Lee, Y., Reichart, G.-J., Tian, H.-A., Wille, F., Middag, R., 2022. The role of the Dotson Ice Shelf and Circumpolar Deep Water as driver and source of dissolved and particulate iron and manganese in the Amundsen Sea polynya, Southern Ocean. Mar. Chem. 246, 104161. https://doi.org/10.1016/j. marchem.2022.104161.

Vernet, M., Sines, K., Chakos, D., Cefarelli, A.O., Ekern, L., 2011. Impacts on phytoplankton dynamics by free-drifting icebergs in the NW Weddell Sea. Deep Sea Res. II 58, 1422–1435. https://doi.org/10.1016/j.dsr2.2010.11.022.

Weis, J., Schallenberg, C., Chase, Z., Bowie, A.R., Wojtasiewicz, B., Perron, M.M.G., Mallet, M.D., Strutton, P.G., 2022. Southern ocean phytoplankton stimulated by wildfire emissions and sustained by iron recycling. Geophys. Res. Lett. 49, e2021GL097538. https://doi.org/10.1029/2021GL097538.