# Platinum and Rare Earth Elements distribution in costal sediments from French western Mediterranean Sea

Chloé Jariel, Stéphane Mounier, Stéphanie Jacquet, Jean-Louis Gonzalez, Christophe Brach-Papa, Teddy Sireau, Nicolas Briant

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Chloé Jariel, Stéphane Mounier, Stéphanie Jacquet, Jean-Louis Gonzalez, Christophe Brach-Papa, et al.. Platinum and Rare Earth Elements distribution in costal sediments from French western Mediterranean Sea. Marine Pollution Bulletin, 2026, 222, pp.118818. 10.1016/j.marpolbul.2025.118818. hal-05318263

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# Platinum and Rare Earth Elements distribution in coastal sediments from French western Mediterranean Sea

Chloé Jariel<sup>a,b,*</sup>, Stéphane Mounier<sup>c</sup>, Stéphanie Jacquet<sup>a</sup>, Jean-Louis Gonzalez<sup>b</sup>, Christophe Brach-Papa<sup>b</sup>, Teddy Sireau<sup>d</sup>, Nicolas Briant<sup>d</sup>

## ARTICLE INFO

Keywords:

TCE

REE Pt

Coastal sediments

Mediterranean Sea

Environmental monitoring

## ABSTRACT

This study presents the first assessment of Technological Critical Elements (TCE), specifically platinum (Pt) and rare earth elements (REE), in coastal sediments along the French western Mediterranean Sea. These trace elements, increasingly used in advanced technologies, raise particular concerns due to their potential environmental impact. Surface sediment samples were collected from 21 stations along the French Mediterranean coast (from Spain border to Italy including Corsica Island) to evaluate TCE concentrations and identify potential sources. Most of the REE concentrations fell within expected natural levels and were mainly influenced by the geological characteristics of local watersheds, indicating a dominant geogenic origin. When normalized to European Shale, REE distribution patterns exhibited convex shapes, typical of coastal environments. In contrast, localized Pt enrichment were detected at specific sites, particularly in port areas, which could be linked to anthropogenic emissions from vehicular catalytic converters. These results highlight platinum as a relevant emerging contaminant in urban coastal environments. However, unexpected REE and Pt concentrations at the mouth of the Hérault river call for further investigation. This study highlights the importance of integrating geological background and anthropogenic pressures in coastal sediment quality assessments and supports the inclusion of TCE in long-term environmental monitoring programs in the Mediterranean Sea.

## 1. Introduction

Over the past decades, Technological Critical Elements (TCE) have gained increasing attention as emerging environmental contaminants (Cobelo-García et al., 2015; Romero-Freire et al., 2019). This group of elements is defined by their strategic economic importance and the high risk associated with their supply, often due to geopolitical or environmental constraints (Nuss and Blengini, 2018). TCE encompass several categories of elements, including; 1) Rare Earth Elements (REE): Sc, La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Y, Gd, Td, Dy, Ho, Er, Tm, Yb, Lu; 2) Platinum Group Elements (PGE): Ru, Os, Rh, Ir, Pd, Pt; 3) Non-traditional metals and metalloids: Ge, Ga, In, Te, Nb, Ta. Due to their unique physicochemical properties, TCE are indispensable to a wide range of advanced technologies, including renewable energy, medical devices, electric mobility, defense, and information technology (Filella and Rodushkin, 2018; Dang et al., 2021). As a result, global demand for these

elements continues to grow, intensifying pressures on their supply chains and raising concerns about their environmental footprint (Ruchter and Sures, 2015).

In aquatic systems, TCE generally occur at trace or ultra-trace levels (Cobelo-García et al., 2015). Despite these low natural concentrations, increasing industrial use is expected to result in growing emissions in terrestrial and aquatic environments (Romero-Freire et al., 2019). Concentrations in the oceanic environment, in dissolved form, are on average 36 ± 35 ng.L⁻¹ for the sum of REE (Pereto et al., 2024) and 39 ± 5 pg.L⁻¹ for Pt (single element) (Colodner et al., 1993; Suzuki et al., 2014, Berezhnaya and Dubinin, 2024). However, the behavior, fate, and potential impacts of most of TCE remain poorly understood, particularly in marine ecosystems.

Among TCE, REE and Pt are of particular concern due to their high technological value and increasing anthropogenic mobilization. For instance, the demand for platinum has increased nearly seven-fold

<footnote><sup>a</sup> Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Marseille, France</footnote>

<footnote><sup>b</sup> Ifremer, COAST, F-83500, La Seyne-sur-mer, France</footnote>

<footnote><sup>c</sup> Université de Toulon, Aix Marseille Univ., CNRS, IRD, MIO, Toulon, France</footnote>

<footnote><sup>d</sup> Ifremer, CCEM Contamination Chimique des Écosystèmes Marins, F-44000, Nantes, France</footnote>

<footnote>* Corresponding author at: Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO, Marseille, France.</footnote>

<footnote>E-mail address: chloe.jariel@univ-amu.fr (C. Jariel).</footnote>
between 1975 and 2025 (www.platinum.matthey.com), driven mainly by the automotive sector through the use of catalytic converters (Matthey and Aesar, 2013). These devices, while reducing emissions, also undergo mechanical wear during vehicle operation, releasing Pt particles that can be transported into the atmosphere and deposited in aquatic environments by runoff. This process has resulted in measurable Pt contamination in both terrestrial (Helmers et al., 1998; Zereini et al., 2001) and marine environments (Neira et al., 2015; Abdou et al., 2019).

Similarly, REE demand has also surged, supported by rapid developments in electric vehicles, offshore wind turbines, and robotics (Guyonnet et al., 2015). Permanent magnets represent a major application, with projected demand rising from 125,000 tons in 2019 to 200,000 tons by 2030. As REE are often poorly recycled and rarely replaced, their release into the environment is becoming increasingly likely.

In this context, the accumulation of TCE in sediments is of particular interest. Sediments constitute an integrative matrix acting as both a potential sink and secondary sources of contaminants in aquatic environments, offering a valuable medium for assessing long-term pollution trends (Zimmermann et al., 2017).

This study focuses on a Mediterranean coastal environment located in southeastern France, encompassing a mosaic of urbanized zones, industrial and port infrastructures (both civilian and military), protected natural areas, and small coastal watersheds. The Mediterranean Sea is subject to intense anthropogenic pressure. It is densely populated, with nearly 450 million inhabitants living along its coasts, corresponding to an average density of approximately 700 inhabitants per square kilometer. In addition to this permanent population, it is the world's leading tourist destination, receiving around 50 million visitors annually. As a result, the Mediterranean basin faces multiple pressures related to human activities, including rapid coastal urbanization, the expansion of agricultural and industrial activities, and the development of an extensive transportation network.

This study investigates the occurrence and distribution of selected TCE, specifically REE and platinum, in coastal sediments of the French Mediterranean coast from Spain border to Italy including Corsica Island. The main objective is to first assess TCE concentrations in sediment for the selected sampling sites and then, investigating their potential sources (natural and/or anthropic) by integrating regional geological characteristics and anthropogenic pressures. This is the first study focuses on TCE elements in this particular area affected by major rivers and strongly affected by human activities.

# 2. Materials and methods

## 2.1. Geological characteristics

The Mediterranean coastline is a geologically diverse region, structured by a complex and active geological history. This diversity has led to a broad range of lithologies, which in turn can influence the natural geochemical background of TCE in coastal sediments. It is therefore essential to understand the geological nature of each catchment area in order to interpret the sedimentary geochemical data presented in this study.

Based on the dominant geological characteristics of the upstream catchments, the sampling stations analyzed in this project can be grouped into three main lithological categories: 1) Magmatic-Morphic soils: stations whose upstream basins are partly composed of crystalline and magmatic rocks, such as granitoids and metamorphic formations originating from the Massif Central, Cévennes, and Alpine chains. This group includes Hérault, Aude, Montpellier, and Rhône sampling sites. 2) Sedimentary soils: these sites are associated with carbonate and/or detrital sedimentary formations, often of Jurassic to quaternary age. These include Cortiou, Marseille, Villefranche, Hyères, St Laurent du Var, Sicié, Fos, Toulon, and Port-la-Nouvelle. 3) Meta-morphic soils: these include sites influenced by the Variscan orogenic

belt (Capitello and Saint-Tropez), and the crystalline massifs of Corsica (Canari, Bonifaccio Posido, Porto-Vecchio, Bastia, Bastia Sud, and Golo) composed primarily of gneiss, schists, and micaschists.

This geological classification provides a framework for interpreting the spatial variability of trace element concentrations observed in the sediment samples, and for distinguishing natural geochemical signatures from potential anthropogenic contributions.

## 2.2. Sampling

21 surfaces sediment samples were collected (Fig. 1) during the oceanographic campaign SUCHIMED (Bouchoucha et al., 2021), conducted from March 15 to July 7, 2021, aboard the R/V L'Europe (French Oceanographic Fleet, operated by Ifremer). Sampling was performed using a Reineck box corer. The uppermost 1 cm of each core was sub-sampled on board, placed into acid-washed polypropylene containers, and immediately frozen at -18 °C. In the laboratory, samples were freeze-dried prior to analysis.

## 2.3. Sample digestion and chemical analysis

Rare earth elements (REE) and platinum (Pt) concentrations were measured at Ifremer Nantes (CCEM Unit) using a triple quadrupole inductively coupled plasma mass spectrometer (TQ-ICP-MS, iCAP-TQ, Thermo®) operated in kinetic energy discrimination (KED) mode with helium as collision gas.

For complete digestion, 200 mg of freeze-dried and sieved (<2 mm) material were digested with a mixture of 250 µL HNO₃, 750 µL HCl, and 6 mL HF in closed Teflon vessels. Acid mixtures with sediment samples were heated at 130 °C for 4 h using a hot block. After evaporation to dryness at 100 °C, a second digestion step was performed using 4 mL HNO₃ and 20 mL ultrapure water for 6 h at 130 °C. The resulting solutions were transferred to 50 mL polypropylene tubes (Falcon®) and diluted to a final volume of 50 mL with ultrapure water.

The following isotopes were measured: REE: ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁴Nd, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁴⁹Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, and ¹⁷⁵Lu, and ¹⁹⁵Pt and trace metals (Cu, Ni and Pb). Elemental concentrations were quantified using external calibration curves based on multi-element standard solutions (Multi-element setup solution, CPAChem®) (Rétif et al., 2024). Indium (¹¹⁵In, 2 ppb) was used as an internal standard to correct for instrumental drift. Quality control was ensured by repeated analysis of instrument blanks (6% HNO₃) and standard reference materials, BCR-723 (Road dust) for Pt and BCR-667 for REE and trace metals. Particle size analyses were performed using liquid-phase laser diffraction (Mastersizer, Malvern Instruments Ltd.) in accordance with standard NF 13320-1 after drying and sieving the samples to 2 mm.

## 2.4. Data processing

Limits of detection (LOD) and quantification (LOQ) were calculated from repeated measurements of procedural blanks using the following formulas:

* **LOD** = concentration equivalent to mean of blanks + 3 × standard deviation of blanks

* **LOQ** = concentration equivalent to mean of blanks + 10 × standard deviation of blanks

Blank values were subtracted from all measured concentrations to correct for potential analytical bias. LOD and LOQ obtained during this study are presented in Table 1. The concentrations were considered as equal to the LOD when the values were lower than the LOQ.
<figure><img src="image_3.png" /><figcaption>Fig. 1. Map of the SUCHIMED 2021 sampling locations. Platinum content (in mg.kg⁻¹) measured: concentrations below 10⁻² mg.kg⁻¹ (light blue), between 10⁻² mg.kg⁻¹ and 2.0 × 10⁻² mg.kg⁻¹ (dark blue and yellow), and above 2.0 × 10⁻² mg.kg⁻¹ (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>

## 2.5. Normalization to aluminum

To take into account the influence of sediment mineralogical and granulometric variability between stations, REE and Pt concentrations were normalized to aluminum (Al), considered a conservative lithogenic tracer, representative of the finest fraction of sediments (clays) (Luoma and Rainbow, 2008). This element is commonly used for the standardization of trace metals in marine sediments, as it is one of the main constituents of the fine-grained fraction, such as aluminosilicates, with which most anthropogenic metals are associated. (Ho et al., 2012; Pougnet et al., 2014; Tessier et al., 2011; Abdou et al., 2019). Normalized concentrations [X]Al were calculated as:

$$[X]_{Al,i} = \frac{[X]i}{[Al]i}$$

Where [X]Al,i is the normalized concentration of element X at station i, and [Al]i is the aluminum concentration at the same station. This normalization enhances inter-station comparison by reducing the influence of terrigenous input or dilution effects by non-lithogenic phases.

For comparison, normalization of REE and Pt concentrations was also performed using iron (Fe), which is likewise considered a lithogenic tracer as clay mineral indicator used as an alternative to Al. The results obtained with this standard, particularly the R² values, were lower than those obtained with aluminum. REE and Pt concentrations therefore appear to be more strongly correlated with aluminum, which was consequently selected as the preferred normalization element.

## 2.6. Rare Earth Element (REE) profile normalization

### 2.6.1. Principle and purpose

REE concentrations were normalized to a reference material to

account for the natural saw-tooth distribution of REE, an effect of the Oddo-Harkins rule, which results in higher abundances of even atomic-number elements compared to odd ones (Banin and Navrot, 1975). Normalization corrects for this pattern, facilitates interpretation, and highlights geochemical processes such as redox conditions or adsorption onto particles (Liu et al., 2021). It also helps in detecting specific anomalies and identifying characteristic trends linked to sources or processes.

### 2.6.2. Normalization reference

REE profiles were normalized to the “European Shale” (ES) reference composition (Bau et al., 2018), a composite representative of typical European shale. This standard is commonly used for normalizing REE data across a range of environmental matrices, including sediments (Picone et al., 2022), biota (Pereto et al., 2020; Figueiredo et al., 2022), and water (Lerat-Hardy et al., 2019), allowing for cross-environmental comparisons.

### 2.6.3. REE classification

For interpretation, REE were grouped into three categories based on their atomic numbers: Light REE (LREE): La to Sm, Middle REE (MREE): Eu to Dy and Heavy REE (HREE): Ho to Lu (Johannesson and Lyons, 1995). This classification supports structured analysis of REE patterns and the detection of enrichment or depletion trends in specific subgroups.

## 2.7. Interpretation of normalized REE profiles

### 2.7.1. Ce and Eu anomalies

Anomalies in cerium (Ce) and europium (Eu) were evaluated by comparing the measured normalized concentrations with expected theoretical values interpolated from neighboring REE (Bau and Dulski,
<table><caption>**Table 1**
Analytical limits of detection (LOD) and quantification (LOQ) (in mg.kg⁻¹).</caption><thead><tr><th></th><th>La</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Eu</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>Pt</th></tr></thead><tbody><tr><td>LOD</td><td>7.8 × 10<sup>-4</sup></td><td>1.0 × 10<sup>-3</sup></td><td>2.5 × 10<sup>5</sup></td><td>5.3 × 10<sup>5</sup></td><td>3.3 × 10<sup>5</sup></td><td>1.6 × 10<sup>5</sup></td><td>2.1 × 10<sup>5</sup></td><td>1.7 × 10<sup>5</sup></td><td>5.6 × 10<sup>6</sup></td><td>1.4 × 10<sup>5</sup></td><td>3.0 × 10<sup>5</sup></td><td>1.7 × 10<sup>5</sup></td><td>1.0 × 10<sup>5</sup></td><td>6.2 × 10<sup>-4</sup></td></tr><tr><td>LOQ</td><td>2.2 × 10<sup>-3</sup></td><td>2.9 × 10<sup>-2</sup></td><td>6.6 × 10<sup>4</sup></td><td>1.5 × 10<sup>-4</sup></td><td>9.8 × 10<sup>5</sup></td><td>3.8 × 10<sup>5</sup></td><td>4.7 × 10<sup>5</sup></td><td>4.4 × 10<sup>5</sup></td><td>8.3 × 10<sup>6</sup></td><td>3.5 × 10<sup>5</sup></td><td>8.2 × 10<sup>5</sup></td><td>4.3 × 10<sup>5</sup></td><td>1.8 × 10<sup>-5</sup></td><td>6.7 × 10<sup>-4</sup></td></tr></tbody></table>

1996):

* Ce anomaly:

$$Ce_i = \frac{[Ce]i}{([La]i + [Pr]i) \times 0.5}$$

* Eu anomaly:

$$Eu_i = \frac{[Eu]i}{([Sm]i + [Gd]i) \times 0.5}$$

Where $[Ce]i$ or $[Eu]i$ is the normalized concentration at station i, and $([La]i + [Pr]i) \times 0.5$ or $([Sm]i + [Gd]i) \times 0.5$ is the theoretical normalized concentration. These ratios reveal specific geochemical processes such as oxidation of Ce(II) to Ce(IV) or reduction of Eu(III) to Eu(II), typically associated with particular redox conditions.

### 2.7.2. MREE Enrichment Index (MREE/MREE)*

The overall shape of the REE pattern was further examined using the MREE/MREE* ratio (Haley et al., 2004), calculated as:

$$(MREE_i/MREE)^* = \frac{mean([Eu]_i, [Gd]_i, [Tb]_i, [Dy]_i)}{mean([Ce]_i, [Pr]_i, [Nd]_i, [Sm]_i, [Ho]_i, [Er]_i, [Tm]_i, [Yb]_i, [Lu]_i)}$$

* A ratio > 1 indicates a convex pattern with relative MREE enrichment.

* A ratio < 1 reflects a concave pattern, suggesting MREE depletion.

* A ratio ≈ 1 corresponds to a flat profile with no preferential enrichment.

This metric is particularly useful for identifying multiple REE sources or distinguishing anthropogenic signatures.

## 3. Results and discussion

### 3.1. Grain size and organic matter content of sediments

The sediments collected during this survey are composed of an average fine fraction (<63 µm) of 42 %. The highest value was observed for Saint-Laurent du var. (98 %) and the lowest for (Bastia Sud and Cortiou) with to 13.5 %, Golo and Hérault (9.5 %, Aude (5.8 %), and Sicié Step (3 %). According to the classification of Ibouily (1981), these sediments span a continuum from pure sands to pure muds. This highly diversified distribution would therefore be synonymous with a heterogeneous capacity to adsorb contaminants on sediment surfaces.

Surprisingly, measured organic matter (OM) contents show high values for all sites. The average OM percentage for these samples was 56 %, ranging from 24 % (Porto Vecchio) to 77 % (Montpellier). This average is higher than typically observed in coastal sediments (Pusceddu et al., 1999; Ausín et al., 2023).

Thus, the high organic matter content could affect surface properties of sediments as their potential for the retention and accumulation of metallic contaminants.

### 3.2. Rare Earth Elements (REE) in surface sediments

#### 3.2.1. Total REE concentrations and normalization to aluminum

Total REE concentrations (ΣREE) for all sites are presented in Fig. 2 (Table 2). Values range from 37 mg.kg⁻¹ at the Canari site (Corsica) to 405 mg.kg⁻¹ at the Hérault River mouth. The average value is 115 mg.kg⁻¹. These concentrations are consistent with the average composition of the upper continental crust, which contains between 150 and 220 mg.kg⁻¹.
<figure><img src="image_4.png" /><figcaption>Fig. 2. Total REE (ΣREE in mg.kg⁻¹) measured in samples collected during the SUCHIMED 2021 campaign along the French Mediterranean coastline.</figcaption></figure>

<caption>**Table 2**</caption>

Aluminum, Platinum and Rare earth concentrations (in mg.kg⁻¹) obtained for the various sites sampled during the SUCHIMED 2021 campaign. The sum of these concentrations is shown in the last column of this table.

<table><thead><tr><th>Station</th><th>Al</th><th>Pt</th><th>La</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Eu</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>ΣREE</th></tr></thead><tbody><tr><td>Villefranche</td><td>6 × 10<sup>3</sup></td><td>0.1 × 10<sup>2</sup></td><td>10</td><td>18</td><td>2</td><td>10</td><td>2</td><td>0.4</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.9</td><td>0.1</td><td>0.7</td><td>0.1</td><td>48</td></tr><tr><td>Saint Laurent du Var</td><td>4 × 10<sup>4</sup></td><td>0.7 × 10<sup>2</sup></td><td>22</td><td>43</td><td>5</td><td>20</td><td>4</td><td>0.7</td><td>4</td><td>0.5</td><td>3</td><td>0.6</td><td>2</td><td>0.2</td><td>1</td><td>0.2</td><td>106</td></tr><tr><td>Saint Tropez</td><td>7 × 10<sup>4</sup></td><td>1.9 × 10<sup>2</sup></td><td>32</td><td>70</td><td>8</td><td>31</td><td>6</td><td>1</td><td>6</td><td>0.8</td><td>4</td><td>0.8</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>165</td></tr><tr><td>Hyères</td><td>2 × 10<sup>4</sup></td><td>0.3 × 10<sup>2</sup></td><td>14</td><td>28</td><td>3</td><td>14</td><td>3</td><td>0.6</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.9</td><td>0.1</td><td>0.7</td><td>0.1</td><td>68</td></tr><tr><td>Toulon</td><td>4 × 10<sup>4</sup></td><td>1.3 × 10<sup>2</sup></td><td>28</td><td>58</td><td>7</td><td>26</td><td>5</td><td>0.9</td><td>4</td><td>0.5</td><td>3</td><td>0.6</td><td>2</td><td>0.2</td><td>1</td><td>0.2</td><td>137</td></tr><tr><td>Cortiou</td><td>5 × 10<sup>4</sup></td><td>&lt;LOD</td><td>10</td><td>19</td><td>2</td><td>9</td><td>2</td><td>0.4</td><td>2</td><td>0.2</td><td>1</td><td>0.3</td><td>0.7</td><td>0.1</td><td>0.6</td><td>0.1</td><td>48</td></tr><tr><td>Marseille</td><td>7 × 10<sup>4</sup></td><td>&lt;LOD</td><td>11</td><td>21</td><td>3</td><td>11</td><td>2</td><td>0.5</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.9</td><td>0.1</td><td>0.7</td><td>0.1</td><td>55</td></tr><tr><td>Fos</td><td>3 × 10<sup>4</sup></td><td>1.0 × 10<sup>2</sup></td><td>24</td><td>50</td><td>6</td><td>23</td><td>5</td><td>0.8</td><td>4</td><td>0.5</td><td>3</td><td>0.6</td><td>2</td><td>0.2</td><td>1</td><td>0.2</td><td>120</td></tr><tr><td>Port La Nouvelle</td><td>4 × 10<sup>4</sup></td><td>1.4 × 10<sup>2</sup></td><td>24</td><td>49</td><td>6</td><td>22</td><td>4</td><td>0.8</td><td>4</td><td>0.5</td><td>3</td><td>0.6</td><td>2</td><td>0.2</td><td>0.6</td><td>0.2</td><td>118</td></tr><tr><td>Aude</td><td>3 × 10<sup>4</sup></td><td>0.3 × 10<sup>2</sup></td><td>19</td><td>39</td><td>5</td><td>18</td><td>4</td><td>0.8</td><td>3</td><td>0.4</td><td>2</td><td>0.5</td><td>1</td><td>0.2</td><td>1</td><td>0.2</td><td>94</td></tr><tr><td>Hérault</td><td>3 × 10<sup>4</sup></td><td>2.6 × 10<sup>2</sup></td><td>84</td><td>177</td><td>21</td><td>78</td><td>14</td><td>2</td><td>11</td><td>1</td><td>8</td><td>1</td><td>4</td><td>0.5</td><td>3</td><td>0.5</td><td>406</td></tr><tr><td>Montpellier</td><td>3 × 10<sup>4</sup></td><td>&lt;LOD</td><td>16</td><td>33</td><td>4</td><td>15</td><td>3</td><td>0.7</td><td>3</td><td>0.4</td><td>2</td><td>0.4</td><td>1.1</td><td>0.1</td><td>0.9</td><td>0.1</td><td>80</td></tr><tr><td>Rhône</td><td>5 × 10<sup>4</sup></td><td>0.8 × 10<sup>2</sup></td><td>26</td><td>51</td><td>6</td><td>23</td><td>4</td><td>0.9</td><td>4</td><td>0.5</td><td>3</td><td>0.6</td><td>2</td><td>0.2</td><td>2</td><td>0.2</td><td>123</td></tr><tr><td>Sicié</td><td>8 × 10<sup>4</sup></td><td>0.9 × 10<sup>2</sup></td><td>52</td><td>109</td><td>13</td><td>48</td><td>9</td><td>2</td><td>7</td><td>0.9</td><td>4</td><td>0.8</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>251</td></tr><tr><td>Canari</td><td>2 × 10<sup>4</sup></td><td>0.3 × 10<sup>2</sup></td><td>6</td><td>13</td><td>2</td><td>7</td><td>2</td><td>0.4</td><td>2</td><td>0.3</td><td>2</td><td>0.4</td><td>1</td><td>0.1</td><td>0.9</td><td>0.1</td><td>37</td></tr><tr><td>Capitello</td><td>6 × 10<sup>4</sup></td><td>1.7 × 10<sup>2</sup></td><td>42</td><td>95</td><td>10</td><td>37</td><td>7</td><td>1</td><td>7</td><td>0.9</td><td>6</td><td>1</td><td>3</td><td>0.5</td><td>3</td><td>0.5</td><td>215</td></tr><tr><td>Bonifaccio Posido</td><td>2 × 10<sup>4</sup></td><td>0.3 × 10<sup>2</sup></td><td>9</td><td>16</td><td>2</td><td>8</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.9</td><td>0.1</td><td>0.9</td><td>0.1</td><td>44</td></tr><tr><td>Porto Vecchio</td><td>3 × 10<sup>4</sup></td><td>0.7 × 10<sup>2</sup></td><td>20</td><td>40</td><td>5</td><td>19</td><td>4</td><td>0.6</td><td>4</td><td>0.6</td><td>4</td><td>0.7</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>102</td></tr><tr><td>Golo</td><td>8 × 10<sup>4</sup></td><td>&lt;LOD</td><td>9</td><td>18</td><td>2</td><td>9</td><td>2</td><td>0.4</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.8</td><td>0.1</td><td>0.7</td><td>0.1</td><td>46</td></tr><tr><td>Bastia South</td><td>2 × 10<sup>4</sup></td><td>&lt;LOD</td><td>13</td><td>27</td><td>3</td><td>13</td><td>2</td><td>0.5</td><td>2</td><td>0.3</td><td>2</td><td>0.3</td><td>0.9</td><td>0.1</td><td>0.7</td><td>0.1</td><td>66</td></tr><tr><td>Bastia</td><td>5 × 10<sup>4</sup></td><td>&lt;LOD</td><td>17</td><td>39</td><td>5</td><td>19</td><td>4</td><td>1</td><td>4</td><td>0.6</td><td>4</td><td>0.7</td><td>2</td><td>0.3</td><td>2</td><td>0.2</td><td>99</td></tr></tbody></table>

¹ of REE (Rudnick and Gao, 2014). One site exceeds this reference range: the Hérault River mouth (405.88 mg.kg⁻¹). Most studies on coastal sediments report ΣREE values below 200 mg.kg⁻¹ (Tranchida et al., 2011; Liu et al., 2021). The Fig. 3 shows a strong positive correlation between ΣREE and Al content, R² = 0.81 and a significance level of p = 0.0092, suggesting a control of REE content by sediment composition and a probably a common geological origin. Due to their sorption capacities, in general, REE concentrations increase from sands to silts and clays (Tranchida et al., 2011).

Only the Hérault mouth deviates from this linear relationship. Its REE concentrations cannot be explained solely by aluminum content. Furthermore, values observed for this site are significantly higher than those reported for some riverine particles from the same region (Roussiez et al., 2013). High concentrations of REE have also been reported in the tissues of bivalves (M. galloprovincialis) at the same station (Briant et al., 2021). Several hypotheses may explain this anomaly. The Hérault River drains a geologically diverse watershed, including the

Massif Central and Cévennes, composed mainly of granitoids and gneiss. Granitic rocks are known to be enriched in REE (Négré et al., 2000). These elements may be transported downstream via erosion and runoff, accumulating in the sediment.

Anthropogenic contamination remains a possible explanation, though unlikely. The distribution pattern of REE at this site is homogeneous, with no selective enrichment of specific REE (Gd, La) typically used in industrial or medical applications. Such patterns are usually a marker of anthropogenic inputs (Kulaksız and Bau, 2011; Garcia-Solsona et al., 2020). To date, widespread REE contamination in the marine environment has not been clearly demonstrated.

### 3.2.2. REE anomalies (Ce/Ce*, Eu/Eu*)

Table 3 presents the Ce/Ce*, Eu/Eu*, and MREE/MREE* ratios calculated for each site. The Ce/Ce* values range from 0.87 (Bonifacio Posido) to 1.11 (Capitello), with an average of 1.00 ± 0.05. This ratio is commonly used to assess redox conditions (Hatje et al., 2024). Ce(III) is
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Sum of rare earth concentrations (in mg.kg⁻¹) as a function of aluminum concentrations (Al%) measured at the various sites sampled during the SUCHIMED 2021 campaign. Sites following a linear correlation are shown in blue, and the Hérault site, which deviates from this correlation, is shown in orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

oxidized to Ce(IV) in oxic conditions. Ce(IV), being particle-reactive, is preferentially removed from seawater and enriched in sediments, producing a negative anomaly in water and a positive one in sediments (Elderfield et al., 1988; Byrne and Sholkovitz, 1996). The values close to or slightly above 1 observed in this study therefore indicate moderate to high oxidation at the sediment-seawater interface (Elderfield et al., 1988).

The average Eu/Eu* ratio is 0.93 ± 0.12. Values range from 0.66 (Porto -Vecchio) to 1.11 (Canari). Negative Eu anomalies (Eu/Eu* < 0.90) were observed at six sites: Saint-Tropez, Porto -Vecchio, Hérault mouth, Capitello, Fos, and Bonifacio Posido. Similar anomalies have been reported in surface sediments near the Têt and Var rivers (Roussiez et al., 2013). In coastal areas, negative Eu anomalies typically indicate a terrigenous origin for REE and are linked to the weathering of igneous rocks (Taylor and McLennan, 1985; Lee and Tanaka, 2021). This behavior reflects feldspar fractionation during magma evolution (Weill and Drake, 1973; Shearer and Papike, 1989), and is not associated with

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

Ce/Ce*, Eu/Eu*, and MREE/MREE* ratios obtained for rare earth concentrations at sites sampled during the SUCHIMED 2021 campaign, normalized to European Shale.

<table><thead><tr><th>Station</th><th>Ce/Ce*</th><th>Eu/Eu*</th><th>MREE/MREE*</th></tr></thead><tbody><tr><td>Villefranche</td><td>0.9</td><td>0.9</td><td>1.3</td></tr><tr><td>Saint Laurent du Var</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Saint Tropez</td><td>1.1</td><td>0.8</td><td>1.1</td></tr><tr><td>Hyères</td><td>0.9</td><td>1.0</td><td>1.2</td></tr><tr><td>Toulon</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Cortiou</td><td>0.9</td><td>0.9</td><td>1.1</td></tr><tr><td>Marseille</td><td>0.9</td><td>0.9</td><td>1.2</td></tr><tr><td>Fos</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Port La Nouvelle</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Aude</td><td>1.0</td><td>1.0</td><td>1.2</td></tr><tr><td>Hérault</td><td>1.0</td><td>0.7</td><td>1.0</td></tr><tr><td>Montpellier</td><td>1.0</td><td>1.0</td><td>1.2</td></tr><tr><td>Rhône</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Sicié</td><td>1.0</td><td>0.9</td><td>1.1</td></tr><tr><td>Canari</td><td>0.9</td><td>1.1</td><td>1.2</td></tr><tr><td>Capitello</td><td>1.1</td><td>0.7</td><td>0.9</td></tr><tr><td>Bonifacio Posido</td><td>0.8</td><td>0.9</td><td>1.0</td></tr><tr><td>Porto Vecchio</td><td>1.0</td><td>0.6</td><td>1.0</td></tr><tr><td>Golo</td><td>1.0</td><td>0.9</td><td>1.2</td></tr><tr><td>Bastia South</td><td>1.0</td><td>1.0</td><td>1.2</td></tr><tr><td>Bastia</td><td>1.0</td><td>1.1</td><td>1.3</td></tr></tbody></table>

anthropogenic inputs.

### 3.2.3. Normalized REE distribution patterns

Two types of REE distribution patterns were identified (Fig. 4): convex and flat profiles.

Convex patterns show a relative enrichment in middle REE (MREE) compared to light (LREE) and heavy REE (HREE). The convexity is quantified by the MREE/MREE* ratio. Most sites exhibit MREE/MREE* > 1 (Fig. 4), indicating convex profiles. Such patterns are frequently reported in coastal sediments (Palmer and Elderfield, 1986; Johannes-son and Zhou, 1999) and are usually associated with terrigenous inputs (Hu et al., 2021).

Convex profiles may result from preferential MREE scavenging during sediment-seawater interactions (Auer et al., 2017; Consani and Cutroneo, 2021). REE are transferred from water to sediments through adsorption onto Fe/Mn oxides, organic matter, or incorporation into carbonate and phosphate mineral lattices (Byrne and Sholkovitz, 1996; Schijf and Byrne, 2021; Du et al., 2022).

The mechanism behind this enrichment remains unclear. It may relate to REE availability or local redox conditions. However, further research is needed to identify the specific processes responsible for

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Calculated MREE/MREE* ratios for the different sites sampled during the SUCHIMED 2021 campaign. MREE/MREE* ratio over than 1 indicates convex profiles, a ratio of 1 (orange line) indicates flat profiles, and MREE/MREE* ratio less than 1 indicates concave profiles.</figcaption></figure>
MREE enrichment (Haley et al., 2021; Hatje et al., 2024).

Flat profiles, characterized by similar normalized concentrations of LREE and HREE, were observed at two sites: the Hérault mouth and Porto -Vecchio. These sites show MREE/MREE* ≈ 1. This pattern suggests a dominant terrigenous REE source, consistent with negative Eu anomalies (Hu et al., 2021).

The REE data from this study highlight a wide diversity of geochemical behaviors and sedimentary processes along the Mediterranean French coast. Each REE profile, characterized by specific anomalies and distributions, and can be used as a geochemical fingerprint or the site. Understanding the processes responsible for these patterns could enhance their use as environmental proxies.

Some features, such as convex profiles and high ΣREE values, may suggest external REE inputs. However, the lack of clear anthropogenic markers and the limited number of comparable studies prevent firm conclusions. The particularly high ΣREE concentrations observed at Hérault site, well above crustal averages, warrant further investigation.

### 3.3. Platinum concentrations in surface sediments

Platinum concentrations measured in sediment samples are presented in Fig. 5 And Table 2. Concentrations range from below the detection limit (<LOD) up to 2.6 × 10⁻² mg.kg⁻¹. These levels are consistent with previous data reported for recent sediments in the area of interest. For instance, concentrations between 1.2 × 10⁻² and 1.6 × 10⁻² mg.kg⁻¹ were recorded in the Toulon Bay (Abdou et al., 2019), while 0.19 × 10⁻² mg.kg⁻¹ was measured in the upper layers of a core from the Rhône estuary (Chastanet et al., 2024).

The Fig. 6 presents Pt vs Al, the later serve to normalize platinum concentrations to the sediment's aluminosilicate content. A positive

correlation was observed between Pt and Al was globally observed (r² = 0.81), suggesting a geochemical control of Pt by sediment mineralogy and granulometry. This correlation holds primarily for sites presenting concentration below <10⁻² mg.kg⁻¹. In contrast, sites with intermediate to high Pt concentrations deviate from this trend, indicating the influence of additional factors.

Based on the measured Pt concentrations, the sampling sites can be grouped into three classes:

* Low concentration (<10⁻² mg.kg⁻¹), with a Pt/Al correlation of 0.81 and a significance level of *p* = 0.0010.

* Cortiou, Baie de Marseille, Montpellier, Golo, Bastia, Villefranche, Saint-Laurent-du-Var, Hyères, Aude, Rhône, Sicié, Canari, Bonifacio Posido, Porto-Vecchio.

* Intermediate concentration sites (10⁻²–2.0 × 10⁻² mg.kg⁻¹): Toulon, Port-la-Nouvelle, Fos, Saint-Tropez and Capitello.

* High concentration site: Hérault mouth

#### 3.3.1. Low-concentration sites: baseline reference for natural background

The majority of stations fall into the low concentration group (<10⁻² mg.kg⁻¹) (in blue in Figs. 5 and 6). At these sites, the strong correlation between Pt and Al suggests that Pt concentrations are largely controlled by the sediment's mineralogical composition and granulometry.

Furthermore, these values align with previously reported historical background levels in the region. In Toulon Bay, pre-catalytic converter sediment horizons contain between 0.6 × 10⁻² and 10⁻² mg.kg⁻¹ of Pt (Abdou et al., 2019). Additionally, in the Rhône estuary, concentrations of 0.16 × 10⁻² ± 0.41 mg.kg⁻¹ were recorded (Chastanet et al., 2024), reflecting a lower baseline likely representative of continental

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Platinum content measured at sites sampled during the SUCHIMED 2021 campaign: concentrations below 0.010 mg.kg⁻¹ (light blue bars), between 0.010 and 0.020 mg.kg⁻¹ (dark blue and yellow bars), and above 0.020 mg.kg⁻¹ (red bar). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Platinum concentrations (in mg.kg⁻¹) as a function of aluminum concentrations (in %) measured during the SUCHIMED 2021 campaign: concentrations below 0.010 mg.kg⁻¹ (light blue), between 0.010 and 0.020 mg.kg⁻¹ (dark blue and yellow), and above 0.020 mg.kg⁻¹ (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>

background inputs. Based on this data set and literature values, a con-
centration threshold of 10⁻² mg.kg⁻¹ is proposed as an indicative natural
background level for French Mediterranean coastal sediments. This
threshold could serve as a reference for assessing anthropogenic
enrichment in surface sediments.

3.3.2. Intermediate and high concentrations with a natural signature

A second positive correlation (r² = 0.9, p = 0.0023) can be observed between Pt and Al among the intermediate sites (Fig. 6). This proportional relationship, with platinum concentration higher than the natural background level proposed, may reflect shared geological characteristics, such as the grain size of the analyzed sediments. However, in the absence of established standard for these areas, this hypothesis cannot be confirmed. The correlation can be subdivided into two coherent groups: (1) Saint -Tropez and Capitello; (2) Toulon, Port -la -Nouvelle, and Fos.

3.3.2.1. *Saint-Tropez and Capitello.* Two sites, Saint-Tropez and Capitello, present Pt concentrations above the proposed natural baseline (1.9 × 10⁻² mg.kg⁻¹ and 1.7 × 10⁻² mg.kg⁻¹, respectively). The Saint-Tropez site is located near the Maures massif, a geological formation rich in mafic to ultramafic rocks, naturally enriched in platinum group elements (PGE) (Balaram et al., 2013). Although Capitello is more distant from this specific formation, it lies in southern Corsica, a region geologically related to the Maures massif. Both units are part of the Variscan orogenic belt formed during the Paleozoic era. This common geological origin is unique compared to the other stations. Various studies have already highlighted the geological similarities between these areas, linked to their common origin (Elter and Pandeli, 2005; Faure et al., 2014). The similarity in Pt concentrations between these two sites may thus reflect a common geological origin and natural Pt linked to Variscan lithology.

While this common geological origin may influence the observed
platinum concentrations,

the possibility of anthropogenic contamination remains entirely
plausible. Indeed, the Capitello site is located near the Ajaccio airport in
an anthropized bay, similarly to Saint -Tropez, which is subject to
intense urbanization and mass tourism.

3.3.2.2. Toulon, Port-la-Nouvelle, Fos. Three sites, Toulon Grande Rade (1.3 × 10⁻² mg.kg⁻¹), Port-la-Nouvelle (1.4 × 10⁻² mg.kg⁻¹), and Fos (10⁻² mg.kg⁻¹) (yellow in Figs. 5 and 6), also exceed the natural Pt baseline proposed (<10⁻² mg.kg⁻¹) and display high Pt/Al ratios.

These locations are major commercial ports with intense industrial
and maritime activity. Previous studies have documented substantial
human pressure in these areas (Tessier et al., 2011; Collignon et al.,
2012; Dang et al., 2015; Misson et al., 2016; Araújo et al., 2019).

In Toulon, the measured concentration aligns with earlier values from recent sediment layers (1.2 × 10⁻² –1.6 × 10⁻² mg.kg⁻¹; Abdou et al., 2019), The core analysis presented, an increase for Pt contents in the upper part of the core, linked to Pt emissions from vehicles equipped with catalytic converters. To assess whether the platinum concentrations measured at these stations originate from anthropogenic activities, the enrichment factor (EF) was calculated for the Toulon site. The formula used is EF = ([Pt]Toulon/[Al]Toulon)/ ([Pt]reference/[Al]reference) (Buat-Menard and Chesselet, 1979) where [Pt]Toulon is the platinum concentration measured at Toulon, [Al]Toulon is the aluminum concentration measured at the same site, and ([Pt]reference/[Al]reference) is the baseline ratio derived from the measurements reported by Abdou et al. (2019) in the Toulon harbor.

An enrichment factor of 3.5 was obtained, placing this station within the “moderate enrichment” category according to Sutherland’s (2000) classification. This result is consistent with an anthropogenic activity, particularly the use of catalytic converters, that could be responsible for the elevated platinum concentrations observed at Toulon. By analogy, the Fos and Port -la -Nouvelle sites, which exhibit similar Pt/Al ratios (Fig. 6) and experience significant human activity, are also likely to present elevated platinum concentrations due to contamination.

3.3.3. *High concentration site: Hérault river mouth*

The Hérault site stands out with the highest Pt concentration of the study (2.6 × 10⁻² mg.kg⁻¹) and a particularly elevated Pt/Al ratio (Figs. 5 and 6, in red). This site clearly deviates from the Pt–Al linear trend (Fig. 7) as from the REE–Al linear trend.

The Hérault watershed drains a wide variety of geological forma-
tions, including the Massif Central and the Cévennes, known for their
mafic and ultramafic lithologies such as basalts and gabbros, which can
be natural sources of platinum (Négrel et al., 2000; Lardeaux, 2014).

Nevertheless, as the high Pt/Al ratio is abnormally high these high
concentrations could also be explained by anthropogenic contamina-
tion. The hypothesis of contamination from catalytic converters remains
to be confirmed. To better constrain the origin of the detected platinum,
a complementary analysis including palladium (Pd) and rhodium (Rh)
concentrations could be undertaken, allowing for the calculation of inter
-element ratios commonly used to trace automotive sources.
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Nickel, Copper, Lead, Total REE and Platinum concentrations, measured during the SUCHIMED 2021 campaign, normalized to aluminum concentration.</figcaption></figure>

### 3.4. Multielement comparison

Finally, to better evaluate the variations in ΣREE and Pt concentrations across the different stations, concentrations Pb, Ni, and Cu, were processed.

The results are presented in Fig. 7. Graphically, classical metals (Pb, Cu, Ni) exhibit pronounced enrichments in industrial -port and fluvial zones (Toulon, Fos, Cortiou, Grand Rhône), reflecting strong anthropogenic influence. In contrast, REE concentrations appear relatively stable and closely correlated with the geological background, with elevated values observed near riverine inputs (e.g., the mouth of the Hérault River) as described before. Platinum, although still present at comparatively low levels, shows marked variations, particularly at sites influenced by port and urban activities, with the highest population densities, consistent with the expected impact of catalytic converters.

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Principal component analysis (PCA) of Pt, REE, Pb, Cu, and Ni concentrations in 21 stations.</figcaption></figure>

A principal component analysis (PCA) was performed on the concentrations of Pt, REE, Pb, Cu, and Ni measured stations (Fig. 8). Projection of stations on the first three components, together explaining 90.2% of the total variance. The graphic highlights distinct geochemical signatures: stations on the one side of the plot are characterized by Pt and REE enrichment, those on the upper side by Ni, and those in the other part by Pb and Cu. Overall, the PCA reveals three major geochemical groups: (i) Pt-REE, (ii) Ni, and (iii) Pb—Cu. This differentiated distribution highlights the complementarity of the investigated tracers: Pb and Cu as indicators of diffuse anthropogenic inputs, REE and Pt as indicator of local geology.

### 4. Conclusions

This study provides a first assessment of platinum and rare earth element (REE) concentrations in surface sediments along the French Mediterranean coastline. The analysis of TCE in surface sediments provides a means to reconstruct environmental conditions, whether of natural origin or related to human activities.

The results reveal a high spatial variability, largely influenced by the geological characteristics of the coastal watersheds. Only few sites seem to be affected by anthropogenic activities. The REE concentration values measured during this study fall within the range previously reported in the literature, except for one site. Both concentrations and normalized distribution patterns appear to be primarily controlled by the natural lithology of the studied areas. The predominantly convex shapes observed in the normalized REE distribution profiles support this interpretation. These findings suggest a predominantly geogenic origin for REE in sediments from French Mediterranean coast. In contrast, probable platinum contamination was identified in sediments from some port areas. We hypothesize that this contamination is likely linked to emissions from vehicular catalytic converters, highlighting platinum as a relevant emerging contaminant in these environments. These findings underscore the need for enhanced environmental monitoring of platinum and associated elements (PGE) in highly urbanized coastal area. Further investigations, including upstream watershed analyses and long -term monitoring, are required to better characterize the spatio
-temporal dynamics of TCE. Given the unexpectedly high concentrations observed at the Hérault River mouth, this site will require particular attention. Such efforts are essential to better understand the sources, transport pathways, and potential impacts of these elements on Mediterranean coastal ecosystems.

### CRediT authorship contribution statement

**Chloé Jariel:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Stéphane Mounier:** Writing – review & editing, Validation, Supervision, Project administration, Conceptualization. **Stéphanie Jacquet:** Writing – review & editing, Supervision, Project administration. **Jean-Louis Gonzalez:** Writing – review & editing, Validation, Supervision, Project administration, Conceptualization. **Christophe Brach-Papa:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization. **Teddy Sireau:** Writing – review & editing, Resources, Methodology, Data curation. **Nicolas Briant:** Writing – review & editing, Validation, Resources, Methodology, Investigation, Data curation.

### Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used ChatGPT in order to improve the readability and language of the manuscript. After using this tool/service, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

### Funding sources

SUCHIMED Campaign was supported by the Toulon -Provence -Mediterranean Metropolis and Rhone -Mediterranean Corsica Water Agency. This work is as part of a PhD funded by the SUD -PACA region and the Institute of Ocean Sciences at Aix -Marseille University (AMU).

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors wish to thank all partners involved in the campaigns since 1996, and especially the SUCHIMED campaign in 2021 (Bouchoucha 2021, SUCHI Med 202 cruise, RV L'Europe, doi:10.17600/18001619). We are also very grateful to all the staff of the Ifremer laboratories (in Toulon and Nantes) for carrying out analysis and Genavir for their valuable assistance with sample preparation and logistics.

### Data availability

Data will be made available on request.

### References

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