# The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy

Kazuyo Tachikawa, François Beny, Marine Cornuault, Vincent Guarinos, Abel Guihou, Pierre Deschamps, Hartmut Schulz, Francisco J Sierro, Marie Boye

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Kazuyo Tachikawa, François Beny, Marine Cornuault, Vincent Guarinos, Abel Guihou, et al.. The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy. Geochimica et Cosmochimica Acta, In press, 10.1016/j.gca.2025.09.012. hal-05247364

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The authigenic and detrital neodymium isotopic composition in the modern
Mediterranean Sea: implications for paleocirculation proxy

Kazuyo Tachikawa, François Beny, Marine Cornuault, Vincent Guarinos, Abel Guihou, Pierre Deschamps, Hartmut Schulz, Francisco J. Sierro, Marie Boye

PII: S0016-7037(25)00479-X

DOI: https://doi.org/10.1016/j.gca.2025.09.012

Reference: GCA 14000

To appear in: *Geochimica et Cosmochimica Acta*

Received Date: 1 April 2025

Accepted Date: 5 September 2025

Please cite this article as: Tachikawa, K., Beny, F., Cornuault, M., Guarinos, V., Guihou, A., Deschamps, P., Schulz, H., Sierro, F.J., Boye, M., The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy, *Geochimica et Cosmochimica Acta* (2025), doi: https://doi.org/10.1016/j.gca.2025.09.012

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The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy

Kazuyo Tachikawa¹*, François Beny¹, Marine Cornuault¹, Vincent Guarinos¹, Abel Guihou¹, Pierre Deschamps¹, Hartmut Schulz², Francisco J. Sierro³, Marie Boye⁴

1: Aix Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Aix-en-Provence, France

2: Department of Geosciences, University of Tübingen, Tübingen, Germany

3: Dept. de Geología, Univ. de Salamanca, Plaza de los Caídos s/n, 37008, Salamanca, Spain

4: Université de Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France
* corresponding author, kazuyo@cerege.fr

Key words

Neodymium isotopic composition, water mass provenance, Mediterranean Sea, authigenic and detrital fractions, benthic flux

**Abstract**

Neodymium isotopic composition ($^{143}$Nd/$^{144}$Nd or $\varepsilon_{Nd}$) has been used as a tracer of water mass provenance in the modern and the past oceans. However, recent studies have questioned its reliability because some phases of detrital sediments are reactive and a partial dissolution of these phases could alter the initial authigenic signals through
porewater Nd. We examined this possibility using core-top sediments from the Mediterranean Sea, an ideal region, because the water circulation and the ε$_{Nd}$ distribution of seawater and detrital fractions are relatively well constrained. Furthermore, smectite, one of the most reactive minerals, is abundant in the easternmost basin due to Nile sediment input. We obtained paired authigenic (leachates and foraminiferal tests) and detrital ε$_{Nd}$ signatures from core-top sediments along the zonal transect from 2°W to 33°E. The authigenic ε$_{Nd}$ values generally agreed with the bottom water signals, except in the easternmost Levantine and the Aegean Seas, where they were higher than the water values. These areas are characterised by high smectite abundance and enclosed nature, making local Nd source influence more visible. The authigenic ε$_{Nd}$ values in these areas were stable at about -5 to -4, while the corresponding detrital values ranged between -10 and -4. The compilation of modern seawater and detrital ε$_{Nd}$ values shows that seawater ε$_{Nd}$ covaries with detrital signals at 0–200 m water depth, and that the correlation is insignificant at depths greater than 1500 m. These results argue against a ubiquitous control of bottom water ε$_{Nd}$ by the benthic flux in the present Mediterranean Sea. Sensitivity tests suggest that the ε$_{Nd}$ values of Mediterranean Sea water are affected by the isotopic composition of Atlantic inflow, deep-water formation zones and local benthic sources, as well as by the size of the Atlantic inflow flux, deep-water formation and zonal water exchange. These results support the use of Nd isotopic composition as a tracer of water mass provenance in the Mediterranean Sea although a combination of isotopic data from various basins is required to distinguish the influence of these factors.

# 1 Introduction
Neodymium isotopic compositions ($^{143}$Nd/$^{144}$Nd or $\varepsilon_{Nd}$) of seawater and authigenic/biogenic phases (sediment leachates, foraminiferal tests, fish teeth and debris, and corals, hereafter referred to as authigenic for convenience) have been used as a tracer of water mass provenance in the modern and the past oceans (Frank, 2002; Goldstein and Hemming, 2003; Lacan et al., 2012; Tachikawa et al., 2017; van de Flierdt et al., 2016 and references therein). The major sources of Nd to the oceans are continental inputs (atmospheric dust, dissolved and particulate river loads, groundwater discharge, marine sediments), and once water masses are marked by the isotopic signals, seawater $\varepsilon_{Nd}$ values in the deep open ocean have been considered to reflect the mixing of water masses (Frank, 2002; Goldstein and Hemming, 2003; Haley et al., 2017; Lacan et al., 2012; Tachikawa et al., 2017; van de Flierdt et al., 2016; von Blanckenburg, 1999). If the authigenic fraction preserves bottom water $\varepsilon_{Nd}$ values without significant modification during burial processes, the authigenic isotopic composition can be used to reconstruct the past circulation patterns. However, these assumptions have recently been challenged by the idea of a ubiquitous benthic Nd flux from the seafloor, which may affect both seawater and authigenic $\varepsilon_{Nd}$ values (Abbott et al., 2015a, 2016; Abbott et al., 2015b; Du et al., 2016; Du et al., 2022; Haley et al., 2017).

The contribution of reactive detrital Nd from sediments to seawater, leading to non-conservative behaviour of seawater Nd isotopic composition, has been recognised for more than two decades, although most cases have been reported for sites close to continental margins (Jeandel et al., 2007; Lacan and Jeandel, 2005; Tachikawa et al., 2003). In contrast, the benthic flux hypothesis includes a ubiquitous Nd flux from the seafloor. The idea is based on high porewater Nd concentrations and consequent diffusive flux to overlying
seawater (Abbott et al., 2015a; Abbott et al., 2015b). If the influence of local sources is dominant compared to water mass advection effects, the use of seawater Nd isotopic composition as a quasi-conservative water mass tracer is not validated. Nevertheless, the amplitude and spatial influence of the local sources is still a matter of debate. It is worth noting that present-day seawater ε<sub>Nd</sub> values are generally correlated with other hydrographic parameters, such as nutrients and radiocarbon, at water depths deeper than 1500 m in the open ocean on a global scale, supporting the usefulness of this proxy for large-scale circulation (Tachikawa et al., 2017).

A compilation of paired ε<sub>Nd</sub> data of authigenic (as defined above) and detrital (residual part after leaching) fractions of marine sediments revealed a broad positive correlation between the two fractions on a global scale (Abbott et al., 2022). Systematic contamination of authigenic ε<sub>Nd</sub> values during the leaching cannot explain the correlation, as the continuous development of leaching techniques with clear criteria for monitoring contamination has successfully demonstrated negligible artificial dissolution of detrital phases (Blaser et al., 2016; Huang et al., 2021; Wilson et al., 2013). Therefore, the observed correlation has been interpreted as a sign of a widespread detrital control of authigenic ε<sub>Nd</sub> values (Abbott et al., 2022). However, considering the fact that the major oceanic Nd source is continental input, the observed large-scale positive correlation is not surprising. If the authigenic ε<sub>Nd</sub> values are exclusively determined by local Nd sources associated with partial dissolution of reactive detrital minerals, a strong correlation should exist on a small basin scale. Such data sets are still scarce.

The Mediterranean Sea is ideal to study the influence of the local Nd inputs and the reliability of authigenic ε<sub>Nd</sub> values as a proxy for bottom water provenance. This is because
the water circulation and the ε<sub>Nd</sub> distribution of seawater (Dubois-Dauphin et al., 2017a; Garcia-Solsona and Jeandel, 2020; Garcia-Solsona et al., 2020; Henry et al., 1994; Montagna et al., 2022; Spivack and Wasserburg, 1988; Tachikawa et al., 2004; Vance et al., 2004) and detrital fractions (Ayache et al., 2016; Blanchet, 2019; Weldeab et al., 2002) are relatively well constrained. Moreover, radiogenic Nile River sediments rich in smectite, one of the candidates for reactive minerals (Abbott et al., 2019; Huang et al., 2024; Wang et al., 2022), are heterogeneously distributed with a high abundance in the easternmost basin (Ehrmann et al., 2007; Hamann et al., 2009; Venkatarathnam and Ryan, 1971). Finally, the distribution of the Mediterranean seawater ε<sub>Nd</sub> has been the target of regional modelling experiments for the present state (Ayache et al., 2016; Ayache et al., 2023) and for the African humid periods when excess freshwater inputs contributed to stagnant circulation (Vadsaria et al., 2020; Vadsaria et al., 2019), leading to the formation of organic-rich layers called sapropels (Rohling et al., 2015).

For an accurate interpretation of the authigenic ε<sub>Nd</sub> records to reconstruct the past
ocean circulation, it is important to clarify the reliability and a possible limit of the proxy.
This study aims at assessing the reliability of authigenic ε<sub>Nd</sub> values as a proxy for bottom
water provenance and how the Nd isotopic signatures of Mediterranean seawater respond
to changes in Nd sources and circulation states. We performed a paired analysis of
authigenic and detrital ε<sub>Nd</sub> for 24 samples as well as the elemental composition of 14
leachate samples obtained from core-top sediments along the zonal transect of the
Mediterranean Sea from 2°W to 33°E. By combining our new results with existing data,
we evaluated the reliability of authigenic ε<sub>Nd</sub> as an indicator of bottom water mass
provenance and identified regions where the impact of local reactive minerals is important.
Finally, fractional factorial design analysis was used to assess the dominant factors controlling the present-day distribution of seawater $\epsilon_{Nd}$ in the Mediterranean Sea.

## 2. Materials and methods

### 2-1. Modern Mediterranean settings and sediment samples

The Mediterranean Sea circulation is characterised by an anti-estuary pattern with warmer and fresher Atlantic inflow at the surface layer and the underlying colder and saltier Mediterranean outflow via the Strait of Gibraltar (Pinardi and Masetti, 2000) (Fig. 1b and c). In the Western Mediterranean Sea (WMS), the Atlantic water is modified by mixing with the underlying Mediterranean water and excess evaporation, and is transformed into saltier modified Atlantic water (MAW, Fig. 1c). This water mass enters the Eastern Mediterranean Sea (EMS) through the Strait of Sicily (Fig. 1b and c). Due to continuous evaporation, the salinity increases, and MAW sinks between Rhodes and Cyprus by intense winter cooling to form Levantine Intermediate Water (LIW, Fig. 1a and c) (Schroeder et al., 2012). The intermediate water mass is the saltiest water mass in the Mediterranean Sea and corresponds to the main westward-flowing water mass. It occupies approximately between 200 m and 600 m in both the EMS and the WMS (Fig. 1c). Part of LIW enters the Adriatic Sea and contributes to the formation of Eastern Mediterranean Deep Water (EMDW). The South Adriatic Sea is the main formation zone of EMDW (Fig. 1a and b), and deep water from the Aegean Sea has a variable contribution to EMDW (Schroeder et
al., 2012). Deep water in the western basin (Western Mediterranean Deep Water, WMDW) is formed in the Gulf of Lions (Millot and Taupier-Letage, 2005) (Fig. 1a and b).

The Mediterranean Sea receives a large amount of mineral dust from North Africa (Bibi et al., 2020; Nabat et al., 2015; Scheuvens et al., 2013). Partial dissolution of dust minerals is one of the Nd sources to the Mediterranean Sea (Ayache et al., 2023; Greaves et al., 1994; Tachikawa et al., 2003; Tachikawa et al., 1999). Rivers contribute both dissolved and particulate loads to the Mediterranean Sea (Ludwig et al., 2009). The construction of the Aswan High Dam in 1965 reduced the discharge of the Nile River by a factor of around 3 and its prevention of flooding has dramatically reduced the sediment input of the Nile into the south-east Levantine basin (Ludwig et al., 2009; Nixon, 2003). Considering water mass ages in the Levantine basin ranging from 50 to 250 years based on mean CFC-12 and tritium ages (Schneider et al., 2014), we consider that our samples and previously reported seawater data (Dubois-Dauphin et al., 2017a; Garcia-Solsona and Jeandel, 2020; Garcia-Solsona et al., 2020; Henry et al., 1994; Montagna et al., 2022; Spivack and Wasserburg, 1988; Tachikawa et al., 2004; Vance et al., 2004) reflect the pre-Aswan conditions.

The Nd isotopic composition of foraminiferal authigenic oxides, leachates, and detrital residue, as well as the elemental composition of the leachates, were analysed for modern sediments along the zonal transect of the Mediterranean Sea (Fig. 1a and Table 1). Surficial sediments were collected during the GEOTRACES-A04N cruise in 2013 (Rijkenberg and participants, 2013ab) using the mono-corer attached to the CTD system that allowed the sediment-water interface to be preserved. They are considered to represent material that is younger than sediments collected by piston corers or ocean drilling. The core-top material
was also analysed for material from ODP Site 977 in the Alboran Sea (Martrat et al., 2004), core SL95 in the Gulf of Sirte in the Ionian Sea and core MD04-2722 in the Levantine Sea (Cornuault et al., 2018) to extend the spatial coverage (Table 1). The core-top age of SL95 was determined at the ARTEMIS facility (Gif-sur-Yvette, France) on a planktonic foraminifer, *Globigerinoides ruber* (white), obtained at 0-1 cm of sediment from the >150 µm size fraction. The conventional radiocarbon age of the core-top was converted to a calendar age based on the MARINE20 calibration (Heaton et al., 2021) using the ¹⁴C calibration software CALIB 8.20 (Stuiver and Reimer, 1993), assuming a reservoir age of 400 yr (Reimer and McCormac, 2002) (Table 1).

## 2-2. Analysis of Nd isotopic composition of the authigenic and detrital fractions

Prior to Nd isotopic analysis, about 30 mg of mixed species of foraminiferal tests were mechanically cleaned to remove clay minerals using an ultrasonication bath (Tachikawa et al., 2014). The cleaned test fragments were then dissolved with step-wise addition of 1 M acetic acid, and the resulting solutions were centrifuged to minimise the contribution of residual detrital particles. The supernatant was purified for Nd using a tandem ion exchange separation with Eichrom TRUSpec™ and Ln.Spec™ resins (Pin and Santos Zalduegui, 1997). Samples from ODP site 977 (>150 µm size fraction) consisted mainly of foraminiferal tests and fragments. Because of the small amount of available tests, we used the whole >150 µm fraction to obtain about 20 mg of carbonates and dissolved the samples with 1 M acetic acid (Gourlan et al., 2010). The supernatant was centrifuged and purified as described above.
Leachates were obtained from about 200 mg of homogenised non-decarbonated samples with 7 mL of 0.02 M hydroxylamine hydrochloride in 25% acetic acid (Wu et al., 2015) for 18 min followed by 12 min of centrifugation to maintain a total leaching time of 30 min (Blaser et al., 2016). After leaching, the residue was rinsed three times with Milli-Q water and recovered by centrifugation. As it mainly consisted of lithogenic material, it was treated as a detrital fraction. The obtained detrital residue was attacked with a mixture of 15.6 M HNO₃ and 28 M HF (1:2) at 120°C for about one week. The following steps included adding 6 M HCl and heating at 120 °C for three days to remove fluorides. The leachates and detrital residue were purified for Nd using Bio-Rad AG50W-X8 cation exchange resin and Ln.Spec™ resin.

Neodymium isotopic analysis was performed on a Thermo Scientific NeptunePlus MC-ICP-MS. The Nd concentration was adjusted to 10 or 20 ng/ml. Samples were measured sequentially, each followed by the reference standard JNdI-1 (Tanaka et al., 2000) prepared at the same Nd concentration. The measured ¹⁴³Nd/¹⁴⁴Nd was normalised to ¹⁴⁶Nd/¹⁴⁴Nd of 0.7219 with an exponential law and then the normalised ¹⁴³Nd/¹⁴⁴Nd was corrected for instrumental bias using a bracketing technique (Albarède et al., 2004). Neodymium isotope data are reported with εNd notation defined as εNd = [(¹⁴³Nd/¹⁴⁴Nd)sample/(¹⁴³Nd/¹⁴⁴Nd)CHUR-1] x 10000 (Jacobsen and Wasserburg, 1980). The analytical uncertainty of each sample measurement is based on both the error propagation of the standard error (SE) of the sample measurement and the bracketing JNdI-1 standards as well as the external reproducibility (2σ) of the JNdI-1 (Table S1). The typical 2SE size is 0.2 to 0.3 ε-units in agreement with the external reproducibility. Repeated measurements of other standards, “AMES” (¹⁴³Nd/¹⁴⁴Nd = 0.5119537 ± 0.0000022) (Chauvel and Blichert-Toft, 2001; Luu et al.,
2022) and the in-house standard “VWR” yielded the mean ¹⁴³Nd/¹⁴⁴Nd values of 0.511971 ± 0.000012 (2σ, n = 27) and 0.512210 ± 0.000011 (2σ, n = 12), respectively. Since the AMES standard value is close to the published isotopic ratio, no further machine bias correction was applied. Procedural Nd blanks were <100 pg for the authigenic (< 1% of the Nd amount in the typical samples) and 10 to 150 pg for the detrital fractions. All the sample preparation and the isotopic analyses were carried out at CEREGE (‘Centre de Recherche et d’Enseignement de Géosciences de l’Environnement’, France).

## 2-3. Analysis of the elemental composition of leachates

The elemental composition of selected leachates was determined using an ICP-MS (Agilent 7500ce). The leachates were diluted by a factor of 2000 and the concentration of Rare Earth Elements (REE), Ca, Sr, Al, Mn and Fe was analysed using In as the internal standard. The accuracy of the measurement was assessed by the analysis of the Geostandard marine mud MAG-1 and was estimated to be better than 5%, except for Er (9%) and Tm (6%), reflecting their low concentration. The blank contribution was less than 1% for all the analysed elements and no correction was applied. We tentatively analysed the phosphorus concentration in the leachates by ICP-OES (Agilent 5800 VDV) using the same solution as for ICP-MS to assess the influence of phosphate phases to the REE cycle (Haley et al., 2004). The phosphorus concentration was below the limit of quantification (about 0.5 µg/mL). All the analyses were carried out at CEREGE.

## 3. Results

### 3-1. Authigenic Nd isotopic composition
The authigenic ε<sub>Nd</sub> values ranged from -9.0 to -3.9 with a clear gradual increasing zonal trend from the west to the east (Fig. 2a and Table S1). Between Station 6a in the Alboran Sea and Station 13a in the southernmost Tyrrhenian Sea, the authigenic ε<sub>Nd</sub> values were within a narrow range at approximately -8.5 (Figs. 1 and 2a). The isotopic composition then increased from -7 at Station 12c in the eastern Tyrrhenian Sea to around -6 in the eastern Levantine Sea (Stations 2c and 19a). The authigenic ε<sub>Nd</sub> values were around -5 in the Aegean Sea (Stations 1c, 32a and 28a). The highest isotopic composition of -4 was found in the easternmost Levantine Sea (Station 24a). This zonal trend is consistent with previous studies (grey symbols in Fig. 2a). A new data point that deviated from this trend was the leachate from Station 6c in the north-eastern (NE) Ionian Sea (ε<sub>Nd</sub> = -4.1 ± 0.2), which was about 2 to 2.5 ε-units more radiogenic than expected. Other leachates and the foraminiferal samples showed a similar zonal ε<sub>Nd</sub> trend, with no systematic offset with the sample type (Fig. 2a). A comparable zonal trend was found for seawater ε<sub>Nd</sub> values at the locations closest to stations where the authigenic ε<sub>Nd</sub> data are available (Fig. 2c). However, the highest seawater value of -5 was observed in the Aegean Sea, and the isotopic composition in the easternmost Levantine Sea is about -7 to -6. Notably, the highest seawater ε<sub>Nd</sub> values were found in the eastern Levantine Sea, but at water depths shallower than 500 m (see Section 4.2).

The authigenic ε<sub>Nd</sub> values generally agreed with the actual seawater isotopic
composition at sites closest to the sediment sites within the same water mass (Fig. 3 and
Table S2). Taking into account the differences in analytical techniques and sample
preparation (van de Flierdt, Pahnke and GEOTRACES-intercalibration-participants, 2012)
as well as the non-exact match of locations/depths, we have set the range for this judgement
at ± 1 ε-unit for this judgement. Three samples showed clear positive offsets: Station 6c in the NE Ionian Sea with a possible leaching problem (see Section 3-2), Station 24a in the easternmost Levantine basin (> 30°E) and Station 1c in the South Aegean Sea. The leachates and foraminiferal εNd values did not show any systematic bias. A positive offset of authigenic Nd isotopic composition from the seawater values has already been observed at sites MD04-2722, MD90-964, MS27PT and ODP site 967 in the easternmost Levantine Sea (Fig. 3) (Cornuault et al., 2018; Duhamel et al., 2020; Scrivner et al., 2004; Vance et al., 2004).

## 3-2. Elemental composition of the leachates

The elemental composition of leachates can be used to assess the potential dissolution of the detrital fraction during leaching. For example, Al/Nd (g/g) <100 has been proposed as a criterion for negligible detrital dissolution during leaching (Gutjahr et al., 2007). The leachates obtained in this study satisfied this criterion with the exception of the sample from Station 6c, which showed Al/Nd of 2615 (Table S3), indicating significant dissolution of the detrital fraction during the leaching. In view of this, the data from Station 6c will be treated with caution. The Mediterranean Sea receives a large amount of Saharan dust (Bibi et al., 2020; Nabat et al., 2015; Scheuvens et al., 2013), which contains detrital carbonates that may be dissolved during leaching and release unradiogenic Nd (εNd of about -10) (Tachikawa et al., 2003). A typical dust Sr/Ca value ranges from 1.0 to 1.5 mmol/mol (Scheuvens et al., 2013), which is comparable to foraminiferal Sr/Ca values (Elderfield et al., 2000). However, other biogenic carbonates have higher Sr/Ca ratios. For instance, the Sr/Ca ratio for coccolithophores is between 1.5 and 3 mmol/mol (Auliaherliaty et al., 2009),
whereas the Sr/Ca ratio for pteropod shells and the calcareous green alga Halimeda is as high as 11 mmol/mol (Reitz and de Lange, 2006). Considering this difference in Sr/Ca, the influence of detrital carbonates on leachate ε<sub>Nd</sub> values was examined by a relationship between Sr/Ca and ε<sub>Nd</sub> in leachates (Fig. 4a). Most of the leachate ε<sub>Nd</sub> of selected samples for elemental analysis varied from -9.0 to -6.5, while leachate Sr/Ca was stable around 1.8 to 2.3 mmol/mol (Fig. 4a and Table S3). Two samples are outside of the cluster: Station 6c in the NE Ionian Sea and Station 32a in the North Aegean Sea. Both samples had high authigenic ε<sub>Nd</sub> values while Sr/Ca was 4.4 mmol/mol at Station 32a and 1.4 mmol/mol at Station 6c (Fig. 4a).

Authigenic Fe-Mn oxides and oxyhydroxides have been considered as one of the main
Nd carrier phases that preserve the seawater-derived Nd isotopic composition. Biogenic
carbonates have also been considered to play an important role because they could prevent
the acid-induced mobilisation of Fe-oxides and the dissolution of volcanogenic material
through buffering effects (Blaser et al., 2016). The Mn/Fe and Fe/Al ratios (g/g) of the
leachates were therefore used to assess whether Mn and Fe in the leachates were associated
with artificial dissolution of detrital minerals during leaching (Fig. 4b). Leachate Mn/Fe
ranged from 0.9 to 2.2 except for two samples with values below the range of 0.2 at Station
6c and of 0.3 at Station 32a (Fig. 4b and Table S3). The general Mn/Fe range is consistent
with the values of non-phosphatised hydrogenetic Fe-Mn crusts (0.9 to 2.8, centred at 1.6)
(Bau et al., 1996) (Fig. 4b). The Fe/Al ratio in the leachates was centred at around 2, except
for a very low value (0.1) at Station 6c and a high value (5.9) at Station 32a (Fig. 4b).

Post-Archean Australian Shale (PAAS)-normalised REE patterns were obtained using
the REE/Ca ratio in the leachates. There is no discernible relationship between the absolute
Nd/Ca values and the REE patterns (Fig. 4c and Table S3). All the leachates showed enrichment of MREE (MREE/MREE* > 1; Fig. 4d), which is typical of non-decarbonated leachates (Wilson et al., 2013). The majority of samples indicated a HREE/LREE ratio greater than 1, with the exceptions of Stations 6c and 32a. These two samples were located close to the detrital array formed between volcanogenic material represented by Mid Ocean Ridge Basalt (MORB) and continental material such as PAAS and Upper Continental Crust (UCC) (Du et al., 2016) (Fig. 4d). Based on these results, it was estimated that the leachate from Stations 6c and 32a contained detrital components and should be treated with caution.

### 3-3. Detrital ε<sub>Nd</sub> distribution and relationship to the authigenic values

The detrital ε<sub>Nd</sub> values ranged from -12.1 to -4.3 (Fig. 2b) and the machine replicates of seven samples generally agreed within uncertainty (Table S1). They showed a zonal increasing trend from west to east as previously reported (Weldeab et al., 2002) (Fig. 2b). The detrital isotopic composition was in the range of -12 to -10 in the WMS (Figs. 2b and 5a). In the western part of the EMS from 18°E to 23°E (the Adriatic and Ionian Seas), it showed a large variability from -12.0 to -5.3 (Fig. 2b). In this longitudinal zone, the detrital ε<sub>Nd</sub> values gradually increased with latitude (Fig. 2d) except for the samples from Station 6c as well as from Station 9c (the Strait of Otranto), where the values were -5.3 ± 0.3 and -7.4 ± 0.2, respectively (Figs. 2b and 5a). The highest detrital ε<sub>Nd</sub> values of -5 to -4 were found in the easternmost EMS (< 30°E) and the Aegean Sea, excluding the value at Station 28a (ε<sub>Nd</sub> = -8.3 ± 0.3) in the Kasos Strait (Fig. 1b) between the South Aegean Sea and the Levantine Sea (Figs. 2b and 5a).
Although the new detrital data were generally consistent with the distribution based on previous studies (Ayache et al., 2016; Blanchet, 2019; Weldeab et al., 2002), our new data revealed a more detailed spatial distribution (Fig. 5a). For example, the limit of the most radiogenic values in the easternmost EMS was better defined (Station 24a and MD04-2722). In the Aegean Sea, the distribution of detrital $\varepsilon_{Nd}$ appeared to be patchy with a high isotopic composition at Station 32a and a rather low value at Station 28a. The data from Stations 6c and 9c showed more radiogenic values than the compilation of existing data.

The relationship between authigenic and detrital $\varepsilon_{Nd}$ values showed a positive correlation across the whole Mediterranean Sea (Fig. 5b), which was similar to the global ocean (Abbott et al., 2022). However, a closer look at the relationship at the sub-basin scale revealed a more complex pattern. The authigenic $\varepsilon_{Nd}$ values were stable at about -5 to -4 in the eastern Levantine and the Aegean Seas, while the detrital values varied from -8 to -4 (Fig. 5b). The observed high authigenic $\varepsilon_{Nd}$ values were in agreement with previously reported values (Cornuault et al., 2018; Duhamel et al., 2020; Scrivner et al., 2004; Vance et al., 2004) (Fig. 5b). Here, only authigenic $\varepsilon_{Nd}$ data obtained from foraminiferal tests and non-decarbonated leachates were considered to minimise potential analytical bias. This selection is based on the importance of carbonates in minimising the artificial dissolution of the detrital fraction through pH-buffer effects (Blaser et al., 2016; Wilson et al., 2013). To improve spatial coverage, we further considered previously published authigenic (only foraminiferal tests, non-decarbonated leachates, and corals) and detrital $\varepsilon_{Nd}$ values when the two isotopic compositions were obtained at the same sites (Table S2). The correlation with the full dataset was high ($r = 0.72$, p-value $<< 0.05$, n = 30). However, when the data from the Levantine and Aegean Seas (data from Station 24a, ODP967, MD04-2722,
MS27PT, stations 28a and 32a, marked by an ellipse in Fig. 5b) were omitted, as the authigenic ε<sub>Nd</sub> values are almost invariable while the detrital values change by about 6 ε-units, the correlation disappeared (r = 0.24, p-value = 0.27, n = 23).

**4. Discussion**

The absence of a systematic bias between leachate and foraminiferal ε<sub>Nd</sub> values and the criteria based on the elemental composition of the leachates (Figs. 2a and 4) indicated that the artificial dissolution of detrital Nd due to the leaching was negligible, except for the samples from Stations 6c and 32a. The zonal changes in authigenic ε<sub>Nd</sub> values were continuous and gradual, which differed from the distribution of the detrital isotopic composition, which was patchy reflecting the influence of local sources (Figs. 2a, 2b and 5a). In addition, the authigenic ε<sub>Nd</sub> values were generally consistent with the corresponding water isotopic composition (Fig. 3). These results suggest that water advection and mixing of different water masses are the main controlling factors of authigenic (and seawater) Nd isotopic compositions. The positive offset of authigenic ε<sub>Nd</sub> from the seawater values was observed mainly in the easternmost Levantine and the Aegean Seas (Fig. 3), suggesting that a strong influence of local Nd is present but limited to specific regions. Regarding the relationship between authigenic and detrital ε<sub>Nd</sub> values, a strong positive correlation (r = 0.72) was observed when considering the whole data set (Fig. 5b). However, closer examination of the relationship at the small basin scale indicates that the relationship between authigenic and detrital ε<sub>Nd</sub> is less apparent than was previously reported.

In the following discussion, we will focus on 1) the influence of the sediment nature, in particular the abundance of reactive detrital minerals – smectite – and bulk carbonate
contents on the authigenic ε$_{Nd}$ values, 2) the impact of the benthic flux on the Mediterranean Nd budget, and 3) the evaluation of main factors controlling the modern Mediterranean Sea water ε$_{Nd}$ distribution using a box model and sensitivity tests.

## 4.1 Authigenic ε$_{Nd}$ values and the sediment nature in the Mediterranean Sea

The conditions under which authigenic ε$_{Nd}$ values can be used as a proxy for water mass provenance are of great interest for the accurate reconstruction of past oceanic circulation patterns. It has been suggested that authigenic ε$_{Nd}$ reflects both bottom and porewater isotopic signals (Abbott et al., 2019; Abbott et al., 2022; Blaser et al., 2016; Creac'h et al., 2023; Du et al., 2016, 2022, 2025; Tachikawa et al., 2013; Wang et al., 2022). A higher Nd concentration in the pore water than in the bottom water reflects not only the addition of labile detrital Nd, but also the recycling of scavenged Nd (release and re-scavenging in the region of the water-sediment interface), with a limited amount of Nd escaping from the sediment due to the strong affinity of Nd for manganese oxides (Patton et al., 2021). Among the possible criteria for the use of authigenic Nd isotopic compositions in paleoceanographic studies, (i) low content of volcanogenic materials and other reactive minerals, (ii) high carbonate content in sediments, (iii) active oceanic currents that increase the influence of Nd inputs by oceanic circulation relative to local inputs, and (iv) high sedimentation rates that reduce diagenetic Nd fluxes following conditions, seem to be critical (Workshop "Goldschmidt 2023"). Here we consider the case of the Mediterranean Sea, with particular attention to points (i) and (ii).

The authigenic ε$_{Nd}$ values are often equivalent or more radiogenic than the detrital isotopic composition (Abbott et al., 2022), possibly reflecting the abundance of a readily
soluble radiogenic detrital fraction. A similar pattern is found in the Mediterranean Sea, and a match between the authigenic and the detrital εNd values was found in the easternmost basin and the Aegean Sea where the Nd isotopic composition of the detrital residue is radiogenic (Fig. 5b). There is an increasing body of evidence for a strong impact of partial dissolution of radiogenic detrital particles on the pore water Nd isotopic composition by the dissolution of smectite, amphibole and/or volcanic glass (Elmore et al., 2011; Huang et al., 2024; Gao et al., 2025; Pearce et al., 2013; Roberts and Piotrowski, 2015; Wang et al., 2022). Clay minerals have been proposed as major reactive detrital phases, due to their high abundance and the formation of authigenic clay minerals that release REEs into pore water (Abbott et al., 2019; Bayon et al., 2023; Du et al., 2022). Indeed, the role of clay minerals, particularly smectites, in determining the Nd isotopic composition of pore water in coastal regions has been proposed. This occurs through partial dissolution along the Pacific margin of West Antarctica (Wang et al., 2022), as well as through the massive delivery of suspended particulate loads from the floodplain in the Bay of Bengal (Huang et al., 2024).

Authigenic carbonate fluorapatite would be another possible Nd carrier (Creac'h et al., 2023; Haley et al., 2004) but its formation in the modern Mediterranean Sea is estimated to be rare due to relatively low porewater $HPO_4^{2-}$ concentrations (Slomp et al., 2002). This is consistent with leachate phosphorus concentrations below the limit of quantification (Section 2-3). Carbonates are estimated to play an important role as buffering reagents because the dissolution of carbonates increases the pH and reduces dissolution of labile detrital fractions (Blaser et al., 2016). They adsorb and/or incorporate REEs during precipitation affecting the REE cycle in the pore water (Smrzka et al., 2019).
Taking these observations into account, we examine the distribution of smectite and carbonates in the EMS, and then the relationship between sediment type and the offset of authigenic ε<sub>Nd</sub> values from seawater signals.

The easternmost Levantine Sea, in particular around Cyprus, is characterised by high smectite abundances (> 60% in the clay fraction) associated with Nile River particulate loads and an anti-clockwise circulation that transports this mineral along the eastern coast (Hamann et al., 2009; Venkatarathnam and Ryan, 1971) (Fig. 6a). The zone of high smectite abundance corresponds to the easternmost Levantine Sea including Stations 24a, MD04-2722 and ODP site 967 (Fig. 5a) and MD90-964 (Duhamel et al., 2020) where authigenic ε<sub>Nd</sub> is high (-5 to -4), while the detrital values range from -10 to -4 (Fig. 5b). On the western side of the Nile River mouth close to site MS27PT, the smectite abundance is lower than in the easternmost basin, ranging from 20 to 60%, reflecting the surface current pattern (Fig. 6a). In the Northern Aegean Sea, smectite abundance data are not available and in the South Aegean Sea it ranges between 20 and 60%. Despite a lower smectite abundance than in the easternmost basin, the authigenic ε<sub>Nd</sub> values in the Aegean Sea (Stations 32a and 28a) are as high as in the area affected by the high contribution of Nile sediments (Figs. 5 and 6a). These observations suggest that the presence of reactive minerals such as smectite is one of the controlling factors of authigenic (and possibly seawater) ε<sub>Nd</sub> values but not the only one.

The concentration of carbonates in bulk sediments in the EMS varies widely from less
than 10% to more than 90% (Venkatarathnam and Ryan, 1971) (Fig. 6b). The concentration
around Cyprus is in the range of 30-50%. In the Aegean, the carbonate concentration varies
within a small spatial scale from less than 10% in the South Aegean to a range of 50-70%
in the North Aegean (Fig. 6b). The highest Fe/Al ratio in leachates was observed at station 32a in the North Aegean Sea (Fig. 4b). These results may indicate that the buffering effect of carbonate, with an abundance of about 50% in the bulk sediment, is not sufficient to prevent natural mobilisation of Fe oxides and dissolution of volcanogenic material. Interestingly, it has been indicated that the influence of sporadic tephra layers on authigenic ε<sub>Nd</sub> values could be minimised by using foraminiferal tests instead of leachates in the case of a core from the Adriatic Sea (Gao et al., 2025). This finding is comparable with the observations in the North Atlantic cores showing less impact of volcanogenic material on foraminiferal ε<sub>Nd</sub> values than on the leachates (Elmore et al., 2011). Therefore, it is possible that the release of radiogenic Nd from volcanogenic material into pore water, and/or its incorporation into foraminiferal tests, is limited in terms of either the amount of Nd or the kinetic aspect. Conversely, both foraminiferal and leachate ε<sub>Nd</sub> values deviate from the bottom water isotopic composition in areas where the reactive detrital fraction is abundant and its sedimentation is continuous, such as in the easternmost Levantine Sea and the Aegean Sea. In such areas, the influence of reactive detrital sources would be dominant.

Considering the above observations, the relationship between authigenic ε<sub>Nd</sub> values,
the abundance of reactive minerals and the carbonate content in bulk sediments appears to
be complex. In the case of the EMS, the presence of smectite is estimated to be the primary
factor leading to high authigenic ε<sub>Nd</sub> values. The precise determination of carbonate content
with a "safe buffer effect" is beyond the scope of this study. It is possible that the semi-
enclosed nature of the Aegean Sea with its complex bathymetry favours the contribution
of Nd from reactive minerals to pore water through limited seawater exchange and possible
resuspension of particles.
## 4.2 Is the abyssal benthic flux an important Nd source to the Mediterranean Sea?

The reactive detrital minerals can affect not only authigenic ε<sub>Nd</sub> values but also the Mediterranean Sea water isotopic composition. Indeed, the budget of the dissolved Nd isotopes in the modern Mediterranean Sea cannot be established without the contribution of local Nd sources characterised by radiogenic ε<sub>Nd</sub> values (Montagna et al., 2022; Tachikawa et al., 2004). In order to investigate the influence of the benthic Nd flux to the modern Mediterranean Sea at different water depths, we examine the relationship between seawater ε<sub>Nd</sub> and detrital isotopic signals at surface (0 – 200 m) and deeper water depths (> 1500 m) using a compilation of available seawater ε<sub>Nd</sub> values (Table S4). Since the detrital ε<sub>Nd</sub> data are not available at exactly the same locations as the seawater ε<sub>Nd</sub> values, the data at the closest locations are selected. In addition to this approximation, we are aware that detrital ε<sub>Nd</sub> values may not accurately represent the local Nd source because the detrital fraction is isotopically heterogeneous and it consists of different minerals with variable reactivity (Blaser et al., 2016; Du et al., 2016; Wilson et al., 2013). Given these limitations, we focus only on the degree of correlation between the seawater Nd isotopic composition and the detrital values, rather than absolute detrital ε<sub>Nd</sub> values.

The seawater ε<sub>Nd</sub> values show a broad positive linear correlation with detrital isotopic composition in the present-day Mediterranean (r=0.57, n=225, p-value <<0.05: Fig. 7a). This correlation is mainly driven by the data from the surface layer. In the surface water (0–200 m), the seawater ε<sub>Nd</sub> values correlate well with detrital Nd isotopic composition with r of 0.74 (n=94), whereas in the deep water (> 1500 m) r = 0.25 (n=34) with p-value of 0.3, showing the correlation is not significant (Fig. 7b and c). The strong detrital
influence in the surface layer in the Mediterranean Sea suggests that the detrital Nd can be released in the surface layer (0–200 m), which is consistent with the higher dissolved Nd concentration in the surface layer in the Mediterranean Sea than at deeper depths (Garcia-Solsona and Jeandel, 2020; Garcia-Solsona et al., 2020; Greaves et al., 1991; Henry et al., 1994; Montagna et al., 2022; Spivack and Wasserburg, 1988; Tachikawa et al., 2004; Vance et al., 2004). Compiled dissolved Nd concentration data show values of 25 to 27 pmol/kg for the surface WMS and EMS whereas the deep-water concentrations are 21 to 24 pmol/kg (Table S5). The Nd concentration in the surface Mediterranean Sea is comparable to that in the Gulf of Cadiz (26 pmol/kg, Table S5) but is clearly higher than the concentration of less than 15 pmol/kg at around 35°N in the Northeastern Atlantic (Piepgras and Wasserburg, 1983, 1987; Rickli et al., 2010). The possible Nd sources in the surface Mediterranean Sea are estimated to be local and will be discussed in the next section.

Our result thus argues against a ubiquitous occurrence of benthic Nd fluxes determining bottom water ε<sub>Nd</sub> values in the present-day Mediterranean Sea. The labile and reactive detrital Nd is released at rather shallower water depths, in spatially restricted areas - mainly in the easternmost Levantine Sea and the Aegean Sea - and the Nd from local sources is transported laterally by oceanic currents and vertically by deep water formation and reversible scavenging. This hypothesis is consistent with the observation that the highest seawater ε<sub>Nd</sub> values are found in the shallow waters, mainly in the upper 500 m in the easternmost Levantine Sea (Fig. 8), rather than at greater water depths (Fig. 2c). These observations suggest that once water masses are characterised by Nd isotopic composition, the isotopic composition of seawater evolves substantially with water mass mixing. The
contribution of Nd from the reactive detrital fraction is an important local source, but its
direct influence is estimated to be spatially limited.

**4.3 Major factors controlling the ε<sub>Nd</sub> distribution of modern Mediterranean Sea water**

Based on the above findings, we assume that the radiogenic local source to the EMS
is partially dissolved smectite in relation to the Nile River sediment load. The Nd isotopic
composition of the surface Mediterranean Sea is influenced by additional local Nd sources,
as demonstrated by the strong correlation between the seawater and detrital isotopic signals
(Fig. 7b) as well as the highly radiogenic ε<sub>Nd</sub> values in the easternmost basin (Fig. 8). These
local sources are considered to be fluxes from sediments, thus benthic flux, even though
they originate from shallow coastal sediments. We treat this source separately from the
contribution of dust (Table S5).

In the following, we quantify the benthic flux in the Mediterranean Sea and compare
it with previous estimates in the Mediterranean Sea using the compilation of Nd (Table
S5). Then, to identify the main factors, we assess the sensitivity of the distribution of
Mediterranean water ε<sub>Nd</sub> to the Atlantic inflow, the formation of deep and intermediate
waters, the water circulation, the dust inputs and the benthic flux.

A 6-box model is used without any changes to the box configuration and water fluxes
(Fig. 9) as initially calibrated by Powley et al. (2016). Since the spatial coverage of
available dissolved seawater Nd concentration, [Nd], and ε<sub>Nd</sub> data is heterogeneous, we
calculate the mean values of [Nd] and ε<sub>Nd</sub> for the 20 boxes in the model proposed by
Bethoux and Gentili (1999). Then, we estimate the values of [Nd] and ε<sub>Nd</sub> for the 6 boxes
using volume-weighted averages (see Table S5). The Mediterranean Sea is assumed to be
in a steady state. The considered Nd influx corresponds to the Atlantic inflow, the dissolution of Saharan dust inputs, and benthic fluxes from shallow water depths, whereas the efflux corresponds to the Mediterranean outflow and the burial of Nd in the deep boxes. The advective Nd fluxes are calculated by multiplying the water flux by the [Nd] values shown in Table S5. The influx of mineral dust is based on Bibi et al. (2020). The net Nd fluxes from Saharan dust are estimated by assuming dust Nd concentrations of 27.5 and 31 µg/g for the WMS and EMS, respectively (Ayache et al., 2023, and references therein), and a dust dissolution rate of 3% (Greaves et al., 1994; Roy-Barman et al., 2021). The Nd isotopic compositions of the dust are -12.5 and -11 for the WMS and EMS, respectively (Ayache et al., 2023, and references therein).

Particulate Nd fluxes represent scavenging, and scavenged Nd is assumed to be totally remineralised in deeper boxes (Tachikawa et al., 2003) (Fig. 9). These fluxes are calculated as the difference between the influx and the efflux. The scavenging coefficients correspond to the particulate Nd flux divided by the [Nd] value in the box in which the particulate Nd flux is generated (Table S5). The benthic Nd flux is determined by the mass balance calculations for each box. If the Nd and $\varepsilon_{Nd}$ values estimated for a given box by a mass balance calculation are consistent with the observations (see Table S5), no additional local sources are considered. For box 1 (surface WMS), the particulate Nd flux is evaluated using a labile particulate Nd concentration of 1 pmol/kg of seawater, based on the analysis of filtered suspension in the Alboran Sea and a settling speed of 75 m/yr (Tachikawa et al., 2004). The benthic Nd flux of box 1 is then quantified using a mass balance calculation.

Since the apparent influence of the local detrital Nd influence is visible at water depths shallower than 500 m (Figs 7 and 8), we consider its contribution to the surface WMS (box
1), the surface EMS (box 4) and the intermediate EMS (box 5) (Fig. 9). Interestingly, the $ε_{Nd}$ value of the deep EMS (box 6) can be explained without any contribution from the benthic flux. This is likely because the most labile and reactive detrital Nd is lost upon contact with surface seawater and in shallow seafloor sediments before reaching abyssal depths, or because the flux of labile and reactive detrital Nd is low at sites of abyssal depths due to the greater distance from the source region. The benthic source in the EMS (boxes 4 and 5) is assumed to be Nile River sediments, for which the $ε_{Nd}$ value is -2 (Tachikawa et al., 2004). All information on the parameters used for the 6-box model is summarised in Table S5.

The total local source flux (benthic flux) obtained for the whole Mediterranean Sea is $4.9×10^5$ mol/yr (Table S5), which is in the same order of magnitude as previous estimates of $2.7×10^5$ mol/yr (Tachikawa et al., 2004) and $6.2×10^5$ mol/yr (Ayache et al., 2023). If the obtained flux is divided by the surface area of the Mediterranean seafloor at depths shallower than 500 m, which is about 500,000 km² according to EMODnet, the benthic flux per unit surface area is estimated at 9 pmol/cm²/yr. This is within the estimated range for the Northeast Pacific of 3–32 pmol/cm²/yr (Abbott et al., 2015). It is important to note that the calculated flux represents the average for the entire Mediterranean Sea. The flux from the easternmost basin should exceed the average, but we do not estimate it due to uncertainty regarding the exact area of the seafloor releasing Nd. The benthic flux to the surface EMS is approximately one order of magnitude greater than that to the surface WMS (Table S5). The estimated $ε_{Nd}$ value of the local source to the surface WMS is -9.1, suggesting that the source may be margin sediments (Ayache et al., 2016). The benthic flux is of a similar magnitude to atmospheric inputs in the WMS, whereas it is around ten times
higher than net dust inputs in the EMS (Table S5). In the EMS, the size of the benthic flux
is comparable with scavenging fluxes, but smaller than some of the advection fluxes
indicated by the thick lines in Fig. 9 (FG1, F14, F45 and F52). Therefore, there is a strong
benthic flux in the EMS, whereas the contribution to the WMS is small. This may be
because the sediment is more reactive in the EMS given that the residence time of water
masses is comparable in the current EMS and WMS (see Table S5 and Fig. 9).

Once the Nd model for the Mediterranean Sea has been calibrated, a fractional factorial
design analysis is performed to determine the parameters (variables) with the greatest
impact on seawater ε<sub>Nd</sub> values. The model variables considered are shown in 'variables
tested' (Fig. 10): scavenging coefficients (1–6); local benthic fluxes (7–9); the ε<sub>Nd</sub> value of
the local benthic fluxes (10–12); Saharan dust (13–14); the ε<sub>Nd</sub> value of the dust (15–16);
the ε<sub>Nd</sub> value of the Atlantic inflow (17) and deep water formation zones (18–19); the
Atlantic inflow (20); the rate of deep water formation (21–22), and water fluxes in the
Mediterranean Sea (23–38). There are thirty-eight variables. As the number of possible
combinations of these variables is enormous, we test only a fraction of the combinations
by focusing on the first-order interactions using a two-level Plackett-Burmann factorial
design (Plackett and Burman, 1946). With this technique, the number of runs (combination
of the variables) should be greater than the number of variables and be a multiple of 4. The
relative influence of the variables is examined by modifying their values by ± 10% from
the initial values for the parameters associated with Nd fluxes, and by ± 0.5 ε-unit for Nd
isotopic composition. This is an arbitrary choice, but it considers analytical uncertainty and
possible natural variation. For each run the dissolved Nd concentration and isotopic
composition of the 6 boxes are calculated using the box model (Fig. 9) until a steady state
is reached. A normal probability plot of the standardised effects is then used to determine
the magnitude, direction and relative importance of the effects (Fig. 10). This is a t-statistic
test that examines the null hypothesis. When the standardised effect (x-axis of Fig. 10) is
zero, the influence of a given variable is insignificant. When the effect is negative
(positive), the seawater ε<sub>Nd</sub> value decreases (increases) with an increase of the variable. It
is important to note that the absolute values of the standardised effects have no precise
meaning as they depend on the number of runs and the number of the variables tested.

The results reveal that the Nd isotopic composition of the Atlantic inflow has a strong
impact on not only the surface waters, but also the intermediate and deep waters in the
WMS and EMS (Fig. 10). Although the surface boxes receive a flux from the local Nd
sources and the Saharan dust, the influence of the Atlantic inflow remains dominant. This
reflects a large advective Nd flux from the North Atlantic as well as a substantial difference
in its ε<sub>Nd</sub> value compared to that of the Mediterranean water masses. For the deep boxes
.box 3 at > 600 m water depth in the WMS and box 6 at > 500 m water depth in the EMS:
(Fig. 9), the influence of the ε<sub>Nd</sub> value of the deep-water formation zone prevails (Fig. 10).
The local Nd sources and the zonal water fluxes (F14: Figs. 9 and 10) also have a strong
influence. It is important to note that the physical processes producing changes of ± 10%
in Nd fluxes and of ± 0.5 ε-units are not comparable. Therefore, the results of flux and
isotopic changes should not be compared directly in this statistical analysis, and the
absolute values of the normalised effect are not meaningful.

Despite this difficulty, we provide the relative sensitivity of the ε<sub>Nd</sub> values of the
Mediterranean waters to different processes. Although the influence of the local source
(benthic flux) is significant, changes in the Atlantic inflow, the rate of deep water formation
and zonal water exchanges can still be traced. Once the isotopic composition of a water mass has been modified, it essentially evolves through the mixing of the water masses. This explains why changes in the influence of benthic flux in the EMS are visible in the intermediate depths in the WMS (Fig. 10).

This test shows that there is a significant impact of local benthic sources (Nile River particles) on the intermediate and deep water masses in the EMS, which is consistent with higher authigenic ε<sub>Nd</sub> values in the eastern basin during Holocene sapropel formation (10.5 to 6.1 thousand calibrated years BP; Grant et al., 2016) compared to present-day values. The authigenic ε<sub>Nd</sub> values of the intermediate and deep water masses in the EMS increased during this period, whereas no such increase occurred in the western basin (Colin et al., 2021; Cornuault et al., 2018; Dubois-Dauphin et al., 2016, 2017; Duhamel et al., 2020; Jiménez-Espejo et al., 2015; Osborne et al., 2008, 2010; Scrivner et al., 2004; Trias-Navarro et al., 2023; Wu et al., 2019). This zonal contrast differs from the present-day situation. It can be explained by reduced zonal water advection, which results in less transport of the radiogenic signal from east to west (Cornuault et al., 2018). This is due to the decline in intermediate/deep water formation caused by strong water column stratification in the EMS (Rohling et al., 2015). The deposition of sapropel layers occurred under oxygen-depleted conditions, necessitating careful interpretation. Sedimentary oxides carrying the seawater ε<sub>Nd</sub> signal would dissolve in such conditions (Skinner et al., 2019), and the contribution of pore water Nd would mask the initial seawater signals. To robustly interpret the authigenic Nd isotopic composition, we strongly recommend examining the spatial variability of the ε<sub>Nd</sub> values by combining records from different sub-basins and using a modelling approach for the comparison. This study supports the usefulness of Nd
isotopes as a proxy for water mass provenance in the Mediterranean Sea although caution
should be exercised.

**5. Conclusions**

In order to assess the reliability of authigenic Nd isotopic composition as a tracer of water mass provenance and the influence of abyssal benthic Nd sources on the seawater and authigenic εNd signatures, we determined paired authigenic and detrital isotopic compositions from core-top sediments along a zonal transect of the Mediterranean Sea. Our major findings are as follows.

The authigenic εNd values generally agreed with the bottom water signals, except in
the easternmost Levantine and the Aegean Seas, where they were higher than the water
values.

The relationship between authigenic and detrital εNd values presents a positive
correlation, but the correlation is likely to be artificially emphasised by the radiogenic
isotopic composition of the authigenic fraction obtained in the easternmost Levantine and
the Aegean Seas. A closer look reveals that there is no correlation between authigenic and
detrital εNd values in the easternmost Levantine and the Aegean Seas, as the authigenic εNd
values were stable at about -5 to -4 while the corresponding detrital values ranged between
-10 and -4.

The compilation of modern seawater and detrital εNd values from the Mediterranean
Sea shows that seawater εNd covaries with detrital signals at 0–200 m depths, and that the
correlation disappears at depths greater than 1500 m. These results argue against a
ubiquitous control of bottom water ε<sub>Nd</sub> by the benthic flux in the present-day Mediterranean Sea.

Sensitivity tests suggest that the ε<sub>Nd</sub> values of Mediterranean Sea water are sensitive to Atlantic inflow at all depths, while local benthic fluxes, the isotopic composition of formation zones and zonal water fluxes, and the rate of deep-water production also have an important influence. Although the influence of local sources is significant, these results support the usefulness of Nd isotopic composition as a tracer of water mass provenance in the Mediterranean Sea.

Data availability

All the data presented in this study are available in S1–S5 in the Supporting Information and through Zenodo at https://zenodo.org/records/16934349.

Acknowledgements

This work is supported by the French national project ANR MedSens (ANR-19-CE01-0019) to KT. Marta Garcia is thanked for her help with the laboratory work. Most of the study samples were collected during the GEOTRACES-A04N cruise and we thank the GEOTRACES programme. Radiocarbon dating was performed at the LMC14 laboratory as a part of the French ARTEMIS programme. We would like to express our deepest
gratitude to David Wilson and the two anonymous reviewers for their insightful comments, which have greatly improved this manuscript.

CRediT authorship contribution statement

KT designed the study; FB, MC, KT and AG performed the laboratory procedures; VG performed the fractional factorial design analysis; KT wrote the manuscript with contributions from all authors.

Appendix A. Supplementary material includes five data tables (S1-S5) presenting the numerical data of this study, the compilation of available data and the parameters used for the box model calculation.

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## Figure captions

**Fig. 1.** Location of the modern sediments studied in this work. (a) Map showing the location and the names of cores investigated in this study (yellow stars), the location where existing authigenic ε$_{Nd}$ values are available (yellow circles: Colin et al., 2021; Cornuault et al., 2018; Dubois-Dauphin et al., 2016; Dubois-Dauphin et al., 2017b; Duhamel et al., 2020; Gao et al., 2025; Jiménez-Espejo et al., 2015; Scrivner et al., 2004; van de Flierdt et al., 2010; Vance et al., 2004) together with the seawater stations of previous publications (small blue dots: Dubois-Dauphin et al., 2017a; Garcia-Solsona and Jeandel, 2020; Garcia-Solsona et al., 2020; Henry et al., 1994; Montagna et al., 2022; Spivack and Wasserburg, 1988; Tachikawa et al., 2004; Vance et al., 2004). The white arrows pointing downwards represent the zones of deep-water formation, while the white ellipse represents the zone of intermediate water formation. (b) Map showing the subdivisions and straits referred to in the main text, with the position of the zonal transect (red line) indicated in c). (c) A salinity zonal transect showing the major water masses and location of the sediments (yellow stars). White arrows indicate zonal circulation patterns. Salinity data are from WOA23 (Reagan et al., 2024). The figures were produced using the Ocean Data View software (Schlitzer, 2025).
**Fig. 2.** Spatial variability of authigenic and detrital ε$_{Nd}$ obtained in this study. (a) authigenic ε$_{Nd}$ as a function of longitude. Grey, small symbols indicate data from previous studies, with squares representing coral, stars representing foraminiferal tests, and circles representing leachates. (b) detrital ε$_{Nd}$ as a function of longitude, (c) seawater ε$_{Nd}$ as a function of longitude with the colour code indicating water depth and (d) detrital ε$_{Nd}$ obtained at sites located between 18 and 23°E as a function of latitude. The vertical dashed lines in panels (a), (b) and (d) indicate the locations of the sample stations shown on the upper x-axis. Some core names are shown in the figure to facilitate identification. Note that the y-axis range differs between the authigenic-seawater panels (a and c) and the detrital panels (b and d), which allows the detailed features of the Nd isotopic composition to be presented more clearly. The data shown in this figure are presented in Table S2. This figure was produced using Ocean Data View software (Schlitzer, 2025).

**Fig. 3.** Comparison between authigenic ε$_{Nd}$ and seawater ε$_{Nd}$ at sites closest to sediment locations within the same water mass in the Mediterranean Sea. The colour code indicates the longitude. The results of this study are shown with closed symbols whereas previously obtained data are shown with open symbols. The authigenic ε$_{Nd}$ values obtained from leachates, foraminiferal tests and corals are marked with circles, stars and squares, respectively. Regarding the leachates, only the values obtained without decarbonation are considered. All data used for this figure are presented in Table S2. The figure was produced using the Ocean Data View software (Schlitzer, 2025).

**Fig. 4.** Elemental composition of the selected leachate samples. (a) Authigenic ε$_{Nd}$ vs. Sr/Ca to assess the contribution of detrital carbonate, (b) Mn/Fe and Fe/Al as a function of
longitude to evaluate the origin of Mn and Fe in the leachates, (c) PAAS normalised REE patterns, and (d) PAAS normalized HREE/LREE and MREE enrichment (MREE/MREE*) compared with detrital array proposed by Du et al. (2016). The blue arrow along the right y-axis (b) indicates the Mn/Fe range of non-phosphatised hydrogenetic Fe-Mn crusts (0.9-2.8 g/g) (Bau et al., 1996). PAAS REE concentrations are taken from (Taylor and McLennan, 1985). The HREE/LREE ratio and the MREE/MREE* values were calculated using the equations proposed by Martin et al. (2010). $HREE/LREE = (Tm + Yb + Lu)/(La + Pr + Nd)$. $MREE/MREE* = (Gd + Tb + Dy)/[(La + Pr + Nd + Tm + Yb + Lu)/2]$.

**Fig. 5.** Detrital $\varepsilon_{Nd}$ values. (a) New detrital $\varepsilon_{Nd}$ data from this study (colour symbols) and previously published detrital values (background colour and white dots indicating the sites: Blanchet, 2019 and references therein; Trias-Navarro et al., 2023). White crosses indicate sites of previously published $\varepsilon_{Nd}$ values discussed in the text. (b) Comparison between detrital and authigenic $\varepsilon_{Nd}$ values obtained from the same sites. Closed symbols indicate the results of this study, while open symbols indicate previously published data. The black line shows a 1:1 relationship to facilitate comparison. An ellipse marks data from the Levantine and Aegean Seas (Station 24a, ODP967, MD04-2722, MS27PT, and stations 28a and 32a) where the authigenic $\varepsilon_{Nd}$ values are high (-5 to -4) while the detrital values range from -10 to -4. See the text for further details. All data used for Figure 5b are presented in Table S2. The figures were generated using the Ocean Data View software (Schlitzer, 2025).
**Fig. 6.** Maps showing the nature of core-top sediments in the Eastern Mediterranean Sea. (a) Smectite proportion in the clay fraction, and (b) calcium carbonate proportion in the bulk sediments. Data are taken from (Hamann et al., 2009; Venkatarathnam and Ryan, 1971).

**Fig. 7.** Comparison between seawater and detrital $\varepsilon_{Nd}$ values at the locations closest to the water sites. The black lines show a 1:1 relationship to facilitate comparison. (a) The whole water column with a colour code indicating the longitudinal positions of the seawater sites, (b) the surface layer (0–200 m) and (c) the deep water (≥ 1500 m). For (b) and (c) the colour code indicates the water depths of the seawater data. All data used for these figures are presented in Table S4. The figures were generated using the Ocean Data View software (Schlitzer, 2025).

**Fig. 8.** Seawater $\varepsilon_{Nd}$ as a function of longitude from the whole water depths. This figure shows all available seawater data, which differs from Fig. 2c. The figure was created using the Ocean Data View software (Schlitzer, 2025).

**Fig. 9.** 6-box model of the Nd cycle in the Mediterranean Sea. Box configuration and the water fluxes between the reservoirs are from Powley et al. (2016). Blue solid arrows indicate water fluxes from box i to box j as $F_{ij}$ in Sv (10$^6$ m$^3$/s) and, “G” indicates “Gibraltar”. The Nd cycle was determined using the compilation of existing data, and numbers in black inside the boxes indicate seawater $\varepsilon_{Nd}$ values (Table S5). Blue dashed
arrows show particulate Nd fluxes (Fp), whereas brown and black dashed arrows present dust Nd fluxes and local Nd sources, respectively.

**Fig. 10.** Factorial design analyses showing the effects of different variables on the seawater ε<sub>Nd</sub> values of the 6 boxes (Fig. 9). Red lines indicate the null hypothesis 'the variable has no effect on the model response'. The black variables are associated with Nd fluxes, while the blue ones correspond to the Nd isotopic composition. The x-axis shows the standardised effects of the different variables on the ε<sub>Nd</sub> values of seawater in the 6 boxes. Variables identified in the figures are statistically significant and discussed in the text for their dominant influence.

<table><caption>Table 1. Core-top materials used in this study</caption><thead><tr><th>Regions</th><th>Station/core</th><th>Depth in core (cm)</th><th>Water depth (m)</th><th>Longitude (E)</th><th>Latitude (N)</th><th>Age (kyr)</th><th>Reference age model</th></tr></thead><tbody><tr><td rowspan="2">Alboran Sea</td><td>6a (GEOTRACES)</td><td>0_2</td><td>1594</td><td>-3.22</td><td>36.04</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>ODP Hole 161-977A</td><td>8_10</td><td>1984</td><td>-1.96</td><td>36.03</td><td>0.38</td><td>Martrat et al. (2004)</td></tr><tr><td rowspan="3">Algero-Provencal Basin</td><td>19c (GEOTRACES)</td><td>0_2</td><td>2631</td><td>4.28</td><td>38.35</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>18c (GEOTRACES)</td><td>0_2</td><td>2799</td><td>5.25</td><td>39.23</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>17c (GEOTRACES)</td><td>0_2</td><td>2820</td><td>5.93</td><td>40.07</td><td>Water-sediment interface</td><td>Not applicable</td></tr></tbody></table>
<table><tr><td></td><td>16c<br>(GEOTRACES)</td><td>0_2</td><td>2791</td><td>6.93</td><td>41.13</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td rowspan="2">Tyrrhenian Sea</td><td>13a<br>(GEOTRACES)</td><td>0_2</td><td>594</td><td>10.88</td><td>38.08</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>12c<br>(GEOTRACES)</td><td>0_2</td><td>3345</td><td>14.50</td><td>39.00</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td rowspan="3">Adriatic Sea</td><td>8c<br>(GEOTRACES)</td><td>4_6</td><td>860</td><td>18.54</td><td>40.96</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>9c<br>(GEOTRACES)</td><td>0_2</td><td>787</td><td>18.82</td><td>40.15</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>7c<br>(GEOTRACES)</td><td>2_4</td><td>930</td><td>19.10</td><td>40.20</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td rowspan="6">Ionian Sea</td><td>10c<br>(GEOTRACES)</td><td>0_2</td><td>2562</td><td>17.98</td><td>38.27</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>17a<br>(GEOTRACES)</td><td>0_2</td><td>3686</td><td>18.03</td><td>34.95</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>SL95</td><td>0_1</td><td>1390</td><td>19.19</td><td>32.77</td><td>0.82</td><td>This study</td></tr><tr><td>6c<br>(GEOTRACES)</td><td>0_2</td><td>1645</td><td>19.74</td><td>38.99</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>5c<br>(GEOTRACES)</td><td>2_4</td><td>3675</td><td>20.16</td><td>37.38</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>4c<br>(GEOTRACES)</td><td>2_4</td><td>3000</td><td>20.55</td><td>35.85</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td></td><td>2c<br>(GEOTRACES)</td><td>4_6</td><td>4236</td><td>22.69</td><td>35.46</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td></td><td>19a<br>(GEOTRACES)</td><td>0_2</td><td>1548</td><td>22.79</td><td>33.66</td><td>Water-sediment interface</td><td>Not applicable</td></tr></table>
<table><tbody><tr><td rowspan="3">Aegean Sea</td><td>1c<br>(GEOTRACES)</td><td>4_6</td><td>921</td><td>24.92</td><td>35.64</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>32a<br>(GEOTRACES)</td><td>0_2</td><td>88</td><td>25.63</td><td>39.77</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>28a<br>(GEOTRACES)</td><td>0_2</td><td>1090</td><td>26.63</td><td>35.28</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td rowspan="2">Levantine Sea</td><td>24a<br>(GEOTRACES)</td><td>0_2</td><td>2396</td><td>31.94</td><td>34.22</td><td>Water-sediment interface</td><td>Not applicable</td></tr><tr><td>MD04-2722</td><td>0_1</td><td>1780</td><td>33.50</td><td>33.10</td><td>3.61</td><td>Cornuault et al. (2018)</td></tr></tbody></table>

**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.
<figure><img src="image_3.png" /><figcaption>Fig. 1</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5</figcaption></figure>

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

<figure><img src="image_10.png" /><figcaption>Fig. 6</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 7</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 7 continued</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 8</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 9</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 10</figcaption></figure>