domain: chim.mate chim phys

text:
Fabien Szczepanski

Alexandre Bayart

Arturas Katelnikovas

Jean-François Blach

Jolanta Rousseau

Sébastien Saitzek
email: sebastien.saitzek@univ-artois.fr

Pr Sébastien Saitzek

Luminescence and up-conversion properties in La2Ti2O7:Eu3+,Er3+ oxides under UV and NIR radiations towards a two-color sensor

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

During the last decades, inorganic or hybrid materials with luminescence properties have been widely studied and are currently used in many devices of our daily life [START_REF] Kitai | Principles of Luminescence[END_REF], [START_REF] Edgar | Luminescent Materials[END_REF]. Among these compounds, inorganic materials based on rare earth ions contained in a hostmatrixes such as oxides, silicates, aluminates, aluminosilicates, aluminoborates, molybdates, tungstates, etc., are those the most studied by the scientific community [START_REF] Blasse | Luminescent Materials[END_REF]. Downconversion (emission in the visible range under a UV light excitation) and upconversion (emission in the visible range under a NIR excitation / absorption of two or more photons) luminescence properties can find many application fields such as fluorescent lamps [START_REF] Lin | Inorganic Phosphor Materials for Lighting[END_REF], radiation sensors [START_REF] Lee | Measurement of Gamma-Rays Induced Luminescence Generated in a Sapphire Based Fiber-Optic Radiation Sensor[END_REF], optical temperature sensors [START_REF] Wang | Luminescent probes and sensors for temperature[END_REF], phosphors for LED industry [START_REF] Mills | Phosphors development for LED lighting[END_REF], anti-counterfeiting ink [START_REF] Kumar | Future prospects of luminescent nanomaterial based security inks: from synthesis to anti-counterfeiting applications[END_REF], incoherent laser light source, display technologies [START_REF] Zhong | Luminescent Materials for 3D Display Technology[END_REF], biological labeling [START_REF] Wang | Upconversion nanoparticles in biological labeling, imaging, and therapy[END_REF] and many others [START_REF] Feldmann | Inorganic Luminescent Materials: 100 Years of Research and Application[END_REF]. Moreover, the upconverting materials are widely used in biological applications [START_REF] Gulzar | Upconversion processes: versatile biological applications and biosafety[END_REF] because their excitation wavelength locates within the NIR optical window of biological tissues and it can thus reach the maximum penetration [START_REF] Chen | Near-infrared deep brain stimulation via upconversion nanoparticle-mediated optogenetics[END_REF]. As such, the upconversion materials are used in the photodynamic therapy [START_REF] Qiu | Recent Progress in Upconversion Photodynamic Therapy[END_REF] or the photoactivated drug delivery systems [START_REF] Yang | Photoactivated drug delivery and bioimaging[END_REF].

Intrinsically, materials that can emit different colors under different types of excitation (UV or NIR) can be particularly attractive for designing multifunctional sensors, anticounterfeiting inks, multi-color fluorescence imaging or for the biomedical field. Recently, it has been shown that La2Ti2O7 oxides can serve as a host matrix for the design of new phosphors with interesting optical properties. Indeed, La2Ti2O7:Eu 3+ oxides show an intense red emission under UV excitation with a quantum yield of 42% [START_REF] Bayart | Synthesis, structural and luminescence properties of (La1-xLnx)2Ti2O7 (Ln=lanthanides) solid solutions[END_REF]. In addition, we have shown that La1.9Er0.1Ti2O7 oxide exhibits upconversion luminescence if excited over a wide NIR excitation wavelength range from 785 to 980 nm [START_REF] Bayart | Upconversion luminescence properties and thermal quenching mechanisms in the layered perovskite La1.9Er0.1Ti2O7 towards an application as optical temperature sensor[END_REF]. In this work, we investigated the possibility to synthesize multifunctional oxides, from a La2Ti2O7 host-matrix co-substituted by Eu 3+ /Er 3+ , which can respond differently according to the excitation wavelength (UV or NIR). In the literature and to the best of our knowledge, studies on co-substituted or co-doped La2Ti2O7 are scarce. The Yb 3+ /Er 3+ system was studied to improve the upconversion properties [START_REF] Tabanli | Solid State Synthesis, Structural and Up-Conversion Properties of Yb 3+ ∕Er 3+ and Yb 3+ ∕Tm 3+ ∕Er 3+ Doped La2Ti2O7 Phosphors[END_REF] and the Pr 3+ /Tb 3+ system was considered in order to get a control of the emission color [START_REF] Zhang | Color tuning and white light emission in a single-phased La2Ti2O7:Pr 3+ , Tb 3+ phosphor[END_REF].

La2Ti2O7 is a multifunctional oxide already known for these features: i) piezoelectric and ferroelectric properties with high Curie temperature (Tc ∼ 1500 °C) [START_REF] Yan | Piezoelectric Ceramics with Super-High Curie Points[END_REF], [START_REF] Nanamatsu | A new ferroelectric: La2Ti2O7[END_REF]; ii) photocatalytic properties, especially used as photocatalyst for H2 production by water splitting [START_REF] Song | Hydrothermal synthesis of flaky crystallized La2Ti2O7 for producing hydrogen from photocatalytic water splitting[END_REF], [START_REF] Onozuka | Perovskite-type La2Ti2O7 mesoporous photocatalyst[END_REF] or for Volatile Organic Compounds (VOCs) degradation [START_REF] Hou | Synthesis and characterization of La2Ti2O7 employed for photocatalytic degradation of reactive red 22 dyestuff in aqueous solution[END_REF], [START_REF] Hwang | Photocatalytic Degradation of CH3Cl over a Nickel-Loaded Layered Perovskite[END_REF]; iii) luminescence properties when doped or co-doped with other lanthanide ions, such as Pr 3+ , Sm 3+ , Eu 3+ , Er 3+ , Tb 3+ [START_REF] Bayart | Synthesis, structural and luminescence properties of (La1-xLnx)2Ti2O7 (Ln=lanthanides) solid solutions[END_REF], [START_REF] Li | Synthesis and luminescence properties of La2Ti2O7:Er 3+ nanocrystals with pyrochlore structure[END_REF], [START_REF] Li | Synthesis and Photoluminescence Properties of La2Ti2O7:Pr 3+ /Tb 3+ Phosphors[END_REF], [START_REF] Shao | Structural Characterization and Photoluminescent Properties of (La1-xSmx)2Ti2O7 Solid Solutions Synthesized by a Sol-Gel Route[END_REF], [START_REF] Diallo | Anti-Stokes luminescence and site selectivity in La2Ti2O7:Pr 3+[END_REF], [START_REF] Zhang | Color tuning and white light emission in a single-phased La2Ti2O7:Pr 3+ , Tb 3+ phosphor[END_REF], [START_REF] Tabanli | Solid State Synthesis, Structural and Up-Conversion Properties of Yb 3+ ∕Er 3+ and Yb 3+ ∕Tm 3+ ∕Er 3+ Doped La2Ti2O7 Phosphors[END_REF]; iv) its good resistance to ionizing radiation and its use as an immobilization matrix of radionuclides for the nuclear industry [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. In the literature, there are several studies that describe techniques for producing La2Ti2O7 oxides. Among them, we can mention the solid state reaction, the sol-gel route [START_REF] Shao | Structural and dielectric/ferroelectric properties of (La1-xNdx)2Ti2O7 synthesized by sol-gel route[END_REF], [START_REF] Prasadarao | Sol-gel synthesis of Ln2(Ln = La, Nd)Ti2O7[END_REF], [START_REF] Atuchin | Electronic structure of layered ferroelectric high-k titanate La2Ti2O7[END_REF], hydrothermal method [START_REF] Song | Hydrothermal synthesis of flaky crystallized La2Ti2O7 for producing hydrogen from photocatalytic water splitting[END_REF], floating zone method for single crystals [START_REF] Nanamatsu | A new ferroelectric: La2Ti2O7[END_REF] and many others [START_REF] Todorovsky | Deposition and characterization of La2Ti2O7 thin films via spray pyrolysis process[END_REF], [START_REF] Suresh | Mixed Hydroxide Precursors for La2Ti2O7 and Nd2Ti2O7 by Homogeneous Precipitation[END_REF], [START_REF] Fuierer | La2Ti2O7 Ceramics[END_REF]. La2Ti2O7 crystallizes in monoclinic/layered perovskite structure with P21 space group (#4). The unit cell consists of four corner-sharing TiO6 octahedra separated by two layers of La-site cations along -axis and infinite chains of corner-sharing TiO6 octahedra along -and -axis. The stability of the monoclinic/layered perovskite structure can be affected during the substitution of La-site by another lanthanide. Indeed, the monoclinic structure is conserved for ionic ratio r(Ln 3+ )/r(Ti 4+ ) higher than 1.78 (i.e., Ln 3+ = La 3+ to Nd 3+ ). While for r(Ln 3+ )/r(Ti 4+ )=1.46 to 1.78 (i.e. Ln 3+ =Sm 3+ to Lu 3+ ), a cubic/pyrochlore structure with Fd m space group (#227) is favored [START_REF] Shao | Stability limit of the layered-perovskite structure in Ln2Ti2O7 (Ln = lanthanide) thin films grown on (110)-oriented SrTiO3 substrates by the sol-gel route[END_REF], [START_REF] Saitzek | Microstructure and Nanoscale Piezoelectric/Ferroelectric Properties in Ln2Ti2O7 (Ln= Lanthanide) Thin Films with Layered Perovskite Structure[END_REF].

In this work, we have synthesized a series of co-substituted La2-x-yEuxEryTi2O7 oxides in order to determine the stability limit of the monoclinic/layered perovskite structure and study the luminescence properties of these oxides under UV and NIR excitation. For that, the classical luminescence and upconversion measurements (excitation and emission spectra, photoluminescence lifetime, quantum efficiency, thermal quenching mechanisms, and CIE 1931 chromaticity coordinates) were performed.

Experimental Section

La2-x-yEuxEryTi2O7 oxides were synthesized by a conventional solid state reaction method. The lanthanide oxides (La2O3, Eu2O3 and Er2O3 -99.99% Strem Chemicals) and titanium dioxide (TiO2 -99.99% Sigma Aldrich) were used as starting materials. Before the synthesis, the lanthanide oxides were annealed at 800 °C for 5 h in order to avoid the lanthanum hydroxide formation which could disturb the good stoichiometry of the final product. The precursors weighed in stoichiometric proportions were mixed for 20 min with a few drops of ethanol and then calcined at 1100 °C for 12 h under air. The calcination and milling steps were repeated twice to improve the sample homogeneity.

The X-Ray Diffraction (XRD) measurements were performed using a Rigaku ULTIMA IV diffractometer equipped with Cu anticathode (λ(Kα) = 1.5418 Å), Soller slits to limit the divergence of X-ray beam, and a nickel foil filter to attenuate the Cu Kβ line. XRD patterns were recorded in the range of 10°-90° (scan speed was 0.1 °/min) using the Bragg-Brentano configuration. The Scanning Electron Microscopy (SEM) images were recorded using Hitachi S4700 microscope equipped with a Field Emission Gun (FEG). The optical properties of La2Ti2O7:Eu 3+ , Er 3+ oxides were characterized using a UV-2600 spectrophotometer (Shimadzu). Room-temperature excitation and emission spectra were recorded on an Edinburgh Instruments FLS980 spectrometer equipped with a double grating Czerny-Turner excitation and emission monochromators. For upconversion experiments, two CW (Continuous Wave) lasers emitting at 808 and 980 nm were employed as the excitation sources. The Thermal Quenching (TQ) measurements were performed using a cryostat "MicrostatN" from the Oxford Instruments coupled to the Edinburgh Instruments FLS980 spectrometer. Liquid nitrogen was used as a cooling agent and the measurements were performed at 77 K and at 100-500 K with 50 K steps. The temperature stabilization time was 90 s and the temperature tolerance was set to ±5 K. During the measurements, dried nitrogen was flushed over the cryostat window to avoid the water condensation on the window surface at low temperatures. Finally, the CIE 1931 chromaticity coordinates (Commission Internationale de l'Éclairage -CIE) were calculated from the emission spectra. The color coordinates were calculated in the range of 360-830 nm.

Results and Discussions

Structural and Optical Characterizations

The XRD patterns of La2-x-yEuxEryTi2O7 powders are presented in Figure 1. According to the substitution percentage, the La2Ti2O7 monoclinic structure (ICSD-1950) [START_REF] Gasperin | Dititanate de lanthane[END_REF] is maintained when the Eu 3+ and Er 3+ contents in the structure are lower than 0.20 and 0.10, respectively. Beyond these substitution values, a phase mixture of a substituted monoclinic phase and cubic/pyrochlore phases such as Eu2Ti2O7 (ICSD-83596) [START_REF] Chtoun | Analyse par diffraction des rayons x, méthode de rietveld, de la structure des solutions solides (1-x)A2Ti2O7 -xFe2TiO5 A= Eu, Y[END_REF] and Er2Ti2O7 (ICSD-249070) [START_REF] Lau | Stuffed rare earth pyrochlore solid solutions[END_REF], which are indicated by a star in Figure 1, is obtained. We can note that the presence of a low concentration of Er 3+ (ca. y=0.05) greatly limits the maximum substitution rate of Eu 3+ . This one is limited to x = 0.20, whereas, in a previous study [START_REF] Bayart | Synthesis, structural and luminescence properties of (La1-xLnx)2Ti2O7 (Ln=lanthanides) solid solutions[END_REF], it was shown that it could reach 0.60 without Er 3+ ions. Knowing that, the stability limit of these structures is linked by the value of the Ti 4+ /Ln 3+ ionic ratio as indicated in the introductory part. The observed structural feature can, therefore, be explained by the ionic radii values of the lanthanides (La 3+ VIII (1.160 Å), Eu 3+ VIII (1.066 Å) and Er 3+ VIII (1.004 Å)) [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Indeed, the ionic radius of the Er 3+ ion is lower than that of the Eu 3+ ion which itself is lower than that of the La 3+ ion. Therefore, the average ionic radius weighted by the substitution percentages (as Vegard's law [START_REF] Tauc | Optical properties and electronic structure of amorphous Ge and Si[END_REF]) quickly reaches the stability limit of the monoclinic phase. Thus, in the interval [x ≤ 0.20; y ≤ 0.10], a single phase isostructural with La2Ti2O7 oxide is observed. The substitution is evidenced by the slight displacement of the (hkl) diffraction peaks towards the wide angle, reflecting a decrease of the unit cell volume. According to x and y values, this displacement is clearly observed for (hkl) reflections with h and k ≠ 0 (enlargement shown in Figures 1c and1d), while the position of the (00l) reflection is not affected. This behavior, already observed for mono-substitution, can be explained by the layered-perovskite structure. Indeed, it is formed by an infinite sequence of TiO6 octahedra along -and -axes and sheets consisting of four TiO6 octahedra separated by an Ln-interleaves along the -axis. Thus, the decrease of the ionic radius of the lanthanide ion has a great influence on the infinite chains of octahedra, explaining the evolution of the a and b cell parameters. Consequently, the Lninterleaves placed between sheets are placed in a less dense layer which can be modified to counterbalance the cationic modification and permits to explain the relative stability of the c cell parameter. In addition, Zhang et al. [START_REF] Zhang | Structural change of layered perovskite La2Ti2O7 at high pressures[END_REF] have shown that the La2Ti2O7 had a high compressibility along the -axis. This direction is, therefore, less sensitive to the modification of the ionic radius and also makes it possible to explain the stability of the c cell parameter.

The morphology of La2-x-yEuxEryTi2O7 observed by SEM images are presented in Figure 2. For the low substitution rates (Figure 2a and2b), aggregates consisting of uniform particles of submicron size are observed with homogenous grain-size distribution. When the substitution rates increase, the grain-size distribution becomes more random (Figure 2c and2d) probably reflecting the multiphasic character observed in the XRD experiments. For substantial substitutions of La by Eu and Er elements (in order to avoid the detection limit), the Energy Dispersive X-Ray (EDX) analyzes confirm also a uniform distribution of La, Ti, Eu and Er elements in the matrix (corresponding to the values expected by the stoichiometric calculations).

The optical properties of La2-x-yEuxEryTi2O7 phosphors were characterized by UVvisible diffuse reflectance spectra. The absorption spectra were recorded for all samples. In Figure 3, the typical room temperature absorption spectra for La1.85Eu0.10Er0.05Ti2O7, Eu2Ti2O7 and Er2Ti2O7 oxides are shown. Except Eu2Ti2O7 sample, each transition assignment in the spectra corresponds to the excited level of Er 3+ ions starting from the ground state 4 I15/2. So, the transitions bands centered at 378, 454, 466, 522, 538, 653 and 798 nm correspond to 4 G11/2, 4 F3/2; 5/2, 4 F7/2, 2 H11/2, 4 S3/2, 4 F9/2 and 4 I9/2 of Er 3+ energy levels, respectively [START_REF] Zhou | Effect of Tm-Er concentration ratio on the photoluminescence of Er-Tm:Al2O3 thin films fabricated by pulsed laser deposition[END_REF]. For Eu2Ti2O7 oxide, the intensities of absorption lines, attributable to the excited states of Eu 3+ ions, are very low. In order to identify them, it is necessary to zoom at the base of peak starting at 350 nm (inset of Figure 3a). The broad and intensive absorption band in the range of 200-340 nm occurs due to Eu 3+ -O 2-Charge Transfer (CT) transition. The sets of lines centered at 394, 465, 427 and 589 nm correspond to transitions originating from the 7 F0,1 ground state of Eu 3+ ions to 5 L6, 5 D2, 5 D1 and 5 D0 excited states, respectively [START_REF] Peijzel | A complete 4f{sup n} energy level diagram for all trivalent lanthanide ions[END_REF]. La2Ti2O7 oxide is a semi-conductor material, thus, the optical band-gap (Eg) can be determined by the Tauc plot based on the following equation [START_REF] Tauc | Optical properties and electronic structure of amorphous Ge and Si[END_REF]:

αhν = A(hν -Eg) n (1)
Here α is the absorption coefficient, hν corresponds to the photon energy, A is a proportionality constant and Eg represents the optical band-gap. Finally, the exponent n denotes the nature of the transition. Here, La2Ti2O7 compound is known to have a direct allowed transition; thus, the equations were solved with n = ½ [START_REF] Pussacq | Reduction of Ln2Ti2O7 Layered Perovskites: A Survey of the Anionic Lattice, Electronic Features, and Potentials[END_REF]. As illustrated in Figure 3b, the (αhν) 2 vs. hν plots, allow to determine the effective optical band-gaps by extrapolating the linear region of the curves to the hν axis intercept. We observed that the Eg values are very close and decrease slowly with the decrease of the Ln 3+ ionic radius. The Eg values are 3.85, 3.78 and 3.77 eV for La1.85Eu0.10Er0.05Ti2O7, Eu2Ti2O7 and Er2Ti2O7, respectively.

Luminescence properties

The room-temperature (RT) excitation and emission spectra recorded in the range from 250 to 800 nm for a La1.6Eu0.10Er0.30Ti2O7 compound are shown in Figure 4. The spectra presented are typical of those recorded on the complete range of La2-x-yEuxEryTi2O7 oxides whose evolution in normalized intensity is showed in supplementary materials (Figure S1). First, in the excitation spectrum, two overlapped broad bands are observed around 280 and 310 nm, which can be attributed to the host-related absorption due to the Inter Valence Charge Transfer (IVCT) and the charge transfer (CT) of Eu 3+ /Er 3+ -O 2-, respectively [START_REF] Zhang | Color tuning and white light emission in a single-phased La2Ti2O7:Pr 3+ , Tb 3+ phosphor[END_REF] (in correlation with the previous optical band-gap measurements). Depending on the emission wavelength, two different behaviors with many transition bands are observed. These transitions, corresponding to the intra-configurational f-f transitions of Eu 3+ and Er 3+ ions, can be indexed using the Dieke diagram [START_REF] Dieke | Spectra and energy levels of rare earth ions in crystals[END_REF]. For λem = 612.5 nm, the transitions observed are mostly due to the excited levels of Eu 3+ ions (i.e., 7 F0 → 5 D4 (361 nm), 7 F0 → 5 L7,8; 5 GJ (375 nm), 7 F0 → 5 L6 (396 nm), 7 F1 → 5 D3 (412 nm) and 7 F0 → 5 D2 (465 nm)), while for λem = 547.5 nm the transitions originating from Er 3+ ions are also rather highlighted (i.e., by

4 I15/2 → 4 G7/2 (355 nm), 4 I15/2 → 2 K15/2; 4 G9/2 (364 nm), 4 I15/2 → 4 G11/2 (377 nm), 4 I15/2 → 2 H9/2
(406 nm)). In the same way, the emission spectra differ according to the excitation wavelength. Under λexc = 310 nm, many transitions with the most intense line centered at 612.5 nm (orange-red spectral range) are observed. These lines can be assigned to Eu 3+ transitions between the lowest excited 5 D0 levels and 7 FJ (J = 0 -5) ground state [START_REF] Ben Amar | Optical spectroscopy of Eu 3+ ions doped in YAl3(BO3)4 crystal[END_REF], [START_REF] Sun | Thermal stable La2Ti2O7:Eu 3+ phosphors for bluechip white LEDs with high color rendering index[END_REF], [START_REF] Atuchin | Synthesis and Spectroscopic Properties of Monoclinic α-Eu2(MoO4)3[END_REF]. The splitting of spectral lines is undoubtedly due to a modification of the electronic states by a Stark effect [START_REF] Rocha | Photoluminescence of new Er 3+ -doped titanosilicate materials[END_REF]. Under λexc = 377 nm, two green emission bands centered at 525 and 548 nm are observed. These bands correspond to the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions of Er 3+ ions, respectively [START_REF] Zhang | Structure and luminescence properties of TiO2:Er 3+ nanocrystals annealed at different temperatures[END_REF]. In addition, at λexc = 377 nm, the ratio between emissions in the green and red ranges varies greatly depending on the x and y values within the La2-x-yEuxEryTi2O7 oxides. Thus, a shift of the emission towards the green spectral domain is observed only for Er 3+ concentrations greater than y = 0.10 and for very low concentrations of Eu 3+ (x ≤ 0.10), as highlighted on the Figure S1 (Supplementary Material). Beyond this x value, the emission is mainly in the orange-red spectral range. The emission is, therefore, mainly dominated by the effect of Eu 3+ ions within La2Ti2O7 host matrix. Further, under λexc = 310 nm, only orange-red emission is observed regardless the x and y values.

In Figure 5a, a photograph displaying the emission color of La2-x-yEuxEryTi2O7 oxides under the excitation of commercial UV excitation lamp (365 nm) is presented. One can note a difference in intensity that is visible to the naked eye according to the values of x and y. In order to quantify this evolution in the luminescence intensity, we plotted the intensity variations of several emission bands under several excitation wavelengths as a function of the

x and y values. The intensity evolution of the 4 S3/2 → 4 I15/2 transition at three excitation wavelengths (i.e., 310, 377 and 393 nm) is presented in Figure 5b. The first result indicates that this transition is active only for an excitation wavelength of 377 nm and that its intensity increases progressively with the Er 3+ ions concentration. The excitation at other mentioned wavelengths resulted in weak to no emission of Er 3+ ions. This also indicates that the emission due to Eu 3+ ions dominates mainly under the UV excitation. The intensity evolution of 5 D0 → 7 F2 (the most intense) and 5 D0 → 7 F4 transitions under the 310 and 377 nm excitation is presented in Figure 5c in order to highlight the x influence and in Figure 5d to highlight the y influence. In the case of La1.95-xEuxEr0.05Ti2O7 oxides, the intensity of the transitions observed goes through a maximum which is different according to the excitation. These are x = 0.30 and x = 0.60 for 310 and 377 nm, respectively. This difference in the quenching concentration can be explained by an energy transfer between the Er 3+ and Eu 3+ ions. Under the excitation at 310 nm, only the Eu 3+ ions act as activators while under an excitation at 377 nm, the Eu 3+ and Er 3+ ions are both activators. Therefore, under an excitation at 377 nm, the concentration quenching appears later than it in the case of a mono-substituted oxide. In a previous study [START_REF] Bayart | Synthesis, structural and luminescence properties of (La1-xLnx)2Ti2O7 (Ln=lanthanides) solid solutions[END_REF], we showed that for mono-substitution with Eu 3+ , the quenching concentration is reached for x = 0.8, that is close to the value determined here. In the case of the oxide La1.90-yEu0.10EryTi2O7, the intensity values of the 5 D0 → 7 F2 and 5 D0 → 7 F4 transitions decrease when the y value increases. Note that for λexc = 377 nm, the relative intensity is significantly lower than that observed for the 310 nm excitation. This also confirms that there is an energy transfer between the Er 3+ and Eu 3+ ions at this wavelength.

In addition, we can estimate the critical distance (Rc) by the following formula given by Blasse [START_REF] Blasse | Energy transfer in oxidic phosphors[END_REF]:

3 / 1 4 3 2         = N x V R C C π (2)
Here V is the unit cell volume, xc represents the concentration of activator ions where the quenching occurs and N is the number of ions whose sites are partially occupied by the activator in the unit cell (in our case N = 8). At λexc = 310 nm and λexc = 377 nm, xc is equal to 0.175 and 0.325, respectively (sum of Eu 3+ and Er 3+ concentration in the oxide). So, the critical transfer distances are about 3.81 and 4.80 , respectively. Knowing that the exchange interaction takes place when the critical distance value is about 3 -4 Å [START_REF] Liu | Structure, luminescence property and energy transfer behavior of color-adjustable La5Si2BO13:Ce 3+ ,Mn 2+ phosphors[END_REF], we can conclude that this mechanism is more accentuated under the excitation at 310 nm.

The room temperature photoluminescence decay curves of La1.95-xEuxEr0.05Ti2O7 phosphors are shown in Figure 6a. The samples were excited at 393 nm and the emission was monitored at 612.5 nm ( 5 D0 → 7 F2 transition). These decay curves can be fitted successfully based on the following multifold exponential equation [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF]:

∑ = - N i t i i e A I t I τ 0 ) ( (3)
where, I and I0 correspond to the luminescence intensity at a given time t and initial intensity, respectively. Ai are pre-exponential constants which are related with the initial intensity, τi are the lifetimes for the each exponential components. The average fluorescence lifetime τave can be calculated by the following equation [START_REF] Pahari | Biophysical Characterization of Genistein in Its Natural Carrier Human Hemoglobin Using Spectroscopic and Computational Approaches[END_REF]:

∑ ∑ = N i i i N i i i ave A A τ τ τ 2 (4)
In our case, the PL decay curves can be fitted with a double exponential function. We can observe a progressive decrease in average life time values with the Eu 3+ concentration increase. This evolution can be explained by resonant energy transfer between Eu 3+ ions (and Er 3+ ions), what leads to decrease of excited state lifetime.

We can also note that this double-exponential modeling reveals the different environments for Eu 3+ cations, as explained by the presence of several crystallographic sites in the monoclinic structure.

In addition, Khalil et al. [START_REF] Khalil | Europium beta-diketonate temperature sensors: Effects of ligands, matrix, and concentration[END_REF] have proposed the following equation to describe thermal and concentration quenching [START_REF] Khalil | Europium beta-diketonate temperature sensors: Effects of ligands, matrix, and concentration[END_REF]:

[ ] + + + = = 3 ) ( 1 Eu K T K K K C nr r t τ (5)
Where Kt is the observed decay rate, Kr and Knr(T) are the radiative and non-radiative decay rates, respectively, and Kc is the quenching term dependent on the concentration of europium ions. Knr(T) is dependent on the environment and temperature. This equation can be transposed to our case by adding a component related to the Er 3+ concentration:

[ ] [ ] + + + + + = = 3 ' 3 ) ( 1 Er K Eu K T K K K C C nr r t τ (6)
If we consider that the measurements are carried out at a constant temperature and that the concentration of Er 3+ ions is constant in samples series, the PL lifetime (or average lifetime) is directly related to the concentration of Eu 3+ ions according to the equation:

] [ 1 3+ + = Eu K A C ave τ (7)
Where A is a constant. The evolution of an average lifetime as a function of the x is shown in Figure 6b. We find a decay that can satisfy the expression (7) followed by a bearing from x = 0.6. This limit is in good relation with the concentration quenching observed under the 377 nm excitation (see Figure 5c). The average lifetime changes from 894 to 674 μs and Kc can be calculated by modeling the first part of the curve (dashed line in Figure 6b). This one is worth 1.299x10 -3 µs -1 .at. % -1 . In the same way, the room temperature PL decay curves of La1.95-yEu0.10EryTi2O7 monitored at 393 nm and correspondent to the 5 D0 → 7 F2 transition were also acquired. These curves are shown in Figure S2. The τave variation of the lifetime as a function of y shows a sudden decrease until y = 0.10, then an increase. This evolution can be related to the structural modification experienced by the sample series (see Figure 1). Indeed, below y = 0.1, only the monoclinic phase is present and, beyond, a phase mixture is obtained (monoclinic and cubic). In order to study the Er 3+ influence, we also recorded the PL decay curves as a function of x and y for the 4 S3/2 → 4 I15/2 transition (Er 3+ ) excited at 377 nm.

The dependences are shown in Figure S3. Besides the fact that these average PL lifetime values are significantly smaller, if compared to those of the 5 D0 → 7 F2 transition of the Eu 3+ ions, the changes in average PL lifetime values show a sharp fall for low values of x and y.

This also tells us that the constant K'c is much higher than the constant Kc. All these results show a dominance of Eu 3+ ions on the spectral emission of La2-x-yEuxEryTi2O7 oxides.

As observed in the photographs of Figure 5a, the La2-x-yEuxEryTi2O7 oxides present an intense emission with orange-reddish color. The CIE 1931 chromaticity coordinates have been calculated for the entire series under λexc = 310 nm. In Figure 7a, the 3D plot of CIE 1931 coordinates as a function of the atomic percent of europium ions are shown. On the 2D plot projection, we observe a linear variation between the X and Y coordinates (Figure 7b) which illustrates a change of color from orange to red. This color shift can be explained by an intensity increase of the line coming from the 5 D0 → 7 F2 transition with respect to those of other transitions. In the similar way, the 3D plot of CIE 1931 chromaticity coordinates as a function of the atomic percent of erbium ions are presented in Figure 7c. On the 2D plot projection (Figure 7d), we observe also a linear variation between the X and Y coordinates. This time, we observe a shift from red to orange with the increase in the proportion of erbium. This behavior is explained by a larger intensity decrease for the 5 D0 → 7 F2 line if compared to that of the 5 D0 → 7 F1 line. It is known that the 5 D0 → 7 F2 transition is hypersensitive to the local environment and the 5 D0 → 7 F1 is insensitive to site symmetry [START_REF] Ben Amar | Optical spectroscopy of Eu 3+ ions doped in YAl3(BO3)4 crystal[END_REF]. Consequently, the ratio between these emission intensities defined by R = I( 5 D0 → 7 F2)/I( 5 D0 → 7 F1), can be used as a sensitive optical probe to determine the symmetry site where Eu 3+ ion is located [START_REF] Maheshvaran | Concentration dependent Eu 3+ doped boro-tellurite glasses-Structural and optical investigations[END_REF]. The evolution of the R value is presented in Figure S4 in supplementary materials. We observe a linear decrease of this value between x = 0.01 and x = 0.1 values of Er 3+ content in synthesized compounds. This can be explained by a lower distortion of the Eu 3+ ion occupancy sites due to the insertion of Er 3+ ions within the layered perovskite structure. Indeed, the Er 3+ ion radius is smaller, it counterbalances the distortion of the sites of occupation in the monoclinic/layered-perovskite structure. Above x = 0.1, a break is observed which may be related to the presence of a phase mixture (monoclinic/cubic), as described in the XRD part. In this case, the cubic phase has centro-symmetric lattice sites that can be occupied by Eu 3+ ions and, therefore, this also contributes to the decrease of the R ratio.

The good thermal resistance and the submicron size of these oxides can allow a homogeneous dispersion in a vitreous matrix to create luminescent glasses for displays, artworks and even anti-counterfeiting markers in luxury flasks. In order to verify this, we carried out a 2% mass dispersion of oxide in a lithium tetraborate matrix. The glass wafer (⌀ 4 cm, thickness 3 mm) is then formed through the fused bead method. As shown in Figure 8, the transmittance of the glass is very slightly affected by the presence of the oxide. The transmittance drop by 2% is observed in the visible range. Photography under natural light, as shown in inset of Figure 8, highlights the good conservation of the glass transparency. Under UV light (365 nm), the luminescence is clearly observed with the naked eye (the intensity increases with the concentration of europium, as observed in the case of oxides). The transmission spectrum also shows characteristic transitions of the Er 3+ and Eu 3+ ions, which are directly annotated in Figure 8. The absorption lines, observed in transmission spectrum of the glass sample, are just overlapping, as observed in excitation spectra. The good chemical and thermal resistance of these oxides allows us to use them as active material within vitreous matrix.

Up-conversion luminescence properties

The upconversion properties of La2-x-yEuxEryTi2O7 oxides were also investigated using two lasers emitting at 808 and 980 nm. In Figure 9a, the emission of the different powders under a laser excitation of 785 nm (power 40 mW) is shown. All samples showed green luminescence visible by naked eye. It is also evident that brightness of emitted light decreases with increasing Eu 3+ concentration for a fixed Er 3+ concentration. The opposite, however, is observed if Er 3+ concentration is increased for the fixed Eu 3+ content. This clearly indicate, that high Eu 3+ concentration quenches upconversion emission of Er 3+ ions in La2-x-yEuxEryTi2O7 oxides. The samples with x = 0.05, y = 0.10 and x = 0.10, y = 0.10 showed the most intensive upconversion luminescence.

The room temperature emission spectra recorded on the La1.70Eu0.20Er0.10Ti2O7 oxide under the excitation at 980 and 808 nm show three main sets of lines attributable to energy levels of the Er 3+ ion. The bands centered at 520, 547 and 665 nm correspond to 2 H11/2 → 4 I15/2, 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, respectively (Figure 9b). As the conventional luminescence, the multitude of peaks observed is certainly due to a modification of the electronic states by the Stark effect [START_REF] Rocha | Photoluminescence of new Er 3+ -doped titanosilicate materials[END_REF]. The mechanism of absorption in upconversion luminescence can follow three main processes [START_REF] Bloembergen | Solid State Infrared Quantum Counters[END_REF], [START_REF] Auzel | Compteur quantique par transfert d'énergie entre deux ions de terres rares dans un tungstate mixte et dans un verre[END_REF]: Excited-State Absorption (ESA), Energy Transfer (ET) and Photon Avalanche (PA). The upconversion mechanisms can be evaluated by measuring the upconversion intensity as a function of the laser power. The number of photons (α) which are required to populate the upper emitting level can be obtained by the following relation [START_REF] Pollnau | Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems[END_REF]:

α P I ∝ (8)
In order to highlight the process involved, we recorded the emission spectra for different laser powers at 980 nm (Figure 9c). Then, we plotted the evolution Ln(I) vs. Ln(P) (see Figure 9d). The slope of this curve indicates the number of photons involved in the absorption process. No inflection point is highlighted, therefore, we can exclude an avalanche process of photons. In our case under the excitation at 980 nm, we observe linear evolutions whose slopes are 1.23 and 1.50 for the 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, respectively.

We can suggest that two photons are required to create these mechanisms. This process has already been demonstrated in a previous study on a monosubstitution of La2Ti2O7 with Er 3+

ions [START_REF] Bayart | Upconversion luminescence properties and thermal quenching mechanisms in the layered perovskite La1.9Er0.1Ti2O7 towards an application as optical temperature sensor[END_REF].

The time-resolved measurements performed for 4 F9/2 → 4 I15/2 transitions under the 980 and 808 nm laser excitation are presented in Figure 10, and this reveals that the main excitation mechanism for Er 3+ ions is excited state absorption regardless the excitation wavelength. The decay curve can be fitted using the sum of two exponential functions, as described above. Concerning the 4 F9/2 → 4 I15/2 transition for the 980 nm laser excitation, we obtained an average PL lifetime value of 61.6 μs, and, concerning the same transition for the 808 nm laser excitation, the average PL lifetime value of 32.3 μs is obtained. It is evident that Er 3+ emission quenches almost twice faster if samples are excited with the 808 nm laser.

Therefore, it can be concluded that the internal efficiency of Er 3+ is higher for the 980 nm laser excitation.

To complete this study, the temperature-dependent upconversion emission spectra of La1.75Eu0.20Er0.05Ti2O7 specimen were recorded under the laser excitation at 980 nm and they are shown in Figure 11a. The conventional emission spectra with the emission bands attributable to 2 H11/2 → 4 I15/2, 4 S3/2 → 4 I15/2, 4 F9/2 → 4 I15/2 and 4 I9/2 → 4 I15/2 transitions, located at 520, 544, 660 and 811 nm, respectively, are observed. For all transitions, we can see a gradual decrease in intensity as a function of temperature over a range of 77 to 500 K. This decrease is induced by thermal quenching effect. This luminescence quenching is due to the increased probability of non-radiative transitions from excited state to ground state at elevated temperatures [START_REF] Cai | Thermal Quenching Mechanisms in II-VI Semiconductor Nanocrystals[END_REF]. The temperature dependence of the luminescence can be described by the Mott-Seitz equation (Ea represents the activation energy for the thermal quenching) [START_REF] Cooke | Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce 3+[END_REF], [START_REF]Theoretical Basis of Luminescence Phenomena[END_REF]:

T k E B a Ce I T I - + = 1 ) ( 0 (9)
Where, I0 and I(T) are the initial intensity and the intensity at the given temperature, respectively. C is constant, kB is Boltzmann's constant (8.617x10 -5 eV/K) and Ea is the thermal activation energy for the non-radiative transition. Note that, in this expression of the luminescence decay, the contribution of the phonon-assisted transitions is neglected. The activation energy can be calculated from a plotting of ln[(I0/I)-1] against 1/kBT, where a slope is equal to -Ea. The activation energy for 4 F9/2 → 4 I15/2 transition quenching was found to be 52.8 meV as shown in the inset of Figure 11b. There are several mechanisms that can describe this thermal quenching phenomenon. The most common is based on energy level diagram evolving with the temperature. At low temperature, the excited levels are populated and their de-excitation leads essentially to the emission of luminescence. At high temperature, the phonon frequency increases leading to increased probability of multi-phonon relaxation and thus, non-radiative de-excitation of excited states.

By normalizing the emitted intensity at 544 nm, we clearly observe an evolution of the intensities corresponding to the 2 H11/2 → 4 I15/2 (IH) and 4 S3/2 → 4 I15/2 (IS) transitions with temperature (Figure S5a in supplementary material). Therefore, the Fluorescence Intensity Ratio (FIR(T)=IH/IS) can be employed in thermal sensing [START_REF] Sidiroglou | Effects of high-temperature heat treatment on Nd 3+ -doped optical fibers for use in fluorescence intensity ratio based temperature sensing[END_REF]. Indeed, the 2 H11/2 and 4 S3/2 levels are in thermal equilibrium and the electrons in them obey Boltzmann distribution law; therefore, FIR(T) can be expressed by the following equation [START_REF] Wang | Effect of Annealing on Upconversion Luminescence of ZnO:Er 3+ Nanocrystals and High Thermal Sensitivity[END_REF]:

        ∆ - = = T k E A I I T FIR B S H exp ) ( ( 10
)
Where A is a pre-exponential constant, ΔE is the energy gap separating the 2 H11/2 and 4 S3/2 levels, kB is the Boltzmann constant, and T is the absolute temperature. The calculated values of IH/IS as a function of temperature for La1.75Er0.05Eu0.20Ti2O7 sample are shown in supplementary material (Figure S5b). The fit of experimentally obtained data with this previous equation yielded ΔE = 565 ± 28 cm -1 and A = 4.539 ± 0.8. From there, the sensitivity (S) that defines the change rate of R(T) per unit temperature, can be determined with following equation [START_REF] Li | Effects of Er 3+ concentration on down-/up-conversion luminescence and temperature sensing properties in NaGdTiO4:Er 3+ /Yb 3+ phosphors[END_REF]: [START_REF] Feldmann | Inorganic Luminescent Materials: 100 Years of Research and Application[END_REF] Since the values of A and ΔE were already obtained, it was possible to model the sensitivity curve. So, as shown in supplementary materials (Figure S5c), the sensitivity reaches maximum of 3.1x10 -3 K -1 at 405 K and the optimal temperature range of use is defined where the sensitivity S is higher than half of its maximum value SMax as illustrated in Figure S5c in supplementary materials. In a previous study based on La1.9Er0.1Ti2O7 oxides (mono-substitution with Er 3+ ), the sensitivity was determined at 1.1x10 -2 K -1 at 512 K and the useful temperature range was estimated between 250 and 1373 K [START_REF] Bayart | Upconversion luminescence properties and thermal quenching mechanisms in the layered perovskite La1.9Er0.1Ti2O7 towards an application as optical temperature sensor[END_REF]. To conclude, these oxides can be used for applications of thermal sensing; but in the case of a co-substitution (Eu 3+ / Er 3+ ), the Eu 3+ ions have a detrimental effect with a reduction of the sensitivity (by a ∼3.7 factor for La1.75Eu0.20Er0.05Ti2O7 oxides) and a reduction in the useful temperature range. This behavior can be explained by non-radiative energy transfer (ET) between energy levels of Er 3+ and Eu 3+ ions.

        ∆         ∆ - = = 2 exp ) ( ) ( T k E T k E A dT T dFIR T S B B

Conclusion

The La2-x-yEuxEryTi2O7 oxides were successfully synthesized using solid-state reaction. These materials were characterized by X-ray Diffraction, Scanning electron microscopy and fluorescence spectroscopies. We have demonstrated that the monoclinic structure is preserved if Eu 3+ and Er 3+ substitution rates are lower than 0.20 and 0.10, respectively. For higher values, a mixture consisting of a substituted monoclinic phase with the layered-perovskite structure and cubic phases, with the pyrochlore structure (Eu2Ti2O7 and Er2Ti2O7) were found. In addition, the morphological features of La2-x-yEuxEryTi2O7 also depends on the substitution rate. For low substitutions, we have noticed the presence of aggregates of uniform submicron sized particles and, when the rates increase, the grain-size distribution becomes random. Concerning the luminescence properties, we note intense lines, assigned to Eu 3+ transitions, centered at 612.5 nm when the samples are excited at 310 nm. However, when the materials are excited at 377 nm, we noticed the presence of green emission bands at 525 and 548 nm which correspond to transitions of Er 3+ ions. Moreover, we noticed the decrease of the average lifetime with the increase of Eu 3+ ion content, which is explained by a resonant energy transfer between Eu 3+ and Er 3+ ions. The CIE 1931 chromaticy coordinates were plotted versus the Eu 3+ (and Er 3+ ) ions concentration. A shift from red to orange was observed with the increase of Er 3+ concentration and a shift from orange to red was observed with the increase of Eu 3+ concentration. The materials were also dispersed in a glass matrix yielding brightly luminescent glass under the UV radiation. The decrease of glass transparency due to phosphor incorporation was negligible. The upconversion luminescence properties were checked by using two laser lines at 808 and 980 nm. For the later, a two photon mechanism was demonstrated and the observed bands correspond to the emission of Er 3+ ions. The average upconversion life times are 61.6 and 32.3 µs for 980 and 808 nm laser excitation, respectively. The evolution of the upconversion luminescence with the temperature was investigated and it follows a Mott-Seitz behavior. To finish, these oxides having these double luminescence properties under UV and NIR excitation, can have several potential applications: i) their good thermal resistance allows them to be dispersed in a glass matrix in order to create anti-counterfeit bottles for the luxury perfume industry. The dispersion carried out in this study on a borate glass confirms this potential; ii) These oxides could also be used to design inks that can be used for official documents, ticketing and banknotes (their activities under two distinct wavelengths will strengthen the document security); iii) After a toxicological study, these oxides could also be used for applications in biology in particular as materials for labeling or as support materials for phototherapy to activate a drug under the NIR excitation and also be detectable with the UV microscopy surface excitation; iv) as materials for coupled detection of NIR and UV radiation on the same material or v) more recently, it has been shown that these oxides, in the form of single crystals, have low extinction coefficients in the visible range [START_REF] Wang | Recent development of A2B2O7 system transparent ceramics[END_REF], [START_REF] Bayart | Optical properties of Ln2Ti2O7 (with Ln = La to Lu) thin films grown on (110)-SrTiO3 substrates by pulsed laser deposition[END_REF]. As such, the luminescence properties can be used for the design of laser crystals. x in La

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. SEM images of (a) La1.94Eu0.05Er0.01Ti2O7, (b) La1.90Eu0.05Er0.05Ti2O7, (c) La1.60Eu0.10Er0.30Ti2O7, (d) La1.55Eu0.40Er0.05Ti2O7.

Figure 3 .

 3 Figure 3. Diffuse reflectance UV-Visible spectra and Tauc plots for La1.85Eu0.10Er0.05Ti2O7, Er2Ti2O7 and Eu2Ti2O7 oxides.

Figure 4 .

 4 Figure 4. Luminescence properties of La1.6Eu0.10Er0.30Ti2O7 oxide: (a) Room-temperature excitation spectra recorded in the range of 250 to 575 nm at λem=547.5 nm and λem=612.5 nm;(b) Room-temperature emission spectra recorded in the range of 450 to 800 nm at λexc=310 nm and λexc=377 nm.

Figure 5 .

 5 Figure 5. a) Photographs highlighting emission color of La2-x-yEuxEryTi2O7 oxides under excitation at 365 nm; b) Intensity evolution of 5 S3/2 → 4 I15/2 transition vs. y in La1.90-yEu0.10EryTi2O7 oxides under λexc=310, 377 and 393 nm ; c) Intensities evolutions of 5 D0 → 7 F4 and 5 D0 → 7 F2 transitions vs. x in La1.95-xEuxEr0.05Ti2O7 oxides under λexc=310 and 377 393 nm ; d) Intensities evolutions of 5 D0 → 7 F4 and 5 D0 → 7 F2 transitions vs. y in in La1.90-yEu0.10EryTi2O7 oxides under λexc=310 and 377 nm.

Figure 6 .

 6 Figure 6. a) Photoluminescence decay curves of La1.95-xEuxEr0.05Ti2O7 oxides b) Evolution of the average lifetime as a function of the x value under an excitation of 393 nm.

Figure 7 .

 7 Figure 7. a) 3D plot and b) 2D projection of CIE 1931 coordinates of La1.95-xEuxEr0.05Ti2O7 oxides; c) 3D plot and d) 2D projection of CIE 1931 coordinates of La1.90-yEu0.10EryTi2O7.

Figure 8 .

 8 Figure 8. Transmittance spectra of glass alone and glass loaded with 2% by weight of La1.60Eu0.30Er0.10Ti2O7 oxide (inset, photograph of the glass wafer under natural light and under UV excitation).

Figure 9 .

 9 Figure 9. a) Photographs highlighting the emission color variation of La2-x-yEuxEryTi2O7 oxides under 785 nm laser excitation (40 mW) b) Upconversion emission spectra of La1.85Eu0.10Er0.05Ti2O7 with two exciting laser (808 and 980 nm); c) Upconversion emission spectra of La1.85Eu0.10Er0.05Ti2O7 (λex=980 nm) at different laser power (from 0.05 W to 0.98 W) recorded at room temperature; d) Power dependence of upconversion luminescence from 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions.

Figure 10 .

 10 Figure 10. Upconversion decays curves performed for 4 F9/2 → 4 I15/2 transition and obtained under 808 and 980 nm laser excitation. The blue curve represents the Instrument Response Function (IRF) for a laser pulse at 808 nm.

Figure 11 .

 11 Figure 11. a) Emission spectra (λex=980 nm) vs. temperature of La1.85Eu0.10Er0.05Ti2O7 oxide in a range of 77 to 500 K; b) Evolution of UC intensity as a function of the temperature for 4 F9/2 → 4 I15/2 transition (inset a plot of ln(I0/I-1) as a function of 1/T for the determination of the activation energy).

2 H

 2

  1.95-x Eu x Er 0.05 Ti 2 O 7

								1.0
												0.369
		a)						0.6 0.8	Eu x Er 0.05 Ti 2 O 7		0.366 0.367 0.368
								0.2 0.4	x in La 1.95-x	Y	0.364 0.365	Y=-0.86614+0.90136 R²=0.9987
												0.363
								0.638			0.362
							0.636
							0.634				0.361
	0.368	0.366	0.364		0.632 0.630	X					0.628	0.630	0.632	0.634 X	0.636	0.638
		Y			0.362	0.628
								0.3
													0.371
									O 7				0.370
								0.2	Ti 2
									Er y
									Eu 0.10				0.369
								0.1	y in La 1.90-y			Y	0.368	Y=-0.89722x+0.93369 R²=0.9997
													0.367
								0.0
							0.632
							0.630					0.366
	0.371	b) 0.370 0.369 Y	0.368	0.367	0.366	0.628	X					0.627	0.628	0.629	0.630 X	0.631	0.632	0.633
															 exc =393 nm
						y=1/(1.08.10 -3 +6.498.10 -4 x)
						R²=0.9952

Acknowledgments

The work was financially supported by « les Ministères de l'Europe et des Affaires Etrangères (MEAE) et de l'Enseignement Supérieur, de la Recherche et de l'Innovation (MESRI) » and by the Ministry of Education and Science and the National Research Council of Lithuania (PHC Gilibert (N° 373027ZC)). Chevreul Institute (FR 2638)", "Région Nord Pas-de-Calais", "FEDER" and "C'Nano Nord-Ouest competence centre" are also acknowledged for supporting and funding this work. The authors also thank Nora Djelal for SEM analyses.


Vincent Roussiez

Wolfgang Ludwig

Jean-Luc Probst

André Monaco

Background levels of heavy metals in surficial sediments of the Gulf of Lions (NW Mediterranean): An approach based on 133 Cs normalization and lead isotope measurements

Keywords: Heavy metals, Coastal sediments, Normalization, 133 Cs, Background levels, Lead isotopes, Gulf of Lions

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Background levels of heavy metals in surficial sediments of the Gulf of Lions (NW Mediterranean): An approach based on 133Cs normalization and lead isotope measurements

Introduction

The first step when analysing the spatial distribution of the heavy-metal pollution in marine sediments is to account for the often considerable variability of natural concentrations that can be related to the variations of grain-size distribution and the mineralogical composition of the samples. It is commonly admitted that elements like metals show a strong affinity with the clay fraction and its coating formations (e.g. organic matter, iron and manganese oxides). From one site to another, the grain-size distribution is rarely homogeneous and greater concentrations of elements do not automatically reflect anthropogenic inputs, but may also be due to a greater clay mineral content or to the abundance of minerals that are naturally enriched in the considered metal [START_REF] Angelidis | Geochemical study of coastal sediments influenced by river-transported pollution: southern Evoikos Gulf. Greece[END_REF]. Based on the metal concentrations alone, it is therefore difficult to distinguish between anthropogenic and natural sources.

For these reasons, various normalization procedures have been proposed. As a result, the normalized concentrations of equally polluted samples, but with different grain-size distributions, should not differ significantly [START_REF] Kersten | Normalization procedures for sediment contaminants in spatial and temporal trend monitoring[END_REF]. The separation of the clay fraction (!2 mm) and the successive analysis of the metal content in this fraction alone would be the ideal normalization procedure. But the process is laborious and for practical reasons, the !63 mm fraction (claysCsilts) is commonly separated. Unfortunately, such method does not avoid the considerable variability of the mineralogical composition and of the clay content between two areas. Based on the results of recent intercomparison exercises, [START_REF] Kersten | Normalization procedures for sediment contaminants in spatial and temporal trend monitoring[END_REF] therefore recommend a two-tiered normalization approach including both sieving (!63 mm) and geochemical correction. This last consists of dividing the measured metal concentrations by the concentrations of certain elements which are considered to be proxies of the clay content (e.g. Al, Fe).

There are no clear rules established for the choice of the normalizer element, although this step is naturally of great importance because the distribution of the normalization element may also be variable in space. Several quality criteria have been defined [START_REF] Luoma | Processes affecting metal concentrations in estuarine and coastal marine sediments[END_REF]: first, the normalizer must covary in proportion with the natural concentrations of the metals of interest. Second, it must be insensitive to anthropogenic sources, and finally, it should not be influenced by changes in the geochemical conditions such as variations of the redox potential. Nevertheless, the choice of the normalizer element still remains somewhat subjective and often varies from one study to another. This makes it difficult to compare the values between different geographic and physiographic areas.

The purpose of our work was to test the power of different normalizer elements to detect natural background levels, and to identify an enhancement of the values due to pollution. The metals we investigated were Co, Cr, Cu, Ni, Pb and Zn, whereas the potential normalizer elements we tested were Al, Fe, Sc, Cs and Co. Sediment samples were taken as surface sediments from the Gulf of Lions in the NW Mediterranean, spreading over the entire platform and the submarine canyons down to a water depth of 1100 m. Our choice of the normalizer element that is the most suitable for tracking pollution in the sediments is based (i) on its degree of correlation with the fine fractions in the samples, (ii) on the degree of detail that can be obtained in the reconstruction of the spatial distribution patterns of polluted and unpolluted samples, and (iii) on their capability to produce natural background levels that are in agreement with other literature sources.

Material and methods

Sampling area and geochemical analyses

The Gulf of Lions is one of the largest continental platforms in the Mediterranean Sea, which receives various sources of particulate matter, both of riverine and atmospheric origins. By far the major part of the terrestrial input originates from the Rhoˆne River, accounting for about 80% of the overall riverine input to the Gulf of Lions (Durrieu de [START_REF] Durrieu De Madron | Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)[END_REF]. Other sources of terrestrial particles are the relatively small coastal rivers, which show a highly variable discharge regime and for which the average sediment discharge is much more difficult to estimate (e.g., the Teˆt, Aude, and He´rault rivers; see Fig. 1). Industrial activity in the Rhoˆne river basin is also clearly more elevated than in the smaller river basins in the Western part of the Gulf of Lions, indicating that most of the riverine metal pollution should be introduced by this river.

The platform sediments in the Gulf of Lions follow a granulometric gradient from the coastline to the outer shelf, as this has been described by [START_REF] Aloisi | Carte ge´ologique du pre´continent languedocien au 1/250000ie`me[END_REF] in a high resolution sampling study. This gradient consists of a sandy band with silty sedimentary units in front of the river mouths (the so-called prodeltas), followed by a middle-shelf mudbank, and finally by a mix of relic sands and modern fine-grained sediments near the canyon heads.

The sediment samples analysed in this study were collected during an oceanographic cruise in November 2002 at 51 stations in the entire Gulf of Lions (Fig. 1) using a Multicorer or box corer. Organized by the Institut de Radioprotection et de Se´curite´Nucle´aire (IRSN) group, this campaign (REMORA 3) was devoted to a radionuclide and trace metal contamination assessment in the entire Gulf of Lions.

At each point, a sediment core was sub-sampled with an acid pre-cleaned PVC tube. The surficial sediments were collected as the first 0.5 cm of the cores. Samples were stored in plastic bags and immediately deep-frozen on board (ÿ15 C). On return to the laboratory, they were lyophilized.

Granulometric analyse were performed using a laser granulometer Sequoia LISST-100 (nZ46). Lyophilised samples were first diluted in Milli-Q water and then passed into a sonification bath for 20 s to eliminate aggregates. Duplicates were realized on each sample to test the reproducibility of the procedure and the results were averaged. The instrument distinguishes the contribution of the different particle diameter classes, ranging from 1.5 to 230 mm. As trace and major elements were analysed in the !63 mm fraction, we only focused on the grain size distribution in this fraction.

Sediment samples for major and trace element determinations were lightly ground in a mortar to break up aggregates and then dry sieved (!63 mm), using an acid pre-cleaned nylon mesh. The fine fraction was totally digested by concentrated HF, HNO 3 and HClO 4 acids to dryness on a hot plate. The residue was solubilized by HNO 3 and diluted to volume. The resulting solution was then analysed for trace metals (nZ49) and stable lead isotopes (nZ17) by a Perkin-Elmer ELAN 6000 ICP-MS at the Laboratoire des Me´canismes de Transfert en Ge´ologie (Toulouse, France). Major element analyses were performed by ICP-AES (nZ47) at the Centre Europe´en de Recherche et d'Enseignement en Ge´osciences (Aix-Marseille, France). The different steps, from sieving to chemical analyses, occurred in clean rooms.

Accuracy was controlled by the use of numerous certified standards, e.g. the MESS-2 and GSMS-3 marine sediments for trace metal determination (see Table 1) and the NIST SRM-981 standard for lead isotopic analyses (not shown).

Statistical determination of the background levels

Based on the assumption that natural metal concentrations should depict a high degree of correlation with the normalizer element, regional background concentrations can be estimated by linear regression of the normalizer against the natural metal concentrations. Ideally, in unpolluted areas, the natural concentration ranges should be reached, whereas in polluted areas, the values should plot significantly above the regression line.

To reach the natural range of the metals, samples suspected to receive any anthropogenic input must hence be removed from regression.

For each of the studied metals, we therefore performed linear regressions with the different normalizer elements, after log-transformations of all data in order to satisfy the assumption of constant variance and normality (Schropp and Windom, 1988). A 95% prediction interval has been drawn for each regression using standard statistical software. Then, all samples that were standing outside the upper limit were eliminated and considered as enhanced [START_REF] Doherty | Natural and enhanced concentrations of trace metals in sediments of Cleveland Bay, Great Barrier Reef Lagoon, Australia[END_REF]. After each elimination step the regression was recalculated. This process was repeated until no sample fell outside the upper limit. Fig. 2 shows some examples of this procedure when using Cs as normalizer element.

Even if the number of the excluded samples varies from one regression to another, one can generalize that by far most of the enhanced samples referred to the same zones in the Gulf of Lions. They correspond on the one hand to the prodeltas of the major rivers (e.g. the Rhoˆne, Aude and Teˆt rivers) and on the other hand to a zone in the south part of the Gulf, stretching along the canyon heads. According to the general circulation scheme [START_REF] Millot | The Gulf of Lions hydrodynamics[END_REF], it is here where most of the particles entering the Gulf via the Rhoˆne River are supposed to be exported from the platform to the open marine system.

To illustrate this general picture, Fig. 3 summarises the location of the enhanced stations according to the statistical analysis of Co, Cr, Cu and Ni content. The technique described above is a frequently applied procedure for the determination of natural background levels in marine sediments. An alternative solution consists of measuring the metal concentrations in the deepest samples of sediment cores [START_REF] Veinott | Baseline metal concentrations in coastal Labrador sediments[END_REF][START_REF] Leivuori | Heavy metal contamination in surface sediments in the Gulf of Finland and comparison with the Gulf of Bothnia[END_REF], based on the hypothesis that early diagenetic processes did not alter the vertical distribution of the considered elements. [START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF] investigated the heavy metal content in several sediment cores in the Gulf of Lions and we therefore also compared our results with those of pre-anthropogenic samples in this study.

Results and discussion

Choice of the normalizer element

Various elements have been proposed in the literature to be clay mineral indicators and hence potential normalizers. Some of them are Al, Fe, Li, Sc, stable Cs and Co. Among these elements, Al and Fe have been the most frequently used in estuarine and coastal studies [START_REF] Din | Use of aluminium to normalize heavy-metal data from estuarine and coastal sediments of Straits of Melaka[END_REF][START_REF] Tam | Normalization and heavy metal contamination in mangrove sediments[END_REF][START_REF] Schiff | Iron as a reference element for determining trace metal enrichment in Southern California coastal shelf sediments[END_REF], while Li, Sc and Cs are more exotic and were used to a lesser extent. Li has been proposed as an alternative for Al in high latitude areas [START_REF] Loring | Lithium e a new approach for the granulometrical normalization of trace metal data[END_REF]. Sc, with the difficulty of its determination because of its close association with Fe hydroxides [START_REF] Loring | Normalization of heavy-metal data from estuarine and coastal sediments[END_REF], has been used in rare cases; a previous study in the Gulf of Lions referred to this element for the normalization of sediment data [START_REF] Grousset | Anthropogenic vs. lithogenic origins of trace elements (As, Cd, Pb, Rb, Sb, Sc, Sn, Zn) in water column particles: northwestern Mediterranean Sea[END_REF].

Caesium, structurally combined in clay minerals [START_REF] Loring | Normalization of heavy-metal data from estuarine and coastal sediments[END_REF], was revealed by [START_REF] Ackermann | A procedure for correcting the grain size effect in heavy metal analyses of estuarine and coastal sediments[END_REF] to be suitable for use in estuarine and coastal sediments, although it is only rarely used in the literature for metal data normalization. In a comparative study, the latter author demonstrated the high dependence of the Cs concentration in the total sediment on the percentage of the fraction !20mm, the phase highly associated with metals, indicating that Cs is a good fine-grained sediment indicator. Also cobalt, associated with silicate and clay minerals [START_REF] Fo¨rstner | Metal pollution in the aquatic environment[END_REF] has recently been reported as a useful normalizer [START_REF] Matthai | Detection of anthropogenic Cu, Pb and Zn, in continental shelf sediments off Sydney e a new approach using normalization with cobalt[END_REF]. Li was not analysed in our study, so we did not test the normalization capacity of this element.

All of the normalizer elements discussed above have in common that they are normally well correlated with the clay content (!2mm) in the This is also the case in our study (Fig. 4 and Table 2). Table 2 presents the affinities of the potential normalizers with the studied metals (before and after the statistical removal of enhanced samples) and clay fraction. For additional information, we presented the correlations with the fraction !20 mm, also considered as a high metalbearing phase [START_REF] Ackermann | A procedure for correcting the grain size effect in heavy metal analyses of estuarine and coastal sediments[END_REF]. All coefficients are positive and significant, except for Cu versus the clay content.

According to our results, Cs depicts the highest correlation coefficients, both with the fine fractions (less than 2 and 20 mm fractions, r 2 Z0.82 and 0.92, respectively, nZ46) and with the heavy metals investigated, compared to the other candidates. This indicates that this element is the most suitable normalizer element in the Gulf of Lions. Moreover, the number of samples that were removed from the regressions due to an enhancement of the values is significantly higher than for the other potential normalizers, suggesting that Cs presents greater potential for the detection of enriched samples. For the other normalizer elements, the detection of outliers may be more difficult because of the generally weaker correlations, whipping out the differences between the natural and the slightly enhanced values.

The better ability of Cs to trace the clay content in the samples is also confirmed when looking at the variability the ratios of the different normalizer elements over the clay fraction in the Gulf of Lions. For all elements, one finds a general trend of decreasing ratios from the coastline to the middle-shelf mudbank, which means from silty sand-rich to clay-rich sediments. It is therefore possible that these trends can be attributed to the nonclay component in the samples, which also may influence the measured normalizer concentrations in the !63 mm fraction. Nevertheless, it has to be pointed out that Cs appears to be clearly less sensitive to this variability in the study area compared to the other elements. Considering all samples, the standard deviation of this ratio is only about 11% of the mean value for Cs, compared to 20% for Al, 21% for Fe, 24% for Sc and 16% for Co (Table 3).

In this context it is also interesting to note that speciation analyses of trace metals in local fine-grained sediments (!63 mm) via sequential extractions [START_REF] Marin | Reproducibility testing of a sequential extraction scheme for the determination of trace metal speciation in a marine reference sediment by inductively coupled plasma-mass spectrometry[END_REF][START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF] revealed that 98% of the total Cs is included in the residual fraction, compared to 93% for Al, 90% for Fe and 80% for Sc. This confirms the detrital origin of Cs and its structural link with the fine particles, which is a necessary condition for the geochemical tracing of these particles in the marine environment.

As a result, Cs will be considered in the following as the most suitable element for the normalization procedure. Also Co may be an alternative choice, since this element has a similar behaviour to Cs in our samples (although the correlations are slightly weaker).

Natural background levels

Our method to distinguish between natural and anthropogenic enhanced metal content is naturally somewhat arbitrary, which means that the so determined natural background values have to be considered with caution. The regression approach we applied is only valid when non-polluted samples co-exist together with polluted ones in the investigated area. When pollution affects almost all of the samples, e.g. by a widespread introduction of the pollutants of atmospheric origin, our method should lead in ''natural'' levels that are too elevated compared to the real background values.

We therefore compared our background levels based on the Cs normalization with the corresponding ratios in commonly cited reference values taken from the literature (Table 4). These values correspond to the terrigenous shales of [START_REF] Taylor | The continental crust: its composition and evolution[END_REF] and of [START_REF] Krauskopf | Introduction to geochemistry[END_REF], as well as to the average composition of the suspended particles in the Parana [START_REF] Depetris | The geological characteristics of the Parana River suspended sediment load: an initial assessment[END_REF] and the Amazon Rivers [START_REF] Gaillardet | Chemical and physical denudation in the Amazon river basin[END_REF]. In addition, the corresponding ratios of sediments from local cores (pre-anthropogenic levels) of [START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF] were considered (Table 4). One can that among the 10 cores analysed by the latter author, only three were long enough to reach the background levels, according to the local sedimentation rates [START_REF] Zuo | Determination of sediment accumulation and mixing rates in the Gulf of Lions, the Mediterranean Sea[END_REF][START_REF] Radakovitch | Etude du transfert et du de´poˆt du mate´riel particulaire par le 210 Po et le 210 Pb. Application aux marges continentales du Golfe de Gascogne (NE Atlantique) et du Golfe du Lion (NW Me´diterrane´e)[END_REF] and the indicated lead contamination in local sediment cores [START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF]. For most metals, and most of the reference values, the metal/Cs ratios are quite similar, indicating that our method is suitable for the detection of the natural background values. Only in the shale of [START_REF] Krauskopf | Introduction to geochemistry[END_REF], the ratios are systematically higher than both in the other reference values, and in most of our nonpolluted samples. This shale has probably an outstanding position and we therefore refer mainly to the other literature reference values in Table 4. One can notice that also the local pre-anthropogenic sediment values are in good agreement with both our values and the other reference ratios.

The good agreement of the natural metal/Cs ratios from the literature with our samples is especially true for Co, Cr, Cu and Ni. Nevertheless, the higher Ni/Cs ratio observed in local pre-anthropogenic sediments is much more difficult to explain. But it is interesting to note that a similar shift between our surficial sediment samples and those of [START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF] is observed, suggesting the influence of the analytical procedure. On the other hand, our ratios are more elevated for Pb and Zn.

The too elevated value for lead is interesting in this context because it seems to reflect the impact of aerosol deposition in the Gulf of Lions. It is well known that this trace metal has contaminated the troposphere, mostly during the 19th century by human activities related to gasoline combustion and metal processing [START_REF] Nriagu | Quantitative assessment of worldwide contamination of air, water and soils by trace metals[END_REF]. Associated with submicron particles, the pollutant can be advected over long distances and reach remote terrestrial and marine environments [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF][START_REF] Settle | Magnitude and sources of precipitation and dry deposition fluxes of industrial and natural leads to the North Pacific at Enewetak[END_REF]. The situation in the study area is quite complex since both anthropogenic and natural sources of aerosols are mixed in the sediment [START_REF] Grousset | Transient Pb isotopic signatures in the Western European atmosphere[END_REF]. But these different origins can be traced by their isotopic signatures [START_REF] Ve´ron | Continental origin and industrial sources of trace metals in the northwest Atlantic atmosphere[END_REF], and the anthropogenic contribution in the overall lead concentrations can therefore be estimated.

Among the four stable isotopes of lead ( 204 Pb, 206 Pb, 207 Pb and 208 Pb), the 206 Pb/ 207 Pb ratio is the most commonly used for the detection of anthropogenic sources because of its generally low standard deviation and its insensitivity to mineralogical changes in the sediments. In this work, lead isotope analyses have been performed on a subset of 17 surficial sediment samples spreading over the entire continental margin (see Fig. 1, Table 5). We focused our attention on both ratios. When plotting our data in a 206 Pb/ 207 Pb and 206 Pb/ 204 Pb diagram (Fig. 5), they globally fall on the mixing line of the main end-members given by the literature: local pre-anthropogenic sediments [START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF], leaded gasoline [START_REF] Monna | Pb isotopic composition of airborne particulate material from France and Southern United Kingdom: implications for Pb pollution sources in urban areas[END_REF] and some commonly used industrial aerosol signatures [START_REF] Monna | Pb isotopic composition of airborne particulate material from France and Southern United Kingdom: implications for Pb pollution sources in urban areas[END_REF][START_REF] Ve´ron | Isotopic evidence of pollutant lead sources in Northwestern France[END_REF]. Only station 35 (Table 5) presents outstanding ratios, falling in the range of the local preanthropogenic levels, which rarely occurs in modern surficial sediments, except after the rapid deposition of Saharan aerosols. But the corresponding Pb/Cs ratio, which is similar to those of the entire study area, does not support such a hypothesis. Station 35 refers to the Teˆt river mouth, in the western part of the Gulf of Lions (Fig. 1). [START_REF] Camarero | Historical variations in lead fluxes in the Pyre´ne´es (northeast Spain) from dated lake sediment core[END_REF] defined the lead isotopic ratios of pre-anthropogenic sediments in Pyrenean lakes, which can be used as local background reference for the Teˆt river basin. The latter author found significantly radiogenic values than those of [START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF]. It is therefore possible that the suspended river material discharged by this river may have a different natural lead isotopic signature compared to the general trend observed in the Gulf of Lions. As a consequence, we estimated the anthropogenic lead contribution in this station differently compared to the other stations (see below).

When looking at Fig. 5, it becomes evident that our surficial samples received an anthropogenic lead contribution, distinguishing them clearly from the local natural levels, even if the degree of contamination is more difficult to determine according to the wide range of anthropogenic signatures revealed by the literature. To do so, we plotted in Fig. 6 the 206 Pb/ 207 Pb ratio versus the inverse of the lead concentration of our data, including also the data for local anthropogenic levels determined by [START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF]. The corresponding regression results in a high correlation coefficient (r 2 Z0.82, nZ22) and an intercept of 1.170G0.009, giving the mean anthropogenic signature of the 206 Pb/ 207 Pb ratio. The distance of some data points from the regression line is probably due to the complex mixing of the different anthropogenic lead sources that affect the entire study area heterogeneously, identifying several geographic units (see Fig. 6). Thus, the mean anthropogenic signature can only be considered as a global estimation.

Following the same procedure, the mean anthropogenic 206 Pb/ 204 Pb ratio can be estimated to be about 18.220G0.22 (r 2 Z0.73, nZ22). Therefore, our mean anthropogenic ratios clearly fall outside of the range of the gasoline isotopic compositions, which are less radiogenic (according to Monna et al. (1997), 206 Pb/ 207 PbZ1.069e1.094 and 206 Pb/ 204 PbZ16.564e 17.068). This indicates that industrial emissions are mainly responsible for the anthropogenic lead input to the Gulf of Lions, even if a mixture of the different endmembers is possible. These results correlate well with the trend observed in recent studies, revealing an increase of the anthropogenic radiogenicity [START_REF] Flament | European isotopic signatures for lead in atmospheric aerosols: a source apportionment base upon 206 Pb/ 207 Pb ratios[END_REF][START_REF] Ve´ron | Isotopic evidence of pollutant lead sources in Northwestern France[END_REF][START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF]. This shift seems to be principally due to the diminution of the automotive lead impact since the use of unleaded gasoline.

Based on the 206 Pb/ 207 Pb ratio, which is considered as an independent index, an estimation of the aerosol contribution to the total lead concentrations is possible, using the following mixing equation: However, due to the different pollution sources that can contribute to the anthropogenic signatures, the plot distribution requires a correction in order to avoid misleading results. We therefore decided for the data points that obviously withdraw from the regression line in Fig. 6, to project them orthogonally to the regression line in order to recalculate the corresponding

ARTICLE IN PRESS

DTD 5

206 Pb/ 207 Pb ratios. The have been averaged and station 35 has been processed with the natural values of [START_REF] Camarero | Historical variations in lead fluxes in the Pyre´ne´es (northeast Spain) from dated lake sediment core[END_REF].

As a result, the overall anthropogenic lead contribution to the Gulf of Lions can be calculated to range between 36 and 71%, with a mean value of 61%. This is in accordance with the results of local investigations [START_REF] Ferrand | Isotopic tracing of anthropogenic Pb inventories and sedimentary fluxes in the Gulf of Lions (NW Mediterranean sea)[END_REF] and recent works on some French forest soils [START_REF] Hernandez | Heavy metal distribution in some French forest soils: evidence for atmospheric contamination[END_REF]. In general terms, the contamination seems to be uniformly spread over the study area, except for a low value observed in the NW part of the Gulf (Fig. 6) and the stations in the eastern part which reveal the highest contributions, probably due to the impact of the nearby city of Marseille. It is interesting to note that the influence of the Rhoˆne, Petit-Rhoˆne and Teˆt (not shown) rivers on the lead input (recorded in the corresponding prodeltas) seem to be of the same magnitude as those in the offshore stations. This indicates that aerosols deposited in the river basins are progressively eroded and rivertransported to the marine environments, where they meet the aerosol inputs coming from direct deposits on the seaeatmosphere interface. Both fluxes are finally mixed and spread over the entire margin by local hydrodynamics and sedimentary processes.

These results allow us now to estimate the background signature for lead on the basis of our samples that were corrected for the atmospheric contribution. The normalization of the resulting ''natural'' Pb by Cs gives the value of 2.20, which is about half the previously calculated value (Table 4). This ratio is now in very good agreement with most of the ratios in our reference literature values, and more precisely the local pre-anthropogenic sediment of [START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF].

For Zn, the slight enhancement of the Cs normalized background value compared to the literature data is more difficult to explain. It is possible that in the Gulf of Lions, the natural Zn levels are greater than in the reference shale of [START_REF] Taylor | The continental crust: its composition and evolution[END_REF], but the deepest samples of local sediment cores of [START_REF] Marin | Re´partition et fractionnement ge´ochimique des e´le´ments traces dans les se´diments marins[END_REF] suggest that our value might be slightly affected by anthropogenic sources. The other possibility is that, like in the case of Pb, also for Zn, pollution is more widespread in the Gulf of Lions and affects the sites offshore the river deltas. It is generally admitted that Zn in aerosols of the NW Mediterranean is essentially of anthropogenic origin (Lo¨ye-Pilot, personal communication), which means that this element may also be widely introduced into the platform via the two pathways (i.e. direct deposit and river transport) described previously. Note that a similar enhancement was observed using the other normalizers, underscoring the trend of a slight anthropogenic influence for Zn in the entire study area. Further work is needed to confirm and/or to reject this hypothesis. Nevertheless, we consider that the local preanthropogenic sediment values calculated from Marin (1998) may probably be the best estimate for the natural background values in the Gulf of Lions.

Conclusions

The purpose of this study was to test the ability of different potential normalizer elements to detect the degree of heavy metal pollution in surface sediments of the Gulf of Lions. Among the different candidates for normalization we tested, Cs appeared to be the most appropriate choice when compared to Al, Fe and Sc, the most currently used normalizer elements. It better reflected the fine-grained sediment and was the least sensitive to the silt fraction in our study area. The normalization capacity of this element has probably been overlooked in previous studies, although this has naturally to be confirmed by additional studies in different environments.

Using this element in the fraction !63 mm, we obtained, with a high confidence level, the local background signal of Co, Cr, Cu, Ni and Pb. This is confirmed both by comparing them to other literature values for non-polluted areas and to local pre-anthropogenic sediments. For lead, however, the aerosol contribution to the overall concentrations has to be estimated and removed first, because of the major influence of the atmospheric particles on the total lead concentrations in the surficial sediments. Directly deposited on the seae atmosphere interface or river-transported with terrestrial particles, these man-made aerosols may be subsequently spread over the entire platform by hydrodynamics and/or sediment resuspension. Also for Zn, it is possible that the atmospheric deposition plays an important role in the introduction of this element in the marine environment, since the normalized Zn values also revealed a slight enrichment compared to local pre-anthropogenic sediment. But the data do not allow further confirmation or rejection of this hypothesis.

Except Zn, for which the deep sediment core signal may be preferred, our results indicate that the backgrounds obtained can be used as the most appropriate references for further regional studies on the heavy metal pollution in the Gulf of Lions and/or similar regions. The appropriate background level choice is crucial for the interpretation of the pollution state in the coastal environment. Non-representative of regional specificities, literature references may yield conflicting conclusions on the cause of enhanced metal levels (i.e. anthropogenic enrichment or natural variability), as suggested by other authors [START_REF] Rubio | Geochemistry of major and trace elements in sediments of the Ria de Vigo (NW Spain): an assessment of metal pollution[END_REF][START_REF] Gibbs | Metals of the bottom muds in Townsville harbour, Australia[END_REF]. Of course, in the case of our samples, one should mention that also the selection of most of the other normalizer elements allow general distinction between the polluted and non-polluted areas in the Gulf of Lions, since the correlations with the fine-grained in the sediments are still reasonably However, the resulting pictures are much less detailed, and some important information can be lost in the interpretation of the general distribution patterns.

Fig. 1 .

 1 Fig. 1. Study area and sampling stations (black points and open stars). Stars refer to samples analysed for lead isotopic composition. The bathymetry has been kindly supplied by S. Berne´, IFREMER (Brest).

Fig. 2 .

 2 Fig. 2. Determination of natural background values for Co, Cr, Cu and Ni based on their correlation with Cs (nZ49). The upper triangles are the samples which have been successively removed from the regressions (see text). Values were log-transformed. The dotted lines represent prediction interval at the 95% confidence level.

Fig. 3 .

 3 Fig. 3. Geographic locations of samples revealing enhanced contents of Co, Cr, Cu and Ni. For additional information, the boundary of the shelf has been represented, showing the complexity of the canyon heads distribution.

Fig. 4 .

 4 Fig. 4. Relationships between clay content (% !2 mm) and Al (%) and Cs (ppm) (nZ46), respectively. The dotted lines represent prediction interval at the 95% confidence level.

Fig. 5 .

 5 Fig. 5. Lead isotopic composition of surficial sediments of the Gulf of Lions (nZ17, black circles). The main end-members are also represented in this diagram. a Ferrand et al. (1999), b Ve´ron et al. (1999), c Monna et al. (1997).

Fig. 6 .

 6 Fig. 6. Diagram of 206 Pb/ 207 Pb ratios versus inverse of lead concentrations in surficial sediments (black circles). Some local pre-anthropogenic values have been added to the linear regression (data from Ferrand et al. (1999), open circles). Isotopic signatures of both metallurgy plants and gasoline are presented by straight lines. xVe´ron et al. (1999), *Monna et al. (1997).

Table 1

 1 Control of analytical accuracy based on the use of MESS-2 and GSMS-3 marine sediment standards. Concentrations are in ppm

	Element MESS-2 Measured Recovery	GSMS-3 Measured Recovery
				(%)			(%)
	Co	13.8	14.3	103.6	53	55.7 105.1
	Cr	106	106.8	100.8	38	36.4	95.8
	Cu	39.3	39.0	99.3	231	242.0 104.8
	Ni	49.3	48.9	99.2	108	107.7	99.7
	Zn	172	172.3	100.2	142	150.2 105.8
	Pb	21.9	23.2	106.0	22	21.2	96.4
	Cs	e	e	e	4.5	4.5 100.0
	Sc	e	e	e	15	14.6	97.3
	Al 2 O 3 e	e	e	77000	74390	96.6
	Fe 2 O 3 e	e	e	38100	36930	96.9

Table 2

 2 Correlation coefficients (r 2 ) between various elements analysed in this study, as well as between these elements and fine fractions in the samples (!2 and 20 mm content)

	Element	Al	Fe	Sc	Cs	Co	!2 mm	!20 mm
	! 2 mm	0.44	0.44	0.30	0.82	0.60	e	0.84
	! 20 mm	0.62	0.59	0.39	0.92	0.59	0.84	e
	Co	0.49/0.53* (3)	0.64/0.67* (1)	0.49 (0)	0.76/0.83* (6)	e	0.60	0.59
	Cr	0.25/0.54* (7)	0.48/0.57* (5)	0.48 (0)	0.48/0.88* (12)	0.74/0.87* (5)	0.58	0.57
	Cu	0.18/0.30* (6)	0.21/0.42* (5)	0.32/0.56* (6)	0.28/0.70* (19)	0.31/0.58* (6)	0.07	0.11
	Ni	0.46/0.62* (6)	0.53/0.69* (4)	0.39/0.46* (1)	0.61/0.89* (10)	0.67/0.91* (2)	0.44	0.42
	Pb	0.38/0.52* (5)	0.49/0.68* (4)	0.51/0.55* (2)	0.67/0.81* (6)	0.67/0.73* (1)	0.43	0.5
	Zn	0.24/0.41* (6)	0.32/0.54* (6)	0.35/0.72* (7)	0.35/0.80* (11)	0.31/0.43* (5)	0.12	0.22
	Values were log-transformed, with the exception of granulometric percentages (nZ47 for major elements; nZ49 for trace metals; nZ46 for 2 and 20
	mm fractions). *Correlation coefficient after the removal of enhanced samples. (x) number of enhanced samples removed from the regressions.

a Without atmospheric contribution. b

[START_REF] Depetris | The geological characteristics of the Parana River suspended sediment load: an initial assessment[END_REF]

. c

[START_REF] Gaillardet | Chemical and physical denudation in the Amazon river basin[END_REF]

.

d

[START_REF] Taylor | The continental crust: its composition and evolution[END_REF]

. e

[START_REF] Krauskopf | Introduction to geochemistry[END_REF]

. f Marin (1998).

Table 5

 5 Lead isotope ratios and inverse lead concentrations of some surficial sediments of the Gulf of Lions (nZ17)

	Stations	Longitude	Latitude	206/204	206/207	1/Pb
	08	4.638	42.822	18.435	1.176	0.022
	09	4.403	43.442	18.408	1.185	0.018
	09X	4.832	43.233	18.319	1.184	0.023
	12	4.591	42.978	18.319	1.182	0.021
	16	4.086	43.217	18.526	1.182	0.024
	21bis	3.534	43.226	18.52	1.188	0.038
	25	3.848	43.048	18.46	1.183	0.022
	26	3.97	42.851	18.384	1.177	0.025
	29bis	3.45	42.91	18.526	1.184	0.024
	35	3.065	42.716	18.771	1.198	0.035
	38bis	3.439	42.542	18.523	1.177	0.023
	39bis	3.511	42.449	18.26	1.176	0.024
	CH20	4.854	43.314	18.628	1.181	0.022
	KB4	4.868	43.318	18.588	1.184	0.021
	KB16	4.824	43.32	18.548	1.186	0.022
	PL1	5.221	43.095	18.346	1.177	0.019
	PL2	5.192	43.021	18.32	1.180	0.016

  Anthropogenic contribution ð%Þ Z ð 206 Pb= 207 PbÞ sample ÿ ð 206 Pb= 207 PbÞ natural ð 206 Pb= 207 PbÞ anthropogenic ÿ ð 206 Pb= 207 PbÞ natural !100

Acknowledgements

The authors would especially like to thank the IRSN group of M. Arnaud and S. Charmasson for having invited us to the REMORA 3 campaign. We would also like to thank IRSN and CEFREM research teams for their assistance during the cruise, together with the captain and crew of the IFREMER Research Vessel ''Europe''. C. Boucayrand and R. Freydier (LMTG) provided useful advice to improve the analytical work. This research was supported by the French research programs ORME (Observatoire Re´gional Me´diterrane´en sur l'Environnement) and PNEC (Programme National sur l'Environnement Coˆtier).