Laurie Bougeois
email: laurie.bougeois@gmail.com

Marc De Rafélis

Gert-Jan Reichart

J De Nooijer

Guillaume Dupont-Nivet

Mg/Ca in fossil oyster shells as palaeotemperature proxy, an example from the Palaeogene of Central Asia

Keywords: palaeoclimate, oyster, Mg/Ca, sclerochronology, Palaeogene, Central Asia

Fossil oyster shells are well-suited to provide palaeotemperature proxies from geologic to seasonal timescales due to their ubiquitous occurrence from Triassic to Quaternary sediments, the seasonal nature of their shell growth and their strong resistance to post-mortem alteration. However,

common use to translate calcitic oxygen isotopes into palaeotemperatures is challenged by uncertainties in accounting for past seawater δ 18 O, especially in shallow coastal environment where oysters calcify. In principle, the Mg/Ca ratio in oyster shells can provide an alternative palaeothermometer.

Several studies provided temperature calibrations for this potential proxy based on modern species, nevertheless their application to palaeo-studies remains hitherto unexplored. Here, we show that past temperature variability in seawater can be obtained from Mg/Ca analyses from selected oyster fossil species and specimens. High-resolution Mg/Ca profiles, combined with δ 18 O, were obtained along 41 fossil oyster shells of seven different species from the Palaeogene Proto-Paratethys sea (Central Asia) found in similar as well as different depositional age and environments providing comparison. Suitable Mg/Ca profiles, defined by continuous cyclicity and reproducibility within one shell, are found to be consistent for specimens of the same species but differ systematically between species, implying a dominant species-specific effect on the Mg/Ca incorporation. Two species studied here (Ostrea (Turkostrea) strictiplicata and Sokolowia buhsii ) provide an excellent proxy

Introduction

The ability to track quantitatively climate seasonality patterns through the geological record is key to understand past, present and future climate processes (Felis et al., 2004;[START_REF] Licht | Asian monsoons in a late Eocene greenhouse world[END_REF].

However, most palaeoclimate proxies yield only qualitative assessments of seasonality or insufficient resolution averaging across thousands of years [START_REF] Eldrett | Continental ice in greenland during the eocene and oligocene[END_REF][START_REF] Evans | Eocene seasonality and seawater alkaline earth reconstruction using shallow-dwelling large benthic foraminifera[END_REF].

In contrast, bivalve shells have long been recognized to potentially represent excellent highresolution (i.e. infra-annual) palaeoclimate archives (e.g. [START_REF] Hudson | Sclerochronology: a tool for interpreting past environments[END_REF]Richardson, 2001).

The shell of bivalves is formed by incremental growth resulting in a record that can be sampled at an intra-annual resolution (Stenzel, 1971;[START_REF] Kirby | Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica)[END_REF]. Moreover, they live over a large range of climate regimes and habitats (Surge et al., 2001;[START_REF] Mouchi | Chemical labelling of[END_REF]. In addition, their generally large and thick calcitic shells promote good preservation of fossils such that oysters represent one the most abundant macro-fossil groups readily available in the sedimentary records (Stenzel, 1971).

When biomineralizing, bivalve species have been shown to build their shell in isotopic equilibrium with sea water and therefore, its stable oxygen isotope ratios (δ 18 O) reflect seawater temperatures in which the organisms formed its shell [START_REF] Killingley | Stable isotopes in a mollusk shell: detection of upwelling events[END_REF]Wefer and Berger, 1991;[START_REF] Ivany | Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary[END_REF]. However, carbonate δ 18 O also varies with the stable oxygen isotopic composition of seawater (δ 18 O sw , e.g. [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF], such that measured isotope ratios corresponds to a combination of past seawater temperature and isotope composition. Estimating seawater isotopic composition and its intra-annual variability in the geologic past remains a major challenge to isolate the temperature signal from shell oxygen isotopes. In principle, seawater oxygen isotope composition can be resolved using a second independent temperature proxy, not primarily depending on salinity or seawater oxygen isotope composition. Such a proxy may be available from minor and trace elements in biominerals, particularly the Mg/Ca and Sr/Ca ratios in carbonates are known to reflect past seawater temperatures in foraminifera (e.g. Nürnberg et al., 1996;[START_REF] Lear | Cenozoic deep-sea temperatures and global ice volumes from mg/ca in benthic foraminiferal calcite[END_REF] and corals [START_REF] Lough | Climate records from corals[END_REF], respectively. Based on Mg/Ca and Sr/Ca, these temperatures can be

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combined with carbonate δ 18 O to estimate past seawater δ 18 O, which in turn reflects seawater salinity [START_REF] Elderfield | Past temperature and δ 18 O of surface ocean waters inferred from foraminiferal Mg/Ca ratios[END_REF][START_REF] Lear | Cenozoic deep-sea temperatures and global ice volumes from mg/ca in benthic foraminiferal calcite[END_REF][START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF].

Accurate minor/trace element-temperature calibration for bivalve shells have long been unavailable by a lack of appropriate controlled growth studies [START_REF] Dodd | Environmental control of strontium and magnesium in Mytilus[END_REF]. More recently, many studies succeeded in relating seawater temperature to bivalve calcitic Mg/Ca ratio in mussels [START_REF] Klein | Bivalve skeletons record sea-surface temperature and δ 18 O via Mg/Ca and 18 O/ 16 O ratios[END_REF]Vander Putten et al., 2000;Freitas et al., 2009;Wanamaker et al., 2008), scallop (Freitas et al., 2006(Freitas et al., , 2009(Freitas et al., , 2012)), fan mussels (Freitas et al., 2005), and oysters (Surge and Lohmann, 2008;[START_REF] Mouchi | Chemical labelling of[END_REF]. These studies revealed relatively large inter-specific differences in temperature sensitivity (see Table 1 for different calibrations). In addition, environmental conditions exert a strong impact on bivalve Mg/Ca even within one species. For example, calibrations differ for M.

edulis when calcifying in estuarine, culture or natural brackish waters (eq. 6, 7 and 8 in Table 1), suggesting that variables other than temperature additionally determine bivalve calcite chemistry (Freitas et al., 2005(Freitas et al., , 2006(Freitas et al., , 2009(Freitas et al., , 2012)). These suggested variables include metabolism (Rosenberg and Hughes, 1991;Vander Putten et al., 2000), intra-shell variability (Freitas et al., 2009), salinity and growth rate (Wanamaker et al., 2008) on Mytilus edulis. Studies also revealed an ontogenyrelated effect in Pinna nobilis (Freitas et al., 2005) and an impact of incorporated organic matter (mainly conchiolin) on bivalve aragonite shell chemistry in Arctica islandica (Schöne et al., 2010).

These effects are negligible in oyster shells in which incorporation of organic compounds into the carbonate crystal lattice, especially in foliated calcite, is much lower than in other bivalve (e.g. mussels and scallops; Stenzel, 1971). Furthermore, the oyster's conchiolin is relatively easily degraded by bacterial activity, so that even the organic content of older parts of the shell has already decreased during the life of the animal and is absent in fossil oysters (Stenzel, 1971). Recent studies also showed that impacts of salinity, ontogeny and growth rate are modest compared to that of temperature on Mg incorporation into the foliated oyster calcite shell (Surge and Lohmann, 2008;[START_REF] Mouchi | Chemical labelling of[END_REF]. Furthermore, duplicate of Mg/Ca ratio analyses in the ligamental area showed that intra-shell small scale variability for magnesium is negligible [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF].

Although these considerations clearly designate oyster shells as prime targets for palaeoclimate reconstructions using Mg/Ca, such analyses have yet to be tested using a thorough analytical protocol. We previously conducted a pilot study on a single fossil oyster shell (Sokolowia buhsii ) from the Eocene Proto-Paratethys sea in Central Asia suggesting that its Mg/Ca may be used to recon-

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struct palaeotemperatures [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. However, to constrain the reliability of Mg/Ca as a palaeothermometer for the geological past and to quantify inter-specimen variability in element composition, additional investigations on specimens and species from a range of environments and ages are necessary. Here, we extend the approach to various Palaeogene oyster species including different environmental and stratigraphic contexts well-constrained by parallel studies of the western Proto-Paratethys sea (Bosboom et al., 2014a,b,c) (Bosboom et al., 2014a,b,c). We aim to determine whether Mg/Ca ratios yield reliable and reproducible environmental intra-annual proxy data in fossil oyster shells from various species and environments and thereby the reliability of the Mg/Ca-temperature calibration established on modern oysters when applied to fossil specimens.

Context of sampling

Environmental and stratigraphic setting

During Palaeogene times, a large shallow epicontinental sea belonging to the Tethyan Realm covered the Eurasian continent from the Mediterranean Tethys (west side) to the Tarim Basin (western China, east side) [START_REF] Dercourt | Atlas Tethys, Paleoenvironmental Maps: Explanatory Notes[END_REF][START_REF] Burtman | Cenozoic crustal shortening between the Pamir and Tien Shan and a reconstruction of the Pamir-Tien Shan transition zone for the Cretaceous and Palaeogene[END_REF]. In the Tarim Basin (Xinjiang, China), the Afghan-Tajik depression (Afghanistan-Tajikistan), the Fergana basin and the Alai Valley (Kyrgyzstan) (Figure 1a, b), regional regression-transgression sequences have been established based on inter-and intra-basin stratigraphic correlations of the deposits with distinct lithological facies and fossil assemblages including bivalves, dinoflagellate cysts, foraminifera and ostracods (see Bosboom et al., 2014a,b,c, and references therein). From the Cenomanian to the final sea-retreat dated late Bartonian-early Priabonian, five major second-order marine incursions have been recognized in the sedimentary record in Central Asia. The marine intervals are dominated by bioclastic wackestones to grainstones, evaporites, clay and mudstones containing ostracods, gastropods, bryozoa, serpulids, echinoids, foraminiferas, algae, fish scales and molluscs. Bivalve calcitic shells such as Ostreidae and Pectinidae mainly remained in sedimentary deposits, whereas aragonite molluscs were leached and not preserved. Sedimentary facies and fossils assemblages are characteristic of shallow coastal environments between offshore to coastal plains, and typical of a carbonate-rich neritic ramp [START_REF] Manceau | Evolution des environnements de dépôts dans le bassin du Tarim (Chine) au cours du retrait Paléogène de la Proto-Paratethys[END_REF].. Continental intervals between marine incursions contain fine to middle grained detrital rocks, red clay to siltstones and evaporitic deposits characterised by nodular and massive gypsum.

These deposits are indicative of flood plains, alluvial plains and playa environments. Sedimentary

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deposits indicate alternation between more humid and dry periods, typical of semi-arid climates with a relatively strong seasonal contrast [START_REF] Manceau | Evolution des environnements de dépôts dans le bassin du Tarim (Chine) au cours du retrait Paléogène de la Proto-Paratethys[END_REF].

Oyster sampling, age and determination

Mollusc macrofauna taxonomic identification largely follows [START_REF] Lan | Late Cretaceous-early Tertiary marine bivalve fauna from the western Tarim Basin[END_REF], [START_REF] Lan | Paleogene bivalve communities in the Western Tarim Basin and their paleoenvironmental implications[END_REF] and Bosboom et al. (2014a,c), representing the most recent revision of regional systematic literature. Fossil ages were estimated using mollusc biostratigraphy [START_REF] Lan | Late Cretaceous-early Tertiary marine bivalve fauna from the western Tarim Basin[END_REF] and adjusted for the two last marine incursions from recent studies combining bio-and magnetostratigraphy in this region (Bosboom et al., 2014a,b) (Figure 1c). Specimens from the oyster species Ostrea bellovacina were extracted from marine sediment strata in Central Asia during field excursions in the summers of 2007, 2010, 2011, 2012 and 2013 (Figure 1). All taxa belong to the Superfamily Ostreaoidea and the Families Ostreidae or Flemingostreidae [START_REF] Carter | A synoptical classification of the bivalvia (mollusca)[END_REF]. Specimens were selected according to the following criteria: (1) good preservation of the shell, particularly of the ligamental area,

(2) sufficiently large ligamental surface to contain growth bands spanning more than ten years and providing a high resolution, (3) specimens fossilized in living position attesting of the living environment. Field sedimentological analyses yield characterisation of the depositional environmental for each species (Figure 2, [START_REF] Manceau | Evolution des environnements de dépôts dans le bassin du Tarim (Chine) au cours du retrait Paléogène de la Proto-Paratethys[END_REF].

Material and methods

To determine the applicability of Mg/Ca as a temperature proxy we have combined a number of analyses on specimens from different species and representative of a range of environments. To detect potential diagenetic alteration and identify the presence of seasonal banding, shells were first analysed with cathodoluminescence microscopy. Trace element analyses were then performed at high resolution yielding Mg/Ca variations with varying reliability criteria defined below. Finally, to provide a reference for comparing and assessing the Mg/Ca results, stable isotope analyses were also performed at high incremental resolution and/or on bulk samples (Table 2).

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Shell description and sample preparation

Oyster shells grow by incremental deposition of calcium carbonate (mainly calcite), resulting in alternating dark and light bands corresponding to the colder and warmer seasons respectively [START_REF] Kirby | Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica)[END_REF]. The foliated calcite forming the ligamental area (or umbo) is very dense and resistant to post-mortem alteration, resulting in conservation of the primary environmental signal.

For this reason, we focused our chemical analyses to the ligamental area, following practice from a number of previous studies [START_REF] Kirby | Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica)[END_REF]Surge and Lohmann, 2008;Lartaud et al., 2010b;Goodwin et al., 2012;[START_REF] Mouchi | Chemical labelling of[END_REF][START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF].

After isolation from the sediment and thorough cleaning of the shell's surface by brushing off loose material and rinsing with deionized water, specimens were prepared for sclerochronological and geochemical analysis by cutting 0.5 cm-thick slices along the maximum growth axis of the left valve in the middle of the resilifer (Figure 3). These sections were subsequently polished and cleaned in an ultrasonic bath with deionized water for ten minutes and dried overnight. Radial sections reveal numerous growth lines providing a continuous record along the oyster's lifetime (Figure 3). Shell slices were then sectioned into slices of maximum 2.5 cm width and 5.0 cm length to fit the sample holder for geochemical analyses.

Cathodoluminescence analyses

Shell preservation and potential impact of diagenesis are commonly detected in bivalve shells using cathodoluminescence (CL) microscopy (e.g. [START_REF] Langlet | Experimental and natural manganese-induced cathodoluminescence in the shell of the Japanese oyster Crassostrea gigas (Thunberg, 1793) from Thau Lagoon (Hérault, France): ecological and environmental implications[END_REF]. In calcite, Mn 2+ is the main luminescence activator, causing emission of yellow to orange light of which the intensity is positively correlated with Mn concentration [START_REF] El Ali | Mn 2+ -activated luminescence in dolomite, calcite and magnesite: quantitative determination of manganese and site distribution by EPR and CL spectroscopy[END_REF][START_REF] De Rafélis | Contribution of cathodoluminescence to the knowledge of manganese speciation into pelagic limestones. Determination of the control of sea-level variations on third-order sequences during the Upper Jurassic[END_REF][START_REF] Habermann | Quantitative cathodoluminescence (CL) spectroscopy of minerals: possibilities and limitations[END_REF][START_REF] Langlet | Experimental and natural manganese-induced cathodoluminescence in the shell of the Japanese oyster Crassostrea gigas (Thunberg, 1793) from Thau Lagoon (Hérault, France): ecological and environmental implications[END_REF]. As Mn is also preferentially incorporated in the oyster shell calcite during summer months [START_REF] Langlet | Experimental and natural manganese-induced cathodoluminescence in the shell of the Japanese oyster Crassostrea gigas (Thunberg, 1793) from Thau Lagoon (Hérault, France): ecological and environmental implications[END_REF]Lartaud et al., 2010b), this results in high luminescence intensities during summer and conversely relatively low luminescence during winter. Thus variations of the CL intensity following the shell's growth axis in each slab and can thus be used as a rapid and non-invasive method to identify the presence of seasonal banding [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. CL

analyses were performed at the Université Pierre et Marie Curie (Paris, France) on a cold cathode device (Cathodyne OPEA) coupled to an optical microscope and a digital camera.

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Trace element analyses

Trace element composition was determined by Laser Ablation-Inductively Coupled-Plasma-Mass Spectrometry (LA-ICP-MS), with a Geolas 200Q Excimer 193 nm laser coupled to a sector field ICP-MS (Element2, Thermo Scientific) at the Utrecht University (The Netherlands). The sectioned oyster shells were ablated at lower energy density (∼1 J.cm -2 ) by moving the sample in x, y, and z axis at a constant speed underneath the laser beam in a He environment. The path was programmed so that subsequent circular ablation spots (120 µm in diameter) overlapped slightly and resulted in a resolution of one data point every ∼7 µm leading in more than 100 measurements for each growth band. All transects were ablated perpendicularly to the growth axis to monitor consistency of obtained profiles. To test data consistency and quantify intra-shell variability inside the umbo, each transect was duplicated by a second parallel transect spaced at approximately 0.5 to 1 mm from the first one. Masses monitored by the ICP-MS included 24 Mg, 26 Mg, 43 Ca, 44 Ca, 55 Mn, 88 Sr and 138 Ba, using the international standard NIST SRM610 (using element concentrations reported in [START_REF] Jochum | Determination of reference values for NIST SRM 610-617 glasses following ISO guidelines[END_REF] and 43 Ca as an internal standard (assuming 40% wt in CaCO 3 ). The glass standard was ablated between measurements of each oyster at a higher energy density (∼5 J.cm -2 ).

Calibration of element/calcium ratios in calcium carbonate samples was obtained with the NIST glass standard yielding accurate values for many elements when using a 193 nm laser as indicated by similar fractionation factors obtained when using glass and carbonate standard material, despite differences in ablation characteristics for these materials [START_REF] Hathorne | Physical and chemical characteristics of particles produced by laser ablation of biogenic calcium carbonate[END_REF]. Here, we mainly focus on the obtained patterns in Mg/Ca (for more details about the LA-ICP-MS protocol, see [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. To account for small scale fluctuations in the Mg/Ca ratio, corresponding values from the two parallel transects were averaged so that a single data-point was obtained for each incremental position. On these data-points, a moving average using a Bartlett window was then run through data from every transect as previously described in [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF].

Stable isotopes analyses

Stable oxygen isotopes were analysed (1) at high incremental resolution aiming to resolve seasonal variability and (2) on bulk samples aiming to average the isotopic record throughout the oyster's life. Bulk carbon isotopes (δ 13 C) were also analysed to assess potential environmental effects on the shell's chemistry. To collect material for these analyses, a high-precision, computerdriven Micromill (New Wave Research) attached to an x, y and z stage was used to follow digitized

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milling path positions. Bulk analyses were performed by milling one continuous line following laser transects across the ligamental area. For incremental analyses, parallel paths were milled every 100-120 µm following the growth bands (Figure 3). At least 30 to 50 µg of calcitic shell powder was collected for each milled sample and analysed for oxygen and carbon stable isotopes (δ 18 O and δ 13 C in % VPDB) using a KIEL-III device coupled online to a Finnigan MAT-253 mass spectrometer at Utrecht University for incrementally resolved data of the specimens AT11-O04, KZ07-O01, KY10-O01. Material from all other specimens and for bulk analyses were analysed using a KIEL-IV carbonate device at the Université Pierre et Marie Curie (UPMC, Paris, France). The international standard NBS-19 and an in-house standard (Naxos marble in Utrecht University and Marceau in UPMC) were used for calibration of all sample sets. For both instruments, long-term analytical precision was better than 0.08% for δ 18 O and better than 0.05% for δ 13 C.

For the oysters analysed at the Utrecht University the entire length of each shell specimen was milled for incremental study at high resolution (one sample every 120 µm) totalling ∼130 samples per shell for ∼20 cycles (or years). Results from this first batch of samples indicate that a slightly lower resolution was sufficient except for intervals with much lower growth rates (and thus thinner bands). This justified a drilling resolution of only 5 to 8 annual cycles into the remaining specimens that all displayed higher growth rates and thereby enabled increasing the total number of specimens analysed. At a resolution of one micro-milled sample every 100 to 200 µm (samples with sufficiently high growth rates were analysed with only one micro-sample every 200 µm) between 42 and 82 samples were thus analysed per specimen. This enabled the incremental stable isotope analyses for 20 carefully-chosen shells (out of the 41 shells initially analysed for trace element composition) while bulk stable isotope analyses was performed on 40 shells, to obtain at least one stable isotopic value for each specimen.

Analytical results

Cathodoluminescence results

In total, 38 oyster shells were analysed with cathodoluminescence microscopy. In general, clear and consistent banding in CL intensities show that diagenetic effects potentially affecting the analysed oyster shells were minimal, indicating a good preservation of the carbonaceous skeleton. A cyclic pattern is well defined for the species S. buhsii, O. (T.) strictiplicata and O. bellovacina with

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alternations between intense luminescence for the light summer bands and lower luminescence intensities for the dark winter growth bands. The luminescence for F. hemiglobosa is very weak and the luminescence for F. bashibulakeensis is inverted with thinnest layers corresponding to winter growth deceleration (Richardson et al., 1993;[START_REF] Kirby | Differences in growth rate and environment between tertiary and quaternary Crassostrea oysters[END_REF] with relatively bright luminescence (Figure 3).

Trace element results

A total of 41 shells were analysed for trace elements. Mg/Ca values range between 1.06 and 14.33 mmol/mol. Average Mg/Ca values for a single specimen varies from 2.74 to 9.75 mmol/mol (Figures 6 7, 8 and 9e-h).

The intra-shell patterns in Mg/Ca vary between specimens and we therefore evaluated the quality of the results using two properties: (1) the small-scale intra-shell variability, which is the difference between patterns in Mg/Ca from the two parallel transects;

(2) the cyclicity in Mg/Ca reflecting whether a primary seasonal signal has been recorded. According to these criteria we grouped the Mg/Ca results into three categories (Table 2):

-suitable (continuously cyclic and consistent between parallel transects, Figure 6)

-partly suitable (only some parts are cyclic and consistent between transects, Figure 7)

-unsuitable (non cyclic and/or not consistent between parallel transects, Figure 8).

For example Mg/Ca ratios from specimen KZ12-O05.1 (Figure 8) display a cyclic pattern but the absolute values of the two transects are clearly different and therefore this sample is qualified as unsuitable. Transects from the specimen MS12-O06.5 (Figure 7) display very consistent values from 7 to 19 cm but not from 0 to 7 cm, therefore it is qualified as partly suitable. These reliability assessments based on elemental results provide a useful criterion for selecting promising specimen before engaging into further isotopic analyses. Unsuitable specimens were discarded altogether from further isotopic analyses. In suitable and partly suitable specimens, well-defined continuous parts were selected for further isotopic analyses. Of all shells analysed (n=41), 12 were considered suitable, 13 partly suitable and 16 unsuitable (Table 2).

Stable isotopes results

All isotopes values obtained (bulk sample analyses and high-resolution incremental analyses) range between -5.45% and -0.57% for δ 18 O and between -0.74% and 2.53% for δ 13 C (Figure

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For each sample, the average value derived from averaging all incremental analyses compares well with values from the bulk sample analyses (Figure 9a-d and Supplementary data). Specimens retrieved from the same stratigraphic bed usually exhibit similar values -standard deviation within 0.3% for δ 13 C and 0.4% for δ 18 O-indicating homogeneous isotope ratios in oyster shells living at the same time and in a similar environment.

The well-resolved cyclicity observed in the oyster's isotopes confirms that sampling resolution (every 100-200 µm; depending on growth rates) was sufficiently high to clearly reveal intra-annual environmental variations with regular cyclicity. However, on two samples with particularly low growth rates in the oldest part of the shell (AT11-O04 and KY10-O01, Figure 5) the expression of seasonality is poor. This most likely results from a sampling bias, mixing material between closely spaced adjacent growth bands rather than an ontogenetic effect or a change in environmental variations or as previously shown in [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. We therefore discarded these parts of the shell record from further consideration.

Discussion

In light of the obtained analytical results, we discuss below (1) the reliability of Mg/Ca records in the various shell species analysed (2) species and environmental effects and (3) the validity of proposed calibrations of the Mg/Ca as a proxy for temperature for various species and depositional environments.

Reliability of Mg/Ca profiles for different species

It appears that reliability of Mg/Ca records varies between species and the species-specific patterns in Mg/Ca are therefore discussed separately.

-O. (T.) strictiplicata and S. buhsii mostly yield suitable and some partly suitable Mg/Ca data (Figures 6 and7, Table 2). Variations are cyclic and correlate well with the other geochemical proxy δ 18 O. No clear ontogenetic effect can be recognized for the Mg incorporation in oyster shells, and CL indicates that diagenesis did not affect the shells analysed here. Furthermore, for specimens with both bulk and incremental stable isotope data, the bulk values compare well with the average of all the incremental δ 18 O values within 0.4±0.2% (see Figure 9ad). This underlines the homogeneity of the isotopic composition within the ligamental area we conclude that all the geochemical tracer analysed in the fossil shells selected here reflect a primary environmental signal. Moreover, the results from these species are all cyclic suggesting seasonal variations in conditions which makes Mg/Ca (and δ 18 O) a promising tool for intraannual palaeoclimate reconstructions.

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-F. bashibulakeensis specimens yielded no suitable profiles, a few partly suitable and mainly unsuitable results displaying a non-cyclic pattern with highly variable Mg/Ca ratios throughout the shell. This may be partly explained by the curvature of the ligamental area affecting the reliability of the positioning of the sampling transects. This is supported by the fact that parts of the transects that may be suitable are perpendicular to the growth direction while unsuitable part of the transects cross the growth bands at an oblique angle. We therefore conclude that species with a curved ligamental area are less suitable for trace element analyses as performed here. Developing a new sampling method following the curvature more closely may yield more reliable results in oysters with strongly curved ligamental areas.

-F. hemiglobosa and O. bellovacina yielded no suitable profiles, a few partly suitable and many unsuitable Mg/Ca patterns characterized by variable intra-shell composition and absence of a well-defined cyclicity (Figure 8). This was observed even from samples from the same stratigraphic level in the Kuhdara section suggesting this is not an actual environmental effect (Table 2). In addition, this unreliability is unlikely caused by diagenesis, since the incremental δ 18 O data of these shells display regular cycles (Figure Figure 5) and that CL values show no alteration through the ligamental area indicating good preservation. For some

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Environmental and species effects

Oyster shells from the same sedimentary level (that have calcified at the same time and location) yielded similar isotopic and Mg/Ca records (Figure 9j). However, these values differ significantly and sometimes systematically when comparing different species, time periods and/or living environments suggesting that the records depend both on species and on environmental conditions. Comparison of the δ 13 C and δ 18 O from bulk and incremental average data (Figure 9i) shows that the samples can be divided into two groups: one with both depleted δ 18 O and δ 13 C values and one with more enriched δ 18 O and δ 13 C values. This may reflect the influence of freshwater (by e.g. continental runoff), resulting in depleted carbon and oxygen isotope rations of shell for species living in the shallowest sites (Lartaud et al., 2010a). This is particularly clear when comparing values from the species F. hemiglobosa in offshore environments at the Aertashi section to values from the same species in a relatively nearshore bay environment at the Mine and Kuhdara sections (Figure 9i).

Furthermore, O. (T.) strictiplicata, S. orientalis and S. buhsii are all collected from fully subtidal marine environments and have similar δ 18 O and δ 13 C to other fully marine specimens and differ strongly from, for example, P. asiatica that lives in a nearshore mangrove environment (Figure 2).

Despite this apparent systematic environmental effect on isotopic fractionation, it remains to be investigated whether this is also reflected in the Mg/Ca since reliable profiles (i.e. suitable and

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reliable parts of partly suitable profiles) were not available from specimens of the same species that calcified in different depositional environments. Only F. hemiglobosa provides this opportunity, and

shows no significant differences between specimens collected from various environments as most analyses are unsuitable.

To estimate potential species-specific effects, multiple reliable Mg/Ca profiles are compared from different species from the same living environments (Figure 9). Mg/Ca results within a species collected from one area and the same geological period are relatively similar (based on the standard de- tiplicata). However, it is more likely due to a species-specific effect on the magnesium incorporation into the calcitic shell as clearly indicated below in our analyses comparing temperatures derived from these results.

Validity of Mg/Ca -temperature calibrations depending on species

Mg/Ca-temperature calibrations have been successfully developed for different bivalve species (e.g. fan mussel, mussel, scallop, and oyster; Table 1, [START_REF] Klein | Bivalve skeletons record sea-surface temperature and δ 18 O via Mg/Ca and 18 O/ 16 O ratios[END_REF]Vander Putten et al., 2000;Freitas et al., 2005Freitas et al., , 2006Freitas et al., , 2009Freitas et al., , 2012;;Wanamaker et al., 2008;Surge and Lohmann, 2008;[START_REF] Mouchi | Chemical labelling of[END_REF]. Given that these calibrations vary strongly between bivalves according on the shell species (Figure 10), we will therefore only focus on calibrations established on modern oyster species. For modern oysters, calibrations have been obtained by Surge and Lohmann (2008) for the estuaries species Crassostrea virginica and by [START_REF] Mouchi | Chemical labelling of[END_REF] for the marine species Crassostrea gigas. To apply these existing calibrations to the extinct fossil species, including those analysed in this study, it is necessary to account for potential inter-species differences. To do so, temperatures obtained using various existing oyster calibrations are compared to temperatures estimated from measured δ 18 O. The carbonaceous shells of most bivalves are precipitated in isotopic equilibrium with the seawater (e.g. [START_REF] Kirby | Stable isotope sclerochronology of Pleistocene and Recent oyster shells (Crassostrea virginica)[END_REF] so that the oxygen isotopic composition of

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different species can be directly compared. Thereby, relationships between seawater temperature, calcite δ 18 O c , and seawater δ 18 O sw are commonly applied to estimate temperatures using fossil bivalve shells (e.g. [START_REF] Ivany | Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary[END_REF]Gillikin et al., 2005). Here we apply calibration established by [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] between seawater temperature T ( • C), bivalve calcite δ 18 O c (% , VPDB), and seawater δ 18 O sw (% , SMOW):

T = 16 -4.14 × (δ 18 O c -δ 18 O sw ) + 0.13 × (δ 18 O c -δ 18 O sw ) 2 (1)
To estimate δ 18 O sw we used a range between the commonly accepted -1% value for global icefree Eocene seawater (e.g. [START_REF] Ivany | Cooler winters as a possible cause of mass extinctions at the Eocene/Oligocene boundary[END_REF] and the more realistic value of 0.65% provided recently by numerical modelling near our study area at an Eocene position of 37.5 Applying the Mg/Ca-temperature calibration for the extant oyster C. virginica (Surge and Lohmann, 2008) to our fossil oysters results in temperatures that are considerably lower than those based on δ 18 O assuming a δ 18 O sw of 0.65% or even -1% (Figure 10). The application of this Mg/Ca-temperature calibration is therefore not suitable for any of the oyster species considered here, as was shown before on S. buhsii [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. In contrast, δ 18 O-derived temperatures compare better with those based for the Mg/Ca calibration of the marine oyster species C. gigas (Mouchi et al., 2013, Figure 10). Previous studies on calcifiers noticed that because seawater Mg/Ca was 50% of modern in the Palaeogene (3 mmol/mol compare to 5 mmol/mol in average), this could be a significant source of uncertainty which may have led to Palaeogene species being characterised by lower Mg/Ca ratios [START_REF] Evans | Deep time foraminifera Mg/Ca paleothermometry: Nonlinear correction for secular change in seawater Mg/Ca[END_REF]. However, the calibration done by [START_REF] Mouchi | Chemical labelling of[END_REF] was established in Arcachon Basin were the seawater Mg/Ca is more or less 5 mmol/mol which is close to the modern seawater Mg/Ca ratio. Furthermore even for modern time, seawater Mg/Ca is highly variable ( 1) between open and coastal environment and (2) at an infra annual scale especially in coastal or near shore environment. According [START_REF] Lorens | The impact of solution chemistry on Mytilus edulis calcite and aragonite[END_REF] However, the validity of the calibration of [START_REF] Mouchi | Chemical labelling of[END_REF] varies between analysed species.

-Mg/Ca values from O. bellovacina, F. hemiglobosa, P. asiatica and F. bashibulakeensis appear to result in unrealistic temperatures using the calibration of [START_REF] Mouchi | Chemical labelling of[END_REF] 11b). An overestimation of δ 18 O sw of around -4% would be required to match the temperatures deduced from both proxies, which is very unlikely in an open marine environment. Furthermore, given the overall warm conditions for the Palaeocene-Eocene and particularly for the Central Asian sample location, temperatures as low as 10 • C are not realistic [START_REF] Lan | Paleogene bivalve communities in the Western Tarim Basin and their paleoenvironmental implications[END_REF]Sun and Wang, 2005;[START_REF] Guo | A major reorganization of asian climate by the early miocene[END_REF]Tindall et al., 2010;Quan et al., 2012). We conclude that the Mg/Ca-temperature calibration of [START_REF] Mouchi | Chemical labelling of[END_REF] does not apply to shells of O. bellovacina, F. hemiglobosa, P. asiatica and F. bashibulakeensis. This suggests that incorporation of Mg differs significantly between species, even within the Ostreidae superfamily. Furthermore, specimens of the species P. asiatica analysed here were living in a marine environment under the influence of continental runoff (mangrove area, restricted marine waters, Figure 2). This may suggest also a potential additional impact from the environment on the magnesium incorporation for this species.

-S. buhsii and O. (T.) strictiplicata, in contrast, appear to yield reliable temperatures using the calibration of [START_REF] Mouchi | Chemical labelling of[END_REF] that are within the same range as those estimated using δ 18 O (around 25 and 30 • C in average with a δ 18 O sw = 0.65% , Figure 11b). Considering that these species also provide the most reliable Mg/Ca records discussed above (Figure 6), we conclude that these species are particularly suitable for estimating palaeotemperatures.

Although average Mg/Ca-derived temperatures are comparable to those based on δ 18 O, the seasonal amplitudes in temperature are significantly higher for Mg/Ca compared to δ 18 O (Figure

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11; also discussed in [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF] this likely relates to seasonal fluctuations in the δ 18 O of seawater as expected in coastal marine living environments of the studied oysters according to the interpreted depositional palaeoenvironments. Indeed sedimentological [START_REF] Manceau | Evolution des environnements de dépôts dans le bassin du Tarim (Chine) au cours du retrait Paléogène de la Proto-Paratethys[END_REF] this indicates that, after careful inspection of the diagenesis effect and independent characterization of the depositional environments, Mg-temperature calibrations based on modern specimens can be used to reconstruct past temperatures and seasonal patterns therein from fossil oyster shells.

Further development and conclusions

By comparing Mg/Ca to δ 18 O high-resolution profiles from various fossil species in different depositional environments we devised a protocol to identify fossil species yielding reliable temperatures when using Mg/Ca-temperature calibrations derived from existing studies on modern oyster species.

The palaeobiogeographic distributional patterns of the reliable oyster species identified here (S.

buhsii and O. (T.) strictiplicata) show an extraordinarily wide dispersal beyond the Tarim Basin into the regional Turkestan stage of Central Asia (Vyalov, 1937), in Northwest Afghanistan [START_REF] Berizzi | Paleogene pelecypods from Kataghan and Badakhshan (North-East Afganistan). Fossils of North-East Afganistan: Italian Expeditions to the Karakorum (K2), and Hindu Kush[END_REF] and in northern Iran [START_REF] Grewingk | Die geognostischen und geographischen Verhältnisse des nordlichen Persiens[END_REF]. Occurrences of Palaeogene oysters have also been reported in the Transylvanian Basin (Rusu et al., 2004) and as far west as the Paris Basin (for O. bellovacina [START_REF] Lan | Late Cretaceous-early Tertiary marine bivalve fauna from the western Tarim Basin[END_REF]. This broad distribution makes these species an ideal choice to track palaeo-seasonality in a large area during the particular Palaeogene period, when global climate changed from greenhouse to icehouse conditions.

Combining Mg-estimated temperature with δ 18 O allows reconstructions of seasonal variabil-

A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT

ity in δ 18 O sw ultimately reflecting changes in salinity (see equation 1 and supplementary data for these calculations, [START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. Seasonally-resolved salinity can be used for palaeoclimate interpretation of the basin hydrological cycle (evaporation and precipitation) at intra-annual resolution thereby significantly improving the comprehension of the environment compared to previous palaeontological studies as exemplified in our study area (Sun and Wang, 2005;[START_REF] Guo | A major reorganization of asian climate by the early miocene[END_REF]Quan et al., 2012). Furthermore, by comparing Mg/Ca and δ 18 O from multiple species from the same stratigraphic level, one could in principle statistically deconvolve impact of differences in sea water Mg/Ca or establish freshwater to seawater mixing lines from δ 18 O.

In principle, the protocol developed here is applicable to oyster (or even more largely bivalve)

fossil species if they are found to yield reliable temperatures using the few existing temperaturecalibrated modern species. The necessity to perform additional calibration studies on modern oyster species to improve calibrations for Mg/Ca and temperature for various Ostreidae species follows is underscored by our inconsistent results with F. hemiglobosa or F. bashibulakeensis species from which no reliable Mg-temperature could be extracted. Further calibration work may also help identify possible specific vital effect on Mg incorporation into the shell (already observed on modern species within a same genus such as Mytilus sp. or Crassostrea sp., Table 1), as well as the environmental impact (observed on same species Pecten maximus in different environment such as estuarine (Vander Putten et al., 2000), culturing (Freitas et al., 2008) or brackish (Wanamaker et al., 2008)). Furthermore, these studies will certainly benefit from the calibration of other trace elements and isotopes such as Ba and δ 13 C recently used as indicators of primary productivity (Goodwin et al., 2012).

Improving the knowledge on biomineralisation of Ostreidae shells is a prime target to develop a reliable and powerful environmental proxies. Indeed, the widespread occurrence of Ostreidae in the fossil records in terms of age (from Triassic until Quaternary) and of geolocalisation (from low to high latitudes, from brackish to fully marine environments) make Ostreidae the most promising group to infer palaeenvironments at intra-annual resolution.

Acknowledgements

This project was partly funded by the Netherlands Organization for Scientific Research (NWO), the A. v. Humboldt foundation and the Marie Curie Career Integration Grant HIRESDAT. We

Tables and Figures

Table 1: Comparison of Mg/Ca-seawater temperature relationships for calcitic bivalves.

Table 2: Summary of experiments performed on the 41 oyster slabs. The three last columns indicate the reliability of Mg/Ca transects based on the two main characteristics : Intra-shell variability ans Signal cyclicity ("+", "=" and "-" correspond respectively to suitable, partly suitable and unsuitable signal). The reliability of the Mg/Ca result for each oyster is expressed by: S = suitable, PS = partly suitable, US = unsuitable. c. Position of analysed species on schematic illustration of the third to fifth marine incursions into the Tarim basin (Bosboom et al., 2014b). d. Examples of analysed fossil oyster left valves with well-developed ligamental area. (i.)

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F. hemiglobosa, (ii.) O. (T.) strictiplicata, (iii.) S. buhsii, (iv.) P. asiatica, (v). F. bashibulakeensis. The first part of the specimen's name indicates the provenance with the code of the stratigraphic sections (see section codes below), and the year of sampling. The complete list of oysters from Central Asia analysed in this study is available in supplementary data. Code for stratigraphic sections -MS: Mine-Bashibulake (39 Living position of species through the carbonate platform. Cathodoluminescence grey scale (light red: raw data, dark red: moving average) results, Mg/Ca ratio for two parallel transects yielding similar results (light blue or green: raw data, dark blue or green: moving average), Oxygen stable isotopes from the selected area. Grey areas correspond to light and dark layers as revealed by CL.

Figure 4: Geochemical data for the specimen KZ12-O03 (S. buhsii): Cathodoluminescence grey scale (light red: raw data, dark red: moving average) results, Mg/Ca ratio for two parallel transects yielding similar results (light blue or green: raw data, dark blue or green: moving average), Oxygen stable isotopes from the selected area. Grey areas correspond to light and dark layers as revealed by CL. incremental analyses per stratigraphic section. When the seasonal pattern could not be defined, we represented only the average (full black square) and a black line between the highest and the lowest values. Red depicts suitable signal, blue partly suitable signal and grey unsuitable signal. When oysters are coming from the same bed, the estimated age is the same. However, to better see the difference between them, we juxtaposed results next to each other. Numbers correspond to the Reference N • in Table 2. i. δ 13 C vs δ 18 O for bulk and averaged incremental results. Two groups can be clearly distinguished between relatively high (full line) and low (dashed line) values of δ 18 O and δ 13 C. j.

Compiled Mg/Ca results from the complete collection with suitable and partly suitable criteria.

Figure 10: a. Infra-annual temperatures (0.2 mm moving average) for the specimen AT13-O20 estimated using various Mg/Ca calibrations established on different species of calcitic bivalves (1 to 10, cf Table 1), or using δ 18 O calibration of [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] and with a constant δ 18 Osw of 0.65% (green plain circles) or -1% (orange empty circles). Grey areas correspond to dark bands on the shells.

b. Comparison between range of temperatures deduced from Mg/Ca [START_REF] Mouchi | Chemical labelling of[END_REF] Sokolowia buhsii [START_REF] Grewingk | Die geognostischen und geographischen Verhältnisse des nordlichen Persiens[END_REF] Platygena asiatica

  the relationship between Mg in the environment, in the biological fluids and in the calcitic part the carbonaceous shell is not clear (compare to Sr). Even if Mouchi et al. (2013) thermodependant equation do not include seawater Mg/Ca, Surge and Lohmann (2008) showed that a large variation of seawater Mg/Ca from 1 to 5 mmol/mol (due to the proximity of Mississipi delta) do not have a A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT significant influence on the oyster shell Mg/Ca. This encourage the sue of such calibrations, even in Palaeocene and Eocene times.

  the CaiYuanpei programme of the French ministry of foreign affairs for collaborative support in field excursion. We would like to thank Roderic Bosboom, Gloria Heilbronn, Bruno Paulet and Jean-Noël Proust for their contribution in the field. We are grateful to Helen de Waard, Arnold Van Dijk for their help during experiments with LA-ICP-MS and stable isotopes analyses in Utrecht University. We are grateful to Nathalie Labourdette and Catherine Pierre for their help during experiments with stable isotopes analyses and Micromill access in Pierre et Marie Curie University.

  Freitas, P.S., Clarke, L.J.,Kennedy, H., Richardson, C.A., 2012. The potential of combined Mg/Ca and δ 18 O measurements within the shell of the bivalve Pecten maximus to estimate seawater δ 18 O composition. Chemical Geology 291, 286-293. Gillikin, D., De Ridder, F., Ulens, H., Elskens, M., Keppens, E., Baeyens, W., Dehairs, F., 2005. Assessing the reproducibility and reliability of estuarine bivalve shells (Saxidomus giganteus) for sea surface temperature reconstruction: Implications for paleoclimate studies. Palaeogeography, Palaeoclimatology, Palaeoecology 228, 70-85. Goodwin, D., Gillikin, D., Roopnarine, P., 2012. Preliminary evaluation of potential stable isotope and trace element productivity proxies in the oyster Crassostrea gigas. Palaeogeography, Palaeoclimatology, Palaeoecology . Goodwin, D.H., Flessa, K.W., Schöne, B.R., Dettman, D.L., 2001. Cross-calibration of daily growth increments, stable isotope variation, and temperature in the Gulf of California bivalve mollusk Chione cortezi : implications for paleoenvironmental analysis. Palaios 16, 387-398.

Figure 1 :

 1 Figure 1: a. Location of sampling area in Kyrgyzstan (yellow), Tajikistan (blue) and China (orange). b. Middle Eocene Palaeographical map showing Central Asia invaded by the Proto-Paratethys sea (map from Licht et al., 2014).

Figure 2 :

 2 Figure 2: a. Environment of the studied species: age, formation, depositional environment and associated fauna. b.

Figure 3 :

 3 Figure 3: Radial sections (top) of oyster ligamental areas showing light and dark alternations, each couplet corresponding to one living growth years. Cathodoluminescence assemblages (bottom) for these specimens show alternation of areas with high and low luminescence corresponding to light and dark bands observed in natural light. Transects followed by laser ablation for trace element analyses are indicated in blue and green. Main paths followed by the Micromill for stable isotopes analyses are indicated in black. Black arrows indicate growth directions. (a.) O. bellovacina, (b.) F. hemiglobosa , (c.) S. buhsii and (d.) F. bashibulakeensis. Supplementary data provides the radial sections and CL images of complete collection. e. Geochemical data for the specimen KZ12-O03 (S. buhsii):

Figure 5 :Figure 6 :

 56 Figure 5: Incremental stable oxygen ratio records (δ 18 O) in oyster shells. Horizontal axis indicates distance in millimetres starting from the first drilled micro-sample. Grey areas correspond to visible dark bands on the shells, white areas correspond to the light bands revealed by CL analyses.

Figure 7 :

 7 Figure 7: Partly suitable Mg/Ca results across the oyster shell ligamental area showing only parts with well-expressedcyclicity and low intra-shell variability between parallel transects (Figure3). Analyses were performed following the growth direction with 0 mm corresponding to the most juvenile part analysed.

Figure 8 :

 8 Figure 8: Unsuitable Mg/Ca results across the oyster shell ligamental area showing poorly-expressed cyclicity and high intra-shell variability between parallel transects (Figure 3) of oyster shells. Analyses were performed following the growth direction with 0 mm corresponding to the most juvenile part analysed. See Figure 8 for explanatory legend of used symbols and patterns.

Figure 9 :

 9 Figure 9: a.-d. δ 18 O results from bulk and incremental analyses per stratigraphic section. e.-h. Mg/Ca results from

  with a constant δ 18 Osw of 0.65% according to modelled values for the study area in the Eocene (Tindall et al., 2010) (11), or of -1% according to the value commonly used for the Eocene ice free world (12). b. Associated temperature range for each calibration.

Figure 11 :

 11 Figure 11: a. Estimated temperature using Mg/Ca ratio with the calibration of Mouchi et al. (2013) (red line; in order to not overcrowd the graphs, only the moving average of Mg estimated temperature are shown) compared to estimated temperature using δ 18 O with the calibration of Anderson and Arthur (1983) and with a constant δ 18 Osw

  Figure1

  Figure2

Figure 7 A

 7 Figure 7

Figure 9 A

 9 Figure9

-

  Figure 11

  R I P T ACCEPTED MANUSCRIPTand further excludes that ontogeny has a large effect on oyster stable isotope ratios. The stable isotope values display a well-defined cyclicity with low values coinciding with light bands and high δ 18 O corresponding to darker bands (Figures4 and 5) revealed by CL. This is in agreement with previous observations showing a seasonal isotopic signal corresponding to variation of the CL intensity(Lartaud et al., 2010b;[START_REF] Bougeois | A high resolution study of trace elements and stable isotopes in oyster shells to estimate Central Asian Middle Eocene seasonality[END_REF]. Similarly, stable isotope values are correlated to the suitable and partly suitable Mg/Ca results with low δ 18 O values coinciding with highest Mg/Ca values as shown in Figure4. Considering that in modern oyster shells Mg/Ca covaries with temperature(Surge and Lohmann, 2008;[START_REF] Mouchi | Chemical labelling of[END_REF], CL intensity increases during summer months[START_REF] Langlet | Experimental and natural manganese-induced cathodoluminescence in the shell of the Japanese oyster Crassostrea gigas (Thunberg, 1793) from Thau Lagoon (Hérault, France): ecological and environmental implications[END_REF] Lartaud et al., 2010b) and δ 18 O is negatively correlated to temperature[START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF],

  R I P T ACCEPTED MANUSCRIPTspecimens of F. hemiglobosa, the apparent mismatch in cyclicity between the stable isotopes and Mg/Ca, may be caused by the position of the cutting plane, which crossed the side of the ligamental area (within a side bead along the resilifer) instead of cutting through the middle. Considering the very low CL intensity (partially correlated to Mn concentration) and the unsuitable Mg/Ca signal, trace element incorporation appear to be altered in the side parts of the ligamental area, making them unsuitable for application of trace elements to environmental reconstructions. However, this can not explain all unsuitable data. A few other F. hemiglobosa and O. bellovacina present substantial intra-shell variability in Mg/Ca (e.g. MS13-O23, KZ12-O12.2 or Kuhdara samples, Table2) although they have been cut properly and CL as well as δ 18 O show cyclic variations with no diagenetic overprint. The cause of the mismatch between δ 18 O and Mg/Ca remains to be investigated for these Palaeocene species. The non-cyclic pattern of trace element incorporation may relate to species-specific or environmental effects and are discussed below.

  viation of their average Mg/Ca). This indicates consistency of the Mg/Ca signal between specimens of one single oyster species. However, absolute values and the range of Mg/Ca ratio vary between species. For instance, O. (T.) strictiplicata and S. buhsii (Middle Eocene) have Mg/Ca averages of approximately 6 mmol/mol with a seasonal amplitude of ∼5 mmol/mol. For F. hemiglobosa (Early Eocene) the average and seasonal amplitude in Mg/Ca is slightly lower around 4 mmol/mol. This difference may be attributed to a palaeoenvironmental effect as these species lived during different

geological time intervals (early Eocene for F. hemiglobosa and the middle Eocene for O. (T.) stric-

  • N, 71.25 • E corresponding to the eastern Proto-Paratethys (Tindall et al., 2010). Resulting calculated seawater temperatures vary from approximately 22 to 41 • C for average temperatures, ∼25 to 46 • C for summer temperatures and ∼20 to 37 • C for winter temperatures (Figure • 11).

  Felis, T., Lohmann, G., Kuhnert, H., Lorenz, S.J., Scholz, D., Pätzold, J., Al-Rousan, S.A., Al- Moghrabi, S.M., 2004. Increased seasonality in middle east temperatures during the last interglacial period. Nature 429, 164-168.Freitas, P., Clarke, L.,Kennedy, H., Richardson, C., 2008. Inter-and intra-specimen variability masks reliable temperature control on shell Mg/Ca ratios in laboratory and field cultured Mytilus edulis and Pecten maximus (bivalvia). Biogeosciences Discussions 5.Freitas, P., Clarke, L.,Kennedy, H., Richardson, C., 2009. Ion microprobe assessment of the heterogeneity of Mg/Ca, Sr/Ca and Mn/Ca ratios in Pecten maximus and Mytilus edulis (bivalvia) shell calcite precipitated at constant temperature. Biogeosciences Discussions 6.

Freitas, P., Clarke, L.,

Kennedy, H., Richardson, C., Abrantes, F., 2005

. Mg/Ca, Sr/Ca, and stableisotope (δ 18 O and δ 13 C) ratio profiles from the fan mussel Pinna nobilis: Seasonal records and temperature relationships. Geochemistry Geophysics Geosystems 6, Q04D14.

Freitas, P., Clarke, L.,

Kennedy, H., Richardson, C., Abrantes, F., 2006

. Environmental and biological controls on elemental (Mg/Ca, Sr/Ca and Mn/Ca) ratios in shells of the king scallop Pecten maximus. Geochimica et Cosmochimica Acta 70, 5119-5133.

  Tindall, J., Flecker, R., Valdes, P., Schmidt, D., Markwick, P., Harris, J., 2010. Modelling the oxygen isotope distribution of ancient seawater using a coupled ocean-atmosphere GCM: implications for reconstructing early Eocene climate. Earth and Planetary Science Letters 292, 265-273. Vander Putten, E., Dehairs, F., Keppens, E., Baeyens, W., 2000. High resolution distribution of trace elements in the calcite shell layer of modern Mytilus edulis: environmental and biological controls. Geochimica et Cosmochimica Acta 64, 997-1011. Vyalov, O., 1937. Sur la Clasification des Ostréidés et leur Valeur stratigraphique. XIIe Congrès Internationnal de Zoologie Session VIII, 1627-1638.Wanamaker, A.D., Kreutz, K.J.,Wilson, T., Borns Jr, H.W., Introne, D.S., Feindel, S., 2008. Experimentally determined mg/ca and sr/ca ratios in juvenile bivalve calcite for Mytilus edulis: implications for paleotemperature reconstructions. Geo-Marine Letters 28, 359-368.

Wefer, G., Berger, W.H., 1991. Isotope paleontology: growth and composition of extant calcareous species. Marine geology 100, 207-248.

  • 51'N, 74 • 32'E), KA: Kansu (39 • 45'N, 74 • 58'E), KZ: Kezi (38 • 26'N, 76 • 24'E), AT: Aertashi (37 • 58'N, 76 • 33'E), YK: Yarkand (37.7 • N, 76.6 • E), KY: Keyliand (37 • 27'N, 77 • 86'E), AB: Ala Buka (41.4 • N, 71.4 • E), TK: Tash Kumyr (41.3 • N, 72.2 • E), AL: Alai Valley (39.6 • N, 72.4 • E), UT: Uch Tobo (39.9 • N, 73.4 • E), DS: Kuhdara-Dushanbe (38.7 • N, 68.9 • E).

  and δ 18 O (Anderson and Arthur, 1983) with a constant δ 18 Osw of 0.65% .

		ACCEPTED MANUSCRIPT
	Species Pecten maximus Mytilus trossolus Mytilus edulis Crassostrea gigas Crassostrea virginica Pinna nobilis	Environment Temperature range culturing 10-20 °C marine 5-19°C culturing 10-20 °C marine 6-23°C estuarine 5-20°C culturing 10-20 °C brakish water 7-19°C marine 5-25°C estuarine 18-32°C Bougeois et al., 2015 Mg/Ca =f(T) Mg/Ca = 2.56 (±0.42) + 0.17 (±0.03) * T Mg/Ca = 4.92 (±2.16) + 0.50 (±0.16) * T Mg/Ca = 9.89 (±2.96) + 0.51 (±0.19) * T Mg/Ca = 2.25 (±0.63) + 0.30 (±0.04) * T Mg/Ca = -0.63 (±0.29) + 0.70 (±0.02) * T Mg/Ca = 1.50 (±0.57) + 0.27 (±0.04) * T Mg/Ca = 5.44 (±0.31) + 0.77 (±0.22) * T Mg/Ca = -0.50 + 0.27 * T Mg/Ca = -0.23 + 0.72 * T Table 1 M A N U S C R I P T marine 10-22 °C Mg/Ca = 17.16 (±1.95) * exp(0.022 (±0.004) * T)	Reference Freitas et al., 2012 (2) Freitas et al., 2006 (3) Freitas et al., 2008 (4) Klein et al., 1996 (5) Vander Putten., 2000 (6) Freitas et al., 2008 (7) Wanamaker et al., 2008 (8) Mouchi et al., 2013 (9) Surge et al., 2008 (10) Freitas et al., 2005 (1)
		A C C E P T E D

Table 2

 2


Member, IEEE Mauro Ettorre
email: mauro.ettorre@univ-rennes1.fr.

Student Member, IEEE Santi C Pavone

Member, IEEE Massimiliano Casaletti
email: massimiliano.casaletti@upmc.fr

M Albani

Experimental validation of Bessel beam generation using an inward Hankel aperture distribution

Keywords: Bessel beams, near field, focusing, radial line slot array, RLSA

In this paper we prove experimentally that nondiffractive Bessel beams can be generated by using inward cylindrical traveling wave aperture distributions. An azimuthally invariant inward traveling wave distribution is defined over the aperture of a Radial Line Slot Array (RLSA) to launch a Bessel beam whose normal electric-field component assumes a truncated, zeroth-order Bessel function. An optimization procedure based on a holographic approach is used for tuning the position and size of the slots of the RLSA. The antenna is centrally fed by a coaxial probe transition. The final structure operates at 12.5 GHz. Full-wave simulations and measurements of the vertical component of the electric field show that a non-diffractive radiation is obtained within a range larger than 12 wavelengths in front of the antenna. The generated Bessel beam presents a stable half power beamwidth of about 20 mm within this range. The proposed system may open new opportunities for planar, low profile Bessel beam generators at millimeter waves, Terahertz, and optics.

I. INTRODUCTION

D IFFRACTION is a basic physics phenomenon that limits the possibility to confine energy in a defined area in any radiating systems. Bessel beams are ideal solutions to Maxwell equations that do not suffer of diffractive spreading [START_REF] Durnin | Exact Solutions for Nondiffracting Beams. I. The Scalar Theory[END_REF]. However, they require an infinite energy and radiating aperture. Nevertheless, practical implementations of Bessel beam generators have shown that the non-diffractive behaviour of such beams can be obtained within a defined limited area in front of the radiating aperture [START_REF] Lapointe | Review of Non-Diffracting Bessel Beam Experiments[END_REF], known as non-diffractive range.

The solutions proposed so far to generate Bessel beams synthesize over a finite aperture a tangential field distribution shaped like a Bessel function either of the zero-th or first order. In particular, in the case of radial polarized zero-th order Bessel beams, the tangential field distribution is azimuthally invariant and is given by J 1 (k a ρ), where J 1 (.) is the first order Bessel function, k a is the transverse propagation constant associated to the Bessel beam, and ρ is the radial coordinate of a cylindrical reference system [START_REF] Durnin | Exact Solutions for Nondiffracting Beams. I. The Scalar Theory[END_REF].

Recently, several works [START_REF] Chávez-Cerda | A new approach to Bessel beams[END_REF]- [START_REF] Inami | Generating Bessel beams using an electrically-large annular slot[END_REF] have highlighted the fact that Bessel beams are generated by the interference of two cylindrical traveling waves, or Hankel waves [START_REF] Chávez-Cerda | Evaluation of focused Hankel waves and Bessel beams[END_REF], one traveling outward and the other inward over a radiating finite aperture. Such combination is achieved in a small bandwidth due to the resonant nature of the obtained aperture distribution. However, in a recent paper, the authors have proven theoretically that Bessel beams can be generated by only inward cylindrical traveling waves [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF] over a wide band of operation opening new opportunities for non-diffractive beams and localized pulses [START_REF] Hernandez-Figueroa | Nondiffractive waves[END_REF]. The goal of this paper is to confirm experimentally the conclusions drawn in [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF].

In the following, we consider the generation of a propagating Bessel beam whose normal component takes on a zero-th order Bessel function J 0 (k a ρ), with k a = 0.4k and k the free space wavelength at the operating frequency of f = 12.5 GHz. This beam is generated by imposing an inward azimuthally invariant traveling wave distribution H

(1) 1 (k a ρ) over a radiating aperture, where H

(1) 1 (.) is the first order Hankel function of the first kind. Here the finite aperture is synthesized by a RLSA antenna. The optimization procedure outlined in [START_REF] Ettorre | On the near-field shaping and focusing capability of a radial line slot array[END_REF] has been used for defining the size and position of the slots of the RLSA antenna. It is worth mentioning that a similar procedure has been adopted in [START_REF] Albani | Automatic design of CP-RLSA antennas[END_REF] for the generation of a circular polarized Bessel beam by a RLSA structure [START_REF] Mazzinghi | Large depth of field pseudo-Bessel beam generation with a RLSA antenna[END_REF]. However in this case the aperture distribution over the slotted plate of the RLSA consists of an azimuthally invariant zero-th order Bessel function.

The final antenna consists of a double grounded substrate where the slots of the RLSA are etched by laser ablation on one of the top metallic layer. The antenna is fed centrally by a coaxial feed. Measurement results of the vertical component of the electric field confirm the non-diffractive capability of the radiated field at the design frequency and the overall antenna design.

The paper is organized as follows. Section II is a brief summary of the results derived in [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF]. Section III gives details on the design procedure used for synthesizing the required aperture field distribution with a RLSA structure. The prototype, full-wave and measurements results are shown in Section IV validating the antenna structure and the analytical derivations in [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF]. Finally several conclusions are drawn in Section V.

II. BRIEF SUMMARY OF THE THEORETICAL RESULTS

Let consider a finite circular radiating aperture of radius a orthogonal to the z-axis of a cylindrical coordinate system (ρ, φ, z) with its origin at the center of the aperture. In the following, Transverse Magnetic (TM) modes with respect to the z-axis are considered; vectors are bold and a hat denotes a unit vector. An inward cylindrical wave is assumed for the tangential electric field on the radiating aperture:

E t (ρ, φ, z = 0) = E t (ρ, z = 0) ρ = H (1)
1 (k a ρ) ρ. The electric field radiated by the aperture is given by [START_REF] Felsen | Radiation and Scattering of Waves[END_REF]

E(ρ, z) = 1 4π +∞ ∞e -jπ k ρ k z H (2) 0 (k ρ ρ)ẑ + jH (2) 1 (k ρ ρ) ρ • E t (k ρ )e -jkz z k ρ dk ρ , (1)
E t (k ρ ) = -2πj +∞ 0 E t (ρ, z = 0)J 1 (k ρ ρ)ρdρ, (2)
where k = 2π/λ is the free-space wavenumber, k ρ and k z = k 2 -k 2 ρ are the transverse and longitudinal spectral wavenumbers, respectively. J n (.) and H (i) n (.) are the n-th order Bessel and Hankel functions of the i-th kind. Equation ( 2) is the Hankel-transform of the tangential field distribution over the aperture and, for the assumed inward finite cylindrical wave distribution, it is given by

E t (k ρ ) = - 4 k a (k 2 ρ -k 2 a ) k ρ k a + πa 2j k ρ H (1) 1 (k a a)J 0 (k ρ a) -k a H (1) 0 (k a a)J 1 (k ρ a) (3)
As an example related to the prototype realized in Sec. IV, let us consider a finite radiating aperture of radius a = 5.33λ = 128 mm with an inward cylindrical field distribution with a transverse propagation constant k a = 0.4k = 104.72m -1 at the operating frequency of f = 12.5GHz.

The first row of Fig. 1 shows the z, ρ components and total electric field arranged in the 1 st , 2 nd , and 3 rd column, respectively. As a comparison, the second row in Fig. 1 provides the field radiated by an ideal finite Bessel distribution over the same aperture and with the same transverse propagation constant of the inward Hankel wave:

E t (ρ, φ, z = 0) = J 1 (k a ρ) ρ.
The dashed lines in the figure represent the Geometrical Optics (GO) shadow boundaries arising from the aperture rim and due to the finiteness of the structure limiting the non-diffractive range to z = a cot θ a ≃ 12.21λ (θ a = sin -1 (k a /k) = 23.58 • ). On the other hand, the dotted line in the inward Hankel case corresponds to the GO shadow boundary generated by the inward wave which, when passing through the caustic along the z-axis, becomes an outward conical wave [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF].

Fig. 1 clearly shows that inward cylindrical aperture distribution creates a non-diffractive Bessel beam close to the antenna axis (namely in the triangular area delimited by the dotted and dashed lines, Fig. 1). On the other hand, the ideal Bessel distribution creates a non-diffractive radiation within a region in front of the aperture with a triangular cross section (area within the dashed line in the second row in Fig. 1). Nevertheless, the depth of focus of the two cases, i.e the nondiffractive range, is the same, and the shape of the realized beam close to the z-axis is very similar. This demonstrates that inward Hankel aperture field distributions can launch efficiently a Bessel beam as standard Bessel distributions. It is worth mentioning that, for infinite radiating apertures, closed form expressions validate the numerical results presented in this section for the finite case, as presented and motivated in [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF]. Here we have considered the finite case since closely related to a realistic implementation of a Bessel beam launcher.

III. SYNTHESIS OF THE APERTURE FIELD DISTRIBUTION

The required azimuthally invariant inward Hankel field aperture distribution is synthesized by a RLSA antenna at the operating frequency of f = 12.5GHz using the optimization procedure outlined in [START_REF] Ettorre | On the near-field shaping and focusing capability of a radial line slot array[END_REF]. This procedure controls the position and size of each slot of the RLSA antenna in such a way to create, over the radiating aperture, the equivalent magnetic current distribution associated to the inward Hankel wave:

M 0 (ρ) = M 0 (ρ) φ = -E ρ (ρ) φ = H (1)
1 (k a ρ) φ. In addition, the antenna spillover efficiency (ratio between the radiated and accepted power by the antenna) is maximized during the design to prevent spurious radiation from the edge of the structure. In the following, an efficiency larger than 95% has been imposed as constraint [START_REF] Ettorre | On the near-field shaping and focusing capability of a radial line slot array[END_REF]. The slots of the RLSA and the associated magnetic dipole moments are oriented along φ as the equivalent magnetic current distribution. A fast and accurate in-house Method of Moments (MoM) [START_REF] Casaletti | Efficient analysis of metallic and dielectric posts in parallel plate waveguide structures[END_REF]- [START_REF] Albani | Rigorous MoM analysis of finite conductivity effects in RLSA antennas[END_REF] is used during the optimization procedure to evaluate the magnetic dipole moment of each slot and overall radiation performance.

The final structure is shown in Fig. 2. The position and size of the slots are given in Table I. The width of the slots (w) is equal to 1.2 mm. A spillover efficiency larger than 97% To estimate the quality of the obtained aperture field distribution, Fig. 3 shows the normalized z-component of the electric field at z = a/2 cot θ a ≃ 6.11λ = 146.64 mm radiated by the RLSA antenna along the xand y-axes and by the ideal inward Hankel wave. In the ideal case, the field is azimuthally invariant. The main and lateral lobes of the field radiated by the RLSA antenna are in very good agreement with the ideal case demonstrating the good accuracy of the synthesized aperture distribution and overall RLSA antenna design. The discrepancies for field levels lower than -30 dB are considered residual effects of the achieved average aperture distribution error. It is worth noting that the plane at z = 146.64 mm passes through the intersection of the two GO boundaries shown in Fig. 1, and corresponds to the maximum extension of the nondiffractive zone along the radial direction for inward Hankel distributions, as outlined in Section II.

IV. PROTOTYPE AND EXPERIMENTAL RESULTS

The structure optimized in Section III has been manufactured and measured at IETR. The final antenna is shown in Fig. 4. The antenna is made on a double grounded Neltec NY9217 substrate with permittivity ǫ r = 2.17 and thickness h = 3.175 mm (refer to Fig. 2). Laser ablation has been used to etch the slots of the RLSA on the upper face of the substrate. The prototype is fed on the back by a SMA connector using the coaxial probe transition shown in Fig. 2. Fig. 5 shows the simulated and measured input reflection coefficient. The antenna is matched at the operating frequency and within the band 12.16-13.00 GHz. However, the measured input reflection coefficient is shifted downward (≈ 1.8%) and presents a higher level with respect to the numerical results [18], but still below the required -10dB level. These differences can be attributed to fabrication constraints related to the proposed probe transition. In fact, a milling process is used to create within the substrate the disk of diameter D 3 in Fig. 2, which is afterwards metalized and connected to the inner conductor of the SMA connector. An accuracy of about 100 µm is expected from the milling process. The normal component of the electric field E z radiated by the antenna has been measured using a short electric probe attached to an automatically controlled 2D translation stage. The short electric probe is made out of a semi-rigid coaxial cable (UT-85) by removing the external conductor and dielectric insulator. The inner conductor extends for a length of λ/10 = 2.5 mm beyond the outer conducting shield.

The normalized E z component at the operating frequency f = 12.5 GHz at z = 150 mm along the xand y-axis is shown in Figs. 6a andb, respectively. These field cuts have been obtained by measurement data over an area of 160x160 mm 2 with a space step of 2 mm. Good agreement is achieved between the measured and simulated results. However, a small difference is observed in the side lobe level (SLL) of the measured fields with respect to the MoM results in both field cuts. Such differences may be due to the fabrication tolerances related to the etching and milling processes used for antenna on the order of 50 µm and 100 µm, respectively. Fig. 7 reports the measured and MoM 2D field plots for the normalized E z components at z = 150 mm. Also in this case, a good agreement can be observed between the measured and simulated results validating the overall antenna design.

The radiated field has been measured in the band 12.2-12.7 GHz corresponding to an input reflection coefficient lower than -10 dB (refer to Fig. 5). Fig. 8 shows the measured and simulated normalized E z component at z = 150 mm at the edges of the considered band. It can be noticed that the field pattern is preserved within the considered band and there is a good agreement between simulation and measurements.

The non-diffractive behavior of the generated beam has been verified by measuring the field along the xzand yz-planes over an area of 300x200 mm 2 with a step of 4 mm. Fig. 9 shows the measured and simulated 2D plots of the normalized E z field along the xzand yz-planes. The main beam of the generated radiation does not change within the considered range (z = 50 -250 mm). This is further highlighted by the contour plots in Fig. 10 for the measured and simulated main beam along the two considered planes. The 3dB beamwidth of the generated Bessel beam is about 20 mm and roughly constant within the measured area in both planes. The operation of the structure within the considered band (12.2-12.7 GHz) is then analyzed. The measured 2D plots of the normalized E z field along the xzand yz-planes at 12.2 GHz and 12.7 GHz are provided in Fig. 11. It is possible to observe that the main beam of the radiated field does not change within the considered space range and operating frequency band validating the large band operation of the proposed structure [START_REF] Albani | Generation of non-diffractive Bessel beams by inward cylindrical traveling wave aperture distributions[END_REF]. For brevity, only measurement results are shown but also for these cases there is a good agreement with simulation results.

V. CONCLUSION

We have presented and validated experimentally that nondiffractive Bessel beams can be generated by using inward cylindrical aperture field distributions. The inward Hankel distribution is synthesized over the radiating aperture by using a recent developed optimization procedure for RLSA antennas. The final RLSA structure is planar and does not require any resonant aperture distribution to generate the non-diffractive

Fig. 1 .

 1 Fig. 1. Electric field radiated by a finite aperture with radius of a = 5.33λ. |Ez| (1 st column), |Eρ| (2 nd column) and total electric field amplitude |E| (3 rd column). The first and second row shows the field radiated by an inward Hankel and ideal Bessel distribution, respectively. The axes are normalized with respect to the wavelength (λ) at the operating frequency f . The dashed line marks the GO boundaries in the two cases [8].

Fig. 2 .

 2 Fig. 2. Cross section of the RLSA antenna and geometrical details of the coaxial probe transition. The slots on each ring of the RLSA have the same length (l i ) and are uniformly distributed along φ. The width of the slots (w) is constant.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Normalized Ez component of the electric field at z = a/2 cot θa radiated by the synthesized RLSA antenna along the xand y-axes. The field radiated by an ideal inward Hankel wave is provided for comparison.

Fig. 5 .

 5 Fig. 5. Measured and simulated input reflection coefficient of the RLSA antenna.

Fig. 6 .

 6 Fig. 6. Normalized Ez component at the operating frequency f = 12.5 GHz at z = 150 mm. (a) x-axis. (b) y-axis. Measurements and MoM results are compared along the two axes.

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. Normalized Ez component at the operating frequency f = 12.5 GHz at z = 150 mm. (a) Measurements. (b) MoM results.

Fig. 10 .

 10 Fig. 10. Contour plots for the measured normalized Ez component at the operating frequency f = 12.5 GHz. (a) xz-plane. (b) yz-plane.

Fig. 11 .

 11 Fig. 11. Normalized measured Ez component at the edges of the considered frequency band. (a) xz-plane at 12.2 GHz. (b) yz-plane at 12.2 GHz. (c) xzplane at 12.7 GHz. (d) yz-plane at 12.7 GHz.

TABLE I POSITION

 I AND DIMENSIONS (in mm) OF THE SLOTS IN EACH RING OF THE RLSA

		Ring number	1	2	3	4	5	6	7	8	9	10	11
			l i	7.42	7.22	7.38	7.59	7.19	7.90	7.55	8.12	7.72	9.05	10.26
			ρ i	8.96	23.20	34.24	47.38	58.62	71.54	82.46	95.18	106.50	118.03	127.02
			n. of slots	5	13	20	28	34	42	48	55	62	69	74
					l i
	h 1	D 3			ρ i
		D 2	h		w
		D 1		z
		Feed		x	y

The authors would like to thank the Ministère des Affaires Etrangéres et Européennes and Campus France (Galileo Projet) for

He is a member of EURAAP and URSI.