Jeremie Berthonneau
email: jeremie.berthonneau@cicrp.fr

Jean-Marc Vallet

Philippe Bromblet

François Martin

Olivier Grauby

Mineralogical origin of the chromatic contrast enhancing the Mount Bego petroglyphs

Keywords: Rock Art, Petroglyph, Metapelite, Iron (hydr)oxides, Clay Minerals, Electron Microscopy

est destinée au dépôt

Introduction

Petroglyphs are created by incising, pecking, or carving rock surfaces. They are among the first marks in the history of mankind [START_REF] Whitley | Introduction to Rock Art Research[END_REF] found in many culturally diverse regions of the world such as northern America (e.g., Mojave Desert, CA, USA, see [START_REF] Whitley | Climate change, rock coatings, and the archaeological record[END_REF], central Asia (e.g., Tamgaly, Kazahkstan, see [START_REF] Hermann | Rock Art of Tamgaly, Kazakhstan[END_REF], southern Africa (e.g., Twyfelfontein, Namibia, see [START_REF] Viereck | Twyfelfontein: A Centre of Prehistoric Art in South West Africa[END_REF], Scandinavia (e.g., Alta fjord, Norway, see [START_REF] Tansem | Red-coated rocks on the seashore: The esthetics and geology of prehistoric rock art in Alta, Arctic Norway[END_REF], and southern Europe (e.g., Valcamonica, Italy, see [START_REF] Anati | Rock art in Valcamonica: Evolution and meaning: A panoramic analysis according to the present research[END_REF]. Despite the great variety of engraving techniques and geological outcrops throughout the world, petroglyphs are generally characterized by a chromatic contrast between the rock surfaces and the brighter engraved areas (Dorn, 1998;[START_REF] Whitley | Introduction to Rock Art Research[END_REF][START_REF] Tansem | Red-coated rocks on the seashore: The esthetics and geology of prehistoric rock art in Alta, Arctic Norway[END_REF].

One of the most exceptional collections of petroglyphs in Europe is located on the slopes of the Mount Bego (alt. 2872 m, Mercantour National Park, Alpes-Maritimes, Fig. 1A), near the border between France and Italy [START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF]. The site displays several thousand open-air petroglyphs, carved on the outcropping rocks of the valleys of Marvels (Fig. 1B) and Fontanalba. Archaeological studies carried out since the beginning of the twentieth century by Clarence Bicknell, Carlo Conti, and Henry de Lumley converged toward a typological classification in which corneas, daggers, agricultural tools, and geometric shapes predominate [START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF][START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF][START_REF] Huet | A study of the roche de l'autel's pecked engravings, les merveilles sector, mont bego area (alpes-maritimes, france)[END_REF]. Most of these petroglyphs has been dated to the Chalcolithic and Bronze Age (1,500 BCE). The iconographic repertoire therefore bears witness to concerns linked to breeding, hunting, and agriculture, emblematic of the Neolithic revolution [START_REF] Demoule | La révolution néolithique dans le monde[END_REF]. The presence of few anthropomorphic figures (Fig. 1C) also led to various anthropological interpretations [START_REF] Begin | Contexte géomorphologique et géologique de la région du Mont Bego, répartition des gravures rupestres de l'âge du Bronze dans le site[END_REF][START_REF] Lumley | Figures of the pleiads stellar cluster on two engraved rocks from the mont bego region z ix. gii. r 4 and z ix. giii. r 6[END_REF][START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF][START_REF] Huet | A study of the roche de l'autel's pecked engravings, les merveilles sector, mont bego area (alpes-maritimes, france)[END_REF]. The site has, moreover, been continuously visited by humans since the Chalcolithic. Therefore, engravings dating from Antiquity, Middle Ages, Renaissance, and contemporary period are also visible. However, the engraving method used during the Neolithic period remains uniform throughout the site. It consists of a succession of percussion made with a hard material, such as quartz fragments collected from the nearby geological formation, on a thin superficial layer (thickness ≤ 0.5 mm) broadly covering the outcropping rocks [START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF][START_REF] Huet | A study of the roche de l'autel's pecked engravings, les merveilles sector, mont bego area (alpes-maritimes, france)[END_REF]. While generally red (Fig. 1B andC), this superficial layer displays a color range from orange to deep red [START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF] depending on its thickness and the nature of the rock substrate. The depth of the engraving incisions varies between 0.5 and 5 mm and reveal a brighter greenish rock substrate, underlying the thin red layer (Fig. 1B). The chromatic contrast thus produced highlights the engraving and testifies for a possible artistic approach.

The presence of a red layer on the surface of rocks may have different origins. The most frequent corresponds to a surface enrichment in iron (Fe) (hydr)oxides from an external source [START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF]. This type of accumulation constitutes a rock coating, commonly referred to as rock or "desert" varnish. Such coating is physically linked to the substrate but without neogenetic relationship with it. Yet, the mobilization and oxidation of Fe present within the minerals composing a rock is also possible during its weathering [START_REF] Meunier | Clays[END_REF][START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF][START_REF] Tansem | Red-coated rocks on the seashore: The esthetics and geology of prehistoric rock art in Alta, Arctic Norway[END_REF]. As such, newly formed particles of Fe (hydr)oxides are present in a wide variety of weathering products including soils [START_REF] Theng | Nanoparticles in the soil environment[END_REF]; river suspensions [START_REF] Poulton | Chemical and physical characteristics of iron oxides in riverine and glacial meltwater sediments[END_REF]; glacial sediments [START_REF] Raiswell | Contributions from glacially derived sediment to the global iron oxyhydroxide cycle: Implications for iron delivery to the oceans[END_REF][START_REF] Bhatia | Greenland meltwater as a significant and potentially bioavailable source of iron to the ocean[END_REF][START_REF] Raiswell | Rusty meltwater[END_REF]; and the surface of building stones [START_REF] Pecchioni | Chemical and mineralogical studies of the red chromatic alteration of florentine pietra serena sandstone[END_REF][START_REF] Wedekind | Damage phenomenon and petrophysical properties of sandstones at the Phnom Bakheng temple (Angkor, Cambodia): first investigations and possible conservation measures[END_REF]. Their nature and the conditions of their formation vary, however, with the physicochemical mechanisms of alteration undergone by the rocks [START_REF] Faivre | Iron Oxides: From Nature to Applications[END_REF]. Finally, the presence of biological organisms colonizing rock surfaces can also lead to the production of red residues in the form of Fe (hydr)oxides [START_REF] Weber | Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction[END_REF][START_REF] Faivre | Iron Oxides: From Nature to Applications[END_REF] or pigments such as carotenoids [START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF][START_REF] Liu | Microbial deterioration and sustainable conservation of stone monuments and buildings[END_REF]. Knowing the nature of the coloring products and their relationship with the rock surface are therefore crucial to understanding their origin and evaluating their stability in the present environment. Their origin being, moreover, conditioned by the geological history of the site.

The engraved valleys of the Mount Bego are made up of vast geomorphological complexes characterized by sets of pediplains (Fig. 1B) inherited from the glacial alternation [START_REF] Begin | Contexte géomorphologique et géologique de la région du Mont Bego, répartition des gravures rupestres de l'âge du Bronze dans le site[END_REF][START_REF] Bigot-Cormier | Chronological constraints on process leading to large active landslides[END_REF][START_REF] Sanchez | Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps[END_REF]. Two geological formations dating from the Permian period (299 -252 Ma) compose the bedrock of these pediplains: the Inferno formation, with arkosic conglomerates lying directly on the crystalline basement; and the Meraviglie Formation, where arkoses, schists and metapelites coexist. Both result from the accumulation of sediments originating from the alteration of preexisting igneous formations such as the crystalline massif of Argentera.

These sediments subsequently underwent intermediate regional metamorphism during successive phases of the Alpine orogeny (66 -5 Ma). Paleoclimatic and geomorphological studies indicate that the outcropping rocks were mostly covered by glaciers since the beginning of the northern hemisphere glaciation during Late Pliocene (~3.2 Ma, [START_REF] Bartoli | Final closure of Panama and the onset of northern hemisphere glaciation[END_REF][START_REF] Sanchez | Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps[END_REF], inducing considerable mechanical stresses [START_REF] Vivian | Hydrologie et érosion sous-glaciaires[END_REF][START_REF] Prestrud Anderson | Chemical weathering in glacial environments[END_REF][START_REF] Siman-Tov | The coating layer of glacial polish[END_REF]. Indeed, the surface of these rocks exhibits a notable polish inherited from the glacial period (Fig. 1C). The retreat of the last glacier has been dated between 13,500 and 6,100 BCE, which corresponds to the transition between the last glacial maximum (19,500 BCE from Jorda et al. (2000)) and the Holocene [START_REF] Sanchez | Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps[END_REF]. This transition was directly followed by the Holocene Climatic Optimum (HCO, 7,050 -3,050 BCE from Bigot-Cormier et al. (2005)) significantly warmer and wetter. The HCO was accompanied by an intense tectonic activity, due to gravitational movements, affecting rocks in the form of faults and folds [START_REF] Sanchez | Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps[END_REF]. The first open-air petroglyphs were carved on the outcropping rocks after this episode (1,800 -1,500 BCE) and, therefore, the last glacial retreat [START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF]. This is confirmed by the absence of ice polish on the greenish substrates exposed in the hollow of the engravings (Fig. 1C). These recently exposed substrates have thus been submitted to weathering for up to 3,800 years. Their greenish color (Fig. 1C) indicates that the red layers have not formed over this period.

The Mount Bego site now constitutes an exceptional but vulnerable archaeological heritage.

The entire area thus benefits from a protection as a Historical Building since 1987. The state of conservation of the petroglyphs is therefore regularly monitored. First, the regulation of access to the site have improved the preservation conditions regarding the acts of negligence, or even vandalism, described by [START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF]. However, many forms of natural weathering are visible. Among these, the presence of erosion phenomena, resulting in the attenuation of engravings by discoloration, is noted in areas of preferential runoff or snow melt [START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF].

Scales of about 5 mm thick are also observed (Fig. in Supporting Information). They could result from the freeze-thaw action [START_REF] Ruedrich | Physical weathering of building stones induced by freeze-thaw action: a laboratory long-term study[END_REF], favored by the presence of networks of micro cracks. The reporting of the alteration phenomena also underlines the heterogeneity of the superficial layer. The thin red layer covers most of the outcropping rocks regardless of their inclination but, its thickness varies slightly (see Fig. 1C and Fig. SI-1). This variation explains the color range from orange to deep red documented by [START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF]. In addition, few purple-colored deposits, associated with areas of pyrite oxidation, are visible on some outcrops (Fig. . All these alteration figures may therefore be inherited from the geological history of the rocks as well as actively linked to the mountain climate prevailing on the site (alt. ≥ 2000 m).

In this complex context, a good understanding of the nature of the red layer, its formation mechanism, and its evolution under local climatic conditions would favor anticipating the evolution of the rock surfaces. This is key to avoid false and misguided conservation measures and thus continue optimizing the preservation of the Mount Bego petroglyphs. This preventive conservation objective is based on the characterization of the mineralogical nature of the chromatic contrast.

Material and Methods

2.1.Description of the rock samples

The preservation of the archaeological, esthetical and spiritual values of the Mont Bego site was ensured in 1987 by its inscription on the list of Historical Buildings. Accordingly, the sampling of engraved rocks is strictly prohibited. For the purpose of this study, unengraved fragments of the Permian geological formations outcropping on the valleys of the Mount Bego were thus collected under the supervision of the Regional Archaeological Service and the Regional Conservation of Historic Buildings (French Ministry of Culture). These samples correspond to three metamorphic rock types, namely arkose, schist, and metapelite. They come in the form of scales invariably composed by a pale green substrate (GS) covered by a thin red layer (RL) on the exposed surface.

Their bulk mineralogy was first studied through X-ray diffraction (XRD) on disoriented powders (≤ 100 μm) using a Bruker D8 Focus diffractometer operating with a Co Kα radiation (λ = 1.789 Å).

The diffracted intensity was collected by a Lynx'Eye detector with an angular aperture of 1°2θ during 2 seconds at each angular interval of 0.02°2θ between 4 and 65°2θ. The diffractograms evidence a similar mineralogical composition where quartz, Na-feldspar (albite), micas, and chlorite coexist in various proportions. The distinction between the rock types is hence based on the relative proportion of each mineral, their crystal sizes, and the intensity of the metamorphism.

2.2.Spectrocolorimetry

The chromatic contrast between the two distinct layers (GS and RL) of each rock types was characterized by a NCS RUBY contact-less spectrocolorimeter with a beam diameter of 8 mm, using the color-system CIE L*a*b* 1976. Specifically, lightness L* and chromatic coordinates a* (greenred axis) and b* (blue -yellow axis) were measured. These parameters were used to define the attribute of chroma C* according to the following equation:

𝐶 * = √𝑎 * 2 + 𝑏 * 2 (1)
Given that most of the Mount Bego petroglyphs were carved on the metapelite outcrops [START_REF] Begin | Contexte géomorphologique et géologique de la région du Mont Bego, répartition des gravures rupestres de l'âge du Bronze dans le site[END_REF][START_REF] Lumley | La montagne sacrée du Bego, Préoccupations économiques et mythes cosmogoniques des premiers peuples métallurgistes des Alpes méridionales[END_REF][START_REF] Huet | A study of the roche de l'autel's pecked engravings, les merveilles sector, mont bego area (alpes-maritimes, france)[END_REF] and that the mineralogy of the three rock types is comparable (Sect. 2.1), only the metapelite scale was further studied.

2.3.Petrographic analysis

A micro-scale petrographic study was thus conducted on the metapelite scale. Several fragments displaying GS and RL were impregnated in a thermo-fusible resin (Aremco Crystalbond TM 509: a mix of phthalic anhydride and ethylene glycol) under vacuum and glued on a glass slide. Once hardened, the preparation was thinned down to 30 μm with a diamond coated saw and several polishing pads of decreasing grain sizes. The petrographic description of the produced thin section was performed using a polarized light microscope (Olympus BX-51). The repartition of the main chemical elements within the thin section was then characterized with a scanning electron microscope (SEM, Jeol JSM-7900F) coupled with a back-scattered electron (BSE) detector and an energy-dispersive X-ray spectrometer (EDS, Bruker QUANTAX FlatQUAD).

A millimetric fragment of the metapelite scale was also studied with the SEM to visualize how the features observed on the thin section are organized in three dimensions. The sample surface (RL) as well as a fresh crack were characterized using the secondary electron (SE) and the BSE detectors.

2.4.Chemical analysis

The bulk chemical composition of the metapelite scale was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). To do so, 10 g of powdered sample was completely mineralized by an alkaline fusion at 1000°C into a Katanax X600. The fusion product was diluted into a 5% nitric acid solution and the major elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe) were quantified by an iCAPTM 7400 ICP-OES Duo spectrometer (Thermo Scientific TM ).

The Fe 3+ /∑Fe ratio as well as the quantity of Fe (hydr)oxides of each layer (GS and RL) were then measured by 57 Fe Mössbauer spectroscopy. The spectra were collected on separated powders (~ 1 mg) of GS and RL, extracted by gentle scraping of the surface, in a constant acceleration conventional spectrometer (WISSEL). This apparatus is equipped with a 57 Co source (Rh matrix) operating at 1.85

GBq over a range of 12 mm.s -1 with 512 channels, and a high count-rate γ-system compact detector.

Note that Fe atoms composing the crystal structure of (hydr)oxides cannot be distinguished from those belonging to clay minerals (such as the micas and chlorite identified in Sect. 2.1) at room temperature due to super paramagnetism [START_REF] Murad | Mössbauer spectroscopy of environmental materials and their industrial utilization[END_REF]. The magnetic field of Fe (hydr)oxides particles is, however, better expressed at low temperature where a sextuplet appears on the Mössbauer spectra [START_REF] Barron | Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite[END_REF]. The spectra were thus recorded at 4.2 K with a Canberra multi-channel analyzer measuring the isomer shifts with respect to α-Fe metal. The obtained spectra were decomposed by adjusting Lorentzian line-shape functions to the doublets and sextuplets with the leastsquares fitting procedures proposed by the Recoil Mössbauer Analysis Software 1.05 edited by Ken

Lagarec & Denis Rancourt (Ottawa University, Canada). Isomer shifts (δ) and quadrupole splittings (Δ) were allowed to vary separately whereas the full width at half maximum (FWHM) of all peaks were coupled to vary in pairs. The χ 2 and misfit values were finally used to evaluate the quality of the numerical models.

2.5.Mineralogical analysis

The mineralogy of the metapelite scale was further studied through a combination of techniques.

First, the disoriented powders (~ 100 µg) of GS and RL were characterized by X-ray Diffraction (Sect.

2.1). Then, a quantitative crystal-chemical analysis of the clay mineral phases composing GS and RL was performed. A fraction of each powder (~ 10 µg) was first dispersed in 1.5 ml of water. 2 μl of the suspensions were deposited onto carbon coated copper slots. The chemical composition of seventyfive particles per sample were assessed from raw micro analytical data recorded by a transmission electron microscope (TEM, Jeol JEM-2010) fitted with a LaB6 electron gun, operated at 200 kV, and coupled with an energy-dispersive X-ray spectrometer (EDS, Bruker X-Flash Silicon Drift Detector 5030). The EDS spectra were acquired at a magnification of 50,000; an angular tilt of 20° toward the detector; a time constant of 60 kcp.s -1 ; an energy range of 40 keV; and a corrected counting time of 30 seconds. The beam diameter was set to approximately 20 nm in order to reach the smallest particles.

These acquisition conditions led to a constant beam density of ~ 60 pA.cm -2 . The acquired EDS spectra were corrected by background subtraction, Gaussian deconvolution, and k-factor correction using values previously computed on a set of clay mineral standards. The main elements (O, Na, Mg, Al, Si, K, Ca, and Fe) were then quantified by applying the Bruker AXS MET line mark procedure [START_REF] Cliff | The quantitative analysis of thin specimens[END_REF]. The cationic compositions, expressed in atomic percent, were used to define the structural formulas on a O10(OH)2 basis for 2:1 clay minerals and O10(OH)8 for 2:1:1 clay minerals [START_REF] Meunier | Clays[END_REF]. Note that the EDS analysis does not allow distinguishing Fe 2+ from Fe 3+ .

The calculation was thus made with the Fe 3+ /∑Fe ratio measured in Mössbauer spectroscopy (see Sect.

2.3) for each layer. The cations were allocated according to Pauling's rules [START_REF] Pauling | The principles determining the structure of complex ionic crystals[END_REF].

Tetrahedrons were filled by Si 4+ and Al 3+ . The remaining Al 3+ was placed in octahedra which were then completed by Fe 3+ , Fe 2+ and Mg 2+ . Finally, alkali and alkaline earth metals (Na + , Mg 2+ , K + , and Ca 2+ ) were used to compose the interlayer space. The morphology and crystallography of the clay mineral particles were also studied with bright-field imaging and selected area electron diffraction patterns (SAED). The SAED patterns were integrated in one dimension using the Fit2D software [START_REF] Hammersley | Twodimensional detector software: From real detector to idealized image or two-theta scan[END_REF] and compared with the ICDD database in order to establish the particles mineralogy.

The different clay mineral phases were then quantified from XRD profile fitting. Oriented deposits were first prepared to promote the expression of 00l reflections over the hkl reflections [START_REF] Moore | X-Ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF]. The oriented deposits were air dried (AD, relative humidity ~ 30%) at room temperature. The acquisition of the diffractograms was performed using the Bruker D8 Focus diffractometer. In this case, the diffracted intensity was collected during 4 seconds at each angular interval of 0.02°2θ between 5 and 55°2θ. The diffractograms were finally fitted over the entire angular domain using the ASN software [START_REF] Drits | X-Ray Diffraction by Disordered Lamellar Structures: Theory and Applications to Microdivided Silicates and Carbons[END_REF]. A trial-and-error approach was implemented in order to converge towards a model including all structural formulas, crystallographic parameters, and proportions of phases. Among the crystallographic parameters, the size of the coherent diffusion domain was used to account for the log normal distribution along the c* axis. This size was characterized by an average value, N, adjusted during the fitting procedure [START_REF] Drits | X-Ray Diffraction by Disordered Lamellar Structures: Theory and Applications to Microdivided Silicates and Carbons[END_REF]. The spatial coordinates of atoms within the crystal lattices have been assigned as established by [START_REF] Moore | X-Ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF]. The instrumental parameters such as the beam divergence, the radius of the goniometer, the length and the theoretical thickness of the oriented deposit were previously introduced into the software algorithm. The mass absorption coefficient and the preferred orientation, σ*, were set at 45 cm 2 .g -1 and 12°, respectively [START_REF] Moore | X-Ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF]. Eventually, the quality of the simulations, and therefore the error associated with the quantification, was estimated using the unweighted profile fitting indicator, Rp [START_REF] Howard | Profile fitting of powder diffraction patterns[END_REF].

2.6.Nanoscale mineralogical analysis

The mineralogical changes between GS and RL were further studied at the nanoscale. A region of interest (ROI) where RL is clearly distinguishable from the underlying GS was thus located on the thin section using a reflective light microscope. A copper slot was fixed on the top of the thin section with an epoxy resin. The surrounding material was circled with a micro-driller to isolate the ROI. A heated soldering iron (~ 200°C) was then placed in contact of the glass slide on the opposite side to locally melt the impregnation resin (melting point of ~ 60°C). The slot containing the ROI was extracted from the parent thin section and submitted to a double flux of Ar in a precise ion polishing system (PIPS).

This procedure was stopped as soon as a hole formed in the center of the ROI. On the periphery of this hole, the bevel was thin enough to allow electron transparency (≤ 100 nm). The nano-texture of the mineral assemblage composing GS and RL was thus further studied in TEM. The chemical composition as well as the crystallographic nature of the particles were determined by coupling the bright-field observations with EDS spectra and SAED (see Sect. 2.5).

2.7.Characterization of the pyrite oxidation products

Finally, the purplish halo surrounding an area of pyrite oxidation was sampled on site, in addition to the previously described rock samples. The products composing this color deposit were then studied through XRD on a disoriented powder (see Sect. 2.1) in order to postulate on the mechanism of oxidation and assess their potential contribution to the formation of the red layer.

Experimental Results

3.1.Chromatic contrast enhancing the petroglyphs

The chromatic contrast between the green substrates (GSs) and the red layers (RLs) of the arkose, leading to chromas ≤ 20 (Fig. 2). The RLs, on the other hand, are marked by higher chromatic coordinates (b* ≥ 20 and a* ≥ 8), and therefore a higher chroma (C* ≥ 20). One can note that the RL of the metapelite displays higher yellow coordinates (b*) while the RL covering the arkose express higher red coordinates (a*). In addition, the metapelite displays a higher lightness (L*) than the rest of the studied rock types (Fig. 2B). These variations support the color ranges from orange to deep red described in situ (see Sect. 1;[START_REF] Bicknell | A Guide to the Prehistoric Rock Engravings in the Italian Maritime Alps[END_REF]. They may be related to the nature of the substrate as well as the variations of the layer thickness. Nonetheless, the superficial layer is generally red and the chromatic contrast between GS and RL, revealed by the engravings (Fig. 1), is due to a significant increase in chroma (Fig. 2B).

3.2.Petrographic description of the metapelite

The chromatic contrast was then described through the petrographic characterization of the thin section of the metapelite in optical microscopy (Fig. 3). At first glance, the metapelite green substrate (GS) shows a homogeneous clay mineral matrix composed by elongated particles (~ 10 μm). Among these particles, mica-like phases are predominant. Chlorite agglomerates are also identified from their greenish color (Fig. 3C). Even if their proportion is limited, these chlorite particles largely contribute to the metapelite green color. In addition to the clay minerals, scarce grains of primary minerals such as quartz, feldspar (albite), and tourmaline are present. In few areas, these primary minerals are noticeably weathered and bordered by secondary phases, which could correspond to a process of sericitization.

RL displays an important difference with GS under the optical microscope (Fig. 3B). The mica-like and chlorite particles composing the clay mineral matrix are covered with a reddish-brown coloring product, most certainly composed of Fe (hydr)oxides. At this scale, this phenomenon seems gradual from the external surface inward, and the transition between GS and RL is diffuse. It occurs at depths ranging from 150 to 500 μm (dashed line on Fig. 3A). Below this limit, the mica-like and chlorite particles are rarely altered, except along the micro crack networks where oxidation is observed (Fig. 3C). These observations suggest a weathering process of the outcropping rocks of superficial origin, conditioned by the circulation of weathering agents, rather than a new deposit.

To verify this hypothesis, the repartition of chemical elements within the metapelite thin section was further studied in SEM-EDS (Fig B). This means that the chromatic contrast is not related to differences in chemical composition. Therefore, the potentiality of a surface enrichment in iron (Fe) (hydr)oxides from an external source, constituting a rock coating [START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF], as a dominant process may be ruled out. However, one can note that most of the Fe is associated with the clay mineral particles, such as the Fe-rich chlorites (and micas, to a lesser extent).

A fragment of metapelite was also observed under the SEM in back-scattered electron mode (Fig. 4). On Fig. 4A, the surface of RL appears composed by finely divided grains. Among these grains, the chemical contrast mode allows distinguishing scarce nanoparticles of Fe (hydr)oxides (red arrows on Fig. 4A) as Fe is the heaviest element composing the metapelite. An important delamination of the clay mineral particles is also noted (Fig. 4A). Moreover, few nanoparticles of Fe (hydr)oxides are observed within these delaminated clay minerals. Even though they seem to cover only a tiny fraction of the metapelite surface, these Fe (hydr)oxides crystals could bestow its red color. A high concentration of Fe (hydr)oxides is also noted along a crack spreading within GS (Fig. 4B). In this case, the clay mineral particles are automorphic, and their faces are covered with Fe-rich agglomerates. These Fe-rich coverings can explain the localized oxidation along the micro cracks previously described by optical microscopy (Fig. 3C). One can note that the accumulation of nanoparticles of Fe (hydr)oxides occurs preferentially on the clay mineral particles. The presence of these nanoparticles does not seem to impact the bulk repartition of Fe within the thin section (Fig. 3D and Fig. SI-2D). It may, however, depict an evolution of the status of Fe between the layers.

3.3.Chemical composition of the contrasted layers

The bulk chemical composition of the metapelite (including both GS and RL) was then determined by ICP-OES. The main chemical elements were quantified and are expressed by oxide weights (mass percentage of SiO2, Al2O3, FeO + Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5, MnO, and CO2) in Table SI-1. Based on the results, the metapelite is mainly composed by alumino-silicates (SiO2 + Al2O3 ~75 wt.%) and a noticeable proportion of iron (FeO + Fe2O3 = 8.2 wt.%). Yet, this chemical analysis does not provide information on the status nor the distribution of iron (Fe) between the two layers.

Mössbauer spectra were therefore acquired on the differentiated powders of GS and RL (Fig. 5).

These spectra allow a clear distinction between the layers in terms of Fe 3+ /∑Fe ratio and presence of Fe (hydr)oxides. The best Lorentzian decomposition of the experimental spectra of GS (Fig. 5A) is obtained with two doublets characterized by distinct isomer shifts (δ) and quadrupole splittings (Δ), as presented in Table 1. The doublet displaying higher δ (1.24 mm.s -1 ) and Δ (2.73 mm.s -1 ) corresponds to Fe 2+ while the second doublet (δ = 0.35 and Δ = 0.85) is interpreted as Fe 3+ . The relative proportions deduced from the decomposition procedure led to a Fe 3+ /∑Fe ratio of 0.375 for GS. Based on this ratio, the chemical composition of GS (FeO + Fe2O3 = 8.2 wt.% from The decomposition model of the Mössbauer spectra obtained on RL (Fig. 5B) also contains two doublets, denoting the coexistence of Fe 2+ and Fe 3+ . The resulting Fe 3+ /∑Fe ratio comes to 0.366, which is slightly lower than for GS. In addition, it clearly displays a sextuplet (Fig. 5B), depicting the presence of Fe (hydr)oxides. The parameters of this sextuplet (δ = 0.28 mm.s -1 ; Δ = 0.00 mm.s -1 and Bhf = 51.20 T, Table 1) are, however, intermediate compared to those of well characterized Fe (hydr)oxides (see values from [START_REF] Murad | Mössbauer spectroscopy of environmental materials and their industrial utilization[END_REF] reported on Table 1). This may be explained by the presence of poorly crystallized Fe (hydr)oxides with particle sizes in the nanometer range and heterogeneous chemical compositions [START_REF] Murad | Mössbauer spectroscopy of environmental materials and their industrial utilization[END_REF]. Assuming a constant FeO + Fe2O3 content from the bulk chemical analysis (Table SI-1), the mass fraction of the Fe (hydr)oxides may be estimated around ~ 1.5 wt.% within RL (see caption of Fig. 5). This confirms that the chromatic contrast (Fig. 1 and 2) only originates from the presence of a small quantity of nanoparticles of Fe (hydr)oxides (Fig. 4).

3.4.Mineralogical composition of the contrasted layers

The powder X-ray diffraction showed that GS is mainly composed by quartz (SiO2), albite (NaAlSi3O8), micas and chlorite (Fig. SI-3). The diffractogram specifically acquired on RL displays the same bulk mineralogical composition where no Fe (hydr)oxides can be clearly identified (inset of Fig. SI-3). This may be due to their limited quantity (estimated at ~ 1.5 wt.%) and low crystallinity (see Sect. 3.3). Yet, the intensity ratios between the reflections attributed to micas and chlorite (Fig.

SI

-3) seem to evolve slightly from GS to RL, suggesting a modification of the clay mineralogy on the exposed surface (RL).

A crystal-chemical analysis was thus performed in TEM-EDS in order to understand the evolution of the clay minerals between the two layers. Examples of EDS spectra acquired on the clay mineral particles are presented in Fig. SI-4. Among the elements quantified from these raw micro analytical data (see Sect. 2.5), the total Fe was shared between Fe 2+ and Fe 3+ based on the Fe 3+ /∑Fe ratios deduced from Mössbauer spectroscopy (Table 1). These ratios were assumed to be constant within the particles contained in each layer. In addition, the site distribution of Fe was considered as exclusively octahedral [START_REF] Lougear | Mössbauer investigations to characterize Fe lattice sites in sheet silicates and Peru basin deep-sea sediments[END_REF] based on the isomer shifts of both Fe 2+ and Fe 3+ (see Table 1). The resulting set of structural formulas were projected on ternary diagrams (Fig. 6A andB).

The M + -4Si-R 2+ diagrams [START_REF] Meunier | Solid solutions in I/S mixed-layer minerals and illite[END_REF] were specifically used to distinguish the main clay minerals according to their interlayer cations compositions (M + = K + (2 × Ca) + Na), tetrahedral charge (4Si = Si/4), and octahedral charge (R 2+ = Mg 2+ + Fe 2+ ). In brief, the magnitude and the location of the particles charge allowed the definition of their phase.

Three clay mineral phases are identified in GS and RL: micas (sericite and phengite) and chlorite (Fig. 6A andB). Their mean structural formulas are given in Table SI-2. As indicated in Table SI-2, all phases contain Fe within their crystalline structure. However, the quantity of Fe is relatively IV Si ≤ 2.8) allow to class them among the ripidolites, a Fe-rich trioctahedral type of chlorite [START_REF] Post | The chlorite series of flagstaff hill area, California: A preliminary investigation[END_REF]. They may accommodate both Fe 2+ and Fe 3+ in their crystalline structure [START_REF] Meunier | Clays[END_REF][START_REF] Bourdelle | Quantification of the ferric/ferrous iron ratio in silicates by scanning transmission x-ray microscopy at the Fe l2,3 edges[END_REF]. Specifically, Fe 2+ is usually distributed within the octahedral sites of the 2: 1 sheet and Fe 3+ within the brucite layer [START_REF] Smyth | Crystal structure refinement and Mössbauer spectroscopy of an ordered, triclinic clinochlore[END_REF][START_REF] Vidal | Deciphering temperature, pressure, and oxygen activity conditions of chlorite formation[END_REF][START_REF] Masci | A XANES and EPMA study of Fe3+ in chlorite: substitutions, importance of oxychlorite and implications for cation site distribution and thermobarometry[END_REF].

Such distribution agrees with the structural formulas deduced from our TEM-EDS micro analysis (Table SI-2). Yet, the chemical compositions of the ripidolites are not significantly different between GS and RL.

3.5.Evolution of the clay mineral particles

The morphology and crystallography of the clay mineral particles composing GS and RL were also studied in TEM-EDS. A series of TEM micrograph acquired on the particles observed in GS is distinguished from their EDS spectra. In both cases, the particles size ranges from 500 nm to 10 µm.

They are well crystallized and display sharp edges, free from any impurity. A TEM micrograph displaying a ripidolite particle of GS is also shown on Fig. 6C. A selected area electron diffraction (SAED) pattern was acquired on the edge of this particle to further study its crystallography (inset of Fig. 6C). The signal integration (Fig. 6E reflections of a pure ripidolite (b = 9.24 Å) expressing only its hk0 reflections.

The TEM micrographs series documenting the clay mineral particles composing RL is also 6D

. The SAED pattern (inset of Fig. 6D) allows defining the nature of these impurities. It shows an array of diffraction points with a three-fold axis of symmetry as well as a series of concentric diffraction circles. The full integration of the signal (Fig. 6F) displays poorly resolved diffraction domes attributed to a ripidolite particle (with dhk0 at 4.62, 2.66, 2.36, 1.76, 1.54, and 1.35 Å). A partial integration of the signal, only accounting for the concentric diffraction circles, was also produced to decipher the contribution of the impurities (dashed red line). The diffraction domes of this partial integration coincide with the hkl parameters of a monocrystal of hematite (ICDD #33-0664) in terms of peak positions and intensity ratio (Fig. 6F). They thus evidence the presence of disordered nanoparticles of hematite on the edges of the ripidolite particles composing RL. This observation agrees with the sextuplet described on the Mössbauer spectra of RL (Fig. 5B) and, therefore, the chromatic contrast between GS and RL (Fig. 1 and2).

To refine the study of a potential mineralogical modification of the clay mineral particles between GS and RL, X-ray diffractograms were acquired on oriented deposits of each layer. In both cases, these diffractograms show two series of rational 00l reflections (Fig. 7). The first includes the diffraction peaks at 14.17 Å (001), 7.07 Å (002), 4.71 Å (003), 3.54 Å (004), and 2.82 Å (005) corresponding to the ripidolite phase. The second is made up of peaks at 9.97 Å (001), 4.98 Å (002), 3.32 Å (003), 2.49 Å (004), and 1.99 Å (005) corresponding to the mica phase, including sericites and phengites. These diffractograms also contain some hkl reflections belonging to accessory minerals (quartz at 4.26 and 3.34 Å; albite at 3.25 and 3.19 Å), but no reflection attributable to Fe (hydr)oxides is visible.

The numerical models fitting the experimental diffractograms are also presented on Fig. 7. The structural parameters defining these models are detailed in Table SI-3. They include the mean coherent scattering domain size, N, the relative contribution (%) of the different clay mineral phases, and Rp allowing to estimate the associated error. This approach produced convincing models using the average structural formulas (Table SI-2). On the one hand, the rational 00l series of reflections attributed to ripidolite was modeled by two distinct phases. First, a ripidolite phase of structural formula as defined in TEM-EDS (Table SI-2) was used. Its relative proportion decreases slightly between GS (9 + 1%) and RL (8 + 1%). Even if the variation seems to overlap the error associated with the quantification, the decrease in peak intensity is clearly visible on RL (see red arrows on Fig. 7B). Second, a weakly crystallized ripidolite (N ~ 30) with the following structural formula ((Al2.05Fe1.3Mg2.75) (Si2.7Al1.3) O10(OH)8) was added. This phase allows reproducing the shoulder observed at low angles, near the 001 reflection of the main ripidolite phase (see inset of Fig. 7). It contributes, however, only very slightly to the calculated intensity (~ 1% in GS and RL). On the other hand, the rational 00l series of reflections associated with the mica phases was modeled thanks to the contribution of the two phases identified in TEM-EDS, namely: sericite and phengite. While the relative proportion of sericites contributing to the models increases (from 10 + 1 to 14 + 2%), the proportion of phengites decreases between GS (81 + 10%) and RL (76 + 11%).

In short, the modeling of the X-ray diffractograms acquired on GS and RL shows a slight decrease in the relative proportion of phengites and ripidolites coupled with an increase in the proportion of sericite in RL. This may be explained by an accumulation of sericite near the surface, or a selective alteration of a fraction (~ 5%) of the phengite and ripidolite particles. The presence of an oxidized and weakly crystallized ripidolite phase can, moreover, be related to the particles of ripidolite lined with Fe (hydr)oxides observed in TEM-EDS (Fig. 6).

3.6.Nano-scale analysis

The nanotexture of the metapelite, as well as the chemical composition and crystallographic nature of the mineralogical phases, were then characterized in TEM-EDS on the ionic thinning of the section observed previously (Sect. 3.2).

At this scale, the finely divided matrix of GS shows a complex mixture of particles. Their chemical compositions allow an easy distinction between grains of quartz, ripidolites, and phengites, in agreement with the mineralogical analyzes. As noted previously (Fig. 6), the clay mineral particles of RL are frequently associated with denser nanoparticles. For example, a phengite particle covered by nanoparticles is displayed on Fig. 8A. The EDS spectra acquired on one of these nanoparticles display a significant enrichment in Fe (Fig. 8B). Similar Fe-rich nanoparticles agglomerated on the surface of another clay mineral particle are shown on Fig. 8C. In order to define their crystallographic nature, a SAED pattern was acquired (inset of Fig. 8C). The pattern clearly shows the contribution of the well crystallized clay mineral (diffraction points), as well as a set of weakly crystallized nanoparticles with random orientations (diffraction circles). The complete integration of the signal (solid black line on Fig. 8D) allows discerning diffraction domes at 4.62 and 1.55 Å, corresponding to the diffraction points of the SAED. These inter-reticular distances can be attributed to the superposition of 003 and reflections (1.55 × 9 = 13.95 Å; 1.55 × 6 = 9.30 Å) of a ripidolite (d00l = 14.15 Å; b = 9.24 Å). A second signal integration was performed on the diffraction circles only (solid red line on Fig. 8D).

This profile shows a series of diffraction domes at 3.68; 2.70; 2.52; 2.21; 1.84; 1.69 and 1.45 Å.

Again, this series of hkl reflections is comparable to the single crystal of hematite in terms of peaks positions and intensity ratios (Fig. 8D). The width of the diffraction domes can be explained by the low crystallinity of the nanoparticles.

The chemical and crystallographic information presented above therefore highlight the presence of agglomerates of Fe (hydro)oxides, in the form of hematite nanoparticles, covering the surfaces of the clay minerals (phengites and ripidolites). These mineralogical associations are observed within the red layer of the metapelites (RL) but also in the networks of micro cracks near the exposed surface. Their presence and absolute quantity constitute a reasonable explanation for the phenomenon of superficial reddening of the metapelite observed macroscopically (Fig. 1 and2). The environmental conditions and the physico-chemical mechanism causing the mobilization of Fe and leading to the formation of these hematite nanoparticles will, therefore, be discussed.

3.7.Pyrite oxidation products

Aside from the widely distributed red layer, the reporting of the alteration figures showed intense purple coloration associated with areas of pyrite oxidation (Fig. SI-1). The X-ray diffractogram acquired on the disoriented powder obtained by grinding the purplish halo surrounding a grain of oxidized pyrite (Sect. One can note that none of these phases were previously identified within the red layer. Therefore, they must all be related with the reaction of pyrite oxidation. Indeed, the coexistence of basic Fe sulfates (szomolnokite) and Fe hydroxides (goethite and lepidocrocite) within pyrite oxidation products agrees with the literature [START_REF] Lowson | Aqueous oxidation of pyrite by molecular oxygen[END_REF][START_REF] Karthe | Oxidation of pyrite surfaces: a photoelectron spectroscopic study[END_REF]Rimstidt and Vaughan, 2003).

The presence of szomolnokite may result from the dehydration of the melanterite (hydrous iron (II) sulfate: Fe 2+ (SO4)•7H2O), more regularly described as the oxidation product of pyrite (Rimstidt and Vaughan, 2003). This reaction is also known to lead to the production of sulfuric acid (H2SO4), conferring an acid pH in the medium. This explains the formation of goethite from the released Fe as it is the stable phase under this acidic pH condition [START_REF] Schwertmann | Effect of pH on the formation of goethite and hematite from ferrihydrite[END_REF]. This pH condition is, however, incompatible with the formation of hematite (favored under 6 ≤ pH ≤ 9, from [START_REF] Schwertmann | Effect of pH on the formation of goethite and hematite from ferrihydrite[END_REF]). All these facts plead toward a distinction between the phenomenon of pyrite oxidation, locally affecting the outcrops at the vicinity of the grains of pyrite, and the general mechanism leading to the formation of the red layer covering the outcrops.

Discussion

The petroglyphs carved on the outcrops of the Mount Bego site are enhanced by a chromatic contrast between a superficial red layer and the rocks' green substrates (Fig. 1). The mineralogical nature of the red layer was therefore studied in order to shed light on its formation mechanism and anticipate its evolution under the local climatic conditions. The present study evidences that this layer is characterized by a significant increase of the chroma (Fig. 2B) originating from the presence of Fe (hydr)oxides (Fig. 4A and5B). The mineralogical nature of these Fe (hydr)oxides corresponds to disordered nanoparticles of hematite (Fig. 6F and8D). These nanoparticles preferentially populate the edges of the clay minerals (Fig. 3, 4, 6, and 8) composing the outcrops from the surface down to ~ 200 μm inward (Fig. 3A andB), as well as within the networks of micro cracks near the exposed surface (Fig. 3C and4B). As a result, while the contrast is clearly visible on the engravings, it appears diffuse on cross sectional thin section (Fig. 3A). Even though they seem to cover only a tiny fraction of the rock surface, the nanoparticles of hematite bestow the superficial red color. In addition to this general red coloration, scarce areas of purple coloration originate from the reaction products of pyrite oxidation. The overall repartition of S within the studied samples (see Fig. SI-2C), the mineralogical nature of the pyrite oxidation products (Fig. SI-6), as well as the chemical conditions of their crystallization suggest, however, a limited contribution of pyrite oxidation to the mechanism leading to the formation of the red layer.

Several mechanisms have been recognized to lead to the superficial reddening of rock surfaces [START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF]. First, the presence of nanoparticles of hematite at the surface of outcropping rocks usually arises from an external supply leading to the formation of a rock coating, also known as rock or "desert" varnish [START_REF] Dorn | Rock varnish[END_REF][START_REF] Garvie | Nanometer-scale complexity, growth, and diagenesis in desert varnish[END_REF][START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF].

Such coating displays Fe-rich layers deposited on the rock surface but, the junction between the layers and the substrate is irrevocably distinct. The deposit is also internally layered due to a repeated process of micro-deposition [START_REF] Whitley | Climate change, rock coatings, and the archaeological record[END_REF]. The presence of series of Fe-rich lamina distinct from the substrate is therefore illustrative of a rock coating. In the case of the red layer covering the outcrops of the Mount Bego site, the Fe (hydr)oxides do not constitute a lamina clearly distinguished from the substrate (Fig. 3). They occur as nanoparticles associated with the clay minerals composing the rock in the first few hundreds of micrometers as well as within the micro crack networks (Fig. 4). Therefore, the Fe (hydr)oxides more likely originate from the superficial weathering of the minerals [START_REF] Schindler | Coatings on rocks and minerals: The interface between the lithosphere and the biosphere, hydrosphere, and atmosphere[END_REF][START_REF] Tansem | Red-coated rocks on the seashore: The esthetics and geology of prehistoric rock art in Alta, Arctic Norway[END_REF]. Interestingly, the formation of Fe (hydr)oxides is combined with a slight decrease in the relative proportion of the phengites and ripidolites particles, as

shown by XRD profile modeling (Fig. 7). This may be explained by a selective alteration of a fraction (~ 5%) of the clay mineral particles containing structural Fe near the exposed surface. The weathering process would thus appear to be conditioned by the circulation of weathering agents, which caused the mobilization of the structural Fe.

The geological history of the Mount Bego site denotes that the surface of the outcropping rocks was mostly covered by glaciers during the last ~ 3.2 Ma. These glaciers, up to 400 m thick during the Würm period [START_REF] Julian | Les Alpes Maritimes Franco-Italiennes: Etude Geomorphologique[END_REF], applied a combination of vertical pressure and semi-horizontal thrust causing shear stresses. These stresses provoked a phenomenon of surface abrasion leading to the ice polish [START_REF] Siman-Tov | The coating layer of glacial polish[END_REF] visible on the outcropping rocks (Fig. 1C). Such glacial environment is also known to induce a considerable chemical weathering at the rock/ice interface as well as along the circulation path of melt waters [START_REF] Prestrud Anderson | Chemical weathering in glacial environments[END_REF][START_REF] Dixon | Chemical weathering and landscape development in mid-latitude alpine environments[END_REF][START_REF] Graly | Chemical weathering under the greenland ice sheet[END_REF]. The summer melt waters, and therefore the base of the glaciers, are known to be enriched in HCO 3-, which gives them a slightly acidic pH (~ 5.5 from [START_REF] Vivian | Hydrologie et érosion sous-glaciaires[END_REF]; [START_REF] Graly | Chemical weathering under the greenland ice sheet[END_REF]).

Conversely, the water circulating at the base of glaciers in winter is rather basic and enriched with dissolved oxygen [START_REF] Vivian | Hydrologie et érosion sous-glaciaires[END_REF]. Water flux has been reported to trigger the disruption of mineral lattices by grinding and thus increases dissolution rates, especially for minerals with high surface areas [START_REF] Prestrud Anderson | Chemical weathering in glacial environments[END_REF]. A progressive weathering of the minerals composing the rocks is therefore plausible near the surface in contact with the glaciers. Specifically, the clay minerals (phengites and ripidolites) identified in the studied metapelite sample are characterized by a high surface area and are thus more inclined to partial dissolution. In addition, the circulation of acid (HCO 3-) has been shown to favor the dissolution of ripidolites [START_REF] Krawczyk-Barsch | Formation of secondary feoxyhydroxide phases during the dissolution of chloriteeffects on uranium sorption[END_REF]. Thus, part of the phengite and ripidolite (~ 5 %) could have been continuously dissolved within the first few micrometers near the rock/ice interface along the period of glacial covering. These particles contain a noticeable proportion of Fe (Table SI-2) that could have been mobilized during this weathering process.

The external surfaces of the outcropping rocks were also eroded during the last episode of glacial retreat (between 13,500 and 6,100 BCE from [START_REF] Jorda | Données nouvelles sur l'âge du dernier maximum glaciaire dans les alpes méridionales françaises[END_REF]). Such an episode induces the circulation of surface melt waters, also acting as highly efficient agents of erosion [START_REF] Cowton | Rapid erosion beneath the greenland ice sheet[END_REF]. This episode was directly followed by the Holocene Climatic Optimum (HCO, between 7,050 and 3,050 BCE, from Bigot-Cormier et al. ( 2005)) significantly warmer and wetter. It is therefore likely that the temperature of the rock outcrops has risen from the last Ice Age to the HCO. This thermal increase, together with the presence of water, could thus have also triggered the release of the structural Fe. Indeed, the crystallographic expansion of clay minerals (especially chlorites) during temperature increase has been studied by [START_REF] Nelson | Inferred limitations to the oxidation of fe in chlorite: A high-temperature single crystal x-ray study[END_REF]. These authors describe a progressive phenomenon occurring between 20 and 550°C and mainly affecting the octahedral sites of the brucite layer. Along this temperature increase, the octahedral sites filled with Fe become the preferred sites for oxidation. At the same time, a progressive repulsion builds up between the octahedral sites which could lead to the breakdown of hydrogen bonds, the rotation of the tetrahedra, and a lateral expansion of the 2:1 sheet [START_REF] Nelson | Inferred limitations to the oxidation of fe in chlorite: A high-temperature single crystal x-ray study[END_REF]. Even if the temperature increase triggered by the HCO is on the low range of the temperature studied by [START_REF] Nelson | Inferred limitations to the oxidation of fe in chlorite: A high-temperature single crystal x-ray study[END_REF], the duration of the warm climatic conditions of the HCO may have resulted in the destruction of the brucite layer of the ripidolite particles and the release of Fe directly at the surface of the clay minerals.

The process of Fe release may thus have occurred progressively on all the rocky reliefs in contact with the glaciers during the extended glacial periods of the last 3.2 Ma, or more punctually during the HCO. Given Fe is the least soluble elements [START_REF] Stefansson | Iron(iii) hydrolysis and solubility at 25°C[END_REF] composing the clay mineral particles, part of the released Fe 2+ and Fe 3+ atoms would remain near the particles' surface. In the presence of water, the Fe 2+ atoms are then easily oxidized according to the following reaction [START_REF] Schwertmann | From Fe(iii) ions to ferrihydrite and then to hematite[END_REF]:

2𝐹𝑒 → 2𝐹𝑒 3+ + 6𝑒 - (2)
6𝐻 3 𝑂 + + 6𝑒 -→ 6𝐻 2 𝑂 + 3𝐻 2 (3)

2𝐹𝑒 3+ + 4𝐻 2 𝑂 → (𝐹𝑒 3+ ) 2 𝑂 3 • 0.5𝐻 2 𝑂 + 3𝐻 2 (4)
This sequence naturally leads to the formation of ferrihydrite ((Fe 3+ )2O3•0.5H2O), a relatively unstable ferric Fe hydroxide. TEM observations have shown that ferrihydrite aggregates produced by the dissolution of chlorite particles consist of a large number of particles ranging in size from 2 to 5 nm, subsequently precipitating on the clay mineral surfaces [START_REF] Krawczyk-Barsch | Formation of secondary feoxyhydroxide phases during the dissolution of chloriteeffects on uranium sorption[END_REF]. The chlorite particles may therefore act as the source of Fe upon weathering as well as the sink from the preferential precipitation of the ferrihydrite colloids on their surfaces [START_REF] Krawczyk-Barsch | Formation of secondary feoxyhydroxide phases during the dissolution of chloriteeffects on uranium sorption[END_REF][START_REF] Brandt | Heterogeneous formation of ferric oxide nanoparticles on chlorite surfaces studied by x-ray adsorption spectromicroscopy (stxm)[END_REF]. The transformation of ferrihydrite into the thermodynamically stable end product hematite (α-Fe2O3) is then favored in the presence of free or surface-bound water [START_REF] Schwertmann | From Fe(iii) ions to ferrihydrite and then to hematite[END_REF]. In laboratory conditions, at a temperature of about 70°C and under pH conditions close to the zero-point charge of ferrihydrite (~ 7), the direct transformation proceeds by crystallization within the aggregates themselves [START_REF] Schwertmann | From Fe(iii) ions to ferrihydrite and then to hematite[END_REF]. This is certainly linked to the minimum solubility of Fe in this pH range [START_REF] Schwertmann | Effect of pH on the formation of goethite and hematite from ferrihydrite[END_REF]. The hematite crystals thus formed are described as being larger than the ferrihydrite particles (~ 50 nm) but remain within the nanometer range. This formation path agrees with the size and quantity of hematite nanoparticles described in the red layer (RL) of the metapelite by Mössbauer spectroscopy (Fig. 5) and TEM (Fig. 6 and8). This physicochemical mechanism provides a reasonable explanation to the formation of the red layer broadly covering the outcrops, and therefore the chromatic contrast of the Mount Bego's petroglyphs.

Conclusions

The conservation of open-air archaeological heritages may be favored by a precise knowledge of their nature and their evolution under the local environmental conditions. The exceptional collection of prehistoric petroglyphs located on the slopes of the Mount Bego is notably enhanced by a chromatic contrast between the superficial red layer covering the outcrops and the rock substrate revealed by the engravings. Throughout the site, it appears that the engravings were voluntary made on the specific areas where this superficial red layer is present. This technic of engraving through a colored superficial layer was potentially selected because it offered high visibility as well as good durability on such substrates exposed to weathering.

The mineralogical origin of the chromatic contrast was therefore studied by coupling crystallographic, spectroscopic, and multi-scale microscopic techniques in order to understand its formation mechanism. Although constrained by the limited sampling possible on this vast archaeological landscape, the results point that the red layer is mainly characterized by the presence of hematite nanoparticles agglomerated on the clay minerals composing the rocks near the exposed surface. Unlike the commonly described rock coatings, this superficial reddening seems to originate from the progressive release of structural Fe from clay minerals along the glacial period and its subsequent oxidation after the last glacial retreat. Additional mechanisms such as the deposition of Fe (hydr)oxides originating from iron sulfides oxidation or chemical weathering of external geological formations could have contributed to the formation of this red layer but to a lesser extent.

Based on the literature, the Holocene Climatic Optimum following the last glacial retreat has created the environmental conditions for the formation of the Fe (hydr)oxides. Such climatic episode is characterized by a warm weather with alternation of precipitation and drought known to promote the formation of hematite. Yet, these environmental conditions could also have prevailed during other interglacial periods and, important concentration of hematite has been noted in Quaternary soils (named Ferretto, Terrae Rossae, or Fersiallitic soils) covering the plains of medium altitude in central Europe [START_REF] Bronger | Rubification of terrae rossae in slovakia: A mössbauer effect study[END_REF][START_REF] Yaalon | Soils in the Mediterranean region: what makes them different?[END_REF]. The exact formation age of the red layer of the Mount Bego rock surfaces could only be determined by advanced dating techniques such as 10 Be Terrestrial cosmogenic nuclide dating [START_REF] Marrero | Cosmogenic nuclide systematics and the CRONUScalc program[END_REF], reflectance spectroscopy [START_REF] Gallach | A new method for dating the surface exposure age of granite rock walls in the mont Blanc massif by reflectance spectroscopy[END_REF], or the recording of the chemical remanent magnetization (CRM) acquired by the hematite nanoparticles during the dehydroxylation reaction chain [START_REF] Gendler | The lepidocrocite-maghemite-haematite reaction chaini. acquisition of chemical remanent magnetization by maghemite, its magnetic properties and thermal stability[END_REF][START_REF] Jiang | Acquisition of chemical remanent magnetization during experimental ferrihydrite-hematite conversion in earthlike magnetic fieldimplications for paleomagnetic studies of red beds[END_REF]. Their limited quantity and crystal size could, however, constrain this objective. These characteristics may also impact the evolution of the red layer under the mountain climatic conditions now prevailing on the site, where snow melt waters frequently circulate on the outcrops.

In a comparable context, [START_REF] Tansem | Red-coated rocks on the seashore: The esthetics and geology of prehistoric rock art in Alta, Arctic Norway[END_REF] recently demonstrated that the red iron film originally covering the outcrops displaying the prehistoric rock art in Alta fjord (Norway) has waned due to land uplift and subsequent biological colonization. In more drastic contexts, important quantities of Fe (hydr)oxides nanoparticles are discharged by glaciers melt waters in Greenland [START_REF] Bhatia | Greenland meltwater as a significant and potentially bioavailable source of iron to the ocean[END_REF][START_REF] Graly | Chemical weathering under the greenland ice sheet[END_REF][START_REF] Hawkings | Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans[END_REF] and the Arctic Ocean [START_REF] Raiswell | Rusty meltwater[END_REF].

These nanoparticles also result from the weathering of the Fe-containing phases (alumino-silicates, sulfides, carbonates, olivines, and pyroxenes) under oxidizing conditions below the glaciers. Their formation mechanism as well as their size thus make them easy to mobilize by water circulation.

  schist, and metapelite samples collected on the Mount Bego site was first evaluated by spectrocolorimetry. The measured chromatic coordinates a* and b* as well as the attributes of lightness L* and chroma C* are presented on Fig. 2. The chromatic coordinates (Fig. 2A) allow a clear distinction between the two layers along the blue -yellow axis (b*) and the green -red axis (a*). On the one hand, the GSs display b* values ranging from 10 to 20 and a* values ranging from -2 to 6,

  . SI-2). The elemental maps acquired at different magnifications (Fig. allow a clear distinction between the particles of quartz, albite, mica and chlorite. Noticeably, the chlorite particles contain the highest content of Fe. Yet, the overall repartition of Fe appears to be relatively homogeneous (around 8 wt. %) within the thickness of the metapelite thin section (see Fig.3Dand Fig.SI-2D). No area of accumulation is noted near the surface, and the red-brown oxidation described in optical microscopy is not associated with a clear enrichment in Fe (nor any other element, see Fig.

  negligible in the sericites and phengites (~ 0.3 atoms per formula unit, pfu) compared to the chlorites (~ 2.3 pfu). Sericites and phengites are common dioctahedral micas found in low grade metamorphic rocks. Their chemical compositions seem relatively homogeneous between GS and RL. The chemical composition of the chlorite particles is more complex. The EDS spectra (Fig.SI-4B) indicate they are mainly trioctahedral and Fe-rich. Their content in Fe 2+ (0.3 ≤ Fe 2+ / R 2+ ≤ 0.4) and tetrahedral Si (2.5 ≤

  presented on Fig. SI-5. Phengites (Fig. SI-5A and B) and ripidolites (Fig. SI-5A and C) are easily

  displayed on Fig. SI-5. Again, phengites (Fig. SI-5D) and ripidolites (Fig. SI-5E and F) are distinguished from the EDS micro-analyzes. The size of these particles does not vary significantly compared to those described in GS. They seem equally well crystallized, but they display a large amount of nanometer scale impurities on their edges (indicated by the white arrows on Fig. SI-5D to F). A TEM micrograph displaying the edge of a ripidolite particle composing RL is also shown on Fig.

  2.6) is displayed on Fig. SI-6. It evidences three phases, namely goethite (α-Fe 3+ O(OH)), lepidocrocite (γ-Fe 3+ O(OH)), and szomolnokite (Fe 2+ (SO4)•H2O) in addition to the residual pyrite (FeS2).

Figure 1 .

 1 Figure 1. A: General location map of the valleys of Marvels and Fontanalba within the Mont Bego

Figure 2 .

 2 Figure 2. Spectrocolorimetric characterization of the samples collected on the Mount Bego outcrops.

Figure 3 .

 3 Figure 3. Petrographic and chemical description of the metapelite thin section. A: Micrograph of the

Figure 4 .

 4 Figure 4. Micro-textural SEM observations of the metapelite fragment in back-scattered electron

Figure 5 .

 5 Figure 5. Decomposition of the Mössbauer spectra (4.2 K) of A: the green substrate (GS) and B: the

Figure 6 .

 6 Figure 6. A and B: Mineralogical differences between the clay mineral particles composing GS and RL. Projections of the chemical composition deduced from TEM-EDS micro-analysis in M + -4Si-R 2+

Figure 7 .

 7 Figure 7. Profile fitting of the X-ray diffractograms acquired on oriented deposits of GS (A) and RL

Figure 8 .

 8 Figure 8. Observation of RL under the transmission electron microscope (TEM-EDS). A/ Phengite

Table 1 .

 1 TableSI-1) may be refined with FeO = 5.1 wt.% and Fe2O3 = 3.1 wt.%. Mössbauer parameters and relative contributions of the doublets and sextuplets used in the decomposition model fitting GS and RL spectra. * Mössbauer parameters of the main Fe (hydr)oxides from[START_REF] Murad | Mössbauer spectroscopy of environmental materials and their industrial utilization[END_REF].

			Isomer shift Quadrupole splitting Magnetic field Relative contribution
			δ (mm.s -1 )	Δ (mm.s -1 )	Bhf (T)	(%)
		Baseline				36.0
	GS	Fe 2+	1.24	2.74		40.0
		Fe 3+	0.35	0.85		24.0
		Baseline				17.6
		Fe 2+	1.24	2.74		42.6
	RL
		Fe 3+	0.38	0.84		24.6
		Fe (hydr)oxides	0.28	0.00	51.20	15.2
		Ferrihydrite*	0.35	-0.07	50.00
		Hematite*	0.37	-0.20	53.50
		Goethite*	0.37	-0.25	50.60
		Magnetite*	0.26	0.02	50.60
		Maghemite*	0.23	0.02	52.00

Acknowledgements

This project was funded by the French Ministry of Culture. The archaeological contextualization and the access to the site was supervised by Franck Suméra from the Regional Archaeological Service of the PACA region. The authors would also like to thank Vasile Heresanu from CINaM (CNRS -UMR 7325) and Jean-Jacques Cochemé from Aix-Marseille University for their precious help regarding the crystallographic and petrographic description.

Author contributions

Jeremie Berthonneau: methodology, data acquisition, writing original draft, and editing.

Jean-Marc Vallet: conceptualization, methodology, and writing review.

Philippe Bromblet: conceptualization, methodology, and writing review.

François Martin: data acquisition (Mössbaeur Spectroscopy).

Olivier Grauby: conceptualization, methodology, data acquisition, and writing review.

Conflict of interest

The authors declare that there is no conflict of interests.

Supporting Information

The Supporting Information provides details on the alteration figures observed on the outcrops (Fig. SI-1

SI-1); the bulk chemical composition of the metapelite from ICP-OES (Table