Guyguy Manangama

Sabyne Audignon-Durand

Lucile Migault

Céline Gramond

Cécile Zaros

Raphaëlle Teysseire

Loïc Sentilhes

Patrick Brochard

Aude Lacourt

Fleur Delva

Maternal occupational exposure to carbonaceous nanoscale particles and small for gestational age and the evolution of head circumference in the French Longitudinal Study of Children -Elfe study

Keywords: Small for gestational age, head circumference, occupational exposure, carbonaceous nanoscale particles, Elfe cohort

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

Introduction

Intrauterine life is highly susceptible to a variety of toxicants, including atmospheric pollutants [START_REF] Li | Impact of ambient pm2.5 on adverse birth outcome and potential molecular mechanism[END_REF]. Atmospheric particulate matter (PM) with an aerodynamic diameter of ≤ 10 μm (PM10) or ≤ 2.5 µm (PM2.5) has been shown to potentially induce reduced birth weight [START_REF] Dadvand | Maternal exposure to particulate air pollution and term birth weight: A multi-country evaluation of effect and heterogeneity[END_REF][START_REF] Li | Impact of ambient pm2.5 on adverse birth outcome and potential molecular mechanism[END_REF][START_REF] Zhu | Maternal exposure to fine particulate matter (pm2.5) and pregnancy outcomes: A meta-analysis[END_REF] and occipitofrontal circumference at birth [START_REF] Clemens | Maternal exposure to ambient air pollution and fetal growth in north-east scotland: A population-based study using routine ultrasound scans[END_REF][START_REF] Jedrychowski | Estimated risk for altered fetal growth resulting from exposure to fine particles during pregnancy: An epidemiologic prospective cohort study in poland[END_REF][START_REF] Lamichhane | Air pollution exposure during pregnancy and ultrasound and birth measures of fetal growth: A prospective cohort study in korea[END_REF][START_REF] Van Den Hooven | Air pollution exposure during pregnancy, ultrasound measures of fetal growth, and adverse birth outcomes: A prospective cohort study[END_REF]. Low birth weight has been associated with high rates of neonatal morbidity and mortality and long-term adverse health effects, such as cardiovascular diseases, metabolic syndrome, high blood pressure, obesity, and type 2 diabetes mellitus [START_REF] Chernausek | Update: Consequences of abnormal fetal growth[END_REF][START_REF] Salam | Impact of intrauterine growth restriction on long-term health[END_REF]. Head circumference (HC) at birth reflects the brain volume and is used as an indicator of in utero brain development [START_REF] Barbier | New reference curves for head circumference at birth, by gestational age[END_REF]. Slow head growth during pregnancy has been correlated with impaired neuropsychological development later in life [START_REF] Gale | The influence of head growth in fetal life, infancy, and childhood on intelligence at the ages of 4 and 8 years[END_REF][START_REF] Veena | Association of birthweight and head circumference at birth to cognitive performance in 9-to 10-year-old children in south india: Prospective birth cohort study[END_REF]). Among the observed effects of exposure to PM from ambient air pollution (e.g. reduced birth weight and HC), nanoscale particles (NPs) have recently been ascribed to play an important role [START_REF] Stone | Nanomaterials versus ambient ultrafine particles: An opportunity to exchange toxicology knowledge[END_REF]. NPs are particles with at least one dimension < 100 nm (EU 2011). They are generally called nanoparticles when intentionally manufactured by humans and ultrafine particles (UFP) or unintentional nanoscale particles (UNPs) when produced unintentionally [START_REF] Oberdorster | Nanotoxicology: An emerging discipline evolving from studies of ultrafine particles[END_REF]. During certain work processes, particles of various chemical families can be unintentionally emitted [START_REF] Audignon-Durand | P095 matpuf: A jobexposure matrix (jem) to unintentional nanoscale particles[END_REF][START_REF] Borm | The potential risks of nanomaterials: A review carried out for ecetoc[END_REF][START_REF] Chen | Source apportionment of sizefractionated particles during the 2013 asian youth games and the 2014 youth olympic games in nanjing, china[END_REF]): carbonaceous particles (polycyclic aromatic hydrocarbon particles and particles of carbon) from the thermal degradation of organic matter and mechanical degradation of carbon, metal and metal oxide particles from the mechanical and thermal degradation of metals, and mineral particles from the mechanical and thermal degradation of minerals.

Unintentional and manufactured particle toxicology are not two distinct fields, but overlap extensively.

Thus, biopersistent NPs share the same general biological mechanisms of adverse effects [START_REF] Oberdorster | Nanotoxicology: An emerging discipline evolving from studies of ultrafine particles[END_REF][START_REF] Stone | Nanomaterials versus ambient ultrafine particles: An opportunity to exchange toxicology knowledge[END_REF]. Based on animal models, maternal exposure to nanoparticles during pregnancy may alter the in and post utero development of offspring [START_REF] Blum | Cadmium associated with inhaled cadmium oxide nanoparticles impacts fetal and neonatal development and growth[END_REF][START_REF] Yamashita | Silica and titanium dioxide nanoparticles cause pregnancy complications in mice[END_REF]. The mechanisms described for the alteration of birth weight are essentially placental [START_REF] Hougaard | A perspective on the developmental toxicity of inhaled nanoparticles[END_REF]. Toxicological studies have shown that adverse effects may occur due to translocation of nanoparticles from the respiratory tract to the placenta, reducing maternal fetal exchange and causing oxidative stress that can induce or aggravate local inflammation or directly alter fetal tissue after crossing the placental barrier. Fetal adverse effects may also indirectly occur through maternal inflammatory responses [START_REF] Ema | Reproductive and developmental toxicity of carbon-based nanomaterials: A literature review[END_REF][START_REF] Hougaard | A perspective on the developmental toxicity of inhaled nanoparticles[END_REF]. Furthermore, NPs can cross the blood-brain barrier [START_REF] Bakand | Toxicological considerations, toxicity assessment, and risk management of inhaled nanoparticles[END_REF][START_REF] Zhang | Exposure to alumina nanoparticles in female mice during pregnancy induces neurodevelopmental toxicity in the offspring[END_REF]) and lead to an increase in oxidative stress and/or a decrease in the number of neurotransmitters in the cerebral cortex, causing central nervous system disorders [START_REF] Zhang | Exposure to alumina nanoparticles in female mice during pregnancy induces neurodevelopmental toxicity in the offspring[END_REF].

We recently published an initial study that showed a significant association between occupational exposure to UNPs and small for gestational age [START_REF] Manangama | Maternal occupational exposures to nanoscale particles and small for gestational age outcome in the french longitudinal study of children[END_REF]). However, this study did not specify the chemical type of NPs involved in the observed effect. Only a few epidemiological studies have examined the association between PM composition and birth weight. The PM components most frequently associated with low birth weight cited by published studies are carbonaceous particles [START_REF] Basu | Effects of fine particulate matter and its constituents on low birth weight among full-term infants in california[END_REF][START_REF] Bell | Prenatal exposure to fine particulate matter and birth weight: Variations by particulate constituents and sources[END_REF][START_REF] Ebisu | Airborne pm2.5 chemical components and low birth weight in the northeastern and mid-atlantic regions of the united states[END_REF][START_REF] Laurent | Sources and contents of air pollution affecting term low birth weight in los angeles county, california, 2001-2008[END_REF][START_REF] Laurent | Low birth weight and air pollution in california: Which sources and components drive the risk?[END_REF][START_REF] Wilhelm | Traffic-related air toxics and term low birth weight in los angeles county, california[END_REF]. Carbonaceous particles constitute a substantial proportion of airborne particulate matter (PM) for both the general population [START_REF] Heal | The application of carbon-14 analyses to the source apportionment of atmospheric carbonaceous particulate matter: A review[END_REF][START_REF] Putaud | A european aerosol phenomenology-2: Chemical characteristics of particulate matter at kerbside, urban, rural and background sites in europe[END_REF][START_REF] Putaud | A european aerosol phenomenology -3: Physical and chemical characteristics of particulate matter from 60 rural, urban, and kerbside sites across europe[END_REF]) and in the work place [START_REF] Leoni | Source impact determination using airborne and ground measurements of industrial plumes[END_REF][START_REF] Morawska | Combustion sources of particles. 1. Health relevance and source signatures[END_REF]. Furthermore, a recently published study reported, for the first time, the presence of carbonaceous particles in the placentas of women exposed during pregnancy in real-life conditions [START_REF] Bové | Ambient black carbon particles reach the fetal side of human placenta[END_REF]. Similarly, a recent review reported that black carbon particles translocate from the respiratory tract to the human placenta at both the maternal and fetal side, causing placental modifications [START_REF] Saenen | Air pollutioninduced placental alterations: An interplay of oxidative stress, epigenetics, and the aging phenotype?[END_REF]. These data confirm that particles can be translocated directly towards the fetus and potentially explain the adverse effects.

The main objective of this study was to investigate the association between maternal occupational exposure to unintentionally emitted carbonaceous NPs during pregnancy and the child's in and postutero development. The two specific objectives were to investigate the association between exposure to unintentionally emitted carbonaceous NPs and SGA and the association between exposure to unintentionally emitted carbonaceous NPs and HC at birth and two years of age.

Methods

Elfe study

The design of the French Longitudinal Study of Children (Elfe cohort) was described in 2019 in a dedicated article [START_REF] Charles | Cohort profile: The french national cohort of children elfe: Birth to 5 years[END_REF]. Briefly, the Elfe cohort is a survey, launched in 2011, that enrolled children born at 33 weeks or more of gestation in maternity hospitals in mainland France for a projected 20-year follow-up. It is a mother-child cohort that aims to characterize the relationship between the environment and the child's health, development, and socialization. The inclusion of participants took place in four waves of four to eight days, spread out over the year to account for possible seasonal variations. The study was offered to all mothers giving birth in 320 maternity units that agreed to participate among the 349 randomly selected in metropolitan France. In total, 18,040 families and 18,329 children were selected. Among the 18,040 families who gave their consent at birth, follow-up surveys were conducted for 16,281 families (90.2%) at two months, 14,439 families (80.0%) at one year, and 13,277 families (78%) at two years. Several types of data were collected, including data on socio-demographic characteristics, lifestyle, health data, environmental exposure data, and data on maternal occupation and occupational exposure. At the maternity ward, data were collected via a face-to-face questionnaire administered by a trained midwife or nurse. These data were supplemented with data from the birth medical record and biological samples taken in the delivery room. During the follow-up, data were collected through telephone surveys and from the children's primary-care physician.

Study participants

We selected consenting mothers who gave birth to a single living infant and worked at least one day during pregnancy.

Small for gestational age

This binary variable (yes/no) was defined according to individual adjusted birth-weight curves, taking into account the sex, birth order, and gestational age of the newborn and maternal parameters (age, height, and weight) [START_REF] Audipog | Calcul des z scores et des centiles en fonction de l'âge gestationnel, du sexe, du rang de naissance de l'enfant; de l'âge[END_REF][START_REF] Mamelle | Identification of newborns with fetal growth restriction (fgr) in weight and/or length based on constitutional growth potential[END_REF]. SGA data were collected in the child's medical file at the maternity ward.

Head circumference

HC corresponds to the occipitofrontal circumference measurements of the child expressed in cm. It was one of the anthropometric parameters measured at birth and during the various follow-ups of the Elfe cohort. HC data were collected from the child's medical file at the maternity ward for the HC at birth and from the child's health record booklet for the HCs during follow-up. The HC values during follow-up were recorded by the child's primary care physician.

2.5. Assessment of occupational exposure to unintentional NPs using a job-exposure matrix Occupational exposure to NPs was characterized using a specific job exposure matrix (JEM) for unintentionally-produced NPs, MatPUF. The methodology used for its construction and practical application has been described previously [START_REF] Audignon-Durand | P095 matpuf: A jobexposure matrix (jem) to unintentional nanoscale particles[END_REF][START_REF] Manangama | Maternal occupational exposures to nanoscale particles and small for gestational age outcome in the french longitudinal study of children[END_REF].

Briefly, the work processes that generate UNPs were identified on the basis of monitoring and bibliographic analyses and the opinion of a multidisciplinary group of experts. In total, 57 work processes were identified and validated as likely to emit UNPs during their implementation. For each work process, the chemical families of potentially emitted UNPs were then characterized. This analysis identified six chemical families as follows: 1) metals and metal oxides from the mechanical and thermal degradation of metals, 2) minerals from the mechanical and thermal degradation of (nonmetallic) minerals, 3) carbonaceous particles (elemental carbon or polycyclic aromatic hydrocarbon) from the mechanical degradation of carbon and thermal degradation of organic matter, 4) wood, cellulose, and plant particles from the mechanical degradation of wood, cellulose, and plants, 5) polymers from the mechanical and thermal degradation of polymers, mainly plastics, and 6) other organic particles (organic food or non-food organic products of animal origin) resulting from the mechanical degradation of organic food or non-food organic products of animal origin. Our study focused on carbonaceous particles. Two parameters were evaluated for each process: the probability and frequency of exposure. The probability of exposure was defined as the proportion of individuals exposed through the implementation of work processes that generate NPs for a given occupation. The frequency of exposure was defined as the proportion of time, daily, during which workers were exposed through the implementation of work processes that generate NPs for a given occupation (in a typical eight-hour working day).

Jobs were characterized by occupations coded according to the International Standard Classification of Occupations (ISCO edition 1968) and industry sectors coded according to the French nomenclature of activities (NAF2000). Maternal occupational exposure was assessed by linking the jobs held by mothers during pregnancies to the JEM. The JEM estimates a probability and frequency of exposure for each occupation and industry sector. We then obtained an estimate for each selected mother of the probability and frequency of occupational exposure during pregnancy for each chemical family assessed in the MatuPUF JEM, specifically carbonaceous particles. According to the probability of occupational exposure to carbonaceous NPs (exposure to elemental carbon and/or polycyclic aromatic hydrocarbon) obtained after using the MatPUF, we classified mothers into three groups: occupationally unexposed (job held during pregnancy associated with a probability of exposure of 0), occupationally exposed (job held during pregnancy associated with a probability of exposure > 50%), and uncertain occupational exposure (job held during pregnancy associated with a probability of exposure > 0 but < 50%). A computed indicator called « the frequency-weighted duration of work », expressed in weeks, was then used only for mothers who were occupationally exposed to carbonaceous NPs (who held a job during pregnancy associated with a probability of exposure > 50%). This indicator corresponds to the total duration of work during pregnancy weighted by the frequency of exposure.

Statistical analysis

We examined the association between maternal occupational exposure to unintentionally emitted carbonaceous NPs during pregnancy and SGA using multivariate logistic regression models. These analyses were carried out on the overall study population and then depending on whether the mothers stopped working during the first, second, or third trimester of pregnancy. All measures of association were reported with their 95% confidence intervals (CI) adjusted for potential cofounders identified from the literature, but omitted variables already used for the construction of the SGA status. Thus, analyses were adjusted for smoking during pregnancy (yes/no), marital status (living alone/with the spouse), residential area (rural, semi-urban, urban), maternal education (university, high school, lower than high school), monthly household income (> 4,100, 2,500-4,100, < 2,500 euros), high blood pressure during pregnancy (yes/no), gestational diabetes (yes/no), and alcohol consumption during pregnancy (yes/no).

We examined the association between maternal occupational exposure to unintentionally emitted carbonaceous NPs during pregnancy and HC at birth using multivariate linear regression models. The basic hypotheses of the linear models were verified by the residuals analysis method. These analyses were carried out on the overall study population and then depending on whether the mothers stopped working during the first, second, or third trimester of pregnancy. All measures of association were reported with their 95% confidence intervals (CI) adjusted for potential cofounders identified from the literature. Thus, analyses were adjusted for maternal education (university, high school, lower than high school), smoking status during pregnancy (yes/no), high blood pressure during pregnancy (yes/no), gestational diabetes (yes/no), residential area (rural, semi-urban, urban), and the child's sex and gestational age.

We modelled the change in HC between birth and the two-year follow-up by applying a mixed model with an interaction with time and random effects on the intercept and slope. The age of the child at the various follow-ups was considered as time. After verifying the linearity hypothesis, the quantitative variables 'age during follow-up' and 'frequency weighted duration of work during pregnancy' were transformed into a logarithm scale. The analyses were carried out on the overall study population and then depending on whether the mothers stopped working during the first, second, or third trimester of pregnancy. Potential cofounders were identified from the literature. Thus, analyses were adjusted for parental smoking status, monthly household income (> 4,100, 2,500-4,100, < 2,500 euros), residential area, maternal education, and child' sex.

For each analysis (regressions and mixed models), two models were generated: the first model included ever-occupational exposure to NPs as the main explicative variable, defining three categories: occupationally exposed, uncertain, and occupationally unexposed, with the last category being the reference category. The second model was only generated for the occupationally exposed group and included the « frequency-weighted duration of work » during pregnancy as the main explicative variable. As the matrix did not provide the intensity of exposure, we used the frequency-weighted duration of work to estimate a quantitative relationship between exposure and outcome.

We considered the missing data for the variables of interest (SGA, HC, and covariables) to be missing completely at random (MCAR) in the Elfe cohort. Thus, we conducted our analyses on the available data [START_REF] Leffondre | Analysis of risk factors associated with renal function trajectory over time: A comparison of different statistical approaches[END_REF]. We verified the MCAR hypothesis by re-estimating the models using the multiple imputation method, MICE (multivariate imputation by chained equations). All analyses were conducted using R-3.5.2 and SAS9.4.

Results

Among the 18,040 mothers included in the Elfe cohort, 12,073 (66%) were selected for our analyses.

Certain mothers were excluded from this selected population because of missing data on the dependent variables. Analyses concerning SGA were conducted for 11,224 mothers and those concerning HC at birth for 10,734 mothers. During follow-up, HCs were recorded for 5,795 children at one month, 5,232 children at two months, 8,050 children at four months, 7,796 children at nine months, 6,991 children at 12 months, and 4,825 children at 24 months (Fig. 1).

The characteristics of the study population in terms of SGA outcome have already been described in our previous study [START_REF] Manangama | Maternal occupational exposures to nanoscale particles and small for gestational age outcome in the french longitudinal study of children[END_REF]. Certain characteristics at birth of the selected population (with available data on HC) according to occupational exposure to unintentionally emitted carbonaceous NPs are shown in Tables 1 and2. The mean age of the mothers was 31.0 years (standard deviation (SD): 4.7). Unexposed mothers worked longer (27.9 weeks, SD: 7.3) than exposed mothers (26.0 weeks, SD: 9.3) during pregnancy. The percentage of newborns who were SGA was 10.6. The proportion of infants who were SGA was significantly higher for the exposed group than the other groups. The mean HC of children at birth was 34.4 cm (SD: 1.4). There was no significant difference in HC between exposure groups. The monthly household income, area of residence, smoking status, maternal education, and duration of work during pregnancy were significantly different depending on the exposure group.

The occupations held by the mothers exposed to unintentionally emitted carbonaceous NPs during pregnancy are shown in Table 3. Among the most highly exposed mothers, approximately 25% were "cooks", 18% were "commercial travelers and manufacturing agents", 9% were "motor vehicle drivers", 7% were "technical salesmen and service advisers", and 7% were "working proprietors (catering and lodging services)".

In the multivariate analyses, mothers occupationally exposed to unintentionally emitted carbonaceous NPs during pregnancy had a higher risk of SGA than unexposed mothers (adjusted odds ratio (ORadj) = 1.80; 95% CI (1.29, 2.46)). Among the occupationally exposed mothers (n = 425), the frequencyweighted duration of work during pregnancy was not significantly associated with SGA (ORad j= 1.02; 95% CI (0.96, 1.08)) (Table 4).

Depending on whether mothers stopped working during pregnancy, the risk was significant when they stopped working during the third (ORadj=1.80; 95% CI: 1.10, 2.95) or second trimester (ORadj=1.84; 95% CI: 1.13, 3.02). This risk was not significant when the mother stopped working during the first trimester of pregnancy (Table 5).

In the adjusted analysis, there was no significant difference in HC between children born to occupationally-exposed mothers and those born to non-exposed mothers (β = -0.06, p = 0.173). Within the exposed group, the mean HC decreased significantly by 0.04 cm when the duration of work weighted by frequency of exposure increased by one week, adjusted for other variables (β = -0.04, p = 0.019) (Table 6). The associations remained non-significant if the analyses were performed considering the trimester of pregnancy during which the mothers stopped working (Table 7).

In the longitudinal data analysis (Table 8), the increase in HC over time was not significantly lower for the exposed versus unexposed groups (β = -0.0004, p = 0.5593). There was also no association (β =-0.002, p = 0.1314) in the analysis conducted only on the exposed group, regardless of the trimester of pregnancy when the mothers stopped working (Table 9).

The results of the multiple imputation analyses were approximately the same as those of the main analyses (Supplementary Tables 1 and2).

Discussion

This study shows a significant association between maternal occupational exposure to unintentionally emitted carbonaceous NPs and SGA at birth for mothers who stopped working during the second or third trimester. There was no significant association between maternal probability of exposure to unintentionally emitted carbonaceous NPs and HC but we found a slight decrease in HC at birth associated with the frequency-weighted duration of work.

In our first study conducted in 2019, we demonstrated a significant association between maternal occupational exposure to unintentional NPs and SGA [START_REF] Manangama | Maternal occupational exposures to nanoscale particles and small for gestational age outcome in the french longitudinal study of children[END_REF], without specifying the chemical composition of the particles. Here, we show an association with maternal exposure to unintentional carbonaceous NPs; the adjusted odds ratios were slightly higher than those when considering any particles, without specifying the chemical composition. To date, there are no other data available on ultrafine particles and birth weight. However, data on coarser particles (PM10) in the occupational setting and on fine particulate matter (PM2.5) in ambient air pollution are available and consistent with our findings. A nationwide cohort study conducted in Sweden showed a significant association between maternal occupational exposure to combustion products (polycyclic aromatic hydrocarbons and diesel) and SGA [START_REF] Norlen | Occupational exposure to organic particles and combustion products during pregnancy and birth outcome in a nationwide cohort study in sweden[END_REF]. This is consistent with our results, given that air pollution compounds from combustion processes are largely composed of ultrafine particles.

Exposure to carbonaceous particles in ambient air pollution has been shown to be associated with reduced birth weight in several studies [START_REF] Basu | Effects of fine particulate matter and its constituents on low birth weight among full-term infants in california[END_REF][START_REF] Bell | Prenatal exposure to fine particulate matter and birth weight: Variations by particulate constituents and sources[END_REF][START_REF] Ebisu | Airborne pm2.5 chemical components and low birth weight in the northeastern and mid-atlantic regions of the united states[END_REF][START_REF] Laurent | Sources and contents of air pollution affecting term low birth weight in los angeles county, california, 2001-2008[END_REF][START_REF] Laurent | Low birth weight and air pollution in california: Which sources and components drive the risk?[END_REF][START_REF] Wilhelm | Traffic-related air toxics and term low birth weight in los angeles county, california[END_REF]. Based on a literature review, the recent expert report published by the French agency ANSES (Agence Nationale de Sécurité Sanitaire de l'Alimentation, de l'Environnement et du Travail) suggested an effect of carbonaceous particles on low birth weight. However, the number of studies on this subject is still limited (ANSES 2019).

In our sub-analysis of whether mothers stopped working during the first, second, or third trimester of pregnancy, the adjusted odds ratios were significant for the second and third trimesters, whereas the association was not significant for the first trimester. Adverse effects on fetal growth have also been shown to be greater following exposure to PM during the second or third trimesters of pregnancy [START_REF] Clemens | Maternal exposure to ambient air pollution and fetal growth in north-east scotland: A population-based study using routine ultrasound scans[END_REF][START_REF] Lamichhane | Air pollution exposure during pregnancy and ultrasound and birth measures of fetal growth: A prospective cohort study in korea[END_REF]. In 2016, Laurent et al. showed a significant association between exposure to elemental carbon during the third trimester of pregnancy and low birth weight in term newborns [START_REF] Laurent | Low birth weight and air pollution in california: Which sources and components drive the risk?[END_REF]. These results suggest that the placental compensation mechanisms that normally develop during mid-gestation had become insufficient to maintain optimal fetal development through the full term, possibly affecting fetal weight [START_REF] Coan | Dietary composition programmes placental phenotype in mice[END_REF]). In addition, mothers working up to the second or third trimesters are exposed much longer than others. However, our results for mothers who stopped working during the first trimester should be interpreted with caution due to the small number of subjects in this category.

Concerning HC, there was no significant association with maternal occupational exposure to unintentionally emitted carbonaceous NPs during pregnancy, despite stratification by the first, second, and third trimesters. There have been no published epidemiological studies that have explored the effect of NPs on HC development. The absence of an association between maternal exposure to unintentional carbonaceous NPs and HC could be explained by the fact that, because the brain is a major consumer of oxygen, it is favored at the expense of fetal growth, which slows if the placenta is unable to provide the required amount of oxygen for development or during a fetal hypoxia [START_REF] Ducsay | Gestational hypoxia and developmental plasticity[END_REF][START_REF] Giussani | The fetal brain sparing response to hypoxia: Physiological mechanisms[END_REF][START_REF]Les variations des vitesses de croissance au cours de la vie foetale[END_REF]. This phenomenon has been observed in other species with large brains (primates), and could explain the lack of an effect or a low effect on HC when there is an effect on gestational weight [START_REF]Les variations des vitesses de croissance au cours de la vie foetale[END_REF]. These results could also be explained by the fact that the Elfe cohort includes only children born at 33 weeks or more of gestation, with the possibility of selecting less-sick children, with the sickest being excluded. A large-scale European study on maternal exposure to PM and the development of HC, including newborns from 22 weeks of gestation, showed a significant effect [START_REF] Pedersen | Elemental constituents of particulate matter and newborn's size in eight european cohorts[END_REF]. This partially reinforces our hypothesis on the selection of our study sample, which could have led to an underestimation of the effect. Furthermore, possible errors in the measurement of HC of infants included in our study, subject to variability depending on the person measuring it, could have led to an underestimation of the effect. Indeed, it was not the same person who measured the HC between children nor the same person between the visits of each child. Thus, a non-standardized measurement was used. There was also no significant effect of prenatal exposure to carbonaceous NPs on HC at two years of age. Indeed, literature data have shown that the HC of children born with a small HC may catch up over time [START_REF] Brandt | distance and velocity standards for head circimference growth before and after term until 18 months--results from a longitudinal study (author's transl)[END_REF][START_REF] Brandt | head circumference and brain development. Growth retardation during intrauterine malnutrition and catch-up growth mechanisms (author's transl)[END_REF][START_REF] Soesanti | Antenatal exposure to second hand smoke of non-smoking mothers and growth rate of their infants[END_REF].

Here, we focused on carbonaceous particles emitted during work processes because the occupational activities of participants of our study (described in Table 3) correspond mainly to exposure from the combustion of organic matter, which leads to carbonaceous particle emission. Moreover, published data suggest that ultrafine particles emitted in occupational settings are generated mainly following combustion processes [START_REF] Manigrasso | Where do ultrafine particles and nanosized particles come from[END_REF], primarily generating carbonaceous particles in high amounts [START_REF] Baraniecka | Emission of polycyclic aromatic hydrocarbons from selected processes in steelworks[END_REF][START_REF] Leoni | Source impact determination using airborne and ground measurements of industrial plumes[END_REF][START_REF] Morawska | Combustion sources of particles. 1. Health relevance and source signatures[END_REF][START_REF] Paul | Optical characterization of nano-sized organic carbon particles emitted from a small gasoline engine[END_REF].

Toxicological data have shown that carbonaceous nanoparticles show biological reactivity for certain tissues [START_REF] Lam | A review of carbon nanotube toxicity and assessment of potential occupational and environmental health risks[END_REF][START_REF] Van Berlo | Investigation of the effects of short-term inhalation of carbon nanoparticles on brains and lungs of c57bl/6j and p47(phox-/-) mice[END_REF], including those of the placenta [START_REF] Campagnolo | Biodistribution and toxicity of pegylated single wall carbon nanotubes in pregnant mice[END_REF][START_REF] Detmar | Fetal growth restriction triggered by polycyclic aromatic hydrocarbons is associated with altered placental vasculature and ahrdependent changes in cell death[END_REF][START_REF] Qi | Damaging effects of multi-walled carbon nanotubes on pregnant mice with different pregnancy times[END_REF]. Indeed, carbonaceous particles, specifically black carbon coming from combustion-derived PM, were found, for the first time, in the placentas of women exposed in real-life conditions during their pregnancy [START_REF] Bové | Ambient black carbon particles reach the fetal side of human placenta[END_REF]). This provides a potential mechanism to explain the adverse effects of carbonaceous particles on pregnancy and offspring. The results of the few existing epidemiological studies on ambient PM2.5 of elemental carbon have shown a significant association with low birth weight [START_REF] Bell | Prenatal exposure to fine particulate matter and birth weight: Variations by particulate constituents and sources[END_REF][START_REF] Ebisu | Airborne pm2.5 chemical components and low birth weight in the northeastern and mid-atlantic regions of the united states[END_REF][START_REF] Laurent | Sources and contents of air pollution affecting term low birth weight in los angeles county, california, 2001-2008[END_REF][START_REF] Pedersen | Ambient air pollution and low birthweight: A european cohort study (escape)[END_REF][START_REF] Wilhelm | Traffic-related air toxics and term low birth weight in los angeles county, california[END_REF]). In the occupational setting, certain occupations described in our study may have exposed mothers to chemical families of NPs other than carbonaceous particles. Nevertheless, as mentioned above, the participants selected in our study were mainly exposed to carbonaceous NPs.

Individual data on extra-occupational exposure to particulate matter from ambient air pollution (PM2.5) that could have been used as adjustment factors are not yet available for the Elfe cohort. However, we used other relevant variables (the area of residence, socio-economic status) to account for potential confounding factors.

Occupational exposure attracted our particular attention because it has been shown that concentrations of UNPs generated by work processes can be much higher than ambient levels [START_REF] Viitanen | Workplace measurements of ultrafine particles-a literature review[END_REF].

A recent literature review showed that exposure to ultrafine particles is higher in the workplace than in the non-professional environment [START_REF] Viitanen | Workplace measurements of ultrafine particles-a literature review[END_REF].

The version of the job exposure matrix that we used (MatPUF) has not yet integrated the intensity parameter. Thus, it was not possible to examine dose-response relationships, which was one of the limitations of our study. Thus, we could not assess the effects associated with high or low intensity exposure, only those associated with a longer duration of exposure. In addition, the MatPUF identified homogeneous groups of exposure defined by the occupation held by mothers during pregnancy, without considering inter-individual differences. This could have led to non-differential misclassification and under-estimation of the association [START_REF] Burstyn | Effects of non-differential exposure misclassification on false conclusions in hypothesis-generating studies[END_REF]). However, this retrospective assessment method, based on a job exposure matrix, prevents recall bias concerning the characterization of the exposure by participants. This matrix, based on expert opinion, also allowed us to characterize the chemical type of the particles to which the mothers were occupationally exposed during pregnancy, without having to question participants, which could have been a source of bias.

We performed our analyses on complete data, excluding subjects with missing data for the variables of interest (SGA, HC, and adjustment variables). Supplemental analyses using the whole dataset by multiple imputation (MICE) showed that subject selection in our main analyses did not likely influence our results.

Conclusion

This study shows a significant effect of exposure to unintentionally emitted carbonaceous NPs on SGA, depending on the period of exposure during pregnancy. These results are consistent with those that have been previously published on PM from air pollution. Preventive measures that account for the different periods of exposure during pregnancy may be necessary in the workplace. Further studies should be conducted using other measurement methods to confirm this effect or should also consider overall occupational and extra-occupational exposure to carbonaceous NPs.

Mothers who requested the destruction of their data, n=57 Adjusted for smoking, marital status, residential areas, maternal education, monthly household income, high blood pressure during pregnancy, gestational diabetes and alcohol consumption during pregnancy; p* p-value (Chi2 of Wald test); ORadj: adjusted odds ratio; CI: confidence interval; N: number;

In bold: statistically significant associations.

Table 2 .

 2 Fig. 1. Flow chart for the selection of the final population analyzed in the study on occupational exposure to NPs and SGA, Elfe Cohort, Metropolitan France, 2019. Characteristics of the participants depending on exposure status. Qualitative variables. N=10,242, Elfe sub-study on occupational exposures to carbonaceous nanoscale particles unintentionally emitted, Metropolitan France, 2019.

	Mothers currently in the cohort
	N=18,272 Mothers
			Mothers of twins, n=574
	Mothers of single children
	N=17,698
			Mothers who did not work
			during pregnancy, n = 4,227
	Mothers of single children who worked during pregnancy N=13,471	ISCO (International Standard Classification of Occupations) and NAF (French nomenclature of
			activities) codes missing
			n = 1,398
	Selected population
	N=12,073
	Missing data on Small for	Missing data on Head Circumference (HC) at birth, n=1,339
	Gestational Age at	Missing data on carbonaceous nanoscale
	birth, n=849	particles, n=1,831
	Missing data on
	carbonaceous			Population with available data on
	nanoscale particles,			HC during follow-up:
	n=1,584			 at 1 month, n=5,795
	Selected population	 at 2 months, n=5,232
	Selected population with SGA at birth and carbonaceous NPs data available n=9,640	with HC and carbonaceous NPs data available at birth, n=10,242	 at 4 months, n=8,050  at 9 months, n=7,796  at 12 months, =6,991  at 24 months, n=4,825

*Kruskall-Wallis test to compare quantitative variables; SE: standard error; min: minimum; max: maximum; n: number of participants; unexposed: probability of exposure = 0; uncertain: probability of exposure between 0 and 50%; exposed: probability of exposure more than 50%.

Table 3 .

 3 The exposing jobs (jobs with probability of exposure to carbonaceous nanoscale particles

	unintentionally emitted > 50%), among exposed mothers (N=458)
	Exposing jobs	N	(%)	Probability of
				exposure
	Cook	114 (24.89)	0.83
	Commercial travellers and Manufacturers' agents	83 (18.12)	0.75
	Motor vehicle drivers	39 (8.52)	0.75
	Technical salesmen and service advisers	33 (7.21)	0.75
	Working proprietors (catering and lodging services)	32 (6.99)	0.75
	Specialised farmers	24 (5.24)	0.94
	Bakers, pastrycooks and confectionery makers	23 (5.02)	0.83
	Street vendors, canvassers and newsvendors	14 (3.06)	0.75
	Orchard, vineyard and related tree and shrub crop workers	14 (3.06)	0.94
	Livestock workers	11 (2.40)	0.96
	Nursery workers and gardeners	11 (2.40)	0.94
	Field crop and vegetable farm workers	8 (1.75)	0.94
	Chemists	6 (1.31)	0.75
	Civil engineering technicians	6 (1.31)	0.99
	Managers (Catering and lodging services	5 (1.09)	0.75
	General farmers	5 (1.09)	0.94
	Mechanical engineers	3 (0.66)	0.92
	Fire-fighters	3 (0.66)	0.83
	Agricultural and animal husbandry workers not elsewhere	3 (0.66)	0.94
	classified
	Jewellery and precious metal workers	3 (0.66)	0.99

Adjusted for smoking, marital status, residential areas, maternal education, monthly household income, high blood pressure during pregnancy, gestational diabetes and alcohol consumption during pregnancy; p* p-value (Chi2 of Wald test); ORadj: adjusted odds ratio; CI: confidence interval; N: number; In bold: statistically significant associations.

Table 5 .

 5 Association between maternal occupational exposures to carbonaceous nanoscale particles unintentionally emitted and small for gestational age, depending on whether mothers stopped working during pregnancies (first, second or third trimester).N=9,640, sub-study Elfe, Metropolitan France, 2019

		Stopped during 1 st trimester		Stopped during 2 nd trimester		Stopped during 3rd trimester
		N ORadj (95% CI)	P	N	ORadj (95% CI)	p	N	ORadj (95% CI)	p
	Occupational exposures to carbonaceous NPs		0.4744			0.0068			0.0117
	Unexposed	611 Ref		3,412 Ref		3,856 Ref
	Uncertain (>0-50%)	186 1.02 (0.53, 1.97)		557 1.49 (1.07, 2.07)		593 1.45 (1.04, 2.04)
	Exposed (>50%)	66 1.71 (0.71, 4.12)		168 1.84 (1.13, 3.02)		193 1.80 (1.10, 2.95)

Acknowlegdement

The Elfe survey is a joint project between the French Institute for Demographic Studies (INED) and the National Institute of Health and Medical Research (INSERM), in partnership with the French blood transfusion service (Etablissement français du sang, EFS), Santé publique France, the National Institute for Statistics and Economic Studies (INSEE), the Direction générale de la santé (DGS, part of the Ministry of Health and Social Affairs), the Direction générale de la prévention des risques (DGPR, Ministry for the Environment), the Direction de la recherche, des études, de l'évaluation et des statistiques (DREES, Ministry of Health and Social Affairs), the Département des études, de la prospective et des statistiques (DEPS, Ministry of Culture), and the Caisse nationale des allocations familiales (CNAF), with the support of the Ministry of Higher Education and Research and the Institut national de la jeunesse et de l'éducation populaire (INJEP). Via the RECONAI platform, it receives a government grant managed by the National Research Agency under the "Investissements d'avenir" programme (ANR-11-EQPX-0038).

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing financial interests declaration

All authors declare they have no actual or potential competing financial interest.


Verdet Cécile
email: cecile.verdet@u-bordeaux.fr

Sirieix Colette

Marache Antoine

Riss Joëlle

Jean-Christophe Portais

Detection of undercover karst features by geophysics (ERT) -Lascaux cave hill

Keywords: karstic morphology, ERT, Geophysics, Geostatistics model

The study of karst features under a detrital cover is difficult to obtain using punctual traditional tools. This paper presents a 3D geostatistical modeling of ERT (Electrical Resistivity Tomography) data to describe undercover karst features. A case study was carried out on the karstic site of the prehistoric decorated Lascaux cave (France). Geophysical measurements were used to define the limit between outcropping Coniacian or Santonian limestones (southwest) and clayey sands (northeast), with a main orientation of N140°. A geometrical description of the scarp was also constructed; pinnacles and notches were found under the clayey sand detrital formation. By combining 3D ERT with geomorphological and geological observations, the geometry of the stratigraphic limit between the Coniacian and Santonian limestone could be determined. This stratigraphic limit separates two domains, one is a potential aquifer, and acts as a feeder for the intermittent spring at the cave entrance, while the other is less permeable, resulting in a permeability contrast with the later rock type. All these observations help define the geological cave environment and ensure better protection for the paintings.

Introduction

In general, a karst landscape can be defined as resulting from the dissolution of carbonated rocks, such as limestone and dolomites [START_REF] Mangin | Apport de l'hydrodynamique pour la compréhension des aquifères karstiques[END_REF][START_REF] Bakalowicz | Connaissance et gestion des ressources en eaux souterraines dans les regions karstiques[END_REF]. The Lascaux cave is developed within the upper cretaceous limestone karst (France). It is a decorated cave that contains prehistoric paintings believed to date back to the Magdalenian era [START_REF] Aujoulat | Approche chronologique de quelques sites ornés paléolithiques du Périgord par datation Carbone 14 en spectrométrie de masse par accélérateur de leur mobilier archéologique[END_REF]. As a UNESCO world heritage site, the cave has many conservation needs: one of them is a better understanding of the surrounding area and especially of its karst. Therefore, this paper focus on the detection of undercover geomorphological karstic features localized at the northeast of the cave, i.e. upstream of the Lascaux cave [START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF].

The complexity of the karst is well known and its high heterogeneity can be seen as a result of a triple porosity at different scales [START_REF] Király | Rapport sur l'état actuel des connaissances dans le domaine des caractères physiques des roches karstiques. Hydrogeol. Karstic Terrains Hydrogéologie[END_REF][START_REF] Halihan | Interpreting flow using permeability at multiple scales[END_REF][START_REF] Worthington | A comprehensive strategy for understanding flow in carbonate aquifers[END_REF][START_REF] Vacher | Eogenetic karst from the perspective of an equivalent porous medium[END_REF][START_REF] Ford | Karst hydrogeology and geomorphology[END_REF]: [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] matrix porosity, (2) fracture and crack porosity and (3) conduit porosity. The water flowing throughout these multiple porosities dissolves the rocks and finally forms characteristic geomorphological features such as sinkholes, pinnacles, karren, karst valleys, and caves.

In some instances, these features are covered (or filled in the case of caves) with allochthonous or autochthonous materials such as sand and clay. Assessing these undercover karst features is quite challenging using only classical techniques like drilling, mainly because of the high variability of the karst medium. In addition, concerning the Lascaux site, only non-destructive methods can be used near the cave, so geophysical methods are suitable.

Currently, geophysical methods are widely used in large-scale investigations. To assess karst, a few techniques have proved effective: [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] ground-penetrating radar (GPR) has been used in some cases [START_REF] Chalikakis | Contribution of geophysical methods to karst-system exploration: an overview[END_REF][START_REF] Carrière | Combining Electrical Resistivity Tomography and Ground Penetrating Radar to study geological structuring of karst Unsaturated Zone[END_REF][START_REF] Kaufmann | Detection and mapping of ghost-rock features in the Tournaisis area through geophysical methods-an overview[END_REF] where the karst is not covered, especially with clay that absorbs the GPR signal; (2) microgravimetry can give good results to detect superficial empty and filled voids and to characterize karst heterogeneity [START_REF] Solbakk | Detecting multiscale karst features including hidden caves using microgravimetry in a Caledonian nappe setting: Mefjell massif, Norway[END_REF];

(3) seismic refraction tomography is also frequently used on karst because it can give information at great depth with a high spatial resolution [START_REF] Guérin | Geophysical characterisation of karstic networks -Application to the Ouysse system (Poumeyssen, France)[END_REF][START_REF] Valois | Karstic morphologies identified with geophysics around Saulges caves (Mayenne, France)[END_REF][START_REF] Valois | Assessment of doline geometry using geophysics on the Quercy plateau karst (South France): morphology of dolines using geophysics[END_REF]; (4) finally, electrical resistivity tomography (ERT) is now widely used to characterize karstic features; to date, most of the karstic structures identified are sinkholes [START_REF] Zhou | Reliability of dipole-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terranes[END_REF][START_REF] Valois | Karstic morphologies identified with geophysics around Saulges caves (Mayenne, France)[END_REF][START_REF] Billi | Hidden sinkholes and karst cavities in the travertine plateau of a highly-populated geothermal seismic territory (Tivoli, central Italy)[END_REF][START_REF] Cueto | Karst-induced sinkhole detection using an integrated geophysical survey: a case study along the Riyadh Metro Line 3 (Saudi Arabia)[END_REF], voids [START_REF] Chávez | Karst Detection Beneath the Pyramid of El Castillo, Chichen Itza, Mexico, by Non-Invasive ERT-3D Methods[END_REF][START_REF] Prins | Testing the effectiveness of an inverse Wenner-Schlumberger array for geoelectrical karst void reconnaissance, on the Swabian Alb high plain, new line Wendlingen-Ulm, southwestern Germany[END_REF], soil and bedrock interface [START_REF] Bermejo | 2D and 3D ERT imaging for identifying karst morphologies in the archaeological sites of Gran Dolina and Galería Complex (Sierra de Atapuerca[END_REF]Cheng et al., 2019b), and karstic aquifer geometry [START_REF] Kaufmann | Detection and mapping of ghost-rock features in the Tournaisis area through geophysical methods-an overview[END_REF][START_REF] Sirieix | Electrical resistivity tomography to characterize a karstic Vauclusian spring: Fontaine d'Orbe (Pyrénées, France)[END_REF][START_REF] Carriere | Feasibility and limits of electrical resistivity tomography to monitor water infiltration through karst medium during a rainy event[END_REF][START_REF] Xu | A geophysical tool for the conservation of a decorated cave -a case study for the Lascaux cave[END_REF][START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF]Cheng et al., 2019a). For the Lascaux case, ERT methods have been shown to be effective [START_REF] Xu | A geophysical tool for the conservation of a decorated cave -a case study for the Lascaux cave[END_REF].

The principle of ERT methods is to inject an electrical current between two electrodes and to measure the potential difference between two other electrodes, from which the apparent resistivity is processed. Next, an inversion process gives an image of the electrical property of the sub-surface. The image can then be interpreted as follows: resistivity depends on both the nature of the soil and on its degree of water saturation [START_REF] Archie | The electrical resistivity log as an aid in determining some reservoir characteristics[END_REF]. Therefore the more saturated the soil is, the lower the resistivity, so are clay-rich soils. Most of the images are produced in 2D, thus the 3D heterogeneity of the underground is not always well sampled. For this reason, 3D ERT tools are currently considered to take account of the variability of the sub-soil.

There are three main ways to achieve 3D electrical resistivity models. First, by using what is called true 3D ERT, where both the acquisition and inversion are done in 3D. This method was applied by [START_REF] Chambers | Three-dimensional geophysical anatomy of an active landslide in Lias Group mudrocks, Cleveland Basin, UK[END_REF] to characterize a landslide, again by [START_REF] Chambers | Bedrock detection beneath river terrace deposits using three-dimensional electrical resistivity tomography[END_REF] to detect a bedrock under river terraces, and by [START_REF] Chávez | Karst Detection Beneath the Pyramid of El Castillo, Chichen Itza, Mexico, by Non-Invasive ERT-3D Methods[END_REF] to detect a karstic void under a Mayan pyramid. True 3D ERT requires a long measuring time, and because it needs a very specific electrode geometrical arrangement in the field, it is not always achievable due to harsh terrain conditions, e.g. presence of dense vegetation. Second,the quasi-3D ERT method, where the data are acquired along 2D lines, then collated and inverted into 3D data sets. This method was used by [START_REF] Kneisel | Application of 3D electrical resistivity imaging for mapping frozen ground conditions exemplified by three case studies[END_REF] to detect the depth of permafrost and its variability, with electrodes set up on a regular grid, and by Cheng et al. (2019a) on karst on a large scale to detect the interface between weathered (or fractured) limestone and the unweathered bedrock. Finally, the third method is a 3D geostatistical modeling of 2D acquired and inverted ERT profile data. Geostatistical methods have been used on other resistivity data, like [START_REF] Riss | A methodology for converting traditional vertical electrical soundings into 2D resistivity models: Application to the Saïss basin[END_REF] who used geostatistical modeling on a large set of VES (Vertical Electrical Sounding) data in order to model apparent resistivity that can be inverted as a classical 2D ERT. Other examples include: [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] the study carried out by De [START_REF] De Benedetto | Integrating geophysical and geostatistical techniques to map the spatial variation of clay[END_REF] who assessed the clay content in the soil using GPR and electromagnetic induction data coupled with a geostatistical analysis;(2) [START_REF] Benoit | Characterization of spatially variable riverbed hydraulic conductivity using electrical resistivity tomography and induced polarization[END_REF] used geostatistical methods to find a correlation between hydraulic conductivity and geoelectrical data acquired with ERT and induce polarization (IP) in a riverbed. The 3D ERT geostatistical modeling method has already been applied to the south of the Lascaux cave by [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF], on a large scale. The authors revealed some interesting features, such as geological limits at a low resolution. The present study will therefore focus on a higher resolution method using 3D ERT geostatistical modeling to detect undercover karst heterogeneity and geological interfaces upstream of the Lascaux cave. The upstream area of the cave is important to acknowledge, as the water present within this area could reach the cave, depending on the spatial distribution of the heterogeneities, and their size, connectivity and permeability [START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF]. Indeed, those characteristics control the water flow transporting biological or chemical compounds that could affect the painted walls. In addition, this water flow changes the internal climatic parameters, like temperature and humidity [START_REF] Lacanette | An Eulerian/Lagrangian method for the numerical simulation of incompressible convection flows interacting with complex obstacles: Application to the natural convection in the Lascaux cave[END_REF][START_REF] Lacanette | Prévision climatique 3D dans la grotte de Lascaux[END_REF], challenging the cave ecosystem.

Study area

The study area is located in southwest France, in the Dordogne department, on the Lascaux hill, upstream of the Lascaux cave (Figure 1.a). The entrance to the cave is located about 100 m above the Vézère River. The karstic plateau that forms the hill consists of Coniacian and Santonian limestone. The northeast of the study site is covered by clayey sands (Figure 1.a) whose origin is still debated, deriving from autochthon alteration [START_REF] Bruxelles | Grotte de Lascaux[END_REF][START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF] or allochthon fillings of the limestone [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF]. Uncovered geomorphological features characterize the karst of the hill. The naturally uncovered features are sinkholes (between the study site and the Regourdou site, Figure 2 At a different scale, a map of the site shows that this study area covers middle and upper Coniacian limestone and clayey sand filling to the northeast (Figure 2). The middle and upper Coniacian (C4b, Figure 1.a) is characterized by a yellow bioclastic limestone and sandy limestone, with a presence of hummocky cross-stratification (HCS) with a thickness of 50 to 70 m. The upper part is made of a compact limestone, with an arenaceous part mixed with gravel and bioclastic debris [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF]. The Lascaux cave developed within this upper part. At the regional scale, the very top of the Coniacian deposit is characterized by an oyster-rich level, 5 m thick [START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF], with a pale yellow limestone formations [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF][START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF]. This level has never been described within the Lascaux cave.

The top of the hill is constituted of Santonian limestone. The lower Santonian is characterized by a chalky limestone that forms small plates, sandstone and yellow sands with a thickness between 40 and 60 m. The base level of the Santonian is formed of very small white-yellow chalky limestone with a nodulous jointing aspect [START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF]. Above, a lithological ensemble is formed of a red sandy bioclastic limestone (30 to 60 m thick [START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF]). The Regourdou site is found within this part [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF].

The Lascaux cave seems to have developed along some of the directions that are recognized on the hill: the structural map (Figure 2) displays four main fracture families [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF] with directions running N178°E (F1), N119°E (F2), N93°E (F3) and N145°E (F4). These fracture directions are known at the regional scale: direction F1 is close to the Larche fault direction, direction F2 is close to that of the Condat and Cassagne faults, formed during the Hercynian orogenesis, and direction F3 is close to that of the Meyssac fault [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF]. The F3 and F1 families can be attributed to the tertiary tectonic and the Pyrenean orogenesis [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF]. Another fracture direction, F1', is proposed by Lopez ( 2009) that was not reported on the stereographic projection. Direction F4 is not found in the other regional faults but is a known Hercynian direction in the region [START_REF] Muller | L'Evolution structurale des Pyrénées (Domaine central et occidental). Le segment hercynien, la chaîne de fond alpine[END_REF]. The directions of the two talwegs situated to the northwest and southeast of the cave are parallel to F4 and F1 respectively (Figure 1 and Figure 2). The main stratification plane above the Lascaux cave shows a direction of N2°E with a dip of 4 to 6°SE (Lopez, 2009, Figure 2). Regarding the geomorphological aspect, it should be noticed that the steepness of the hill slope becomes smoother at the top.

Currently, there are two main uncertainties, represented by the question marks in Figure 1d. First, the thickness of the clayey sand fillings, which is mainly unknown because of the high variability of the bedrock geometry. Second, the Santonian/Coniacian limit around the cave, the eastern clayey sand filling masks the limit at the location where the slope steepness changes. As the climatic condition of the cave depends on the water path upstream the cave, these two uncertainties will be studied and discussed further in this paper. [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF], [START_REF] Xu | Caractérisation de l'environnement karstique de la grotte de Lascaux par couplage de méthodes géophysique[END_REF] and [START_REF] Houillon | La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux[END_REF], from [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] 2009) on a map by [START_REF] Vouvé | Travaux préliminaires dans le cadre de l'aménagement d'un nouvel accès dans la grotte de Lascaux -Creusement d'un puits[END_REF]. "n" is the number of fractures measured on site and "Dm" is the mean direction for each fracture family.

Methodology

Electrical Resistivity Tomography

Survey design and inversion

First of all, it should be noted that on site time-lapse monitoring indicated that the best season to carry out ERT measurement on the Lascaux epikarst is from January to July, as the soil is at its highest saturation point [START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF]. A high saturation will enhance the contrast between resistive and less resistive materials, making it interesting to target such a period for the ERT measurements. We therefore chose to do the survey in March 2018.

Secondly, the surveyed area is densely forested with many anthropic features. This was the main constraint to the positioning of the ERT profiles and it conditioned the survey type. Because the true 3D process needs a precise electrode arrangement, usually a square with electrodes equally spaced on a regular mesh, the use of a true 3D technique is not easily feasible on our site. Moreover, the presence of anthropic features such as fences and buried electrical cables made 3D electrode implantation more difficult. We therefore carried out a 2D survey designed to take into account the main known directions on site, positioning the ERT lines perpendicularly to the directions. We also chose to produce a 3D geostatistical model, since [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] demonstrated that it gave good results on this site. Xu et al.

(2016) used a setup where the profiles were 20 m spaced among each other, with 96 electrodes spaced at 1.5 m. The spacing between profiles is large, and, as we know, the karst heterogeneity is so variable that information can be easily missed between the profiles. We reduced this spacing from 20 m in [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] to 5-10 m between two parallel profiles. As [START_REF] Sirieix | Electrical resistivity tomography to characterize a karstic Vauclusian spring: Fontaine d'Orbe (Pyrénées, France)[END_REF] showed, the smaller the electrode spacing, the better we can detect karstic features. Later, [START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF] empirically demonstrated that with a 1 m electrode spacing, features could be better defined on the Lascaux hill.

In order to improve the geostatistical model accuracy, it is interesting to acquire data in at least two directions. The setup we chose is made up of 14 profiles (Figure 3). 11 profiles are made up of 72 electrodes spaced at 1 m, for a total length of 71 m (WE2-1, WE2-3, WE2-4, WE2-5; NS2-1, NS2-2, NS2-NS2-3, NS2-4, NS2-5; N100, S100); 1 profile is made up of 96 electrodes spaced at 1 m, for a total length of 95 m (WE2-2). Two profiles are refined near the cave, N50 and S50, made up of 72 electrodes spaced at 0.5 m for a total length of 35.5 m. The spacing between profiles is about 5 to 10 m, covering an area of about 5,500 m². The measurements were performed with an Iris Instrument Syscal Pro®. On each profile, the current injection is carried out during 500 ms for pole-dipole and 250 ms for gradient electrical array. The inversion is the result of a concatenation of three types of array: (1) a gradient ;

(2) forward and (3) reverse pole-dipole. The gradient array was selected because of its robustness, its sensitivity to the vertical variations, its high spatial resolution and its velocity. The pole-dipole arrays were used because of their velocity, their sensibility to voids and their good penetration depth. After many tests, it appears that on the study site, the array that fitted best the depth of the compact limestone is the concatenation of those two arrays [START_REF] Xu | Caractérisation de l'environnement karstique de la grotte de Lascaux par couplage de méthodes géophysique[END_REF]. Data were filtered in order to remove points with a quality factor above 5% [START_REF] Peter-Borie | Electrical resistivity monitoring with buried electrodes and cables: noise estimation with repeatability tests[END_REF]. As demonstrated by [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF], the site anthropic noise is very low and can be found insignificant, the size of the electrode spacing is adapted and a very small number of datum points were filtered. As a result, most of the profiles kept their total number of datum points. On the worst-case scenario (profile WE2-4), only 9 datum points out of 9,133 were removed. The inversion was performed with Res2DInv® software [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF] v. 4.05.38. with L1 norm as it is adapted on a heterogeneous medium with abrupt variability, which is the case for the karst. The mesh was refined to half the electrode spacing. The absolute error of the models was between 0.34% and 3.9%. After inversion, the total number of resistivity datum points was 32,845. The experimental one-off sampling plan comprised cone penetrometer test (CPT) data acquired by [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF] near the area of investigation (Figure 3). It was completed by superficial auger boreholes along the WE2-1 and WE2-2 profiles. These data (Figure 4) are to be used later in the paper to complete the interpretation of the 3D geostatistical model, alongside the other geomorphological observations made on the hill (see "study area" description). Also, two small pits were dug in July 2019, marked D1 and D3 in Figure 3

2D results

Each 2D electrical resistivity image displayed the same pattern of resistivity (Figure 5): -In the northeast area of the profile (to the right of limits B and C), there is a part with a globally low resistivity, and with a high inner variability. Field observations, augers and pit D1 (Figure 4.b) showed that this part is mainly formed of clayey sand. At the surface, some highly resistive patches are present, which are the signature of clearer sand.

-In the southwest, at depth (below limits A and C) there is a globally high resistivity part with a low variability. As [START_REF] Verdet | Détermination du fonctionnement hydrogéologique d'un site karstique par un suivi temporel en tomographie de résistivité électrique -Epikarst de la grotte de Lascaux[END_REF] and [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF][START_REF] Xu | A geophysical tool for the conservation of a decorated cave -a case study for the Lascaux cave[END_REF] showed, such resistivity is due to the massive limestone bedrock. This is also supported by penetrometer P1 (Figure 4.c) data that gave a refusal depth of 15 m (Figure 3).

-In the southwest, near the surface (to the left of limit B, above limit A), there is a mildly resistive part with an inner variability. [START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF] indicated that this part is made up of different limestones from the previous one. They also indicated that this limestone has a resistivity that varies with time; therefore, its water content evolves. This superficial part was called weathered Coniacian limestone by [START_REF] Houillon | La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux[END_REF] and [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF], corresponding to the epikarst as defined by [START_REF] Mangin | Apport de l'hydrodynamique pour la compréhension des aquifères karstiques[END_REF].

3D geostatistical modelling

Several steps make up the geostatistical modeling process. The first is to statistically observe the raw inverted data. The mean resistivity is 164 Ω•m, varying from 3 to 5,119 Ω•m. In order to perform the variographic calculus, giving an understanding of the spatial structure of our data, and to avoid any influence from extreme values, we kept values between 10 and 1,000 Ω•m (representing 98% of all values, i.e. 32,250 values). As these extreme values could have a physical significance, they are added for the final 3D kriging modeling process. The variogram study and the kriging model are made from the log10 of the resistivity and are transformed into resistivity at the end for the final analysis.

The first tool we use is a variographic map (variomap), enabling us to observe the data structure in all directions. In particular, it makes anisotropy stand out (if anisotropy exists for the studied variable), and gives correlation length(s). The first map is computed using the ERT data from the whole study area. On this map, two anisotropic directions are unveiled: one in the horizontal plane, with direction N140°, and one vertically (Figure 6.a and c). We can see that the variomap (Figure 6.a) remains isotropic under 10 m in the horizontal plane.

A second variomap is built using ERT data from the limestone part only, in the southwest of the study area. The N140° direction present on the whole site does not appear, nor does any other anisotropy direction in the horizontal plane (Figure 6.b). Only a vertical anisotropy appears (Figure 6.d), as the one found over the whole study area. The experimental variogram is then calculated in two directions: one in the horizontal plane and one in the vertical plane. The experimental variogram in the horizontal plane is calculated isotopically as the neighborhood will have a radius of 10 m for the kriging process (see below). The corresponding theoretical variogram is then fitted on the experimental variogram as a combination of four elementary models (Figure 7), providing that the lag stays under 10 m so the N140° anisotropy direction can be ignored:

-A nugget effect with a sill at 0.015; After this variographical study, the next goal is to estimate a resistivity value at each node of a regular grid by the kriging method. The chosen grid has an elemental cell size of 0.5m x 0.5m x 0.5m, because of the size of the inversion grid used with the geophysical software. The kriging needs a defined neighborhood to determine the volume around a grid node for which the resistivity value will be estimated. The experimental points within this neighborhood will be used for the estimation at the grid node. In our case, this neighborhood is a 10 m diameter sphere. The total number of estimated points after kriging is 1,246,592.

Then the model is intercepted with the Digital Elevation Model (DEM, X. Muth, personal communication) in order to remove estimated points above the topographical surface.

Results and discussion

The 3D electrical model obtained is shown in Figure 8. Already, a few features stand out, such as the N140° orientation. The geological limits are outlined later in this section ; so are the geomorphologic karstic features under the cover of other detrital material.

Geological limit precision

First of all, the 3D resistivity model can be divided into two main domains, separated by the N140° vertical limit already found with the variogram maps. The two domains can be interpreted with the help of auger boreholes, penetrometer data, small pits (Figure 3 and Figure 4) and old pictures of the cave entrance, before being widened (Figure 9). The N140° direction could correspond to the N145° structural direction cited earlier. Looking at the structural map (Figure 2), the N145° (F4) direction seems to separate the limestone part from the detrital part. [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] also found a structural direction of N145° by geostatistics, to the south and next to our study site, on the Lascaux hill.

Secondly, in the northeast, the first domain has a mainly low resistivity with a median of 68 Ω•m (Figure 8). In the southeast of the domain, a very superficial zone appears with a maximum thickness of 2 m and a very high median resistivity of 640 Ω•m. The low resistivity part is made up of detrital clayeysand, found with augers 6, 7, 8, 11, 12, 15 and 16 (Figure 3). The high resistivity part is composed of sands that can be observed at the surface, directly on the floor and in pit D1 (Figure 3). These formations were also characterized in the southern part of the site by [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] and Bruxelles and Camus (2014), [START_REF] Xu | Caractérisation de l'environnement karstique de la grotte de Lascaux par couplage de méthodes géophysique[END_REF]. In the southwest, the second domain has a globally higher resistivity, attributed to limestone, as it has been found in augers 1, 2, 3, 4, 5, 10, 13 and 14 (Figure 3). It can be separated into two subzones (Figure 8.b.):

-a superficial one, above 181 mNGF (French ordnance datum), heterogeneous with a median resistivity of 140 Ω•m. The entrance of the cave develops within this subzone; The deeper part with a higher resistivity (493 Ω•m) is made up of a more compact limestone, as can be observed within the cave and on the historical photo in Figure 9.a, Figure 10 and Figure 11, below the approximate 183 mNGF limit. It is mostly covered with calcite, and where visible, it forms multidecimetric slabs. We also observed an oyster-rich level in the roof of the "Hall of the Bulls", at an altitude of 181-182 mNGF (Figure 11), making it a part of the massive limestone. The oysters observed measure about 1 to 2cm, so they could be Ceratostreon pliciferum auricularis. We have also found this oyster-rich level in other parts of the cave, always between 178 mNGF and 182 mNGF. The geological log for the Perigordian Region [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF][START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF] shows that such an oyster-rich level is the last stratigraphic level of the upper Coniacian stage, with no oysters found in the lower Santonian limestone. So, the oyster-rich level observed in the cave defined the limit between Coniacian limestone and the Santonian limestone on the site.

Besides the limit altitude of 182 mNGF defined by the oyster-rich level, the altitude is quite constant across the whole site in the ERT model (this study, [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF]), with a very small dip, if any. The N140° direction -or any other horizontal anisotropy direction -has not been found on the variographic map computed on the limestone part (Figure 6.c), showing that the two kinds of limestone, each with its own lithology, must be homogeneous -horizontally and at our scale -with a sub-horizontal dip.

Previously, other authors [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF][START_REF] Houillon | La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux[END_REF][START_REF] Verdet | Cluster analysis of time-lapse resistivity data to identify a privileged feed zone within the epikarst of Lascaux Cave (France)[END_REF][START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF][START_REF] Xu | A clustering approach applied to Time-Lapse ERT interpretation -case study of Lascaux Cave[END_REF] said that the difference between the two parts (top with a resistivity of 140 Ω•m, and bottom with a resistivity of 493 Ω•m) was due only to the difference in weathering effect. On the top was a limestone identified as the epikarst, and below a limestone identified as the infiltration/transmission zone [START_REF] Houillon | La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux[END_REF]. In this present study, the dip of the limit between the two limestones is sub-horizontal (close to the one found by Lopez ( 2009)) and the thickness of the top layer is not constant (thicker at the top than at the bottom of the site, and the top layer of limestone was never found to the west of the studied site). These observations are not consistent with the hypothesis of a difference in weathering according to depth as the only explanation for this top layer.

Karst geomorphological aspect

The analysis of the whole resistivity model showed there are two main resistivity groups. They are delimited by an arbitrary limit, based on the resistivity histogram: one above 250 Ω•m and the other below 250 Ω•m. The materials left when keeping the resistivity of the blocks (from the 3D model) below 250 Ω•m comprise mainly the detrital clayey-sand formations and the platy limestone (Santonian limestone). We chose here to focus on the shape of the compact limestone that emerged when we keep the blocks with resistivity higher than 250 Ω•m. The depth of the compact limestone under the detrital formation is confirmed by penetrometer P1 (Figure 3) by [START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF] showing a refusal depth at the same depth, i.e. at 176 mNGF. Most of the decorated part of the cave is located under this limit, and therefore develops within the massive Coniacian limestone.

The vertical limit between the limestone and the detrital domains becomes clear and stands as a scarp with a few notch zones (Figure 12), as observed at the Regourdou site (Figure 1.b.), to the south of the hill site (Figure 1.c.), and at the Balutie site. This is also supported by earlier on site work by Vouvé (Chapter Vouvé in Leroi-Gourhan and Allain (1979)) that showed the limit between the two domains must form a scarp. We can also observe that the Coniacian limestone roof forms what could be interpreted as karren morphologies between the scarp and the cave, with the karren channels highlighted with dotted lines.

At the bottom of the detrital domain, the limestone shape becomes clearer as well and defines what can be assimilated as pinnacles (Figure 12). This interpretation is coherent with the observations made at the Regourdou site (Figure 1.b.) where pinnacles are visible. The area upstream of the Lascaux cave shows a precise organization of its karstic features as described earlier in this paper, such as under-cover pinnacles, a notch and the limestone depth and geometry.

The geometry of the superficial limit of the limestone and clayey sand formation (Figure 8.a) can be reported at the scale of the hill (Figure 13.a). We observe that the northwest part of the limit has the same N140° orientation as the NW talweg (N137°) and prolongs it (Figure 13). We can assume that N140° is a fracture direction, which allowed the NW talweg to form (Figure 13). We can also assume that this fracture allowed the formation of a depression. Vouvé (chapter Vouvé in Leroi-Gourhan and Allain, 1979) showed there are two paleo-tributaries of the Vézère paleo-river present on the Lascaux hill, one at each side of the cave. We can assume that the south bank of the northern tributary could be the scarp that we found delimiting the limestone and the detrital clayey sand formation. The geological section is modified, as shown in Figure 13.b.

Conclusion

The construction of a geostatistical 3D model based on ERT data provided a better understanding of the Lascaux cave karstic environment geometry. The study area upstream of the cave shows two areas, with a NW-SE limit separating limestones in the west from a clayey sand formation in the east. This limit is an extension of the north talweg and is a structural direction. The high resolution of the 3D ERT model uncovered unknown features of the site, such as pinnacles and a limit between the two kinds of limestone. The limit between the upper Coniacian limestone and the lower Santonian limestone has been redrawn thanks to geological and geomorphological observations on site along with the 3D ERT model in order to match the 182 mNGF isohypse. We now assume that the entrance to the cave develops within the small elongated chalky Santonian limestone plates; while the rest of the cave, which is decorated, develops within the massive Coniacian limestone. Due to its constitution, the Santonian limestone is more prone to weathering than the Coniacian, and therefore has a higher permeability. It is likely a potential aquifer acting as a feeder for the intermittent spring at the cave entrance. The water flowing through this intermittent spring can transport biological or chemical compounds that could affect the painted walls, inducing potential conservation matters. In the northeast of the site, the clayey sand formation lies above a limestone that forms undercover pinnacles and a scarp.

Geological knowledge of the area around the cave is of great interest for its conservation. The limit between the limestone and the detrital clayey sand formation to the northeast of the cave is well known and could be used for protection purposes. The changes we have made to the Coniacian and Santonian limit (more fractured, with clayey sand joint fillings and a higher water content) can be used to adapt models of the cave environment, e.g. the thermal properties of the rocks around the cave or the hydrogeological models applied to water circulation around the cave. Finally, the contrast in permeability allows the presence of a perched aquifer, which feeds the spring at the entrance of the cave. The permeability contrast is not due to a marly horizon; but to the contact between the lower platty and permeable Santonian limestone and the upper compact shelly Coniacian limestone with a lower permeability. It explains why the emergence in the cave entrance is located at this altitude and at the limit between the Santonian and Coniacian limestones.

  ), scarps (at the Regourdou site, Figure 1.b)) and a notch (Balutie archeological site and southeast of the Lascaux cave, Figure 1.c). Other known karstic features on site are few caves: Lascaux cave, small caves at the Balutie site and in the scarp southeast of the Lascaux cave. At the top of the hill, there is a site called Regourdou where the clayey sand filling was removed manually, uncovering pinnacles, caves, and scarps (Figure 1.b).

Figure 1 :

 1 Figure 1: (a) Study area location with the geological map and site of interest[START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF],[START_REF] Xu | Caractérisation de l'environnement karstique de la grotte de Lascaux par couplage de méthodes géophysique[END_REF] and[START_REF] Houillon | La dynamique du carbone inorganique dans le continuum sol-épikarst-cavité du site de la Grotte de Lascaux[END_REF]

  ), (b) Regourdou scarp and pinnacles, (c) notch, (d) section along A-A' line. Note that the vertical scale is twice the horizontal one. Question marks indicate the areas of uncertainty.

Figure 2 :

 2 Figure 2: Known sinkhole locations superimposed on the fracture report by Lopez (2009) on a map by[START_REF] Vouvé | Travaux préliminaires dans le cadre de l'aménagement d'un nouvel accès dans la grotte de Lascaux -Creusement d'un puits[END_REF]. "n" is the

Figure 3 :

 3 Figure 3: ERT profiles, augers (numbered 1-16), penetrometers (labeled P1, P10, P15 ), and small pits (labeled D1-D3) location on a DEM (Digital Elevation Model) by Muth (2017). SAS1 is the first airlock, composed of three compartments C1, C2 and C3 and SAS2 is the second airlock. No paintings are present in the two airlocks.

  , and gave a direct view of the first 40 centimeters. Pit D1 was dug in the clayey sand area, D3 in the limestone area (Figure 1.a).

Figure 4 :

 4 Figure 4: description of (a) auger boreholes with (a.i) augers n°1 and 2 in example, and (a.ii) synthesis of the refusal depth for augers showing limestone materials and depth investigated (not to the refusal) for augers showing clayey sand formation with flint nodules; (b) description of pits D1 and D3; (c) penetrometer P1 result, with a refusal depth at 15m.

Figure 5 :

 5 Figure 5: WE2-4 profile with the geological interpretation reported.

Figure 6 :

 6 Figure 6: Variographic maps for (a and c) the whole study area, and (b and d) the limestone area only. (a and b) represents

-

  A spherical model with a sill at 0.035, a range of 7 m in the horizontal plane and 1.5 m in the vertical plane; -A second spherical model with a sill at 0.018, a range of 6 m in the horizontal plane and 500 m in the vertical plane; -A cubic model with a sill of 0.19, a range of 500 m in the horizontal plane and 25 m in the vertical plane.

Figure 7 :

 7 Figure 7: Experimental (dotted lines) and theoretical (continuous lines) variograms in both horizontal and vertical planes.

Figure 8 :

 8 Figure 8: 3D resistivity model interpreted using auger and Vouvé boreholes. (a) View from above, with the superficial limits and the A-A' section position. (b.) View from the south, with a section along A-A' that shows the internal structure.

Figure 9 :

 9 Figure 9: Historic picture of the cave entrance before it was widened. (a) SAS1 C3 is now located here (see Figure 3). The limits of approx. 185.5 and 183 mNGF are estimated from the recent 3D model of the cave (picture Windels 1940-1941). (b) It is known that the feet of Laval are above 183mNGF, so the whole outcrop is above 183mNGF. The SAS1-C1 is now located here (picture Larivière 1940-09, Brive, from right to left: M. Breuil, Marsal (discoverer), Ravidat (discoverer) and Laval)). We can observe in the upper part that the limestone forms elongated small plates with a filling in between the joints.

Figure 10 :

 10 Figure 10: Limit between elongated small plates and compact limestone observed in the Vouvé pit south of the cave (downstream). (Picture Verdet).

Figure 11 :

 11 Figure 11: Detail of the oyster-rich level seen in the roof of the "Hall of the Bulls" (Picture Studio Guichard/Pérazio Engineering).

Figure 12 :

 12 Figure 12: View of the compact limestone. The plain arrows point to the pinnacles, the scarp is well defined. Karren morphology can also be observed (dotted lines) at the top of the limestone, north of the cave.

Figure 13 :

 13 Figure 13: (a) Modified geological map according to the findings of this present study. (b) Section along A-A' line. Note that the Santonian limit and the depth of the NE sand filling are determined. The nature of the pinnacle is unknown, but could be made up of Santonian. (c) Close-up of the study zone.

Acknowledgements

The authors would like to thank the French Ministry of Culture (DRAC Nouvelle-Aquitaine) for accessing the cave surroundings and for their financial support. We especially thank Philippe Malaurent for his help on the site and on the history of the site. He forms part of the memory of the Lascaux cave. We also thank Fabien Naessens and Sylvain Mateo for their help with the field surveys (ERT and sampling).

We are very grateful to Jean-Pierre Platel for sharing his geological knowledge of the region and Laurent Londeix for his help on the geology.

-a deeper one, below 181 mNGF, and more homogeneous, with a higher median resistivity of 493 Ω•m. The Hall of the Bulls (Figure 3), one of the decorated chambers of Lascaux, as well as the rest of the cave develops within this resistive subzone. [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] also identified these two domains at a wider scale and south of our study site, with a median resistivity of 150 and 556 Ω•m respectively, and an elevation of the limit of 179 mNGF. These values are virtually the same, allowing for measurement accuracy. Auger reconnaissance (n°1, 2, 3, 4, 13, 14, Figure 4.a) and Vouvé boreholes (sampled from 1965[START_REF] Vouvé | Travaux préliminaires dans le cadre de l'aménagement d'un nouvel accès dans la grotte de Lascaux -Creusement d'un puits[END_REF], described and published in Xu (2015)) show that the whole zone is made up of limestone. The top part (above 181 mNGF) has a low, variable resistivity, probably indicating a limestone with a globally high porosity and water or clay content; while the lower part has a higher resistivity, thus more homogeneous, indicating a more massive limestone. The limit between these two kinds of limestone found with the geophysical ERT method is therefore between 181 mNGF ± 1.5 m (this study) and 179 mNGF ± 2 m [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF].

The uncertainty about the altitude is determined from the size of the inversion blocks at this depth.

We observe an outcrop on historical pictures of the site (Figure 9). At the top of Figure 9.a (above approx. 185 mNGF) and in Figure 9.b, we can see a limestone made up of elongated small plates with a filling in between the joints. Between 185 and 183 mNGF (Figure 9.a), the limestone is covered with calcite, but seems to consist of the same elongated small plates. Such a limestone, which is easily weathered, could perfectly explain the low resistivity of 140 Ω•m found previously for the top part. The top limestone can also be observed in the recent pit D3 (190 mNGF) as centimetric chalky, slightly red and yellow, with elongated small plates, and a clayey-sand filling in the joints. In the south, an old pit (Vouvé pit in Figure 2), dug in the 1960's and partially filled since, allows direct observation of the limit between the two limestones (Figure 10). This clearly delineated boundary is estimated at around 182 mNGF in the Vouvé pit, in good agreement with a sedimentary origin well known in the literature [START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF] rather than an erosion origin.

Moreover, the nature of the two kinds of limestone is not the same. Platel [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF][START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF] completed with a personal communication, 2019) described the lower Santonian limestone as isotropic and chalky, forming centimetric chips separated by marly beds. [START_REF] Platel | Le crétacé supérieur de la plate-forme septentrionale de Bassin d'Aquitaine. stratigraphie et évolution géodynamique[END_REF]personal communication, 2019) described the top of the upper Coniacian as a shelly limestone, rich in oysters 2 to 3 cm in size, marking a visible limit with the Santonian limestone. Furthermore, on the [START_REF] Guillot | Notice de la carte géologique de ka France à 1/50 000 -Terrasson[END_REF] geological map, the geological limit clearly follows the topography around the hill, forming a smoother plateau at the top. The limit is roughly at an altitude between 180 and 190 mNGF.

What we observe throughout this study is consistent with the Platel description. Therefore, we conclude that the top part, with a low resistivity, is made up of Santonian limestone, whereas the bottom part, with a higher resistivity, is made up of Coniacian limestone. Moreover, the Santonian limestone constitution makes it prone to a higher alteration than the Coniacian, and it is highly weathered on site. The elevation of the limit between these two kinds of limestone is determined from the 3D model (this study, the [START_REF] Xu | 3D geostatistical modeling of Lascaux hill from ERT data[END_REF] study) and the field observations between 179 and 183 mNGF on the study site. The synthesis of all our observations allows us to estimate the altitude of this limit at around 182 mNGF. From this point, the Coniacian/ Santonian limit on the geological map is modified, as shown in Figure 13. This changes the Vouvé idea (chapter "Vouvé" in Leroi-Gourhan and Allain (1979)) that described the permeable limit as a marly horizon as the origin of the waterflow in the SAS1 C3. Originally, [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] described this horizon as a small layer, 30-40 cm thick, made up of a high water content limestone. Recently, [START_REF] Houillon | Assessing cave internal aerology in understanding carbon dioxide (CO2) dynamics: implications on calcite mass variation on the wall of Lascaux Cave (France)[END_REF] described a contrast in permeability -rather than a marly horizon-allowing the water to flow into the cave at the roof of SAS1 C3 (Figure 3). Our study confirms the [START_REF] Schoeller | Étude géologique, hydrogéologique et climatologique de la grotte de Lascaux pendant le cycle 1964-1965[END_REF] description; besides, we think the contrast in permeability is due to the contact between the compact Coniacian limestone limit and the Santonian limestone with the weathered Santonian limestone acting as a potential aquifer for the water flowing at the intermittent spring at the cave entrance.