Jean-François Deconinck

Danny Boué

Francis Amédro

François Baudin

Ludovic Bruneau

Emilia Huret

Philippe Landrein

Jean-David Moreau

Anne Lise Santoni

First record of early Aptian Oceanic Anoxic Event 1a from the Paris Basin (France) -Climate signals on a terrigenous shelf

Keywords: Lower Cretaceous, Aptian, Oceanic Anoxic Event, Paris Basin, Clay minerals, Organic carbon isotopes

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

Introduction

Identified for the first time by [START_REF] Schlanger | Cretaceous oceanic anoxic events: causes and consequences[END_REF], Oceanic Anoxic Event (OAE) 1a (early Aptian) is characterised by the worldwide preservation of organic matter in black shales as a consequence of intensification in marine primary productivity under oceanic oxygen-depleted conditions [START_REF] Coccioni | Descrizione di un livello guida "radiolaritico-bituminoso-ittiolitico" alla base delle Marne a Fucoidi nell'Appennino Umbrio-Marchigiano[END_REF][START_REF] Coccioni | Barremian-Aptian calcareous plankton biostratigraphy from the Gorgo a Cerbara section (Marche, Central Italy) and implication for planktonic evolution[END_REF][START_REF] Erba | Nannofossils and superplumes: the early Aptian "nannoconid crisis[END_REF][START_REF] Erba | Calcareous nannofossils and Mesozoic oceanic anoxic events[END_REF][START_REF] Erbacher | Evolution patterns of radiolarian and organic matter variations: a new approach to identify sea-level changes in mid-Cretaceous pelagic environments[END_REF][START_REF] Leckie | Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous[END_REF][START_REF] Jenkyns | Geochemistry of oceanic anoxic events[END_REF][START_REF] Föllmi | Early Cretaceous life, climate and anoxia[END_REF][START_REF] Erba | Environmental consequences of Ontong Java Plateau and Kerguelen Plateau volcanism[END_REF]. A global perturbation in the carbon cycle is associated with this event, recorded by a prominent negative excursion followed by a shift towards more positive values [START_REF] Menegatti | High-resolution δ 13 C stratigraphy through the early Aptian "Livello Selli" in the Alpine Tethys[END_REF].

The negative excursion is generally explained by an intense volcanic episode at the origin of the formation of the Ontong-Java oceanic plateau [START_REF] Tarduno | Rapid Formation of Ontong Java Plateau by Aptian Mantle Plume Volcanism[END_REF][START_REF] Larson | Onset of the mid-Cretaceous greenhouse in the Barremian-Aptian: igneous events and the biological, sedimentary and geochemical responses[END_REF][START_REF] Méhay | A volcanic CO2 pulse triggered the Cretaceous Oceanic Anoxic Event 1a and a biocalcification crisis[END_REF][START_REF] Tejada | Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event[END_REF][START_REF] Bottini | Osmium-isotope evidence for volcanism, weathering, and ocean mixing during the early Aptian OAE 1a[END_REF][START_REF] Erba | Environmental consequences of Ontong Java Plateau and Kerguelen Plateau volcanism[END_REF][START_REF] Charbonnier | Mercury enrichments in lower Aptian sediments support the link between Ontong Java large igneous province activity and oceanic anoxic episode 1a[END_REF] while the following positive excursion is seen as a consequence of organic matter burial in oceanic sediments. The duration of the negative excursion, first underestimated between 27 and 44 kyr [START_REF] Li | Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~120 Ma)[END_REF], is likely longer with a duration estimated between 280 and 350 kyr [START_REF] Lorenzen | A new sediment core from the Bedoulian (Lower Aptian) stratotype at Roquefort-La Bédoule, SE France[END_REF]. The duration of OAE1a itself is estimated between 1 and 1.3 myr [START_REF] Li | Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ~120 Ma)[END_REF]. These environmental changes were associated with rising sea surface temperatures [START_REF] Mutterlose | High sea-surface temperatures during the early Aptian Oceanic Anoxic Event 1a in the boreal realm[END_REF][START_REF] Bottini | Climate variability and ocean fertility during the Aptian Stage[END_REF] and a calcification crisis in calcareous nannoplankton known as the 'nannoconid crisis' [START_REF] Erba | Nannofossils and superplumes: the early Aptian "nannoconid crisis[END_REF]. Despite its widespread extension and its recognition in numerous sedimentary basins [START_REF] Bréhéret | L'Aptien et l'Albien de la Fosse vocontienne (des bordures au bassin). Évolution de la sédimentation et enseignements sur les événements anoxiques[END_REF][START_REF] Bellanca | Palaeoceanographic significance of the Tethyan "Livello Selli" (early Aptian) from the Hybla Formation, northwestern Sicily: biostratigraphy and high-resolution chemostratigraphic records[END_REF][START_REF] Ando | Timing and magnitude of early Aptian extreme warming: Unraveling primary δ 18 O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean[END_REF][START_REF] Vincent | Carbon-isotope stratigraphy, biostratigraphy and organic matter distribution in the Aptian -Lower Albian successions of southwest Iran (Dariyan and Kazhdumi formations[END_REF][START_REF] Giraud | The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)[END_REF] among others) and shallow water carbonate platforms [START_REF] Baudin | Enregistrement de l'événement anoxique aptien inférieur dans les faciès péritidaux du Guyot Resolution (océan Pacifique Nord-Ouest)[END_REF][START_REF] Luciani | Regional record of a global oceanic anoxic event: OAE1a on the Apulia Platform margin, Gargano Promontory, southern Italy[END_REF][START_REF] Najarro | Major palaeoenvironmental perturbation in an Early Aptian carbonate platform: Prelude of the Oceanic Anoxic Event 1a?[END_REF][START_REF] Graziano | Sedimentology, biostratigraphy and event stratigraphy of the Early Aptian Oceanic Anoxic Event (OAE1A) in the Apulia Carbonate Platform Margin -Ionian Basin System (Gargano Promontory, southern Italy)[END_REF][START_REF] Godet | Aptian palaeoclimates and identification of an OAE1a equivalent in shallow marine environments of the southern Tethyan margin: Evidence from Southern Tunisia (Bir Oum Ali section, Northern Chott Chain)[END_REF][START_REF] Pictet | Three successive phases of platform demise during the early Aptian and their association with the oceanic anoxic Selli episode (Ardèche, France)[END_REF][START_REF] Amodio | Palaeoenvironment and palaeoecology before and at the onset of Oceanic Anoxic Event (OAE)1a: Reconstructions from Central Tethyan archives[END_REF][START_REF] Hueter | Central Tethyan platform-top hypoxia during Oceanic Anoxic Event 1a[END_REF], OAE1a has not yet been described in the Paris Basin mainly because lower Aptian clayey deposits are poorly outcropping. In 2013, several boreholes were drilled by Andra (the French National Radioactive Waste Management Agency) to the south-east of the Paris Basin in order to characterise Aptian and Albian clayey formations including Argiles à Plicatules, a so-called formation because of the occasional occurrence of Plicatula placunea Lamarck, 1819, a common warm water bivalve [START_REF] Squires | Review of the bivalve genus Plicatula from Cretaceous and lower Cenozoic strata of California and Baja California[END_REF]. One of these boreholes (AUB 121) intersected the Argiles à Plicatules Formation with excellent recovery allowing detailed sedimentological, mineralogical and geochemical studies to be performed; it also provided an opportunity to search for a record of OAE1a in the Paris Basin. As a result, facies descriptions as well as mineralogical, Rock-Eval and δ 13 Corg analyses were performed to characterise the Aptian sediments deposited in the Paris Basin. The main objectives were to precisely describe the whole succession of the Argiles à Plicatules Formation, to highlight the record of OAE1a in the terrigenous shelf environment of the Paris Basin and to specify climatic conditions, notably the fluctuations in the hydrological cycle that prevailed during the Aptian through a high resolution study of the clay minerals.

I -Geological context and biostratigraphy

The Paris Basin is a Meso-Cenozoic intracontinental basin that, today, is bordered by Palaeozoic massifs including the Armorican Massif to the west, the Massif Central to the south, the Vosges to the east and the Ardennes Massif to the north. During the Aptian, the Paris Basin was occupied by a narrow epicontinental sea (strait) between the London-Brabant Massif (LBM) and the Rhenish Massif (RM) to the north and a landmass comprising the Armorican Massif (AM) and the Massif Central (MC) to the south (Fig. 1). This narrow epicontinental sea, situated at a latitude comprised between 30 and 35°N, connected the Tethys Ocean located to the south-east to the boreal realm toward the north-west [START_REF] Masse | Map 13. Early Aptian. (112-114 Ma) Peri-Tethys programme, paleogeographical atlas[END_REF].

After a long period of continental evolution of the Paris Basin starting at the Jurassic/Cretaceous transition (Purbeckian facies), transgressive marine sediments were deposited above Early Cretaceous variegated continental and deltaic sediments, the so-called Wealden facies [START_REF] Allen | Purbeck-Wealden (early Cretaceous) climates[END_REF][START_REF] Guillocheau | Meso-Cenozoic geodynamic evolution of the Paris Basin: 3D stratigraphic constraints[END_REF][START_REF] Radley | The Wealden (non-marine Lower Cretaceous) of the Weald Sub-basin, southern England[END_REF]. To the southeast of the Paris Basin, the Early Cretaceous continental sedimentation (Berriasian to Barremian) was occasionally interrupted by the deposition of shallow marine sediments including the early Hauterivian "Calcaires à Spatangues" Formation and the early Barremian "Argiles Ostréennes" Formation, as a consequence of transgressions originating from the south-east (Tethyan domain), whereas the late Barremian continental deposits consist of variegated clays showing common root traces. From the Aptian, a continuous marine sedimentation took place until the end of the Cretaceous.

I-1. Lithology of the AUB 121 borehole

The AUB 121 borehole is situated to the south-east of the Paris Basin, at Juzanvigny, near Brienne le Château (Fig. 2). The lithological succession has been described in detail by [START_REF] Amédro | Les forages de Juzanvigny (Aube, France) : litho-biostratigraphie des formations du Barrémien à l'Albien moyen dans l'est du bassin de Paris et datations par les ammonites[END_REF]. The 31 m-thick Argiles à Plicatules Formation is underlain by upper Barremian continental deposits (Wealden facies) corresponding to weathering profiles (laterite) and overlain by upper Aptian glauconitic sands corresponding to the "Sables Verts de l'Aube" Formation (Fig. 3). Three distinct intervals (1 to 3) were defined within the Argiles à Plicatules Formation:

-Unit 1, from 116.76 m to 111.90 m, includes black to grey clays with traces of palaeosoils at the base. Woody debris and coalified compressions of pteridophytes are common in this part of the formation (Fig. 4A). Some iron oolites-rich layers (Fig. 4B) occasionally filling burrows occur at the base of this unit, while the top is marked by the presence of prominent shelly beds (between 112.40 m and 111.90 m) (Fig. 4C).

-Unit 2, from 111.90 m to 106.30 m, consists of a grey-brown bioturbated (Chondrites) clay, which appears to be homogeneous (no cycles can be distinguished). Some Plicatula occur occasionally and ammonites are common in the upper part of this unit.

-Unit 3, from 106.30 m to 86.10 m, corresponds to bioturbated khaki clays (Chondrites) in which the cycles are clearly individualised and limited at the base by bioturbated surfaces likely corresponding to firmgrounds. This poorly fossiliferous unit shows scattered Plicatula, some oysters and ammonites which allow a biostratigraphic division to be esdetablished. Coarser quartz grains and glauconite occur at the topmost part of the formation announcing the overlying sandy glauconitic sedimentation (Sables Verts de l'Aube Formation).

I-2. Biostratigraphy

Ammonites have been systematically collected along the core (Fig. 3). Although they are not abundant, their study provides valuable information. A total of 19 ammonites were found in the 30.66 m of the Argiles à Plicatules Formation recovered in borehole AUB 121, of which 15 could be determined, at least at the generic level. They occur mostly as imprints in clays, and in a few cases as internal moulds preserved either in the form of pyritic nuclei or calcium phosphate nodules [START_REF] Amédro | Les forages de Juzanvigny (Aube, France) : litho-biostratigraphie des formations du Barrémien à l'Albien moyen dans l'est du bassin de Paris et datations par les ammonites[END_REF]. All recognised taxa are cosmopolitan, with a geographic distribution that includes the boreal, Tethyan and sometimes even southern realms. This wide geographic distribution is explained by the fact that the summit of the early Aptian corresponds to a high sea level (transgression of a second order eustatic cycle, see [START_REF] Jacquin | The north-Atlantic cycle: an overview of second order transgressive-regressive facies cycles in Western Europe[END_REF]. The vertical distribution of the ammonites permits to establish a biostratigraphic scheme.

The lower three meters of the Argiles à Plicatules Formation did not provide determinable ammonites in the Juzanvigny boreholes and therefore the stratigraphic attribution of the lowermost part of this formation remains uncertain. The occurrence of several Roloboceras transiens [START_REF] Casey | The stratigraphical paleontology of the Lower Greensand[END_REF] The carbon isotopic composition is expressed in delta notation and reported in per mil (‰) relative to the Vienna Pee Dee Belemnite (V-PDB) standard; the external reproducibility based on duplicate analyses of the samples is better than ± 0.2‰ (1σ).

The proportion of CaCO3 is deduced by the difference in weight before and after decarbonation.

II-2. Organic matter characterisation

A Rock-Eval 6 Turbo apparatus (Vinci Technologies) was used for the quantitative and qualitative study of organic matter from 34 selected samples distributed over the entire section. The method, described in detail by [START_REF] Lafargue | Les applications de Rock-Eval 6 dans l'exploration et la production des hydrocarbures, et dans les études de contamination des sols[END_REF] and [START_REF] Behar | Technologie Rock-Eval 6 : performances et développements[END_REF], consists of a two-step analysis with programmed temperature: pyrolysis, under inert atmosphere (N2), followed by oxidation. Samples were analysed at the Institut des Sciences de la Terre de Paris (UMR7193/Sorbonne Université). Crushed samples are first subjected to a 3 min isotherm at 300°C at which free hydrocarbons are volatilised (peak S1). Then, a heating step with a ramp rate of 25°C/min leads to the vaporisation of products via the thermal cracking of organic matter up to 650°C (peak S2). Pyrolysis effluents are continuously detected by a flame ionisation detector (FID) and expressed in mg per g of sample. Organic CO and CO2 are measured online as well by an infrared cell (peak S3). At the end of the pyrolysis step, samples are automatically transferred into an oxidation oven where they are subjected to a 1 min isotherm at 300°C then a ramp rate of 20°C/min up to 850°C. The total signals of both organic and mineral CO and CO2 are expressed in mg per g of samples. All these parameters can be used to calculate the TOC content. The hydrogen index (HI), corresponding to the quantity of hydrocarbon compounds released during pyrolysis relative to the TOC (S2/TOC) in mg of HC per g of TOC as well as the oxygen index (OI) corresponding to the organic CO2 released during pyrolysis relative to the TOC (S3/TOC) in mg of CO2 per g of TOC are also calculated. HI and OI are correlated to the H/C and O/C atomic ratios respectively, which can be used to determine the origin of the organic matter.

Tmax is defined as the pyrolysis temperature at which the maximum amount of hydrocarbon is yielded by kerogen [START_REF] Espitalié | Rapid method for source rocks characterization and for determination of petroleum potential and degree of evolution[END_REF]. Tmax increases linearly with the maturation degree of the organic matter, thus giving a rapid estimate of the thermal maturity of sedimentary basins [START_REF] Espitalié | Use of Tmax as a maturation index for different types of organic matter. Comparison with vitrinite reflectance[END_REF].

II-3. Clay mineralogy

A total of 80 samples were analysed using X-Ray Diffraction (XRD). After moderate grinding in a mortar, powdered samples were decarbonated with a 0.2N HCl solution. The < 2 µm fraction (clay-sized particles) was extracted with a syringe after deflocculation and decantation of the suspension for 95 minutes following Stokes' law; this fraction was then centrifuged. Clay residue was then smeared on oriented glass slides and run in a Bruker D8 diffractometer with CuKα radiations, a LynxEye detector and a Ni filter with a voltage of 40 kV and an intensity of 25 mA (Biogéosciences laboratory, Université Bourgogne/Franche-Comté in Dijon, France). Goniometer scanning ranged from 2.5° to 28° for each analysis.

Three runs were performed for each sample to discriminate the clay phases: 1) air-drying; 2) ethylene-glycol solvation at room temperature during 24 hours; and 3) heating at 490°C during 2 hours, as recommended by [START_REF] Moore | X-Ray Diffraction and the Identification and Analysis of Clay Minerals[END_REF]. Clay minerals were identified using their main diffraction (d00l) peaks and by comparing the three diffractograms obtained. The relative proportions of the clay minerals are estimated using peak intensity ratios. The error margin of the method is approximately ± 5 %.

III -Results

III-1. CaCO3, Organic matter characterisation and δ 13 Corg

In the Argiles à Plicatules Formation, the proportion of CaCO3 is very low (around 3%) in all of the samples studied (Fig. 5). The total organic carbon (TOC) content measured on the 34 selected samples using Rock-Eval pyrolysis is also very low, comprised between 0.1 and 0.52% (Table 1). As the quantity of organic matter (OM) is low, the Rock-Eval parameters are only partly reliable. However, some reliable Tmax values lower than 420°C indicate that OM is immature (below the oil window). Organic matter is more abundant in lithological unit 1 at the base of the section; this was expected since macroscopic woody debris and fern pinnules preserved as coalified compressions were observed (Fig. 4B).

The %Corg obtained after isotopic analyses is between 0.15 and 0.82 (Fig. 4), systematically showing slightly higher values than those obtained using Rock-Eval pyrolysis, but with the same trends. Higher values are recorded at the base of the core in lithological unit 1 which is enriched in continental organic matter. By comparison with unit 3, a slight increase is recorded in the interval corresponding approximately to lithological unit 2 (Fig. 5); however, significant enrichment in marine organic matter, which would have indicated it was preserved in a significantly oxygen-depleted environment, is not recorded.

The δ 13 Corg values fluctuate between -24.19‰ and -27.91‰ (Fig. 4). A prominent negative excursion is recorded between 112 and 110 m at the base of lithological unit 2, followed by a shift to higher (close to -25‰) and relatively constant values up-section.

III-2. Clay mineralogy

The following main clay minerals were identified: a R0 type illite-smectite mixedlayer (17 Å based on a glycolated run condition) referred to as smectite for the following sections; chlorite (14.2 Å, 7.1 Å, 4.7 Å and 3.54 Å peaks); illite (10 Å, 5 Å, 3.33 Å peaks); kaolinite (7.18 Å and 3.58 Å peaks) and vermiculite (14 Å in air-drying conditions, 14.5-15 Å after ethylene-glycol solvation and 10 Å peaks after heating). In addition, traces of pyrophyllite are occasionally suspected by the occurrence of a small peak at 9.3 Å. The clay mineral assemblages are predominantly composed of illite generally between 40 and 50% and kaolinite with proportions comprised between 20 and 45%. Minor amounts of chlorite (less than 10%), vermiculite (around 10%) and smectite (generally less than 10%, except at the top of the formation where the proportions of this mineral may reach 50%) are associated with these two clay species (Fig. 6). Traces of pyrophyllite are only identified at the base of the core. Four stratigraphic intervals can be distinguished according to the clay mineralogy.

Interval 1 corresponds to the lowermost part of the formation below the bioclastic accumulation and coincides with lithological unit 1. In this interval, the proportion of vermiculite is relatively high and decreases up-section while the proportion of illite increases.

The second interval, from 112 to 104.5 m is characterised by the relative abundance of kaolinite (30 to 45%) notably at the base of this interval where the proportions of this mineral show a maximum (45%), and by the absence of smectite. The third interval from 104.5 m to 87.5 m shows a monotonous clay assemblage without any significant change and is composed of approximately 5% of chlorite, 45% of illite, 10% of vermiculite, 10% of smectite and 30% of kaolinite. The uppermost part of the formation (interval 4) shows a sharp increase in the proportions of smectite balanced by significant decreasing proportions of illite and kaolinite.

IV -Discussion

IV-1. Depositional environments of the Argiles à Plicatules Formation

The lowermost part of the formation overlying the continental Wealden facies and consisting of dark grey clays with abundant iron oolites (Fig. 4A) and common terrestrial plant remains (woods and foliar remains, Fig. 4B) likely corresponds to transgressive sediments deposited in coastal environments. Similar facies were described in boreholes drilled close to Soulaines to the east of Juzanvigny (Fig. 2) where transgressive facies are clearly separated by emersion features suggesting, several transgressive events at the beginning of the Aptian transgression [START_REF] Ferry | Analyse sédimentologique de la série albo-aptienne de Soulaines -Projet site TFA[END_REF]. Foliar meso-and macro-remains exclusively consist of fragments of pinnae and isolated fern pinnules preserved as coalified compressions (Fig. 4B). The largest pinna fragment is 22 mm long. The pinnae bear up to 8 mm long and 3 mm wide alternate to subopposite, oblong, thick and leathery pinnules. They are attached by a broad base to the rachis and display a rounded apex and an entire margin.

Sometimes the pinnules are slightly inclined forward. The midrib is straight and ends before the apex of the pinnule. These features as well as the reticulate veins allow to ascribe these specimens to Weichselia reticulata (Stokes and Webb) Fontaine which is a common fern in the Early Cretaceous flora from Europe [START_REF] Blanco-Moreno | Palaeobiogeographic and metric analysis of the Mesozoic fern Weichselia[END_REF]. In France, W. reticulata was previously reported from the Wealden facies of the Nord department [START_REF] Carpentier | La flore wealdienne de Féron-Glageon (Nord)[END_REF] and the Oise department [START_REF] Carpentier | Recherches sur les végétaux fossiles des argiles éocrétaciques du Pays de Bray[END_REF]. The abundance of plant remains in oolithic facies shows that the depositional environment was influenced by strong terrestrial inputs. Although Weichselia mostly occurred in a continental environment in Europe [START_REF] Blanco-Moreno | Palaeobiogeographic and metric analysis of the Mesozoic fern Weichselia[END_REF], this report shows that W. reticulata was one of the components of littoral flora during the earliest Aptian.

As they occur immediately above the upper Barremian lateritic palaeo-alteration, iron oolites may be reworked from the lateritic soils (Bhattacharyya, 1983). However, iron oolites may also have formed as a replacement of a pre-existing ooid structure or via the precipitation of an iron aluminosilicate (chamosite/berthierine) grain in shallow marine environments [START_REF] Velde | Phyllosilicate formation in berthierine peloids and iron oolites[END_REF]. Iron may be provided in the sedimentary environment by the weathering of the proximal continental areas. In most cases iron oolites are associated with transgressive deposits, which is the case in the lowermost part of the Argiles à Plicatules Formation, and are indicators of condensed sections [START_REF] Collin | Depositional environments and iron ooid formation in condensed sections (Callovian-Oxfordian, south-eastern Paris basin, France)[END_REF][START_REF] Clement | Formation and stratigraphic facies distribution of early Jurassic iron oolite deposits from west central Nevada, USA[END_REF].

Up-section, the deposits increasingly less silty show common bivalves. The first ammonite occurs at 113.70 m indicating open-marine environments. The prominent shell horizon intercalated in the clayey succession at the top of lithological unit 1 between 112.40 m and 111.90 m (Fig. 4C) can be divided into two parts. The first corresponds to a bed comprising pluri-centimetric bivalve shells forming the base of a 30 cm-thick fining up sequence. The top of this bed, which is 112.05 m (Fig. 4C), is bioturbated. Above, a second accumulation of bivalves is associated with centimetric clasts. The succession may correspond to two successive storm deposits separated by a bioturbated surface. This suggests a deposition above storm wave base in upper offshore environments and a deepening trend of the basin as a consequence of the Aptian transgression. This trend continues up-section from 111.90 m to 106.30 m, as the homogenous faintly bioturbated clays without any sedimentary structure suggest a deposition in a quiet environment, below the storm wave base (lower offshore). Rare calcareous nannofossils observed in smear slides occur in most of the samples and are likely responsible for the carbonate content (close to 3%) of the sediment. From 106.30 m to the top of the formation, the clays contain more silt. Bioturbated surfaces indicate sedimentation breaks that are consistent with the common occurrence of phosphate (carbonate-apatite) nodules.

The Argiles à Plicatules Formation encompasses a transgressive interval from the base to a maximum flooding surface located in the interval between 111.90 m and 106.30 m. The uppermost 20 m of the formation corresponds to a highstand system tract terminated by a prominent bioturbated surface between the Argiles à Plicatules and the Sables Verts de l'Aube formations deposited at a shallower water depth in shoreface environments.

IV-2. Evolution of the δ 13 Corg and organic matter contents Several of the isotopic segments initially distinguished by [START_REF] Menegatti | High-resolution δ 13 C stratigraphy through the early Aptian "Livello Selli" in the Alpine Tethys[END_REF] across the "Livello Selli", which includes OAE1a, or its equivalent in the Tethys Ocean, and which were subsequently recognised worldwide and used as a reliable tool for chemostratigraphic correlations, are clearly identified in the studied borehole: C2 at the base of the section, C3 corresponding to the negative excursion showing the lowest values of δ 13 Corg between 111.80 and 110 m, C4, C5 and C6 characterised by a shift to higher values and finally C7 (Fig. 5).

The 3‰ negative excursion that occurs immediately before OAE1a and corresponding to the C3 segment is clearly identified. It occurs probably in the deshayesi Zone just above the bioclastic accumulations interpreted as storm deposits, with an amplitude comparable to that observed in many sedimentary basins [START_REF] Menegatti | High-resolution δ 13 C stratigraphy through the early Aptian "Livello Selli" in the Alpine Tethys[END_REF][START_REF] Ando | Timing and magnitude of early Aptian extreme warming: Unraveling primary δ 18 O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean[END_REF]. The C3 segment is relatively thick (1.80 m) indicating a relatively high sedimentation rate and shows two distinct negative excursions separated by a positive excursion with a small amplitude, which can also be observed in the δ 13 Ccarb curves from sections from south-east France [START_REF] Kuhnt | Carbon isotope stratigraphy of the lower Aptian historical stratotype at Cassis-La Bédoule (SE France)[END_REF]2011). This may suggest the existence of, at least, two successive volcanic CO2 pulses at the origin of the emplacement of the Ontong Java-Manihiki-Hikurangi Plateau [START_REF] Lorenzen | A new sediment core from the Bedoulian (Lower Aptian) stratotype at Roquefort-La Bédoule, SE France[END_REF]Naafs et al., 2016).

Surprisingly, the interval corresponding to the δ 13 Corg positive excursion is not significantly enriched in organic matter and the facies of the Argiles à Plicatules Formation are extremely homogeneous, unchanged in colour and without any laminations. The absence of significant organic matter enrichment is surprising since the deep, quiet lower offshore depositional environment was favourable to the preservation of marine organic matter. Some authors report an absence of significant marine organic matter enrichment in several sedimentary basins of northern Spain including the Basco-Cantabric Basin [START_REF] Garcia-Mondejar | The OAE1a in Cuchia (Early Aptian, Spain): C and O geochemistry and global correlation[END_REF] and the Organyà Basin (Sanchez-Hernandez and Maurasse, 2016) although they correspond to oxygen-depleted environments but without prolonged anoxia.

In the Paris Basin, the absence of OM enrichment can be explained in several ways.

1) The primary productivity was probably low as suggested by the very low carbonate content of the succession. The carbonate fraction of the Argiles à Plicatules Formation, which does not exceed 4%, is composed of rare nannofossils including the genus Watznaueria, Zeugrhabdotus, Rhagodiscus, Biscutum, Discorhabdus and Cyclagelosphaera (F. Giraud, in Ferry, 2000). The scarcity of fauna and flora suggests that the environment must have been oligotrophic.

2) The absence of anoxia in the Aptian sea-way between the LBM/RM and the AM/MC (Fig. 1) may be also explained by the presence of marine currents flowing through the strait thus favouring relatively well-oxygenated environments. However, except the occasional occurrence of phosphate nodules that may indicate low sedimentation rate due to the winnowing of fine particles, there is no evidence of hydrodynamic influences expressed in the sedimentary facies.

3) The oxygen-minimum zone that was clearly expressed in the Tethys Ocean was deeper and did not reach the relatively shallow environments of the Paris Basin. This is plausible, given that [START_REF] Coccioni | Descrizione di un livello guida "radiolaritico-bituminoso-ittiolitico" alla base delle Marne a Fucoidi nell'Appennino Umbrio-Marchigiano[END_REF] and [START_REF] Westermann | Rapid changes in the redox conditions of the western Tethys Ocean during the early Aptian oceanic anoxic event[END_REF] show that anoxic to euxinic conditions developed in the deeper part of the Tethys Ocean during OAE 1a, while conditions were not as reducing in shallower environments.

By comparison with several sections from the Tethys ocean, the δ 13 Corg positive excursion is less pronounced than expected, what could be the result of several gaps coeval with the positive excursion like those observed in carbonate platform environments (Wissler et al., 2005). However, except the occurrence of bioturbated surfaces and some scattered phosphate nodules, there is no clear evidence of significant sedimentation breaks.

IV-3. Significance of clay mineral assemblages

IV-3-1. Diagenetic influences Before examining the significance of clay mineral assemblages in terms of the palaeoclimate and palaeoenvironment, it is essential to ensure that they are predominantly detrital in origin without any significant influence of burial diagenesis, thermal influences or authigenic growth during early or late diagenesis. The post-Aptian sedimentary successions probably never exceeded 1100 m [START_REF] Robaszynski | Stratigraphy and stage boundaries in a type-section of the Late Cretaceous chalk from the East Paris basin: The "Craie 700 boreholes[END_REF], which is consistent with a weak thermal impact [START_REF] Torelli | Thermal evolution of the intracratonic Paris Basin: Insights from 3D basin modelling[END_REF] compatible with the occurrence of smectite and immature organic matter. Consequently, it is unlikely that clay minerals may have been transformed during burial diagenesis. This conclusion is consistent with many studies performed to the east of the Paris Basin on upper Jurassic deposits showing that the thermal influences remain very weak even when these deposits are more deeply buried [START_REF] Pellenard | Contrôle géodynamique de la sédimentation argileuse du Callovien-Oxfordien moyen dans l'Est du Bassin de Paris : influence eustatique et volcanique[END_REF][START_REF] Pellenard | Mineralogical variability of Callovo-Oxfordian clays from the Paris Basin and the Subalpine Basin[END_REF][START_REF] Blaise | Reconstruction of low temperature (<100° C) burial in sedimentary basins: A comparison of geothermometer in the intracontinental Paris Basin[END_REF][START_REF] Mangenot | An emerging thermochronometer for carbonate-bearing rocks: Δ47 /(U-Pb)[END_REF].

Small quantities of glauconite commonly occur in the uppermost part of the Argiles à Plicatules Formation at the transition with the overlying Sables verts de l'Aube Formation.

Glauconite is an iron-rich illite and the granules are coarser than the clay fraction and normally not included in the clay fraction. However, as the samples are ground in preparation for XRD analyses, it is possible that glauconite was partly introduced in the clay fraction and slightly increases the total proportion of illite at the top of the formation. Therefore, we assume that most clay minerals are detrital in origin and may be useful to precise environmental conditions that prevailed during the early Aptian around the Paris Basin.

IV-3-2. Environmental control of the clay sedimentation

Primary minerals, including illite, chlorite and pyrophyllite constitute more than half of the clay fraction. The abundance of these minerals suggests a relatively intense erosion on the continental areas bordering the Paris Basin. Although clay minerals may be transported over long distances, they likely originated from the Palaeozoic massifs surrounding the Paris Basin. Illite, chlorite and pyrophyllite originated from the erosion of igneous and metamorphic rocks from the AM/MC and from the low grade metamorphic rocks from the LBM. Vermiculite is also common, notably at the base of the formation. This mineral also occurs in lower to middle Albian clayey strata of the Paris Basin and likely originated from the north, i.e. from the LBM [START_REF] Gale | The stratigraphy and petrography of the Gault Clay Formation (Albian, Cretaceous) at Redcliff, Isle of Wight[END_REF][START_REF] Corentin | Environmental and climatic controls of the clay mineralogy of Albian deposits in the Paris and Vocontian basins (France)[END_REF]. Vermiculitic clays corresponding to illite/vermiculite mixed-layers were also identified together with illite and kaolinite in the Aptian deposits of the Lower Saxony Basin of Germany [START_REF] Benesch | Mineralogy of the Lower Aptian sediments from the borehole Hoheneggelsen KB 40, with special emphasis on the clay mineralogy[END_REF] and likely originate from the moderate weathering of chlorite.

Kaolinite is also abundant with maximum values in interval 2 characterised by the homogeneous clay deposited in the deepest environments. Therefore, in the present case, the abundance of kaolinite cannot be a consequence of its preferential deposition in nearshore environments as suggested by [START_REF] Godet | Platform-induced clay-mineral fractionation along a northern Tethyan basin-platform transect: implications for the interpretation of Early Cretaceous climate change (Late Hauterivian-Early Aptian)[END_REF]. The stratigraphic interval coinciding with the δ 13 Corg negative excursion is strongly depleted in smectite suggesting that a particularly hydrolysing (warm and humid) climate was established at that time [START_REF] Chamley | Clay sedimentology[END_REF][START_REF] Ruffell | Comparison of clay mineral stratigraphy to other proxy palaeoclimate indicators in the Mesozoic of NW Europe[END_REF]. By contrast, in the topmost part of the Argiles à Plicatules Formation, the decreasing proportions of kaolinite balanced by increasing proportions of smectites suggest increasingly arid climate conditions confirming a certain variability in the Aptian climate (Dumitrescu et al., 2006;[START_REF] Bottini | Climate variability and ocean fertility during the Aptian Stage[END_REF]Naafs and Pancost, 2016). These arid climatic conditions also seem to have prevailed during the Aptian-Albian in the Algarve Basin (Southern Portugal) where sedimentary rocks are depleted in kaolinite and show a high amount of the xerophytic pollen Classopollis [START_REF] Heimhofer | Coastal sediments from the Algarve: low-latitude climate archive for the Aptian-Albian[END_REF]. Aridity may be a consequence of colder climate (cold snaps) recorded at the Aptian/Albian transition [START_REF] Mutterlose | The Aptian -Albian cold snap: Evidence for "mid[END_REF][START_REF] Maurer | Late Aptian long-lived glacio-eustatic lowstand recorded on the Arabian Plate[END_REF][START_REF] Millán | Expression of the late Aptian cold snaps and the OAE1b in a highly subsiding carbonate platform (Aralar, northern Spain)[END_REF].

IV.4. Climate evolution during the deposition of the Argiles à Plicatules Formation

A comparison of δ 13 Corg and the mineralogical data shows that the C3 segment corresponding to the negative excursion recorded prior to the OAE1a is characterised by an increase in the proportions of kaolinite (Fig. 7) at the same time as a decrease in the proportions of smectites. This suggests the installation of a more humid climate as a consequence of an acceleration of the hydrological cycle induced by the warming triggered by the increase in pCO2 [START_REF] Keller | A volcanically induced climate warming and floral change preceded the onset of OAE1a (Early Cretaceous)[END_REF][START_REF] Bodin | Large igneous provinces and organic carbon burial: Controls on global temperature and continental weathering during the Early Cretaceous[END_REF]Naafs and Pancost, 2016). There are several lines of evidence for such a climatic change. [START_REF] Wortmann | Altered carbon cycling and coupled changes in Early Cretaceous weathering patterns: Evidence from integrated carbon isotope and sandstone records of the western Tethys[END_REF] suggested that the widespread occurrence of sandstones in the Tethys-Atlantic seaway was a consequence of increasing erosion rates due to the acceleration of the hydrological cycle in the wake of the early Aptian carbon cycle perturbation. Increasing global weathering intensity coeval with OAE1a is also suggested by calcium isotopes [START_REF] Blättler | Significant increases in global weathering during Oceanic Anoxic Events 1a and 2 indicated by calcium isotopes[END_REF] measured in carbonate deposits from several areas including southern England, Italy and Resolution Guyot from the mid-pacific mountains. [START_REF] Hochuli | Episodes of high productivity and cooling in the early Aptian Alpine Tethys[END_REF] also describe global warming resulting in increasing riverine fluxes. The warming is evidenced by a drop in δ 18 O, TEX86 data and by pollen records. The δ 18 O data from many sedimentary basins including the South Pacific shows, that the ocean waters warmed strongly during the negative excursion and that temperatures then decreased probably due to the storage of organic matter in the sediments [START_REF] Ando | Timing and magnitude of early Aptian extreme warming: Unraveling primary δ 18 O variation in indurated pelagic carbonates at Deep Sea Drilling Project Site 463, central Pacific Ocean[END_REF][START_REF] Stein | Progressive palaeoenvironmental change during the Late Barremian-Early Aptian as prelude to Oceanic Anoxic Event 1a: evidence from the Gorgo a Cerbara section (Umbria-Marche basin, central Italy)[END_REF][START_REF] Zakharov | Late Barremian-early Aptian climate of the northern middle latitudes: Stable isotope evidence from bivalve and cephalopod molluscs of the Russian Platform[END_REF]. These observations are in agreement with the maximum proportions of kaolinite and the decrease in the proportion of Classopollis, (xerophytic pollen) in the Maestrat Basin (eastern Spain) during OAE1a [START_REF] Cors | Climatic evolution across oceanic anoxic event 1a derived from terrestrial palynology and clay minerals (Maestrat Basin, Spain)[END_REF]. Tex86 data obtained in the Selli level (or other equivalents of the OAE1a) also indicate particularly warm sea-surface temperatures [START_REF] Schouten | Extremely high sea-surface temperatures at low latitudes during the middle Cretaceous as revealed by archaeal membrane lipids[END_REF]Dumitrescu and Brassel, 2006;[START_REF] Mutterlose | TEX86 and stable δ 18 O paleothermometry of Early Cretaceous sediments: implications for belemnite ecology and paleotemperature proxy application[END_REF][START_REF] Mutterlose | High sea-surface temperatures during the early Aptian Oceanic Anoxic Event 1a in the boreal realm[END_REF].

The global warming that occurred prior to OAE1a is therefore likely responsible for an acceleration of the hydrological cycle and for the increase of kaolinite because of increasing runoff and hydrolyses on the continental areas bordering the Paris Basin. The high proportions of kaolinite in Aptian deposits of the Paris Basin could be explained by the formation of kaolinite in soils, but the duration of the climatic perturbation is relatively short, between 280 and 350 kyr [START_REF] Lorenzen | A new sediment core from the Bedoulian (Lower Aptian) stratotype at Roquefort-La Bédoule, SE France[END_REF], while the formation of kaolinite in soils required a longer time estimated at least to 1 myr [START_REF] Thiry | Palaeoclimatic interpretation of clay minerals in marine deposits: an outlook from the continental origin[END_REF]. It is more likely that kaolinite was principally reworked from kaolinite-rich weathering profiles developed over the Palaeozoic massifs during the Early Cretaceous (Jurassic-Cretaceous transition to Barremian) [START_REF] Meyer | Continental sedimentation, soil genesis and marine transgression in the basal beds of the Cretaceous in the east of the Paris Basin[END_REF][START_REF] Gilg | D-H evidence for the timing of kaolinization in Northeast Bavaria, Germany[END_REF][START_REF] Quesnel | Paleoweathering and paleosurfaces from northern and eastern France to Belgium and Luxembourg: geometry, dating and geodynamic implications[END_REF][START_REF] Yans | An overview of the saprolites of Belgium and their potential kaolinitic supplies to Mesozoic and Cainozoic sediments[END_REF][START_REF] Thiry | Altération bauxitique associée aux argiles à chailles sur la bordure sud-est du bassin de Paris[END_REF][START_REF] Thiry | Continental France and Belgium during the early Cretaceous: paleoweatherings and paleolandforms[END_REF][START_REF] Corentin | Environmental and climatic controls of the clay mineralogy of Albian deposits in the Paris and Vocontian basins (France)[END_REF]. As detrital clays including kaolinite may be reworked from older rocks or from palaeo-alterations, their climatic significance may be questioned (Jeans et al., 2001), but overall, illite and kaolinite dominated clay assemblages of the Aptian in the Paris Basin point to a significant runoff and consequently to humid climate conditions. It is probable, that fluctuations in the hydrological cycle controlling fluvial inputs occurred before and during OAE1a as evidenced by changing environmental conditions in terms of anoxia/dysoxia (Socorro and Maurasse, 2018;2020).

Although the reliability of δ 18 O data has recently been confirmed [START_REF] O'brien | Cretaceous sea-surface temperature evolution: Constraints from TEX86 and planktonic foraminiferal oxygen isotopes[END_REF], the acceleration of the hydrological cycle following the very hot episode preceding OAE1a can be used to address the question of the significance of the low δ 18 O values recorded prior to OAE1a. δ 18 O-values may also be a result of a slight decrease in ocean salinity. This question, already addressed for other periods, shows that it is essential to take better account of the fluctuations in the hydrological cycle in the context of climatic reconstructions. In addition, higher runoff may increase thermohaline stratification that may be involved in the development of an OAE (e.g. [START_REF] Erbacher | Increased thermohaline stratification as a possible cause for an ocean anoxic event in the Cretaceous period[END_REF][START_REF] Van Helmond | Freshwater discharge controlled deposition of Cenomanian-Turonian black shales on the NW European epicontinental shelf (Wunstorf, northern Germany)[END_REF].

Conclusions

For the first time, OAE1a is clearly identified in argillaceous deposits, the so-called Argiles à Plicatules Formation, deposited in the Paris Basin. Most of the C-isotope segments defining the OAE 1a interval are identified in the stratigraphic succession. However, the section is devoid of any significant marine organic matter enrichment likely because the Tethyan oxygen-depleted deep-water masses did not reach the epicontinental areas of the Paris Basin. The global warming preceding OAE1a is likely responsible for an acceleration of the hydrological cycle, an increase in runoff and the reworking of kaolinite from palaeoweathering profiles cropping out on the Palaeozoic massifs bordering the Paris Basin.

As indicators for an acceleration of the hydrological cycle are also recorded in other areas, a slight decrease in ocean salinity prior to OAE1a may be responsible in part for its development and questions the reliability of the sea surface temperature deduced from the δ 18 O values. The hydrolysing conditions were so intense prior to and during OAE1a that smectite totally disappeared from the clay fraction. However, smectite increases in the upper part of the succession suggesting, that more arid conditions prevailed at that time possibly in relation with a colder climate during the Aptian/Albian transition. The changing clay mineral assemblages suggest, that the Aptian stage was characterised by a climate instability marked not only by changes in temperature but also in the hydrological cycle.

Figure captions

  in the neighbouring department of Yonne and Megatyloceras at the base of the Argiles à Plicatules Formation (unpublished data) suggests that the interval could be attributed to the Deshayesites forbesi Zone, as these ammonites are characteristic of this biozone. The Deshayesites deshayesi Zone is undoubtedly recognised from 113.70 m to 105.15 m by the presence of an association including Aconoceras (Aconoceras) nisoides Sarasin, 1893, Pseudosaynella bicurvata Michelin, 1838, Deshayesites deshayesi d'Orbigny, 1841 and Lithancylus grandis J. de C. Sowerby, 1828. The Dufrenoya furcata Zone sensu Reboulet compiler (2018) [= the Tropaeum bowerbanki Zone described by Casey (1961)] is clearly identified from 102.50 m to 90.60 m by the occurrence of Dufrenoyia cf. formosa Casey, 1964 and Dufrenoyia cf. furcata J. de C. Sowerby, 1836. II -Analytical methods II-1. Carbon stable isotopes, Total Organic Carbon and calcimetry Organic carbon isotope compositions (δ 13 Corg) and Total Organic Carbon (TOC) were measured on carbonate-free residues of 80 samples at the Biogéosciences Laboratory, Université Bourgogne/Franche-Comté in Dijon. Sample powders were reacted with HCl (2N) at room temperature for 24 h to remove the carbonate phases. Residues were rinsed with deionised distilled water until neutral, centrifuged (4500 rpm for 15 min), and then dried at 50°C overnight. Aliquots of dried decarbonated samples (~7-50 mg) were then weighed in tin capsules. TOC content and δ 13 Corg measurements were performed at the Biogéosciences Laboratory of the University of Bourgogne/Franche-Comté (Dijon, France) on a Vario MICRO cube elemental analyzer (Elementar, Hanau, Germany) coupled in continuous flow mode to an IsoPrime stable isotope ratio mass spectrometer (Isoprime, Manchester, UK). USGS40 L-Glutamic acid (C = 40.8 wt%; δ 13 CVPDB = -26.39 ± 0.04‰) and IAEA-600 Caffeine (δ 13 CVPDB = -27.77 ± 0.04‰) certified reference materials were used for calibration.

Fig. 1 .

 1 Fig. 1. Palaeogeographic map modified from R. Blakey: https://www2.nau.edu/rcb7/. AM = Armorican Massif, MC = Massif Central, LBM = London Brabant Massif, RM = Rhenish Massif, BM = Bohemian Massif, CSM = Corso-Sardinia Massif, AB = Algarve Basin, PB = Paris Basin, LSB = Lower Saxony Basin.

Fig. 2 .

 2 Fig. 2. Simplified geological map of the area to the east of the Paris Basin and location of the AUB 121 borehole near Juzanvigny.

Fig. 3 .

 3 Fig. 3. Lithology of the AUB 121 borehole from the lower Barremian "Argiles ostréennes" Formation to the lowermost part of the "Argiles tégulines de Courcelles" Formation (lower Albian) (modified after Amédro et al., 2017). The lower Aptian Argiles à Plicatules Formation comprises three distinct lithological units and are dated by ammonites. The deshayesi and furcata ammonite Zones are clearly identified while the

Fig. 4 .

 4 Fig. 4. Base of the "Argiles à Plicatules" Formation. A. Thin section showing an iron oolite in a clayey matrix with quartz grains (Q) (sample AUB 121 116.70 m). B. sample AUB 121, at an altitude of 116.60 m, shows abundant plant debris and notably Weichselia reticulata. C. Picture of the core showing the top of Unit 1 (grey clays) with the prominent shelly beds, and the base of Unit 2 (grey-brown clays).

Fig. 5 .

 5 Fig. 5. CaCO3 content, %Corg and δ 13 Corg stratigraphic trends along the "Argiles à Plicatules" Formation. The prominent δ 13 Corg negative excursion occurring prior to the OAE1a is compared with the δ 13 Corg of the Roter Sattel reference section. Most of the segments (C2 to C7) defined by Menegatti et al. (1998) are recognised in the Argiles à Plicatules Formation. W.F. = Wealden facies, D. forb = Deshayesites forbesi, Barr. = Barremian, Alb. = Albian. MFS = Maximum Flooding Surface, TST = Transgressive System tract, HST = Highstand system tract.

Fig. 6 .

 6 Fig. 6. Clay mineralogy of the Argiles à Plicatules Formation (the proportions of chlorite, approximately 5% throughout the formation, are not plotted). D. forb = Deshayesites forbesi, W.F. = Wealden facies. MFS = Maximum Flooding Surface, TST = Transgressive System tract, HST = Highstand system tract.

Acknowledgements: We would like to thank Claude Aurière, who is in charge of storing Andra's cores, for his help in studying the boreholes. We would like to warmly thank the two reviewers, Jochen Erbacher and Helmut Weissert for their constructive and helpful comments on the initial version of the manuscript.


Clément Le Bihan

MD Vincent Faucherre

MD Vincent Le Moing

PhD ; Aurélie Mehenni

MD Damien Nantes

MD Antonio Da Silva

MD Cyril Jaume

MD ; Fabienne Lassalle

MD Alain Makinson
email: makinson@chu-montpellier.fr

COVID-19: The Forgotten Cases of Hidden Exiles

Keywords: COVID-19, infection, homeless, migrant

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

Introduction

France has been marekdly affected by the SARS-CoV-2 pandemic, and authorities ordered a lockdown on the 17 th of March 2020. Incidence of hospitalizations decreased nationally starting on the 31 st of March [START_REF] Chêne | COVID-19 : point épidémiologique du 30 avril 2020[END_REF]. Health authorities adopted a strategy of search and isolation of clusters, with extensive testing of symptomatic and closely exposed persons, and isolation of positive cases.

Exiled persons from resource-poor countries, some of whom are dismissed asylum-seekers, are a particularly vulnerable population in France. Many live in dire conditions, commonly in squats, due to major social and economic difficulties. These people are difficult to reach, particularly for health-care prevention programs, and given their social conditions, encounter more difficulties in confining [START_REF] Bhopal | Covid-19: undocumented migrants are probably at greatest risk[END_REF].

To effectively limit the spread of the SARS-Co-V2 into the community, information, screening, and isolation of index and contact cases in all populations are essential. We report a COVID-19 screening experience in two squats following the diagnosis of symptomatic cases, and present the different efficacy outcomes of these interventions.

Methods

Montpellier Metropole is a metropolitan area with approximately 450,000 inhabitants located in the South of France. Medical associations, such as Médecins du Monde, monitor squats in Montpellier with the objective of providing assistance in agreement with each squat's representative, who is usually a social worker. The squats are primarily inhabited by exiled people in economic and social hardship. Some are asylum seekers or rebuffed asylum seekers, while others are economic exiles. Since the COVID-19 outbreak, clinical assessment and testing for SARS-CoV-2 on nasopharyngeal swabs have been proposed when appropriate.

Following monitoring, COVID-19 was diagnosed in five moderately symptomatic residents dwelling in two squats. On the 23 rd of March 2020, four residents were diagnosed with COVID-19 in squat A, as was one resident in squat B. Spearheaded by "Médecins du Monde", a strategy to extensively diagnose and confine contaminated individuals, similar to responses to COVID-19 outbreaks in nursing care facilities [START_REF] Arons | Presymptomatic SARS-CoV-2 Infections and Transmission in a Skilled Nursing Facility[END_REF] or cruising ships [START_REF] Baraniuk | What the Diamond Princess taught the world about covid-19[END_REF], was planned out. The plan brought together the regional health authorities, medical associations, infectious diseases specialists from the University Hospital, social workers coordinating the squats and local political authorities (representatives of the Mayor's office and the Prefect). Subsequent to decisions reached at these meetings, positive cases were to be monitored, if willing, in a medicalized homeless facility, or else hospitalized according to clinical severity, while negative comorbid residents at risk were to be sheltered in a hotel.

Prior to intervention, accompanied by the social workers, medics repeatedly visited the two squats and gave flyers on COVID-19 symptoms and risks, advice on preventive measures, and information on the testing procedures; they also estimated the number of residents, and enlisted volunteers.

Living conditions and sanitary situations of each squat were recorded, as were demographic data on the screened population. Percentage of residents screened, percentage of residents positive for SARS-Co-V2 and the number of positive cases referred to the COVID shelter were evaluated as outcomes. Oral consent for data use and publication was obtained, and only data useful for clinical care were recorded.

Results

The populations screened in squats A and B were mainly young exiles with median ages of 31 and 30 years respectively (Table 1), with an estimated 200 and 120 residents respectively.

French-speaking people were more frequently present in squat B. In squat A, there were no showers, only one water source and three toilets. In squat B, there were three showers and four toilets, with access to water. Distancing and containment after the first positive case was possible in single rooms and carried out in squat B, whereas following the initial cases, no isolation or barrier measures had been implemented in squat A.

On the 21 st of April 2020, a mobile unit for nasopharyngeal testing arrived in squat A. Out of the 75 of the estimated 200 residents who had initially volunteered for testing (Figure 1), 35 actually came to be tested. All in all, 50 persons were tested, as 15 additional subjects agreed to be tested on the day of the intervention (Table 1). Only two tested subjects had a cough (4%). On the 23 rd of April, results were handed back to 34 of the 50 tested persons, including 16 of the 19 positive cases, who thereby accounted for 38% of the screened population. The medical team had to leave before having completed its work, the reason being that two residents did not accept their presence on the premises. Other test results were communicated by phone on the 24 th and 25 th of April. All in all, eight residents, one of whom had tested positive, did not initially receive their results. Three of the positive cases went to the COVID-shelter for homeless persons. Interactions with the medical association were resumed a few weeks later, and two months after screening, six additional toilets and showers were set up. Even though systematic screening was not undergone, no hospitalizations or new cases were diagnosed after our intervention. On the 31 st of August 2020, authorities decided to dismantle the squat.

In squat B, screening sessions were carried out three times during a period of 14 days. On the 14 th of May, 49 persons were screened, and none were found positive. Five days later, ten other persons were tested, and two positive cases were identified. Six more subjects underwent testing ten days later, and the results were negative. All in all, 65 persons were tested (54% of estimated residents), nine of whom were tested twice at different sessions.

None of them went to the COVID shelter. Positive cases accounted for 3% of the screened population. Three showers and two toilets were provided in the days following screening as were masks and sanitizers. Acceptance of the intervention was very good. After which, no new cases were diagnosed and no positive subjects were hospitalized, even though systematic screening of the residents had not been provided.

Discussion

Our experience of testing for COVID-19 in squats revealed a high level of asymptomatic disease transmission, particularly in squat A. Monitoring squats and testing only subjects with symptoms may favor inter-resident spread of the virus before the first symptomatic case diagnosis; in addition, prior transmission to comorbid or older residents may occur. The high degree of asymptomatic and paucisymptomatic contaminations in squat A can be explained by the young median ages and the healthy populations residing on the premises. However, systematic and regular screening of squat residents is challenging to organize, and acceptance among exiles may be limited, given that some of them may not have legal resident status. This contrasts with routine surveillance strategies in other settings, such as homeless shelters in the state of Washington (USA) [START_REF] Rogers | Characteristics of COVID-19 in Homeless Shelters[END_REF]. High transmission rates, particularly in squat A, could also have resulted from dire sanitary conditions, difficulties in achieving adequate confinement, and the time-lapse between the diagnosis of the initial symptomatic cases and the planned screening interventions.

Our experiences underscore low acceptance of the intervention, particularly in squat A, notwithstanding painstaking careful planning and the experience of professionals, and the individual and community benefits of the intervention were correspondingly limited. These results contrast with those reported in a publication on a homeless care facility in Boston [START_REF] Baggett | Prevalence of SARS-CoV-2 Infection in Residents of a Large Homeless Shelter in Boston[END_REF], in which all of the 408 residents were assessed and tested. Local political authorities were closely involved, and the population was composed of homeless but not exiled persons. So it is that context, political engagement, pre-existing sanitary conditions and legal issues are major determinants of the success of these interventions. The difficulty in upgrading sanitary conditions may have contributed to the low acceptance of our intervention in squat A. In addition, some residents may have been wary of any kind of governmental representation, even that of health-care medics, given their fear of impending expulsion procedures of dismissed asylum-seekers. Cultural misrepresentations of COVID-19 [START_REF] Hargreaves | Targeting COVID-19 interventions towards migrants in humanitarian settings[END_REF] might also explain some of our results, particularly in squat A. Meaningful community engagement and community-based interventions are a key element in ensuring that health strategies are widely accepted and reasonably effective.

Other obstacles to controlling outbreaks in our settings include the high mobility of residents, whose frequent moves from one squat to another may entail a high risk of viral spread. Since the end of lockdown, the double objective of caring for patients and controlling the epidemic in these settings may have been rendered even more difficult than was the case during our study.

Acknowledgement

This work did not receive any specific funding

encountered at the individual and the aggregate levels amongst the exiled populations in France. Cross-curricular strategies are essential to appropriate and culture-centered medical and sanitary assistance for COVID-19 diagnosis and management. They will necessitate social and political measures that might help to reduce reducing the prevalence of SARS-Co-V2 and risk of viral spread in this population.

Contribution