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La géoarchéologie française au xxie siècle

 | 
Nathalie Carcaud
, 
Gilles Arnaud-Fassetta

Partie II. Les hydrosystèmes fluviaux, entre climat et anthropisation/Section 2. Fluvial hydrosystems, between climate and “anthropogenic processes”

Chapter 8. Holocene fluvial dynamics of the middle Loire River (Val d’Orléans, France)

Responses to climatic variability and anthropogenic impacts

Cyril Castanet, Joëlle Burnouf, Christian Camerlynck, Nathalie Carcaud, Anne-Laure Cyprien-Chouin, Manuel Garcin et Michel Lamothe

Texte intégral

Financial supports for this study was provided by the French INEE – CNRS (Institut Écologie et Environnement) programme Zone atelier bassin-versant de la Loire, by the ARCHEA Région Centre programme Val d’Orléans, by the Programme Opérationnel Plurirégional Loire Féder, Etablissement Public Loire and Agence de l’Eau Loire Bretagne. The authors thank INRAP (Institut National de Recherches Archéologiques Préventives) and the CEMEX enterprise (ex-Morillon-Corvol) for their precious collaboration, and the anonymous reviewers for their suggestions.

Introduction

1Current and future fluvial risk management requires a good understanding of the complexity of fluvial responses to climate and societal changes. In the current context of global change, the reconstruction of fluvial dynamics in response to climate and societal changes during the past, are key for a better perception of current and future dynamics (Gregory et al., 2006). During the last few decades, several works on European catchments have revealed important variability in fluvial activity during the Holocene (Rotnicki, 1991; Starkel, 1991; Bravard, 1992; Jorda and Provansal, 1996; Miramont et al., 1999; Benito, 2003; Brown, 2003; Pastre et al., 2003; Starkel, 2003; Macklin et al., 2006). Climate change is most frequently suggested as the main driver of fluvial activity during the Holocene. For the last three to five millenniums, societies through impacts on sediment and water flows in the catchment and climate have controlled fluvial dynamics. Only rarely have autocyclic evolution been proposed to explain fluvial evolutions (Erkens, 2008). This case study aims to improve our understanding of the interactions between climatic and anthropogenic impacts as the drivers of fluvial dynamics and autocyclic evolution. Our knowledge of the Holocene history of fluvial dynamics of large French hydrosystems is mainly centred on the Rhône catchment (Bravard, 1987; Salvador, 1991; Bravard, 1997; Arnaud-Fassetta, 1998; Provansal et al., 1999; Arnaud-Fassetta et al., 2005; Salvador, 2005; Arnaud-Fassetta, 2007) and on the Seine catchment (Pastre et al., 2003). The Holocene fluvial dynamics of the rivers Loire and Garonne have as yet been rarely studied. During the last decade, works have been initiated in the floodplain of the upper Loire River (Cubizolle et al., 2002; Straffin and Blum, 2002), middle Loire River (Garcin et al., 1999), and lower Loire River (Carcaud et al., 2002; Carcaud, 2004). This study concerns the Holocene fluvial dynamics of the Middle Loire River near Orléans. It focuses on three questions: How did climatic variability influence fluvial dynamics during the Holocene? What were the fluvial responses to the interactions between climate changes and land uses by societies? What was the importance of autocyclic processes in these fluvial dynamics?

Study area

2The selected study area is situated in the south of the Paris basin in the northern sector of the Loire River roughly midway between the river source and the estuary (Fig. 1). It is the main tract of the floodplain of the Middle Loire River (length: 50 km; maximal width: 8 km) between 120 m and 90 m asl (average slope: 0.5‰; Fig. 1). The Loire valley results from Weichselian incision into the Tertiary bedrock (clayey sandy detritical tertiary deposits, i.e. Formation de Sologne, and karstic Tertiary limestone, i.e. Formation de Beauce; Gigout and Desprez, 1970; Gigout et al., 1970). This geomorphological context was favourable for the preservation of Holocene fluvial archives. Tectonic activity (i.e., neotectonics) in this sector of the Paris basin is considered as very low (Freytet et al., 1989). The hydrosedimentary flows in transit in this floodplain sector come from an upstream catchment of 37000 km2 underlain by endogenous rocks at the level of the Central Massif and sedimentary rocks at the level of the Paris basin. The maximum height of the upstream, mountainous catchment is around 1900 m asl. The current climatic conditions and the physical properties of the catchment induce a current flow regime of the Middle Loire River characterised with strong seasonal and inter-annual variations (Dacharry, 1974). The current fluvial pattern of the river is a single sinuous to meandering channel. Hydrological and meteorological studies indicate that the strongest Middle Loire River loods during the last two centuries are under Atlantic and Mediterranean atmospheric controls (Dacharry, 1974). he last strong loods inundating the loodplain occurred during the middle part of the 19th c. (Dion, 1934).

Fig. 1. Location map of the Loire River and the studied area in the floodplain. A: The Loire River and the major fluvial hydrosystems of Western Europe; B: The Loire catchment; C: The Middle Loire floodplain near Orléans and methodological investigations.

Materials and methods

3In this study we investigate how Holocene luvial dynamics of the Loire River had impacts on fluvial pattern and frequency, and the magnitude of the floods in the floodplain (Castanet, 2008). Materials and methods aim at characterising the palaeohydrography and morphostratigraphy of the deposits, and sedimentary processes in the floodplain, focusing on the sedimentary archives and written sources. Morphostratigraphic analysis are based on two main cross-sections of the Loire loodplain (upstream section CS1 “Saint-Benoît-sur-Loire” and downstream section CS2 “Sandillon”; Fig. 1), the study of three quarries (alluvial extraction), 15 archaeological surveys (Institut National de Recherches Archéologiques Préventives) and 120 boreholes. A geological database of 1200 geological borings included in the study area was analysed (BSS-BRGM; Giot and Gaultier, 1999). Palaeohydrographical reconstructions were derived from the analyses of a LiDAR DEM (Ramirez, 2003), morphostratigraphy, and chronostratigraphy of the deposits. As a supplement to the identiication of the main lithofacies and architectural elements of the Holocene deposits, six sedimentary series distributed on both transverse profiles were analysed. This aims to (i) consider the spatial and temporal discontinuities of the sedimentary records and the conservation potential of the deposits, (ii) estimate the lateral and longitudinal variability of alluviation in the floodplain, and (iii) discriminate between the local and regional influences on middle Loire River fluvial dynamics. For each sedimentary series (palaeochannel fills), one borehole was selected in the most representative sector and the most dilated series. Selected sedimentary records are: (i) rates of organic matter and carbonates; (ii) silicates and oxyhydroxydes, mass accumulation rates of mineral matter and organic matter (Macaire et al., 2006); (iii) sediment grain size; and (iv) bulk magnetic susceptibility. Series SB1 and SB2 were also studied in using palynology. The chronostratigraphic frame was based on 38 radiocarbon dates, 1 Optically Stimulated Luminescence date (IRSL; Huntley and Lamothe, 2001), and biochronological elements (palynology). A flood database of the Middle Loire floodplain near Orléans during historical times (6th to 20th c. AD) was developed for this study (Champion, 1858-1864; Dion, 1934; Dacharry, 1996).

Results

4Three sets of results reveal the morphostratigraphy of the deposits, the palaeohydrography of the Loire River, and the hydrosedimentary variability of the Loire River during the Holocene (Castanet et al., 2007; Castanet, 2008).

Morphostratigraphy of Holocene fluvial archives

5Several units of coarse detrital deposits (boulders, gravels, sand) with gravel bars and bed form architectural elements (GB of Miall, 1996) represent the main part of the current alluvial storage of the floodplain. They date from the Weichselian period (Pleniglacial and Lateglacial). The main part of the Holocene deposits is nested in Wechselian deposits (Fig. 2) due to the vertical and lateral incision controlled by Holocene flows. Some Holocene deposits are vertically stacked on Weichselian deposits at the level of Pleniglacial and Lateglacial palaeochannels (lithofacies of fine flood-plain deposits) or laterally to Holocene channels (bank lithofacies). The main Holocene architectural elements described are channels (CH of Miall, 1996), lateral accretion macroforms (LA of Miall, 1996), and fine flood deposits (FF of Miall, 1996; Fig. 2). Lithofacies associated with CH channels suggest Holocene palaeochannels with coarse bed loads (gravels, sand) and with a strong lateral and longitudinal variability. Channel widths vary from 350 m and 220 m (Fig. 3). Holocene Scroll bars identified near the current river and palaeochannels of the Loire River include boulders with gravely-sand matrix at the bottom and silty-sand at the top of the series (Figs. 2 and 3). The thickest FF deposits are the most important particularly in the Holocene and in Weischelian palaeochannels (thickness between 2 m and 4 m, in the series SB2, SB 3, SB 4, SAN1, SAN2, and SB1). FF deposits are not observed at the level of the highest points of the floodplain, in-filled by Weichselian deposits named “montilles”. The lithofacies of FF is an organo-mineral deposit (clayey-peaty silt, peat) and silty sand. Organic deposits are thus very rare in this sector of the valley.

Palaeohydrography of the Holocene middle Loire River

6The strong correlation between floodplain microtopography and the morphostratigraphy of the deposits (between the relief of palaeochannels and the lithofacies of the architectural elements CH) allowed us to identify and to date fluvial forms (Fig. 3). Their conservation within the plain results essentially from several avulsions, meander cut-offduring the Holocene, and a relatively low rate of lateral migration of the active channel. The Loire channel pattern has evolved toward a sinuous to a meandering single channel since the Preboreal period (channel SBCH3), during the old Atlantic period (channel SBCH2B), and the late Atlantic period (channel SBCH2A; Fig. 3). The sinuosity (1 and 1.5) and the slope (0.5 and 0.6‰) of the channels dated from Preboreal, Atlantic and Subatlantic periods were the same as those of the current channel. During the Subatlantic (particularly during the Middle Ages), the channel pattern was sinuous to meandering with a single channel. At the end of the Middle Ages (15th c.) and the Modern Epoch (beginning of 19th c.), a pattern with multi-channels is identified from sedimentary archives, written sources and old maps. During the first half of 19th c., a change of channel pattern was characterised by a shiftto a single channel (Dacharry et al., 2002). The present channel of the Loire River and Subatlantic palaeo-channels have constructed natural levees and breaches. Preboreal and Atlantic palaeochannels show very small or non-existent natural levees and breaches (Fig. 3). The first embankments of the Middle Loire River are dated to the Middle Ages (Fig. 2). In this area, the river was entirely dyked (embanked) from 16th c. (Dion, 1934; Garcin et al., 2006).

Fig. 2. Model of fluvial architecture of the Loire valley transect CS1 (location, Fig. 1): Methodology, lithofacies, architectural elements and genetic interpretation in terms of depositional environments.

Holocene variability in the frequency and the magnitude of the Middle Loire River floods: Sedimentary archives and written sources studies

7The analysis of the loods database during historical times (written sources for 54 loods counted for the loodplain of the Loire loodplain near Orléans) highlights two periods marked by greater frequencies of large loods of the Loire River (Fig. 4). he irst period is between the second half of the 9th and the irst half of 11th c. he second period begins at the beginning of 15th c. and extends until the 19th c. he frequencies are maximal during the second period, during which the average lood recurrence varies from 10 to 12 years. No major lood is counted between these two periods. Furthermore, the temporal distribution of zones characterised by increased frequencies and/or magnitudes of loods for each sedimentary series is analysed (Fig. 4). For historic periods, it is completed by lood events mentioned by the written sources. he partial or total simultaneity of several zones recorded in the floodplain is analysed. We identify seven periods characterised by an increase in luvial dynamics during the Holocene within the loodplain of the Middle Loire River near Orléans. Their estimated duration varies from 150 to 1350 years. Two durations are proposed for each period. The smaller range corresponds to the stepping of the largest number of zones concerned by the period. The wider range corresponds to the stepping of at least two sedimentary series at the level of which the period is recorded (Fig. 4). Each period characterised by an increase of luvial activity was recorded in at least three series, and sometimes in ive or six sedimentary series. It means a record of every period of increased fluvial activity included between 50% (E5) and 85% (E3A) of the sedimentary series. Furthermore, apart from E6, all the periods were identiied in the upstream section (CS1) and the downstream section (CS2) that are 22 km apart (Fig. 1). For a given period, zones have a length considered widely superior to that of the considered period (example of Z4 of the series SB2 for the period E5). he non simultaneity of zones attributed to the same period can result from limits due to (i) the deinition of the zone within the series (resolution of the sedimentary record and/or the resolution of the analysis), (ii) the chrono-stratigraphic frame (uncertainty of the age – depth model), and/or (iii) the spatiotemporal variability of the record and the conservation of a period of higher luvial activity only on one site. Considering the relatively low thickness of the deposits and the resolution chosen for the sedimentary proxies, the longest periods could include several periods of higher luvial activity of shorter duration (e.g., E3 including probably E3A and E3B).

Fig. 3: Palaeohydrography of Holocene (and Weichselian) fluvial forms in the middle Loire floodplain near Orléans.

Discussion: Interactions between geodynamic and anthropogenic factors in the control of luvial dynamics and sedimentary environments

8During the Holocene, the dynamics and sedimentary environments of the Middle Loire River near Orléans have evolved at multi-millennium and multicentennial scales (Castanet, 2008; Fig. 5).

Multi-millennium scale evolution

9A long-term trend is identified at the Holocene scale (11650 cal. BP-Present). It begins during the Preboreal with the individualisation of a single, incised channel. It is a consequence of the rapid global warming of the Preboreal and increasing forest cover (Reille and Beaulieu, 1988; Cyprien et al., 2004). The irst part of the Holocene (between 11500 and ~ 4500 cal. BP) is marked by a general trend to decreased luvial activity. It is characterised by the reduction of mass accumulation rates of mineral matter, a ining (reduction) of textures, and increase in the rates of organic matter deposition on the lood plain. Forest development (Reille and Beaulieu, 1988; Argant and Cubizolle, 2005) reduced the sediment yield in the upstream part of the catchment. Sediment discharges are the lowest of the Holocene during this period (Sifeddine et al., 1996; Fourmont, 2004). Climatic conditions evolving towards the “hermal Maximum of the Holocene” (Johnsen et al., 2001; Heikkilä and Seppä, 2003; Davis et al., 2003) control the stability of the vegetated and pedologically altered soils covers. Climatic conditions control indirectly the decreasing clastic discharges of the Loire River. The second part of the Holocene (from ~ 4500 cal. BP) is characterised by the transformation of sedimentary environments in the floodplain. Increased mass accumulation rates of mineral matter, and flood-deposits grain size, indicate increased fluvial activity from the period E4, and clearly increased from the period E3. This Holocene fluvial change of the Middle Loire River is relatively simultaneous with the climate change identified around 4500 cal. BP. Sometimes referred to as the “Big Wet Shift(Hughes et al., 2000)” or “Neoglaciation”, it is characterised by a progressive decrease in summer temperatures (Davis et al., 2003) and change in rain distribution and volume (Annoy et al., 2003). It comes after the “Mid Holocene Climate Change” identified by Steig (1999) around ~ 5500/6000 cal. BP. In the upstream catchment, the first tracts of deforestation and non-permanent cultivations of cereals appear around 5500 cal. BP one millennium before the observed fluvial change (Fig. 5). The first long lasting and continuous anthropogenic impacts on vegetation and soils become widespread only during the Iron Age (800 BC – AD; Reille and Beaulieu, 1988; Miras et al., 2004; Argant and Cubizolle, 2005). The response of the main fluvial change of the Middle Loire River around 4500 cal. BP seems to be driven by climatic control. This increase in sedimentation appears related to the impact of climate change (colder and wetter) on the processes of erosion. However, the dynamics of settlements and societal exploitation of soils, which increased during the last three millenniums, almost probably contributed to intensifying fluvial response to climate change.

Fig. 4. Increased fluvial activity periods during the Holocene in the middle Loire River reconstructed from sedimentary (cores SB1, SB 2, SB 3, SB 4, SAN1 and SAN2) and written archives. Grey bands correspond to higher fluvial activity.

Multi-centennial scale evolution

10This long-term trend is modulated by a multicentennial scale variability characterised by (i) an increase of flood frequencies and flood magnitude in the floodplain and, sometimes, (ii) a change of the channel pattern of the Middle Loire River. For example, during the period E1 (15th -19th c. AD), the increased frequencies and magnitude of the floods was synchronous of the development of unstable, multiple channels (Figs. 3 and 5). These periods result from the modification of solid vs. liquid discharges (Qs/Qw) that controlled the spatial adjustment of channel geometry (Schumm, 1977). These periods have a duration varying from 0.15 to 1.35 ka and a recurrence varying from 0.6 to 2.4 ka (average of 1.34 ka): E7, 10.25-9.35 ka cal. BP; E6, 8.7-7.7 ka cal. BP; E5, 6.8-5.9 ka cal. BP; E4, 4.45-3.55 ka cal. BP; E3, 3-1.65 ka cal. BP; E3B, 2.9-2.7 ka cal. BP; E3A, 2.25-1.9 ka cal. BP; E2, 1.1-0.9 ka cal. BP; E1, 0.55-0.1 ka. cal. BP). During the Holocene, the dynamics of the active channel of the Loire River (lateral migration, avulsion, meander cut-off, etc.) does not seem to control the major modifications of the sedimentary environments in the two studied sections. For example, if the avulsion had arisen during period E6 it could, maybe, have locally modified hydro-sedimentary dynamics in the upstream section (CS1), but the avulsion between E5 and E6 did not induce such consequences. According to seven identified periods, the hydrological variability of the Loire River does not seem to result from auto-cyclic evolution. Each period of higher fluvial activity is in phase with a period of high lacustrine level as identified by Magny et al. (2003) in the Northern Alps and Jura, with the exception of E2. Holocene, hydrological variability of the Middle Loire River seems to be under climatic control. The colder and wetter conditions during a part of these periods (increase in annual rainfalls, the reduction of summer temperatures; Magny et al., 2001) contributed to increase the frequency and the magnitude of the floods. For example, the hydrogeomorphological modifications arisen during E1 result partially from hydro-climatic conditions of the Little Ice Age manifest in the upstream catchment (Stebich et al., 2005). Besides, at the southern limit of the Loire catchment, two rivers (Gard and Ardèche) experienced numerous floods of high magnitude during at least the last three millenniums. Three main periods of severe floods (Sheffer, 2003; Sheffer et al., 2007, units 11, 6 and 4) are synchronous with the periods E3B, E3A and E1 recorded in the Middle Loire River (this study). These periods would be controlled by the Mediterranean heavy rainfall regime (Sheffer et al., 2007). It suggests major rainfall events of Mediterranean origin in the upstream catchment of the Loire River were responsible for these periods of flooding. The origin of large floods in the Middle Loire River over the least three millennia appears to be under both Mediterranean and Atlantic atmospheric influences.

Fig. 5. Holocene evolution of flood frequencies, flood magnitude and channel pattern in the middle Loire River. Correlation with agricultural expansion, deforestation and land-use changes in the upstream Loire catchment (a: Reille and Beaulieu, 1988; b: Argant and Cubizolle, 2005). Grey bands correspond to higher fluvial activity.

Conclusions

11This integrated approach involving geomorphology, geoarchaeology, geochronology and palynology to Holocene alluvial formations has allowed us to propose a palaeohydrographical and palaeohydrological reconstruction of the Middle Loire River during the last twelve millennia. The active channel of the Holocene Loire River was relatively narrow (2 to 3 km) compared with the width of the floodplain (up to 8 km). The nested geometry of the Holocene deposits into the Weichselian deposits results from process of incision, lateral migration and avulsion of the Loire channel, and meanders cut-off. The Holocene sedimentary lithofacies are associated with the architectural elements as channels (CH), through lateral accretion (LA) and fine flood-plain deposition (FF). The bed load deposited in the palaeochannels is gravely sand to sandy gravel. The deposits of the floodplain are mainly mineral. Organo-mineral sediments of abandoned channels fills include an organic component varying from 1 to 85% of the mass of the sediment. The Middle Loire river pattern evolved toward a sinuous-to-meandering single channel at the beginning of the Preboreal, during the Atlantic and a part of the Subatlantic period. Holocene variability of hydro-sedimentary flows induced a fluvial change characterised by multiple channels in the floodplain, as for example occurred again during the Little Ice Age.

12During the Holocene, the hydrosedimentary dynamics of the Middle Loire River evolved according to a long-term trend punctuated by multi-centennial scale perturbations. Between 11500 and ~ 4500 cal. BP, a general trend characterised by a decrease in fluvial activity is observed. This trend is controlled by the evolution of the climate towards the thermal maximum of the Holocene, in a context of unimportant human influence on the vegetation and pedological stability. Between ~ 4500 cal. BP and today, the inversion of this long-term trend is characterised by a transformation of the sedimentary environments of the floodplain due to increased fluvial activity. This major change seems initially controlled by hydroclimatic change (Big Wet Shift; Mid Holocene Climate Change; neoglaciations). The impact of societies on sediment yield and transport could have increased the fluvial response to the hydroclimatic change from ~ 800 BC. This long-term trend is punctuated by seven periods of multi-centennial scale variability (duration varying from 0.15 to 1.35 ka and recurrence interval varying from 0.6 to 2.4 ka, with an average of 1.34 ka). Autocyclic evolution does not seem control this hydrosedimentary variability. Climatic variability is the main control parameter. The colder and wetter conditions during these periods contributed to increased flood frequency and the magnitude. During at least the last three millennia, the large floods of the Middle Loire River seem to have been under the double control of atmospheric circulations of both Atlantic and Mediterranean origin, in a context of increasing anthropogenic impacts in the catchment.

13This study illustrates the complexity of fluvial dynamics in response to climatic and human factors. Our research into the complex interrelations between geodynamic and anthropogenic factors that control the fluvial system, aims at a better perception of the current and future dynamics for improved management of the fluvial risk in the context of global change. With this aim in mind research is in progress to characterise the evolution of interactions between the societies living in the Loire floodplain and fluvial dynamics during the Holocene (Programme AGES Évolutions géomorphologiques anciennes de l’hydrosystème ligérien, 2012-2016, coordinated by C. Castanet).

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Table des illustrations

Légende Fig. 1. Location map of the Loire River and the studied area in the floodplain. A: The Loire River and the major fluvial hydrosystems of Western Europe; B: The Loire catchment; C: The Middle Loire floodplain near Orléans and methodological investigations.
URL http://books.openedition.org/editionscnrs/docannexe/image/22062/img-1.jpg
Fichier image/jpeg, 412k
Légende Fig. 2. Model of fluvial architecture of the Loire valley transect CS1 (location, Fig. 1): Methodology, lithofacies, architectural elements and genetic interpretation in terms of depositional environments.
URL http://books.openedition.org/editionscnrs/docannexe/image/22062/img-2.jpg
Fichier image/jpeg, 251k
Légende Fig. 3: Palaeohydrography of Holocene (and Weichselian) fluvial forms in the middle Loire floodplain near Orléans.
URL http://books.openedition.org/editionscnrs/docannexe/image/22062/img-3.jpg
Fichier image/jpeg, 577k
Légende Fig. 4. Increased fluvial activity periods during the Holocene in the middle Loire River reconstructed from sedimentary (cores SB1, SB 2, SB 3, SB 4, SAN1 and SAN2) and written archives. Grey bands correspond to higher fluvial activity.
URL http://books.openedition.org/editionscnrs/docannexe/image/22062/img-4.jpg
Fichier image/jpeg, 236k
Légende Fig. 5. Holocene evolution of flood frequencies, flood magnitude and channel pattern in the middle Loire River. Correlation with agricultural expansion, deforestation and land-use changes in the upstream Loire catchment (a: Reille and Beaulieu, 1988; b: Argant and Cubizolle, 2005). Grey bands correspond to higher fluvial activity.
URL http://books.openedition.org/editionscnrs/docannexe/image/22062/img-5.jpg
Fichier image/jpeg, 379k

Auteurs

Professor Emeritus, Panthéon-Sorbonne (Paris 1) University, Mixed Research Unit (UMR 7041) CNRS/Universities of Paris 1 & Paris 10/Ministry of Culture and Communication (Archaeologies and Sciences of Antiquity – ARSCAN), Nanterre, France (joelleburnouf@orange.fr).

Associate Professor, Pierre & Marie Curie (Paris 6) University, Mixed Research Unit (UMR 7619) CNRS/University of Paris 6/EPHE/ENSM Paris (Structure and Functioning of Continental Hydrosystems – SISYPHE), Paris, France (christian.camerlynck@upmc.fr).

Palynologist, University of Nantes, Mixed Research Unit (UMR 6566) CNRS/University of Nantes (Laboratory of Atlantic Ecology and Palaeoenvironments–LEPA), Nantes, France (anne.laure.cyprien@free.fr).

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