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International audience; Wetland archaeological sites carry especially high cultural value, and are inscribed on the list of UNESCO World Heritage Sites. Ongoing climate change caused by human activity poses a threat to these exceptional heritage sites. It is important that we evaluate them with a view to better managing and preserving them.With that in mind, the recent investigations around Lake Annecy in the Savoie department give us a chance to observe how sensitive variations in this body of water are to climate change over the long term. Using a set of archaeological, chronological and sediment data collected from lakeshore sites, we can reconstruct lake level changes over the Holocene period. These data highlight a range of over 5m in the low water mark over the Holocene Climate Optimum (around 9000-7000 cal BP), characterized regionally by an average summer temperature high point of around 2/2.5°C compared to the preindustrial era. Results from Annecy are supported by others from investigations done on sedimentary fillings from Lake Cerin in the Jura department.These observations suggest that the present-day outlet stream system regulating Lake Annecy’s water levels could prove insufficient in the future to maintain the average water level, if global warming intensifies and affects the lake’s water supply in a more severe and permanent way. More broadly, the results of these investigations highlight the issue of long-term preservation and conservation of archaeological remains in wetlands in the subalpine and Jura regions at a time when global warming is faster and more intense than ever.; Les sites archéologiques en milieux humides constituent un bien culturel particulièrement précieux inscrit sur la liste du patrimoine mondial de l’UNESCO. Les changements climatiques en cours provoqués par les activités humaines font peser sur ce patrimoine d’exception des menaces qu’il importe d’évaluer pour en assurer au mieux la gestion et la préservation. Dans ce contexte, les investigations menées récemment autour du lac d’Annecy en Savoie offrent la possibilité d’observer dans la longue durée la sensibilité des variations de ce plan d’eau aux changements climatiques. Un corpus de données sédimentologiques, archéologiques et chronologiques collectées sur les sites littoraux permet de reconstruire les variations du niveau du lac au cours de l’Holocène. Ces données mettent notamment en évidence un étiage du plan d’eau de plus de 5 m d’amplitude au cours de l’Optimum climatique holocène (vers 9000-7000 cal BP) caractérisé régionalement par un maximum thermique de l’ordre de 2° à 2,5°C en termes de température moyenne estivale par rapport à la période pré-industrielle. Les résultats obtenus à Annecy sont confortés par d’autres obtenus à partir d’investigations effectuées dans les remplissages sédimentaires du lac de Cerin dans le Jura. Ces observations suggèrent que la régulation contemporaine du niveau du lac d’Annecy par les aménagements de l’émissaire pourrait s’avérer insuffisante à l’avenir pour le maintien du niveau moyen du plan d’eau, si le réchauffement climatique actuel devait s’accentuer et affecter plus fortement et plus durablement encore l’alimentation du lac. Plus généralement, les résultats de ces investigations nous alertent sur la question de la préservation et de la conservation à long terme des vestiges archéologiques en milieux humides dans le domaine subalpin et jurassien dans un contexte où le réchauffement du climat s’accentue et s’accélère.

The Geological Regional Natural Reserve (GRNR) of Pontlevoy, inaugurated in 2018, comprises two ancient quarries (theFour à Chaux quarry and the Hauts de la Plaine Saint-Gilles quarry). It shows the main Miocene formations, overlaying one on theother, which are characteristic of the area located between the Loire and the Cher valleys at the South of Blois: Aquitanian “Calcaire deBeauce”, Burdigalian “Sables et marnes du Blésois” and Langhian “Falun du Blésois”, the two last formations being fossiliferous. Aftera brief synthesis on the Miocene formations of this area, we present sedimentological data on the outcropping formation of the GRNRand results of new paleontological excavations in the “Sables et marnes du Blésois” of the reserve. The geophysical prospection usingelectrical conductivity method as well as borings and core drillings, performed before the preservation works in the area located betweenthe two quarries showed (1) a new fluvial formation filling a deep channel cut in the Langhian falun; this fluvial formation can be datedof the Serravalian or the Upper Miocene, and (2) a senestral strike-slip fault, also regarding to the east, which affects the whole Mioceneformations., La Réserve Naturelle Régionale Géologique (RNRG) de Pontlevoy inaugurée en 2018 comprend deux anciennes carrières (carrièredu Four à Chaux et carrière du Haut de la Plaine Saint-Gilles). On peut y observer en superposition stratigraphique les principalesformations miocènes du secteur situé entre Loire et Cher au sud de Blois : Calcaire de Beauce de l’Aquitanien, Sables et marnes duBlésois du Burdigalien et Faluns du Blésois du Langhien, les deux dernières étant fossilifères. Après une brève synthèse sur les formationsmiocènes de la région, sont présentées ici des données sédimentologiques sur les formations visibles dans la RNRG et les résultats denouvelles fouilles paléontologiques dans les Sables et marnes du Blésois de la réserve. La prospection géophysique par conductivité électrique,ainsi que les sondages et forages carottés effectués entre les deux carrières avant l’aménagement de la réserve, ont révélé (1) laprésence d’une nouvelle formation fluviatile comblant un profond chenal incisé dans le falun langhien ; cette formation peut être rapportéeau Serravallien ou au Miocène supérieur et (2) la présence d’une faille à regard est et décrochement sénestre affectant l’ensemble desformations miocènes.

International audience; Climatic oscillations and anthropogenic perturbations have a great influence, at multiple spatial and time scales, on hydrosedimentary dynamics and the morphology of rivers. In particular, numerous studies emphasize on changes in fluvial regimes occurring during the post-LGM deglaciation period. Though most of the Holocene evolution of rivers seems to be linked to climatic fluctuations, it is essential, when focusing on historical periods (i.e. post- Bronze Age), to take into account and try to quantify the influence of anthropic activities on such an evolution.The Cosson watershed, located south of the middle Loire river section, flows on the Miocene fluvio-lacustrine formations of the Sologne region.Alluvial terraces in the watershed give evidence of a common evolution between the Loire and Cosson rivers before they separate during mid-Pleistocene times. In the Chambord National Domain, analyses of high-resolution Digital Elevation Models derived from airborne Lidar data show a complex network of multiple channels on the Cosson river (channeled since 15th century times) floodplain surface. Core drillings have been carried out in the floodplain to precise the Holocene hydrosedimentary dynamics of the river. Ages of alluvial deposits have been obtained using radiocarbon dating. Results show that the Holocene period can be divided in two principal sequences : a basal sequence, several metres thick, composed of coarse alluvium (coarse sand and gravels) and a top sequence about 1m-thick consisting in silty-clayey materials alternating with sandy layers. Datings place the transition between the two sequences around 1500 and 1800 yrs BP, whereas no major change seems to occur in hydrosedimentary dynamics at the base of the Holocene. Complementary work on the alluvium, such as grain-size, organic matterand palynological analyses are being carried out. Such analyses will allow to characterize sedimentary sources, better constrain the watershedpaleoenvironments, and precise the respective influence of climatic and anthropogenic forcings on the Holocene hydrosedimentary dynamics in theCosson lowland setting, where anthropic activities (prior and posterior to the edification of the Chambord royal castle) have been well documented by archeological and historical studies.

International audience; Les variations climatiques naturelles et les perturbations liées aux activités anthropiques ont une grande influence, à différentes échelles spatiotemporelles, sur la dynamique hydrosédimentaire et la morphologie des cours d'eau. De nombreuses études précisent notamment les modalités de modifications de régime fluviatile postérieurs au dernier maximum glaciaire. Si la majeure partie de l'évolution holocène des cours d'eau semble liée aux fluctuations climatiques, il est fondamental, pour la période historique (i.e., post-Age du Bronze), de prendre en compte et d'estimer l'influence anthropique, via la mise en culture, sur cette évolution. Le bassin versant du Cosson, au sud de la Loire entre Orléans et Blois,draine les formations miocènes de la Sologne. Les terrasses alluviales du Cosson témoignent d'une histoire commune avec la Loire, avantindividualisation de ces cours d'eau au cours du Pléistocène moyen. Dans le Domaine National de Chambord, l'analyse de MNT issus d'une acquisition LIDAR aéroportée montre en surface de la plaine d'inondation du Cosson (canalisé depuis le 15ème siècle) la trace de multiples chenaux. Lors de cette étude, nous avons acquis des données de résistivité électrique et analysé des sondages carottés. Les dépôts alluviaux ont été contraints chronologiquement par datations au radiocarbone.Les premiers résultats permettent de distinguer pour la période Holocène deux séquences majeures : une séquence basale épaisse de plusieurs mètres d'alluvions grossières (sables grossiers et graviers dominants) et une séquence sommitale constituée d'environ 1m de matériel silto-argileux. Les âges obtenus situent la transition entre ces deux séquences entre 200 et 500 ap. J-C, alors qu'aucune modification majeure du régime fluviatile ne semble intervenir au début de l'Holocène. Des analyses fines des alluvions sont en cours : granulométrie, nature et degré de maturation de la matière organique, palynologie... Ces analyses permettront de caractériser les sources sédimentaires et contraindre les paléoenvironnements dans le bassin versant, afin de préciser l'influence respective des facteurs forçants climatiques et anthropiques sur la dynamique fluviatile, dans ce contexte de cours d'eau de plaine où l'activité humaine (antérieure et postérieure à la construction du château de Chambord) est bien documentée par les prospections archéologiques et les écrits historiques.

International audience; Soil erosion plays an important role in sediment and carbon storage within, and exports from, catchments. In cultivated landscapes, field borders can improve the temporary storage of eroded soil particles and associated carbon, by impeding lateral soil fluxes. These local soil accumulations can lead to the development of linear landforms (such as headlands and lynchets) which will keep evolving after field border removal. A recent study performed in a representative cultivated hillslope of the SW Parisian Basin showed that 39% of the area corresponds to landforms resulting from soil accumulation induced by former and present field borders. This study demonstrated that field borders influence greatly the landscape morphology, but also the spatial distribution of soil thickness, and locally the A-horizon thickness, which are essential parameters for the prediction of SOC stocks. This study aims at characterizing and quantifying the effect of field borders and their removal on medium term topsoil erosion and deposition rates in a cultivated hillslope of the SW Parisian Basin, consolidated in 1967. Here, we used the Cs-137 technique to assess recent patterns of soil redistribution. We measured the Cs-137 inventories of 68 soil cores sampled along transects covering the area and, more specifically, linear landforms identified along present and past field borders (i.e. lynchet and undulation landforms, respectively). Then, we used a spatially-distributed Cs-137 conversion model that simulates and discriminates soil redistribution induced by water and tillage erosion processes over the last fifty years. Finally, observations and model outputs were confronted. Our results show that tillage erosion dominate the soil redistribution in the study area for the 1954-2009 period and generated about 95% (i.e. 4.50 Mg.ha −1 .yr −1) of the total gross erosion. Soil redistribution was largely affected by the presence of current and former field borders, where hotspots areas of deposition and erosion (>20 Mg.ha −1 .yr −1), respectively, were observed. Land consolidation contributed to the local acceleration of topsoil erosion through the conversion of storing areas into sediment generating areas. Though the general patterns of Cs-137 inventories in the area were correctly reproduced by the model, this latter performed weakly with a r2 of 0.20. Important discrepancies were associated with sampling points located along current field borders where data suggests that tillage erosion processes cannot be described as elsewhere, i.e. as a diffusive process. These specific processes implied here should be characterised and implemented into erosion models for simulating rates and patterns of topsoil redistribution in fragmented cultivated landscapes. In addition, the use of a DEM of the present-day morphology leads to the underestimation of soil erosion and storing within linear landforms which morphology seems to have greatly evolved since 1967. This study highlights the importance of present and former field borders on the patterns and intensities of topsoil erosion and deposition processes at landscape scale. This is of particular interest concerning the improvement of our knowledge on soil organic carbon patterns and on estimation of SOC stocks.

National audience; Les réseaux de bordures de parcelles structurent les paysages cultivés et exercent un rôle important sur la variabilité spatiale des processus d’érosion-dépôt de sols. Les bordures végétalisées (haies, bandes enherbées…) affectent les connectivités hydrologiques et sédimentologiques à travers les versants, et l’ensemble des bordures de parcelles fait obstacle aux transferts de sols induits par le labour. Ces phénomènes locaux d’érosion-dépôt entraînent le développement de figures morphologiques linéaires et décamétriques (par exemple des banquettes agricoles ou crêtes de labour) qui continuent d’évoluer après la disparition des bordures associées. Ceci se vérifie particulièrement en Europe de l’Ouest où la mécanisation et les politiques agricoles ont mené à la disparition massive de bordures de parcelles via de nombreuses campagnes de remembrements (années 1960-1990). Dans un contexte de changement climatique et d’évolution des pratiques agricoles, la compréhension de l’effet des structures paysagères et de leur évolution sur la redistribution des sols paraît essentielle pour l’avenir. Des informations sont en effet nécessaires afin d’envisager quelles configurations paysagères optimiseraient la conservation des sols. Le but de cette étude est donc d’évaluer l’effet des bordures de parcelles et de leur disparition sur la redistribution des sols au sein d’un versant cultivé.

International audience; Field borders induce landscape fragmentation and have an important role on the spatial variability of soil erosion and deposition processes. Vegetated borders affect the hydrological and sedimentological connectivity across hillslopes, and all types of field borders act as lines of zero-flux against tillage translocation. This leads to the development of linear anthropogenic landforms (e.g., ridges-and-furrows, headlands, and lynchets) which will keep evolving after field border removal. This is particularly the case in western Europe where agricultural policy and mechanisation led to the massive removal of field borders through numerous land consolidation schemes (1960-1990). The aim of this study is to assess the effect of field borders and their removal on soil redistribution in a cultivated hillslope of the SW Parisian Basin. Recent patterns of soil redistribution are derived from 137Cs inventories and analysed with respect to the different topographical settings, especially linear anthropogenic landforms. Then, erosion and deposition rates are calculated using a conversion model that integrates the dominant processes of soil redistribution over the last fifty years. Results show that areas submitted to intensive erosion or deposition are mainly concentrated on linear landforms induced by field borders. The use of the conversion model highlights that soil redistribution is strongly dominated by tillage-induced processes. Land consolidation enhanced soil redistribution through the conversion of depositional areas into sediment delivering areas. Erosion and deposition processes induced by field borders and their removal appear to be very important when compared to mean soil redistribution in the entire study area.

Owing to the automation of the measurements, geophysics is now commonly used to predict soil thickness over large areas in addition to punctual information given by soil augering. Among the various geophysical tools, the methods based on electrical resistivity measurements give good results providing that the bedrock is homogeneous. In this study we use electric and electromagnetic methods to map soil thickness over an area with heterogeneous bedrock. Geostatistical analysis allows eliminating the bedrock variability influence. The most efficient predictions are obtained when integrating topographic slope to the resistivity data., Avec l'automatisation des mesures, la géophysique est de plus en plus utilisée pour prédire l'épaisseur des sols sur de grandes surfaces, en complément des informations ponctuelles fournies par les sondages pédologiques. Parmi les différentes méthodes, les prospections basées sur la mesure de la résistivité électrique donnent de bons résultats lorsque le substrat est homogène. Dans cette étude nous testons l'utilisation des méthodes électriques et électromagnétiques pour cartographier l'épaisseur du sol sur substrat hétérogène. Un traitement géostatistique des données permet de s'affranchir de l'influence de la variabilité du substrat. Les meilleurs résultats sont obtenus en intégrant aux mesures de résistivité la pente topographique.

International audience; Soil apparent electrical resistivity, or its converse soil conductivity, is a parameter commonly used to predict soil properties, such as porosity, water content, particle size, clay content... It has also been used for soil thickness mapping, but the resulting data can be misinterpreted, due to interrelationships between soil resistivity and the physical and chemical properties of soils, which may be related to the bedrock lithology. Soil thickness mapping using resistivity measurements thus gives results only when the bedrock is electrically homogeneous and presents a high resistivity contrast related to soil. It therefore appears necessary to precisely characterise the bedrock resistivity variability before interpreting soil resistivity measurements. In this study, the relationships between surficial apparent resistivities at different depths of investigation and soil thickness-defined as the summation of organo-mineral and structural (A+B) horizons-were tested to predict soil thickness over large areas. The study site corresponds to a 100 ha cultivated hillslope located near the village of Seuilly (SW Parisian Basin, France). It covers 3 types of the Upper Cretaceous sedimentary formations: (a) Lower and Middle Turonian white chalk, (b) Upper Turonian yellow sandy limestone and (c) decarbonated yellow sandy limestone enriched in clay by deep weathering. The site shows a wide range of soil thicknesses (from 0.3 m to more than 2 m in lynchets) due to the fragmentation by field limit networks. The resistivity of the bedrock was measured using an electromagnetic survey with an EM31 conductivity meter (Slingram method), which gives a large investigation depth (about 5m), making this instrument quite insensitive to soil variability. 35 electrical soundings were also performed along a transect covering 800 m from top to bottom of the hillslope, allowing the establishment of a 2D resistivity cross section of the bedrock. The resistivity of the soil was measured using an ARP (Automatic Resistivity Profiling) survey at 3 different depths of investigation (0.5, 1 and 2 m). Inside the study site, a 16 ha test zone representative of the whole site was chosen for the establishment of the soil thickness / resistivity correlations. Soil thickness was measured at 686 points thanks to manual augering. Soil resistivity was also measured directly on 241 soil augerings using a Wenner array and the results were compared to the ARP interpolated data. Finally, soil properties (particle size, organic carbon and carbonate content) were analysed at 248 points and compared to soil resistivity to assess the relationships between soil resistivity and each soil property. The electromagnetic survey results and the electrical soundings show that the 3 bedrock types are characterized by different resisitivity values. The Upper Turonian yellow sandy limestone presents the highest resistivity (50 to 100 ohm.m). In this area, soil thickness / resistivity correlation is good (R2=0.66), allowing high resolution digital soil thickness mapping from ARP measurements. The Lower and Middle Turonian white chalk presents lower resistivity values (20 to 50 ohm.m) and is electrically heterogeneous, making the soil thickness / resistivity correlation insufficient (R2=0.3) to map soil thickness correctly. However, the ARP mapping gives precise information on bedrock heterogeneities. Finally, the decarbonated yellow sandy limestone is characterized by low resistivity values (< 20 ohm.m) similar to soil resistivity, making impossible soil thickness prediction. In this area the ARP results seem more correlated with the soil particle size. These results shows the importance of characterising precisely the electrical response of the bedrock (variability and resistivity contrast related to soil) before using soil apparent resistivity as a tool for digital soil thickness mapping, and more generally for soil properties mapping.

International audience; The quality of soils strongly depends on their thickness resulting from equilibrium between soil development and erosion. Soils water and organic carbon contents are examples of properties which are sensitive to thickness variation, and then has a direct impact on crop quality and yields in cultivated areas. So, the assessment of soil thickness variability in landscapes appears of major interest to quantify soil quality indicators and for soil mapping. This work aims to characterize linear morphological features of decametric width, identified within cultivated hillslopes, and their connection with soil thickness variability. The 16 ha study area is located on the southwestern Parisian Basin (cretaceous chalks) and has a general rolling topography. Two types of linear morphological features were observed within the area: the well-known lynchets and what we called “undulations”. Accurate elevation records were carried out in parallel to an intensive campaign of soil thickness measures within the features andover the whole study area. An accurate DEM and a soil thickness map were computed. An expert method based on the observation of slope gradient variation at each sample neighbourhood was then developped in order to classify the sampling locations into three classes (lynchets, undulations and areas outside features influence). The spatial variability of soil thickness appears more important within lynchets and undulations than within others areas, especially towards the main slope direction. These features show local soil thickenings of different intensity: meanthickness values are about 1.10 m, 0.62 m and 0.45 m within lynchets, undulations and other areas respectively. Moreover, the shape of soil thickenings within a lynchet and within an undulation has different specific characteristics. We used then the classification tree (CT) algorithm to assess whether the features can be discriminated through their respective landform attributes and soil thickness (CTsoil), and through their respective landform attributes only (CTtopo), e.g. slope, curvature, profile curvature and planform curvature. The CT approach has the advantagesto be a non-parametric analysis method and to allow the establishment of predictive classification models to map the different types of features all over the study area. The validation results of CTsoil and CTtopo models showed respectively an 83 % and 67 % model efficiency. The two models performed well for lynchets mapping. These features present statistical differences with the two other classes for all the predictor variables. The most encountered errors in models application are confusions between areas outside features influence and undulations. These confusions tended to increase when soil thickness is not involved as predictor variable because undulations are statistically distinguishable from areas without features influence by curvature, profile curvature and soil thickness only.The mapping all over the study area of the features lynchets and undulations reveals that undulations are associated to former field limits that disappeared mainly during the last important land consolidation, in comparison to lynchets which are associated to present field limits. 39 % of the study area are constitued by lynchets and undulations which then constitute morphological indicators of human-induced soil accumulations. The method developed here confers new possibilities regarding soil thickness and dependant properties quantification and mapping.

International audience

International audience

International audience; Water and tillage erosion, combined with the effect of successive regrouping of the land campaigns, have sensibly modified the morphology of cultivated hillslopes. Nowadays, we can still observe over cultivated landscapes various anthropogenic structures (e.g. lynchets) that correspond to former and/or present field limits. The aim of this work is to spatially characterise the geometry of these various structures, and to assess their relation with soil variability. The 10 ha studied site corresponds to an hillslope located in a small calcareous watershed near Tours in the Parisian Basin (France). Soils are mainly Cambisols (calcaric), Epileptic cambisols (calcaric) and Colluvic cambisols. This watershed is characterised by the presence of many soil accumulation structures and has been submitted to an important regrouping of the lands since the 1960's. The existing structures were accurately located and defined through a topographical analysis (slope, profile curvature...), and then related to past and present field limits using aerial photographies and cadastral maps. Two main morphological structures, deeply marked in the topography, are identified. The most remarkable type corresponds to well developed lynchets located at the lower part of field limits. Secondary structures, less marked than lynchets, are linked to field limits which existed at least since 1836, but that disappeared from the 1960's. These secondary structures look like longitudinal bulges placed perpendicularly to the direction of the main slope. A soil survey on these two geomorphic structure types has been performed in order to determine their specific geometry. Lynchets are characterised by a large increase in the thickness of soils: from 35 cm at 24 m upslope to more than 120 cm on its top. But, in some cases, the elevation study shows a topographical discontinuity between the top of the accumulation and the field or road below, discontinuity that is more important than the maximum soil thickness observed in the lynchet. This implies that the substrate, which is mostly homogeneous, has been largely excavated below these limits certainly due to repeated tillage operations. Concerning the secondary structures, soil thickness increases slightly from 35 cm at 16 m up- and downslope the former field limit to 70 cm at the maximum of the bulges convexity. However, the slope morphology seems to show larger soil accumulation considering a regular substrate morphology along the hillslope profile. Here too, by combining soil thickness and surface topography, we show that the substrate has certainly been strongly remodelled at these former field limits by tillage erosion. The spatial variability of the various soils types is closely linked to the sequence of structures oriented perpendicularly to the direction of the main slope. Although the current topography is clearly marked by various structures linked to former and present field limits, it appears that soil thickness is not the only factor explaining these large variations in the slope morphology: long-term agricultural practices, certainly tillage, "shape" the substrate as well. It is thus important to take into account these substrate excavations for sediment budget studies. The use of tracers such as 137Cs will allow to understand the intensity of these morphological changes at the slope scale within the last decades.

International audience

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International audience; Ce travail examine : (i) comment les corrélations spatiales entre une variable du sol et les attributs topographiques peuvent être améliorées en mettant l'accent sur les relations entre ces deux types d'information à des échelles spatiales spécifiques ; (ii) l'effet des sources des attributs topographiques utilisées comme variables explicatives pour la modélisation de la variable considérée ; (iii) la possibilité de l'extrapolation du modèle pour la cartographie au-delà de la zone où il a été établi.

National audience; Le Lac Afourgagh, situé en contexte karstique dans le Moyen Atlas marocain, présente un enregistrement sédimentaire holocène qui constitue un exemple caractéristique d'une sédimentation carbonatée contrôlée par l'environnement. Ce lac peu profond, eutrophe, est alimenté par une nappe relativement enrichie en Mg lequel résulte du lessivage du substratum dolomitique jurassique. Etant épisodiquement confiné, il est caractérisé par des eaux alcalines qui présentent un rapport Mg/Ca variable mais élevé. L'extension maximale des rives du lac coïncide avec un niveau de stabilisation tributaire de l'exutoire. Reposant sur un éventail alluvial essentiellement composé de silt, les sédiments de la terrasse étudiée se sont déposés au cours des derniers 2500 ans. Ces sédiments comprennent quatre principaux faciès caractérisant différents milieux depuis le littoral vers les eaux plus profondes : (1) une croûte littorale, (2) des paléosols, (3) des silts palustres et (4) des tufs à charophytes. Dans les parties distales les tufs à charophytes présentent une lamination régulière, dont la rythmicité est corrélée au développement de microstromatolithes sur les thalles des algues. La composition minéralogique des carbonates est liée au faciès : alors que les tufs à charophytes sont caractérisés par une teneur élevée en aragonite à côté de la calcite, la croûte littorale est principalement formée de magnésite. Ces particularités sont liées à l'évolution de la chimie des eaux sous l'influence de la prolifération des charophytes durant les étés secs. La précipitation bioinduite des carbonates sur les thalles algaires entraîne progressivement un accroissement dans le rapport Mg/Ca des eaux lacustres, alors que l'évaporation des eaux de la nappe remontée par capillarité sur le rivage engendre la précipitation d'un précurseur de la magnésite, avec pour conséquence la formation de magnésite durant la diagenèse précoce. Ce processus est manifeste pour deux épisodes : la période chaude romaine pour partie, et le début de la période froide médiévale. La minéralogie des carbonates des différentes séquences de dépôt illustre ainsi les fluctuations du niveau lacustre et de la chimie des eaux sous contrôle climatique.

International audience; Geophysics is commonly used to predict soil parameters (e.g. soil thickness, soil constituents and properties) over large areas in addition to punctual information given by soil augering. Among the various geophysical tools, the methods based on electrical resistivity measurements give good results providing that the bedrock is homogeneous. However, in the case of heterogeneous bedrock, the resulting data can be misinterpreted, due to interrelationships between soil resistivity and some soil properties, which may be related to the bedrock lithology. In this study we use electric and electromagnetic methods to (1) map soil thickness over an area with heterogeneous bedrock and (2) assess the relationships between soil resistivity and some constituents of soils (contents in silt, sand, organic carbon and CaCO3). The study site corresponds to a 100 ha cultivated hillslope located in the SW Parisian Basin (France) and covering 3 types of the Upper Cretaceous sedimentary formations characterised by contrasted resistivity values. The resistivity of the bedrock was measured using an electromagnetic prospection with an EM31 conductivity meter and 44 electrical soundings, whereas soil resistivity was precisely known from an ARP (Automatic Resistivity Profiling) survey at 3 different depths of investigation. Soil thickness was measured at 650 points by manual augering, and soil analyses were performed at 248 points. Geostatistical analyses were tested to assess the relative influence of the different soil properties on the resistivity data for each bedrock type.

International audience; Among the various existing soil erosion models, the Landsoil model has recently been developed to spatially simulate both water and tillage erosion at the event scale. The aim is to apply this model to an intensively farmed area that has been submitted to an important land consolidation in the 1960s, in order to assess the evolution of past and still existing lynchets. The 240 ha study site is located in Seuilly, in the south western Parisian basin (France). The area is mostly under crop cultivation. Monthly calendars of soil surface characteristics have been established for each main crop, and are for the first time adapted to account for the major soil variations observed in the study area. A detailed DEM has been computed using LIDAR data. These data had to be carefully filtered to remove any non-perennial microtopographic feature. 6 mn rainfall data were used to characterize the rainfall events. As the site is not instrumented, the effect of infiltration capacity and suspended sediment load values is approximated. Results show a very contrasted soil redistribution in the study area, mainly due to the different soil types, thus leading to a contrasted morphological evolution of the landscape. The upper, predominantly flat part of the study site with silty soils is mainly under the influence of water erosion, whereas the lower, calcareous and clayey part of the site is mostly under the influence of tillage redistribution processes, leading a progressive flattening of the former lynchets, whereas current lynchets are still developing.

International audience; Soils and landscapes evolve simultaneously. Soil evolution is controlled by redistribution and transformation processes influenced by topographic and climatic parameters, with also a major contribution of management strategies. The perennial landscape features have a strong influence on soil spatial distribution (geometry) and soil genesis. Building landscapes which enhance soil resilience to degradation processes and increase soil services appears as a promising way to adapt to forthcoming climatic and land use evolutions. The presentation aims to synthetize major results from a research program nicknamed Landsoil which focused on the evolution of agricultural soils over medium time scales (decades to centuries) in relation to changing conditions of land use and climate. Precise study of the soil 3D organization in three contrasted landscapes (Brittany, Touraine, Languedoc-Roussillon) enabled to link soil redistribution in space to landscape components (field geometry, hedges or ditches network) and their past evolution. A dynamic and high resolution spatial modeling approach was developed coupling erosion processes and soil organic matter evolution and was calibrated over past evolution using dating techniques (Cs137, C14, OSL). The resulting Landsoil model was afterwards applied in a prospective manner under different scenarios of land use and climate change over the 21th century. Indicators of soil vulnerability and soil resilience were defined and tested by the comparison of several prospective scenarios applied on a same landscape and by comparison of the contrasted landscapes.

International audience; Ce travail avait pour objectif de montrer comment la prédiction spatiale peut être améliorée en capitalisant sur la meilleure corrélation entre les variables à des échelles spatiales spécifiques. L'objectif de ce travail a été examiné à travers l'estimation spatiale de l'épaisseur du sol (ES), ponctuellement mesurée sur 12ha au Nord-est de Seuilly (Touraine) en utilisant comme dérives externes dans les équations de krigeage des mesures de résistivité électrique des sols (ARP) et de pente. Les deux dérives externes ont été exhaustivement mesurées sur la zone d'étude. L'idée de base est de remplacer les valeurs de l'ARP et de la pente dans les équations de krigeage par les valeurs des composantes spatiales qui sont les plus fortement corrélés avec l'épaisseur du sol.

National audience

International audience; Grâce à l’automatisation des mesures, les méthodes géo-électriques sont de plus en plus utilisées pour aider à la cartographie des sols sur de vastes surfaces. En effet, ces méthodes permettent une mesure rapide, depuis la surface du sol, de la résistivité électrique apparente du sol, ou de son inverse, la conductivité électrique. La résistivité peut renseigner sur certains paramètres ou propriétés du sol, qui peuvent être variables dans le temps, comme la teneur en eau, ou bien pérennes, comme la composition (ex : teneur en argile), la granularité, ou encore l’épaisseur des différents horizons (Carroll et Oliver, 2005 ; Chaplot et al., 2010). La résistivité dépendant de nombreux facteurs, il apparaît alors difficile de mettre directement en relation les valeurs de résistivité apparente avec un paramètre du sol en particulier sans un traitement statistique nécessitant de nombreux points d’échantillonnage et d’analyse de sol. Dans cette étude nous testons les relations qui existent entre la résistivité électrique de surface et plusieurs paramètres du sol (dont l’épaisseur) sur un versant de 16 ha situé à Seuilly (SW du Bassin Parisien) présentant un substrat sédimentaire hétérogène. L’objectif est d’analyser, pour l’ensemble du site d’une part, et pour chaque type de substrat d’autre part, quels sont les facteurs influençant le plus la résistivité électrique du sol.

National audience

International audience; Minimum rates of solid (SSY) and dissolved (DSY) sediment yield (SY) were evaluated in t/km2 per yr from sediments stored in the Sarliève palaeolake (French Massif Central) for seven phases of the Lateglacial and Holocene up to the seventeenth century. The catchment (29 km2), mainly formed of limestones and marls, is located in an area rich in archaeological sites in the Massif Central. The respective impacts of human activities and climate on SY were compared by quantification of human settlements through archaeological survey and palynological data. During the Lateglacial and early Holocene up to about 7500 yr cal. BP, variations in SSY and DSY rates were mainly related to climate change with higher rates during colder periods (Younger Dryas and Preboreal) and lower rates during warmer periods (Bölling-Alleröd and Boreal). However, CF1 tephra fallout induced a sharp increase in SY during the Alleröd. During the middle and late Holocene after 7500 yr cal. BP, SSY and DSY greatly increased (by factors of 6.5 and 2.8, respectively), particularly during the Final Neolithic at about 5300 yr cal. BP when the climate became cooler and more humid. After this date, at least 75% of the SSY increase and more than 90% of the DSY increase resulted from human activities, but SSY rates showed little variation during Protohistoric and Historic Times up to the seventeenth century. SSY and DSY rates and DSY/SSY ratio indicate that catchment soils began to form during the Lateglacial and Preboreal, thickened considerably during the Boreal and Atlantic, finally thinning (rejuvenation) mainly as the result of human-induced erosion during the sub-Boreal and sub-Atlantic. Increased mechanical erosion during the late Holocene also induced an increase in chemical erosion.; Les teneurs minimales en sédiments solides (SSY) et dissous (DSY) (SY) ont été évaluées en t / km2 par an à partir des sédiments stockés dans le paléolac de Sarliève (Massif Central français) pour sept phases du Tardiglaciaire et de l'Holocène jusqu'au XVIIe siècle. Le bassin versant (29 km2), principalement formé de calcaires et de marnes, est situé dans une zone riche en sites archéologiques. Les impacts respectifs des activités humaines et du climat sur SY ont été comparés par la quantification des établissements humains à travers des relevés archéologiques et des données palynologiques. Au cours du Tardiglaciaire et du début de l'Holocène jusqu'à environ 7500 ans cal. BP, les variations des taux SSY et DSY étaient principalement liées au changement climatique avec des taux plus élevés pendant les périodes plus froides (Dryas récent et Préboréal) et des taux plus bas durant les périodes plus chaudes (Bölling-Alleröd et Boréal). Cependant, les retombées de CF1 tephra ont induit une forte augmentation de SY durant l'Alleröd. Au milieu et à la fin de l'Holocène après 7500 ans cal. BP, SSY et DSY ont fortement augmenté (par des facteurs de 6,5 et 2,8, respectivement), en particulier pendant le Néolithique final à environ 5300 ans. BP quand le climat est devenu plus frais et plus humide. Après cette date, au moins 75% des SSY augmentent et plus de 90% de l'augmentation des DSY résultent des activités humaines, mais les taux des SSY montrent peu de variation au cours de la Protohistoire et des Temps historiques jusqu'au XVIIe siècle. Les taux SSY et DSY et le ratio DSY / SSY indiquent que les sols des bassins hydrographiques ont commencé à se former durant le Tardiglaciaire et le Préboréal, s'épaississant considérablement durant la période boréale et atlantique, s'amincissant finalement du fait de l'érosion causée par l'activité humaine sub-boréale et sub-atlantique (rajeunissement). L'érosion mécanique accrue à la fin de l'Holocène a également provoqué une augmentation de l'érosion chimique.

International audience; This work is part of the French CNRS ECLIPSE program « Impact anthropique sur l'érosion des sols et la sédimentation dans les zones humides associées durant l'Holocène ». It aims to reconstitute the evolution of human impact on soil erosion at various periods via the study of Holocene sedimentary archives. In this framework the Choisille catchment (288 km²; elevation: 50 - 200 m), tributary of the River Loire near Tours (France), has been the subject of an interdisciplinary study (sedimentology, geophysics, archaeology, palynology). 3 areas are investigated: a downstream stretch, a silicated sub-catchment area and a carbonated sub-catchment area. In the downstream stretch, located near ancient populated areas, drillings were performed along cross sections through valley floor alluviums. They show that a more or less organic clayey silty sedimentation started at the beginning of the Holocene. The sedimentation rates strongly increased at the beginning of the Subbatlantic (Bronze Age), simultaneously with the anthropogenic pressure advent (on set of agriculture), as shown by archaeological and palynological evidences (agricultural settlements, massive loggings on slopes, stockbreeding on valley-floor grasslands). In the silicated sub-catchment area, located upstream, drillings have shown that clayey silty sedimentation began at the end of the Roman Period, continued during the Early Middle Ages and increased during the High Middle Ages. Spatial archaeological prospecting has revealed a faint anthropogenic presence at the Roman Period, then a decline of population until the High Middle Ages, characterised by an agricultural revival. Palynological analyses have shown that, in this area, grasslands were dominant since the Early Middle Ages, with an increase in cereal cultures at the beginning of the High Middle Ages. In the carbonated sub-catchment area, drillings have shown that the more or less organic clayey silty sedimentation has begun during the Bronze Age. Sedimentation rates have increased during the Modern era and the Contemporary history. The high fine sediment storage appeared and evolved differently, depending on the considered period and catchment valley area, due to variation of soil erosion. The difference between a fine, early and regularly increasing sedimentation in the downstream site and the later, intense and non-univocal sedimentation in the sub-catchments doesn't seem to be strictly resulting from natural factors. This idea and the palaeoenvironnemental dataset show that the fine sedimentation basically results from an anthropogenic impact, notably in the sub-catchments. Therefore soil exploitation by humans seems to be the main sedimentary production factor. This work mainly shows that anthropogenic impact (age of appearance, intensity) highly varies spatially, even into a little catchment. This variability would be led by the agricultural potential of the considered catchment valley area.

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International audience; This paper presents the results of a geophysical survey which was performed in the Sarliève marsh, located in the north of the Massif Central (France). The studied area corresponds to an ancient lake filled with 6 m of Lategla-cial to historic sediments. The numerous cores and excavations previously performed in basin areas show the presence of thick fine grained and carbonated lake deposits. The mineralogical study of these deposits allows us to distinguish two main types of formations, which differ in their composition and origin. In the lower part of the sedimentary filling, the SDLF formation is generally characterized by abundant chalky and dolomitic layers, probably formed by chemical precipitation in an endoreic environment. It was found only in the distal (or depocenter) parts of the ancient lake. The upper part of the filling corresponds to calcitic clayey silty sediments (CF), mainly of detrital origin. They can be found both in the distal and proximal parts of the ancient lake. In order to precise the geometry of the marsh sedimentary filling and to discriminate between these two fine grained sediment types (SDLF and CF), an electromagnetic survey was carried out using an EM31 conductivity meter. It was completed by 16 electrical soundings performed within the marsh. The electromagnetic survey enables us to draw a large-scale conductivity map, which highlights the presence of a highly conductive zone (HCZ) in the central part of the marsh, more pronounced in the south. The HCZ is bordered by more resistive and heterogeneous fields, where conductive structures are visible. We show that there is no clear correlation between the measured conductivity and the total lacustrine sediment thickness. However, the HCZ is related to the distal parts of the ancient lake, locally identified from borehole data. In the corresponding sedimentary filling, the SDLF formation appears as a very low resistive layer (about 1.2 ⍀.m) on the electrical soundings. It explains the high conductivity of the distal lacustrine sediments. We finally combine the cores and the geophysical data to estimate the spatial extent and the volume of the SDLF formation of the whole marsh. This study shows that geophysics can contribute to distinguish, in a globally homogeneous fine grained sedimentary filling (carbonated clayey silts), lacustrine sediments formed in very different contexts, one as chemical precipitation, the other of detrital origin; Dans cette note, nous présentons les résultats d'une prospection géophysique réalisée dans le marais de Sar-liève, situé au sud de Clermont-Ferrand, dans le Massif central. La zone étudiée correspond à un ancien lac d'une super-ficie d'environ 5 km 2 , qui a été drainé et asséché au cours du XVII e siècle. Les nombreux forages carottés réalisés dans le marais ont montré la présence d'un remplissage sédimentaire épais (maximum 6 m) allant du Tardiglaciaire à l'époque historique. Des zones de deltas et de bassins lacustres ont été identifiées. Dans les zones de bassins lacustres, les sédiments sont argilo-silteux et carbonatés, et d'apparence assez homogène. Deux grands types de formations sédi-mentaires y ont été distingués d'après leur composition minéralogique et leur origine. Dans la partie inférieure du rem-plissage, la formation SDLF est caractérisée par des sédiments dolomitiques surmontés d'une épaisse couche constituée de séquences de lamines carbonatées. Ces dépôts se seraient formés par précipitation chimique en milieu lacustre peu profond et confiné (milieu endoréique). Cette formation n'a été repérée que dans la partie centrale du marais, correspon-dant aux zones de bassins les plus profondes de l'ancien lac (faciès distaux). Les dépôts plus superficiels (formation CF) sont principalement de nature détritique argilo-silteuse et sont riches en calcite. Ils traduisent un milieu exoréique. Ils se retrouvent dans presque toute l'ancienne zone lacustre, aussi bien en position distale que proximale (deltas et bassins). Afin de connaître la géométrie des dépôts dans le marais et de distinguer SDLF de CF dans les zones de bassin, une prospection géophysique électromagnétique a été effectuée au moyen d'un conductivimètre EM31 (méthode slingram). Seize sondages électriques ont aussi été réalisés sur différents secteurs du marais. Les mesures électromagnétiques permettent d'établir une carte de conductivité apparente du site dans ses parties nord et sud. Nous mettons ainsi en évidence la présence d'une zone fortement conductrice dans l'axe central N-S du ma-rais, plus prononcée au sud. Cette zone est entourée de terrains plus résistants et hétérogènes, où des poches conductri-ces sont repérées. Aucun paléochenal surcreusé dans le substrat n'est mis en évidence. Nous montrons qu'il n'existe pas de corrélation nette entre la conductivité apparente et l'épaisseur totale de sédiments lacustres. Par contre, l'axe central o 6 1. UPRES EA 2100, Lab. GéEAC, Univ. François Rabelais, Parc de Grandmont, 37200 Tours, France. Florent.hinschberger@univ-tours.fr 2. UMR 7619 Sisyphe, Univ. Pierre et Marie Curie (Paris 6), case 105, 4 place Jussieu, 75252 Paris cedex 05, France. Guerin@ccr.jussieu.fr Manuscrit déposé le 19 décembre 2005 ; accepté après révision le 29 mai 2006. très conducteur du marais correspond aux zones distales de l'ancien lac, reconnues ponctuellement grâce aux carottages. Dans le remplissage des zones distales, la formation SDLF se signale sur les sondages électriques par une très faible ré-sistivité (environ 1,2 ⍀.m), expliquant la forte conductivité apparente des faciès distaux. Nous utilisons enfin les don-nées géophysiques, en complément des informations fournies par les carottages, pour évaluer l'étendue et le volume de la formation SDLF sur l'ensemble du marais. Cette étude montre finalement qu'il est possible d'utiliser la géophysique pour différencier entre eux, au sein d'un remplissage sédimentaire à faciès peu différenciés (silts argileux carbonatés), des dépôts lacustres formés dans des conditions très différentes, par précipitation chimique dominante d'une part, et par sédimentation détritique dominante d'autre part.

Quantification in grams per metres squared per year of the sediment accumulation in a flood plain (‘marsh’) located in the southwestern Parisian basin showed that there is no close relationship between the accumulation of organic matter (OM) and mineral matter (MM) during the late Holocene, and provided an accurate view of the distinct yield and storage conditions of both sediment components. Endogenic OM accumulation in peaty sediments is not related to the climate but to felling of the alder forest and its substitution by Cyperaceae and paludal taxa in the marsh (Iron Age and Middle Ages). MM accumulation expresses mainly the sediment yield on the slopes, determined by landuse. During an initial phase (from the late Neolithic to the early Middle Ages), land-use change from crop cultivation to pastureland, possibly related to climate deterioration, led to a decrease in the sediment yield. During a second phase, since the early Middle Ages, the considerable development of crop cultivation over pasturing, even during periods of climate deterioration (such as the ‘Little Ice Age’), led to a sharp increase in sediment yield. However, although sediment yield was high, the hydrodynamics in the fen did not favour particle retention. Thus, since the Neolithic, yield and storage of OM and MM sediment have been marked by human activities, initially with high climatic stress, but since the Middle Ages without significant climatic stress.

National audience; An electromagnetic profiling survey (EM31) completed with vertical electric soundings has been applied in the study of the spatial distribution of near surface deposits along a valley side of about 10 ha. The study area is located in a metamorphic hilly region south of La Châtre, in the northern part of Massif Central (France). We explain the observed high conductivity contrasts by the interaction of (1) the bedrock lithology (an E-W conductive zone lies parallel and just above the gneiss-amphibolites boundary) and (2) the topography (the convex areas show higher resistivities). These two factors may have influenced the degree of weathering of the bedrock and, consequently, the nature and distribution of alterites.

International audience; Accumulation of organic matter in fens of fluvial valleys is often related to a low terrigenous matter delivery and to palaeoenvironmental conditions inducing low mechanical erosion. These assumptions come from the interpretation of contents in organic (MO) and mineral (MM) matters in sediments, expressed in percents, and then exactly anticorrelated. Calculation of mass accumulation rates of MO (TaMO) and MM (TaMM), expressed in g m−2 yr−1, shows that TaMO and TaMM generally are not anticorrelated and that high values of TaMO and TaMM could appear simultaneously. That expression of MO and MM accumulation makes it possible to precise the climatic and human impact on sedimentation.

The Taligny marsh is located downstream in the Négron River catchment (southwestern Parisian basin). 33 boreholes performed in the marsh show that it contains a maximum thickness of 5.80 m sediments. Their composition varies from a sandy-gravely end in the lower part of the pile to a peaty end in the upper part, the silty phase being the most abundant in the whole sediment store. The sedimentation has been continuous since at least 4360 yrs cal. BC. The accumulation rates of organic matter (TaOM) on one hand and of mineral matter (TaMM) on the other were calculated in g.m-2.yr-1 from cores located in the middle of the marsh (47° 07'812 N; 0° 11'494 E). These values have been correlated with that of the pollen and malacofauna analysis, and to the palaeoclimate data (temperature: T and precipitation: P). There is no significant negative correlation between TaOM and TaMM during the Upper Holocene. For exemple, high values of TaMM can be synchronous with high values of TaOM, particularly during the Subatlantic. OM accumulation does not depend on the climate variations expressed by P. The high values of TaOM are clearly related to Alnus forest clearance and to its substitution by Cyperaceae and paludicol taxa in the marsh (Iron and Middle Ages). Cyperaceae and paludicol taxa development results in an increase of OM supply on the soil surface and the rising of the water table, confirmed by the malacofauna data, due to the evapotranspiration decrease. MM accumulation, mainly related to the sediment yield by the catchment, does not show any one-to-one significant relation neither to climatic parameters (P or T) nor to the vegetation assemblages (forest vs. meadows or cultivation). TaMM was higher during period of cultivation development (dry and warm Neolithic, and humid cold or cool second part of the Middle Age and Modern times). TaMM was lower and little varying during the other periods (Bronze, Iron, Gallo-roman and first part of the Middle Age) due to the antagonistic effects or the little variations of the climatic parameters and the landuse (forest vs. meadows).The hydrodynamic during periods of fen development (mainly during the Middle Age) seems to have been little favorable to the retention of suspended matter in the marsh, even if the sediment yield was high in the catchment.