Long term evolution and internal architecture of a high-energy banner ridge from seismic survey of Banc du Four (Western Brittany, France)

International audienceThe recent completion of a coupled seismic and swath bathymetric survey, conducted across the sand ridge system of the Banc du Four located on the Atlantic continental shelf of Brittany (Mer d’Iroise, France), provided new data for the study of the long term evolution of deep tidal sand ridges. Five seismic units are distinguished within the ridge, separated by pronounced major bounding surfaces. The basal unit is interpreted to be shoreface deposits forming the core of the ridge. It is overlaid by a succession of marine sand dunes fields forming the upper units. Sandwave climbing, which combines progradation and accretion, is the major process controlling the growth of the ridge. The elevation of the preserved dune foresets reaches values of about 20–30 m within the ridge. The foresets indicate a combination of giant dunes characterized by numerous steep (up to 20°) clinoforms corresponding to a high-energy depositional environment. Moreover, the presence of scour pits linked to the 3D geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines. All of the radiocarbon ages of the biogenic surficial deposits of the Banc du Four range from 10,036 to 2748 cal years B.P. and suggest the Banc du Four has grown during the last sea-level rise. The apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non-conservation of recent deposits. In contrast, the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge. Moreover, the long-term evolution of the ridge appears strongly controlled by the morphology of the igneous basement. The multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy, consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement.Therefore, the Banc du Four should be thought of as a representative example of a large-scale high-energy banner bank

Morphological and internal variability of sand banks on a macro tidal dispersive shelf

We investigated the morphological and architectural variability of four sandbanks, from the same area, at intermediate depths (from 50 to 120m) along the continental shelf of Western Brittany (France) in the Iroise Sea, a dispersive shelf exposed to large storms. In order to identify the parameters controlling the variability of sandbanks morphologies and architectures, we analyzed geophysical data (seismic, reflectivity and bathymetry) from several oceanographic cruises. Preliminary results reveal internal architectures composed of 4 to 5 seismic units for each bank. They are interpreted as controlled by the stepwise flooding of the Armorican basement shelf during the last transgression. This igneous basement that outcrops on the seafloor up to 100m deep has constrained local tidal circulation and swell exposure during the progressive flooding of the shelf, which explains the very contrasting morphologies and internal architectures of these coeval banks on the Iroise Sea shelf

GIANT DUNE MORPHOLOGIES AND DYNAMICS IN A DEEP CONTINENTAL SHELF ENVIRONMENT: EXAMPLE OF THE BANC DU FOUR (WESTERN BRITTANY, FRANCE)

The aim of this paper is to improve the knowledge of spatio-temporal variability of very large to giant dunes in deep tide dominated environment. Their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations. This motivated the realisation of three recent swath bathymetry surveys across the wide dune field of the Banc du Four located offshore the western Brittany. The Banc du Four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2D to 3D shapes and reaching the largest sizes of such sedimentary structure ever described (wavelength over 1000 m and height over 30 m). The analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters. Our results show a good height-spacing power law correlation but do not strike the usual statistical regression observed in previous studies. The steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime

De l’usage de certaines pêcheries à l’âge du Bronze ancien dans l’archipel de Molène

International audienceThe abundant ichthyofauna on the archaeological site of Beg ar Loued reflects a high consumption of fish by people occupying the island of Molène during the Early Bronze Age. This raises the question of fishing techniques employed, especially from nearby stone fish traps, which are very numerous on the foreshores of Brittany. This study provides an inventory of these structures on the foreshore of the archipelago of Molène, based on aerial, pedestrian, underwater, and high-resolution bathymetric surveys. Functional stone fish traps during the Early Bronze Age were identified from their position in relation to former tide levels. A total of 36 (proven or likely) structures have been identified around the archipelago. Only 10 of them were functional at the Early Bronze Age. Other traps are located at a lower depth, indicating an earlier use, probably from the early Neolithic.La présence d’une abondante ichtyofaune sur le site de Beg ar Loued témoigne d’une forte consommation de poissons par les populations ayant occupé l’île de Molène à l’âge du Bronze ancien, soulevant la question des techniques de pêche employées et la présence éventuelle de barrages de pêcherie à proximité. Cette étude dresse l’inventaire de ce type de structure sur les estrans de l’archipel, en s’appuyant sur une série de prospections aériennes, pédestres et subaquatiques, ainsi que sur un ensemble de relevés bathymétriques à haute résolution. Les pêcheries fonctionnelles à l’âge du Bronze ancien ont été repérées en se basant sur leur position par rapport aux anciens niveaux de marée. Un total de 36 structures, avérées ou possibles, a été identifié autour de l’archipel. Seulement 10 d'entre elles étaient fonctionnelles au Bronze ancien. Les autres pièges sont situés à une altitude plus basse, indiquant une utilisation plus ancienne, probablement dès le début du Néolithique

Short-term morphodynamics of a submarine dune bank in a macrotidal environment: observations from the Creïzic bank (South Brittany, France)

International audienceMarine sand dunes and sand banks can exhibit rapid morphological changes in coastal areas, which may posea hazard for navigation in shallow water or generally in areas near marine infrastructures. Understanding theirdynamics is still a scientific challenge, especially their migration processes and its consequences on the seabedtopography and on sediment transport. In this study, the morphodynamics of a submarine sandbank in a macrotidalenvironment is investigated at short time scale. The aim is to quantify the morphological changes and migrationof the bank and overlying dunes (primary and secondary sedimentary structures) in order to estimate sedimenttransport corresponding to bedform migration. The study site is the Creïzic submarine dune bank, located in theGulf of Morbihan (South Brittany, France). The Creïzic bank is about 1200 m long, 600 m wide with dune heightsfrom 0.1 m to 3.5 m. It is situated in water depths between 5 m and 20 m and is mainly composed of shellyand coarse sand. The main hydrodynamic forcing factor at play on this sand bank is a strong and unsteady tidalcurrent. During storm events, waves, mostly from fetch, also have an impact as the bank is shallow. The bankfeatures three main sedimentary structures: a central dune field at the middle of the bank and two dune fields oneither side of the bank. Former studies at yearly time scales have shown that the central area is highly mobilewith significant net migration of the dunes. To investigate the morphodynamics of the Creïzic bank, a 37-day fieldcampaign has been carried out in the fall of 2017. A high-resolution (50 cm grid size) bathymetric dataset has beenacquired using a mulitibeam echosounder (MBES), with repeated surveys at the time scales of a month (entirebank), a tidal cycle and hour scales (portions and patches on the bank). From this data, velocities and directionsof migration are estimated by computing differential Digital Terrain Models (DTM), and quantified with a spatialcross-correlation algorithm. The preliminary analysis of the data set confirms the high mobility of the primaryand secondary sedimentary structures at short time scales, with migration distances up to 1 m over one hour forbedforms with metric heights and wavelengths of the order of 30 m in the central zone, and reaching 50 cm overtwo hours for dunes with heights between 0.2 m and 0. 5 m and wavelengths between 2 m and 4 m on the south-eastside of the bank

Short-term morphodynamics of a submarine dune bank in a macrotidal environment: observations from the Creïzic bank (South Brittany, France)

International audienceMarine sand dunes and sand banks can exhibit rapid morphological changes in coastal areas, which may posea hazard for navigation in shallow water or generally in areas near marine infrastructures. Understanding theirdynamics is still a scientific challenge, especially their migration processes and its consequences on the seabedtopography and on sediment transport. In this study, the morphodynamics of a submarine sandbank in a macrotidalenvironment is investigated at short time scale. The aim is to quantify the morphological changes and migrationof the bank and overlying dunes (primary and secondary sedimentary structures) in order to estimate sedimenttransport corresponding to bedform migration. The study site is the Creïzic submarine dune bank, located in theGulf of Morbihan (South Brittany, France). The Creïzic bank is about 1200 m long, 600 m wide with dune heightsfrom 0.1 m to 3.5 m. It is situated in water depths between 5 m and 20 m and is mainly composed of shellyand coarse sand. The main hydrodynamic forcing factor at play on this sand bank is a strong and unsteady tidalcurrent. During storm events, waves, mostly from fetch, also have an impact as the bank is shallow. The bankfeatures three main sedimentary structures: a central dune field at the middle of the bank and two dune fields oneither side of the bank. Former studies at yearly time scales have shown that the central area is highly mobilewith significant net migration of the dunes. To investigate the morphodynamics of the Creïzic bank, a 37-day fieldcampaign has been carried out in the fall of 2017. A high-resolution (50 cm grid size) bathymetric dataset has beenacquired using a mulitibeam echosounder (MBES), with repeated surveys at the time scales of a month (entirebank), a tidal cycle and hour scales (portions and patches on the bank). From this data, velocities and directionsof migration are estimated by computing differential Digital Terrain Models (DTM), and quantified with a spatialcross-correlation algorithm. The preliminary analysis of the data set confirms the high mobility of the primaryand secondary sedimentary structures at short time scales, with migration distances up to 1 m over one hour forbedforms with metric heights and wavelengths of the order of 30 m in the central zone, and reaching 50 cm overtwo hours for dunes with heights between 0.2 m and 0. 5 m and wavelengths between 2 m and 4 m on the south-eastside of the bank

The Stone Tidal Fish Weirs of the Molene Archipelago, Iroise Sea, Brittany, Western France: a long-term tradition with early megalithic origins

This reports on a project that combined evidence gleaned from aerial photographs, place-names, interviews, topography, LIDAR data, and sonar bathymetry to locate stone tidal fish weirs in the Molene Archipelago. The results were verified by diver and pedestrian visual surveys. Models of Holocene sea-level change allowed a group of possibly Late Mesolithic-Early Neolithic weirs to be recognized, with a second group broadly dated to the later Neolithic-Early Bronze Age. The construction of these long megalithic structures is compared to the funerary monuments for which the Molene Archipelago is well known, in terms of technique, cost, and societal organization

De l’usage de certaines pêcheries à l’âge du Bronze ancien dans l’archipel de Molène

International audienceThe abundant ichthyofauna on the archaeological site of Beg ar Loued reflects a high consumption of fish by people occupying the island of Molène during the Early Bronze Age. This raises the question of fishing techniques employed, especially from nearby stone fish traps, which are very numerous on the foreshores of Brittany. This study provides an inventory of these structures on the foreshore of the archipelago of Molène, based on aerial, pedestrian, underwater, and high-resolution bathymetric surveys. Functional stone fish traps during the Early Bronze Age were identified from their position in relation to former tide levels. A total of 36 (proven or likely) structures have been identified around the archipelago. Only 10 of them were functional at the Early Bronze Age. Other traps are located at a lower depth, indicating an earlier use, probably from the early Neolithic.La présence d’une abondante ichtyofaune sur le site de Beg ar Loued témoigne d’une forte consommation de poissons par les populations ayant occupé l’île de Molène à l’âge du Bronze ancien, soulevant la question des techniques de pêche employées et la présence éventuelle de barrages de pêcherie à proximité. Cette étude dresse l’inventaire de ce type de structure sur les estrans de l’archipel, en s’appuyant sur une série de prospections aériennes, pédestres et subaquatiques, ainsi que sur un ensemble de relevés bathymétriques à haute résolution. Les pêcheries fonctionnelles à l’âge du Bronze ancien ont été repérées en se basant sur leur position par rapport aux anciens niveaux de marée. Un total de 36 structures, avérées ou possibles, a été identifié autour de l’archipel. Seulement 10 d'entre elles étaient fonctionnelles au Bronze ancien. Les autres pièges sont situés à une altitude plus basse, indiquant une utilisation plus ancienne, probablement dès le début du Néolithique

Dunes morphodynamics superimposed on sand banks in a shallow macro-tidal environment: the example of the Haut Fond de Ouessant

This study investigates the morphological parameters and migration rates of dune systems associated with a set of banner banks located at intermediate depths (50 m to 120 m) in the context of a highly dispersive shelf, offshore Brittany (Western France). The work mainly focuses on the Haut Fond d’Ouessant bank. New geophysical data (bathymetry, seismic, reflectivity) from Speedunes (2015) and Bankable (2019) cruises are analysed. We examine how the large range in basal surface depth over which the bank lies affect the dune morphodynamics. We also characterize sediment fluxes between these sediment structures, toward an overview map of sediment pathways across the Iroise shelf, in relation with the straits and the irregular shelf botto

Apport des données alti-bathymétriques (Litto3D et bathymétrie SMF) dans la détection et l'étude des vestiges archéologiques inter- et sub-tidaux : premiers résultats et perspectives le long des côtes du Finistère

International audienceLes rivages français de la Manche et de l’Atlantique comptent un nombre très important de barrages en pierre de type « pêcheries » destinés à capturer les poissons lors du jusant. Ces engins de pêches fixes ont été utilisés depuis le Mésolithique et jusqu’au Moyen-âge en Europe. Sur les seules côtes de Bretagne, plus de 700 vestiges archéologiques de ce type ont été inventoriés à partir de prises de vue aériennes de l’IGN et de quelques prospections pédestres localisées. Toutefois, un travail récent mené dans l’archipel de Molène a montré que cet inventaire sous-estimait le nombre réel de structures, certaines d’entre elles n’ayant pu être détectées en raison d’une forte couverture algale ou d’une profondeur importante. Contrairement aux pêcheries en bois de Basse-Normandie, ces barrages ne livrent pas d’éléments ligneux qui permettent une attribution chronologique par le biais de méthodes de datation conventionnelles. Pour contourner ce problème, Daire et Langouët (2011) ont proposé une approche permettant de déduire l’âge de chaque vestige en les positionnant par rapport aux anciens niveaux marins relatifs. Cette tentative s’est néanmoins heurtée à deux principaux verrous méthodologiques : (i) l’absence de données fiables sur les niveaux marins holocènes dans la région, (ii) un positionnement altitudinal approximatif des structures sur l’estran. Toutefois, des recherches ont été menées ces dernières années afin d’affiner les courbes de remontée du niveau marin relatif pour les 8000 dernières années dans l’ouest de la Bretagne. En outre, plusieurs sources de données alti-bathymétriques peuvent désormais être mobilisées afin de détecter plus aisément les barrages en pierre et de les documenter de façon précise. Parmi elles, les données issues du programme Litto3D sont disponibles sur l’ensemble des côtes du Finistère. Cette étude a donc cherché à répondre à trois objectifs : (i) proposer un inventaire réactualisé des barrages de pêcheries du Finistère en testant, à partir des données Litto3D, diverses approches de détection semis automatiques ; (ii) affiner localement l’étude de ce type de vestiges par des campagnes de mesures bathymétriques au SMF ; (iii) proposer un cadre chronologique pour l’édification de ces structures.Notre inventaire recense plus d’une centaine de structures archéologiques inédites le long du littoral finistérien. L’utilisation conjointe de plusieurs traitements numériques appliqués aux Modèles Numériques de Terrain (insolation, pente, rugosité, local relief model) apparaît comme le moyen le plus approprié pour la détection de ces vestiges. Deux campagnes de mesures bathymétriques au SMF réalisées dans l’archipel de Molène en 2014 et aux alentours de l’île de Batz en 2016 renseignent sur les stratégies d’implantation de ces barrages. Enfin, l’attribution chronologique des vestiges met en évidence (i) l’ancienneté de certaines structures, remontant au tout début du Néolithique ancien, et (ii) leur disparition probable à la fin de l’âge du Bronze