Sediment dynamics and Holocene evolution of a semi-closed macrotidal system : the example of the bay of Brest

The bay of Brest is a tide-dominated, semi-enclosed coastal basin, which is connected with two rivers (Aulne and Elorn). Its jagged shoreline and seabed morphology constitute an ideal setting to understand sedimentary transfer at the land-sea interface. In this aim, this study is divided in two different time scale. The first (a) focuses on the present-day sedimentary bedload dynamic and the second (b) on the reconstruction of the Holocene infilling, during the last marine transgression. (a) A analyse combining a sedimentology approach and numerical has highlighted the major control of the tidal currents. Swells and rivers have a minor impact, because the inherited morphology accentuates and promotes tidal currents. There are responsible of a shelly sandy exchange between the oceanic domain (Iroise Sea) and confined (bay of Brest). At present, the bay of Brest suffers from an imbalance of shelly sand. Fine sediments are eroded and redistributed in the shallow parts of the bay. In ideal conditions (strong tidal current) they can be exported.(b) From the stratigraphic and chronostratigraphic analyses, the successive landward retreat of estuarine processes, during the last marine transgression, has been observed. The sea level acts as the major factor control of the sedimentary geometry deposits. However, the morphological particularity of the bay of Brest influenced their preservation and it is possible to determinate the longitudinal and transversal asymmetry. After the sea level stabilisation (3 000 BP), the Holocene climatic variations and anthropic activities have disturbed the suspension dynamic.The bay of Brest had a different impact on the sedimentary transfer during the last marine transgression. At present, it acts as a « sediment trap » for the fine particles and has not enough sandy sediment. This configuration tends to an erosion of the system.La rade de Brest constitue un cadre idéal pour étudier et restituer les transferts sédimentaires terre-mer dans un contexte de bassin côtier semi-fermé macrotidal. Dans cette perspective, deux axes majeurs, d’échelle temporelle différente, ont été développés avec pour objectifs (a) de caractériser le modèle sédimentaire et sa dynamique par charriage à l’actuelle, (b) de déterminer la mise en place des dépôts sédimentaires au cours de la dernière transgression marine.(a) L’analyse combinant une approche sédimentaire et analogique a permis de montrer le contrôle majeur des courants de marée sur la dynamique sédimentaire actuelle. La morphologie du littoral et des fonds marins renforcent l’intensité des courants tidaux qui facilite les échanges en sables coquilliers entre la rade interne et la mer d’Iroise. Le système est néanmoins en érosion et enregistre un déséquilibre en apports sédimentaires. Les sédiments fins sont remobilisés par les courants de flot et les actions anthropiques colmatant les secteurs peu profonds.(b) L’étude litho- et chronostratigraphique de la séquence sédimentaire, a mis en évidence le recul successif des processus estuariens lors de la dernière remontée du niveau marin (holocène). La morphologie héritée du substratum, combinée à la transgression marine, a fortement influencé l’architecture des dépôts, leur préservation et leurs variations lithologiques latérales et longitudinales. Après la stabilisation du niveau marin (3 000 BP), les variations climatiques (optimum climatique médiéval, et petit âge glaciaire) et les activités anthropiques impactent d’avantage la dynamique des particules en suspension.La rade a donc eu un impact différent sur le transfert des sédiments au cours de la remontée du niveau marin. Actuellement, elle acte comme un « piège à sédiment fin » et enregistre dans le même temps un déficit de stock sédimentaire grossier potentiellement remobilisable. Cette configuration favorise une érosion des parties les plus profondes du système

Dynamique sédimentaire et évolution holocène d'un système macrotidal semi-fermé : l'exemple de la rade de Brest

The bay of Brest is a tide-dominated, semi-enclosed coastal basin, which is connected with two rivers (Aulne and Elorn). Its jagged shoreline and seabed morphology constitute an ideal setting to understand sedimentary transfer at the land-sea interface. In this aim, this study is divided in two different time scale. The first (a) focuses on the present-day sedimentary bedload dynamic and the second (b) on the reconstruction of the Holocene infilling, during the last marine transgression. (a) A analyse combining a sedimentology approach and numerical has highlighted the major control of the tidal currents. Swells and rivers have a minor impact, because the inherited morphology accentuates and promotes tidal currents. There are responsible of a shelly sandy exchange between the oceanic domain (Iroise Sea) and confined (bay of Brest). At present, the bay of Brest suffers from an imbalance of shelly sand. Fine sediments are eroded and redistributed in the shallow parts of the bay. In ideal conditions (strong tidal current) they can be exported.(b) From the stratigraphic and chronostratigraphic analyses, the successive landward retreat of estuarine processes, during the last marine transgression, has been observed. The sea level acts as the major factor control of the sedimentary geometry deposits. However, the morphological particularity of the bay of Brest influenced their preservation and it is possible to determinate the longitudinal and transversal asymmetry. After the sea level stabilisation (3 000 BP), the Holocene climatic variations and anthropic activities have disturbed the suspension dynamic.The bay of Brest had a different impact on the sedimentary transfer during the last marine transgression. At present, it acts as a « sediment trap » for the fine particles and has not enough sandy sediment. This configuration tends to an erosion of the system.La rade de Brest constitue un cadre idéal pour étudier et restituer les transferts sédimentaires terre-mer dans un contexte de bassin côtier semi-fermé macrotidal. Dans cette perspective, deux axes majeurs, d’échelle temporelle différente, ont été développés avec pour objectifs (a) de caractériser le modèle sédimentaire et sa dynamique par charriage à l’actuelle, (b) de déterminer la mise en place des dépôts sédimentaires au cours de la dernière transgression marine.(a) L’analyse combinant une approche sédimentaire et analogique a permis de montrer le contrôle majeur des courants de marée sur la dynamique sédimentaire actuelle. La morphologie du littoral et des fonds marins renforcent l’intensité des courants tidaux qui facilite les échanges en sables coquilliers entre la rade interne et la mer d’Iroise. Le système est néanmoins en érosion et enregistre un déséquilibre en apports sédimentaires. Les sédiments fins sont remobilisés par les courants de flot et les actions anthropiques colmatant les secteurs peu profonds.(b) L’étude litho- et chronostratigraphique de la séquence sédimentaire, a mis en évidence le recul successif des processus estuariens lors de la dernière remontée du niveau marin (holocène). La morphologie héritée du substratum, combinée à la transgression marine, a fortement influencé l’architecture des dépôts, leur préservation et leurs variations lithologiques latérales et longitudinales. Après la stabilisation du niveau marin (3 000 BP), les variations climatiques (optimum climatique médiéval, et petit âge glaciaire) et les activités anthropiques impactent d’avantage la dynamique des particules en suspension.La rade a donc eu un impact différent sur le transfert des sédiments au cours de la remontée du niveau marin. Actuellement, elle acte comme un « piège à sédiment fin » et enregistre dans le même temps un déficit de stock sédimentaire grossier potentiellement remobilisable. Cette configuration favorise une érosion des parties les plus profondes du système

Sediment dynamics and Holocene evolution of a semi-closed macrotidal system : the example of the bay of Brest

La rade de Brest constitue un cadre idéal pour étudier et restituer les transferts sédimentaires terre-mer dans un contexte de bassin côtier semi-fermé macrotidal. Dans cette perspective, deux axes majeurs, d’échelle temporelle différente, ont été développés avec pour objectifs (a) de caractériser le modèle sédimentaire et sa dynamique par charriage à l’actuelle, (b) de déterminer la mise en place des dépôts sédimentaires au cours de la dernière transgression marine.(a) L’analyse combinant une approche sédimentaire et analogique a permis de montrer le contrôle majeur des courants de marée sur la dynamique sédimentaire actuelle. La morphologie du littoral et des fonds marins renforcent l’intensité des courants tidaux qui facilite les échanges en sables coquilliers entre la rade interne et la mer d’Iroise. Le système est néanmoins en érosion et enregistre un déséquilibre en apports sédimentaires. Les sédiments fins sont remobilisés par les courants de flot et les actions anthropiques colmatant les secteurs peu profonds.(b) L’étude litho- et chronostratigraphique de la séquence sédimentaire, a mis en évidence le recul successif des processus estuariens lors de la dernière remontée du niveau marin (holocène). La morphologie héritée du substratum, combinée à la transgression marine, a fortement influencé l’architecture des dépôts, leur préservation et leurs variations lithologiques latérales et longitudinales. Après la stabilisation du niveau marin (3 000 BP), les variations climatiques (optimum climatique médiéval, et petit âge glaciaire) et les activités anthropiques impactent d’avantage la dynamique des particules en suspension.La rade a donc eu un impact différent sur le transfert des sédiments au cours de la remontée du niveau marin. Actuellement, elle acte comme un « piège à sédiment fin » et enregistre dans le même temps un déficit de stock sédimentaire grossier potentiellement remobilisable. Cette configuration favorise une érosion des parties les plus profondes du système.The bay of Brest is a tide-dominated, semi-enclosed coastal basin, which is connected with two rivers (Aulne and Elorn). Its jagged shoreline and seabed morphology constitute an ideal setting to understand sedimentary transfer at the land-sea interface. In this aim, this study is divided in two different time scale. The first (a) focuses on the present-day sedimentary bedload dynamic and the second (b) on the reconstruction of the Holocene infilling, during the last marine transgression. (a) A analyse combining a sedimentology approach and numerical has highlighted the major control of the tidal currents. Swells and rivers have a minor impact, because the inherited morphology accentuates and promotes tidal currents. There are responsible of a shelly sandy exchange between the oceanic domain (Iroise Sea) and confined (bay of Brest). At present, the bay of Brest suffers from an imbalance of shelly sand. Fine sediments are eroded and redistributed in the shallow parts of the bay. In ideal conditions (strong tidal current) they can be exported.(b) From the stratigraphic and chronostratigraphic analyses, the successive landward retreat of estuarine processes, during the last marine transgression, has been observed. The sea level acts as the major factor control of the sedimentary geometry deposits. However, the morphological particularity of the bay of Brest influenced their preservation and it is possible to determinate the longitudinal and transversal asymmetry. After the sea level stabilisation (3 000 BP), the Holocene climatic variations and anthropic activities have disturbed the suspension dynamic.The bay of Brest had a different impact on the sedimentary transfer during the last marine transgression. At present, it acts as a « sediment trap » for the fine particles and has not enough sandy sediment. This configuration tends to an erosion of the system

Nouvelle phase porteuse de plomb anthropique dans les sédiments fluviaux et estuariens à proximité d'une ancienne mine (Huelgoat, France)

International audienc

Unlikely lead-bearing phases in river and estuary sediments near an ancient mine (Huelgoat, Brittany, France)

International audienc

Nouvelle phase porteuse de plomb anthropique dans les sédiments fluviaux et estuariens à proximité d'une ancienne mine (Huelgoat, France)

International audienc

The intertidal sanding up of the Seine-Maritime coast (Normandy, France): Sedimentological and geochemical approaches

International audienceThe Seine-Maritime coastline (France) is a macro-tidal environment (8 m tidal range), developingalong an epicontinental sea, the English Channel. The SW-NE coast is opened to westerlyatmospheric flows, generating occasionally wind sea with energetic waves (Hs: 4.65 m decennialreturn). High chalk cliffs and a wide marine erosion platform partially hidden on its upper part by aflint pebble beach, characterise this 130 km long coast.Observations since the end of the 1990’s show a recent and massive sanding up of the marineerosion platform. This raises the question of the origin of the sandy fraction and the sedimentarydynamics on the intertidal area.We present herein an innovative method that combine grain-size and geochemical analysis inorder to highlight sand sources and transport direction along these rocky coast.Sixteen beaches were sampled during low tide and fair-weather conditions. At each site, 3 sampleswere collected along the cross-shore beach profile (from the pebbly upper beach to the low tidelimit).Grain-size results show that for all sites, medium to coarse-grained sands dominate in the upperbeach (mode 315-400μm) while fine sands dominate in the middle and low foreshore (mode160-250μm). A decrease in grain-size is thus evidenced from the upper beach to the low foreshore.The geographical variability of the sand composition and consequently sources was determinedon the basis of geochemical data. In order to avoid the granulometric effect on the data, X-Rayfluorescence analysis (xSORT, SPECTRO AMETEK) were performed on the two major grain-sizemodes of each sample. Eighteen calibrated chemical elements (Si, S, K, Ca, Ti, V, Mn, Fe, Ni, Ga, As,Br, Rb, Sr, Y, Pb, Th and U) were measured at each station. Statistical processing performed step bystep on the data allows to gradually reduce the number of significant geochemical parameters.Finally, 4 major elements (Si, Ca, Sr, K) as well as the ratio Sr/Ca have been considered as the bestproxies of sample discrimination and potential source.The first results indicate a longshore gradient of Si and Ca, especially for the finest sands 60-200μm). From SW to NE, i.e. in the direction of the littoral drift, and whatever the positionacross the beach profile, there are an enrichment in Si (sands are more siliciclastic) and animpoverishment in Ca.This gradient highlights differentiated longshore sediment transport and sorting, in relationprobably with sediment sources (siliclastic sands vs bioclastics sands)

The intertidal sanding up of the Seine-Maritime coast (Normandy, France): Sedimentological and geochemical approaches

International audienceThe Seine-Maritime coastline (France) is a macro-tidal environment (8 m tidal range), developingalong an epicontinental sea, the English Channel. The SW-NE coast is opened to westerlyatmospheric flows, generating occasionally wind sea with energetic waves (Hs: 4.65 m decennialreturn). High chalk cliffs and a wide marine erosion platform partially hidden on its upper part by aflint pebble beach, characterise this 130 km long coast.Observations since the end of the 1990’s show a recent and massive sanding up of the marineerosion platform. This raises the question of the origin of the sandy fraction and the sedimentarydynamics on the intertidal area.We present herein an innovative method that combine grain-size and geochemical analysis inorder to highlight sand sources and transport direction along these rocky coast.Sixteen beaches were sampled during low tide and fair-weather conditions. At each site, 3 sampleswere collected along the cross-shore beach profile (from the pebbly upper beach to the low tidelimit).Grain-size results show that for all sites, medium to coarse-grained sands dominate in the upperbeach (mode 315-400μm) while fine sands dominate in the middle and low foreshore (mode160-250μm). A decrease in grain-size is thus evidenced from the upper beach to the low foreshore.The geographical variability of the sand composition and consequently sources was determinedon the basis of geochemical data. In order to avoid the granulometric effect on the data, X-Rayfluorescence analysis (xSORT, SPECTRO AMETEK) were performed on the two major grain-sizemodes of each sample. Eighteen calibrated chemical elements (Si, S, K, Ca, Ti, V, Mn, Fe, Ni, Ga, As,Br, Rb, Sr, Y, Pb, Th and U) were measured at each station. Statistical processing performed step bystep on the data allows to gradually reduce the number of significant geochemical parameters.Finally, 4 major elements (Si, Ca, Sr, K) as well as the ratio Sr/Ca have been considered as the bestproxies of sample discrimination and potential source.The first results indicate a longshore gradient of Si and Ca, especially for the finest sands 60-200μm). From SW to NE, i.e. in the direction of the littoral drift, and whatever the positionacross the beach profile, there are an enrichment in Si (sands are more siliciclastic) and animpoverishment in Ca.This gradient highlights differentiated longshore sediment transport and sorting, in relationprobably with sediment sources (siliclastic sands vs bioclastics sands)