Les Diatomées Sub-fossiles de L’estuaire du Saloum, Sénégal: inventaire floristique et paléo-environnement fournis par la carotte de Ndangane Babou

Dans le programme de reconstitution de l’histoire des estuaires et lagunes du Sénégal et de l’inventaire de la microflore à diatomées de la Séné-Gambie, une carotte de sondage de 400 cm de long a été réalisée à Ndangane Babou sur un des bras de l’estuaire du Saloum (14°07′53.86″ N/16°17′40.08″ W) à 95 km de l’embouchure. L’analyse comparée de la lithologie et de la microflore a montré une corrélation significative entre la fraction pélitique, l’abondance absolue des diatomées et leur diversité spécifique. Le faible taux de sable, majoritairement d’origine marine tout au long de la carotte pourrait s’expliquer par l’éloignement du site de prélèvement par rapport à l’embouchure du fleuve. L’étude a permis d’inventorier 89 espèces et variétés de diatomées appartenant à 48 genres. Les genres les plus représentés sont Nitzschia (11 espèces), Coscinodiscus (7 espèces), Amphora et Navicula (4 espèces chacune). Huit espèces sont citées pour la première fois en Séné-Gambie. La dominance des espèces euryhalines tout au long de la carotte ainsi que la présence constante des formes d’eau douce témoignent de variations de la salinité entre les saisons hivernales marquées par des apports d’eau douce et les saisons sèches. D’un milieu deltaïque ouvert aux influences marines (Zone A), le Saloum a été marqué par des upwellings qui ont apportés des sels nutritifs jusqu’à la hauteur de Ndangane Babou durant le dépôt de la zone B. Par la suite, l’installation d’un climat chaud et humide a entrainé une augmentation de la pluviométrie et permis d’atteindre une paléo-productivité primaire et une paléo-biodiversité maximales au sommet de la zone C, avant l’installation d’un climat aride à l’origine du caractère inverse de l’estuaire (zone D). Dans ce milieu, l’hydrodynamisme serait à l’origine de l’augmentation du taux de sable et corrélativement la diminution des diatomées suivie de leur disparition.   As part of the program to rebuilt the estuaries and lagoons history in Senegal and the inventory of the diatom microflora in Senegal and Gambia, a 400 cm long core was taken at Ndangane Babou on one of the arms from the Saloum estuary (14°07′53.86″ N/16°17′40.08″ W), 95 km from the ocean. The comparative analysis of the lithology and the microflora showed a significant correlation between the lutite fraction, the absolute abundance of diatoms and their specific diversity. The low rate of sand, mostly of marine origin throughout the core, could be explained by the distance of the site from the mouth of the estuary. The study allowed to inventory 89 species and varieties of diatom belonging to 48 genera. The most represented genera are Nitzschia (11 species), Coscinodiscus (7 species each), Amphora and Navicula (4 species each). Eight species are cited for the first time in Sene-Gambia. The dominance of euryhaline species throughout the core as well as the constant presence of freshwater forms attest to variations in salinity between the rainy seasons marked by freshwater inflows and the dry seasons. From a delta environment open to marine influences (Zone A), the Saloum estuary was marked by upwellings brought nutritive salts up to the height of Ndangane Babou during Zone B deposit. Subsequently, the installation of a hot and humid climate led to an improvement in rainfall and allowed to achieve maximum primary paleo-productivity and paleo-biodiversity at the top of zone C, before the installation of an arid climate at the origin of the opposite character of the estuary (zone D). In this environment, hydrodynamics would be the cause of the increase in the rate of sand and correlatively the decrease in diatoms followed by their disappearance

Sub-fossil diatoms from the Saloum Estuary, Senegal, West Africa: Floristic inventory and paleo-productivity evolution along the Ndangonkha core

International audienceAs part of our program to reconstruct the estuaries and lagoons history in Senegal and the diatom microflora inventory in Sene-Gambia, a 400 cm long core was taken at Ndangonkha in the Saloum estuary, near Foundiougne. The dating obtained show that sedimentation began in the Nouakchottian age, around 6000 years BP, with an average rate of sedimentation higher at the base than at the top where the high hydrodynamics of the environment would have caused sediment reworking. The comparative analysis of the lithology and the microflora shows a perfect correlation between the lutite fraction, the absolute abundance of diatoms and their specific diversity. The study allowed to inventory 110 species and varieties belonging to 62 genera; the most represented genera being Nitzschia (10 species), Coscinodiscus, Cyclotella and Diploneis (5 species each). Nine species are cited for the first time in Sene-Gambia. The microflora is dominated by euryhaline forms headed by Cyclotella striata. This dominance would be linked to the frequent variations in salinity between the rainy and dry seasons, while the abundance of marine species is related to the dominant influence of marine waters. The rare freshwater species encountered could be brought by wind, insects or birds. The analysis of the microfloristic and lithological data shows that the primary paleo-productivity and the specific diversity are weak at the base but increasing upwards (zone A), maximum in zone B whereas in zone C they are decreasing before disappearing in the last 10cm, in relation to the hydrodynamics of the environment

Sub-fossil diatoms from the Saloum Estuary, Senegal, West Africa: Floristic inventory and paleo-productivity evolution along the Ndangonkha core

International audienceAs part of our program to reconstruct the estuaries and lagoons history in Senegal and the diatom microflora inventory in Sene-Gambia, a 400 cm long core was taken at Ndangonkha in the Saloum estuary, near Foundiougne. The dating obtained show that sedimentation began in the Nouakchottian age, around 6000 years BP, with an average rate of sedimentation higher at the base than at the top where the high hydrodynamics of the environment would have caused sediment reworking. The comparative analysis of the lithology and the microflora shows a perfect correlation between the lutite fraction, the absolute abundance of diatoms and their specific diversity. The study allowed to inventory 110 species and varieties belonging to 62 genera; the most represented genera being Nitzschia (10 species), Coscinodiscus, Cyclotella and Diploneis (5 species each). Nine species are cited for the first time in Sene-Gambia. The microflora is dominated by euryhaline forms headed by Cyclotella striata. This dominance would be linked to the frequent variations in salinity between the rainy and dry seasons, while the abundance of marine species is related to the dominant influence of marine waters. The rare freshwater species encountered could be brought by wind, insects or birds. The analysis of the microfloristic and lithological data shows that the primary paleo-productivity and the specific diversity are weak at the base but increasing upwards (zone A), maximum in zone B whereas in zone C they are decreasing before disappearing in the last 10cm, in relation to the hydrodynamics of the environment

Sub-fossil diatoms from the Saloum Estuary, Senegal, West Africa: Floristic inventory and paleo-productivity evolution along the Ndangonkha core

International audienceAs part of our program to reconstruct the estuaries and lagoons history in Senegal and the diatom microflora inventory in Sene-Gambia, a 400 cm long core was taken at Ndangonkha in the Saloum estuary, near Foundiougne. The dating obtained show that sedimentation began in the Nouakchottian age, around 6000 years BP, with an average rate of sedimentation higher at the base than at the top where the high hydrodynamics of the environment would have caused sediment reworking. The comparative analysis of the lithology and the microflora shows a perfect correlation between the lutite fraction, the absolute abundance of diatoms and their specific diversity. The study allowed to inventory 110 species and varieties belonging to 62 genera; the most represented genera being Nitzschia (10 species), Coscinodiscus, Cyclotella and Diploneis (5 species each). Nine species are cited for the first time in Sene-Gambia. The microflora is dominated by euryhaline forms headed by Cyclotella striata. This dominance would be linked to the frequent variations in salinity between the rainy and dry seasons, while the abundance of marine species is related to the dominant influence of marine waters. The rare freshwater species encountered could be brought by wind, insects or birds. The analysis of the microfloristic and lithological data shows that the primary paleo-productivity and the specific diversity are weak at the base but increasing upwards (zone A), maximum in zone B whereas in zone C they are decreasing before disappearing in the last 10cm, in relation to the hydrodynamics of the environment

Sub-fossil diatoms from the Saloum Estuary, Senegal, West Africa: Floristic inventory and paleo-productivity evolution along the Ndangonkha core

International audienceAs part of our program to reconstruct the estuaries and lagoons history in Senegal and the diatom microflora inventory in Sene-Gambia, a 400 cm long core was taken at Ndangonkha in the Saloum estuary, near Foundiougne. The dating obtained show that sedimentation began in the Nouakchottian age, around 6000 years BP, with an average rate of sedimentation higher at the base than at the top where the high hydrodynamics of the environment would have caused sediment reworking. The comparative analysis of the lithology and the microflora shows a perfect correlation between the lutite fraction, the absolute abundance of diatoms and their specific diversity. The study allowed to inventory 110 species and varieties belonging to 62 genera; the most represented genera being Nitzschia (10 species), Coscinodiscus, Cyclotella and Diploneis (5 species each). Nine species are cited for the first time in Sene-Gambia. The microflora is dominated by euryhaline forms headed by Cyclotella striata. This dominance would be linked to the frequent variations in salinity between the rainy and dry seasons, while the abundance of marine species is related to the dominant influence of marine waters. The rare freshwater species encountered could be brought by wind, insects or birds. The analysis of the microfloristic and lithological data shows that the primary paleo-productivity and the specific diversity are weak at the base but increasing upwards (zone A), maximum in zone B whereas in zone C they are decreasing before disappearing in the last 10cm, in relation to the hydrodynamics of the environment