No abstract

Les écosystèmes naturels ne sont pas indifférents des fortes variabilités climatiques intersaisonnières de ces dernières années. L'évaluation des impacts actuels de ces variabilités nécessitent la connaissance des paléoenvironnements. Ce travail de synthèse fait l'état des connaissances sur l'évolution des environnements géologiques et paléoclimatiques des 20000 ans B.P. dans l'estuaire de l'Ouémé au Bénin. De façon spécifique, il s'agit, de caractériser l'environnement de dépôt de différentes séquences sédimentaires, d'analyser le contenu palynologique des sédiments et de situer le paléoclimat dépôts dans le contexte intertropical africain et global. L'approche utilisée est basée sur la collecte et la synthèse des coupes géologiques et palynologiques. Il ressort que depuis le dernier maximum glaciaire jusqu'à 14000 ans B.P., les forêts et les niveaux marins ont régressé. Une colonisation forestière a été notée entre 13400-12500 ans B.P., mais elle a été très tôt interrompue par le Younger Dryas. Dès l'Holocène (10300 ans B.P.) les forêts ont proliféré, l'estuaire de l'Ouémé a été transformé en ria. Il s'en est suivi vers 4200 ans B.P., une péjoration climatique au cours de laquelle l'influence anthropique a été exprimée. Il est envisagé une étude qui combine la sédimentologie et les espèces de dinoflagellés, un outil de reconstitution du paléoclimat.

Due to the inherent damages of trace elements to living organisms, the amount of bio available metal is considered to be a quantity that conditions the becoming of aquatic ecosystems. The aim of this work is to evaluate the proportion of Pb, Cu and Cd that are biologically available in the superficial sediments of the Ouémé estuary as well as the influence of environmental factors on the becoming of these metals. Fractionation of the metals was performed according to the sequential extraction method and their measurement was done by inductively coupled plasma mass spectrometer (ICP-MS). Physicochemical parameters evaluated are: granulometry, pH, cation exchange capacity (CEC), organic carbon (Corg), iron, aluminum and calcium oxides (Fe2O3, Al2O3 and CaO). Risk indices related to metal fractionation were evaluated followed by statistical processing in R 3.3.2. It appears that: 39.44% of the Cd are preferentially bound to the exchangeable fraction, 37.25% of the Pb are for the reducible fraction and 47.47% of the Cu for the oxidizable fraction. Risk levels are in the low to very high range. The richness of materials is revealed as a factor limiting the bioavailability of metals while CaO facilitates the bioavailability in high risk sites. Liquid and solid wastes from the Dantokpa market are the potential sources of metals.

The objective of this research is to determine the physicochemical elements contained in the groundwater and in the deposits originating from these waters of the Arrondissement of Dêkin, Commune of Dangbo in the South of Benin. Physical parameters were measured from the multi-parameter and the chemical parameters by reagent assays. The precipitate from the groundwater was studied by speciation. The different assays were carried out using the Hach Lange DR2800 Spectrophotometer. The data processing was done with different software including ArcGis 10.1, Diagram.Ink of Roland SIMLER and PHREEQC 2.17.4137. Strong mineralization of the deposit from the drilling water was indicated in these three elements: Copper (220.1 mg/kg), iron (2528 mg/kg) and zinc (239.5 mg/kg) whose contents are higher than the French guideline values for heavy metal content in sediments in mg/kg. The calculation of the Saturation Indices of the drilling water shows saturation with respect to Goethite and Hematite. All these results have led us to conclude that the high content of Metallic Trace Element (MTE) in the deposit and especially of the iron in the drilling water originates from the dissolution-precipitation of the aquifer rock.