ISOTOPIC EVOLUTION OF FLUIDS AND BIOGENIC ARAGONITE IN AN INTERTROPICAL COASTAL LAGOON - POSSIBLE USE FOR PALEOENVIRONMENTAL STUDIES

The salinity and isotopic composition of sea waters which penetrate th e lagoon of Mbodiene (South Senegal) during each tidal cycle, are modi fied by a strong evaporation in the lagoon. The variation of the O-18 composition between low and high tide was nearly 1 parts per thousand 700 m from the mouth. The O-18 content of the biogenic aragonite preci pitated by Anadara senilis (a common bivalve in West African lagoons) in this environment varies with temperature following the equation of Grossman and Ku equation. The average O-18 content of the water calcul ated from this equation is related to coastal hydrodynamics which are in turn related to the balance between the trade winds and the monsoon winds and to the general climate of the region. Thus, isotopic analys is of the fossil shells of A senilis might be considered for calculati on of the O-18 content of coastal sea waters. They may consequently be used for the determination of Quaternary climatic variations.