Résumé
This study is located in Niger in the Sahelian area, close to the city of Niamey and its aim is to determine the conditions for the formation of carbonaceous nodules which may be found on the old terraces of the Niger River. The chemistry, the isotopic geochemistry (radioactive and stable) and mineralogical studies of the carbonates were used to date the nodules and reconstruct the precipitation conditions. The results show that the nodules were formed in the first metre of soil and demonstrate the contribution of the shallow groundwater in this process. The nodules seem old; they appear to have been formed between 1500 and 7000 BP. This time interval could agree with the transitional period between the present dry conditions and the regional humid phases that occurred between 10 000 and 4500 BP. However the diversity of dating and the different types of nodules show that local parameters (groundwater, vegetation, soil texture) must also have contributed in the precipitation process of nodules.