Origin of late Holocene fine-grained sediments on the French Guiana shelf

Marine muds deposited on the French Guiana coast mostly originate in the Amazon. Recent sediments are composed of (a) illite and chlorite of Andean origin and (b) kaolinite and smectite, principally from the Amazonian lowlands but also from the Guiana shield. The Sr–Nd isotopic studies confirm that these deposits are the result of the mixing between Andean and lowland materials. The importance of the source material can be estimated. coastal mud prism, the comparative study of clay assemblages and Sr–Nd isotopic compositions during the late Holocene shows two assemblages: (a) grey muds with illite, chlorite and a higher Sr content appear to originate mainly in the Andean environment, (b) beige muds with kaolinite, smectite and a lower Sr content show a decrease of the input in favour of a bigger part of the lowland environment. The most important event is a reduction of the Andean source, derived from a dry phase which occurred in Western Amazonia confirming that regional decreases in rainfall, water discharge and erosion were associated with climatic change. As a result particulate flux from the ocean decreased, which had an influence on the sedimentation on the Guiana coasts.