CARBON ISOTOPE COMPOSITION AND GEOCHEMISTRY OF PARTICULATE ORGANIC-MATTER IN THE CONGO RIVER (CENTRAL-AFRICA) - APPLICATION TO THE STUDY OF QUATERNARY SEDIMENTS OFF THE MOUTH OF THE RIVER

Organic carbon isotopes and the geochemistry of organic matter have been studied in suspended material from the Congo River and some of its tributaries and in a core located at 2000-m depth, off the mouth of the river. The organic carbon isotopic composition of suspended material from the Congo tributaries identifies the floral cover and the lithologic and pedologic characteristics of their watersheds. Suspended material from the Congo River in the Stanley Pool emphasizes the sole influence of organic matter derived from C3-rich plants. Outside the Congo estuary, differences of isotopic composition between suspended material near the surface and that near the bottom emphasize the respective marine or continental origin of particulate organic carbon. The carbon isotopic composition of the marine deposits records biozonal fluctuations of the recent Quaternary. Shifts in the isotopic composition of organic matter may be explained by an increase of terrigenous organic matter derived from C3-type plants during the wet biozones Z and X whereas during the drier biozone Y the associated organic matter shows a greater influence of the productivity of the oceanic biomass. However, a possible contribution of C4 plant-derived particulate organic matter can be assumed for the very top of the Y biozone, introducing a C-13-enriched signal, indicative of marine planktonic input.