Gulf of Guinea continental slope and Congo (Zaire) deep-sea fan: Sr-Pb isotopic constraints on sediments provenance from ZaiAngo cores

Sediment accumulation in the continental margin of Equatorial Central Africa is mostly derived from terrigenous flux of the larger (Congo, ex-Zaire) and smaller coastal rivers. In order to determine the present-day respective contributions of these different sources, we analysed the Pb-Sr isotopic compositions of late Quaternary sediments accumulated on the continental margin and Congo deep-sea fan. The isotopic data of eight piston-cores from the ZaiAngo project (Ifremer-Total) show a strong geochemical similarity between sediments from distant cores situated on the slope near the Congo estuary. That can be explained by the spreading of the Congo sediment plume over extensive areas of the margin. However, the core located at the northern boundary of the Congo River influence has higher Sr-87/Sr-86 ratio, suggesting a melange with a more radiogenic source probably provided by the small but numerous rivers of this coast. The differences observed between present-day samples from Congo channel and from the terminal depositional lobe suggest a segregation of the sand along the channel during the transit which creates a Pb isotopic fractionation. Therefore, the comparison between Sr and Pb isotopes shows a strong discrimination of Pb according to sedimentation processes while Sr isotopes seem directly influenced by the coastal rivers contributions. (c) 2005 Elsevier B.V. All rights reserved.