LATE QUATERNARY PALEOPRODUCTIVITY CHANGES OFF THE CONGO DEDUCED FROM STABLE CARBON ISOTOPES OF PLANKTONIC-FORAMINIFERA

A detailed record (approximate to 2-kyr intervals) of the difference i n stable carbon isotopes (Delta delta(13)C) between Globigerina bulloi des and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comp arisons of the oxygen and carbon isotope data from this core with reco rds from the Niger Fan and from pelagic cores in the eastern equatoria l South Atlantic indicate that the Congo Fan isotope records do not co ntain a strong freshwater signal as is described off other major river s. The temporal pattern of the Delta delta(13)C Signal correlates with the marine organic carbon record from the Congo Fan. Thus the plankto nic Delta delta(13)C record, reflecting past changes in upwelling inte nsity and nutrient content, corroborates the signal provided by sedime ntary organic carbon, which is presumed to indicate changes in the amo unt of biological productivity and export flux to the seafloor. The pl anktonic Delta delta(13)C Signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwellin g fluctuations off the Congo to precessional forcing. Minima in the De lta delta(13)C record are aligned with periods of minimum boreal summe r insolation over Central Africa reflecting an increase of upwelling a nd biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in rive r discharge probably did not occur, indicating that fertilization by r iver-derived nutrients has played only a minor role with respect to La te Quaternary changes in the total amount of primary productivity off the Congo.