MIDDLE TO LATE QUATERNARY SEDIMENT FLUX AND POSTDEPOSITIONAL PROCESSES BETWEEN THE CONTINENTAL-SLOPE OFF GABON AND THE MID-GUINEAN MARGIN

Two cores from the continental slope off Gabon (2330 m of water) and one core from the Mid-Guinean margin (4000 m of water) were studied in order to examine climatic and paleoceanographic fluctuations during the middle and late Quaternary. Oxygen isotope variations in one shallow core were correlated with global isotope stages, and extended to the two other cores on the basis of microfaunal and mineralogic contents. Interglacial stages have been characterized by high carbonate content. low carbonate dissolution effect and moderate flux organic matter derived from primary oceanic production. Glacial stages have included dissolution processes on carbonate tests, related to increased CO2, content near the seabed due to the sink of organic matter from both planktonic and benthic fauna; at a depth of 4000 m of water, this phenomenon has controlled the vertical carbonate distribution through time. Many interglacial intervals record high terrigenous inputs (increased quartz and kaolinite content), while some glacial intervals contain peaks of both well-crystallized smectites (probably reworked from Mesozoic Cenozoic outcroppings on the adjacent continental shelf) and poorly crystallized smectites (land derived). At a depth of 4000 m of water, quartz-rich layers deposited during some glacial internals include surface textures indicative of an aeolian origin. A long-term climatic change is recognized, consisting of a progressive cooling of sea-surface waters and of climatic conditions since 400,000-350.000 yrs B.P.