Abstract:
Glacial/interglacial climatic cycles triggered variations in the atmospheric and oceanic circulation. As one consequence the wind system above West Africa was strongly enforced during glacial periods causing elevated dust supply from Africa to the central Equatorial Atlantic. Wind-driven changes in ocean productivity and lateral displacement and/or increasing current velocities of prominent bottom currents correspond to these glacial/interglacial cycles. In this study we present results from grain size analysis (sedigraph and settling velocity measurements) performed on Plio/Pleistocene deposits from ODP Hole 959C. ODP Hole 959C was drilled on the upper flank of the Cote d'Ivoire/Ghana transform margin, close to the African continent. Biostratigraphic and stable isotopic stratigraphy indicate low sedimentation rates throughout the Plio/Pleistocene probably reflecting enhanced bottom current activities causing persistent winnowing at the exposed position of Hole 959C. Informations on eolian supply of terrestrial material and bottom water velocities are deduced from grain size characteristics. Distribution patterns of the sortable silt fraction of bulk samples (2-63 mu m) enable to identify (a) erosional, (b) accumulating, and (c) sorted sediment types, which, in turn, reflect variable sediment transport at the sea floor in response to changing bottom current velocities. Eolian supply is inferred using the relationship between the modal grain size of siliciclastic silt and the percentage of carbonate- and opal-free silt >6 mu m in the bulk siliciclastic sediment fraction. Data are used to distinguish between (I) well-sorted, wind-borne material, (II) clay-rich, fluvial-discharged material, and (III) clay-depleted, locally-winnowed material.