ALBERT

Description: Sediments sampled in three lateral traverses from the outer continental shelf to the continental rise off the Sahara Desert were analysed for their grain size distribution and composition. The carbonate content of the shelf and upper slope sediments increases from about 10-15 % in the south to about 50-90 % in the north and changes to about 40-45 % in deeper water. The high carbonate content in the north generally reflects the high proportion of relict material in the sediments. The main change in the composition of the sediments - defined mostly as hemipelagic - from the outer shelf to the rise is in an increase of coccoliths and clays and a decrease of quartz. The cumulative grain size curves indicate that each of the samples is a mixture of different proportions of two approximately log normal populations. The size of the truncation point between the populations, the proportion between the coarse and fine populations and the mean, sorting, skewness and kurtosis decrease with increasing depth. Correlations between the parameters indicate that the moments express mainly the mixture proportions of the two populations. The two populations may, however, indicate an artificial effect of deflocculation during sample treatment rather than reflecting some hydraulic factor. The cumulative grain size distribution curves of eolian dust collected on ship's surface during cruises along the Saharan shelf and that of other published data is divided into a two-population and a onepopulation type. The two populations may be attributed to either gravitational settling of larger dust particles and normal dust fall or to disaggregation effect. The moment statistics of the dust differ mainly from those of the marine sediments in that they have a lower standard deviation. The composition and the textural parameters, combined with modal analysis of the dust and the separated soluble and insoluble components of the sediments indicate that the downslope transportation of the coarser sediment particles is a result of marine currents rather than of wind. The distribution pattern of the relict material in the northern outer shelf and upper slope is attributed to an extinct current regime which was stronger than the present one. The gravitational settling of coarse dust, mainly over the shelf, most probably accounts for the principal input of sediment that is transported further downslope by marine currents. Part of the fine, as well as a small portion of the coarse terrigenous material may be derived from direct settling of dust over the open sea. The recent sedimentation process results in a continental margin prograding off the Sahara Desert. In the south sedimentation occurs on the outer shelf, slope and rise, whereas in the north main sedimentation occurs on the lower slope and rise. The deflection of the Canary current away from the Cape Blanc coast is proposed as an explanation for the apparent reduced deposition rate over the outer shelf and upper slope in the northern area.

Description: Surface sediments from the continental slope and rise off North-West Africa between the Canary Islands and the Cape Verde Islands are mainly composed of silt-sized material (2-63 µm). A number of sampling profiles were run normal to the coast and the composition of the silt fraction was determined quantitatively by scanning electron microscope analysis. The carbonate portion of the sediment was found to be nearly exclusively of biogenic origin. The most important contributors are planktonic foraminifers and coccoliths with minor contributions derived from pteropods. Plankton-produced biogenic opal such as diatoms and radiolarians play a very minor role. The high production rates of opal-silica plankton which exists in the surface waters of the NW-African upwelling system does not give rise to corresponding increases of opal accumulation in the bottom sediment. Benthic producers consist mainly of foraminifers and molluscs but the entire input from benthic producers is extremely small. An exception to this occurs in the prodelta sediments of the Senegal river. Downslope particle transport is indicated by the occurrence of shallow-water coralline algae, ascidian sclerites and cliona boring chips and can be traced as far down as the continental rise. The non-carbonate silt fraction mostly consists of quartz which is derived as eolian dust from the Sahara desert by the Harmattan and the NE-Trade-wind system. The percentage of carbonate in the surface sediments directly indicates the relative proportions of autochthonous biogenic components and terrigenous allochthonous quartz particles.