ALBERT

Description: Late Quaternary sediments on the West African continental margin between 24°N and 15°N were studied with RV METEOR (1971) and VALDIVIA (1975). Cores on the shelf were taken with a 6-m-vibrocorer, in deeper water mainly with a 12 m Kastenlot corer. During Holocene, and up to the present time, more or less and climatic conditions north of the Senegal River area reduced terrigenous supply. Therefore, the biogenic-carbonate content exceeds about 50%. Wüstenquarz numbers (red + yellow quartz : white quartz x 100) are high (20 to mmore than 200), indicating eolian input. The Senegal River supplied fine grained, green colored, terrigenous material with some plant debris. During Würm, the Mediterranean climatic zone with winter rains was shifted more than 5° to the South and was reaching Banc d'Arguin (at about 20°N). Therefore, the terrigenous supply was increased in this northern part and consequently the carbonate content and the Wüstenquartz numbers dropped below 50% and 10, respectively. The arid zone was also shifted to the south; as a consequence, the Senegal River did not reach the sea, eolian supply diluted the biogenic carbonates, and increased Wüstenquartz numbers to more than 200. Eolian dunes covered parts of the shelf. Ratios of radiolarians/plankonic foraminifera and planktonic/benthonic organisms and sedimentation rates of organic carbon indicate stronger upwelling in the northern region. Turbidity currents were more frequent, eroding as much as a third of the material supplied ba pelagic sedimentation.

Description: In the sediments of the NW African continental margin the mainly biogenic carbonate constituents become increasingly diluted with terrigenous material as one approaches the coast, as indicated by the carbonate-C02 content, the Al20a/Si02-ratios, and the presence of ammonia fixed to alumino-silicates, predominantly to illites. In the northern area of investigation - off Cape Blanc and Cape Bojador - the terrigenous constituents are mainly quartz from the Sahara Desert, whereas in the south - off Senegal - more alumino-silicates as clay minerals are admixed with the carbonate constituents. The organic carbon content of the continental slope sediments off Senegal is higher than in samples of the continental rise or of the shelf, indicating optimal conditions for the preservation of organic matter as a result of high production and relatively rapid sedimentation. The zone of manganese-oxide enrichment follows the redox potential of + 330 m V from the surface (0-5 cm) into the sediments (20-30 cm deep) at 2000-3000 m and 3700 m of water depths, respectively. At shallower water depths, low redox potentials preclude deposition of manganese oxides and cause their mobilization from the sediments. About 1/3 of the total sedimentary Zn and 1/4 of the Cu is associated with the carbonate mineral fraction, probably in calcium phosphate overgrowths as a result of the mineralization of phosphorus-containing organic matter. Besides the precipitation of calcium phosphate, the mineralization of organic matter mediated by bacterial sulfate reduction also results in calcium carbonate precipitation and the exchange of ammonia for potassium on illites. Because of these simultaneous reactions, the depth distribution of all mineralization constituents in the interstitial water can be determined using the actual molar carbon-to-nitrogen-to-phosphorus ratios of the sedimentary organic matter. The amount of sulfide sulfur in this process indicates the predominance of bacterial sulfate reduction in the sediments off NW Africa. This process also preferentially decomposes nitrogen- and phosphorus-containing organic compounds so organic matter deficient in these elements is characteristic for the rapidly accumulating sediments. During the last glacial stage there were more biogenic carbonates and organic matter accumulating in the sedimcnts than today, indicating there was increased production of organic carbon compounds and more favorable conditions for their preservation. During the last interglacial times conditions were similar to those of today. This differentiation with time has also been observed in sediments from thc Argentine Basin and from the slope off South India indicating perhaps world-wide environmental changes throughout Late Quaternary times.

Description: Surface sediments from 5 profiles between 30 and 3000 m water depth off W Africa (12-19° N) have been studied for their sand fraction composition and their total calcium carbonate and organic matter contents to evaluate the effect of climatic and hydrographic factors on actual sedimentation. On the shelf and upper slope (〈 500 m), currents prevent the deposition of significant amounts of finegrained material. The sediments forming here are characterized by high sand contents (〉 60 % ; in most samples 〉 80 %), low organic carbon contents (in most samples 〈 0.8 %), high median diameters of the sand fraction (120-500 μm), and by a predominance of quartz and biogenic relict shells (most abundant: molluscs and bryozoans) in the sand fraction. Median diameters of total sand fraction and of major biogenic sand fraction components (biogenic relict material, benthonic molluscs, benthonic and planktonic foraminifers) co-vary to some extent and show maximum values in 100-300 m water depth, reflecting the sorting effect of currents (perhaps the northward flowing undercurrent; MITTELSTAEDT 1976). In this water depth, biogenic relict material is considerably enriched relative to quartz, the second dominating sand fraction component on the shelf and upper slope, resulting in distinct calcium carbonate maxima of the bulk sediments. The influence of the undercurrent is also reflected in a northward transport of finegrained river load and perhaps in the distribution of the red stained, coarse silt and sand-sized clay aggregates, which show maxima in 300-500 m water depth. They probably originate from tropical soils. Abundant coarse red-stained quartz on the shelf off Cape Roxo (12-13° N) suggests a southward extension of last glacial dune fields to this latitude. Below about 500 m water depth, current influence becomes negligible - as indicated by a strong decrease in sand content, a concomitant increase in sedimentary organic carbon contents (up to 2.5-3.5 %), and the occurrence of high mica/quartz ratios in the sand fraction. Downslope transport, presumably due to the bioturbation mechanism proposed by BEIN & FüTTERER (1977), is indicated by the presence of coarse shelfborne particles (glauconite, relict shells) down to about 1000 m water depth. The fine/coarse ratio (clay + silt/sand) of the sediments from water depths 〉 500 m never exceeds a value of 11 in northern latitudes (19°-26° N), but shows distinct maxima, ranging from 50 to 120, at latitudes 18°, 17°, 15° 30', and 14° N in about 2000 m water depth. This distribution is attributed to the deposition of fine-grained river load at the continental slope between 18° and 14° N, brought into the sea by the Senegal and southern rivers and transported northward by the undercurrent. Strang calcium carbonate dissolution is indicated by the complete disappearance of pteropods (aragonite) and high fragmentation of planktonic foraminifers (calcite) in sediments from water depths 〉 300-600 m. Fragmentation ratios of planktonic foraminifers were found to depend on the organic carbon/carbonate ratio of the sediment suggesting that calcite dissolution at the sea bottom may also be significant in shelf and continental slope water depths if the organic matter/carbonate ratio of the surface sediment is high and the tests remain long enough within the oxidizing layer on top of the sulfate reduction zone. The fact that in the region under study intensity and annual duration of upwelling decrease from north to south is neither reflected in the composition of the sand fraction (i. e. radiolarian and fish debris contents, radiolarian/planktonic foraminiferal ratios, benthos/plankton ratios of foraminifers), nor in the sedimentary organic carbon distribution. On the contrary, these parameters even show in comparable water depths a tendency for highest values in the south, partly because primaty production rates remain high in the whole region, particularly on the shelf, due to the nutrient input by rivers in the south (SCHEMAINDA et al. 1975). In addition, several hydrographic, sedimentological and climatic factors severely affect their distribution - for example currents, dissolution, grain size composition, deposition of river load, and bulk sedimentation rates.