Pollen abundance in Late Quaternary sediment cores off West Africa

The pollen record of three marine late Quaternary cores off Senegal shows a juxtaposition of Mediterranean, Northern Saharan, Central Saharan elements, which are considered transported by the trade winds from a winter-rainfall area, and Sahelian, Soudanese, Soudano-Guinean elements, considered transported both by winds and mostly by the Senegal River, and coming from the monsoonal, summer tropical rainfall area of southern West Africa. Littoral vegetation is either the edaphically dry and saline Chenopodiaceae from sebkhas at the time of the main regression, or the warm tropical humid mangrove with Rhizophora during the humid optimum period. Four stratigraphic zones reflect, from basis to top: Zone 4. A semi-arid period with a balanced pollen input. Zone 3. A very arid period with the disappearance of monsoonal pollen, probably from the disappearance of the Senegal River, a very saline littoral plain with Chenopodiaceae, a larger input of northern Saharan pollen from intensified trade winds. Zone 2. A quite humid period, much more so than today, very suddenly established, with a northward extension of the monsoonal areas, a rich littoral mangrove, and weakening of the trade winds. Zone l. A slow and steady evolution toward the present semi-humid conditions with regression of the mangrove, and of the monsoonal areas toward the south. Tentative datations and correlations with the Tchad area suggested: zone 4: 22,500 to 19,000 years BP; zone 3: 19,000 to 12,500 years BP; zone 2: 12,500 to 5,500 years BP; zone 1: 5,500 years BP to top of core. Dinoflagellate cysts display a tropical assemblage with mostly estuarine neritic elements and also a weak oceanic component, mostly in the lower slope core 47. Cosmopolitan taxa dominate the assemblage and only a few species point to more specialized environments. Quantitative variations of the assemblage are the basis of stratigraphy which is not similar to the pollen stratigraphy, and an inshore-outshore gradient has to be taken into account to correlate the three cores.

Chemistry of Late Quaternary sediments and their interstitial waters of sediment cores from the Nort-West African continental margin

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-CO2 content, the Al2O3/SiO2-ratios, and the presence of ammonia fixed to alumino-silicates, predominantly to illites. In the norther area of the 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 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 mV 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 than today, indicating there was increased production of organic carbon compounds and more favorable conditions of their preservations. During the last interglacial times conditions were similar to those to today. This differentiation with time has also been observed in sediments from the Argentine Basin and from slope off South India indicating perhaps world-wide environmental changes throughout Late Quaternary times.

Distribution of benthic foraminifera in surface sediments of the Portuguese continental margin

Living and dead benthic Foraminifera of 26 sediment surface samples from the East Atlantic continental margin (off Portugal) are studied. The stations are located on two profiles off Cape Mondego and off Cape Sines, ranging in water depth from 45 to 3905 meters. The highest values of standing crop are on the shelf (200 m) (up to 420 specimens/10 cm**3). Below 1000 m water depth standing crop is low (5 -24 specimens/10 cm**3). 151species and species groups are distinguished. Most of the living species do occur in a wide depth range. Faunal depth boundaries are at 50/100m, at 600/800 m, and at 1000 m. Results published from the North Atlantic and the East Mediterranean do not differ from those obtained in samples off Portugal. Depth of water (e.g. hydrostatic pressure) or another factor being controlled by depth (e.g. limitation of food supply) seems to be the most important factor of the benthic foraminiferal distribution.

Age determination of sediment cores from the shelf off Mauretania, West Africa

The 60 km wide shelf off Mauritania is cut by several submarine canyons. Its water-circulation is controlled by the cool Canary current and upwelling. Its Recent sediments show faunal associations remarkably related to the grain size distribution which in water depths between 40 and 80 m is strongly influenced by reworking of older coarse sand or sandstone. In this depth range a mixed biofacies originating from Pleistocene and Recent material is encountered. The present lateral faunistic and sedimentological facies change, including horizons of mixed provenance, can be recognized in vertical sequences taken by vibro-coring. This correlation combined with 14C-datations on molluscs enable the reconstruction of the history of the last glacial regression and transgression. Due to the arid climate, the emerging calcareous shelf sediments are indurated and, therefore, protected from subaerial and submarine erosion. During low sea level eolian sand migrates over the shelf, but only about 1/10 of this material remains there and is later incorporated into the sandy shelf sediments. The calculated average rate of total sedimentation during Holocene is 15 cm, and the production rate of carbonate is 5 cm/1000 years.