Quantitative composition and distribution of the silt fraction in surface sediments of the North-West African continental margin

Surface sediments from the continental slope and rise of North-West Africa between the Canary lslands 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.

Paleomagnetic and rock magnetic properties of ODP Leg 173 sites, west Iberia continental margin

We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly.

Benthic foraminifera of surface samples and sediment cores off Senegal and Gambia, West Africa

Water depth zonation of fifty nine benthonic foraminiferal species in marine sediment surfaces has been described. Some species are combined to groups which mark particular depth zones: an upper and lower shelf-fauna, an upper and lower slope fauna, and a shelf-slope fauna. Dependence on latitude could be ascertained for Textularia panamensis, and submergence effects for Hyalinea balthica.