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.

(Table 2) Mineralogy, stable isotopes, and mineral percentages of the carbonate fraction of samples from the South China Sea

Chemoherm carbonates, as well as numerous other types of methane seep carbonates, were discovered in 2004 along the passive margin of the northern South China Sea. Lithologically, the carbonates are micritic containing peloids, clasts and clam fragments. Some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids. Dissolution and replacement is common. Mineralogically, the carbonates are dominated by high magnesium calcites (HMC) and aragonite. Some HMCs with MgCO3 contents of between 30-38 mol%-extreme-HMC, occur in association with minor amounts of dolomite. All of the carbonates are strongly depleted in d13C, with a range from -35.7 to -57.5 per mil PDB and enriched in d18O (+ 4.0 to + 5.3 per mil PDB). Abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements, likely fossils of chemosynthetic microbes involved in carbonate formation. The microbial structures are intimately associated with mineral grains. Some carbonate mineral grains resemble microbes. The isotope characteristics, the fabrics, the microbial structure, and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes. From the succession of HMCs, extreme-HMC, and dolomite in layered tubular carbonates, combined with the presence of microbial structure and diagenetic fabric, we suggest that extreme-HMC may eventually transform into dolomites. Our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the South China Sea. Characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes, dissolution, and early transformation events.

Grain-size distributions of the terrigenous fraction of two deep-sea sediment cores GeoB3375-1 and MD96-2094

In this study, we present grain-size distributions of the terrigenous fraction of two deep-sea sediment cores from the SE Atlantic (offshore Namibia) and from the SE Pacific (offshore northern Chile), which we 'unmix' into subpopulations and which are interpreted as coarse eolian dust, fine eolian dust, and fluvial mud. The downcore ratios of the proportions of eolian dust and fluvial mud subsequently represent paleocontinental aridity records of southwestern Africa and northern Chile for the last 120,000 yr. The two records show a relatively wet Last Glacial Maximum (LGM) compared to a relatively dry Holocene, but different orbital variability on longer time scales. Generally, the northern Chilean aridity record shows higher-frequency changes, which are closely related to precessional variation in solar insolation, compared to the southwestern African aridity record, which shows a remarkable resemblance to the global ice-volume record. We relate the changes in continental aridity in southwestern Africa and northern Chile to changes in the latitudinal position of the moisture-bearing Southern Westerlies, potentially driven by the sea-ice extent around Antarctica and overprinted by tropical forcing in the equatorial Pacific Ocean.

Mineralogy of the clay fraction from Cenozoic Strata at DSDP Leg 94

Leg 94 Sites are located in a large geographic area of the northeastern Atlantic. Clay mineral analyses of the sediments recovered on Leg 94 (Eocene to the present), together with results obtained from previous DSDP legs (47B, 48, 80, 81, 82), provide greater insight into the paleoenvironmental evolution of the northeastern Atlantic. The characteristics of Eocene clay sediments are regional, reflecting, in the absence of strong bottom currents, the influence of neighboring petrographic environments: basic volcanic rocks (Sites 403-406, 552, and 608) and acid volcanic rocks (Sites 508 to 510). During the Oligocene, atmospheric circulation patterns left their mineralogical signatures in the southern part of the area investigated (Sites 558 and 608), whereas during the Miocene the intrusion of northern water masses led to the gradual homogenization of the clay sedimentation throughout the North Atlantic. In the late Pliocene, input from glacial sources became widespread.