Congo Deep-sea Fan Research Articles

Gulf of Guinea continental slope and Congo (Zaire) deep-sea fan: Sr–Pb isotopic constraints on sediments provenance from ZaiAngo cores

Sediment accumulation in the continental margin of Equatorial Central Africa is mostly derived from terrigenous flux of the larger (Congo, ex-Zaire) and smaller coastal rivers. In order to determine the present-day respective contributions of these different sources, we analysed the Pb–Sr isotopic compositions of late Quaternary sediments accumulated on the continental margin and Congo deep-sea fan. The isotopic data of eight piston-cores from the ZaiAngo project (Ifremer–Total) show a strong geochemical similarity between sediments from distant cores situated on the slope near the Congo estuary. That can be explained by the spreading of the Congo sediment plume over extensive areas of the margin. However, the core located at the northern boundary of the Congo River influence has higher 87Sr / 86Sr ratio, suggesting a melange with a more radiogenic source probably provided by the small but numerous rivers of this coast. The differences observed between present-day samples from Congo channel and from the terminal depositional lobe suggest a segregation of the sand along the channel during the transit which creates a Pb isotopic fractionation. Therefore, the comparison between Sr and Pb isotopes shows a strong discrimination of Pb according to sedimentation processes while Sr isotopes seem directly influenced by the coastal rivers contributions.

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

Africa displays a variety of continental margin structures, tectonic styles and sedimentary records. The comparative review of two representative segments: the equatorial western Africa and the SW Africa margins, helps in analysing the main controlling factors on the development of these margins. Early Cretaceous active rifting south of the Walvis Ridge resulted in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led to the formation of the equatorial western Africa margin, characterised by a wide zone of crustal stretching and thinning, and thick, extensive, syn-rift basins. Contrasting lithologies of the early post-rift (salt vs shale) determined the style of gravitational deformation, whilst periods of activity of the decollements were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW Africa accounts for high clastic sedimentation rate during Late Cretaceous to Eocene, while dominant carbonate production on equatorial western Africa shelf suggests very little erosion of a low hinterland. The early Oligocene long-term climate change had contrasted response in both margins. Emplacement of the voluminous terrigenous Congo deep-sea fan reflects increased erosion in equatorial Africa, under the influence of wet climate, whereas establishment of an arid climate over SW Africa induced a drastic decrease of denudation rate, and thus reduced sedimentation on the margin. Neogene emplacement of the African superswell beneath southern Africa was responsible for renewed onshore uplift on both margins, but it accelerated erosion only in the Congo catchment, due to wetter climatic conditions. Neogene high sedimentation rate reactivated gravitational tectonics that had remained quiescent since late Cretaceous.

Lipid biomarkers as major source and preservation indicators in SE Atlantic surface sediments

Surface sediments from the eastern South Atlantic were investigated for their lipid biomarker contents and bulk organic geochemical characteristics to identify sources, transport pathways and preservation processes of organic components. The sediments cover a wide range of depositional settings with large differences in mass accumulation rates. The highest marine organic carbon (OC) contributions are detected along the coast, especially underlying the Benguela upwelling system. Terrigenous OC contributions are highest in the Congo deep-sea fan. Lipid biomarker fluxes are significantly correlated to the extent of oxygen exposure in the sediment. Normalization to total organic carbon (TOC) contents enabled the characterization of regional lipid biomarker production and transport mechanisms. Principal component analyses revealed five distinct groups of characteristic molecular and bulk organic geochemical parameters. Combined with information on lipid sources, the main controlling mechanisms of the spatial lipid distributions in the surface sediments are defined, indicating marine productivity related to river-induced mixing and oceanic upwelling, wind-driven deep upwelling, river-supply of terrigenous organic material, shallow coastal upwelling and eolian supply of plant-waxes.

Dinoflagellate cysts as environmental indicators in surface sediments from the Congo deep-sea fan and adjacent regions

Forty-nine surface sediment samples from the Congo deep-sea fan and adjacent hydrographic environments were prepared palynologically and analyzed for fossilizable dinoflagellate cysts. The main objective was to investigate the extent to which cysts reflect present-day hydrography, and therefore may be used downcore as paleoceanographic indicators. The region sampled covers a range of marine environments including the Congo River plume, the area of pronounced upwelling off West-Central Africa, and a nearshore–offshore gradient from coastal to oceanic waters. This is the first study of the effect of a major tropical river plume on associated cyst assemblages. Cyst counts were low, particularly near the river mouth and furthest offshore. A combination of statistical treatments was used to analyze the surface cyst assemblages, including cluster analysis, correspondence analysis and canonical correspondence analysis. This helped to identify five distinctive sample groupings representing the main ecological signals in the cyst assemblage data. Group A, dominated by spherical brown protoperidinioid cysts, is an upwelling signal. Group B, dominated by the cosmopolitan species O. centrocarpum, is the main signal for the river plume effect, interpreted as reflecting the environmental instability this creates. Group C, dominated by S. delicatus with accessory O. centrocarpum, characterizes the outer plume, somehow reflecting the mixing of progressively diluted river plume water and the surrounding oceanic waters. Group D, with high proportions of Impagidinium and Nematosphaeropsis together with O. centrocarpum and S. delicatus, characterizes the location of the Angola–Benguela Front. Group E, with mainly Impagidinium species, is an oceanic signal. The identification of nutrients and water stability as main determining factors influencing cyst signals has implications for cyst-based paleoenvironmental interpretations using transfer functions, which are developed almost exclusively from comparisons with more easily available data for sea-surface temperature, salinity, and sea ice.

Late-Quaternary supply of terrigenous organic matter to the Congo deep-sea fan (ODP site 1075): implications for equatorial African paleoclimate

Late-Quaternary sections (about 1 Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter supply to the easternmost equatorial Atlantic Ocean. Variations in quantity and quality of the riverine organic matter reflect the interaction between the paleoclimatic development within the continental catchment area and the paleoceanographic conditions in the Congo river plume. To characterize the delivery of organic matter from terrigenous and marine sources, we used elemental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemistry, and organic petrology. High-amplitude fluctuations occurring about every 15–25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as would be anticipated in front of a major river. Fluctuations in Tmax from Rock-Eval pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine production offshore of the Congo. Organic petrological observations confirm the existence of abundant terrigenous plant tissues, both non-oxidized (vitrinite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterland, and implies more arid climatic conditions. Results from ratios of specific phenolic lignin components suggest that terrigenous organic matter in Late-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specific lignin phenols and the bulk δ13Corg signature fosters the conclusion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbon isotopic signatures alone.

Abstract: Congo deep-sea fan: Recent high resolution data bridges the gap between exploration plays and reservoir understanding

Abstract: Congo deep-sea fan: Recent high resolution data bridges the gap between exploration plays and reservoir understanding