Composition of sediments from the north-west African continental margin

Entlang dreier Profile vom NW-afrikanischen Kontinentalrand wurden Oberflächensedimente aus Wassertiefen zwischen 39m und 1514m auf ihre Zusammensetzung der Sandfraktion, auf ihre Gehalte an Karbonat und organischer Substanzen sowie auf ihre mineralogische Zusammensetzung hin untersucht. 1) Die auf dem Schelf und dem oberen Hang abgelagerten Sedimente (<500m) zeichnen sich durch hohe Sandgehalte (>70%) und durch hohe Grob/Fein-Verhältnisse aus. Unterhalb dieses Bereiches nimmt der Einfluß von Strömungen, die die Ablagerung von wesentlichen Mengen an Feinmaterial oberhalb 500m verhindern, ab, wie die starke Abnahme des Sandgehaltes, des Quarz/Glimmer und des Grob/Fein-Verhältnisses zeigen. Die Sedimente aus diesen Wassertiefen werden zum großen Teil aus Partikeln der Siltfraktion aufgebaut. Mit zunehmender Tiefe ist auch eine Zunahme der Tonfraktion zu beobachten, wobei höhere Tonanteile (>10%) erst in Tiefen unterhalb von 1200m auftreten. 2) Die quantitative Komponentenanalyse der Sandfraktion zeigt, daß der karbonatische Anteil fast ausschließlich biogener Herkunft ist. Er besteht zum wesentlichen Teil aus planktonischen Komponenten, vorwiegend Foraminiferen und mengenmäßig nur sehr untergeordnet auftretenden Pteropoden. Das opalkieselige Plankton (Diatomeen, Radiolarien) ist nur in geringen Mengen in den untersuchten Proben vorhanden. Auch das Benthos stellt nur eine untergeordnete Komponente der Sandfraktion dar. Vor allem der Anteil von Foraminiferen und Mollusken nimmt mit zunehmender Wassertiefe relativ deutlich ab. Die übrigen benthonischen Komponenten sind im Sediment nur in geringen Anteilen vertreten. 3) Hauptsedimentbildner im Profil Nouakchott sind die nichtbiogenen, terrigen-detritischen Sandkomponenten. Sie bestehen vorwiegend aus Quarz und mit zunehmender Wassertiefe aus Kotpillen bzw. Kotpillenaggregaten. Je nach Tiefe treten vor allem Glimmer (>1000m) und Glaukonit (<800m) hinzu. Die restlichen Komponenten treten nur gelegentlich und in äußerst geringen Mengen im rezenten Oberflächensediment auf. 4) Quarz wird als Windstaub mit dem NE-Passat und vor allem durch den "Harmattan" aus der Sahara heraustransportiert und vorwiegend über dem Schelfbereich sedimentiert. Windstaubmaterial besteht primär weitgehend aus Siltkorngrößen, die vor Nouakchott über die Schelfkante hinaustransportiert werden und zu einer Grobsiltanreicherung am mittleren Hang führen. 5) Das Verhältnis zwischen den karbonatischen Biogenkomponenten und den nichtbiogenen Partikeln spiegelt sich deutlich in der Karbonatverteilung sowohl des Gesamtsedimentes als auch der Sandfraktion wider. Relativ hohe Karbonatgehalte vor Cap Leven im Norden stehen sehr geringen Anteilen von Nouakchott gegenüber. Mit zunehmender Wassertiefe ist eine deutliche Abnahme des Karbonatanteils zu verfolgen. 6) Die Tatsache, daß das Profil Cap Blanc im Bereich des ganzjährigen Auftriebs liegt, spiegelt sich nicht in der Zusammensetzung der Sandfraktion wider. Südlich der Zone des ganzjährigen Auftriebs weisen verschiedene Parameter (Radiolarien, Diatomeen, Verhältnis von Radiolarien zu planktonischen Foraminiferen, Benthos/Plankton-Verhältnis der Foraminiferen) trotz abnehmender Auftriebsintensität eher steigende Werte auf. Dies ist wesentlich auf eine infolge des Nährstoffeintrages durch Flußzufuhr bedingte Verschiebung der maximalen Primärproduktion weit in südliche Richtung zurückzuführen. 7) In den aufgeführten Parametern zeigen sich von Profil zu Profil sehr deutliche fazielle Unterschiede, obwohl der großklimatische Hintergrund im gesamten Untersuchungsgebiet etwa gleich ist. Vor Cap Leven bildet sich eine Fazies, die im wesentlichen aus planktonischen Foraminiferen besteht, während das Sediment vor Nouakchott zum überwiegenden Teil aus nichtbiogenen Komponenten aufgebaut wird. Im Übergangsbereich vor Cap Blanc bildet sich eine Mischfazies, die keinerlei Prägung durch das Auftriebsgeschehen erhält. Die Ursachen dieser faziellen Unterschiede werden auf fehlenden Terrigeneinfluß vor Cap Leven einerseits und hohe Terrigenanlieferung vor Nouakchott andererseits zurückgeführt. 8) Die Zusammensetzung und Verteilung der rezenten Grobfraktionssedimente am Kontinentalrand vor Nw-Afrika wird somit im wesentlichen als Ergebnis einer Überprägung der Biogenanlieferung durch nichtbiogene Komponenten angesehen. Wesentlicher steuernder Faktor ist demnach das hier vorherrschende Windsystem.

Stable isotope record of planktic foraminifera sampled by plankton haul and in surface sediments off North-West Africa

Foraminifera shells from modern sediments document the hydrography of the coastal upwelling region off Northwest-Africa (12-35° N) through the stable isotopic composition of their shells. Oxygen isotopes in planktonic foraminifers reflect sea surface temperatures (SST) during the main growing season of the differnt species: Globigerinoides ruber (pink and white) and G. sacculifer delineate the temperatures of the summer, Globorotalia inflata and Pulleniatina obliquiloculata those of the winter. Oxygen isotopes on Globigerina bulloides document temperature ranges of the upwelling seasons. d18O values in planktonic foraminifera from plankton hauls resemble those from the surface sediment samples, if the time of the plankton collection is identical with that of the main growing season of the species. The combined isotopic record of G. ruber (white) and G. inflata clearly reveals the latitudinal variations of the annual mean SST. The deviation of the d18O values from both species from their common mean is a scale for the seasonality, i.e. the maximum temperature range within one year. Thus in the summer upwelling region (north of 25° N) seasonality is relatively low, while it becomes high in the winter upwelling region south of 20° N. Furthermore, the winter upwelling region is characterized by relatively high d18O values - indicating low temperatures - in G. bulloides, the region of summer upwelling by relatively low d180 values compared with the constructed annual mean SST. Generally, carbon isotopes from the plankton hauls coincide with those from sediment surface samples. The enrichment of 13C isotopes in foraminifers from areas with high primary production can be caused by the removal of 12C from the total dissolved inorganic carbon during phytoplankton blooms. It is found that carbon isotopes from plankton hauls off Northwest-Africa are relatively enriched in 13C compared with samples from the western Atlantic Ocean. Also shells of G. ruber (pink and white) from upwelling regions are enriched in the heavier isotope compared with regions without upwelling. In the sediment, the enrichement of 13C due to high primary production can only be seen in G. bulloides from the high fertile upwelling region south of 20° N. North of this latitude values are relatively low. An enrichment of 12C is observed in shells of G. ruber (pink), G. inflata and P. obliquiloculata from summer-winter- and perennial upwelling regions respectively. Northern water masses can be distinguished from their southern counterparts by relatively high oxygen and carbon values in the "living" (=stained) benthic foraminifera Uvigerina sp. and Hoeglundina elegans. A tongue of the Mediterranean Outflow water can be identified far to the south (20° N) by 13C-enriched shells of these benthic foraminifera. A zone of erosion (15-25° N, 300-600 m) with a subrecent sediment surface can be mapped with the help of oxygen isotopes in "dead" benthic specimens. Comparison of d18O values in aragonitic and calcitic benthic foraminifers does not show a differential influence of temperature on the isotopic composition in the carbonate. However, carbon isotopes reflect slightly differences under the influence of temperature.

West & North Africa, ca. 1710 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: Carte de la Barbarie, le [sic] la Nigritie et del la Guinée, par Guillaume de l'Isle de l'Academie Royale des Sciences. It was published by ex officina Nicolai Visscher, ca. 1710. Scale [ca.1:9,000,000]. Covers West and North Africa. Map in French.The image inside the map neatline is georeferenced to the surface of the earth and fit to the Africa Sinusoidal projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, routes and roads, territorial boundaries, shoreline features, and more. Relief shown pictorially. Includes notes.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

West & North Africa, ca. 1730 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: Carte de la Barbarie, le la Nigritie et de la Guinée, par Guill[au]me Del'Isle, de l'Academie Royale des Sciences. It was published by Chez Jean Cóvens et Corneille Mortier, Geographes ca. 1730. Scale [ca.1:9,250,000]. Covers West and North Africa. Map in French.The image inside the map neatline is georeferenced to the surface of the earth and fit to the Africa Sinusoidal projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, roads and routes, territorial boundaries, shoreline features, and more. Relief shown pictorially. Includes notes.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

North & West Africa, 1655 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: L'Afrique, ou Lybie ulterieure : ou sont le Saara, ou desert, le pays des Negres, la Guinee, et les pays circonv. tirée en partie de Sanut, et de l'Arabe de Nubie, en partie de diverses cartes veües jusques a present par le S. Sanson d'Abbeville, geog. ordre. du Roy ; I. Somer Pruthenus, sculp. It was published by Chez Pierre Mariette, rue St. Iacques a l'Esperance in 1655. Scale [ca. 1:10,500,000]. Covers portions of North and West Africa. Map in French.The image inside the map neatline is georeferenced to the surface of the earth and fit to the World Miller Cylindrical projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, territorial boundaries, shoreline features, and more. Relief shown pictorially.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

West & Central Africa, ca. 1690 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: Effigies ampli Regni auriferi Guineae in Africa siti, extensum inde ad insulis Atlanticis, vulgo dictis, de Cabo Verde : ad flumen Benin, us[que], ad cujus ripam sita est Regia urbs et magna Benin, at[que] inde ad promontorium Lopi Gonsalvi, delineata per S. Rovelascum, et politioribus lineamentis figurata per Lodovicum Texeram, protocosmographum Regis Hispaniarum ; Baptista Doetechomius sculpsit ; Carolus Allardt excudit. It was published by Carolus Allardt ca. 1690. Scale [ca. 1:8,000,000]. Covers portions of West and Central Africa. Map in Latin and Portuguese.The image inside the map neatline is georeferenced to the surface of the earth and fit to the World Miller Cylindrical projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, shoreline features, and more. Includes also illustrations and text.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

West & Central Africa, ca. 1752 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: Guinea propria : nec non Nigritiae vel terrae Nigrorum maxima pars : Geographis hodiernis dicta utraque Aethiopia Inferior, & huius quidem pars australis ex delineationibus Anvillianis itineri Guineensi D. de Marchais insertis secundum Leges proiectionis stereographicae Hasianae, designata & edita studio & labore Homannianorum Heredum A. 1743 = Guinée de meme que la plus grande parties du pais des Negres : appellées par les geographes modernes Ethiopie inferieure et meridionale, tirées des morceaux geographiques de Mr. d'Anville, qu'il a inseres au voyage du Chev. de Marchais, & puis dessinées suivant les loix de la nouvelle projection de feu Mr. le prof. Has, par les Heritiers d'Homan. A 1743. It was published by edita studio & labore Homannianorum Heredum ca. 1752. Scale [ca. 1:7,400,000]. Covers portions of West and Central Africa. Map in Latin.The image inside the map neatline is georeferenced to the surface of the earth and fit to the World Miller Cylindrical projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, territorial boundaries, shoreline features, and more. Relief shown pictorially. Includes also notes.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.

West & Central Africa, ca. 1753 (Raster Image)

This layer is a georeferenced raster image of the historic paper map entitled: A new and correct map of the coast of Africa : from Cape Blanco lat. 20°40'. N. to the coast of Angola lat. 11°. S. : with explanatory notes of all the forts and settlements belonging to the several European powers, R. W. Seale, sculp. It was published by J. & P. Knapton ca. 1753. Scale [ca. 1:10,000,000]. Covers portions of West and Central Africa.The image inside the map neatline is georeferenced to the surface of the earth and fit to the World Miller Cylindrical projected coordinate system. All map collar and inset information is also available as part of the raster image, including any inset maps, profiles, statistical tables, directories, text, illustrations, index maps, legends, or other information associated with the principal map. This map shows features such as drainage, cities and other human settlements, territorial claims, forts, factories, shoreline features, and more. Includes also notes, index, and inset: A separate map of the Gold Coast, upon a larger scale.This layer is part of a selection of digitally scanned and georeferenced historic maps from the Harvard Map Collection. These maps typically portray both natural and manmade features. The selection represents a range of originators, ground condition dates, scales, and map purposes.