[1] The influence of the large-scale ocean circulation on Sahel rainfall is elusive because of the shortness of the observational record. We reconstructed the history of eolian and fluvial sedimentation on the continental slope off Senegal during the past 57,000 years. Our data show that abrupt onsets of arid conditions in the West African Sahel were linked to cold North Atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with Heinrich Stadials. Climate modeling suggests that this drying is induced by a southward shift of the West African monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric African Easterly Jet.

[1] We present a suite of new high-resolution records (0-135 ka) representing pulses of aeolian, fluvial, and biogenic sedimentation along the Senegalese continental margin. A multiproxy approach based on rock magnetic, element, and color data was applied on three cores enclosing the present-day northern limit of the ITCZ. A strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13°N. These millennial-scale dust fluxes are synchronous with North Atlantic Heinrich stadials. Fluvial clay input driven by the West African monsoon predominates at 12°N and varies at Dansgaard-Oeschger time scales while marine productivity is strongly enhanced during the African humid periods and marine isotope stage 5. From latitudinal signal variations, we deduce that the last glacial ITCZ summer position was located between core positions at 12°26 0 and 13°40 0 N. Furthermore, this work also shows that submillennial periods of aridity over northwest Africa occurred more frequently and farther south than previously thought.

We combine environmental magnetism, geochemical measurements and colour reflectance to study two late Quaternary sediment cores: GeoB 4905-4 at 28 30 0 N off Cameroon and GeoB 4906-3 at 08 44 0 N off Gabon. This area is suitable for investigating precipitation changes over Central and West Africa because of its potential to record input of aeolian and fluvial sediments. Three magnetozones representing low and high degree of alteration of the primary rock magnetic signals were identified. The magnetic signature is dominated by fine-grained magnetite, while residual haematite prevails in the reduced intervals, showing increase in concentration and fine grain size at wet intervals. Our records also show millennial-scale changes in climate during the last glacial and interglacial cycles. At the northern location, the past 5.5 ka are marked by high-frequency oscillations of Ti and colour reflectance, which suggests aeolian input and hence aridity. The southern location remains under the influence of the Intertropical Convergence Zone and thus did not register aeolian signals. The millennial-scale climatic signals indicate that drier and/or colder conditions persisted during the late Holocene and are synchronous with the 900 a climatic cycles observed in Northern Hemisphere ice core records.

S U M M A R YIn complex sedimentary environments with different sources of the magnetic particles, it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data. Palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources, and diagenesis may alter the primary magnetic mineral assemblage. Rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages. In this study of core-retrieved sediments from the last 150 kyr off the coast of Senegal (NW Africa), the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant. In glacial stages, ferrimagnetic iron and iron-titanium oxides magnetically dominate with few signs of a diagenetic overprint. Low-and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low-temperature AC susceptibility measurements and SEM observations. In contrast, the interglacial samples appeared to have been severely affected by diagenesis, with pyrite making up over 50 per cent of the iron-bearing phases. The Verwey transition observed at 110 K confirms the presence of magnetite. The glacial and interglacial contrast reveals the close link between northwest African climate and the magnetic mineralogy of the sediments.

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