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A relatively high resolution pollen analysis (a sample each ca 5 ka) has been made on ODP site 658, off Cap Blanc, N-W Africa. In the same cores detailed palaeomagnetism, biostratigraphy, sedimentology and stable isotopes provided a chronology based on the cyclic forcing by the astronomical parameters. The period previous to 3.5 Ma corresponds to a warmer and wetter climate from northwestern Europe as far south as equatorial West Africa. Long term variation shows a first step towards a drier climate between 3.5 and 3.2 Ma. At 3.2 Ma, the pollen record establishes the aridification of northwestern Africa probably in connection with enhanced Trade winds. This phase may correspond to a preliminary stage of the Northern Hemisphere glaciation. Spectral analyses show a positive correlation between low deep sea temperatures in the Atlantic Ocean and aridity in northwestern Africa. After a period of reestablished humidity, a second and stronger step to aridification started at ca. 2.6 Ma. Before that date, there is a northern emplacement of savannah and tropical forest that probably shifted southwards afterwards. Already from 2.8 Ma, a reduction of savannah vegetation took place, probably as a result of the development of a desert in West Africa. However, mean percentages of steppe elements of North Africa are five times lower during the Pliocene than during the Late Pleistocene indicating that arid periods were still less prolonged and/or less severe during the Late Pliocene. Spectral analyses suggest that forcing of the climate by obliquity started after or ca. 2.6 Ma.