VEGETATIONAL AND CLIMATIC CHANGES AT THE NORTHERN FRINGE OF THE SAHARA 250,000-5000 YEARS BP - EVIDENCE FROM 4 MARINE POLLEN RECORDS LOCATED BETWEEN PORTUGAL AND THE CANARY-ISLANDS

Four Atlantic marine pollen records located between 38-degrees and 29-degrees-N offshore Portugal and Morocco together represent the period from about 250 ka to 5 ka BP. Comparing their oxygen isotope stratigraphy with the recognized pollen stratigraphy a great coherence is evident, which indicates that vegetational history is a useful proxy to evaluate paleoclimatic change and emphasises the potential of marine palynology. A fluctuating Mediterranean vegetation zone is evidenced: a widespread distribution is registered from 240-190 ka BP, 125-70 ka BP, and after 10 ka BP. During intervening (glacial) periods a much reduced distribution is noted. However, some interstadials (dated 180-160 ka BP and 55-40 ka BP) permitted some expansion of Mediterranean oak forests. The northwestern Saharan boundary shifted over about 4-degrees of latitude, the massive relief of the Atlas Mountains being its northernmost limit. A gradual northward expansion of the desert during the last two glacials is registered, indicating a gradual aridification of the northwestern Sahara towards Termination II and I. The Artemisia rich, partly Pinus wooded steppe-like vegetation formed a narrow zone between the Sahara desert and the Mediterranean vegetation during interglacials. During glacials, however, the steppe-like vegetation strongly expanded and formed a transition zone from the Sahara to the treeless Westeuropean tundra's. Regional and temporal diversity in the glacial composition of this Artemisia dominated steppe-like vegetation zone is expressed by fluctuations in the records of grasses and Mediterranean shrub elements. The Mediterranean-Saharan transition zone was narrowest shortly after Termination II and I, indicating a steep gradient in humidity across the Atlas Mountains. The much discussed climatic change in the northwestern Sahara and Maghreb during the last 40 ka BP is addressed: cold and moderately dry climate from 40 ka to 30 ka BP, increasing aridity from 30 ka to 20 ka BP, arid conditions from 20 ka to 15 ka BP, gradually less arid conditions from 15 ka to 8.5 ka BP, and warm and humid conditions after 8.5 ka BP are evidenced.