Henry Hooghiemstra

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° and 29°N offshore Portugal and Morocco together represent the period from about 250 ka to 5 ka BP. Comparing their oxygen isotope stratigraphy with 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 forest. The northwestern Saharan boundary shifted over about 4° of latitude, the massive relief of the Atlas Mountains being its nothernmost 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 tundras. 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 frrm 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.

Variations of the NW African Trade Wind Regime during the Last 140 000 Years: Changes in Pollen Flux Evidenced by Marine Sediment Records

Pollen was analyzed in 7 eastern Atlantic cores located between 37° and 9°N. The cores lie along a transect in the main trajectory of the northeast trade winds. The northern sector of the transect is close to source areas of pollen; whereas, the southern sector is distant (<2000 km) from the pollen source areas. Winds dominate in the transport of pollen from the northernmost part of the trade wind belt to the coring sites. Therefore, the pollen in the cores is a potential monitor of eolian transport. 18O and 14C based time control of the sediment sections made it possible to display the pollen data as influx records. After having estimated latitudinal shifts of the different pollen source areas, spatial and temporal variations in the pollen flux have been used to evaluate changes in the trade wind vigor.

Changes of major wind belts and vegetation zones in NW Africa 20,000–5000 yr B.P., as deduced from a marine pollen record near cap blanc

The improved understanding of the pollen signal in the marine sediments offshore of northwest Africa is applied to deep-sea core M 16017-2 at 21°N. Downcore fluctuations in the percentage, concentration and influx diagrams record latitudinal shifts of the main northwest African vegetation zones and characteristics of the trade winds and the African Easterly Jet. Time control is provided by 14C ages and 18O records. During the period 19,000–14,000 yr B.P. a compressed savanna belt extended between about 12° and 14–15°N. The Sahara had maximally expanded northward and southward under hyperarid climatic conditions. The belt with trade winds and dominant African Easterly Jet transport had not shifted latitudinally. The trade winds were strong as compared to the modern situation but around 13,000 yr B.P. the trade winds weakened. After 14,000 yr B.P. the climate became less arid south of the Sahara and a first spike of fluvial runoff is registered around 13,000 yr B.P. Fluvial runoff increased strongly around 11,000 yr B.P. and maximum runoff is recorded from about 9000-7800 yr B.P. Around 12,500 yr B.P. the savanna belt started to shift northward and became richer in woody species: it shifted about 6° of latitude, reached its northernmost position during the period of 9200-7800 yr B.P. and extended between about 16° and 24°N at that time. Tropical forest had reached its maximum expansion and the Guinea zone reached as far north as about 15°N, reflecting very humid climatic conditions south of the Sahara. North of the Sahara the climate also became more humid and Mediterranean vegetation developed rapidly. The Sahara had maximally contracted and the trade winds were weak and comparable with the present day intensity. After about 7800 yr B.P. the southern fringe of the Sahara and accordingly the savanna belt, shifted rapidly southward again.

Land-sea comparisons during the last glacial-interglacial transition: pollen records from West Tropical Africa

Abstract Land-sea correlation off NW Africa is investigated from both modern and late Pleistocene/Holocene pollen records. Pollen spectra from modern soil surface samples from Senegal and Mauritania and from modern bottom sediment samples from the offshore eastern Atlantic (21-12°N) record accurately the latitudinal vegetation zonation of West Tropical Africa, by showing the maximum percentages of the phytogeographical groups and the same characteristic taxa as the adjacent vegetation zones. A strong correlation between marine (core M-16017-2 near Cap Blanc) and continental (sebkha of Chemchane, Mauritania) pollen signals is also observed for the evolution of the late Glacial/Holocene vegetation zonation at 21°N. The differing sensitivities of each sedimentary environment for recording local and/or subcontinental vegetation changes are discussed.

Changes in the vegetation and trade winds in equatorial northwest Africa 140,000–70,000 yr B.P. as deduced from two marine pollen records

Abstract The deep-sea cores M 16415-2 and M 16416-2 at about 9°N off Sierra Leone were analysed palynologically for the time interval 140,000–70,000 yr B.P. Results were presented in absolute (pollen concentration and pollen influx) and relative diagrams (pollen percentage). In a previous study it was evidenced that in northwest Africa pollen is mainly transported to the Atlantic by wind, so that the efficiency of aeolian pollen transport (pollen flux) could be used to evaluate changes in the intensity of the northeast trade winds. The glacial episodes (represented by the oxygen isotope stages 6 and 4) are characterized by strong northeast trade winds, whereas the last interglacial (stage 5) is characterized by weak trace winds. The pollen influx diagram shows that the intensity of the trade winds increased slightly during the relatively cool intervals of stage 5 (viz. 5.4 and 5.2). Tropical forest had maximally expanded around 124,000 yr B.P. (stage 5.5), around 98,000 yr B.P. (transition of stage 5.3 to 5.2), and around 70,000 yr B.P. (first part of stage 4): an increasing delay of the response of tropical forest to global intervals with maximum temperature is apparent during the last interglacial. As tropical forests need continuous humidity, the record of tropical forest monitors changes in climatic humidity south of the Sahara. During the last interglacial, the southern boundary of the Sahara shifted only little: expansions and contractions of the tropical forest area are correlated with contra-oscillations of the grass-dominated savanna zone. Great latitudinal shifts of the desert-savanna boundary, on the contrary, occurred during the penultimate glacial-interglacial transition (around 128,000 yr B.P.) to the north, and during the last interglacial-glacial transition (around 65,000 yr B.P.) to the south.