Age, pollen and spores of ODP Site 175-1082

The present-day condition of bipolar glaciation characterized by rapid and large climate fluctuations began at the end of the Pliocene with the intensification of the Northern Hemisphere continental glaciations. The global cooling steps of the late Pliocene have been documented in numerous studies of Ocean Drilling Program (ODP) sites from the Northern Hemisphere. However, the interactions between oceans and between land and ocean during these cooling steps are poorly known. In particular, data from the Southern Hemisphere are lacking. Therefore I investigated the pollen of ODP Site 1082 in the southeast Atlantic Ocean in order to obtain a high-resolution record of vegetation change in Namibia between 3.4 and 1.8 Ma. Four phases of vegetation development are inferred that are connected to global climate change. (1) Before 3 Ma, extensive, rather open grass-rich savannahs with mopane trees existed in Namibia, but the extension of desert and semidesert vegetation was still restricted. (2) Increase of winter rainfall dependent Renosterveld-like vegetation occurred between 3.1 and 2.2 Ma connected to strong advection of polar waters along the Namibian coast and a northward shift of the Polar Front Zone in the Southern Ocean. (3) Climatically induced fluctuations became stronger between 2.7 and 2.2 Ma and semiarid areas extended during glacial periods probably as the result of an increased pole-equator thermal gradient and consequently globally enhanced atmospheric circulation. (4) Aridification and climatic variability further increased after 2.2 Ma, when the Polar Front Zone migrated southward and the influence of Atlantic moisture brought by the westerlies to southern Africa declined. It is concluded that the positions of the frontal systems in the Southern Ocean which determine the locations of the high-pressure cells over the South Atlantic and the southern Indian Ocean have a strong influence on the climate of southern Africa in contrast to the climate of northwest and central Africa, which is dominated by the Saharan low-pressure cell.

Pollen analysis of surface sediments from the Gulf of Guinea and offshore NW-Africa

The areas of marine pollen deposition are related to the pollen source areas by aeolian and fluvial transport regimes, whereas wind transport is much more important than river transport. Pollen distribution patterns of Pinus, Artemisia, Chenopodiaceae-Amaranthaceae, and Asteraceae Tubuliflorae trace atmospheric transport by the northeast trades. Pollen transport by the African Easterly Jet is reflected in the pollen distribution patterns of Chenopodiaceae-Amaranthaceae, Asteraceae Tubuliflorae, and Mitracarpus. Grass pollen distribution registers the latitudinal extension of Sahel, savannas and dry open forests. Marine pollen distribution patterns of Combretaceae-Melastomataceae, Alchornea, and Elaeis reflect the extension of wooded grasslands and transitional forests. Pollen from the Guinean-Congolian/Zambezian forest and from the Sudanian/Guinean vegetation zones mark the northernmost extension of the tropical rain forest. Rhizophora pollen in marine sediments traces the distribution of mangrove swamps. Only near the continent, pollen of Rhizophora, Mitracarpus, Chenopodiaceae-Amaranthaceae, and pollen from the Sudanian and Guinean vegetation zones are transported by the Upwelling Under Current and the Equatorial Under Current, where those currents act as bottom currents. The distribution of pollen in marine sediments, reflecting the position of major climatic zones (desert, dry tropics, humid tropics), can be used in tracing climatic changes in the past.

The late Pliocene pollen record of ODP Site 108-658 off Cape Blanc, Atlantic Ocean

A 200 m long marine pollen record from ODP Site 658 (21°N, 19°W) reveals cyclic fluctuations in vegetation and continental climate in northwestern Africa from 3.7 to 1.7 Ma. These cycles parallel oxygen isotope stages. Prior to 3.5 Ma, the distribution of tropical forests and mangrove swamps reached Cape Blanc, 5°N of the present distribution. Between 3.5 and 2.6 Ma, forests occurred at this latitude during irregular intervals and nearly disappeared afterwards. Likewise, a Saharan paleoriver flowed continuously until isotope Stage 134 (3.35 Ma). When river discharge ceased, wind transport of pollen grains prevailed over fluvial transport. Pollen indicators of trade winds gradually increased between 3.3 and 2.5 Ma. A strong aridification of the climate of northwestern Africa occurred during isotope Stage 130 (3.26 Ma). Afterwards, humid conditions reestablised followed by another aridification around 2.7 Ma. Repetitive latitudinal shifts of vegetation zones ranging from wooded savanna to desert flora dominated for the first time between between 2.6 and 2.4 Ma as a response to the glacial stages 104, 100 and 98. Although climatic conditions, recorded in the Pliocene, were not as dry as those of the middle and Late Pleistocene, latitudinal vegetation shifts near the end of the Pliocene resembled those of the interglacial-glacial cycles of the Brunhes chron.

High-resolution pollen record of ODP Hole 175-1078C

To address the connection between tropical African vegetation development and high-latitude climate change we present a high-resolution pollen record from ODP Site 1078 (off Angola) covering the period 50-10 ka BP. Although several tropical African vegetation and climate reconstructions indicate an impact of Heinrich Stadials (HSs) in Southern Hemisphere Africa, our vegetation record shows no response. Model simulations conducted with an Earth System Model of Intermediate Complexity including a dynamical vegetation component provide one possible explanation. Because both precipitation and evaporation increased during HSs and their effects nearly cancelled each other, there was a negligible change in moisture supply. Consequently, the resulting climatic response to HSs might have been too weak to noticeably affect the vegetation composition in the study area. Our results also show that the response to HSs in southern tropical Africa neither equals nor mirrors the response to abrupt climate change in northern Africa.

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

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 trade 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.

(Fig. 9) Pollen analysis of sediment core GIK16017-2

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 180 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.