The climatic conditions of the Middle Holocene (5,000 yr BP) can be derived quite realistically through numerous proxy-data from different sources (sea-level changes, lake sediments, etc.). The vegetation cover of 5,000 yr BP was reconstructed from dated pollen and macro-remains. The distribution of available data is illustrated in the map of the data-base for 5,000 yr BP. Unfortunately no SST data are available for this time slice because of the low time-resolution of deep-sea-drilling-cores.

 

Although the climatic optimum phase persisted since 9,000 BP, the 5,000 yr BP time-slice shows a regional differentiated situation.

The East African highlands showed evidence for increasing aridity in its northern parts, whereas on the southern hemisphere the humid conditions persisted.
Regional characteristics:

• pronounced lake level oscillations in the Afar-Depression between 6,000 and 4,000 yr BP (Gasse & Street 1978), dessiccation after 4,000 BP
• lowering of lake levels of Ziway-Shala (Ethiopia; Gillespie et al. 1983), Lake Turkana (Kenya; Mäckel et al. 1989),  after 5,000 BP
• Lake Abiyata, at 6,000 yr BP still receiving about 450-500 mm/a of precipitation, runs almost dry in the the following 1,000 years (Lézine & Bonnefille 1982, Gasse 1977)
• changing vegetation: north of Lake Victoria, semideciduous rainforest appeared southwest of Lake Mobutu (with a 3 to 4 months dry season) at the expense of evergreen rainforest (Kendall 1969, Ssemanda & Vincens 1993)
• the mountainous forests of Uganda and Burundi (Rukiga Highlands, Muchoya Swamp, Ruwenzori) showed a vegetation comparable to today's natural vegetation, with similar humidity conditions, but increased temperatures (Taylor 1990, Morrison 1968, Livingstone 1967, Bonnefille et al. 1991)

The shifting of drier conditions from North to South can also be determined for West- and Central-Africa. In the southern parts more humid conditions persisted as well.

• humid evergreen rainforest at Niari, Congo (Vincens et al. 1998)
• semideciduous rainforest at Pointe Noire, S-Congo (Elenga et al. 1992, Schwartz et al. 1992)
• after marine pollen cores off SW-African coast between 5,600 and 4,200 yr BP occurred the most humid and warm conditions (Shi et al. 1998)
• Expansion of the mangroves in West-Africa due to sea level rising of about 3 m and increaesed temperatures (Faure & Elouard 1967, Sowumni 1979); evergreen rainforests in the hinterland
• for the last time, semideciduous rainforest species dominated the depressions along the atlantic coastline of Senegal (Niayes), the inland was characterized by dry forests (Lézine 1988, Frankenberg & Anhuf 1989)
• already drier conditions prevailed within the Sahel: open dry savanna at Ari Kourkouri (S-Niger; Schulz et al. 1995, Fontes & Gasse 1991), sahelian savanna at Tjéri (Niger; Maley 1977,1981)

In the Sahelian zone (at about 16°N) trees and shrubs, especially the sudanian floristic elements, decreased, showing increasing aridification.

• grass savannah with Acacia and Capparidaceae at Termit Ouest (Niger; Faure et al. 1963, Schulz et al. 1995), Sahelian tree savannah at Oyo (Sudan; Ritchie et al. 1985)
• shifting of the aharo-sahelian border of about 200 km southwards (compared to its position at about 20°N at 8,000 yr BP) (Schulz 1987, Schulz 1991b)
• decline of lake lavels, intensified after 3,700 yr BP (Lauer & Frankenberg 1980)

Sahara

• In the Eastern Sahara, north of 20° N, the strengthened aridification started around 6,500 yr BP (Pachur & Wünnemann 1996). At 5,000 yr BP a desert-like vegetation occurred in the Libyan Desert of Sudan (Burg et Tuyur, Wadi Shaw & Wadi Sahal; Neumann 1993). Further south more humid conditions lasted until 4,500 yr BP, providing a thorn- and grass savannah (Oyo, Gebel Tageru, Sudan; Pachur et al. 1990). At Wadi Howar, Sudan (16/17°N) increasing aridity started at 5,300 yr BP and a sahelian savannah established, but already restricted to wadi areas (Neumann 1991a, 1993).
• The dry depressions of the Central Sahara showed a decisive aridity at 5,000 yr BP. The former existing lakes have disappeared (Zoo Baba, Seguedine; Baumhauer 1987, Schulz 1994). The predominant vegetation is the Achab-flora and Acacia-Panicum-savannah in the water-favoured wadis. The montainous regions (Tassili N'Ajjer, Hoggar, Tibesti) show also dry conditions, dominated by herbage, tussock-grasses and small shrubs (Ballouche et al. 1995, Messerli & Winniger 1992)
• In the western Sahara numerous lake sediments can be found (Taoudenni-Agorgott, Schulz 1991b; Tanezrouft, Petit-Maire et al. 1990, Ine Kousamene Depression, Hillaire-Marcel et al. 1983), providing a second, but  weaker, holocene humid phase in this region. On the basis of pollen analysis no changes to a more humid vegetation type can be stated, but a slow transition to the today's conditions (Schulz 1991a).

The mediterranean part of Northern Africa is also influenced by a noticeable climatic deterioration, i.e. a lowering of temperatures in the western Maghreb and increasing temperatures in the eastern region, perhaps also an increasing aridity:

• deciduous forests were restricted to the High and Middle Atlas, the Rif-Mountains, the Tell (Algeria) and the Kroumirie (Tunisia)
• coniferous forests (i.e. with Cedrus atlantica) can be found in the same regions as at 8,000 yr BP, but in reduced extension (Middle Atlas, Rif, Aures mountains), in the High Atlas they have been replaced by evergreen oak forests. These forests spred also into the zone of deciduous oak forests, indicating an enhanced anthropogenic impact (Lamb et al. 1989, Ballouche & Damblon 1988). Also Quercus suber began to spread in the whole region.
• shrub formations (macchie) with Qu. suber and coccifera characterized the coastal regions of Morocco and Tunisia (Ben Tiba & Reille 1982). Artemisia became much more frequent in the Tunisian steppe, but the general aspects had not changed between 8,000 and 5,000 yr BP (Brun 1983).

Go to data-base for 5,000 yr BP
 

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© 2000 Department of Physical Geography, University of Mannheim
Editor: Birgit Schröder