Geoscience Reference
In-Depth Information
deeply weathered mantles, creating a major erosion surface in north-east Africa and
adjacent parts of peninsular Arabia. This surface was later disrupted by tectonic
movement along zones of pre-existing crustal weakness, leading in stages to the
formation of the Red Sea, the Gulf of Aden, the Afar Rift, the Ethiopian and Kenya
rifts, and the Dead Sea Rift.
Superimposed on the trend of late Cenozoic desiccation, which is evident in the
pollen record and that of large lakes and river systems, there were short-term climatic
fluctuations. These were modulated by changes in orbital geometry, which controlled
the length of glacial-interglacial cycles and changes in the strength of the summer
monsoon. In intertropical Africa, glacial maxima were generally times of greater
aridity and of temperatures 4-8
C lower than the present. In a number of instances,
local hydrologic and topographic conditions counteracted the influence of regional
changes in climate, as, for example, at Lake Masoko in Tanzania, which was relatively
high when Lake Malawi nearby was relatively low during the LGM (Gasse et al.,
2008
).
In the southern Negev, desert dunes were active during the LGM, although the
climate in that area was wetter than it is today. Stronger winds appear to have been
the primary cause of the linear dune activity. In the Kalahari and Namib, the climatic
signal conveyed by desert dunes is blurred by the influence of other factors, such as
sand supply, wind strength and changes in surface cover, rather than aridity alone.
Desert dunes were active along the southern margin of the Sahara during the cold, dry
LGM and again during the cold, dry Younger Dryas. The LGM Nile was reduced to a
seasonal trickle during the prolonged LGM drought, as were other big rivers, like the
Niger and Senegal. The lakes in East Africa became shallow, and both Lake Victoria
and Lake Tana in the White Nile and Blue Nile headwaters, respectively, dried out
completely and soils formed on their exposed lake floors.
The abrupt return of the summer monsoon at 14.5 ka ushered in an era of plant
and animal abundance, of widespread small lakes in East Africa, the Sahara and
Arabia, and of Mesolithic hunter-fisher-gatherers and their cattle-herding Neolithic
successors. Climatic desiccation set in anew from about 5 ka onwards, forcing the
Neolithic pastoralists to migrate to wetter latitudes or favourable environments like
the Nile Valley.
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