Geoscience Reference
In-Depth Information
events. Slow northward movement of the African plate during the last 100 million
years (late Mesozoic and Cenozoic) resulted in the migration of North Africa from
wet equatorial into dry tropical latitudes. A slight clockwise rotation of Africa began
about 15 million years ago and continued through the Miocene and Pliocene, bringing
Africa into contact with Europe. This displacement was accompanied by crustal
deformation and rapid uplift in the Atlas region and by volcanic eruptions and gentle
updoming in Jebel Marra (3,042 m), Tibesti (3,415 m), the Hoggar (2,918 m) and the
Aır Mountains. Owing to their altitude, the high mountains of the central and southern
Sahara have always been wetter than the surrounding desert plains and so may have
served as refugia for plants, animals and humans throughout the Quaternary.
Two additional factors were responsible for the late Cenozoic desiccation of the
Sahara-Arabia and Asian deserts. One was the gradual expansion of continental ice in
high latitudes, which was associated with the cooling of the Southern Ocean. The final
separation of Australia from Antarctica some 45 Ma ago culminated in the establish-
ment of a large ice cap on Antarctica by 34 Ma ago. The closure of the Panama
Isthmus and diversion of warm water into the North Atlantic, in conjunction with
high northern latitude cooling, provided the impetus for a sudden increase in the
volume of Northern Hemisphere ice caps around 2.5 Ma ago.
One effect of the progressive build-up of high latitude ice sheets, noted in the pr-
evious section, was to steepen the temperature and pressure gradients between the
equator and the poles, resulting in increased Trade Wind velocities. Faster Trade
Winds were better able to mobilise the alluvial sands of an increasingly dry Sahara
and to fashion them into desert dunes. For example, the first appearance of wind-
blown quartz sands in the Chad Basin occurs towards the end of the Cenozoic, when
they were interstratified among late Pliocene to early Pleistocene fluviatile and lacus-
trine sediments (Servant,
1973
; Servant and Servant-Vildary,
1980
; Sepulchre et al.,
2006
). The associated lacustrine diatom flora indicates temperatures cooler than those
now prevalent in this region. The combined evidence suggests that the late Pliocene
was both cooler and drier along the tropical borders of the Sahara. The diatom and
pollen evidence from a large late Pliocene lake at Gadeb in the south-eastern uplands
of Ethiopia (Gasse,
1980
; Bonnefille,
1983
) is also consistent with the inference
that intertropical cooling and desiccation may have been closely bound up with the
expansion of Northern Hemisphere ice caps around 2.5 Ma ago.
A further factor contributing to the drying out of the Sahara and Arabian deserts
was the late Cenozoic uplift of the Tibetan plateau and the ensuing creation of the
easterly jet stream that brought dry subsiding air to the incipient deserts of Pakistan,
Iran, Arabia, Somalia, Ethiopia and the Sahara. Isotopic analysis of fossil soils and
fossil herbivore teeth collected from the Potwar Plateau of Pakistan indicates a major
change in flora and fauna between 7.3 and 7 Ma ago. Until about 7.3 Ma ago, forest
and woodland dominated the landscape. After 7 Ma, there was a rapid expansion
of tropical grassland at the expense of the forest. This change in vegetation may
indicate the inception (or strengthening) of the Indian summer monsoon 7 Ma ago
