Geoscience Reference
In-Depth Information
Northern Africa moved away from wet equatorial latitudes into the dry subtropics.
Growth of the great continental ice sheets and cooling of the oceans led to a decrease
in precipitation and an increase in the strength of the Trade Winds. At the start of the
Oligocene, there was a sharp drop in sea surface temperatures, which had eventual
global repercussions. The late Cenozoic desiccation of the Sahara was not a continu-
ous process but took place in stages. Evidence from marine cores collected off the
west coast of the Sahara indicates that the terrestrial climate over the Sahara was rel-
atively cold and dry at 24-20, 18-14, 13-9.5, 7.5-5.3, 3.2-1.9 Ma and from 0.73 Ma
onwards (Sarnthein et al.,
1982
). During these intervals, the ITCZ displayed seasonal
migrations similar to those of today, meridional winds were stronger and zonal winds
were weaker. River loads were much reduced between latitudes 10
N, and
dust flux into the Atlantic was increased. During the intervening stages (20-18, 14-
13, 9.5-7.5, 5.3-3.2 and 1.9-0.73 Ma), the climate seems to have been less arid,
with intervals of intense river discharge into the ocean. These climatic fluctuations
were superimposed on the long-term desiccation caused by the northward drift of the
African plate during the Neogene. Apart from tectonic uplift, what other factors were
responsible for this dramatic change from a landscape of lowland equatorial rainforest
to one of bare, rocky inselbergs and desert dunes?
Three additional influences seem to have contributed to the late Cenozoic desic-
cation of the Sahara. These were the uplift of the Tibetan Plateau, the build-up of
continental ice in Antarctica and the Northern Hemisphere, and the cooling of the
world's oceans. We have already considered the possible causes of these phenomena
in
Chapter 3
. Our concern here is with their effects on the Sahara.
Uplift of the vast Tibetan Plateau was an important factor contributing to the late
Cenozoic desiccation of the Sahara and was associated with the intensification or even
the onset of the easterly jet stream that today brings dry subsiding air to the deserts of
Arabia and northern Africa, including the Horn of Africa (Flohn,
1980
). A significant
change in the late Miocene climate is evident in East Africa (Cerling et al.,
1997
)
and Pakistan. Quade et al. (
1989
) identified a major change in the flora and fauna
of the Potwar Plateau in the Siwalik foothills of Pakistan between 7.3 and 7.0 Ma,
which may be related to Himalayan uplift and is consistent with the intensification or
perhaps the inception of the Indian summer monsoon. This interval is coeval with one
of the drier intervals identified by Sarnthein et al. (
1982
) for the west Sahara.
Although remote from the Sahara, the accumulation of continental ice in Antarctica
also contributed to Saharan desiccation. There were mountain glaciers in Antarctica
early in the Oligocene, and a large ice cap was well-established by at least 10 Ma
(Shackleton and Kennett,
1975
). Continental ice was slower to form in the Northern
Hemisphere but was present in high northern latitudes by 3 Ma, and possibly by 5 Ma
or even well before then, with a rapid increase in the rate of ice accumulation around
2.5-2.4 Ma (Shackleton and Opdyke,
1977
; Shackleton et al.,
1984
). Closure of the
Panama Isthmus around 3.2 Ma paved the way for rapid accumulation of continental
°
and 28
°
