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and ensuing tropical climatic desiccation are better constrained than the older glacial
events described in
Chapter 3
, but they still offer scope for differing interpretations.
The same is true of when and why East Africa became arid.
For example, Cane and Molnar (
2001
) proposed that closure of the Indonesian
seaway 3-4 Ma ago as a result of northward displacement of New Guinea in the
early Pliocene would have triggered a change in the source of water flowing through
Indonesia into the Indian Ocean from previously warm South Pacific waters to cooler
North Pacific waters. The concomitant decrease in sea surface temperatures in the
Indian Ocean could have reduced rainfall over East Africa. However, it seems unlikely
that closure of the Indonesian seaway was the sole cause of late Pliocene desiccation
in East Africa, since this region derives its moisture from both the South Atlantic and
the Indian Ocean. It is equally possible that the late Pliocene increase in aridity evident
in East Africa and Ethiopia 3-4 Ma ago (Feakins et al.,
2005
) may have arisen from
the closure of the Panama Isthmus and the northward diversion of the warm equatorial
water which until then had flowed westwards from the Atlantic into the Pacific Ocean.
The presence of warm, moist air over the North Atlantic, coupled with a decrease in
insolation that was linked to increased orbital eccentricity and a decrease in the tilt of
the earth's axis (leading to cooler high-latitude northern summers and milder northern
winters), was a prerequisite for widespread and persistent snow accumulation over
North America (Williams et al.,
1998
).
The rapid accumulation of ice over North America at 3.5-2.5 Ma was accom-
panied by global cooling and intertropical aridity, revealed in the drying out of the
large late Pliocene/early Pleistocene tropical lakes and rivers of the Sahara and East
Africa. The emergence of stone tool-making at this time in East Africa may have
been an adaptation by our ancestors to the increase in seasonality and the need to
diversify their sources of food protein. As we have seen in
Chapter 9
this was also
a time of widespread loess accumulation in central China (Heller and Liu,
1982
)
and of the first appearance of stony desert plains in central Australia (Fujioka et al.,
2005
). The region around the Mediterranean also developed its now characteristic
dry summer, wet winter climatic regime. The net effects of these late Cenozoic
environmental changes were an increase in the temperature gradients between high
and low latitudes, a more seasonal rainfall regime, a reduction in forests and the
replacement of woodlands by deserts in North Africa, Arabia and Australia, and the
emergence in Africa some 2.5Ma ago of upright-walking, stone tool-making ancestral
humans.
18.6 Cenozoic uplift of the Ethiopian Highlands and Blue Nile incision
The history of the Nile is closely tied up with tectonic, volcanic and climatic events
in its Ethiopian and Ugandan headwaters (Talbot and Williams,
2009
; Williams and
Talbot,
2009
). Between Lake Tana in the Ethiopian Highlands and the modern Sudan
border, the Blue Nile is entrenched into a plateau about 2.5 km in elevation and has cut
