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(Quade et al., 1989 ). Other factors have probably contributed to intertropical cool-
ing and desiccation during the past 30 Ma. One was the progressive shrinkage of
the Paratethys Sea. This warm, shallow sea once stretched across Eurasia but shrank
gradually during the Oligocene and Miocene. As the once extensive sea shrank, the
rainfall that was previously well-distributed throughout the year became progressively
more seasonal. A further agent of late Cenozoic cooling was the decrease in atmo-
spheric carbon dioxide associated with increased erosion, weathering and associated
consumption of carbon dioxide caused by the late Cenozoic uplift of the Himalayas,
the Rockies, the Andes, the Ethiopian uplands and perhaps also the Transantarctic
Mountains. The global increase in plants using C 4 photosynthesis and the reduction
in C 3 plants between about 8 and 6 Ma ago (Quade et al., 1989 ) is certainly consistent
with a decrease in the concentration of atmospheric carbon dioxide. The threshold
for C 3 photosynthesis is higher at warmer latitudes, and so it is not surprising that
the initial change from C 3 to C 4 plants occurred in the lowland tropics first. Climatic
cooling was probably also triggered by the eruption of the voluminous Ethiopian flood
basalts over a period of no more than 1 million years around 30 Ma ago (Pik et al.,
2003 ; Pik et al., 2008 ).
Changes in the Cenozoic flora and fauna of the Sahara show a similar trend to
that inferred for the Himalayan foothills of Pakistan. During the Palaeocene and
Eocene, much of the southern Sahara was covered in equatorial rainforest, and there
was widespread deep weathering at this time. During the Oligocene and Miocene,
much of what is now the Sahara was covered in woodland and savanna woodland, but
by Pliocene times many elements of the present Saharan flora were already present
(Maley, 1980 ;Maley, 1981 ;Maley, 1996 ). Pollen preserved in scattered localities in
northern Africa shows that the replacement of tropical woodland by plants adapted
to aridity was already underway during the late Miocene and early Pliocene (Maley,
1980 ;Maley, 1981 ;Maley, 1996 ), a conclusion consistent with the pollen evidence
preserved in deep-sea cores off the north-west coast of Africa (Leroy and Dupont,
1994 ; Leroy and Dupont, 1997 ).
From about late Pliocene times onwards, the great tropical inland lakes of the
Sahara, Ethiopia and Arabia began to dry out. The formerly abundant tropical flora
and fauna of the well-watered Saharan uplands became progressively impoverished
as entire taxa became extinct, and a once integrated and efficient network of major
rivers became increasingly obliterated by wind-blown sands. In the Chad Basin there
is good evidence of wind-blown desert dune sands deposited between alluvial and
lacustrine sediments that were laid down more than 2 million years ago (Servant,
1973 ). Further north, in the Tibesti and Hoggar mountains of the central Sahara, the
evidence from fossil pollen grains shows that some of the plants growing in this region
were already adapted to aridity at about the time that the desert sands made their first
appearance in the Chad Basin (Maley, 1980 ;Maley, 1981 ).
In central China, the first appearance of wind-blown desert dust was initially dated
to around 2.4 Ma ago (Heller and Liu, 1982 ), but further to the north-west in central
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