Geoscience Reference
In-Depth Information
ice sheets in high northern latitudes during the late Pliocene (Schnitker,
1980
; Loubere
and Moss,
1986
; Prentice and Denton,
1988
). Oxygen isotope evidence from deep sea
cores indicates that the onset of major Northern Hemisphere continental glaciations
at 2.4
±
0.1 Ma (Shackleton et al.,
1984
) also coincided with cooling in high southern
latitudes (Kennett and Hodell,
1986
).
The 2.5-2.3 Ma temperature drop is also evident in the south-eastern uplands of
Ethiopia (Bonnefille,
1983
) and the dry northern interior of China, with the beginning
of widespread loess accumulation in the Loess Plateau of central China dated to 2.4Ma
(Heller and Liu,
1982
). In the north-western Mediterranean region, the presence of
a Mediterranean vegetation adapted to winter rains and summer droughts is already
evident at 3.2 Ma, but it is not developed in its modern form until about 2.3 Ma (Suc,
1984
). Magnetic susceptibility measurements of deep sea cores from the Arabian Sea
and the eastern tropical Atlantic also reveal a change in the length of astronomically
controlled climatic cycles at this time. Before 2.4 Ma, the dominant cycles are the 23-
ka and 19-ka precession cycles, but after 2.4 Ma, the 41-ka obliquity cycle becomes
dominant (Bloemendal and deMenocal,
1989
).
As the two poles became progressively colder, high-latitude sea surface temperat-
ures also declined. As a result, the temperature and pressure gradients between the
equator and the poles increased. There was a corresponding increase in Trade Wind
velocities, and therefore in the ability of these winds to mobilise and transport the allu-
vial sands of the Saharan depocentres and to fashion them into desert dunes (
Chapter
8
). Higher wind velocities were also a feature of glacial maxima during the Pleisto-
cene and, as we saw in
Chapter 9
, were responsible for transporting Saharan desert
dust far across the Atlantic (Parkin and Shackleton,
1973
; Parkin,
1974
; Williams,
1975
; Sarnthein,
1978
; Sarnthein et al.,
1981
).
The late Cenozoic cooling of the ocean surface would have helped reduce precip-
itation in the intertropical zone. About two-thirds of global precipitation now falls
between latitudes 40
S and depends on evaporation from the warm tropical
seas (Galloway,
1965a
). The reduction in evaporation from the tropical ocean that was
associated with the global cooling resulting from high-latitude continental accumula-
tion and enhanced cold bottom-water circulation would also lead to reduced rainfall
across North Africa.
°
N and 40
°
18.4 Uplift and erosion of the Afro-Arabian dome
During the Oligocene, there was a prolonged phase of slow crustal doming of the
region centred on the northern Red Sea and southern Levant to form the Afro-Arabian
dome (Bowen and Jux,
1987
). This approximately 1,500 km wide elliptical dome
extended across Ethiopia and Yemen, with its long axis aligned from south-south-
west to north-north-east over a distance of about 3,000 km, and was located above the
Afar plume (Avni et al.,
2012
). The uplift led to reactivation of pre-existing faults and
