Geoscience Reference
In-Depth Information
Scott and Woodborne (
2007
) used pollen preserved in hyrax dung to reconstruct
late Quaternary vegetation changes in the winter rainfall area of south-west Africa.
They deduced that the climate was cold and dry at 21-20 ka, warmer and wetter at
21-19 ka, cold and dry at 19 ka, wetter at 17.5 ka, warmer and drier at 16 ka, dry
at 13-12 ka, warmer and wetter at 12-9.5 ka, slightly cooler at 11 ka and showing a
decrease in summer rainfall at 5.6-4.9 ka. They noted that there could have been a
slight increase in summer rain during the LGM and again possibly during the mid-
Holocene, which would imply a possible southward displacement of the westerly
wind belt, a conclusion contrary to that of Chase and Meadows (
2007
), which future
work should resolve.
The marine sediment record has contributed greatly to our knowledge of late
Quaternary (and older) climates in southern Africa along both the western and eastern
coasts (Stuut et al.,
2002
; Stuut and Lamy,
2004
; Stuut et al.,
2004
; Dupont et al.,
2005
; Pichevin et al.,
2005
; Schefu
et al.
(
2011
) analysed a marine sediment core located about 100 km off the Zambezi delta.
They used the hydrogen isotopic composition of certain lipid biomarkers derived from
higher plants to reconstruct the hydrologic changes in the Zambezi basin during the
last 18,000 years. These changes closely reflected changes in
β
et al.,
2011
; Maslin et al.,
2012
). Schefu
β
18
O in the Greenland
NGRIP ice core, showing that during cold events in the Northern Hemisphere such
as the Younger Dryas (around 12.8-11.6 ka) and Heinrich stadial 1 (16.8-14.6 ka),
rainfall was heavier and discharge was greater in the Zambezi drainage basin, probably
as a result of the southward displacement of the ITCZ in the austral summer. Rainfall
was also high in the last 4 ka, when local summer insolation was high. Pichevin et al.
(
2005
) analysed the sediments in a 190 ka marine core off the southern Namib Desert
and concluded that the mean accumulation rate of quartz grains could serve as a long-
term indicator of aridity on land. They inferred that aridity in the Namib was greatest
when summer insolation was low over the southern tropics. Stuut et al. (
2002
;
2004
)
and Stuut and Lamy (
2004
) also used changes in particle size in marine cores off the
south-west coast of Africa to reconstruct past changes in Trade Wind strength related
to changes in orbital precession, obliquity and eccentricity. An overriding problem
with such an approach involves distinguishing eolian silts and fine sands from fluvial
silts and fine sands. The issue is compounded by the presence in many Namib valleys
of Pleistocene loess that has been reworked by running water and deposited as valley
fills.
18.11 Conclusion
The long-term desiccation of the Sahara, Arabia and southern Africa is linked to the
northward movement of the Afro-Arabian plate, uplift and volcanism in East Africa
and southern Arabia, rifting, drainage disruption and the development of internally
drained depressions. Oligocene and later uplift and erosion stripped away previous
