Geoscience Reference
In-Depth Information
Younger Dryas (
Chapter 8
). Limited periglacial and glacial deposits in the Saharan
and East African uplands are consistent with colder temperatures during the LGM in
northern Africa, with estimated temperature lowering of 4-8
°
C relative to present (see
Chapter 13
).
18.8 Quaternary environments in the Nile Basin and adjacent areas
TheNile Basin occupies the north-east quadrant of Africa and contains a generous slice
of that continent's climatic history. The Nile is the longest river in the world and carries
a large volume of water and sediment from its tropical headwaters through Sudan and
Egypt to the arid north-east coast of Africa. Since at least the Middle Pleistocene,
it has acted as a corridor for and occasional barrier to human dispersal. Holocene
floods fostered the advent of Neolithic farming in the Nile Valley and the subsequent
emergence of one of the world's greatest urban civilizations. The Nile Basin also
provides a unique offshore record of global climatic history. Marine sediment cores
collected from the floor of the eastern Mediterranean show a repetitive sequence of
alternating calcareous muds with a significant content of Saharan wind-blown dust
and dark, organic-rich sediments termed sapropels (Larrasoana et al.,
2003
; Ducassou
et al.,
2008
). Each sapropel unit spans up to about 10 ka in duration and accumulated
during times of enhanced freshwater inflow from the Nile and now inactive Saharan
rivers (Wehausen and Brumsack,
1998
;Scrivneretal.,
2004
). Sapropel 1 (S1) is
coeval with the early Holocene wet phase evident across the eastern Sahara (Kuper
and Kropelin,
2006
). Wetter intervals in the main Nile Valley appear to coincide with
S2 (55 ka), S3 (81 ka) and S4 (102 ka) but are still quite poorly dated (Williams
et al.,
2010b
). S5 (around 124 ka: Lourens et al.,
1996
; Kroon et al.,
1998
) coincides
with a time when the Western Desert of Egypt was a lake-studded savanna occupied
by Middle Palaeolithic hunter-gatherers (Wendorf et al.,
1993
). Recently mapped
lakes and drainage channels in southern Egypt point to periodic overflow from the
Nile into these former lakes (Maxwell et al.,
2010
), prompting us to ask what was
happening upstream at that time. It was also a time when an integrated drainage
network connected the Chad Basin with the Mediterranean, allowing free movement
of
Homo sapiens
across the Sahara (Osborne et al.,
2008
; Drake et al.,
2011
).
Herodotus (ca. 485-425 BC) surmised correctly that the black alluvial clays depos-
ited each year by the Nile came from Ethiopia and were quite unlike the red desert
soils of Syria and Libya, but he was puzzled by the cause of the Nile summer floods
(Herodotus,
1960
). In fact, three different rivers contribute to this flood. The Blue Nile
and Atbara rivers flow from the Ethiopian Highlands and, until dams were built on
both rivers, used to contribute 97 per cent of the annual sediment load and 90 per cent
of the peak discharge in August but only 17 per cent of the June low-season discharge
to the main Nile (Garzanti et al.,
2006
). The White Nile provides 83 per cent of the
low flow but only 10 per cent of the peak discharge to the main Nile.
