Geoscience Reference
In-Depth Information
Table 18.1.
Late quaternary environments in the Sahara and Nile Basin (Modified
from Williams,
2012a
.)
I. 250-25 ka
(Interval includes two glacial-interglacial cycles, each
100 ka long)
High White Nile floods at
>
240, 210
±
30, 200
±
40, 166
±
30 and
125 ka coeval with
intervals of stronger Northern Hemisphere (NH) summer monsoon
Sapropel accumulation in Eastern Mediterranean Sea at 217 ka (S8), 195 ka (S7), 172 ka
(S6), 124 ka (S5), 102 ka (S4), 81 ka (S3) and 55 ka (S2), broadly coincident with phases
of very high Nile discharge
Lakes full and fresh during last two interglacial phases
Integrated drainage across Sahara during last interglacial phase
II. 25-17 ka
(Interval includes the Last Glacial Maximum (LGM: 21
2 ka))
Lakes drying out in headwaters of Blue and White Nile rivers coeval with southward
displacement of the ITCZ relative to today during NH summer and weakening of the
summer monsoon; soils form on floor of Lake Albert at 20.7-17.7 ka and 16.5-15.1 ka
White Nile deprived of overflow from Ugandan lakes and reduced to a seasonal trickle and
its lower reaches blocked by desert dunes
Lakes drying across the Sahara and East Africa
Temperatures 4-8
±
C cooler than today in the Ethiopian Highlands, with local glaciation in
the Semien and Bale Mountains and intense periglacial action and mass movement on
mountain slopes; upper tree-line lowered
°
1,000 m; bare unstable slopes supply abundant
coarse debris to highly seasonal Blue Nile and Atbara rivers
Widespread deposition of coarse sand and fine gravel across the Gezira alluvial fan in
central Sudan and along the main Nile Valley in Sudan and Egypt; main Nile probably
dries out during winter months; humans mostly abandon the main Nile Valley and migrate
south in search of more reliable supplies of water
Sand dunes active up to 500 km south of present southern limit of the Sahara
Dust storms active, with export of desert dust to Europe, Negev Desert and Amazon Basin
III. 17-5 ka
Lakes in Blue and White Nile source regions begin to rise at
17 ka and overflow
perennially at 15-14.5 ka
Stronger summer monsoon and ITCZ extends
500 km further north than today during
the NH summer
Upper catchments of Blue and White Nile densely vegetated and soil formation active
Perennial channel flow re-established in Blue and White Nile and main Nile, which now
carry a large seasonal suspension load of silt and clay
Blue Nile incised
>
10 m into its former floodplain since 15 ka and
>
4 m since 9 ka,
5ka
High White Nile flood levels at ca.14.7-13.1, 9.7-9.0, 7.9-7.6, 6.3 and 3.2-2.8 ka; high
Blue Nile flood levels at ca.13.9-13.2, 8.6, 7.7 and 6.3 ka
Sahara is once more studded in sporadic lakes and supports a human population of
Mesolithic hunter-fisher-gatherers and later Neolithic pastoralists
A composite sapropel (S1) accumulates in the Eastern Mediterranean Sea between
beheading its Gezira distributary channels, which dry out by
13.7
and12.4 ka near the base and between
9.9 and 8.9 ka near the top; the gap between the
two sapropel subunits may denote the influence of the Younger Dryas (YD) episode
