Geoscience Reference
In-Depth Information
fundamentally, it was also a time when Lakes Victoria, Albert and Edward in Uganda
were dry or at very low levels and no longer flowing into the White Nile (Beuning
et al.,
1997b
;Lærdaletal.,
2002
;Stageretal.,
1986
; Stager and Johnson,
2000
;Stager
et al.,
2002
; Stager and Johnson,
2008
). The resumption of overflow from Lakes Vic-
toria and Albert at 15-14.5 ka (Williams et al.,
2006c
; Talbot and Williams,
2009
)
is indicated by the presence of cross-bedded fluvial sands in the lower White Nile
Valley, dated by OSL to 13.3
0.9 ka (Williams et al.,
2010b
),andinvolvedavery
different flow regime for the White Nile before the Sudd swamps in southern Sudan
had time to become established once more, thereby acting as a gigantic physical filter
only allowing the passage downstream of fine silt and clay particles. The abrupt return
of the summer monsoon at 14.5 ka was not only seen in overflow from the Ugandan
headwaters of the White Nile (Williams et al.,
2006c
) but also from Lake Tana in
the Ethiopian headwaters of the Blue Nile (Lamb et al.,
2007
; Marshall et al.,
2011
).
This was also a time when the summer monsoon became intensified across tropical
Africa (Williams et al.,
2006c
;Lambetal.,
2007
; Gasse et al.,
2008
; Williams,
2009b
;
Lezine et al.,
2011
).
An independent record of climatic fluctuations near the White Nile headwaters is
provided by Lake Challa, a crater lake on the eastern flank of Mount Kilimanjaro,
which was very high from 10.5 to 8.5 ka (Verschuren et al.,
2009
), consistent with very
wet conditions in the White Nile headwaters at this time and the presence of a large
freshwater lake in north-west Sudan at about the same time (Hoelzmann et al.,
2000
).
The four periods of low Holocene lake levels (8.0-6.7 ka, 5.9-4.7 ka, 3.6-3.0 ka,
0.7-0.6 ka) identified by Verschuren et al. (
2009
) at Lake Challa also coincide with
times of low flow in the White Nile. The dry interval starting at 3.6 ka may be coeval
with the sharp decrease in rainfall along the southern Dead Sea at around 3.9 ka
(Frumkin,
2009
), possibly indicating that this arid phase may have been widespread,
but until a great deal more evidence is forthcoming, this must remain speculative.
More than fifty years ago, Grove and Pullan (
1963
) mapped the ancient shorelines
of a greatly expanded Lake Chad, later shown to be of late Pleistocene and early to
mid-Holocene age (Servant,
1973
; Servant and Servant,
1980
). Over thirty years later,
Armitage et al. (
2007
) obtained last interglacial OSL ages for the Bama Ridge, which
was built during their Lake Mega-Chad phase, and OSL ages of 100-110 ka for Lake
Megafezzan in south-west Libya.
Six years after his pioneering work on Lake Chad and more than 4,000 km fur-
ther south, Grove (
1969
) mapped the dunes and lake shorelines associated with the
distal sector of the Okavango River in the Kalahari Desert of Botswana. Cooke
and Verstappen (
1984
) used air photographs to map what they termed lake 'Palaeo-
Makgadikgadi' in the depression of that name, today occupied by a series of shallow
ephemeral lakes and pans. They estimated that this former lake occupied an area of
about 37,000 km
2
at an elevation of 945 m, and they identified two main lower lake
levels at 920 m and 912 m. Within the Makgadikgadi depression, they also mapped
±
