Geoscience Reference
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the parent sediments originated in the volcanic highlands of Ethiopia, were ferried
down by the Blue Nile and were later reworked as source-bordering dunes (Williams
and Adamson,
1973
; Williams,
2009b
). The channels consist of a veneer of clay over
sands and gravels. The channel sands become older with depth and have OSL ages
between 70 ka and 115 ka, indicating a prolonged phase of fluvial sand entrainment
and deposition. Within the error terms, the
10
Be age of 109
±
8 ka for the strandline
is statistically similar to the OSL age of 115
10 ka for the fluvial Blue Nile channel
sands and is consistent with backfilling of the White Nile Valley from Blue Nile
floodwaters brought in by the former channel during the last interglacial (Barrows
et al.,
2014
). The dimensions of the 386 m White Nile strandline suggest that the last
interglacial Blue Nile peak floods were more extreme than the 14.5 ka floods that gave
rise to the 382 m lake in the lower White Nile Valley. However, the last interglacial in
North Africa was not uniformly wet but instead showed significant climatic variation
(Rohling et al.,
2002
).
Independent evidence that the peak of the last interglacial was significantly wetter
than the early Holocene comes from four quite separate sources: (A) The speleothem
records from the Negev and peninsular Arabia provide unequivocal evidence that the
last interglacial was much wetter than the early Holocene (Vaks et al.,
2006
;Vaks
et al.,
2007
;Vaksetal.,
2010
; Fleitmann et al.,
2011
). (B) The record of last interglacial
lakes from southern Libya and the eastern Sahara also shows that the climate was far
wetter than it was during the early Holocene (Wendorf et al.,
1993
; Geyh and Thiedig,
2008
; Maxwell et al.,
2010
). (C) The rivers that flowed across the central and eastern
Sahara during the last interglacial never attained the same degree of integration during
the Holocene (Osborne et al.,
2008
; Drake et al.,
2011
). (D) Finally, global sea level
was 5.5-9 m higher during the last interglacial (MIS 5e, around 125 ka), consistent
with significant ice melting in Greenland and the Antarctic (Dutton and Lambeck,
2012
). Age calibration of the Red Sea last interglacial sea level record with the fine
resolution Soreq Cave speleothem chronology indicates peak sea level at 132-126 ka
(Rohling et al.,
2008
; Grant et al.,
2012
), somewhat older than but broadly consistent
with the OSL age of 115
±
10 ka for the Blue Nile paleochannel and the
10
Be age of
±
109
8 ka for the 386 m White Nile strandline. Higher Nile discharge evident in the
Mediterranean sapropel record (Lourens et al.,
1996
; Wehausen and Brumsack,
1998
;
Larrasoana et al.,
2003
; Scrivner et al.,
2004
) accords with a warmer global climate
and stronger summer monsoon during MIS 5e. The 109 ka
10
Be age is the first direct
age for the 386 m strandline and represents the maximum extent of this 'reservoir
lake'. As the lake gradually receded, sandy sediments were laid down between the
high strandline and the present White Nile (Williams et al.,
2003
, fig. 2). Vertical
incision in the main Nile after the last interglacial (Butzer,
1980
, Williams et al.,
2010b
) would have lowered the White Nile base level from 386 to 382 m by the
terminal Pleistocene. Renewed Nile incision in the mid-Holocene lowered the base
level by a further 6-7 m on the main Nile (Bell,
1970
) and 8-10 m at Khartoum
±
