Geoscience Reference
In-Depth Information
Figure 7.1. Strontium isotopic composition of Blue and White Nile waters and of
lakes in the White Nile headwaters. The numbers are radiocarbon ages of dated late
Quaternary samples from sites along the Blue and White Nile valleys. (After Talbot
et al.,
2000
.)
Victoria and Albert overflowed into the upper White Nile. They also analysed the
strontium isotope ratio (
87
Sr/
86
Sr) preserved in freshwater gastropod shells collected
from Blue and White Nile sediments ranging in age from terminal Pleistocene to
present-day and compared the values obtained with the strontium isotope ratios from
the Ugandan lakes (
Figure 7.1
). Because these ratios are not changed by weathering
and fluctuating hydrological cycles, the strontium ratios of river and lake waters give a
weighted average for the type of rocks within the various basins making up the overall
river system. For example, in volcanic catchments, the
87
Sr/
86
Sr ratios will generally
be
0.706, and in those on Precambrian metamorphic basement complex rocks, the
ratios will be
<
0.722 (Williams et al.,
2006c
). All of the shells analysed had been
tested for carbonate recrystallization using X-ray diffraction. The results showed
that overflow from Lake Victoria was underway by around 14.5 ka and provided
an independent confirmation that the present-day integrated Nile drainage network
became re-established at that time, a conclusion confirmed by later studies (Williams
et al.,
2000
; Johnson et al.,
2000
; Williams et al.,
2006c
; Williams
2009b
; Williams
et al.,
2010b
).
Fluctuations in the
87
Sr/
86
Sr values preserved in Nile Delta clays have been used
to reconstruct a Holocene history for the main Nile (Krom et al.,
2002
; Stanley et al.,
2003
; Woodward et al.,
2007
), as well as for Lake Albert (Williams et al.,
2006c
).
One dramatic event recorded by the strontium isotopic data is the sudden collapse of
>
