Geoscience Reference
In-Depth Information
This lake attained a maximum elevation of 382 m and was up to 20 km wide before
it began to recede, leaving behind a scatter of dead aquatic snails. The shells were
rapidly buried in several metres of terminal Pleistocene and early Holocene alkaline
alluvial clays, and they were therefore preserved as dateable evidence of the 382 m
White Nile paleo-lake (Williams,
2012a
).
The isotopic record preserved within lake shells, lake sediments and groundwater
carbonates also provides an invaluable paleoenvironmental record and can be used to
distinguish between lakes fed primarily or solely by local run-off from those fed by
groundwater (Cerling et al.,
1977
; Williams et al.,
1987
; Abell and Williams,
1989
;
Gasse and Fontes,
1989
; Talbot,
1990
; Ayliffe et al.,
1996
; Ito,
2001
; Talbot,
2001
). It
can also be used to detect major climatic changes within the lake basin. For example,
Cerling et al. (
1977
) deduced that rainfall decreased sharply around Lake Turkana
in northern Kenya 1.8-2.0 Ma ago and in the Olduvai Gorge region of semi-arid
Tanzania 0.5-0.6 Ma ago. This sudden reduction in precipitation in these two regions
was shown by a major increase in the proportion of the heavier isotope of oxygen
(
18
O, see
Chapter 7
) within pedogenic and groundwater carbonates.
A younger example comes from a series of shallow clay pans in the presently arid
region 15 kmwest of the lower White Nile, in latitude 15
N (Ayliffe et al.,
1996
).
The clay pan sediments contain abundant freshwater gastropod shells dated between
9.9 and 7.6 ka, with most ages (eleven out of fourteen) concentrated at 9.0-8.4 ka
(Williams and Jacobsen,
2011
), together with some land snails and several semi-
aquatic species of snail. Stable carbon and oxygen isotope analysis of the gastropod
shells shows that the shell carbonate is highly depleted in
13
Cand
18
O (see
Chapter
7
). The strongly negative oxygen isotope values show that the region at that time had
much lower rates of evaporation than it does today. In addition, the extreme variability
in isotopic composition (up to 6-7 per cent PDB) indicates a seasonal rainfall regime
with a high degree of variation from year to year. Ayliffe et al. (
1996
) concluded
that there was a stronger south-west monsoon and an associated northward shift of
the summer rainfall zone during the time when the clay pans contained water and
that this part of the eastern Sahara was both wetter and possibly cooler with lower
rates of evaporation around 9.9-7.6 ka. This was also a time when lake levels were
high elsewhere in northern Africa, pointing to a regionally wetter climate at that time.
Later work has vindicated this claim (Hoelzmann et al.,
2004
; Williams et al.,
2010b
;
Marriner et al.,
2012
; Blanchet et al.,
2013
).
Johnson et al. (
1990
) provided a concise report on the aims of the International
Decade for the East African Lakes (IDEAL) drilling project, together with a sum-
mary of the past variations in eight of the main target lakes (Mobutu, Edward, Kivu,
Tanganyika, Rukwa, Malawi, Turkana, Victoria) and a brief review of their poten-
tial as archives of climatic history. The type of proxy climatic data that the sedi-
ments of these (and other) large lakes can provide includes temperature, precipitation,
seasonal variability, wind and cloudiness. Temperature can be inferred from grass
°
22.5
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