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of reduced rainfall. For instance, Cerling et al. (
1977
) used variations in the oxygen
isotopic composition of pedogenic and groundwater carbonates (see
Chapter 15
)to
infer a sharp reduction in precipitation at 2.0-1.8 Ma in the vicinity of Lake Turkana
in northern Kenya and at 0.6-0.5 Ma around Olduvai Gorge in Tanzania.
As we approach the present, the evidence of former wet and dry episodes is increas-
ingly well-preserved and more abundant. Remains of numerous late Pleistocene and
Holocene lakes are scattered across the Sahara (Faure et al.,
1963
;Faure,
1966
;
Faure,
1969
; Williams,
1971
; Williams,
1973
; Fontes et al.,
1985
; Ritchie et al.,
1985
;
Pachur et al.,
1990
; Hoelzmann,
1993a
; Hoelzmann,
1993b
; Hoelzmann et al.,
1998
;
Hoelzmann et al.,
2000
; Bonfils et al.,
2001
; Hoelzmann et al.,
2001
; Hoelzmann
et al.,
2004
; Pachur and Altmann,
2006
; Drake et al.,
2011
). The lake sediments
often contain mollusc and ostracod shells, biogenic tufas and even charcoal (Williams
et al.,
1987
; Gasse,
1990
; Gasse,
2000a
; Gasse,
2000b
; Gasse,
2002
), so dating these
lakes by radiocarbon analysis is usually fairly reliable, provided that reservoir effects
from ancient carbon can be assessed (see Chapters
6
,
11
and
12
). The majority of the
younger former lakes in the central and southern Sahara are early to middle Holocene
in age, with aridity setting in from around 5 to 4 ka and onwards. In the Afar Desert
and the Kenya Rift, the late Quaternary lakes were low during the LGM, high for
perhaps 10,000 years before then and high again during the early to mid-Holocene
(Butzer et al.,
1972
; Gasse,
1975
; Williams et al.,
1977
; Street and Grove,
1979
;
Williams et al.,
1981
; Gasse,
2000a
; Gasse,
2000b
;Chalie and Gasse,
2002
). Zerboni
(
2005
) analysed tufas in the Acacus massif, lake carbonates from the Edeyen dune
field near Murzuq in the Fezzan and rock varnish from sites in the Messak massif.
These localities are in the now hyper-arid Fezzan region of south-west Libya. The
tufas showed that springs were active from 9.8 ka until the 8.2 ka cold event. The
small lakes were highest between 10 and 8.2 ka and high again between 7.8 and
5.0 ka. Changes in rock varnish composition showed moist early to mid-Holocene
environments, with dry conditions from 5.5 ka onwards and sustained eolian dust
accretion in the past 1-2 ka (Zerboni,
2005
; Zerboni,
2008
).
More than thirty years ago, Rognon and Williams (
1977
) showed that events along
the northern margins of the Sahara were not always synchronous with those in the
central and southern Sahara and even in the southern Sahara there were regional
variations linked to elevation. For example, from around 40 to 23 ka, lakes were
generally high along the tropical southern margins, consistent with a wetter-than-
present climate, glaciers were active in the Atlas and rivers flowed considerable
distances from these mountains, in accord with a cool and wet full glacial climate
along the temperate northern margins. From 23 to 15 ka, lake levels were in general
low along the tropical margins, dunes were active and the climate was cold and dry.
However, small lakes were present in Tibesti and Jebel Marra, perhaps fed by winter
depressions. The summer monsoon became re-established after 14.5 ka and extended
well into the central and eastern Sahara (Williams et al.,
2006c
), and lakes were
