Desert lakes - Climatic Change in Deserts - page 198

11.6 Desert lakes of Africa
Scattered across the Sahara are the remains of abundant former lakes ranging in age
from Holocene to Pleistocene (Faure et al., 1963 ;Faure, 1966 ;Faure, 1969 ; Williams,
1971 ; Williams, 1973b ; Fontes et al., 1985 ; Drake et al., 2011 ). Dating the Holocene
and late Pleistocene lakes by radiocarbon analysis is usually straightforward, because
the lake sediments often contain mollusc and ostracod shells, biogenic tufas and even
charcoal (Williams et al., 1974 ; Williams et al., 1987 ; Gasse, 1990 ; Gasse, 2000a ;
Gasse, 2000b ; Gasse, 2002b ). Dating the earlier lake deposits has proven to be more
difficult and the results are sometimes hard to interpret, so there are very few detailed
studies of older lakes and aquatic ecosystems from this region (Karim, 1968 ; Williams
et al., 1981 ; Petit-Maire, 1982 ; Williams, 1984a ; De Deckker and Williams, 1993 ;
Wendorf et al., 1993 ). By way of example, Pleistocene Lake Shati in south-east Libya
has provided uranium-series ages considered by Gaven et al. ( 1981 ) and by Petit-
Maire ( 1982 ) to be around 130 ka in age, which would place the lake in Marine
Isotope Stage 5 (MIS 5). However, Williams ( 1984a , p. 440) noted that the uranium-
series ages obtained from this site by Gaven ( 1982 )on Cerastoderma glaucum shells
showing little or no recrystallization (Icole, 1982 ) fall into four distinct groups. The
four oldest samples range in age from 173 to 158 ka, with error terms of up to 20 ka,
eleven out of twenty-one samples are dated to136-132 ka, five are close to 90 ka and
the youngest dates to 40 2 ka, so there could equally be four lake phases rather than

the single episode inferred by Petit-Maire ( 1982 )andGaven( 1982 ).
A further example underlines the need for great care when dating lake carbonates.
Causse et al. ( 1988 ) corrected for the effects of detrital thorium and obtained uranium-
series ages of 100-80 ka for lake sediments in the western Sahara considered to
belong to the last major wet phase in that area, widely regarded as early Holocene.
Szabo et al. ( 1995 ) used uranium-series dating of lacustrine carbonates in an effort to
obtain ages from the Pleistocene lakes at Bir Sahara and Bir Tarfawi in the Western
Desert of Egypt and other former lake sites in the eastern Sahara ( Figure 11.4 ). They
identified five discrete lake phases dated to about 320-250, 240-190, 155-120, 90-
65 and 10-5 ka. Crombie et al. ( 1997 ) subsequently obtained uranium-series ages
on travertines from Kurkur Oasis in the Western Desert of Egypt that fell into three
broad groups:

±

260, 220-191 and 160-70 ka. However, all of these ages need to
be viewed with considerable caution in light of the earlier experience of Wendorf
et al. ( 1993 ) in seeking to date the complex of lakes at Bir Tarfawi and Bir Sahara.
Wendorf and his colleagues had carefully identified a suite of successive lake deposits
associated with Acheulian and Middle Palaeolithic artefacts. They applied a number
of different dating methods to these deposits, including luminescence (TL and OSL),
uranium-series, amino acid racemisation and electron spin resonance. Out of all of
these methods, they found that only the OSL ages yielded stratigraphically consistent
results (Wendorf et al., 1993 , pp. 552-573). The other methods gave an age range

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