Geoscience Reference
In-Depth Information
encouraged French geologists to investigate the former lake deposits that are so
widespread along the margins of the Sahara as well as around the central Saharan
uplands (Faure et al.,
1963
). Such investigations also had potential economic value, for
certain of the lake evaporites (salt, natron) and diatomites were sufficiently pure and
extensive to warrant commercial extraction (Faure,
1963
). Well-defined shorelines,
formed at a time when now dry or shallow, brackish lakes were deep bodies of fresh
water, are sometimes more easily detected in the narrow fault-troughs of the Afar
(Fontes et al.,
1973
) than they are in the vast plains of Mauritania (Chamard,
1973
),
Chad (Servant et al.,
1969
) and Niger (Faure et al.,
1963
;Faure,
1966
). Nevertheless,
within about a decade, many former strandlines were mapped and dated, fossil hippos
and fish were collected (Hugot,
1962
; Williams,
1971
; Clark et al.,
1973
; Hugot,
1977
)
and the paleoecological potential of fossil pollen, spores and diatoms was soon to be
exploited. Pioneer studies of small lake basins,
chotts
and
sebkhas
by Roger Coque in
Tunisia (Coque,
1962
), by Hugues Faure in Niger (Faure,
1966
;Faure,
1969
) and by
numerous others throughout North Africa, often incidentally to their main research
(Monod,
1958
; Biberson,
1961
; Elouard,
1962
; Pias,
1971
), soon gave way to detailed
treatises on major lake basins, such as those of Chad and the Afar. (
Chotts
and their
bordering clay or gypsum dunes are similar to the playas of the North American and
Mexican deserts and to certain of the more saline pans and lake-lunette complexes in
Australia;
sebkhas
are coastal salt flats in arid areas.)
By 1973, Michel Servant had demonstrated that Lake Chad had been high between
approximately 40 ka and 23 ka, low from 23 ka to 14 ka and high again after 14-
12 ka, with maximum levels around 9 ka. His findings were solidly based on detailed
stratigraphic descriptions of deep well sections - a difficult and often dangerous task -
backed up by seventy-seven radiocarbon dates - an unusually large number for that
time (Servant,
1973
). A comparison with the history of the East African lakes (Butzer
et al.,
1972
) strengthened the growing realisation of other researchers that lake level
fluctuations in the intertropical zone of Africa during the last 40,000 years had been
broadly synchronous from the Atlantic to the Red Sea (
Chapter 11
).
One important but too little appreciated aspect of the chronology of high and low
lake levels just outlined concerns groundwater recharge. Assuming that Saharan lakes
were full during periods of sustained high rainfall and low or dry during long dry
spells (an assumption supported by the historic fluctuations of Lake Chad), we might
expect major transgressions to coincide with phases of maximum aquifer recharge
and minimum lake levels to coincide with minimum recharge. Isotopic dating of
groundwater in Libya, Sudan and Chad verified this expectation (Sonntag et al.,
1980
) and showed that many Saharan wells were tapping water dated to 40-25 ka for
the deeper aquifers and to 14-6 ka for the shallower aquifers. Sure enough, little or
no recharge took place during the long, late Pleistocene dry spell between about 23 ka
and 17 ka during which the valleys of mighty rivers like the Senegal (Michel,
1969
),
Niger and White Nile (Williams and Adamson,
1974
) were invaded by wind-blown
