Geoscience Reference
In-Depth Information
sands and active sand dunes were widespread (Mainguet and Canon,
1976
)upto
800 km beyond the present confines of the 100-150 mm isohyets, which roughly
delineate the boundary between active and fixed dunes today (see
Chapter 8
).
The diatoms in the lakes of the Afar and the Sahel are sensitive to variations in
water chemistry, depth and temperature (Gasse,
1975
; Gasse,
1976
). The uptake of
dissolved silica by diatoms ensures that the siliceous diatom cells, or frustules, accu-
mulate on the bed of the lake as resistant, usually well-preserved fossils (see
Chapter
16
). Identification to species level may be possible, particularly with recourse to a
scanning electron microscope (Gasse,
1974
). Diatomaceous lake clays or even pure
diatomites may be many metres thick, and because accumulation rates are compar-
atively slow (0.1-0.5 mm/year is not unusual: Washbourn-Kamau,
1971
), a wealth
of paleoecological information may be contained within several 20-metre sections
sampled at 5 cm or closer intervals. Evolutionary changes in diatoms and the presence
of extinct or archaic fossil diatoms within a given lake deposit (Rognon and Gasse,
1973
; Servant-Vildary,
1973
) allow biostratigraphic correlations to be made with lake
deposits much further afield, some of which may lie on or beneath lavas suitable for
potassium-argon dating (see
Chapter 6
).
Gasse (
1975
) conducted a detailed study of the Plio-Pleistocene evolution and
paleolimnology of the lakes of the central Afar Desert. Three types of lakes occur in
the Afar: those dependent mostly on surface flow (e.g., Lake Abhe, which is fed by the
Awash River); those maintained by a combination of surface and subsurface flow (e.g.,
Lake Asal); and those sustained by subsurface seepage only (e.g., Lake Afrera). The
first type fluctuates mainly in response to variations in the precipitation/evaporation
ratio in the elevated headwaters region; the second acts likewise, with a time lag
which smooths out minor oscillations; but the third class of lake depends only very
indirectly on climatic conditions in the headwaters. Evidence from the Afar lakes
shows that during the Holocene, there were three, or perhaps four, periods of high
lake level, the fourth apparently coinciding with the latter phases of the 'Little Ice
Age' of north-west Europe, dated between roughly 1350 and 1850 AD. The Afar late
Pleistocene sequence of three high lake-level phases, dated at
40 ka to 30 ka
and 30 ka to 23 ka, confirmed that major lacustrine transgressions in East Africa were
broadly in phase, as was the trans-Sahelian arid interval of 23 ka to 15 ka (Burke
et al.,
1971
; Butzer et al.,
1972
; Delibrias et al.,
1973
; Williams,
1975
). Dominantly
biogenic deposition in Lake Abhe during the interval from
>
40 ka,
40,000 to 30,000 BP
raised the question of what the vegetation cover was like in the headwaters of the
Awash River at this time, a question that future pollen analysis could answer.
5.2.3 Vegetation history of the Sahara reconstructed from pollen analysis
Almost two decades before paleo-lake studies reached the level of sophistication
outlined in the previous section, French scientific explorers were already trying to
