Geoscience Reference
In-Depth Information
Figure 8.8. Map showing active and fixed dunes in and beyond the Sahara. The
present-day limit of active dunes is bounded by the 150 mm isohyet. Fixed dunes
extend up to 500 km south of the Sahara, locally into areas that now receive 1,000 mm
of mean annual rainfall. (Modified from Williams et al.
1998
, fig. 9.8.)
today is broadly delineated by the 200 mm isohyet (
Figure 8.8
) (Grove,
1958
; Grove
and Warren,
1968
; Mainguet and Canon,
1976
; Mainguet et al.,
1980
; Talbot,
1980
).
Based on an array of
14
C ages of fluvial and/or lacustrine sediments overlying and
underlying the fixed dunes, most workers concluded that the time of peak dune activity
coincided with the cold, dry and windy Last Glacial Maximum, some 20,000 years
ago. This was also a time when the great lakes of East Africa dried up or became
saline (see Chapters
11
and
12
) and when exports of desert dust from the Sahara
were exceptionally high (see
Chapter 9
). However, the reality may be somewhat more
complex.
Based on his Tunisian experience, Swezey (
2001
;
2003
) has argued that dune
sediments are best preserved if succeeded by a humid phase but have less chance of
preservation if followed by an arid phase. We noted in
Section 8.3
that glacial age
wind velocities were stronger in the Sahara than they are today (Lancaster et al.,
2002
),
an inference confirmed by the abundance of desert dust in marine cores off the west
Saharan coast (deMenocal et al.,
2000
). However, in the most comprehensive review
of dune activity in the Sahara yet attempted, Swezey (
2001
) determined that the vast
majority of dated eolian records from the Sahara were in fact younger than the Last
Glacial Maximum (21
2 ka). He concluded that this apparent absence of evidence
for LGM dune activity was probably caused by subsequent reworking during the arid
Younger Dryas episode (12.8-11.5 ka) and made the important point that this portion
±
