Geoscience Reference
In-Depth Information
12
°
16
°
18
°
20
°
22
°
24
°
26
°
28
°
14
°
12
°
S
Angolan Highlands
Angolan Highlands
10
o
S
ANGOLA
ANGOLA
20
o
S
14
°
S
KEY
30
o
S
ZAMBIA
ZAMBIA
Kalahari
Sands
Okavango
Delta
40
o
E
10
o
E
20
o
E
30
o
E
16
°
S
Linear
Dunes
Faults
Alluvial
Deposits
Active
Fluvial
Channel
Basin
Sumps
18
°
S
Fossil
Channel
ZIMBABWE
ZIMBABWE
Mababe
Mababe
Depression
Mababe
Depression
Depression
20
°
S
Lake
Ngami
Lake
Ngami
Lake
Makgadikgadi
Makgadikgadi
Pans
Makgadikgadi
Pans
NAMIBIA
NAMIBIA
Ngami
Pans
N
22
°
S
BOTSWANA
BOTSWANA
km
0
200
Figure 3.7
Makgadikgadi catchment map from Burrough, Thomas and Bailey (2009). This dryland basin clearly has a catchment
that extends to wetter tropical locations, complicating climatic interpretations of lake-level changes in the past.
3.4.1
Aridity during glacial times?
mechanisms responsible for their development. Because
the evidence is clearest, though complex, this will be done
with respect to the period of the last glacial (c.80 000 BP to
c.12 000 BP with maximum ice extent at c.24-18 000 BP).
Global ice expansion had a number of profound effects
on oceanic and atmospheric circulation and temperature
regimes. Among the most important of these (cf. Nichol-
son and Flohn, 1980) were the steepening of meridional
(pole-equator) temperature gradients; a latitudinal dis-
placement of climatic belts and baroclinic zones includ-
ing the intertropical convergence zone; and the weakening
of the oceanic thermohaline circulation. The southward
displacement of the northern hemisphere baraclinic zone
and the associated upper westerly jet stream caused an in-
crease in moisture-bearing weather systems reaching parts
of the southwestern United States and the Mediterranean
region for at least part of the late glacial period.
It has also been suggested that the southward shift of
As previously noted, shifts in low-latitude aridity are in
many cases strongly correlated to variations in orbitally
induced solar radiation and its hemispheric distribution.
Increasing availability of both palaeoenvironmental evi-
dence and its associated chronologies, however, has also
demonstrated that the dynamics of environmental condi-
tions in desert regions can in fact be regionally diverse
due in part to feedback mechanisms and the interaction
and reorganisation of circulation systems at a number
of temporal and spatial scales. In addition, the superpo-
sition of abrupt climatic events can sometimes masque
broader temporal patterns of aridity. Despite this recogni-
tion of variability, there has been a continued affirmation
of the paradigm that equates high-latitude glaciations with
trends in low-latitude aridity (Sarnthein, 1978; Benson
et al.
, 1997). This makes it appropriate to examine the
