Geoscience Reference
In-Depth Information
sand accumulation at 104 ka and further evidence of dune activity in spatially discrete
locations at 77-76, 57-52, 35-27, 21-19 ka and again at 15-9 ka, when the climate
in this region was unusually arid and linked to intensification of the continental
anticyclone (Telfer and Thomas,
2007
). Additional OSL dating, giving a total of 136
OSL ages for the southern Kalahari, including additional samples collected at closer
vertical intervals of 0.5 m, suggested that the dunes in the south-western Kalahari
had been partially active throughout much of the past 120 ka and that apparent age
clusters could be produced spuriously as a function of reducing the sampling frequency
with depth within the dunes (Stone and Thomas,
2008
). Stone and Thomas (
2008
)
concluded that the dunes in this region had been close to their threshold of reactivation
throughout much of the late Quaternary and that earlier work invoking discrete phases
of dune sand accumulation needed to be reassessed.
Chase (
2009
) reviewed the evidence for dune activity across southern Africa and
concluded that three primary phases of dune activity centred on 60-40, 35-20 and
17-4 ka could be identified. Given the error terms associated with these ages, one
could argue equally plausibly for more or less continuous dune activity from 60 ka
onwards. Chase (
2009
) discounted aridity as the sole or even primary control over sand
movement and invoked changes in wind strength as the main forcing factor. Chase and
Brewer (
2009
) compared the output of coupled Ocean-Atmosphere Global Circulation
Models with empirical studies of potential sand transport during the LGM (24-18 ka)
and found little theoretical evidence for LGM dune mobility across southern Africa,
in opposition to actual dated evidence of dune activity at this time obtained from
field studies. They concluded that the model outputs could be unreliable, as could the
empirical indices for dune activity. They also questioned the validity of using dune
records as paleoclimatic proxies at millennial scales.
The evidence from desert dunes in southern Africa has yielded highly equivocal
information about the Quaternary climates in this region, with some workers arguing
that sediment supply and wind strength are more powerful determinants of dune
activity than increased aridity. In addition, the search for pulses in dune accretion has
been bedevilled by the conflicting results arising from differences in the sediment
sampling resolution in regard to depth, with apparent pulses vanishing once dunes
were sampled at closer vertical intervals. It also appears that the Kalahari linear
dunes have been close to their threshold for reactivation throughout much of the late
Quaternary, so that only minor changes in dune-forming agents, such as wind speed,
plant cover, sand supply and effective precipitation, would have been needed to trigger
renewed dune movement.
8.15 Desert dunes of Australia
The Australian sand deserts are the second largest sand deserts on earth and have been
investigated in some detail, as will be evident from the following summary. Dunes
and sand plains presently cover about two-fifths of Australia and have long attracted
