Geoscience Reference
In-Depth Information
reduced rates of accumulation and widespread soil development during the warmer
wetter interglacial phases (Liu,
1985
;Liu,
1987
;Liu,
1991
).
Not all loess mantles are in primary context, and once deposited on hill slopes and
valley sides, the loess will be subject to erosion and movement downslope under the
influence of mass movement and running water, as discussed in
Chapter 10
. Indeed,
many of the fine-grained, late Pleistocene valley-fill deposits in the mountains of
the Namib Desert (Eitel et al.,
2001
; Heine and Heine,
2002
; Eitel et al.,
2005
), the
Sinai (Rogner et al.,
2004
), the Negev (Avni,
2005
; Avni et al.,
2006
), the presently
semi-arid Flinders Ranges of South Australia (Williams et al.,
2001
; Chor et al.,
2003
;
Williams and Nitschke,
2005
; Williams and Adamson,
2008
; Haberlah et al.,
2010a
;
Haberlah et al.,
2010b
), as well as the Matmata Hills of Tunisia (Coude-Gaussen et al.,
1987
) and the semi-arid Chifeng region of Inner Mongolia in northern China (Avni
et al.,
2010
) consist primarily of reworked loess, and so have more to tell us about
fluvial activity at that time than about causes of primary dust deposition.
These fine-grained, late Pleistocene valley fills are up to 20 m thick and are wide-
spread within dissected arid uplands in Africa and Australia. Such deposits are not
accumulating today (Williams et al.,
2001
). The dominant lithology is silty clay, with
minor lenses of fine to medium gravel. In the arid Flinders Ranges of South Aus-
tralia, the valley fills consist of a massive lower unit and a finely laminated upper
unit. The lower unit contains abundant unbroken ostracod and aquatic mollusc shells,
indicating deposition under perennially wet conditions. The uniform stable carbon
and oxygen isotopic composition of the shells also confirms deposition under stable
climatic conditions (Glasby et al.,
2007
). In contrast, the individual fining-upwards
laminae in the upper unit contain broken shells and fragmented plant remains, and
are best interpreted as slackwater deposits (Haberlah et al.,
2010a
; Haberlah et al.,
2010b
). The valley fills have been dated using paired charcoal and shell samples for
AMS
14
C analysis combined with optically stimulated luminescence dating. They
were laid down between about 35 ka and 15 ka (Williams et al.,
2001
;Glasbyetal.,
2010
; Haberlah et al.,
2010a
; Haberlah et al.,
2010b
).
Particle size analysis, strontium isotope analysis and rare earth element composition
show that the silty clays in the Flinders Ranges valley fills were largely derived from
reworked loess. The loess was blown in from the south-west by stronger westerlies
and accumulated along the ridges and slopes of the north-south aligned ranges. The
atmospheric carbon dioxide concentration at this time was as low as 180-200 ppmv.
Such low concentrations favoured the expansion of grasses and herbs at the expense of
eucalypt trees (Williams and Adamson,
2008
). The grass cover would have provided
an effective dust trap. The demise of the deep-rooted river red gums (
Eucalyptus
camaldulensis
) along the valley bottoms would have resulted in a slow rise of the
local water-table, leading to swampy conditions. Low summer temperatures and much
reduced evaporation were conducive to persistence of these wetlands (Chor et al.,
2003
). A much weaker summer monsoon regime in the tropical north would have
