Geoscience Reference
In-Depth Information
Table 17.5
Criteria used in the literature to designate dune
since dunes were active, Talbot (1994), Wasson (1984),
Lancaster (1988) and Chase and Brewer (2009) have all
proposed the use of dune mobility indices (Figure 17.14).
Similar indices may also be used with modelled climate
data to predict sand sea responses to climatic changes in
the future (Thomas, Knight and Wiggs, 2005; Wang
et al.
,
2009).
Relict dune systems have also been used to reconstruct
palaeocirculation patterns as well as the former extent
of aridity (Lancaster, 1981; Wells, 1983; Wasson, 1984;
Thomas and Shaw, 1991b). The orientation of the relict
dunes is compared to the resultant direction of modern
sand-moving winds, calculated from wind data. Results
have frequently suggested that circulation patterns dif-
fered from those of today when dune development oc-
curred. Because dunes form in a wide variety of wind
regimes today, often with considerable seasonal or diur-
nal directional variability (see Chapter 19), the conclu-
sions that have been drawn are frequently speculative.
'relict' status.
1. A cover of vegetation sufficient to inhibit sand transport
2. Dune form degraded (slope angles) compared to active
dunes
3. Subject to pedogenesis
4. Presence of surface crusts/lithification
5. Dune slopes gullied
6. Internal structures destroyed by burrowing animals
7. Sediment size distribution bimodal due to post-formation
dust inputs
8. Dunes overlain by other depositional landforms
(including second generation dunes)
9. Drowned in valleys/lakes/off-shore
Wasson, 1983; Wasson, 1984; Bullard
et al.
, 1996). For
example, as Williams (1985, p. 226) has noted:
17.5.1
Dating aeolian landscape change
In the case of the source bordering dunes of Australia,
Sudan and, presumably, of Zaire and Amazonia, the
three prerequisities for dune formation seem to be
a seasonally abundant supply of river sands, strong
unidirectional seasonal winds, and a less dense tree
cover than today. Such palaeo dunes do not connote
extreme aridity.
Major advances have occurred in understanding Quater-
nary period sand sea dynamics through the advent and ap-
plication of thermoluminescence (TL) (Singhvi, Sharma
and Agrawal, 1982) and subsequently optically stimulated
luminecscence (OSL) (e.g. Stokes, 1992; Stokes, Thomas
and Washington, 1997) dating to aeolian sediments (see
Box 17.1). Prior to the advance of these dating meth-
ods, quartz-rich aeolian sands and silts were datable only
if bracketed by other sediments to which other dating
methods could be applied. For example, the alternating
loess-palaeosol sequences in the Loess plateau of China
have been dated by radiocarbon or uranium-series dat-
ing of the palaeosol units, of which over 30 occur, by
palaeomagnetic studies (Liu and Ding, 1998) or by 'curve
fitting' of sedimentological properties of the loess with
isotope records from polar ice cores (Balsam, Elwood
and Ji, 2005).
After analysis of a range of variables, Hesse and Simp-
son (2006) concluded that over millennial timescales, it is
vegetation cover that is the primary control on linear dune-
field activity and on dune accumulation, rather than wind
strength. A recent study by Mason
et al.
(2008) also sug-
gested that wind strength is not a primary control of activ-
ity, though some authors attach primary importance to this
parameter (e.g. Tsoar, 2005). In an attempt to resolve the
issue of the relative roles of erodibility (vegetation cover)
and erosivity (wind energy) in explaining dunefield inac-
tivity and the level of climatic changes that have occurred
Box 17.1
Luminescence dating
The origins of luminescence dating lie more than four decades ago within the discipline of archaeological science and
a need to determine the age of pottery. Since then it has undergone a rapid series of methodological and technological
developments and has emerged in the last decade as an important and effective Quaternary environmental dating
method. It is particular useful in arid zones where it is used to date directly the depositional age of sediments
