Geoscience Reference
In-Depth Information
17
Aeolian landscapes and bedforms
David S.G. Thomas
17.1
Introduction
nologies now available from space; e.g. Washington
et al.
,
2003) and (b) the internal structures of aeolian deposits
to be understood (i.e. via ground penetrating radar, e.g.
Bristow
et al.
, 2000); and (4) chronometric advances, such
as optically stimulated luminescence dating (e.g. Aitken,
1994), which are allowing the timing and frequency of
aeolian deposit accumulation to be established, as well as
the relationships to drivers of system dynamics.
Given these advances, we are now better placed than
ever to understand the spatial and the temporal behaviour
of aeolian systems, the drivers of dynamic behaviour
and the relationships in behaviour across scales from the
movement of individual sand and silt grains to the de-
velopment of dunefields and sand seas. To develop an
understanding of how aeolian systems behave and accu-
mulate, it is nonetheless necessary to make separate con-
siderations of processes, landforms and landscapes. In this
chapter the larger spatial and temporal scales are exam-
ined, by looking at the distribution of aeolian deposits at
the global scale and what determines that distribution and,
by examining the long-term Quaternary timescale, the de-
velopment of aeolian systems and their relationships to
climatic changes. The chapter considers both sandy de-
posits - sand seas and their constituent parts - and the
landscape features comprised of wind-lain silt, termed
loess.
Arid zone aeolian deposits have long attracted the at-
tention of geographers, geomorphologists and allied sci-
entists. They represent for many people the seminal
landscape of deserts, though the actual geographical dis-
tribution and extent of, for example, dune fields is no more
than 20 % of the total area of drylands today, and in some
deserts, such as the southwestern USA, they cover less
than 1 % of the total area. Scientifically, Bagnold's (1941)
seminal work on sand transport processes and dune forma-
tion established a depth of understanding and rigour that
set the framework for research in the ensuing half-century.
Subsequently, at markedly different scales, the availabil-
ity of remotely sensed data coving otherwise difficult to
access desert areas (e.g. Breed and Grow, 1979; Paisley
et al.
, 1991) and the growing number of reductionist stud-
ies involving the detailed monitoring of wind and sand
flow over dunes (e.g. Weng
et al.
, 1991; Wiggs, 1993) led
to advances in aeolian research at very contrasting spatial
scales (Thomas and Wiggs, 2008) (Figure 17.1).
More recently, four scientific developments have pro-
vided the opportunity for across-scale enhancements in
understanding the nature and behaviour of aeolian systems
(Thomas and Wiggs, 2008): (1) further advances, largely
driven by technological developments (e.g. Arens, 1996;
van Boxel, Sterk and Arens, 2004), in empirical mea-
surement that allow the relationships between sediment
movement and wind power to be elucidated; (2) mathe-
matical modelling of dune (e.g. Baas and Nield, 2007)
and atmospheric dust (e.g. Zender, Bian and Newman,
2003) dynamics and their interactions with key controls;
(3) technological developments in remote monitoring sys-
tems that allow (a) high-resolution observations of sedi-
ment movement (via the plethora of remote sensing tech-
17.2
Aeolian bedforms: scales and
relationships
Because of their fine-grained nature, loessic deposits (pri-
marily silt-sized material with 3.9-63 µm particle diam-
eters) tend to be rather amorphous, landscape-blanketing
