Geoscience Reference
In-Depth Information
particular, aridity is a primary control on the distribution
of vegetation using either C3 or C4 photosynthetic path-
ways. C4 plants are adapted to arid conditions while C3
plants are generally found in more humid climates, so
that temporal fluctuations in the proportional abundance
of these types can be determined via the analysis of phy-
tolith assemblages (silicified plant cell fossils). In Arabia,
the use of phytoliths has helped to establish the abrupt
onset of arid conditions at 4100 ca yr BP (Parker
et al.
,
2004). The short life cycle of grasses makes them partic-
ularly good indicators of rapid change as they are able
to adapt quickly to prevailing environmental conditions.
C4/C3 vegetation change is complicated as a proxy for
periods of major climate change due to the equally im-
portant influence of ambient CO
2
and temperature on the
adaptive advantage of the differing photosynthetic path-
ways (Ficken
et al.
, 2002). In addition to identification
of the plant cells themselves and because of the distinct
photosynthetic pathways, stable carbon isotope ratios of
organic matter in soils, lake sediments, hyrax middens
and speleothems can also be used to reflect the isotopic
composition of the dominant vegetation assemblage that
produced it (e.g. Olago
et al.
, 1999, 2000).
A broad framework for arid zone expansion has been
achieved by the placement of aeolian sediments in the
long timescales of oceanic cores. Together, the palaeo-
magnetic and isotopic evidence that they preserve has led
to the establishment of the 'master chronology' of Ceno-
zoic temperature and ice volume changes. Within this,
major layers of aeolian sediments have been correlated to
periods of global cooling extending back 38 million years
(Sarnthein and Diester-Haass, 1977; Sarnthein, 1978).
This contributed directly to the replacement of the 'glacial
equals pluvial' hypothesis with one that equates glacia-
tion with tropical aridity (Street, 1981; Goudie, 1983;
Williams, 1985). Like its predecessor, this general frame-
work is not without its problems (see Shaw and Cooke,
1986) and, as more records of past arid zone fluctuations
have developed, so more complex patterns of change have
been revealed (see recent reviews such as Gasse
et al.
,
2008).
3.4
Climatic interpretations and issues
Resolving the changes in climatic parameters that con-
tributed to fluctuations in the extent of arid zones is of-
ten attempted from the terrestrial data sets that are used
for environmental reconstructions, and which are sum-
marised in the preceding sections. Most reconstructions
focus on proxy data that inevitably point towards precipi-
tation changes. There are limited terrestrial proxies in the
arid context that facilitate temperature reconstructions, so
studies may resort to data sets, e.g. from marine cores,
where isotopic data can be used for temperature recon-
struction (e.g. Schefuβ, Schouten and Schneider, 2005).
Two examples from major terrestrial data sources can
be used to illustrate some of the issues that arise when
attempting terrestrial environmental and climatic recon-
structions.
In the case of sand dune evidence, a key issue is to
try and ascribe the relative roles of temperature and pre-
cipitation in leading to dune surface vegetation reduction
(erodibility). For dunes dated, for example, to the LGM,
precipitation is the likely leading candidate given global
temperature reductions. Even if erodibility is increased,
sufficient wind energy is required for aeolian activity to
occur. The relative role of surface cover change and wind
energy change is currently a hotly debated topic (Thomas,
Knight and Wiggs, 2005; Chase and Brewer, 2009; Tsoar,
2005; see also Chapter 17) and in such instances other
independent proxies, such as records of increased windi-
ness from dust accumulations in offshore marine cores,
may be important (Tiedemann, Sarnthein and Shackleton,
3.3
Dating arid zone fluctuations
Establishing a chronology of arid zone expansions and
contractions during the Quaternary Period is subject to
the application of relative and absolute dating techniques.
In this respect, the advances in isotopic dating methods
of the last few decades (see Walker, 2005; Bradley, 1999;
and other Quaternary texts for discussions of these meth-
ods) have contributed to the overall picture of dynamic
environments, to the recognition that some deserts are
extremely old (Street, 1981) and to a clearer picture of
the geomorphic responses of current desert and peridesert
areas to climatic changes. Recent developments and appli-
cations of some dating methods, such as the luminescence
family of techniques (see Chapter 17 for an explanation of
luminescence dating), and the application of radiometric
techniques to new data sources, such as hyrax middens
(e.g. Chase, 2009), have had major implications for the
development of chronologies of climate and environmen-
tal changes in drylands (e.g. Singhvi and Porat, 2008).
This in part helps to overcome the problems in a dryland
context associated with, and relative paucity of materials
available for, more generally used methods such as radio-
carbon dating (e.g. Deacon, Lancaster and Scott, 1984;
Williams, 1985; Thomas and Shaw, 1991; Nanson
et al.
,
