Geoscience Reference
In-Depth Information
Using soils to infer past climate is a difficult task, given that soil formation involves
a number of independent influences or soil forming factors, only one of which is
climate. The aim of this chapter is to consider the utility and disadvantages of using
soils and duricrusts to identify past climatic changes in deserts and their margins.
Before doing so, it is necessary to discuss the factors of soil formation and briefly to
review some of the diagnostic characteristics of so-called desert soils and soils found
in what are now deserts. We begin by defining what a soil is.
15.2 What is a soil?
Given the understandable but sometimes misleading emphasis among soil scientists
on describing the two-dimensional soil profiles revealed in soil pits and the one-
dimensional view afforded by soil augers, it is easy to forget that soils are in fact
three-dimensional features of the landscape (Birkeland,
1999
; Retallack,
2001
). More
than half a century ago, the
Soil Survey Manual
(1951, p. 7) pointed out that soils
are 'landscapes as well as profiles' and emphasised that 'soil is the natural medium
for the growth of land plants, whether or not it has “developed” soil horizons'. The
Manual
went on to define soil as 'the collection of natural bodies occupying portions
of the earth's surface that support plants and that have properties due to the integrated
effects of climate and living matter, acting upon parent material, as conditioned by
relief, over periods of time' (op. cit., p. 8). This definition remains just as valid today,
and it will be the one adopted here.
In this comprehensive definition, the five classic factors of soil formation are
clearly identified: parent material, topography, biological activity, climate and time
(Jenny,
1941
; Paton,
1978
, pp. 96-108). In the early stages of soil development,
parent material and topography exert the greatest influences on soil characteristics,
and theymay be considered as relatively passive factors of soil formation (Paton,
1978
,
p. 96) (
Figure 15.1
). As a result, we can recognise the varying influence exerted by
different igneous, metamorphic and sedimentary rocks on the physical and chemical
characteristics of immature soils that developed, for example, on the more common
rocks, such as granite, basalt, sandstone, siltstone and limestone. In some cases, soils
may occur at the top of a deep weathering profile, while in other instances, there may
be a series of buried soils within successive layers of alluvium, loess and volcanic ash
or lava flows. Many soils found in deserts are formed on relatively young sediments,
including alluvial and lacustrine parent materials, as well as eolian deposits such
as sand and desert dust or loess. Sequences of buried soils are common in such
environments.
Topography plays an important role in soil development in several ways (Milne,
1936
). Water percolates through weathered rock laterally as well as vertically, and is
a potent agent of mechanical eluviation of finer soil particles and their redeposition
along the base of hills and inselbergs to form an aureole of finer-textured soils, as in
