Geoscience Reference
In-Depth Information
Brewer (
1964
) was among the first to establish soil micromorphology as a rigorous
scientific discipline, but the specialised names now widely used in this field of soil
science are not for the faint-hearted. The techniques of soil fabric analysis have been
widely used in soil science, as well as in other disciplines such as geo-archaeology
and glacial geomorphology, and they are an invaluable adjunct to field-based soil
description, in that the thin sections can show the actual amounts of illuvial clay
within a given soil horizon.
15.5 Examples of desert soils
Given the iconic status of desert dunes, it seems fitting to begin with soils developed
on and within desert dunes and sand plains, especially since many dune soils are
hard to recognise as actual soils. Since 1977, Neil Munro and his colleagues have
conducted extensive soil surveys throughout Saudi Arabia, Yemen, Oman, Jordan,
Bahrain and the United Arab Emirates, combined with a program of radiocarbon and
luminescence dating and archaeological research (Munro et al.,
2012
). It is worth
quoting one of their conclusions:
At the present time on the Yemen Tihama dunes, moderate summer rainfall allows substantial
rainfed millet production on mobile dunes (and concomitant dune stabilisation) and thus the
requisite for a developed soil horizon to allow a vegetation cover to form is false . . . and one
does not have to search for true paleosols with Cambic [i.e., clay-rich] horizons to show that
lands were grassed or wooded and supporting [prehistoric human] populations. (op. cit., p. 32)
This comment would equally apply to the sand deserts of Africa, Asia, Australia and
the Americas. Such soils would qualify as arenosols (see
Table 15.1
), which are sandy
soils with minimal texture contrast and weak or no soil horizons.
However, many dune soils do show some degree of horizon development, and they
often have modest amounts of clay at shallow depths, together with soft calcium
carbonate (CaCO
3
) aggregates or hard, irregular carbonate nodules and rhizocretions
(Williams,
1968a
; Williams et al.,
1991a
;Amitetal.,
2007
). Rhizocretions are roughly
cylindrical or downward-tapering carbonate pipes that formed around tree roots, a
process that can be observed on living tree roots today in regions as far apart as
Algeria, Sudan, the Thar Desert and Australia. They are especially common in coastal
eolianites (Sprigg,
1959
; Sprigg,
1979
). Dust storms are the primary sources of the
clay and calcium carbonate found within dune soils, although some carbonate may
have travelled in solution in groundwater. Analysis of the strontium and neodymium
isotopes (
Chapter 7
) within desert soil carbonates is consistent with deflation from
distant sources, including continental shelves exposed during times of low glacial sea
level (Dart et al.,
2007
). Carbonate rich soils in varying stages of development are
common on pediment surfaces in areas as far-removed as the Mojave Desert in the
south-west United States, the southern Negev Desert, the Thar Desert of Rajasthan in
