Geoscience Reference
In-Depth Information
The question of climate change is a vital one, given the
present interest in the two environmental processes of
'desertification' and 'global warming'. During Pleistocene
glaciations, the higher mountain ranges of the Alps,
Pyrenees, Sierra Nevada and Tauros Mountains experi-
enced ice caps. The Pleistocene seems to have been dry in
the Mediterranean with steppe vegetation, becoming
wetter by 8000 BP in the early Holocene. Thereafter it is
difficult to identify climatic cycles because of changes
brought about by cultural and land use events, i.e. to
disentangle natural climatic trends from anthropogenic
effects. However, in the historical period there are
identifiable trends over long periods of time. For example,
the mean rainfall in south-east Spain halved between
1890 and 1940. There have also been significant wetter
and drier periods on a regional scale. Although the
evidence is still debatable, the so-called Mediterranean
Oscillation gives periods of lower rainfall in the western
Mediterranean associated with higher rainfall in the
eastern Mediterranean, and vice versa. Some see these
trends as entirely random, whilst others detect clear cycles,
even though the causes are as yet unknown.
winter months will be leaching, the removal of weathered
products (cations and anions) and any free calcium
carbonate (decalcification) from the soil profile. The rates
of soil formation and thickening vary considerably
between different rock types. Hard igneous and metamor-
phic rocks weather slowly, owing to the restricted length
of the moist season and the generally low precipitation
totals; in such situations there is usually a sharp and clear
interface between solum and rock, and so the profiles have
no C horizon. On softer rocks (e.g. chalks and marls)
weathering proceeds fast enough to give a deeper profile,
with fragments of rock and stones in a C horizon.
In addition to the leaching of ions from the soil, winter
precipitation causes the leaching of clay and silt particles
from the A into the B horizon to give a clay-enriched or
textural Bt horizon. This is very evident in the field
because of the clay and silt coatings (cutans) on stones and
soil structural units, giving typically prismatic structures
in the subsoil. The process is referred to as argillation .
The results of weathering and leaching in the winter
months are thus the dissolution of the parent rock, the
formation of a Bt horizon, and the production of
secondary weathering products of clay minerals, oxides
and hydroxides of iron and aluminium ('sesquioxides'),
and silica. In the ensuing hot and dry season the non-
crystalline (amorphous) iron and aluminium oxides
become dehydrated and crystallize to form crystalline
oxides; where the soil retains some moisture only partial
dehydration takes place, and the browner hydrated oxides
of iron are formed ( goethite , a FeO.OH, and lepidocrocite ,
g FeO.OH). Where dehydration is complete (drier soil
climate, well drained profile, porous parent material) the
iron oxides take the form of anhydrous haematite (Fe 2 O 3 )
which imparts a strong red colour to the soil. As this
chemical reaction is irreversible, the development of a
red hue will increase with time, and thus the degree
of reddening can be used as an indicator of the age of a
soil. In 1853 in Italy these red soils were first called terra
rossa and the designation has remained ever since. The
whole set of processes producing reddening is termed
rubefaction ( Plate 25.1 ).
SOIL FORMATION AND
DISTRIBUTION
Soils form an integral part of all ecosystems. They reflect
the influences of their specific site (the factors of soil
formation: climate, vegetation, topography, geology, time)
and they strongly influence many surface processes
(infiltration, overland flow, surface erosion). Also, they
have important effects on land use through the supply of
a rooting medium, water and plant nutrients for any
cultivated crops.
Soil-forming processes
The Mediterranean climate exerts a powerful influence on
soil-forming processes. In the moist winter season, rates
of weathering and leaching are at a maximum. Minerals
in rocks and unconsolidated parent materials are
subjected to chemical weathering along cracks and fissures
in the subsoil. The weathering processes of hydrolysis and
hydration are carried out by rainwater charged by carbon
dioxide (CO 2 ) both from the atmosphere and from soil
air, whose higher content of CO 2 comes from the activities
of soil fauna and soil micro-organisms. pH values for
rainwater of 5ยท5 readily attack soil minerals, and, where
the parent rock is limestone, cause rapid dissolution by
carbonation. Simultaneous with weathering during the
Soil types
Figure 25.5 shows three profiles which are widespread in
the Mediterranean region and where the relative imprint
of weathering, leaching, argillation and rubefaction varies.
The Brown Mediterranean soil (FAO: Calcic Luvisol; terra
fusca ) is characteristic of more humid sites (higher
rainfall, cooler summers, higher elevation, impervious
parent material). By contrast the Red Mediterranean soil
 
 
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