Geoscience Reference
In-Depth Information
now is to see why the general climate could change (cf. § 5.5.1).
Also, the gradual differentiation of the cuirass could limit the
downward circulation of water and facilitate the formation of a
perched water table. In short, the cuirass becomes the instrument
of its own degradation.
3
)
Intervention of phytoliths
. So long as the cuirass still contains
large quantities of aluminium, it is only silica that is lacking for
neoforming kaolinite. Silicon is taken up by tree roots in large
quantities. It forms
phytoliths
in the leaves, opal deposits that
affect the cell walls, the intracellular voids and the intercellular
spaces. Some plant species contain 2 per cent Si in their leaves.
In the equatorial zone, annual additions to the soil of 30 to 60 kg
silicon per ha through various plant debris and litters have been
measured (Alexandre
et al.
1994; Lucas
et al
. 1993, 1996). In the
soil these phytoliths are rapidly dissolved as shown by electron-
microscope studies. All calculations show that only 200,000 years
are needed to pump from below the quantity of Si necessary
to form a kaolinite layer 4 or 5 metres thick. We will see later
that this period is not incompatible with that involved in the
formation of these soils.
4
)
Lateral transport
. Kaolinite could have been removed from
nearby reliefs during their planation and deposited on top of
the cuirassed soils (Truckenbrodt
et al
. 1991). This hypothesis
resurfaces regularly but is equally regularly disproved by careful
studies that show the lithodependence of the constituent layers
of the soil (Morras
et al
. 2008).
5
)
Instability of kaolinite.
Till recently it has been argued that kaolinite
is very stable. But the
18
O/
16
O ratio of the mineral appears to be
in equilibrium with that of meteoric water. Thus the kaolinite
could be formed and destroyed rapidly (Grimaldi 2004). It will be
dissolved (slightly!) during episodes of rainfall and reprecipitated
in the dry season. In the end, its disappearance from the profiles
and its reappearance will be easy…
The soil studied surely has highly contrasting horizons, but this must
not hide the progression of changes at the mineralogical and chemical
levels from the bottom to the top of the profile. This progression is
evident when the essential mineralogical and chemical features are
represented in triangular diagrams. Figure 5.5 presents, through an
Chemical and mineralogical aspects
