Geoscience Reference
In-Depth Information
The tongues of degradation have a fine-clay/total-clay ratio about
4 times lower than that of the coatings (Jamagne 1973). They are also
richer in quartz. They correspond to the qualifier
Glossalbic
(WRB).
8.1.3 Degradation
Many observations lead us to think that there is loss of clay by
solubilization or lateral translocation at least in the upper layers of the
differentiated profiles.
Losses of substances calculated by mass-balance analysis (Chap. 1, §
1.2.4) are large. An example is given in Table 8.1.
Loss from the profile
Table 8.1
Calculated losses of substances in an Alfisol (Hapl-ud-alf, Enon series,
North Carolina) by the isozircon method (Oh and Richter 2005).
Losses (export from the profile)
Si
Al
Fe
Surface soil (order of magnitude; cf. curves)
40%
25%
50%
9%
Precisely calculated over 4-m thickness
39%
13%
If the movement of clay between E and B had been the only
phenomenon involved in evolution of the soil, losses of aluminium and
silicon obviously must have been zero.
Many authors have obtained similar results. The difficulty lies in
determining if the losses were in the form of particles in suspension or
as dissolved ions. As streams carry sediments and products in solution at
the same time, the two forms of depletion certainly coexist. However, we
have stated in Chapter 3, § 3.4.4 that we lacked arguments for thinking
that the lateral transport of clay only is very important, particles of other
sizes being excluded.
Many authors have observed that the B horizons of Luvisols or
Albeluvisols are rich in smectites. On the contrary, the surface horizons
are often richer in kaolinite (Dudal 1953; Eimberck-Roux 1977; Revel
1982; Favrot
et al
. 1992; Delgado
et al
. 1994; Feijtel
et al
. 1988…). The
smectite Æ kaolinite evolution is confirmed by recent studies (White
et
al
. 2008). Considering what we have seen earlier (Chaps. 3, 5 and 6), this
is a very typical sign of desilication related to geochemical depletion.
Lastly, the tongues seem to be sites of degradation because the largest
Mineralogical evolution
