Geoscience Reference
In-Depth Information
The solubilized calcium is partly displaced to the middle of the
soil profile, where it precipitates for reasons we have indicated above.
The accumulations are at first diffuse and localized. In soils with very
coarse texture, they are first deposited on the lower surface of pebbles.
In other cases, they are located in the vicinity of roots. Sometimes, the
accumulations end up forming a fully hardened layer. At this stage,
the vertical circulation of water is slowed down in the humid season
and replaced by lateral movement. The upper part of the crust is then
redissolved and redeposited as dictated by episodes of rain or drought.
It assumes a horizontally layered structure:
laminar crust
.
Lime accumulations are accompanied in arid climate (50-100 mm
annual rainfall) by sepiolite or palygorskite, for example in Morocco or
the footslopes of the mountains of the Mojave desert in USA (Amundson
et al
. 1989; Jiménez-Espinosa and Jiménez-Millán 2003). Palygorskite
is a fibrous clay mineral rich in Mg and Si. It is formed in very dry
environments and is solubilized when the climate becomes slightly moist
(Moinereau and Robert 1976). The lime accumulations are accompanied
by smectite and haematite in semi-arid and dry Mediterranean climate
(100-500 mm). Crusting is possible in a humid climate once there is
a short season without vertical drainage of water. This happens in
Champagne (France) where more than 800 mm of rain falls per year
(Durand 1980a).
Dating
Let
c
be the age of the initial deposits on the bottom of pebbles and
l
the age of the laminar crust that represents the final stage. The
duration of deposition is then
l
-
c
. By using the uranium/thorium
method, about 100,000 years has been found for a crust 50-cm thick in
Spain (Candy
et al.
2004). Other authors give greater ages. In fact, many
crusts have developed during the entire Quaternary. Use of
14
C for
estimating the age of a crust is theoretically possible. But the phenomena
of dissolution and reprecipitation could have followed each other in the
course of time, with atmospheric CO
2
participating every time.
Very often the quantity of calcium present exceeds what can be provided
by the horizons overlying the crust. It is then necessary to invoke
the oblique movement of carbonate-bearing solutions along the slopes
(Ruellan 1972).
Origin of the calcium
