Geoscience Reference
In-Depth Information
In Mediterranean climate, the amount of calcium extracted increases very
markedly after a dry period. Observations have shown that desiccation
provokes retraction of the clays and detachment of a thin weathered
film on the surface of the coarse calcite particles buried in the soil. This
film is composed of calcite crystals that dissolution has loosened but
has not yet eliminated totally. This promotes their later solubilization
(Lamouroux 1970, 1971). Then the phenomenon starts over again. This
is not observed in humid climate where the dissolution is direct.
Role of seasonal alternations
7.3.3 Kinetics of Dissolution
Many calcareous soils have surface horizons with rounded lime pebbles
and lower horizons with angular ones. A careful observer can perceive
this in alpine grasslands for example where cultivation with the plough
is impossible and cannot be responsible for any modification of the shape
of the coarse elements. This shows that after having infiltrated a few
decimetres into the soil, rainwater is no longer aggressive even though
it is not saturated with calcium. For example, during periods of intense
drainage, the Ca
2+
content in the water in soils on chalk in Champagne
is just 4-8 mg l
-1
, whereas the system at equilibrium (open to the air)
should correspond to a Ca
2+
concentration of 16 mg l
-1
(Durand 1979).
This has been confirmed by laboratory experiments (Kaufmann and
Dreybrodt 2007). Several explanations can be given:
Firstly, when the water penetrates the soil and moves away from
the surface, the replenishment of CO
2
is not assured. The pH rises and
aggressiveness of the water decreases.
Secondly, it is necessary to consider the dynamics of the phenomena.
The model most commonly used is the following (Morse and Arvidson
2002):
dt
=
(
A
V
k
)
(1 - W)
n
,
R
= -
dm
___
__
Where:
R
is the quantity of calcite dissolved in
ยต
mol m
-2
h
-1
,
m
is the number
of moles of calcite,
t
is the time,
A
is the surface area of the solid,
V
is the volume of the solution,
k
is a constant, W is the saturation index
given by W = ion-activity product/solubility product, and
n
is a positive
constant known as
order
of the reaction (in this particular case its value
is close to 3).
