Geoscience Reference
In-Depth Information
its cations for H
+
and Al
3+
, but this is not always true. If the imbibed
water is Na
+
-rich, this cation remains in large amounts on the exchange
complex. Be that as it may, the medium is rich in aluminium that is
present in various soluble forms:
Al
(
OH
)
2
+
and
Al
6
(
OH
)
15
3 +
in particular.
It is known that aluminium has a protective effect on organic substances,
by forming stable complexes. It is therefore not surprising that these
mangrove soils are rich in peaty layers.
All in all, brown colour and a 'chestnut purée' (fluid to very plastic)
consistence are characteristic of these milieus rich in organic matter
and in iron, when they are oxidized (Aubrun 1988). The structure is
continuous. It is necessary to dry the material fully before prismatic
or blocky structure can be seen. The salinity of the groundwater can
increase considerably on drying, through concentration, and may reach
5 to 10 times that of seawater.
These processes are extremely rapid. For example, such a soil sample
may have a pH of 6
in situ
, then go to pH 3 after a few hours of drying
in air.
12.6.3 Ecology
Figure 12.14 shows, at the world scale, the coasts where mangrove
swamps are most commonly found (Weiss 1967, in Vieillefon 1977).
Fig. 12.14
Coastal regions with mangrove swamps (Weiss 1967, in Vieillefon 1977).
Herbaceous formations or lands devoid of vegetation are found at
the back of mangrove swamps, in a slightly higher position, in regions
having a long dry season, particularly on the western coast of Africa.
Here, submergence occurs only sporadically. These are evaporating
systems in the dry season, resulting in occurrence of acid sulphate as
well as supersaline soils (Marius and Lucas 1991). The tidal regime (more
