Geoscience Reference
In-Depth Information
Actually, much progress remains to be made for obtaining reliable
scales, concordant between continents. It is also necessary to understand
better certain spectacular variations in the magnetic field.
Using these palaeomagnetic techniques in the tropical zone, a rate
of weathering of the base of soils of nearly 11.3 mm per thousand
years has been demonstrated (Théveniaut and Freyssinet 1999). Thus in
100,000 years, a metre of rock can be weathered. This is only an order
of magnitude, but apparently is typical.
This rarely used method is replete with lessons to be learned. It consists
of calculating the time that was necessary to give rise to a given soil by
studying the export of one constituent. Let us consider an example that
could be found in tropical conditions.
A 50-cm deep soil on a limestone with 10 per cent decalcification
residue is studied. The specific gravity of the limestone is 2.6 and
that of the soil 1.3. Thus a column of this soil with cross-section 1
cm
2
weighs 50 × 1.3 = 65 g. It had been contained in a rock column
65 × 10 = 650 g, representing 650/2.6 = 250 cm (it should be noted that
the soil is produced by dissolution
in situ
of 250 cm of the rock and not
of 5 metres as could be presumed if we forget the difference in density.
This column of rock contains 650 - 65 = 585 g of calcium carbonate.
Knowing that the current annual rainfall at the site is 200 cm and annual
evapotranspiration 100 cm, in a year 100 cm
3
of rainwater is left to
ensure the evacuation of the calcium carbonate (all calculations are done
for a 1-cm
2
area of soil). Knowing the mean annual temperature at the
site is 20 °C, the solubility of calcium carbonate in rainwater saturated
with carbon dioxide at atmospheric pressure is then 10
-3.4
moles of Ca
++
per litre or 0.02 g CaCO
3
per litre, which represents 0.002 g evacuated
annually by the 100 cm
3
of water available. Thus 585/0.002 or 282,500
years will be required. In short, the soil could be between 200,000 and
400,000 years old.
These calculations are very approximate. They do not take into
consideration climatic changes that possibly affected rainfall as well
as temperature. They assume that there had been no erosion, no
additions and no lateral circulation of water. They also assume that the
water passing through the soil has enough time to get saturated with
carbonates, which is not always the case (Egli and Fitze 2001). They do
not consider the 'cation pump' represented by plants. Also ignored are
the daily and seasonal variations of temperature, humidity and soil CO
2
.
Rates of export
