Geoscience Reference
In-Depth Information
We can now write:
F
Al
r
-
Al
s
T
=
Al
e
F
Si
r
-
Si
s
T
=
Si
e
(dissolved)
(residual in
(exported)
the soil)
From these two equations the two unknowns
F
and
T
can be
calculated. And thence at least five parameters may be obtained:
Annual subsidence of the surface in m:
S
=
F
-
T
Time required for formation of the soil profile studied:
t
=
D/T
Number of metres of rock weathered to form the profile:
tot
F
=
t·F
Total subsidence of the surface during time
t
: tot
S
=
t·S
The ratio
VR
=
F/T
represents the ratio of the thickness of
weathered rock to the thickness of the soil formed. Lucas calls
it 'volumetric ratio'. It is mathematically correct, but can lead to
confusion because it is practically considered as the ratio of two
lengths (unidimensional).
To these parameters mentioned by Lucas, we could add the
following:
t
hlp
=
D/
(2·|
S - F|
·10
6
). This is, in millions of years,
the time required for the profile to gain or lose 50 per cent of
its original height or, in other words, the
half-life of the profile
. It
enables us to gauge, from the concept of progressive descent of
weathering fronts, whether the profile is in stable equilibrium
or not (§ 3.1.2).
The accuracy of the calculations depends on a good understanding
of the Si and Al contents of the rock, the soil and the water. Lucas
emphasized the difficulty of finding within the rock an unweathered
and representative sample. Moreover it was presumed that there was
only one weathering front giving rise to a single pedological material.
For this to be acceptable, the values
Al
s
and
Si
s
taken for calculation
should be the calculated means of those of all the soil horizons taken
together. Lucas even took the mean of values calculated for an entire
series of profiles. To go further, it would be necessary to computerize
the procedure and involve in succession several transformation fronts
in the same profile. The values
Al
e
and
Si
e
pertaining to the waters are
also very approximate, considering the method of sampling from the
stream.
But the method remains relevant. It provides orders of magnitude.
Table 5.4 concerns two equatorial soils in Manaus and Juruti in
Amazonia, Brazil.
