Geoscience Reference
In-Depth Information
Table 5.4
Simplified results of weathering calculations for two Xanthic Ferralsols
of Amazonia (Lucas 1989).
Manaus soil Juruti soil
Annual descent of the weathering front in 10
-6
m
F
1.6-2.8
1.7-4.4
Thickness of profile formed per year in 10
-6
m
T
0.13-1.3
0.2-2.4
Annual subsidence of the surface in 10
-6
m
S
1.5
1.5-2
Time taken for formation of the soil in 10
6
y
t
13-122
11-145
totF
Metres of rock weathered to form the profile
35-200
47-248
totS
Total subsidence of the surface in m
19-184
21-222
VR
Sediment weathered/profile formed
2.2-12.5
1.8-9.5
Based on theoretical calculations done by varying the quantity of
water that drains through the profile, Tardy (1993a) estimated that the
rock/soil boundary could subside by 20 m per million years.
The rate of deepening of the soils towards their base at the expense
of the parent material can be estimated from the amounts of dissolved
ions released to the streams. Simultaneously, surface erosion can be
evaluated from the materials suspended in the same water. The method
needs calculations, precautions and hypotheses. The authors cited took
into consideration the fact that the silica exported does not come from
quartz, which is presumed to be stable and is left in place. Table 5.5
gives the results obtained.
Weathering/erosion balance
Table 5.5
Deep weathering and surface erosion of profiles in the equatorial
zone (Boeglin
et al.
1997); My = million years.
Drainage basin
Erosion
Weathering
D
thickness Soil depth
thlp
(m/My)
(m/My)
(m)
(m)
(My)
Milo (Guinea)
11.2
12.4
1.3
20
7.5
Niger (Siguiri)
6.0
6.2
0.2
5
12.5
Niger (Bamako)
2.4
6.1
3.6
5
0.5
Chari (upstream)
4.9
4.7
-0.2
20
50
Chari (downstream)
2.4
2.5
0.1
20
100
Amazon (upstream)
273
57
-216
25
0.05
Amazon (downstream) 128 26 -102 25 0.1
The last two columns have been added by the author of the present topic;
thlp
has been defined
earlier.
