『Abstract
The comparison between contemporary and long-term weathering
has been carried out in the Small Experimental Watershed (SEW)
of Nsimi, South Cameroon in order to quantify the export fluxes
of major and trace elements and the residence time of the lateritic
weathering cover. We focus on the hillside system composed of
a thick lateritic weathering cover topped by a soil layer. This
study is built on the recent improvements of the hillside hydrological
functioning and on the analyses of major and trace elements.
The mass balance calculation at the weathering horizon scale
performed with the parent rock as reference indicates (i) strong
depletion profiles for alkalis (Na, K, Rb) and alkaline earths
(Mg, Ca, Sr, Ba), (ii) moderate depletion profiles for Si, P,
Cd, Cu, Zn, Ni and Co (iii) depletion/enrichment profiles for
Al, Ga, Ge, Sn, Pb, LREE, HREE, Y, U, Fe, V, Cr, Mn. It is noteworthy
that (i) Mn and Ce are not significantly redistributed according
to oxidative processes as it is the case for Fe, V and Cr, and
(ii) Ge is fractionated compared to silica with enrichment in
Fe-rich horizons. The calculations performed for the topsoil with
iron crust as parent material reference reveal that the degradation
of the iron crust is accompanied by the loss of most of the constituting
elements, among which are those specifically accumulated as the
redox sensitive elements (Fe V, Cr) and iron oxide related elements
like Rh.
The overall current elemental fluxes from the hillside system
at the springs and the seepage zones are extremely low due to
the inert lateritic mineralogy. Ninety-four percent of the whole
Na flux generated from the hillside corrected from atmospheric
deposits (77 mol/ha/yr) represents the current weathering rates
of plagioclase (oligoclase) in the system, the other remaining
6% may be attributed to the dissolution of hornblende. The silica
hillside flux is 300 mol/ha/yr and can be mostly attributed to
the plagioclase and kaolinite dissolution. Al and Ga are exported
from the lateritic regolith and maybe due to the dissolution of
kaolinite crystals. Compared to the other immobile elements (Zr,
Hf, Nb and Th), Ti is significantly exported. Among redox-sensitive
elements (Fe, V, Cr, Mn, Ce), only Ce and Mn are exported out
of the hillside system. The other elements (Fe, V, Cr) are likely
able to be mobilized but over a short distance only. Rb, Sr, Ba,
Ni, Cu, Zn and affected by export processes. LREE and Y are exported
but in very low amounts (in the range from μmol/ha/yr to mmol/ha/yr)
while HREE and U are exported in negligible quantities.
A first attempt is carried out to compare the mature ridge top
profile from Nsimi SEW with the immature ridge top weathering
profile from the Mule Hole SEW (South India), developed on similar
granodioritic basement, in order to get deeper insight into (i)
the contemporary saprolite production rates and (ii) the combined
effect of precipitation (in terms of Mean Annual Rainfall, MAR)
and evapotranspiration on the aggressiveness of the draining solutions.
Considering (i) the contemporary Na flux as representative of
the dissolution of plagioclase crystals and conservative during
saprolitization processes and (ii) steady state of the inter-annual
recharge (R) over a 10 years period, the current saprolite production
rates (σr) are of 22 mm/kyr for Mule Hole
SEW and 3 mm/kyr for Nsimi SEW, respectively. Even with a very
low R/MAR ratio (0.04) compared to Nsimi, the chemical weathering
at Mule Hole is active and related to the groundwater exports.
At Mule Hole, plagioclase crystals are still present in the saprolite
and the soil cover leading to a diffuse weathering front. The
high Nsimi R/MAR ratio (0.2) allows the solution to be still aggressive
with respect to the plagioclase and other weatherable minerals
at the bedrock interface resulting in their complete breakdown
in a few centimetres (sharp weathering front) leading to a mature
saprolite.
For the Nsimi SEW, if we consider (i) the low contemporary saprolite
production rate (2 mm/kyr), (ii) the Miocene age (average 15 Myr)
of the South Cameroon Plateau landscape and (ii) the limited movement
of Africa continent since Eocene, the long term saprolite production
rate should have remained in its lower range, from 2 to 10 m/Myr.
This suggests that, for thick weathering profiles the migration
of the weathering front into the bedrock occurs at a relatively
uniform rate regardless of present-day climatic conditions. Climate
variation leading to the alteration of setup of savanna or humid
forest will have an effect on physical erosion rather than chemical
erosion for such deep weathering profiles.』
1. Introduction
2. Settings
3. Material and methodology
3.1. Water sampling and analyses
3.2. Sampling, analytical procedures of parental rock and regolith
4. Results
5. Discussion
5.1. Elemental distribution in bedrock and weathering
5.2. Long-term elemental fluxes in the weathering lateritic profile
5.2.1. Selection of the inert element, bulk density of the saprolite
and strain in the weathering profile
5.3. Element mass transfer profiles within the in situ weathered
horizons
5.4. Elemental mass transfer profiles within the topsoil
5.5. Contemporary elemental fluxes in the hillside lateritic
system
5.5.1. Origin of the contemporary fluxes in the hillside spring
and seepage
5.5.2. Determination of the recharge rate on the hillside system
and quantification of the contemporary elemental fluxes
5.5.3. Assessment of the contemporary saprolite production rate
and aggressiveness of the draining waters with respect to the
primary mineral weathering assemblage
5.6. Implication on the long-term evolution of lateritic terrains
in the humid Tropics
6. Conclusion
Acknowledgements
References