The effect of organic matter on chemical weathering. Study of a small tropical watershed: Nsimi-Zoetele site, Cameroon
The effect of organic mater during soil/water interaction is still a debated issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, the authors focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetele, South Cameroon). The lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones: unsaturated slope soils on the hills and/or elevated areas; and water-saturated soils in the swamp zone which represent 20% of the basin surface. The study presents chemical analysis performed on water samples collected monthly from different localities between 1994--1997 and on soil samples taken during a well drilling in December 1997. The results suggest the existence of chemical and spatial heterogeneities of waters in the basin: colored waters flooding the swamp zone have much higher concentrations of both organic matter (i.e., DOC) and inorganic ions (e.g., Ca,Mg,Al,Fe,Th,Zr) than those from springs and groundwater from the hills. Nevertheless, these organic-rich waters present cation concentrations (Na,Ca,Mg,K) which are among the lowest compared to that of most world rivers. The main minerals in the soils are secondary kaolinite, iron oxi-hydroxides, quartz, and accessory minerals (e.g., zircon, rutile). The authors mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through SEM observations the textural characterization of weathered minerals such a kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. In order to explain the increase of element concentration in the organic-rich waters, the authors suggest that organic acids enhance dissolution of minerals such as kaolinite, goethite, and zircon and also favors the transport of insoluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. Geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone. Quantitatively they propose that the increasing amount in total dissolved solid (TDS) due to organic matter and vegetation effect is about 35%. In summary, this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements. Thus, the low concentration of major elements in these waters is a direct consequence of the depleted nature of the soils.
- Research Organization:
- Univ. Paul Sabatier, Toulouse (FR)
- OSTI ID:
- 20006269
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 63, Issue 23-24; Other Information: PBD: Dec 1999; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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