The effect of organic matter on chemical weathering: study of a small tropical watershed: nsimi-zoétélé site, cameroon

• Published in: Geochimica et cosmochimica acta 1999-12, Vol.63 (23), p.4013-4035
• Main Authors: Oliva, Priscia, Viers, Jerôme, Dupré, Bernard, Fortuné, Jean Pôl, Martin, François, Braun, Jean Jacques, Nahon, Daniel, Robain, Henri
• Format: Article
• Language: English
• Subjects: CAMEROON; DISSOLUTION; ENVIRONMENTAL SCIENCES; GEOCHEMISTRY; MINERALS; ORGANIC MATTER; ROCK-FLUID INTERACTIONS; SOIL CHEMISTRY; SWAMPS; WATER CHEMISTRY; WATERSHEDS; WEATHERING
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(99)00306-3

The effect of organic matter 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, we focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetélé, South Cameroon). This site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry. 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 present 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). We 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 as kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. Water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin. Thus, in order to explain the increase of element concentration in the organic-rich waters, we 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. SiO 2(aq) concentrations in these waters are above saturation with respect to quartz. Dissolution of phytholithes (amorphous silica) may be responsible for this relatively high SiO 2(aq.) concentration. Al/Mg ratios obtained for the soil and the Me