Abstract:This study concerns the problem of water erosion in the Sahel. Surface water and sediment yields (suspended matter and bedload) were monitored for 3 years (1998)(1999)(2000) at the outlet of a small grazed catchment (1Ð4 ha) in the northern part of Burkina Faso. The catchment consists of about 64% sandy deposits (DRY soil surface type), which support most of the vegetation, and about 34% of crusted bare soils (ERO soil surface type). The annual solid-matter export is more than 90% suspended sediment, varying between 4Ð0 and 8Ð4 t ha 1 . The bedload represents less than 10% of soil losses. In a single flood event (10 year return period), the sediment yield can reach 4Ð2 t ha 1 . During the period studied, a small proportion (20 to 32%) of the floods was thus responsible for a large proportion (80%) of the solid transport. Seasonal variation of the suspended-matter content was also observed: high mean values (9 g l 1 ) in June, decreasing in July and stabilizing in August (between 2 and 4 g l 1 ). This behaviour may be a consequence of a reorganization of the soil surfaces that have been destroyed by trampling animals during the previous long dry season, vegetation growth (increase in the protecting effect of the herbaceous cover) and, to a lesser extent, particle-supply limitation (exhaustion of dust deposits during July). The particle-size distribution in the suspended matter collected at the catchment outlet is 60% made up of clay: fraction <2 µm. The contribution of this clay is maximum when the water rises and its kaolinite/quartz ratio is then close to that of the ERO-type surfaces. This indicates that these surfaces are the main source of clay within the catchment.

Three soil profiles developed on a serpentinite body (La Rochel'Abeille, near Limoges) show three stages of weathering. All soils contain iron-rich smectites and secondary chlorites. The latter are very silica-rich, more so than 14 A chlorites from crystalline rocks. In the (B)lg horizon of the hydromorphic profile, these minerals seem to give a reaction of the type:This reaction, typical of a closed system, appears to be operative in a soil profile which is certainly, in part, open to chemical migration. The chemistry of the weathered serpentinite and the chemical composition of newly formed minerals as well as those of the serpentinite are used to indicate the chemiographic relations of clay minerals formed in the weathering profiles.

Summary The sandy microdune systems of the Sahel are important for biomass production, in that they trap and store water. We have studied the movement of water over and in a dune and the chemistry of the water to understand this aspect of the systems. We experimented with simulated rain using a field sprinkling infiltrometer. We applied demineralized water with a chemical composition similar to that of the natural rain on a 1‐m2 plot. The plot was delimited by a metallic two‐level setting: the first enabled us to collect surface runoff, while the second measured subsurface flow. Water samples were taken at 5‐ to 10‐minute intervals throughout each simulation for chemical analysis (alkalinity, SO42–, F–, NO3–, Ca2+, Mg2+, Na+, K+ and Si). Mass balances, combined with a simple mixture model involving one tracer (chloride) and two reservoirs (old and new waters), were calculated. The equilibrating pressures of the CO2 (pCO2) and the saturation index with respect to specified minerals (e.g. calcite, fluorite, silicates) were also calculated by the AQUA ion‐pair model. The solute concentrations decrease in surface runoff as well as in subsurface water, except for F– and Si in the subsurface. The pCO2 decreased to a pressure less than the atmospheric pressure. The difference between measured concentrations and concentrations computed with the mixing model highlighted interactions between the soil and water. The dissolution of calcite which consumes CO2, and the cation exchanges, dominated, whereas the dissolutions of fluorite, silicates and gypsum appear secondary. Reactive mineral stocks were quickly exhausted, especially in the surface flow.

R E S U M E: Dans les encrofitements calcaires sous galet de la Plaine Poitevine, nous avons mis en Evidence, par des m&hodes microscopiques, une s~quence min~ralogique 6volutive de la calcite. Par spectrom&rie d'absorption infrarouge des ~l~ments de la s~quence pr~lev~s en hiver, nous caract~risons, /t c6t~ de la calcite (bande ~i 1420 cm 1), du carbonate amorphe hydrat~ (bandes /t 1405 et 1485 cm -1 et un autre type de calcite (bande ~t 1445 cm-~). Les r~sultats d'essals de laboratoire nous permettent d'envisager une ~volution du carbonate amorphe vers la calcite par l'interm~diaire d'une phase calcitique transitoire, et d'aragonite toujours en faible proportion. La formation de ces carbonates est li~e aux conditions bio physico-chimiques du sol.ABSTRACT: Micro-profiles of carbonate accumulations developed beneath pebbles in calcretes of the Plaine Poitevine, France, were studied by microscopy and IR spectroscopy. Samples were collected in winter and were found to contain (i) hydrated amorphous calcium carbonate, (ii) normal calcite (IR vibration band at 1420 cm -~) and (iii) a calcite showing a vibration band at 1445 cm 1. Laboratory experiments showed that the formation of amorphous carbonate depended on: (i) a low temperature (<15~ (ii) a high calcium bicarbonate concentration in the initial solution, and (iii) the presence of foreign substances dissolved or dispersed in the bicarbonated soil solutions. Moreover, amorphous carbonate was quickly destabilized in a CO z atmosphere and formed abnormal (1445 cm -a) calcite or, under certain conditions, aragonite. Depending on the nature of the substances initially present in the solutions, the abnormal calcite was with time more or less completely transformed into normal calcite. These experimental results have been used to explain the chronological and genetic succession of the calcium carbonates in soils of the Plaine Poitevine.

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