Résumé
The present dynamics of the Fouta Djalon massif is investigated from a geochemical study of surface waters which drain this old lateritic Guinean mountainous landscape, referred to be as the "water tank" of West Africa. The purpose consists in determinating, at a landscape scale, the geochemistry of natural elements transported by the rivers in the Sahelo-Sudano-Guinean zone, where the geological formations are mainly constituted of sandstones, granits and dolerites. The dissolved and suspended river transports are calculated in the upstream parts of the Senegal, Gambia and Niger basins. The increase of the drought contrasts more and more a long dry season with a short hot and rainy season. All the isohyets slid down to the South, of about 200 km, because of the drought which is affecting this country since 1970. The water deficit exists everywhere, except in the Guinean zone which concerns the major part of the Fouta Djalon massif. This massif, which covers a surface of 59 000 km2, receives a mean annual precipitation of 1380 mm/y ; the annual average runoff is of 380 mm/y and represents a runoff deficit of 75 %.
At present in West Africa, atmospheric dust is an important seasonal climatic event as rainy season. Their depositions are about 200 g/m2/y in the Sahelian zone and about 40 g/m2/y in the Guinean zone ; 40 % of these dust inputs are remobilized. Their contribution to input-output sediment budget is negligible and doesn't change the mechanical erosion balance. But their impact is important in the chemical weathering budget because they modify the chemical quality of the rain waters which are not acid and which contain bicarbonates and calcium. In comparison with the world average, these rain waters are strongly mineralized (from 10 mg/1 to 22 mg/1) with an annual average of 13 mg/1. These results note the importance of the Saharan dust to understand the present dynamics of these West Africa landscapes. The pyramidal collecting system, a new instrument perfected in this study, is the most adapted to these difficult countries such as the arid regions.
The surface waters of the Sahelo-Sudano-Guinean zone are weakly loaded from 20 mg/1 to 100 mg/1. HCOJ, Mg2+, Ca2+, Na+ contents are diluted by the discharge increase whatever the season ; the suspended load increases with the discharge and also depends on the surface condition of the drainage basin ; the silica concentration is regulated by biogeochemical processes. The potassium concentration is invariant, the loads of chlorides and sulphates show an important variability characteristic of atmospheric inputs. The chemistry of river suspended sediments reflects the chemical composition of soils impoverished by an intense leaching. The particulate organic carbon content decreases with the suspended matter concentration, and the organic matter represents 5% to 10% of the total river suspended sediment transport.
The suspended transport by rivers is mainly due to slope erosion by the surface runoff. In the upstream part of the Senegal river, surface runoff, estimated using a hydrograph separation method by spectral analysis, represents on average 23 % of the annual river discharge. The annual suspended load in the surface runoff is calculated to 1 g/1 after correction of bank erosion. This initial concentration is diluted in the river water by subsurface and groundwater flows. Then the river suspended sediment load is increased by the reworking of stream bed sediment during the first flows and by the bank erosion contribution. The first contribution produces the maximum concentration in river waters before the peak discharge. The relation between the cumulative sum of river suspended sediment fluxes and the cumulative sum of river discharges shows a change in the response capacity of the drainage basin owing to the erosion between an early erosion period and a period of erosion and transport. The first period (1 or 2 months) is related by the reworking sediment, and the suspended load depends on the studied hydrological cycle and on the intensity of the last hydrological cycle. The second period concerns the erosion of the drainage basin and finishes always on the fourth month after the beginning of the river water rising ; the suspended load appears to be an intrinsic constant of the drainage basin, and of the order of 160 mg/1 for the upstream Senegal river.
In the upstream parts of Gambia and Tinkisso basins, the mechanical erosion is calculated to 7 t/km2/y. It's one of the lowest of the African continent, and so in the world. It's owing to the ferralitic soils which cover entirely this upstream zone corresponding to the lateritic Fouta Djalon massif. In the upstream parts of the Niger, the Faleme and the Senegal basins, the mechanical erosion is more important, from 10 t/km2/y to 18 t/km2/y, because the middle course of these rivers drains the Mandingue plateau, which is a zone of ferruginous soils, very sensitive to gully and channel erosion. The specific chemical weathering increases from 1 t/km2/y in Sahelian zone to 5 t/km2/y in Guinean zone. It's also one of the lowest in the world. The chemical weathering is highly influenced by the hydrology and so depends on climatic zone.
The main characteristic of matter fluxes coming from the Fouta Djalon massif is the low concentration of particulate load (21 mg/1) in comparison to the dissolved load (35 mg/1). Only 25% of the dissolved flux result from the rock weathering, the other part is supplied by the atmospheric inputs. These results underline the weakness of the chemical weathering which affects at present these Guinean landscapes often duricrusty. Globally, the Fouta Djalon massif is at the present time in "geochemical balance" : the deepening of the soil profiles (4.6 mm/1000 y) is only just superior to the mechanical erosion in surface (4 mm/1000 y). Lastly, the study of the influence of the hydroclimatic parameters fluctuations on landscape dynamics proves that hydrological parameter is the best to explain the " geochemical respiration " of drainage basin in these tropical regions with two contrasted annual seasons. As a result, if a long-term climate is assumed, for an annual runoff under 380 mm/y, the soil formation by chemical weathering won't be sufficient to balance the soil losses by mechanical erosion.
Finally, this work underlines the necessity to study the surface water geochemistry in relation to the hydroclimatology. In the SaheloSudano-Guinean climatic zone, the runoff intensity is the main hydro-climatic parameter which controls the chemical weathering rate. So in this region, the atmospheric CO2 flux consumed by rock weathering may be expressed as a direct function of the drainage intensity. This approach of Hydroclimatology and Global Geochemistry of Continents (HGGC) shows also that the years of climatic type "Wet-Cold" allow to detect an influence of the air surface temperature on the river water bicarbonate loads, and then on the atmospheric CO2 flux consumed by weathering, increasing with the temperature for a same discharge. In conclusion, at time where the problem of CO2 concentration evolution in the atmosphere is important, one often forget that the rock chemical weathering can play a major role in the evolution of these atmospheric CO2 concentrations as buffering effect (regulating or not ?) between natural partitions, which are lithosphere, biosphere and atmosphere. The HGGC appears to be a necessary approach to understand and to calculate the CO2 consumption by climatic zone. For the Sahelo-Sudano-Guinean zone, the present average consumption is estimated to 18. 103 mol/km2/y of atmospheric CO2.
Orange Didier. Hydroclimatologie du Fouta Djalon et dynamique actuelle d'un vieux paysage latéritique (Afrique de l'Ouest) Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1992. 222 p. (Sciences Géologiques. Mémoire, 93)