Physical and chemical weathering rates and CO2 consumption in a tropical lateritic environment: the upper Niger basin

The chemical composition of Niger river water measured bimonthly at Bamako Mali. during the period 1990–1992 provides an estimate of present weathering rates in the upper Niger basin. The dominant weathering process is kaolinite formation ‘monosiallitization’.. However, seasonal variations promote gibbsite formation in the rainy season September. and smectite development in the dry season May. The results show that lateritic profiles continue to develop even during very dry episodes. The rate of profile development, calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface, is about 1.3 to 3.7 mrMyr. A comparison between 43 river basins of the world shows that, for similar runoff, the CO2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones

Géochimie, degrés d'évolution et lithodépendance des cuirasses ferrugineuses de la région de Gaoua au Burkina Faso

Cet article présente une étude géomorphologique, pétrographique et géochimique des cuirasses ferrugineuses de la région de Gaoua (Burkina Faso). Les résultats des analyses minéralogiques et chimiques permettent de caractériser différentes familles de cuirasses, d'étudier les relations entre la composition chimique des roches mères et celle des cuirasses qui les recouvrent et de discuter le problème de la lithodépendanc

Transports fluviaux de matières dissoutes et particulaires sur un bassin versant en région tropicale : le bassin amont du Niger au cours de la période 1990-1993

Entre janvier 1990 et juillet 1993, 85 échantillons ont été prélevés sur les eaux du Niger à la station de Bamako (Mali). A partir des analyses effectuées et des données de débits disponibles sur cette période particulièrement sèche, ont pu être calculés les flux spécifiques pour les différentes espèces dissoutes (par ordre décroissant : HCO3 > SiO2 >> COD > Na > Ca > K > Mg >> Cl > SO4 >> Fe > Al) et le flux spécifique des matières en suspension. Le flux spécifique de TDS, caractéristique de l'intensité de l'altération chimique, est de 8 t/km2/an, alors que le flux de TSS, représentatif de la vitesse de l'érosion mécanique, est de 5 t/km2/an. Ces flux de matières dissoutes et particulaires sont ensuite comparés à ceux de 16 bassins fluviaux d'Afrique tropicale. On a pu enfin déterminer des relations TDS-drainage et TSS-drainage caractéristiques de cette zone tropicale. (Résumé d'auteur

Terrestrial and fluvial carbon fluxes in a tropical watershed: Nyong basin, Cameroon

The Nyong watershed, with an area of 27 800 km2 and a mean annual discharge of 390 m3 s−1, is the second largest river in Cameroon. The Nyong watershed serves as an outstanding study area for the examination of carbon fluxes in humid tropical environments because of its limited anthropogenic impact and homogeneous silicate bedrock. Between April 2005 and April 2007, we sampled water at seven stations, from the small watershed of the Mengong (0.6 km2) to the Nyong at Edea (24 500 km2), and monitored temperature, pH, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) contents, as well as the isotopic composition of DIC (δ13CDIC)andDOC(δ13CDOC).We estimated terrestrial net ecosystemproductivity in theNyong River watershed and measured fluvial fluxes of carbon to the ocean and the atmosphere. The Nyong River basin sequesters significant amounts of carbon on an annual basis: ~7 920 000t C year−1 (300 g C m−2 year−1). The combined dissolved organic, dissolved inorganic and atmospheric fluxes of carbon from the Nyong River only export 3% of this flux fromthe basin on an annual basis. This includes a minimumCO2 outgassing of 1487 g Cm−2 year−1, comparable to 115% of the annual flux of DOC and four times greater than the flux of DIC

Physical and chemical weathering rates and CO2 consumption in a tropical lateritic environment : the upper Niger basin

The chemical composition of Niger river water measured bimonthly at Bamako (Mali) during the period 1990-1992 provides an estimate of present weathering rates in the upper Niger basin. The dominant weathering process is kaolinite formation ("monosiallitization"). However, seasonal variations promote gibbsite formation in the rainy season (September) and smectite development in the dry season (May). The results show that lateritic profiles continue to develop even during very dry episodes. The rate of profile development, calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface, is about 1.3 to 3.7 m/Myr. A comparison between 43 river basins of the world shows that, for similar runoff, the CO2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones. (Résumé d'auteur