Biogeohydrodynamic in the Forested Humid Tropical Environment

Séance spécialisée Bull. Soc. géol. France, 2002, t. 173, no 4, pp. 347-357 Sol, altération, érosion Toulouse, 30-31 mai 2000

Biogeohydrodynamic in the forested humid tropical environment : the case study of the Nsimi small experimental watershed (south Cameroon) JEAN-JACQUES BRAUN1,2,BERNARD DUPRÉ2,JÉRÔME VIERS2,JULES RÉMY NDAM NGOUPAYOU3, JEAN-PIERRE BEDIMO BEDIMO4,LUC SIGHA-NKAMDJOU4,RÉMI FREYDIER2,HENRI ROBAIN1,5, BRUNOT NYECK3,JACQUES BODIN1,PRISCIA OLIVA2,JEAN-LOUP BOEGLIN1, SÉBASTIEN STEMMLER6 and JACQUES BERTHELIN6

Key words. – Lowland humid tropics, Small experimental watershed (SEW), Mass balance, Biogeochemistry, Hydrological functioning, Chemical and physical weathering, Laterite Abstract. – This paper summarizes a six-year study of the Nsimi Small Experimental Watershed (SEW), considered as a model for the South Cameroon humid tropical ecosystem. When this small watershed was set up, no similar survey of input/output hydrobiogeochemical fluxes in granitoid rocks in stable cratonic environment was available, to our knowledge, on any site close to the Equator. Moreover, this is the first attempt, world-wide, to combine different ap- proaches in hydrology, (bio)geochemistry, mineralogy, crystallography, microbiology, geophysics and pedology. Re- search is based on (1) regular hydrobiogeochemical surveys in various water reservoirs of the SEW ecosystem (atmospheric deposits, groundwater and stream), (2) surveys related either to the organisation and composition of different reservoirs in the superficial layers (basement rocks, saprolite, soils) or to various hydrological, biological and geochemical processes. These surveys aim at (1) finding the main parameters involved in the chemical and physical erosion processes of the humid tropical ecosystem, (2) understanding the source of a particular chemical composition in groundwater and rivers, (3) documenting accurately the different exportation processes of chemical elements in water and soil (4) investigating the possible relation between the biodegradation of soil organic matter and the leaching of metals (especially iron) and (5) comparing the long and short term weathering rates using mass balance calculations. An other important objective of this study is to provide a new scientific and engineering database for the future development of South Cameroon, which is still nowadays a relatively preserved ecosystem. One of the major results is the essential role played by the biological cycle (vegetation and soil organic matter) in the fractionation, exportation or storage of the chemical elements in humid tropical environments. Moreover we are able to propose a model of the current erosion for this SEW from the database obtained on (1) the mineralogy of the basement rocks and the soil layers, (2) the geochemistry of the soluble and colloidal phases of waters and (3) the hydrology within the different reservoirs of the hydrosystem. This model has been confirmed and extended on a regional scale (Nyong river basin). It emphasized the behaviour of the main elements of the tropical soil layers (Fe, Al, Si), the nutrients (C, Ca, Mg, K, Sr) and specific tracers of the weathering processes either with strong mobility (Cl, Na) or on the contrary with an extremely low mobility (Zr, Th, REEs). On the SEW scale, a strong geochemical contrast occurs between the different groundwater zones flooding (1) the hill slope lateritic profiles, (2) the weathering front (interface between the saprolite and the basement rocks), and (3) the swampy zone in which the Mengong brook flows. High DOC contents (15 mg/L) but also high Fe, Th, Al, Zr contents characterize the swampy zone waters. Na and Si have mainly a deep origin (exfiltration), Al, Th, Zr and REEs are strongly linked with colloidal organic matter located in the upper horizons of the swamp. Fe has a much more complex behaviour due to its change of redox state which can be independent of organic matter complexation. Concerning the major base cations, their origin can be constrained by the biological cycle (storage or leaching). K is typically influenced by the biological cycle. During the floods, Cl has the same behaviour as K : it is one of the most striking points of this study. However, the Cl annual budget is balanced. These characteristics can be understood as the consequence of the weathering of the minerals present in the saprolite (kaolinite, goethite, zircon, Th-oxide). This chemical weathering allows the leaching of base cations and also Al and Fe. It has been demonstrated that the microbial populations of the swampy zone can play an important role in the mobilization of transition metals (e.g. Fe). This study point out the role of humic acids in the transport and the weathering budget of elements usually considered as immobile in the superficial cycle (e.g. Al, Th, Zr, Fe). It must be mentioned that worldwide the SEW and even the Nyong network waters are among the least concentrated river waters. It means that even if the organic matter plays an important role in the mobilization and transport of some elements in the swampy zone, its action is limited in term of major cation fluxes on the SEW scale. The reason invoked is that the cation fluxes are directly linked to the pedological history and the geomorphology of the watershed. The presence of thick soil layers composed of saprolite and latosol on the hillsides and of hydromorphic soils in the swampy zone with constant mineralogy lead to isolating the bedrock. The long residence time of water close to the weathering front plays a major role in preserving the parent rock from the hydro-chemical outputs. Moreover, the topsoil layers are stabilized by the vegetation cover, which limits mechanical erosion. This should be taken into account for the carbon mass balance calculation because of the wide areas on stable shields concerned by the humid tropical

1 IRD, BP 1857, Yaoundé, Cameroun. 2 LMTG, UMR 5563, 38 rue des 36 Ponts, 31400 Toulouse. 3 Université de Yaoundé, BP 812, Yaoundé, Cameroun. 4 IRGM-CRH, BP 4110, Yaoundé, Cameroun. 5 IRD LFS, Centre Ile de France, 32 Avenue H. Varagnat, 93143 Bondy cedex. 6 CPB CNRS, 17, rue Notre Dame des Pauvres, BP 5, 54501 Vandoeuvre-lès-Nancy. Manuscrit déposé le 21 décembre 2000 ; accepté après révision le 26 février 2002. Bull. Soc. géol. Fr., 2002, no 4 348 J.J. BRAUN et al.

ecosystems. Moreover, comparison between long and short-term weathering allows us to suggest that paleo-climatic conditions did not change since the Miocene (6-20 Ma) in this part of the world.

Le petit bassin versant experimental de Nsimi (sud Cameroun) : exemple d’observatoire naturel du fonctionnement biogéohydrodynamique de l’écosysteme forestier tropical humide sur roche ignée silicatée

Mots clés. – Région tropicale humide, Bassin versant expérimental, Bilan de masse, Biogéochimie, Fonctionnement hydrodyna- mique, Altération chimique et physique, Latérite Résumé. – Cet article propose une synthèse des travaux réalisés au cours des six années d’étude du petit bassin versant expérimental (PBVE) de Nsimi considéré comme observatoire naturel de l’écosystème tropical humide du Sud-Came- roun. Lorsque nous avons installé ce PBVE, il n’existait, à notre connaissance, aucun site proche de l’équateur étudié en termes de flux hydrobiogéochimiques entrées/sorties sur roches alumino-silicatées en zone cratonique stable. De plus, ce site est le premier au monde où a été entreprise une approche combinée en hydrologie, (bio)géochimie, miné- ralogie, cristallographie, microbiologie, géophysique et pédologie. Les actions de recherche sont basées sur (1) des suivis hydrobiogéochimiques réguliers dans les différents réservoirs aquifères à l’échelle de l’hydrosystème PBVE (dépôts atmosphériques, nappes phréatiques et ruisseau) et (2) des études plus ponctuelles concernant, soit l’organisation et la constitution des différents réservoirs des formations superficielles (roche-mère, saprolite, sols) soit divers processus intervenant dans les cycles hydrologique, biologique et géochimique. Ces actions visaient à (1) rechercher les paramètres prépondérants qui contrôlent l’érosion chimique et mécanique de l’écosystème tropical humide, (2) comprendre l’acquisition d’une composition chimique donnée par les eaux de nappes et de rivières, (3) connaître pré- cisément les différentes formes de transfert des éléments chimiques dans les eaux et les sols, (4) mettre en évidence la relation possible entre la biodégradation des matières organiques du sol (MOS) et la solubilisation d’éléments miné- raux (en particulier le fer) et (5) comparer les bilans de l’érosion actuelle et passée. Un autre aspect important de cette étude est d’apporter une nouvelle base de données aux scientifiques et ingénieurs pour le développement futur du Sud Cameroun qui est encore, de nos jours, une région relativement préservée des actions anthropiques. Le résultat majeur est la mise en évidence du rôle moteur essentiel joué par le cycle biologique (végétation, ma- tière organique des sols (MOS)) dans les processus de transfert et/ou de stockage des éléments dans les régions tropicales humides. Il a été en outre possible, grâce aux données acquises (1) en minéralogie sur le substratum rocheux et les formations superficielles, (2) en géochimie sur les phases soluble et colloïdale des eaux et (3) en hydrologie sur les différents réservoirs de l’hydrosystème, de proposer un modèle de l’érosion actuelle pour ce PBVE. Ce modèle a été confirmé et étendu à l’échelle régionale du bassin fluvial du Nyong. Il rend plus particulièrement compte du comportement des principaux éléments constitutifs des formations superficielles tropicales (Fe, Si, Al), des nutriments (carbone, cations de base majeurs ou en trace Ca, Mg, K, Sr) et de traceurs spécifiques des processus d’altération/érosion, soit à forte mobilité hydrolitique (Cl et Na) soit, au contraire, à mobilité extrêmement limitée (Zr, Th, terres rares). A l’échelle du PBVE, il existe un fort contraste géochimique entre les différentes zones de la nappe phréatique baignant (1) les profils latéritiques des collines, (2) le front d’altération (interface entre saprolite et roche-mère) et (3) le bas-fond marécageux dans lequel s’écoule le ruisseau Mengong évacuant les eaux du PBVE. Des expériences d’ultrafiltration (jusqu’à 1 KD) ont permis d’appréhender le comportement des éléments dans les différentes fractions par- ticulaire, colloïdale et dissoute des différents réservoirs. Dans les eaux de nappes de versant baignant les profils latéritiques les concentrations de silice et d’aluminium sont en équilibre avec le quartz et la kaolinite alors que le fer est en équilibre avec un hydroxyde de fer amorphe. Le thorium, le zirconium et les terres rares sont principalement contrôlés par les phases solides colloïdales minérales. Au front d’altération, les concentrations en silice sont quatre fois plus élevées que dans les parties supérieures des nappes de versant et ne correspondent plus à des concentrations en équilibre avec le quartz et la kaolinite. Toute- fois, les concentrations en aluminium sont très faibles et peuvent être considérées comme étant en équilibre avec la kaolinite. Les fortes concentrations en silice et en cations de base (Ca, Na, K) sont expliquées par la dissolution des plagioclases et des feldspaths primaires. Les eaux de nappes de bas-fond sont caractérisées par des quantités importantes de carbone organique dissous (COD ; 15 mg/L) mais aussi de fortes concentrations en Fe, Al, Th et Zr. L’origine des éléments a pu être déterminée en échantillonnant en continu une crue. Il se dégage de l’analyse de celle-ci que Na et Si ont essentiellement une origine profonde (exfiltration de nappe), que Al, Th, Zr et les terres rares sont fortement liés à la matière organique col- loïdale localisée dans les horizons supérieurs du bas-fond. Le fer a un comportement plus complexe lié au changement possible de son degré d’oxydation qui peut être indépendant de la complexation par la matière organique. Pour les cations majeurs, l’origine est un peu plus complexe à déterminer et l’augmentation des concentrations peut être due à des processus de déstockage par le couvert végétal. Le potassium est l’élément type de la mobilisation par le cycle biologique. Lors de la crue, le Cl a le même comportement : c’est l’un des résultats les plus novateur s de cette étude. Toute- fois à l’échelle du cycle hydrologique annuel, le bilan du Cl est bouclé. Ces caractéristiques peuvent être interprétées par l’altération dans les bas fond des minéraux présents dans les saprolite (kaolinite, gœthite, zircon, oxyde de thorium). Cette altération a pour conséquence de libérer, outre Al et Fe, des cations. Il a été montré que les populations microbiennes des bas-fonds marécageux peuvent jouer un rôle non né- gligeable surtout en ce qui concerne la mobilisation des métaux de transition (exemple de Fe). L’un des résultats fon- damentaux de cette étude est la confirmation du rôle joué par les acides humiques dans le transport des éléments réputés à mobilité hydrolitique extrêmement faible (par exemple Al, Th, Zr, Fe) et l’importance de ces mêmes acides dans les bilans d’altération.

Bull. Soc. géol. Fr., 2002, no 4 BIOGEOHYDRODYNAMIC IN THE FORESTED HUMID TROPICAL ENVIRONMENT 349

Si l’on compare les concentrations des cations majeurs (Ca, Mg, Na, K) à celles d’autres rivières de la planète, force est de constater que les eaux du PBVE de Nsimi et du bassin fluvial du Nyong sont parmi les moins chargées du monde. Ceci sous-entend que, même si la matière organique joue un rôle prépondérant dans la mobilisation et le transfert de certains éléments dans les zones marécageuses, cela n’a pas un effet majeur en termes de flux de cations pro- duits à l’échelle du bassin. La raison invoquée pour expliquer cela est directement liée à l’histoire pédologique et à la géomorphologie du PBVE. La présence d’une formation superficielle épaisse constituée de saprolite, de latosols sur les collines et de sols hydromorphes dans les bas-fonds dont la composition minéralogique est monotone isole la roche-mère des agents d’érosion chimique (matière organique, eaux météoriques), limite les circulations des fluides et par conséquent protège la roche mère de l’érosion chimique et limite le flux de cations majeurs. En outre, ces couver- tures de sols sont par ailleurs stabilisées par le couvert végétal qui limite l’érosion. Ce rôle protecteur des sols vis-à-vis de l’érosion chimique, déjà mis en évidence à plus petite échelle par Stallard [1983] est ici clairement montré tant à l’échelle du PBVE de Nsimi que du bassin fluvial du Nyong. Ceci est à prendre en compte dans les calculs de bilan du cycle du carbone pour les surfaces considérables que les écosystèmes forestiers tropicaux sur craton stable représen- tent à l’échelle du globe. Par ailleurs, les comparaisons entre érosion chimique court et long terme permettent de sup- poser que les conditions paléo-climatiques n’ont que peu changée depuis au moins le Miocène (20 Ma) dans cette partie du monde.

INTRODUCTION Braucher et al., 2000]. From 1994 onwards, the research was carried out in the Nyong river basin (27.8 × 103 km2) Humid tropical environments cover about 23.106 km2 and mostly on the Mengong experimental watershed at (17 %) of the continental surface and play a very important Nsimi (South Cameroon plateau) (fig. 1). In its entirety, the role in maintaining the physico-chemical equilibrium of Nyong basin is covered by humid tropical forests. Its sub- our planet. Half of the continental flow drains these regions strate is made of silicate rock belonging either to the Congo [Tardy, 1993]. By way of example, we can draw attention to craton (2 800 Ma) or the Pan-African belt (600 Ma). 70 % the fact that the major portion of the two largest fluvial wa- of the soil layers are made of laterites and the remaining tersheds of the world, the Amazon and the Congo-Zaire are 30 % is composed of either hydromorphic soil found in situated in the humid tropical zone. As a general rule, these swamps or soil that is hardly mature on high slopes or/and regions are covered with thick weathering formations of the surface of the substratum. These soils constituted on the complex history [Tardy, 1993; Tardy and Roquin, 1998]. internal African surface date back relatively to the Miocene Called laterites, these soils are extremely poor in alkalines period (6-20 Ma) [Tardy and Roquin, 1998]. and alkaline earths and present a constant mineralogical When we set up the Nsimi SEW, there was, to our composition, mainly composed of a mixture of secondary knowledge, no other site located close to the Equator on minerals such as kaolinite, iron and aluminium oxides and granitoids, that enabled to study the input/output fluxes [cf. oxy-hydroxides (goethite, hematite, gibbsite) and residual Bruijnzeel, 1990]. Although there are many comprehensive minerals like quartz. Although precise models are available studies on ecosystems referring to temperate environments today regarding (1) petrographic and geochemical modifi- [Likens et al., 1977; Drever and Clow, 1995], the only site cations of the lateritic cover [Tardy 1993; Nahon, 1991; situated in a humid tropical zone was the small Rio Icacos Muller et al., 1995; Lucas et al., 1996; Théveniaut and watershed in Puerto Rico [White and Blum, 1995; Berner et Freyssinet, 1999; Beauvais, 1999; Girard et al., 2000] and al., 1998; White et al., 1998]. The aforementioned site is (2) the level of chemical and physical weathering on the located on hilly terrain in an active margin while the Nsimi inter-tropical river basin scale [Stallard, 1988; Stallard, SEW is in a stable cratonic zone. We would like to point 1995a; Stallard, 1995b; Edmond et al., 1995; Gaillardet et out that the Puerto-Rican site is today used as a reference al., 1999], the same cannot be said about the interfaces be- for the study based on the influence of climate (temperature tween the soils and the hydrographic network in small wa- and runoff) on chemical weathering of silicates and conse- tersheds. The manner in which the waters drained acquire quently on the quantity of atmospheric CO2 consumed. their mineral and geochemical composition and the In order to make sure that the Nsimi SEW would cover flux/transfer budget are known only partially [Thomas, the entire South Cameroon plateau and to take into account 1994 ; Freyssinet and Farah, 2000]. However the informa- the possible variations of water quality with scaling, we tion revealed by the study conducted on small experimental carried out a systematic study on rivers of the Nyong river watersheds (SEW) are essential for an understanding of the basin [Ndam et al., 1998; Viers et al., 2000] and bio- laws of physical and chemical weathering of continents on geohydrochemical monitoring on interlinked basins from a small scale and their consequences on the past and pres- Mengong (0.6 km2) up to the Nyong river (Olama station, ent climate of the planet [White and Blum, 1995; Velbel, 18510 km2). These studies allowed us to show that on the 1995; Berner et al., 1998]. scale of the large Nyong river basin, the geochemical signa- Within the framework of the National Programs ture of the river waters is acquired right from the elemen- PROSE/PEGI and PNSE (INSU/CNRS/IRD) and DYLAT- tary functional unit and that the seasonal variations are the Cameroon (IRD), we carried out various studies on the same whatever be the order of the rivers and streams con- tropical, humid ecosystem of South Cameroon since 1993. sidered with higher concentrations of chemical elements Our research began on the scale of a topo-sequence at a site during rainy seasons. In the SEWs, chemical weathering Goyoum, found in South East Cameroon by a study carried takes place mainly in the swampy areas. These zones con- out on the behaviour of geochemical and mineralogical stitute water reservoirs that are rich in organic matter mobi- tracers of the weathering processes [Braun et al., 1998; lized during rainfall events [Viers et al., 2000]. Bull. Soc. géol. Fr., 2002, no 4 350 J.J. BRAUN et al.

CAMEROON 0¡ NYONG

RIVER BASIN Long

Kom

Ayos Nyong 4° Edea Mfoumou Abong-Mbang Kelle Yaoundé Mefou Long Mafog Eseka Nyong Mbalmayo Nyong Akonolinga Dehane Kaya Soo Olama NSIMI SEW

Fala Ebolowa 100 km Kribi Sangmelima ATLANTIC OCEAN

10° 12°

FIG. 1. – Location of the Nyong river basin and the SEW of Nsimi, South Cameroon. FIG. 1. – Localisation du bassin fluvial du Nyong et du petit bassin versant expérimental de Nsimi (Sud Cameroun). This article is a short synthesis of the papers and re- basin sources was equipped with a level recorder scale. The ports of our group during the six year study of the Nsimi main source was equipped with a level meter. The level of SEW. More detailed information will be found in Braun et ground water was identified every two days in a piezometer al. [2002]. The objectives retained were to understand (1) network. the organization and the constitution of different reservoirs of soil layers (parent rock, saprolite, soils, groundwater) through a coupled approach taking into consideration pe- GENERAL FEATURES trography, geophysics and pedology, (2) the main physical and chemical weathering processes, using various geo- Geological settings and soils chemical and crystallographic tracers, (3) soluble and solid The geophysical measurements (electric resistivity, radar) forms of transfer, through coupled (bio)geo-chemical, crys- and observations carried out on the pedologic pits and bore- tallographic and mineral studies, (4) the possible relation- holes (specially all along the toposequence L6) allowed to ship between biodegradation of the soil organic matter and the fractionation of mineral elements (iron in particular) et 300

L6 700 m (5) the present and past weathering budget. 1 200 L7 L5 700 L4 692 L8 100 2 L3 700 LOCATION AND EQUIPMENT AT THE NSIMI 692 L9 L2 692 0 3 SEW 692 L10 692 684 692 4 -100 S 684 The SEW is constituted by two low hills (700 m altitude) 676 5 with convex slopes. These hills are separated by a swampy 6 676 676L1 -200 700 SPZ HSB S MENGONG gauge station zone whose surface represents 20 % of the total surface SWAMP area of the basin (60 hectares) (fig. 2). Many perennial -300 7 L11 springs, starting points of the Mengong brook, flow at the 8

level of the basin head. Eleven pathways regularly main- -400 9 OPZ weir 692 tained were created in the forest to facilitate access to the 10 sites for assessment and sampling. The different compart- -500 11 ments of the Nsimi basin were equipped for a pedologic, -600 geophysical, hydro-chemical and hydrological follow-up. A site for sampling atmospheric deposits was set up with the -100 0 500400300 800700600200100 900 1000 m following equipment : an automatic sampler of wet depos- pits S spring with weir its, samplers of total deposit and aerosols, gas probes and a piezometers rain gauges tracks L6 SPZ - OPZ pits (Oliva et al., 1999) rain gauge. The non saturated zone was studied using other atmospheric deposition specific equipment (tensiometers, porous candles). Quan- station tities of rain were measured using 19 rain gauges of which three were automatic. The flow of the Mengong was re- FIG. 2. – Equipment of the Nsimi watershed. corded continuously with a daily level recorder. Each of the FIG. 2. – Équipement du bassin de Nsimi. Bull. Soc. géol. Fr., 2002, no 4 BIOGEOHYDRODYNAMIC IN THE FORESTED HUMID TROPICAL ENVIRONMENT 351 constrain the three dimensional structure of the soil cover (63-69 % SiO2) belonging to the TTG group of Ntem of the bedrock and the piezometric level [Robain et al., (2 800 Ma) [Nédélec, 1990] is associated to a leucocrate 1999; Robain et al., 2001; Camerlynck et al., 2001]. Figure granite (75 wt% SiO2). The boundary between the two 3 illustrates a synthetic section of the toposequence L6. A rocks is diffuse and progressive. The clear-cut weathered normal east-west fault appears at the fracture of the north front shows thresholds that have a direct effect on the flow hill slope. In relation to the soil cover, two systems can be of the ground water on the basin scale. The bedrock out- identified : the slope system where the reddish yellow crops in a few places in the swamp area and reaches a depth polycyclic laterite soils develop and the swamp system con- of 40 m at the top of the hill. The granodiorite is fine to me- taining hydromorphic clayey soils. dium grained. It is composed of oligoclase (An 25), quartz, alkaline feldspar, augite/hornblende and titaniferous biotite. The bedrock The accessory minerals are apatite, zircon, monazite, magne- An estimated average bulk chemical and modal composi- tite, titanomagnetite, ilmenite and rutile. The leucocratic tion of the bedrock is shown in table I. On the Nsimi basin granite on the other hand is coarse grained. The major min- scale, two granitoids occur. A greyish blue granodiorite erals are quartz, K-feldspath and albite (Ab95), epidote and

FIG. 3. – L6 toposequence : soil horizon organisation and mineralogical composition of the bed rock, saprolite and soils and average depth of the groundwater table. The location of the reference profiles of tophill and swamp are indicated. FIG. 3. – Toposéquence L6 : organisation pédologique et composition minéralogique de la roche mère, du saprolite et des sols et niveau piezométrique moyen. Est également indiquée la localisation des profils de référence du sommet de toposéquence et de la zone marécageuse.

TABLE I. – Average chemical composition of the bedrock and estimation of the modal abundance of the main minerals. Tabl. I. – Composition chimique moyenne de la roche mère et estimation de l’abondance modale des principaux minéraux.

RECONSTRUCTED PARENT MATERIAL AVERAGE ± 1σ VARIATION MODAL ABUNDANCE (wt%) n = 9 %

SiO2 wt% 69.01 ± 3.59 5 QUARTZ 25 Al 2O3 wt% 15.18 ± 0.96 6 K-FELDSPAR 20

Fe2O 3 wt% 2.93 ± 1.56 53 OLIGOCLASE 26 MnO wt% < DL ALBITE 16 MgO wt% 1.11 ± 1.04 94 HORNBLENDE 4 CaO wt% 3.03 ± 1.35 45 BIOTITE 3 Na20 wt% 4.10 ± 0.54 13 K2O wt% 3.00 ± 1.61 54 TiO2 wt% 0.29 ± 0.15 52 P2O5 wt% 0.14 ± 0.07 50 LOI wt% 0.70 ± 0.25 36

wt% 99.49 CIA 0.49

Zr ppm 95 ± 35 37 Th ppm 5.6 ± 1.9 34

Bull. Soc. géol. Fr., 2002, no 4 352 J.J. BRAUN et al.

Ti-biotite. The accessory minerals are sphene, magnetite, sand material whose thickness varies from 0.5 to 2 meters. ilmenite and zircon. The coarse zones are made of It is essentially composed of kaolinite and round-shaped re- pluricentimetric crystals of quartz and K-feldspar. sidual quartz, Ti-oxides and zircon grains. Field observations suggest that the sandy material is derived from Saprolite colluvial accretion rather than in situ illuviation of the saprolite [Oliva et al., 1999]. All along the water year, the The saprolite constitutes the most important volume of the swamp is flooded and constitutes a seepage zone (free soil layers. The texture and the structure of the parent mate- groundwater table), the extension of which depends on the rial are preserved. Its thickness reaches 20 m on the summit seasonal distribution of rain. The seepage waters and super- of the north hill and about 40 m in the fault zone. The ficial draining water (1 meter depth) have a higher saturated saprolite shows a sharp contact with the parental rock hydraulic conductivity (up to 12 m/day) than the groundwa- (weathered front). Within a few centimeters, the fresh rock ter at 2 meters depth (3 m/day). is transformed into loose sandy loamy to loamy material, depending on the initial texture. Saprolite is mainly com- Climatic and hydrological features posed of secondary minerals (kaolinite, hematite, goethite) and residual minerals (quartz, illite, Ti-oxides, zircon). It During the year two dry and two rainy seasons of unequal constitutes the main aquifer of the basin groundwater. duration follow each other. The short and the long dry seasons correspond respectively to the northern (July) and the southern (January) boundary positions of the inter-tropical The hillside system front. The hydrological year extends from 1st March to Three main pedological horizons may be roughly identified 28/29 February. The hydrological assessment was made af- in the hillside lateritic system namely mottled clay, nodular ter carrying out a study during six hydrological years (from ferruginous and soft clayey topsoil. Their development de- 1993 to 1998) through continuous follow-ups of rainfalls, pends on the topographic position. The soil thickness may water flow at the source and at the outlet of the basin. Many reach about 15 m at the hilltop (cf. profile type, fig. 3) and floods as well as piezometric variations of the saturated decreases progressively towards the flat swamp. The zone were also taken into account. The water budget is saprolitic material is transformed progressively into a mot- summarized in table II. tled clay horizon displaying the juxtaposition of phases either rich in iron oxides, from yellow to purple in color, or Precipitations rich in kaolinite, white bleached in color. The saprolitic The mean annual precipitation on the watershed is 1751 ± structure is partly preserved. The mottled clay is overlain 143 mm. During the four water years, the number of rainy by more ferruginous horizons. The boundary between both day ranges between 103 and 129. Usually there is only one horizons is smooth and gradual. Two domains may be dis- stormy event per day, mainly originating from oceanic tinguished according to their topographic position. monsoon fluxes. However, some rain events are originated Upslope, the ferruginous horizon is discontinuous, made of from a regional cycle of precipitation/re-evaporation from indurate ferruginous boulders (iron crust relicts), nodules the rainforest itself. The rain reaches its maximum intensity and pebbles from centimeter to millimeter in size sur- after about 10 min. Fog and drizzles are the other forms of rounded by red clayey-sandy matrices. Towards the foot of wet precipitation. They do not exceed 1 mm per day. The the slope, this horizon decreases in thickness and trans- 5 mm rainfall event class represents from 25 to 35 % of the forms progressively into another ferruginous horizon called total events, the 5-15 mm class from 20 to 40 %, the “carapace”. It develops progressively at the expense of the 15-30 mm from 20 to 30 % and the 30 mm class from 10 to mottled zone. A soft clayey porous topsoil horizon tops the 15 %. The canopy interception, measured during the water ferruginous horizons. On the other hand, the boundary be- year 95-96, is about 15 % of the annual precipitation tween the soft clayey topsoil horizons and the underlying (1 490 mm/yr) while stem flow was assumed less than 1 %. layers is undulating and clear. The roots of the hillside vegetation are essentially concentrated in this topsoil horizon. Mengong runoff Fauna activity (e. g. termites and worms) is intense. A thin organo-mineral horizon, depleted in clays and covered by a The mean annual runoff at the Mengong outlet is of 380 ± thin litter, tops the soft clayey horizon. Hydrologically, the 88 mm. It represents between 17 and 25 % of the annual lateritic domain is divided into (i) a vadose zone corre- precipitation. The daily Mengong discharge is character- sponding to the topsoil, the ferruginous nodular horizon, ized by a strong variability in relation with the rain events. and the top of the mottled clay and (ii) a waterlogged zone The flood hydrographs show usually a quick response to composed of the bottom of the mottled clay and the entire the rain event whatever the period of the year. In general, saprolite. the water level starts to rise just after the beginning of the rainstorm and returns to the base flow level 12 hours after The hydromorphic swamp system TABLE II. – Water budget of the Nsimi SEW. The thickness of the swamp weathering cover is highly TABL. II. – Bilan hydrologique du bassin de Nsimi. variable from nil where bedrock crops out, to about 30 m in the faulted zone (cf. profile type fig. 3). Where it exists, the WATER YEARS WATER AMOUNTS yellow saprolitic material is truncated and displays alternat- 94-98 mm/yr ing domains from sandy to loamy in texture, the top portion Open Field Precipitations 1751 ± 143 Mengong brook output 380 ± 88 of which is composed of a bleached sandy clayey kaolinitic evapotranspiration 1212 material. The whole swamp is overlain with a gray clayey Flow deficit 1375 78.50% Bull. Soc. géol. Fr., 2002, no 4 BIOGEOHYDRODYNAMIC IN THE FORESTED HUMID TROPICAL ENVIRONMENT 353 the end of the rainfall. The permanent swampy zone satura- able part of these elements can be attributed to the tion constitutes a seepage zone. The overland saturation Harmattan input coming from the Sahara in the form of flow generated during the rain events explains the quick dust [Pelassy, 1990], the major portion originates from the flood response. All along the year, the Mengong base flow canopy emissions owing to the high biogenous recycling of is supported by the swamp groundwater and hillside this ecosystem. Chlorine, normally considered a conserva- springs, of which the mean annual discharge represents tive, is equally influenced by the biological cycle in the hu- about 20 % of the total outlet discharge. The mean dis- mid deposits [Viers et al., 2001]. However, on the yearly charge of the springs is less variable than that of Mengong. scale, the chlorine input-output statement is balanced. So it The swampy zone regulates the discharge of the brook. is possible to use the annual weighted average of Cl to cor- rect the raw concentrations of the elements in the different Vegetation and human activities SEW waters of the marine contribution. The silica concentration is inferior to the detection threshold in the total and Semi-deciduous rainforest (Sterculiaceae-Ulmaceae) (60 %), humiddepositsbuttheyvarybetween5to40µMinthe damaged by food crops (40 %), covers the rounded hills. throughfalls. This proves the mobilization of silicon by Crops are composed of tubers, manioc, peanuts, palm-trees plants. Sodium, present in small concentrations in the hu- and plantain. The farming method consists to slash and burn mid deposits (1.3 µM), has an essentially marine origin. Af- the preexisting vegetation followed by annual or biannual ter correction of the marine part of sodium in the basin cultivation. Then the farmers leave the fallow fields during waters, this element is to be used as a geochemical tracer of periods from 2 to 4 years. The clearing of the early forest is bedrock chemical weathering. the most important perturbation brought about this ecosystem. The population uses any chemical fertilizer. The whole swamp zone is preserved. Dominant tree popu- The Mengong and the groundwater table lations of both Gilbertiodendron deweverei (caesalpiniaceae) At the SEW scale, it is possible to identify three kinds of and raphia palm-trees (Raphia monbuttorum) cover the water. Figure 4 presents the zones where these samples well-drained zones (i.e. towards the outlet) while in the poorly have been collected. Table III summarizes the chemical drained zones (roughly from the spring to the track L5), compositions (< 0.45 µm filtration) of the rain, ground and only raphia palm-trees exist. The whole swamp is covered surface waters. The water types are (1) the clear waters with various species of semi-aquatic plants among which from the weathering front (hillside and swamp), (2) the those from the Araceae family dominates. The watershed clear waters from the upper fringe of the hillside aquifer has been preserved from burning and clearing during the and (3) the coloured water (tea colour) from the swamp sur- time of study. face and the Mengong brook. Filtration experiments from 0.45 µm to 1 KD (tangential and frontal ultra-filtration) were also carried out in order to find out (1) the chemical CHEMICAL AND PHYSICAL WEATHERING AND and the mineral composition of the particulate and colloidal THE TRANSFER FORMS phases and (2) the chemical composition of the «true» dissolved phase [Viers et al., 1997; Olivie Lauquet, 1996; Atmospheric deposit Olivie Lauquet et al., 1999; Rose et al., 1998; Olivie Lauquet et al., 2000]; the knowledge of this distribution is Three kinds of sampling were carried out at the SEW in or- necessary to set up a thermodynamic model. der to understand the distribution of atmospheric input during the year, identify sources and integrate them in the «input-output» budget. Sampling was carried out with re- Clear waters at the weathering front gard to wet atmospheric deposit, the total deposit taking Waters originating from the contact zone between saprolite into account the wet and the dry deposit and the throughfall and fresh rock were sampled at a depth of two meters at the [Ndam, 1997; Braun et al., 2002; Freydier, 1997; Freydier level of the swampy zone and at a height of 40 meters on et al., 1999]. The comparison of marine molar ratios and at- the hillside. These waters are the most concentrated and are mospheric deposit shows that the rains and the throughfall the richest in SiO2 compared to all the SEW [TSS (total are rich in Ca, Mg, K and SO4 , showing that these elements suspended sediment) < 1 mg/L; TZ+ (total cations) = have essentially a continental origin. Although a consider- 800 µeq/L ; [SIO2] = 400 µM (70 % of TDS)]. The high

TABLE III. – Chemical composition of the water reservoirs of the Nsimi SEW (average and volume weight mean – VWM). TABL. III. – Composition chimique des différents réservoirs du bassin de Nsimi (moyenne et moyenne pondérée par les volumes).

- - 2- 2+ 2+ + + EC T pH ANC Cl NO3 SO4 Ca Mg K Na Al Fe Ti Zr Th TZ+ SiO2 TDS TSS DOC POC µS.cm -1 °C µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 µm.L -1 nm.L -1 nm.L -1 nm.L -1 µeq.L -1 mg.L -1 mg.L -1 mg.L -1 mg.L -1 mg.L -1

WET DEPOSITIONS VWM 50 5.6 5.50 2.6 7.4 2.4 3.5 1.1 3.6 1.3 < DL < DL < DL < DL < DL 17.0 < 0.1 < 0.2 ND BULK DEPOSITIONS VWM 13 8.5 4.69 8.4 7.8 1.3 10.7 4.1 5.9 5.2 < DL < DL < DL < DL < DL 60.0 0.2 0.5 ND THROUGHFALL DEPOSITIONS VWM 15 14.0 5.24 7.8 25.2 2.8 19.4 23.3 37.8 5.1 < DL < DL < DL < DL < DL 134.0 1.0 2.7 ND

UPPER FRINGE HILLSIDE AQUIFER (SPRING) VWM 41 14.4 23.6 4.90 30.0 11.5 12.4 0.4 12.8 13.4 3.5 26.7 0.55 0.07 6 < DL < DL 112 5.4 8.4 1.1 0.7 < DL UPPER FRINGE HILLSIDE AQUIFER (TOPHILL) AVERAGE 6 20.8 23.6 5.08 46.0 19.8 4.3 4.1 22.2 7.9 13.9 15.6 1.47 1.95 < DL < DL < DL 99 5.9 9.0 < 1 1.1 ND WEATHERING FRONT (HILLSIDE) AVERAGE 7 6.05 23.7 17.0 8.1 111.7 34.6 43.9 248.7 0.23 0.32 < DL < DL < DL 585 17.0 29.2 < 1 1.7 ND WEATHERING FRONT (SWAMP) AVERAGE 5 65.5 23.4 5.81 572 12.7 1.9 0.8 96.1 91.4 18.3 179.1 1.64 7.03 41 0.174 0.035 572 23.9 35.4 < 1 2.4 ND

MENGONG OUTLET VWM 44 18.9 22.6 5.41 35.3 9.7 2.2 1.3 34.0 25.3 7.8 47.4 12.6 7.3 54 4.755 0.520 177 7.8 11.5 9.0 14.5 2.4

Bull. Soc. géol. Fr., 2002, no 4 354 J.J. BRAUN et al.

FIG. 4. – Reference profile of the hill top (A) and of the swamp (B) and location of the sampling waters : (1) clear waters of the weathering front; (2) clear water of the upper fringe of the hillside groundwater; (3) colored water of the surface swamp groundwater and of the Mengong brook. FIG. 4. – Profil de référence du sommet de colline (A) et du marécage (B) et localisation de l’échantillonnage des eaux : (1) eaux claires du front d’alté- ration ; (2) eaux claires de la partie supérieure de la nappe de colline ; (3) eaux colorées de la nappe affleurant dans le marécage et du ruisseau Men- gong. concentrations of major cations are to be linked to the The average concentration of Na is 15 µM, and is fully of weathering of the primary silicates. The waters have small atmospheric origin within the upper fringe at the top of the suspended loads [TSS (total suspended sediment) 1 mg/L] hill. At the bottom of the toposequence, the average con- and are poor in COD (0.5-3 mg/L). The total average Na centration is 25 µM, 65 % of which can be attributed to content is 180 µM, of this 90 % originates mostly from the chemical weathering. On the hillside scale, there is a high dissolution of oligoclase and albite of the parent rock. FeT concentration of gradient owing to the distance of the bed- and AlT are present in small contents. It is to be noted that rock and the topographic position. Th and Zr concentrations are inferior to the ICP-MS detection threshold, which is a few pM. These waters have a long Colored waters from the rising swamp groundwater residence time. table and the Mengong brook The Mengong water has a slightly higher mineral content Clear waters of the upper fringe of the hillside aquifer than the upper fringe aquifer of the hillsides (TDS = Groundwaters flooding the most mature soil profiles 12 mg/L, TZ+ = 180 µeq/L). The average concentration of (saprolite, mottled clay horizon and ferruginous horizons) DOC is 15 mg/L. The average suspension matter amount is were taken as samples (1) from the hill top (pits and low (TSS = 9 mg/L). The swamp surface waters have simi- piezometers) and (2) from the interface between hills and the lar characteristics. The particle phase is composed of min- swampy zone (spring and piezometers). They have a very erals originating from soil profiles (quartz, kaolinite, small mineral content (TDS = 8 mg/L; TZ+ = 100 µeq/L), goethite) but also of amorphous silica of biogenous origin small quantities of suspension (TSS 1 mg/L) and are poor in (phytoliths, diatom frustules) and organic matter (COP = COD (0.5-3 mg/L). The particle phase is composed of crys- 30 % of TSS). tals originating from soil profiles (quartz, kaolinite and The colloidal phase is composed essentially of organic goethite). The colloidal phase is constituted of ferrous gel colloids of different molecular weight. Ultra-filtration ex- (Fe(OH)3) and kaolinite micro-crystals. These solid phases periments of 0.45 µm to 1 KD and the thermodynamic cal- are rare and in fact control the small concentrations of FeT culations show clearly that the organic matter, found in and AlT (< 0.5 µM) in the filtrate [Viers et al., 1997]. The colloidal form, plays a very important role. The functional pH is acidic between 4.3 and 5.5. These waters have a short acid groups increase significantly the speed of silicate and residence time. oxy-hydroxide dissolution and thus favor the transfer of el- Whatever the topographic position of the upper fringe ements under a complex soluble form, adsorbed on the or- of the groundwater, silica concentration is always around ganic colloidal surface outside the watershed [Viers, 1998; 100 µM and represents 65 to 80 % of the TDS. All these Viers et al., 1997; Dupré et al., 1999; Oliva et al., 1999]. waters are in equilibrium with the residual quartz and the The organic colloids serve as vector to transport iron and secondary kaolinite [Viers et al., 1997; Oliva et al., 1999]. aluminum (FeT =7µM;AlT =13µM)andevenelements Bull. Soc. géol. Fr., 2002, no 4 BIOGEOHYDRODYNAMIC IN THE FORESTED HUMID TROPICAL ENVIRONMENT 355 known for their extremely weak hydrolitic mobility like ering rates in the Nsimi watershed are among the lowest re- thorium and zirconium (Th = 500pM, Zr = 4700 pM). corded in the world. Actually, when there is a small quantity of DOC, Th and Zr are found in extremely weak concentrations (< 3 pM). The Comparison with other SEWs on a granite substratum major cations (K, Ca, Mg) are mostly in the soluble phase. Although an important part comes from the weathering of To determine the influence of temperature and rainfall on the silicates, they are highly influenced by the biological chemical weathering and as a result on the consumption of cycle. For potassium for example, the plants act as a very atmospheric CO2, White and Blum [1995] compiled com- efficient sink, removing it from the soil solution. The prehensive existing data of input-output flux, on about 60 Mengong Na volume weight mean is 40 µmole of which small watersheds on silicate rocks, distributed all over the world. It is to be noted that the watersheds in tropical hu- 80 % can be attributed to chemical weathering. The average o silica concentration in the soluble phase is 120 µM. It is mid zones (temperature close to 25 C and rainfall thus slightly higher than in the upper part of the hillside 1 500 mm) are under-represented. Sodium and silica have been retained as tracers for chemical weathering as they are groundwater (100 µM) but lower than the measured concen- essentially in a dissolved form in the ground and surface trations at the weathering front (400 µM). This slight increase of concentration can be attributed to the proximity of water and participate on a small scale in the biological cy- the bedrock in the level of the swampy zone and the disso- cle. We have plotted in figure 5, the SiO2 at Nsimi SEW lution of the silicate minerals more easily weatherable than [White and Blum, 1995; Berner, 1998]. It is clear that the quartz. The participation of biological recycling (dissolving relation established by these authors does not apply to the of phytoliths) remains to be determined and ought not to humid tropical ecosystems of South Cameroon. We believe exceed a certain percentage [Meunier et al., 2001]. Both, that the thick soil cover and the residence time in the differ- silica and sodium, originating mostly from the weathering ent zones of the aquifer limit the hydrochemical outputs. of the silicates, have the same behavior during floods. They This is in accordance with Stallard [1995a], Edmond et al. originate from the deep groundwater and their concentra- [1995] and Gaillardet et al. [1999] who demonstrated, on tions diminish during the flood peak. This behavior differs the scale of the large tropical river basins (Amazone, greatly from that of potassium for example, which is highly Orénoque, Congo) the influence of soil protection on chem- influenced by the biological cycle. This element originates ical weathering and on the atmospheric CO2 consumption from the superficial zones of the groundwater and its con- flux. centration increases at the peak of the flood [Braun et al., 2002]. Long-term elemental transfers and comparison with The most active zone for both physical and chemical current chemical fluxes weathering is therefore the swampy zone. Element mobilization and export is favoured by (1) the hydrological re- The mass balance calculations, following the model of gime (seepage zone), (2) the production of organic colloids Brimhall et al. [1991], were only performed in the hill top by the slow mineralizing process of the dead swamp vegetation and (3) the presence of the bedrock at a shallow depth at certain places of the swamp. On the other hand, the permanent groundwater table induces conditions of anaerobic functioning [Stemmler, 2001]. There is therefore a high anaerobic degradation of the organic matter as well as a high dissolution of iron (as divalent iron) involving ferri-reduc- ing bacteria.

PHYSICAL AND CHEMICAL WEATHERING BALANCE

The present day physical and chemical weathering The total flux of exported matter in particle form, expressed 2 in tons/km /yr is 3.42 of which about 1 is of POC, 1.5 SiO2 (quartz, kaolinite, biogenous silica), 0.26 aluminium (kaolinite) and 0.10 iron (goethite). The physical weathering rate is generated mainly at the top of the organic-rich swamp horizon, the bulk density of which is 1.44. The esti- FIG. 5. – Correlation showing the influence of temperature and pluviome- mation gives a rate of 2.4 mm/kyr. The chemical weather- try on the chemical erosion of the silicate from White and Blum [1995] ing rate is estimated using the chemical weathering flux of and Berner et al. [1998], where is reported the Nsimi SEW (black dot). a base cation completely leached during the weathering This is out of the correlation showing the limitation of the hydrogeoche- mical output linked to the thickness of the soil cover, on chemical weathe- process and its fraction into the parent rock. The calcula- ring in humid tropics and cratonic areas. tion is performed using Na. The estimated chemical FIG. 5. – Corrélation montrant l’influence de la température et de la weathering rate is in the range 4.0-5.2 mm/kyr (average pluviométrie sur l’érosion chimique des silicates d’après White et Blum 4.6 mm/kyr). The average total weathering rate is about [1995] et Berner et al. [1998]. Les points (noirs) correspondant au bassin de Nsimi ont été reportés. Ils sont en dehors de la corrélation attestant la li- 7 mm/kyr, typical of the transport-limited weathering re- mitation des sorties hydrogéochimiques liées à l’épaisseur des sols sur l’al- gime [Stallard, 1995]. These estimates show that the weath- tération chimique dans les zones de craton sous climat tropical humide. Bull. Soc. géol. Fr., 2002, no 4 356 J.J. BRAUN et al. profile. We propose that this is the most evolved profile on CONCLUSIONS the watershed scale; therefore it had to record most of the soil cover history. Thus an integrated estimate of the ele- • The present day chemical and physical weathering mental mass transfers through the constituting horizons rates at the Nsimi SEW are among the lowest recorded with knowledge of the profile age should inform us about worldwide, with chemical dominating physical weathering the mean leaching chemical elemental fluxes. Despite the rates. large cumulative errors connected to the planation surface • The chemical weathering rate is enhanced in the age (Miocene) and the relative heterogeneity of the bed- swamp area by the role played by the biological cycle (veg- rock, this calculation allows giving us a range for the ele- etation, organic matter). The humic acids produced by the mental mass transfer per unit surface and time. Silica, base slow decay of the swamp vegetation are strongly involved cations and phosphorus are strongly leached in the whole in the transfer of elements considered to have extremely soil column. The loss of aluminum is also high and reaches weak hydrolitic mobility (for example Al, Th, Zr, Fe). The 75 % in the ferruginous horizons. Iron is depleted in both bacteria from swampy zones can also play a strong role saprolite and the mottled clay horizon and accumulated in with regard to the mobilisation of these transition metals. the ferruginous nodular horizon with however high varia- • The water of the Nsimi SEW and the Nyong river ba- tion ranges caused by local redistributions between soil sin are among the least concentrated in base cations (Ca, phases. Zr and Ti are also leached from 10-20 % in the Mg, Na, K) worldwide. This implies that, even if organic saprolite up to 50 % in the ferruginous nodular horizon. matter plays a role in the mobilization and the transfer of The past average chemical fluxes give us an idea about the certain elements in the swampy zones, it has no major ef- mean intensity of chemical weathering in the lateritic envi- fect in terms of base cations fluxes produced on the SEW ronment since the Miocene. Although it has been shown scale. that the swamp processes are predominant to provide chem- • The presence of thick soil layers made up of ical elements to the solute+colloidal fluxes, we may con- saprolite, latosol on the hills and hydromorphic soils in the sider that the current soluble+colloidal input-output budget swampy zone with constant mineral composition, lead to measured at the outlet of the watershed integrate the chemi- basement rock isolation in terms of hydrological function- cal weathering processes in the hillside and swamp sys- ing. The residence time of waters in the aquifer upper tems. Hence the comparison between the integrated past fringe is rapid compare to those at the weathering front. fluxes and the current chemical fluxes corrected from atmo- The parent rock is thus protected from the chemical weath- spheric inputs has to be done with caution especially for el- ering agents (organic matter, meteoric water). This is to be ements of which the behaviour is strongly influenced either taken into account in the calculations of the carbon cycle by the vegetation cycle (e. g. K) or transferred as organic balance for large surfaces like that of the tropical forest complexes (Al, Fe, Ti and Zr). Nevertheless the current sil- ecosystems on stable cratons at the global level. ica flux (not much influenced neither by atmospheric input • Moreover, the comparisons between short and long nor vegetation cycling) falls into the range of the past aver- term chemical weathering of silica allows one to assume age fluxes. For Ca, Mg and Na, the current chemical weath- that the paleo-climatic conditions have hardly changed ering fluxes are higher. This could be explained by the since at least the Miocene (20 Ma) in this part of the world. better efficiency of the chemical weathering in the swamp area due to the present geomorphic features. This would Acknowledgements. – Apart from the specific support from the IRD, the suggest that since the Miocene, the weathering regime on DYLAT-Cameroon project benefited from funding from the national pro- an average has hardly varied. The paleo-climatic conditions grams PEGI, PROSE, and PNSE (INSU/CNRS/IRD). We deeply thank Mathieu Zang, Mathurin Amougou, Patrice Messi, Claude Akono of the would hardly have changed since this period. We will stress Nsimi village for the care that they took to set up the watershed and to the fact that the position of the equator has not varied a lot maintain it. Our warm thanks to Philippe De Perceval, Michel Valladon, since this period [Tardy and Roquin, 1998]. Bernard Reynier of LMTG (Toulouse), James Rouiller and Emmanuel Jeanroy of CPB for their technical help.

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