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Pediments have been regarded as smooth, gently inclined, surfaces that front receding escarpments, are epigene forms shaped by running water, and are well represented in arid and semi-arid lands. Covered pediments carrying a cover of coarse allochthonous detritus largely conform to this model, but are of limited distribution. The more common mantled and rock forms however do not. Many of the mantled pediments developed in sedimentary terrains are associated with scarps that have been worn back, but the mantle comprises a surficial wash over a weathered mantle in situ: they are first-stage etch forms. Many mantled pediments shaped in granite front scarps that demonstrably recede only slowly, and others lack a backing upland and scarp. Though many are drained by stream networks some lack any surface drainage, and in any event, though diffuse drainage can erode friable materials, they do not greatly affect coherent rocks such as fresh granite. The mantle consists of a veneer of introduced material over weathered granite in situ: the bedrock is shaped by subsurface moisture attack or mantle-controlled planation. They too are first-stage etch forms. Diffuse wash can remove friable grus and where the mantle has been stripped the weathering front is exposed in rock pediments and platforms, both of which are etch forms. Les pédiments, qui ont été considérés comme des surfaces lisses en pente douce constituant le front d’escarpements en recul, sont des formes épigènes façonnées par les eaux courantes et, du reste, fortement représentées dans les régions arides et semi-arides. Les pédiments couverts, qui portent une couverture de débris grossiers allochtones, se conforment pour la plupart à ce modèle mais leur répartition est limitée. Les formes couvertes (autochtones) et rocheuses, plus répandues, ne s’y conforment pas. Un grand nombre de pédiments à couverture autochtone, qui se sont développés dans des terrains sédimentaires, sont associés à des escarpements en recul, mais leur couverture est assurée par le ruissellement de surface au-dessus d’une couverture de débris érodés in situ ; ce sont les premières étapes des formes d’altération. Beaucoup de pédiments à couverture autochtone se sont formés au pied d’escarpements granitiques reculant lentement. Bien que beaucoup soient drainés par un réseau hydrographique, certains ne le sont pas et, de toute façon, bien que le ruissellement diffus ait une capacité érosive, il n’a pas de grands effets sur les roches cohérentes comme le granite sein. La couverture consiste en un placage de débris reposant sur du granite altéré in situ : le substratum est façonné par l’action de l’humidité sous la surface, correspondant là aussi aux premières étapes de formation des formes d’altération. Le ruissellement diffus peut enlever du « grus » friable et là où la couverture a été décapée, le front d’altération est exposé dans des pédiments et les plateformes rocheuses, qui sont des formes d’altération de second ordre.

International audience; Soils in boundary conditions of contrasting ecosystems generally show unique features. Transition often leads to changes in soil-forming processes, whereby the environment never comes to equilibrium and therefore the soil chemistry and mineralogy show different influences. Such an environment was analysed in the Saloum River basin, west-central Senegal. The objective was to identify the main pedogenic processes prevailing in this saline and acid pedoenvironment and to assess the influence of environmental factors (climate, topography, soil salinity and acidity) on local soil formation and mineral distribution. The terrace landscape is built up by a floodplain, a low terrace, which is still influenced by groundwater, and a middle terrace. The results show that soil properties are strongly influenced by hydrology, salinity and acidity in the entire toposequence: Gleyic Hyposalic and Hypersalic Solonchaks (Sulfatic) in the floodplain, Haplic Gleysols (Thionic) in the low terrace, and Endogleyic Arenosols in the middle terrace. The oxidation of pyrite followed by the redistribution of the main products (Fe2+ and SO42-) represents the major chemical process responsible for iron oxide and jarosite formation. Mineral distribution and crystallinity are linked to the landscape position, which controls the hydrological behaviour and reactions of Fe and S ions. Finally, we observed intrapedon processes such as gleysation, sulfidisation and sulfurisation, as well as interpedon processes such as salinisation, colluvio-alluviation and lateral eluviation. The combination of processes depends strongly on the landscape positions.

The distribution of trace elements (Sr, Mo, Ba, Rb, U, Mn, Ni, Cu, Zn, and Cd) was investigated in surface waters and associated particulates in two upper Amazonian basins (Mamore and Beni). Dissolved Sr, Ba, Mo, Rb, U and to a lesser extent Zn and Cd correlate with major ions and appear to be predominantly derived from soluble rocks (carbonates, evaporites or sulfides essentially contained in shales). These elements are conservative in waters except in highly alkaline or sulfate-rich waters where calcite and barite can precipitate. The concentrations of transition element are generally not correlated with major ion chemistry. Their behaviour can be largely explained by equilibrium reactions in solution and solid–liquid exchanges. Additionally, the distribution of Mn and Cu reflects also the lithological differences between the Mamore and Beni basins. The occurrence of mineralisation in the Beni basin explains the high Zn, Cd as well as the concentrations of Cu in some rivers of the Beni basin. The higher content of Mn in the Mamore basin may reflects the predominance of carbonate rocks. The fractionation of trace elements in SPM has been attributed to the sorting of feldspars during transport in water, and to the mixing between clay minerals and quartz. These minerals, refractory to weathering and introduced in water rivers by mechanical erosion, have not contributed to the dissolved load.

Dating weathering processes is an innovative approach to understanding climate-dependant modifications of the Earth's surface. Potassium-bearing manganese oxides are a ubiquitous feature of weathering mantles developed from Mn-bearing parent rocks. Recent studies have demonstrated the possibility of dating K-rich manganese oxides from the hollandite group, such as cryptomelane (K1-2Mn8O16, nH2O), using 40Ar/39Ar argon-ion laser probe method. This work applies that approach to cryptomelanes formed in the lateritic manganese ore deposits of Tambao (northern Burkina Faso), a present unexploited deposit among the cryptomelane-richest in the world. Two samples of massive Mn-rich concretions (10 mm and 4 mm in thickness) were collected from the massive 80-m high Mn-rich hill at Tambao. A detailed petrographical study shows that these cortical formations are composed of successive Na-rich cryptomelane layers, with Na/(Na+K+Ba) up to 20%. Seven and five subsamples from these two concretions were studied with a 40Ar/39Ar laser probe. The age spectra indicate ages that are coherent with the general organization of the concretions, showing a regular decrease from cores towards rims. Within the two concretions, ages range from 49.8 + or - 0.4 Ma to 48.1 + or - 0.1 Ma, and from 47.5 + or - 0.3 Ma to 44.5 + or - 0.4 Ma, respectively, the youngest ages being obtained for the most external layers. The integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm/Ma. These ages clusters show that weathering at Tambao was functioning during the early Eocene and thus suggest that there is a relationship between the formation of the bauxites of the African Surface and the Mn-Fe laterites. (Résumé d'auteur)

In a lateritic bauxite formed by weathering of nepheline syenite at Passa Quatro, Minas Gerais State, Brazil, bauxites on the hill-tops directly develop from the syenite bed-rock, while downslope, a kaolinitic layer occurs between bauxite and synetie. A petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit. The undisturbed weathered rocks were chemically analysed for major trace elements including REE and Zr. Mass balance calculations were applied, and the behaviour of the REE in the Passa Quatro weathering system was established compared to REE reference chondrite and to REE reference parent rock. In the lateritic bauxite, the results suggest that the first stages of weathering induce a volumetric change of 50%, i.e. collapse, with respect to the parent rock, and remove REE with a slightly larger loss of the LREE, except Ce, compared to the HREE. In the upper layers, where bauxite is more mature, a net mass gain in REE is observed relative to the underlying layers. This gain takes place during the reduction of the upper layer during the downward progression of the weathering front. Very significant REE losses occurs during the bauxitization processes throughout the upslope profile. In addition, the downslope kaolinitic system is demonstrated to be depleted in REE in the same proportions as the upslope bauxite. We proposed that the REE exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments.

9 pages, 2 tableaux; National audience; La sensibilité potentielle des roches cristallines à l'hydrolyse dépend très fortement de leur composition minéralogique. Un indice d'altérabilité (Ia) calculé à partir de la composition chimique globale des matériaux non altérés permet de prendre indirectement en compte le facteur minéralogique. Cet indice est particulièrement utile dans le cas des roches cristallophylliennes, pour lesquelles l'observation des lames minces renseigne mal sur la composition modale. Un indice de sensibilité à la décohésion chimique (Id), au caractère plus empirique, intègre l'influence de l'agencement des minéraux.

The chemical weathering of ultramafic rocks has resulted in eluvial concentration of Pt-group minerals (PGM) in lateritic weathering profiles of southern New Caledonia. The Pt mineralization interpreted as being primary consists of Pt-group minerals included within chromite crystals. The oc- currence of PGM as free particles in the weathering profile results from the supergene dissolution of Pt-bearing chromite (Traore et al. 2008). Following their release in the profile, supergene dissolution processes variably affect the PGM particles. The behavior of Pt-group elements in the weathering profile is characterized by significant loss of Pd and relative enrichment of Pt indicating that Pd is more mobile than Pt in the exogenous cycle. Unstable Pt-Fe-Cu-Pd alloys and PGE oxides undergo chemical and mineralogical changes to acquire the chemical configuration of isoferroplatinum (Pt3Fe), which is the most stable Pt phase in a lateritic environment. The isoferroplatinum phase may also be dispersed throughout the weathering mantle and/or accumulated in the lower parts of profiles ac- cording to a translocation mechanism of residual Pt-rich fine particles driven by percolation of water through the connected pore spaces.

The study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored. In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (

International audience; The long‐term effects of intermittent flooding on soil properties were studied in field experiments on a Vertisol cropped with rice in Senegal. The dominant clay minerals were smectite and kaolinite. When the soil was reduced after flooding, its cation exchange capacity (CEC) increased to twice that of its oxidized, unflooded state. Mössbauer spectroscopy showed an increase in smectite structural FeII upon reduction, which explained a part of the increase in CEC. The rest of the increase was attributed to the removal of iron oxyhydroxide coatings by reductive dissolution. The reduction and dissolution of oxides under the field conditions were substantiated by analysis of the surfaces of vermiculites buried in the Ap horizons of the cropped and the non‐cropped soils. The redox‐induced CEC changes were found to be reversible after 22 cycles of rice cropping. Nevertheless, the structural Fe and free Fe contents of the rice field Ap horizon were less than those of soil in uncropped neighbouring land, suggesting that inundation induced weathering and eluviation of the minerals. The observed changes in CEC and related redox reactions may substantially modify proton, anion and cation balances in intermittently flooded soils.

Due to the scarcity of water resources in semiarid sedimentary basins, hill reservoirs are often constructed torechargegroundwaterandlimitrunoffinducedwaterloss.TheimpactofsuchreservoirsongroundwaterchemistryisinvestigatedintheaquifersoftheElGouazinewatershed,CentralTunisia.Threegroundwatertypesarerecognised,Ca–HCO 3 ,Na–ClandCa–SO 4 .Thestrongsimilaritybetweenhostrockandgroundwaterchemistriesindicatessig-nificantrock–waterinteraction.Aflowpath,alongwhichthechemicalcompositionofthegroundwaterevolves,canbeidentified using the contrast in stable isotope signature between upstream and downstream groundwater. Shallowupstreamgroundwaterisrechargedbytheinfiltrationofrainwaterwiththerateofrechargestronglylinkedtothepermeability of the host lithology. Calcium and HCO 3 are supplied to an alluvial aquifer from a more rapidlyrechargedlimestoneaquiferwiththeconcentrationofCaandHCO 3 ionsdecreasingbydilution.ThealluvialaquiferisalsoenrichedinCaandSO 4 duringthedownstreamflowofgroundwaterthroughgypsiferousmaterials.Thereisevi-dence of mixing between meteoric groundwater and evaporated reservoir water. Below the reservoir and partlyresponsibleforreservoirleakageisasandyaquifer,formedbyweatheringanderosionofasandstonehostwhichalsosupplieswatertothealluvialaquifer.# 2002ElsevierScienceLtd.Allrightsreserved.1.IntroductionA perennial problem for farmers and herdsman insemiaridareaswithoutpermanentriversisthescarcityofwaterowingtothelowanderraticrainfall.Inthelastcentury,globalpopulationexpansionhastremendouslyincreased the pressure on water resources. To securewater supply and intensify agricultural production,water harvesting is traditionally used to combat theproblemsofvaryingrainfallratesandpopulationden-sities.One technique of water harvesting, floodwater har-vesting(FWH),alsocalledlargecatchmentwaterhar-vesting or spate irrigation, is widely developed. Whenfloodwaterisharvestedinthestreambed,thewaterflowisdammedandstoredintemporaryorpermanentsur-face reservoirs. Water is forced to infiltrate and,asaresult,torechargetheaquifer,withrechargeconcentratedmainly in the valley downstream from the reservoir(Prinz,1999).FWHisarelativelyinexpensivetechniqueprovidingwaterforthedevelopmentofagricultureandagroforestry, and reducing the use of other valuablewater resources such as groundwater. Water supply isnotalwaysevenandsecureduringtheyeardependingontheclimaticvariability.Largeevaporationlossesandlossofstoragebysiltationandpollutionarethemainlimitations of FWH. Social problems like conflictbetweenwaterusersareafurtherdrawbacktothisstyleofwaterharvesting.IntheMediterraneanbasin,waterharvestingplayedan important role in ancient civilisations such as theRomanandEgyptianempires(Prinz,1999).Duringthe20thcentury,moderntechniques(groundwaterdrilling,largereservoirconstructionetc.),whicharefavouredbygovernmentpolicy,slowlyreplacedtraditionalharvest-ing techniques. High demands for water during theagricultural growing season, which coincides with thedryperiodoftheMediterraneanclimate,hasresultedinthe depletion of the groundwater table in some areas.Thelowrenewalrateofgroundwaterresourcesandthe

International audience; In the Serra do Mar region, in southeastern Brazil, the soil mantle is mainly characterised by (i) a gibbsitic saprolite, (ii) various kaolinitic horizons within the gibbsitic material, (iii) kaolinito-gibbsitic topsoil horizons. This organisation does not match with the thermodynamic stability of gibbsite and kaolinite accompanying the solution percolation through soil profiles. A study of the micromorphological, mineralogical and chemical properties of the soil mantle reveals that this organisation arises from the in situ development of the soil from the crystalline bedrock. The bauxitic weathering of the bedrock, even if it is rich in quartz, can be explained by a fast renewal of the solutions and/or a high solubility of the kaolinite. Recycling of Si and Al by the forest can maintain a dynamic equilibrium of Kaolinite in the topsoil horizons, as observed in Amazonia. The kaolinitic compact horizons evolve upslope at the expend of the gibbsitic material. At the contact between kaolinitic and gibbsitic material, dissolution patterns of quartz and gibbsite are observed, indicating that this evolution is in process. These observations and the organisation of the soil mantle set the problem of the apparent stability of gibbsite and kaolinite in this environment. Various assumptions that could explain this organisation of the soil mantle are discussed. Changes in the activity of water due to the pore size diminution, and displacement of the gibbsite-kaolinite equilibrium appear insufficient to explain the stability of kaolinite. However, it could be alloted to the slow down of water flows in the soil mantel. Lastly, the eventual role of the complexing organic matter is presented. More investigations on the biogeochemical cycle of Si and Al and on the physico-chemical processes at the soil solution-mineral interface are necessary to explain the stability and dynamic of gibbsite and kaolinite in this environment.

Le Mont Cameroun est un volcan actif situé en bordure du continent africain. Le climat tropical humide régnant sur ses flancs crée des conditions privilégiées pour l'étude des processus d'altération des basaltes. L'altération chimique a_ d'abord été examinée dans des coulées massives (d'âge < 100 ans) puis dans des sols formés à partir de dépôts pyroclastiques récents (810 - 4530 ans). Les coulées massives ne montrent pas de transformations chimiques notables liées à l'altération, ainsi qu'en témoignent les concentrations en éléments majeurs et traces et les isotopes du Sr, Nd, Pb et' U. Toutefois, deux processus associés à l'interaction eau/roche se produisent à la surface des coulées: (1) un échange de cations entre les ions Na+ de la lave et les ions H+ des eaux d'altération et (2) une oxydation partielle du . Fe2+ en Fe3+. Ces modifications chimiques restent très limitées au regard des précipitations dans la zone d'étude (2"à 12 m/an). Nos résultats démontrent que les effets de l'altération sont négligeables sur la composition chimique de laves basaltiques massives soumises à des conditions tropicales humides sur une période de 100 ans. Plus perméables, les sols formés à partir de dépôts pyroclastiques constituent un matériel plus réactif vis-à-vis des solutions d'altération. Ces sols sont significativement enrichis en Cs, Ba, Pb, U, Th, P et appauvris en Na, K, Rb, Mg, Ca, Sr, Ni, Co et Si. Les enrichissements élémentaires, ainsi que les variations isotopiques du Sr, Nd et Pb dans les horizons superficiels, s'expliquent par l'adjonction périodique de poussières sahariennes. Nous montrons que la proportion massique d'aérosols sahariens dans les sols n'excède pas 7%. Les appauvrissements élémentaires résultent de l'hydrolyse des minéraux magmatiques. L'intensité de l'altération ne dépend pas de l'âge des dépôts pyroclastiques mais du temps pendant lequel ils . restent en surface avant d'être recouverts par des laves plus récentes et des conditions paléoclimatiques., no abstract

Core MD77-169, located in the Andaman Sea, is characterized by eNd(0) ranging between −9.5 and −11, implying that the Irrawaddy River is the main contributor of detrital material. The pedogenic clays (smectite and kaolinite) to primary mineral (feldspar, quartz, illite and chlorite) ratios show strong precessional cycles, suggesting that they are controlled by past changes in the monsoon intensity. Wet periods of summer monsoon reinforcement are characterized by the increase of chemical weathering and/or physical erosion of the Irrawaddy plain soils.

This paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments. We studied the Nyong River basin (27,800 km2) located on the northwestern part (Ntem Complex) of the Congo craton (central Africa). The dissolved concentrations (i.e.

Dans la région birimienne de Piéla, l'or est associé à des filons de quartz d'une centaine de mètres de longueur qui se situent dans une large zone de cisaillement. La présence d'or a également été mise en évidence dans les matériaux superficiels. Ceux-ci sont étudiés dans deux plaines d'épandage présentant des contextes géomorphologiques différents. Un site proximal, où l'or est mal trié, présente des particules peu évoluées avec une faible proportion de particules évoluées héritées de formations superficielles anciennes. Un site distal, situé à 2,5 km de la minéralisation primaire, piège des particules d'or grossières, de forme très évoluées malgré la modicité du transport. (Résumé d'auteur)

The aim of this study was to give some new constrains on the Himalayan erosion, using the geochernistry of dissolved and particulate matter carried by Himalayan rivers. The main results are : - The supended load is not representative of the total erosion load. For the Ganges-Brahmaputra (G-B), it represents less than 50% of the total particulate discharge. The total erosion in the G-B is therefore 2.4 ± 0.5 109 ton/yr. - The location of intense erosion corresponds to the location of heavy rain in the range or at the G-B scale. The erosion rate are 3.4 ± 0.7 mm/yr. for the Brahmaputra Himalaya and 2.3 ± 0.6 for the Ganges Himalaya while the run off of these 2 domains are 2.1 and 1.2 m/yr., respectively. In central Nepal, erosion rates follow the climatic contrast between the south and the North flank of the range : the erosion of the South flank of the high range is more than 6 times higher than that of the North flank.- The highest erosion rate is located just above the crustal ramp of the main Himalayan thrust, bellow the heaviest rain and on glaciated terranes. Tectonic convergence and the geometry of the crustal thrust focus the vertical uplift in the narrow zone of the south flank of the high range, where the erosion is twice more important than everywhere else. - The Himalayan erosion is buffered by physical processes. For the G-B, more than 55% of the eroded carbonate are carried by the particulate phase and no more than 1.3% of the eroded silicate have been dissolved. - The high erosion rate acts as a restriction of the weathering. In the south flank of the high range, where the erosion rate is the highest, the weathering of rocks is the lowest. This result implies that there is not a general and positive relationship between tectonic and weathering. This link is even more complex for the whole orogenie process since the weathering of highly divided material in the foreland basin has to be taken into account. - The alkalinity flux corresponding to silicate weathering is 2. 7 1011 mol/yr. corresponding to 2.3% of the global flux. The budget of the weathering implies a dominant contribution of the carbonate dissolution to the riverine chemistry and that weathered silicate are mostly sodic. The long term atmospheric co2 consumption by silicate weathering has been estimated to be 6.4 10 10 mol/yr. for the whole G-B. - 14 Eq% of the alkalinity corresponds to an abiotic alteration. The main is the sulfide oxidation that produces around 70% of the dissolved sulfate load of the G-B. Some metamorphic C02 degassing, along Tibetan graben account for less than 1% of the who le alkalinity, but is loc ally significant. - Dissolved Sr budget and silicate alkalinity budget are uncoupled. An incongruent dissolution of carbonate in the Tibetan part of the range is the main source of dissolved Sr while the small weathering of old crustal terranes (> 2Ga) on the south flank acts as a 87Sr spike for the Himalayan rivers. The G-B (6.5 108 mol/yr. of dissolved Sr with 87Sr/86Sr- 0.730) plays a key role of the Sr oceanic budget but has a few impact on the alkalinity. - The global impact on the carbon cycle by the Himalayan erosion lies in its ability to preserve and bury organic carbon. This burial is one order of magnitude higher than the long term atmospheric CO2 consumption by silicate weathering and represents 5% of the global C burial. This enhanced C removal from the ocean-atmosphere reservoir by the erosion of the Himalaya could be sufficient to destabilize the global carbon cycle., L'étude de la géochimie des particules exportées par les rivières himalayennes ainsi que la chimie des éléments dissous ont permis de préciser la quantification du couplage entre la tectonique active, caractéristique de l'Himalaya et le climat ainsi que de définir les principaux processus qui caractérisent l'érosion actuelle de la chaîne himalayenne. Les caractéristiques principales de l'érosion de la chaîne himalayenne sont : - Un important fractionnement minéralogique entre la matière en suspension et la charge de fond des rivières. Pour l'ensemble du Gange-Brahmapoutre (G-B), le transport par les MES est < 50% de l'érosion totale qui est de 2.4 ± 0.5 109 tonnes/an pour le bassin du G-B. - La corrélation entre le taux d'érosion et l'intensité annuelle des précipitations, tant a l'échelle des larges bassins (G-B) qu'à l'intérieur de la chaîne. Le taux d'érosion de la chaîne est de 2.3 ± 0.6 mm/an pour la partie drainée par le Gange et de 3.4 ± 0.7 mm/an pour celle drainée par le Brahmapoutre alors que les écoulements spécifiques des parties himalayennes de ces fleuves sont respectivement de 1.2 et 2.1 m/an. Au Népal central, le flanc sud, soumis aux précipitations de mousson, s'érode plus de 6 fois plus vite que le flanc nord, soumis à un climat aride et sec. - Un taux maximal d'érosion physique localisée à l'endroit où la surrection tectonique, les précipitations et les glaciers sont les plus importants. C'est à l'aplomb de la rampe crustale du chevauchement himalayen (MHT) que se trouve la haute chaîne, les plus fortes précipitations et les glaciers. A cet endroit, l'érosion physique est au moins 2 fois plus élevée que dans le reste de la chaîne. Une érosion dominée par le broyage mécanique et non l'altération et la pédogenèse. Pour l'ensemble du G-B, plus de 55% des carbonates sont érodés et transportes sous forme particulaire et seulement 1.3% des silicates sont dissous. - La limitation de l'altération par la forte érosion physique. Le flanc sud de la haute chaîne est caractérisé par la plus faible altération et la plus forte érosion. Dans la chaîne, le couplage tectonique-altération montre un seuil au delà duquel la tectonique limite l'altération. L'altération du matériel himalayen a principalement lieu dans le bassin d'avant chaîne. - Le flux d'alcalinité liée a l'altération des silicates est de 2.7 10 11 mol/an ce qui représente 2.3% du flux global. Le bilan de l'altération implique une forte dominance de l'altération des carbonates et un caractère surtout sodique des silicates dissous. La consommation a long terme de C02 atmosphérique par l'altération dans le bassin du G-B est de 6.4 10 10 mol/an. - Une altération abiotique qui représente 14 Eq% de l'alcalinité. L'oxydation des sulfures des séries sédimentaires, principalement, dans la partie tibétaine de la chaîne produit 70% des sulfates dissous du G-B. Des émanations de C02 métamorphique, lie à la décarbonatation en profondeur, peuvent localement contribuer à l'altération. - Le découplage du budget du Sr dissout et celui de l'alcalinité liée aux silicates. L'altération des carbonates dans la partie aride de la chaîne est in congruente et libère une quantité importante de Sr faiblement radiogénique alors qu'une faible altération des terrains silicates anciens (> 2Ga) joue le rôle de Spike de 87Sr. Ainsi le G-B (6.5 108 mol/an de Sr et un 87Sr/86Sr- 0.730) a un impact primordial sur le budget océanique du Sr. - La prépondérance de l'enfouissement de C organique par rapport à l'altération vis à vis du cycle du carbone. L'érosion physique de l'orogenèse maintient des taux de sédimentation très élevés dans le bassin d'avant chaîne et le cône du Bengale. L'enfouissement et la préservation de carbone organique actuel est d'un ordre de grandeur plus élevée que la consommation de C02 atmosphérique via l'altération. Elle représente une faible quantité (5%) de l'enfouissement global, cependant suffisante pour engendrer une perturbation dans le cycle naturel du carbone.

L'altération chimique intense des péridotites de Nouvelle Calédonie génère des sols oxydiques essentiellement goethitiques et un relief de type karstique. Un rabattement brutal d'une nappe d'eau au fond d'une doline conduit à une induration rapide, massive et continue des oxydes de fer à l'interface de deux milieux oxydant et réducteur. La goethite précipite sous une forme réticulée, pseudomorphose des cellules végétales et peut être associée à de la lépidocrocite, de la sidérite et de la rhodochrosite. Ces indurations ne peuvent être considérées comme des cuirasses ou des ferricrètes au sens strict. (Résumé d'auteur)

In this paper it is proposed to quantify the importance of some physical parameters responsible for stone decay on monuments. The most common decay process is the crystallisation of salt near the surface of the rocks or inside their porous network. Therefore, the water balance in rocks submitted to these special saturation and position conditions has been studied specifically, using the general concepts of water transfer in unsaturated porous media, and using the capillary imbibition kinetics of different rocks. Different parameters have been taken into account for the calculation of the salt crystallisation position: on one hand, several external parameters such as relative humidity, air convection and the presence of solute in solution, and on the other hand, the intrinsic water transfer properties of the rocks. Their relative importance is discussed, considering the potential values that each parameter can reach in nature.

A global regression model is established between bicarbonate river fluxes and different environmental factors. A relationship calculated at a global scale between bicarbonate concentration and drainage allows the determination of a transfer function between the global flux of atmospberic CO2 consumed by rock weathering and the drainage intensity. The fluctuations of this global C02 flux are reconstituted during the last century using the continental runoff variations.

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)

Geochemical and mineralogical studies carried out on weathered material at three cupriferous deposits (Chapada Grande - Goias, Salobo 3A - Para and Santa Blandina - Sao Paulo) 100 to verify copper behavior in lateritic weathering profiles. It has been showed that copper retention or mobilization depends on the nature and variety of supergene neoformed phases and on the nature of weathering solutions. In Chapada Grande, goethite is the main secondary phase able to retain copper in weathered mantle, percolating solutions are copper depleted. So, as goethite is not abondant and as its capacity to retain copper is limited, copper is almost totally removed from the weathered material. In Salobo 3A, vermiculite, interestratified biotite-vermiculite and goethite are the main secondary phases that retain copper, and percolating solutions are little richer in copper. So, as phyllosilicates can retain much copper and as there is high contents on goethite through the profile, much of primary copper is fixed on weathered mantle. In Santa Blandina, peculiar conditions lead to very rich copper solutions and to copper silicate (chrysocola) formation at the bottom of the profile. So, copper is accumulated at the bottom of weathered mantle.