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La particularité des sols côtiers de Nouvelle-Calédonie est d’être essentiellement formés de latérites, exploitées depuis des décennies pour leurs teneurs en éléments traces métalliques (ETM) tels que le nickel, le chrome, le cobalt ou le manganèse. De par sa forte productivité en matière organique, la mangrove constitue un réservoir important de matière organique dissoute (MOD) qui peut être transportée vers l’Océan sur de grandes distances. Les observations menées soulèvent la question du rôle puit/source que joue la mangrove vis-à-vis de la dynamique des ETM à l’interface Terre-Mer à l’aval des massifs miniers de Nouvelle-Calédonie. Le projet TREMOR a ainsi, pour objectif d’apporter des éléments de réponses à cette question en s’appuyant sur les propriétés de fluorescence d’une fraction de la MOD, appelée Matière Organique Dissoute Fluorescente ou MODF, pour mieux comprendre l’origine, les sources et les puits de la MODF identifiée dans les rivières et dans le lagon de Nouvelle-Calédonie et évaluer l’influence de cette MODF sur la dissémination et la toxicité des ETM au sein du littoral à l’aval des sites miniers de Nouvelle-Calédonie.

Isotopic compositions are used to study the biogeochemical cycle of Zn, which is greatly impacted by anthropic activities. However, the interpretation of the measurements performed on natural or synthetic samples requires the knowledge of Zn isotope properties in equilibrium conditions (as reference) and the understanding of the mechanisms that are at the origin of the isotopic composition variations. In this work, we determined by performing quantum calculations, equilibrium Zn isotope fractionation constants in various phases including solids and liquids. We highlighted the crystal-chemical parameters controlling the isotopic properties : Zn interatomic force constant, Zn-first neighbours bond lengths and the electronic charge on atoms involved in the bonding. We carried out a methodological development in order to calculate isotopic properties in liquid phases from molecular dynamics trajectories at a reduced computational cost. We showed through the modelling of aqueous Zn that a reasonable description of van der Waals forces using a non-local exchange correlation functional is required to stabilise the experimentally observed hexaaquo zinc complex over other complexes at room temperature. This work provides a consistent database of equilibrium Zn isotope fractionation constants for experimental works.; La mesure de composition isotopique est utilisée dans l'étude du cycle biogéochimique du zinc ; cycle largement impacté par les activités anthropiques. Toutefois, l'interprétation de ces mesures réalisées sur des échantillons naturels ou synthétiques requiert la connaissance des propriétés isotopiques du zinc en condition d'équilibre (qui servent de référence) et la compréhension des mécanismes à l'origine des variations de composition isotopique. Dans ce travail, nous avons déterminé en réalisant des calculs quantiques, les constantes d'équilibre de fractionnement isotopique du zinc dans des phases solides et liquides. Nous avons mis en évidence les paramètres cristallochimiques qui contrôlent les propriétés isotopiques : la constante de force interatomique du zinc, les longueurs de liaisons Zn-premiers voisins et la charge électronique de ces premiers voisins. Nous avons réalisé un développement méthodologique permettant de calculer les propriétés isotopiques dans les phases liquides à partir de trajectoires de dynamique moléculaire à un coût réduit en temps de calcul. Nous avons montré à travers la modélisation du zinc aqueux, qu'une description raisonnable des forces de van der Waals par une fonctionnelle d'échange et de corrélation non locale est nécessaire à la stabilisation à conditions ambiantes du complexe hexaaquo zinc par rapport à d'autres complexes, réconciliant ainsi calculs et expériences. Ce travail met à disposition des études expérimentales une base de données cohérente de constantes d'équilibres isotopiques.

International audience; Lacustrine records hold considerable archives providing access to valuable information. However, to decode this information it is necessary to know how the lake in question and its catchment area function. In this study we propose to highlight the different factors that govern sedimentation by studying different core samples covering the Holocene taken from lake Ossa, Cameroon. To do this we chose to use organic matter as a tracer of changes in sedimentation by coupling two complementary methods: spectrophotometry for color measurement and Rock Eval pyrolysis for bulk geochemical analysis. Spectrophotometry is used classically to quantify color and obtain data very rapidly, at very high resolution and non-destructively. Rock Eval data give geochemical indication on organic matter composition. Thus, it was possible to highlight different types of organic matter at different stages of transformation, including different sources that can be responsible of this transformation (distal, proximal, autochtonous). The synthesis of this information led to the formulation of a scenario of how Lake Ossa was filled. Although this lake had already been used for paleoclimatic/environmental purposes, we show here that most of the forcing responsible for the sedimentation in the lake was due to the paleotopography and the rise in the lake level during the Holocene.

During the Middle–Late Miocene, tidal sedimentation was the rule in the Amazon foreland basin. The Amazon foreland basin became emerged in the Late Miocene–Pliocene times. In this study, trace element chemistry ( n = 104) and Nd–Sr isotopic systematic ( n = 30) are used to constrain the provenance of the Miocene tidal sediments and of the Late Miocene–Pliocene fluvial sediments. Neogene Amazonian foreland basin sediments are the result of a mixing between Andean andesitic volcanic rocks and cratonic shield rocks. The south Amazonian foreland basin (SAFB) sediments are the result of long-term weathering, recycling, and erosion of the Brazilian shield and Andean Paleozoic/Mesozoic rocks also Brazilian shield in provenance. Compared with north Amazonian foreland basin (NAFB) sediments, SAFB sediments are more felsic. NAFB sediments have 3 distinct signatures: i) an upper crust signature with trace element characteristics similar to PAAS and UCC and with ɛ Nd(0) values between − 8 and − 11.9; ii) an arc andesitic rock signature with high Cr / Th ratios, low Eu anomalies, low Th / Sc ratios and ɛ Nd(0) values between − 3 and − 5; and iii) a cratonic signature with high Eu anomalies and Zr / Sc ratios and with a very low ɛ Nd(0) value (− 15.5). Our data indicate that most of the basic detritus came from the Ecuadorian Andes. In the distal part of the NAFB, the successive uplifts of the Iquitos forebulge were responsible for Late Miocene to Pliocene divide between Andean and cratonic drainage systems. The modern “Amazon River” drainage network is no younger than the Pliocene.

The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, eNd150 Ma ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low eNd150 Ma values (from � 6.4 to � 9.5), and high initial 87 Sr/ 86 Sr and 206,207,208 Pb/ 204 Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high eNd150 Ma (>+10) and low initial Pb isotopic ratios. The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed

The origin of Ngaye River alluvial sediments and the evaluation of the weathering degree of their source rocks are assessed using trace and rare-earth element geochemistry in three bulk sediments and their different size fractions (2000–200 lm, 200–50 lm, 50–2 lm and

The Titicaca Lake is the most important water resource over the Andean plateau and the ecological equilibrium of this region is nowadays perturbed by recent changes in land use and management practices. The Katari watershed encompasses mining area, cities representing over 1.2 million habitants, and agricultural zones before ending in Cohana bay in the Titicaca Lake. Cohana Bay is known to be one of the most eutrophic bay of the Titicaca Lake. The objective of the study was to evaluate the impact of anthropic activities along the watershed on the river quality and on the bacterial diversity. Both mining activities and release of wastewater in river systems impacts greatly the surface water quality, with level of As exceeding limits for drinking water, and phosphate over the European guidelines for bad quality rivers. Antibiotic from the sulfonamide family was detected in the watershed in high concentrations downstream of the two main cities and bacterial resistance occurred in nearly all the sampled water points.

Suspended matter (SM) from the Nyong basin (Cameroon, Africa), a tropical watershed, was collected by tangential flow ultrafiltration to separate particulate (>0.45 μm) and colloidal ( 20 kDa) fractions. In this basin, two distinctive systems in a selected small catchment (Nsimi–Zoetele) of the Nyong river basin have been considered: (i) colourless water (groundwater and spring) with a low suspended load ( 10 mg/l) and contains higher amounts of SM (10–20 mg/l) than the colourless water. Freeze-dried samples of SM have been analysed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), electron paramagnetic resonance spectroscopy (EPR), and visible diffuse reflectance spectroscopy (DRS). Colourless water mainly contains mineral phases, such as poorly ordered kaolinite, plus quartz and goethite in the particulate fraction, and euhedral kaolinite plus amorphous iron oxyhydroxides in the colloidal fraction. In contrast, the SM in coloured water is mainly organic in nature. The mineral phases in the particulate fraction are similar to those from clear water, but with additional phytoliths and diatom frustules composed of biogenic opal. In the colloidal fraction, complexation of Fe3+ and Mn2+ with organic matter is evidenced by EPR, together with significant occurrence of Fe oxyhydroxides associated with organic matter. The sites of Al, Si, Fe, Mn in colloidal fractions derived from spectroscopic analyses are discussed with reference to chemical analyses performed by inductively coupled plasma mass spectrometry. Most of the observed solid phases or species correspond to those expected from published thermodynamic calculations for the same hydrosystem, except the colloidal iron oxyhydroxides in the coloured water. The presence of such iron phases is emphasised since they are expected to have large sorption capacities for numerous trace elements. The crystal chemistry of SM is used to discuss the origin of the mineral particles transported from the soil to the main rivers in terms of mechanical and chemical erosion processes.

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.

Le volcanisme récent de l'arc insulaire des Nouvelles Hébrides (Vanuatu) est influencé par sa collision avec la ride d'Entrecasteaux. La géochimie des éléments en trace et l'étude des compositions isotopiques des laves de cet arc permettent de nuancer les interprétations antérieures faisant état d'un gradient de composition, au nord et au sud de la zone de collision. Les données géochimiques montrent une variation nette, évoluant de compositions calco-alcalines à l'aplomb de la zone de collision à tholéiitiques de part et d'autre. Par contre, les signatures isotopiques varient de façon progressive, définissant des sources de moins en moins enrichies à des distances croissantes de la zone de collision, évoluant vers des sourcse "normales" de type MORB. Ce gradient isotopique suggère l'existence de deux contaminations, l'une magmatique, par une lithosphère ancienne au front de la zone de collision (variation du 206Pb/204Pb), et l'autre mantellique, par un manteau de type "enrichi" (type Dupal) débordant largement cette dernière. (Résumé d'auteur)

Trace metal elements (TME) can be real threats for living organisms. However, few studies dealt with TME in reservoirs in rural areas where farming practises could induce negative effects. Mae Thang res- ervoir (northern Thailand) has been studied for 3 years to understand the seasonal behaviour of dissolved TME: Fe, Mn, Cd, Al, Pb, V, Cr, Co, Ni, Cu, Zn, Mo, U and As and associated physicochemical parameters. In situ measure- ments of these parameters were done during the dry and the wet seasons as well as water samples along the water column for further analyses and TME determination by inductively coupled plasma-mass spectrometry (ICP- MS). In the dry season, the water column was character- ized by a strong stratification and anoxic conditions in the hypolimnion. High rain and water input from the water- shed during the wet season induced mixing of the water. All TME, except Ni, Co and Cr were less concentrated in the wet season indicating a dilution effect by water input. There was thus no important dissolved pollution coming from the watershed. The anoxic conditions in the dry season enhanced the reduction of Fe and Mn and the desorption processes. Depth, and thus oxic-anoxic con- ditions were the main drivers of TME in the dry season, while in the wet season, dissolution processes from par- ent rocks of watershed were favoured. The average con- centrations of TME in the reservoir were in the limit of the international and Thai standards. Only localized val- ues in the bottom of the reservoir for Fe and Mn were higher than the limits.

International audience; Trace-element and REE data of glass and pumices of acidic tuffs and related fall deposits erupted in southern Peru and northern Bolivia between 20 and 0.36 Ma display typical characteristics of subduction related continental arc magmatism of the CVZ with strong LILE/HFSE enrichment and non enrichment of HREE and Y. Geochemical variations of these tuffs are linked to subduction processes and controlled by changes in tectonic regimes which occured with each Quechua tectonic pulse and affected the astenospheric wedge and both the dowgoing and the overriding lithospheres. During Neogene — Pleistocene times, tuffs erupted in northern Bolivia are typically enriched in Zr, Hf, Th, Ba, LREEs and other incompatible elements and incompatible /Yb ratios are much higher relative to those erupted from southern Peru, at a given SiO2 content (65–67 wt. for dacites, 72–73 wt.% for rhyolites). ZrHf ratios increase eastward from 27 to 30 and CeJbN ratios from 11 to 19 reflecting the variation of degree of wedge contribution. Fractionation of the LREE over the HREE and fractionation of incompatible elements may be due to their heterogeneous distribution in the magma source. More highly fractionated REE patterns of Bolivian tuffs than Peruvian tuffs are attributed to variable amounts of contamination of magmas by lower crust. After the Quechua compressional event at ~7 Ma, SrY ratios of tuffs of the same age, erupted at 150–250 km or 250–400 km from the Peru-Chile trench, increase from southern Peru to northern Bolivia. These differences may be attributed to the subduction of a swarm oceanic lithosphere under the Bolivian Alti-plano, leading to partial melting of the sudbucted lithosphere. New FT dating of obsidian fragments of the sillar of Arequipa at 2.42 ± 0.11 Ma. This tuff dates the last Quechua compressional upper Pliocene phase (~2.5 Ma) and confirms that the sillar is not contemporaneous with the Toba 76 tuff or the Perez ignimbrite of northern Bolivia. Geochemical characteristics of tuffs erupted before and after this last compressional phase remained the same and provide evidence that the upper Miocene (~7 Ma) compressional deformations played the most important role on the variability of the geochemical characteristics of the southern Peruvian and northern Bolivian tuffs.

La zone d'étude correspond à la nappe libre de l'aquifère quaternaire du BLT au sud-est du Niger (le Kadzell) et au nord-est du Nigeria (Etat du Bornou) où se démarquent deux dépressions piézométriques majeures. L'objectif de ce travail est de contribuer, par une double approche géochimique (ions majeurs et traces, 18O, 2H) et hydrodynamique, à l'identification des processus d'acquisition de la minéralisation dans les deux dépressions, en réponse à l'hydrodynamique actuelle et aux pulsations humides/arides qui se sont succédées au BLT depuis l'Holocène moyen. La recharge actuelle est marquée par une forte variabilité spatio-temporelle. Elle est active et discontinue en bordures de la Komadougou Yobé (rivière intermittente) et du lac Tchad. L'étude de l'évolution des éléments majeurs et traces en fonction des teneurs en 18O suivant les directions des flux hydrodynamiques a permis de déceler un processus de mélange entre des eaux «évaporées» à faibles teneurs en chlorures à proximité des zones de recharge actuelle et une eau chlorurée à faibles teneurs isotopiques au centre de chaque dépression piézométrique. A ce processus de mélange, s'ajoutent l'altération des aluminosilicates et un échange de base avec la matrice argileuse de l'aquifère. L'eau aux centres des dépressions, isotopiquement appauvrie et riche en chlorure, est l'empreinte d'une paléo-recharge massive remontant au moins au dernier maximum humide holocène et ayant entraîné et piégé les sels accumulés pendant les phases arides antérieures. Un Mégalac Tchad, dont la présence est avérée par de nombreuses études est le vecteur le plus probable de transfert de ces sels au sein de l'aquifère. La modélisation hydrodynamique été calée et validée à partir des mesures piézométriques disponibles depuis 1960. Le modèle montre la nécessité d'une évapotranspiration dominante au centre des dépressions piézométriques et de faibles perméabilités pour reproduire la nappe en creux. Il confirme les résultats isotopiques avec une recharge uniquement latérale avec des apports réduits depuis la rivière et le lac Tchad. Le régime transitoire depuis l'Holocène moyen (6300 ans B.P.) montre l'évolution de la nappe libre après la disparition du Mégalac comme il apporte des premières caractéristiques au mélange. (Résumé d'auteur)

International audience; The Fada N?Gourma area in Burkina Faso is underlain by Paleoproterozoic rocks that make the northeastern West-African Craton. This region is composed of NE-trending volcano-sedimentary belts and foliated tonalites, affected by several shear zones. A generation of younger, ?2100 Ma-old, non-foliated biotite-bearing granites intrudes the former rock units. We have investigated the younger granite pluton of Kouare that was previously considered as forming a single body with the pluton of Satenga to the west, a pluton which likely belongs to the ?20 Ma more recent Tenkodogo-Yamba batholith. Magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data. The granite encloses angular enclaves of the host tonalites. Magmatic microstructures are preserved inside the pluton and solid-state, high-temperature deformation features are ubiquitous at its periphery. The presence of steeply plunging lineations in the pluton of Kouare and its adjacent host-rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact. Around Kouare, the foliation in the host tonalites conforms with a map-scale, Z-shaped fold in between NNE-trending shear zones, implying a bulk clockwise rotation of the material contained in-between the shear zones, including the emplacing pluton. Regionally, the Fada N?Gourma area is concluded to result from NW-shortening associated with transcurrent shearing and vertical transfer of granitic magmas. This study concludes that the ?2200 Myears old juvenile crust of Burkina Faso was brittle before the intrusion of the biotite-granites, became softened close to them and that gravity-driven and regional scale wrench tectonics were active together.

The geochemical development of laterites, a very common superficial formation in the tropics, is still a matter of debate. To determine the main steps of their formation, and to interpret lateral geochemical variations often observed within laterites, we studied the Kaya ferricrete in northern Burkina Faso by analysing two profiles in contrasting topographic positions. We determined the mineralogy and the composition in major and trace elements of whole rocks and of < 0.2 mu m granulometric fractions. The nature and proportion of relictual primary minerals and of secondary clays and Fe-oxyhydroxides control the distribution of major and soluble trace elements. The distribution patterns of Fe, transition metals, lanthanides, U and Th in the two profiles require (i) an initial accumulation in the top ferruginous horizon during its formation and (ii) a secondary redistribution downwards in the underlying horizons. Lanthanides, Ni and Co were remobilized to a much greater extent than Cu and Sc, whereas Fe, V, Cr and Th accumulated in the ferruginous horizon. The uphill better drained profile showed more intense redistribution than the downhill profile. Uranium in particular is poorly redistributed in the downhill profile, whereas it was redistributed like the lanthanides in the uphill profile. Remobilizations are also more intensely recorded in the fine fractions than in the whole rocks. These results allow us to propose a scenario for the formation of the Kaya laterite that accounts for both vertical and lateral chemical distributions. They also highlight the potential of multimethod geochemical studies to uncover the sequence of evolution of weathering profiles.

Resume Quatre unites ignimbritiques sont reconnues dans la region d'Arequipa : (1) celle de « rio Chili » a 13,33 Ma ; (2) l'ignimbrite « La Joya », la plus etendue, qui a 4,9 Ma ; (3) l'ignimbrite « aeroport d'Arequipa », qui s'est mise en place vers 1,65 Ma a partir de la zone ou s'est edifie le volcan Chachani ; (4) enfin, les « Tufs de Yura », vers 1,02 Ma, qui affleurent a l'ouest du Chachani. Il s'agit de rhyolites calco-alcalines a plagioclase, biotite, quartz, sanidine et opaques. La presence d'amphibole caracterise les ignimbrites de rio Chili et La Joya. Les elements en traces indiquent une contamination crustale plus forte pour les ignimbrites anciennes. Pour citer cet article : P. Paquereau et al., C. R. Geoscience 337 (2005).

National audience

Geochemical and mineralogical data from ancient sedimentary strata can be reliable indicators of the provenance of sediments. The heavy mineral assemblages and the major and trace element contents of sedimentary rocks from the Neogene continental successions of the Central Depression in Chile and the Mauri and Corque Basins in the Altiplano of Bolivia reflect volcanic source rocks with different degrees of magmatic differentiation and alkalinity. These results indicate that the source rocks of the Central Depression were less differentiated (andesite to rhyodacite) than those of the Altiplano basins (rhyodacite to rhyolite). The low concentration (cations per formula unit) of total Al ( 0.15 pfu) and [Ca+Na] (f0.92 pfu), the variable Ti content (0.01–0.04 pfu) of the clinopyroxenes and the high [Nb/Y] (>0.7) in whole-rock analyses of sandstones from the Mauri Basin indicate erosion of alkaline rock sources. Locally, the low concentration of total Al (

This thesis relates to the study of parameters which influence the dynamic of nickel and its radionuclide 63 Ni in soils. At the Saille time, the study of the chemical and biological characteriza- tion of these elements availability in the soils is done. The available pool of Ni and 63 Ni (chemical estimation) was measured by chemical and isotopic methods and soil incubations; by evaluating the interest and limits of each roethod. The factors which influence the mobility of metal in soils were thus determined. With through this study, we showed that the bio- and phyto-availability of Ni depend on the soil mineralogy, on the soil weathering, on the type of metal associations with the mineraIs, and on the quantitative presence of these mineraIs. The transfer of Ni and 63 N i from the soil to the plant depends on the metal availability in the soil and on the roots behaviour to the soil offe1, in relation to the strategies of hyperaccumulation, accumulation or exclusion of N z. Plants are able to modify the metal availability in soils by influencing the sorption processes of Ni from the solid phase to the soil solution. They are also able to change the speciation of Ni in solution however the majority form of Ni in the soil solution was N i2+. Hyperaccumulators regulate Ni and 63 N i concentration in soil solution and their migration to the ground water. If the physico-chemical properties of soils, the pollutant availability and the choice of the hyperaccumulators are taken into account, the se plants are able to decrease and control durably the ecotoxicological effects of trace elements and radionuclides contamination in soils., Cette thèse porte sur l'étude des paramètres qui influencent la dynamique du nickel et de son radionucléide 63 Ni dans les sols. L'objectif principal de ce travail est la caractérisation à la fois chimique et biologique de la disponibilité de ces éléments dans les sols. Le pool disponible du Ni et du 63 Ni (estimation chimique) a été mesuré par des méthodes chimiques, isotopiques et des incubations (méthode des sachets); en évaluant l'intérêt et les limites de chaque méthode. Les facteurs qui influencent la mobilité du métal dans les sols ont été ainsi déterminés. A travers cette étude, nous avons démontré que la biodisponibilité et la phytodisponibilité du Ni dépendent de la minéralogie des sols, du niveau d'altération des sols, du type des associations du métal avec les minéraux et la présence quantitative de ces minéraux. Le transfert du Ni, ou du 63 N i du sol jusqu'à la plante dépend de la disponibilité du métal dans le sol et de la diversité de comportement des racines face à l'offre du sol, en relation avec les stratégies d'hyperaccumulation, d'accumulation ou d'exclusion du Ni. Les plantes sont capables d'influencer la disponibilité du métal dans les sols en intervenant aux processus de désorption du Ni par la phase solide et sa mise en solution. Elles sont aussi capables de modifier la spéciation du Ni en solution cependant la forme majoritaire du Ni dans la solution du sol était le Ni2+. Une culture d'hyperaccumulateurs régulent la concentration de Ni et de 63 Ni dans la solution du sol ainsi que la migration vers la nappe phréatique. En tenant compte des propriétés physico-chimiques des sols, la disponibilité du polluant et le choix des hyperaccumulateurs, ces plantes sont capables de diminuer et de contrôler durablement les effets écotoxicologiques d'une contamination par les éléments en traces ou/et les radionucléides.

International audience; The chemical composition of the Amazon River results from the mixing of two water types: black water and white water. On-site fractionation by sequential tangential ultrafiltration (STUF) was used to differentiate transported organic carbon and to determine the distribution and association of major and trace elements with different size fraction of the organic carbon (OC). Several sampling campaigns (1994-1996) allow a monthly quantification of particulate (OCP, Me-P), colloidal (OCC, Me-C) and dissolved (OCD, Me-D) organic carbon and metal ions inputs. In white rivers the OC is mainly concentrated in the low molecular weight fraction (OCD 5 kDa). For Mg, Ca and K, 50% of the total amount of each element is found in fraction MeD while 15% and 35% are found in fractions Me-C and Me-P, respectively. Al and Fe are in the particulate fraction at 99% of the total metal concentration for all river samples. This work emphasizes the coagulation processes and the sink for elements in the mixing zone. These physicochemical transformations of the organic matter vary seasonally. The changes happen during the transition periods: before high-level waters and before low-level waters. By way of flux measurement, a seasonal carbon loss was observed. The estimated annual organic carbon flux of the Amazon at Obidos is 28 x 10(6) t. At the same time, an average of 9 x 10(6) t of organic carbon per year is retained in the reach between Manaus and Obidos, probably via coagulation processes. Copyright (C) 2003 John Wiley Sons, Ltd.

We studied the changes in major elements and organic carbon concentrations during the initial stage of the mixing of the black (Rio Negro) and the White (Rio Solimoes) waters in the Amazon River basin to understand the geochemical processes that could control the redistribution between particulate and dissolved fractions. Water samples were collected at six stations including the Rio Negro and the Rio Solimoes and four stations downstream from the confluence. The relative contributions of the two tributaries were determined using a triple tracer approach (delta(18)O, deltaD, Cl-). Particulate (>0.2 mum) and dissolved (

The trace metal (V, Cr, Mn, Co, Cu, Zn, As, Rb, Sr, Mo, Cd, Sb, Cs, Ba, U) concentrations and temporal variabilities of the Amazon river and its main tributaries are studied on time series basis in the major tributaries of Amazon river (Negro, Madeira and Solimoes Rivers) and at Obidos station on the Amazon mainsteem which represents 90% of the total discharge of Amazon river to the Ocean. Variations of river chemistry may reflect variations of the sources. The Shield rivers (as the Rio Negro) have typically depleted concentrations in As, Sr, Ba, Cu, and V as compared with Andean rivers. Elements such Mn and As are mainly transported by the flood flows. These elements are known to be concentrated in lateric (ferricrete) soils which represent 80% in the Amazon basin, suggesting that these elements are washed away in solution during the high discharge. Moreover, these elements can be stored in the surrounding floodplain areas (varzea) where deposition/resuspension cycles as well as the exchange rate between floodplain and mainstream channel may control at least partially the temporal variation of redox element concentrations such Mn and As. Implication on these results on the trace element flux from Amazon River to the Atlantic Ocean is discussed.