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Using new mass spectrometry techniques developed for the analysis of Ra isotopes, we present 238U230Th226Ra disequilibria data from a variety of volcanic settings, and we compare them with previously published data. Two correlations are observed with alkali volcanic data, one between [230Th238U] and [230Th226Ra] and another between the intensity of the disequilibria and the buoyancy flux of the underlying plume. These two correlations prove that partial melting is the major cause of UThRa fractionations in this volcanic context. The 238U230Th226Ra disequilibria then place new constraints on some parameters of the classical melting models (batch melting and dynamic melting). We show, nevertheless, that these estimates remain very speculative. The comparison of 238U230Th226Ra disequilibria in alkali volcanics, carbonatites and subduction zones shows a clear parallel between the disequilibria value and the type of volcanic context. Such a parallel reflects the diversity of the conditions of magma generation, and shows that the 238U230Th226Ra disequilibria systematics are very dependent on the chemical composition of liquids produced during magmatic processes. A systematic difference is observed between disequilibria in MORB and in alkali volcanics, which could indicate that the melting processes in these two volcanic contexts are very different.

In this study, we tested experimentally the influence of plant and bacterial activities on the calcium (Ca) isotope distribution between soil solutions and plant organs. Abiotic apatite weathering experiments were performed under two different pH conditions using mineral and organic acids. Biotic experiments were performed using either apatite or Ca-enriched biotite substrates in the presence of Scots pines, inoculated or not with the rhizosphere bacterial strain Bulkholderia glathei PML1(12), or the B. glathei PML1(12) alone. For each experiment, the percolate was collected every week and analyzed for Ca concentrations and Ca isotopic ratios. No Ca isotopic fractionation was observed for the different abiotic experimental settings. This indicates that no Ca isotopic fractionation occurs during apatite dissolution, whatever the nature of the acid (mineral or organic). The main result of the biotic experiments is the 0.22 ‰ (44)Ca enrichment recorded for a solution in contact with Scots pines grown on the bacteria-free apatite substrate. In contrast, the presence of bacteria did not cause Ca isotopic fractionation of the solution collected after 14 weeks of the experiments. These preliminary results suggest that bacteria influence the Ca isotopic signatures by dissolving Ca from apatite more efficiently. Therefore, Ca isotopes might be suitable for detecting bacteria-mediated processes in soils.

New fossil material was recovered at Tam Hang during the second fieldwork season conducted in 2007, by a Lao-French-American team. The Tam Hang site is located in northern Laos, in the Hua Pan province, about 260 km from Vientiane. The breccias of the Tam Hang South locality produced 404 new fossil specimens which complete the previous material recovered in 2003. The fossiliferous deposits are described geologically. In terms of palaeontology, the mammalian fauna is essentially composed of isolated teeth of middle- to large-sized mammals (Artiodactyla, Perissodactyla, Proboscidea, Carnivora, Rodentia, Primates). The overall fauna is compared to the six well-documented Middle to Late Pleistocene faunas from the Indochinese province (Yenchingkuo, Phnom Loang, Thum Wiman Nakin, Lang Trang, Duoi U’Oi, Ma U’Oi). On the basis of the faunal composition and evolutionary stages of the species, a biochronological framework at the transition between the Middle Pleistocene and the Late Pleistocene is proposed.

The geochemical study of springs and lake waters from the Ol’khon Region, Siberia, confirms the strong chemical variability of these water samples, more particularly regarding their salinity. Such variability does not result from a simple mixing process between surface freshwaters and deeper saline waters. The variability, observed at the scale of a few square kilometers, would preferentially result from a secondary concentration processes (evaporation and/or cryogenesis) of lake waters of variable intensity from one lake to another. The U-disequilibria data suggest that the duration of the secondary process is certainly an important parameter to be considered to account for the variable salinity of these lakes. The data indicate that the lakes, however modest in size, could be as old as several ky, confirming therefore that the lake sedimentary deposits could represent relevant paleoenvironmental recorders for the last thousands years.

In the Critical Zone where rocks and life interact, bedrock equilibrates to Earth surface conditions, transforming to regolith. The factors that control the rates and mechanisms of formation of regolith, defined here as material that can be augered, are still not fully understood. To quantify regolith formation rates on shale lithology, we measured uranium-series (U-series) isotopes ( 238 U, 234 U, and 230 Th) in three weathering profiles along a planar hillslope at the Susquehanna/Shale Hills Observatory (SSHO) in central Pennsylvania. All regolith samples show significant U-series disequilibrium: ( 234 U/ 238 U) and ( 230 Th/ 238 U) activity ratios range from 0.934 to 1.072 and from 0.903 to 1.096, respectively. These values display depth trends that are consistent with fractionation of U-series isotopes during chemical weathering and element transport, i.e., the relative mobility decreases in the order 234 U > 238 U > 230 Th. The activity ratios observed in the regolith samples are explained by i) loss of U-series isotopes during water–rock interactions and ii) re-deposition of U-series isotopes downslope. Loss of U and Th initiates in the meter-thick zone of “bedrock” that cannot be augered but that nonetheless consists of up to 40% clay/silt/sand inferred to have lost K, Mg, Al, and Fe. Apparent equivalent regolith production rates calculated with these isotopes for these profiles decrease exponentially from 45 m/Myr to 17 m/Myr, with increasing regolith thickness from the ridge top to the valley floor. With increasing distance from the ridge top toward the valley, apparent equivalent regolith residence times increase from 7 kyr to 40 kyr. Given that the SSHO experienced peri-glacial climate ∼ 15 kyr ago and has a catchment-wide averaged erosion rate of ∼ 15 m/Myr as inferred from cosmogenic 10 Be, we conclude that the hillslope retains regolith formed before the peri-glacial period and is not at geomorphologic steady state. Both chemical weathering reactions of clay minerals and translocation of fine particles/colloids are shown to contribute to mass loss of U and Th from the regolith, consistent with major element data at SSHO. This research documents a case study where U-series isotopes are used to constrain the time scales of chemical weathering and regolith production rates. Regolith production rates at the SSHO should be useful as a reference value for future work at other weathering localities.

International audience; In the southern Upper Rhine Valley, groundwater has undergone intensive saline pollution caused by the infiltration of mining brines, a consequence of potash extraction carried out during the 20th century. Major and trace elements along with Sr and U isotopic ratios show that groundwater geochemical characteristics along the saline plumes cannot reflect conservative mixing between saline waters resulting from the dissolution of waste heaps and one or more unpolluted end-members. The results imply the occurrence of interactions between host rocks and polluted waters, and they suggest that cationic exchange mechanisms are the primary controlling process. A coupled hydrogeochemical model has been developed with the numerical code KIRMAT, which demonstrates that cationic exchange between alkalis from polluted waters and alkaline-earth elements from montmorillonite present in the host rock of the aquifer is the primary process controlling the geochemical evolution of the groundwater. The model requires only a small amount of montmorillonite (between 0.75% and 2.25%), which is in agreement with the observed mineralogical composition of the aquifer. The model also proves that a small contribution of calcite precipitation/dissolution takes places whereas other secondary mineral precipitation or host rock mineral dissolution do not play a significant role in the geochemical signature of the studied groundwater samples. Application of the model demonstrates that it is necessary to consider the pollution history to explain the important Cl, Na and Ca concentration modifications in groundwater samples taken over 2 km downstream of waste heaps. Additionally, the model shows that the rapidity of the cationic exchange reactions insures a reversibility of the cation fixation on clays in the aquifer.

International audience; 238U–234U–230Th radioactive disequilibria were analyzed in suspended sediments (collected at different depths) from the Ganges River and one of its main tributaries: the Narayani–Gandak River. Results associated with bedload sediment data suggest that uranium-series (U-series) disequilibria in river sediments of the Ganges basin vary with grain size and sampling location. The range of observed U-series disequilibria is explained by a mixing model between a coarse-grained sediment endmember, represented by bedload and bank sediments, and a fine-grained end-member that both originate from Himalaya but undergo different transfer histories within the plain. The coarse-grained sediment end-member transits slowly (i.e. >several 100's ky) in the plain whereas the fine-grained sediment end-member is transferred much faster (

Over the last decade it has become apparent that Li isotopes may be a good proxy to trace silicate weathering. However, the exact mechanisms which drive the behaviour of Li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy. In this study, we analysed the Li concentrations and isotopic compositions in the various surface reservoirs (soils, rocks, waters and plants) of a small forested granitic catchment located in the Vosges Mountains (Strengbach catchment, France, OHGE http://ohge.u-strasbg.fr ). Li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin, atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals (which together represent a minimum of 70% of dissolved Li). Li isotope ratios in soil pore waters show large depth dependent variations. Average dissolved δ 7 Li decreases from −1.1‰ to −14.4‰ between 0 and −30 cm, but is +30.7‰ at −60 cm. This range of Li isotopic compositions is very large and it encompasses almost the entire range of terrestrial Li isotope compositions that have been previously reported. We interpret these variations to result from both the dissolution and precipitation of secondary phases. Large isotopic variations were also measured in the springs and stream waters, with δ 7 Li varying from +5.3‰ to +19.6‰. δ 7 Li increases from the top to the bottom of the basin and also covaries with discharge at the outlet. These variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks. We suggest that the dissolved δ 7 Li increases with increasing residence time of waters through the rocks, and so with increasing time of interaction between waters and solids. A dissolution precipitation model was used to fit the dissolved Li isotopic compositions. It was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of −5‰ to −14‰ (best fit −10.8‰), in agreement with Li incorporation into clay. In soil solutions, it was found that isotopic fractionation during secondary precipitation is larger (at least −23‰), suggesting a major role for different secondary phases, such as iron oxides that maybe incorporate Li with a higher isotope fractionation.

Natural mass-dependent fractionation of calcium isotopes is a promising tool for investigating Ca pathways and cycling in geological and biological materials. But since natural isotope fractionation of Ca appears to be extremely limited (∼1.25‰/amu), excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a new Ca purification procedure that consists of a high selectivity automated ionic chromatography separation protocol, which is suitable for Ca isotope measurements by mass spectrometry and applicable to multiple natural matrixes (waters, mineral and organic samples). The analytical progress in this automated technique are multiple: (1) saving time with a minimum of handling, (2) unique operating protocol whatever the nature of the sample, (3) complete separation of Ca from K, Mg and Sr, avoiding isobaric interferences which are critical during TIMS analysis, and (4) Ca separation by peak recognition optimising the full recovery of Ca even if its retention time is shifted from one sample to another. The two latter advantages ensure a Ca recovery yield close to 100%, leading to the absence of any fractionation of Ca isotopes during the chemical clean-up. Thus, this chemical separation will be of special interest for applications not compatible with the use of the double spike technique such as MC-ICP-MS and 40Ca excesses measurements. Additionally this procedure leads to a twofold improvement of the long-term repeatability of the Ca isotopes determination by TIMS (±0.11 δ44/40Ca, 2SD) as compared with a classical resin chemistry protocol and is similar to the best repeatability published so far (±0.10 δ44/40Ca, 2SD).

International audience; Leaf litter contains not only important quantities of elements derived from decomposition of organic matter, but also atmospheric compounds. This is illustrated by Sr and Nd isotope data from soils of the Strengbach catchment, which show that soil litter contains more than 50% of atmospheric Sr and Nd. Downward migrating soil water becomes strongly acidic during flow through litter and humus layers (pH 3.8–4 at 5–10 cm depth), causing not only intense mobilization of elements in the underlying acid brown and podzolic soils at depths between 10 and 50 cm, but also the downward transfer of some litter derived compounds. The Sr and Nd isotope data allow to quantify that soil water from 10 to 70 cm depth contains 45 to 65% of litter derived Nd and Sr. Sequential leaching experiments have been performed on soil samples in order to recover adsorbed trace elements or those fixed in acid soluble mineral phases such as Fe–Mn oxyhydroxides and phosphate minerals. The isotope data of these leachates of the uppermost soil horizons point similarly to the presence of important quantities of litter derived Sr and Nd in the uppermost 30 cm of the soils. With the exception of the litter layer, the soil profiles are substantially depleted in Ca and P down to 50 cm depth, which can be related to low apatite and plagioclase contents. These minerals have probably been preferentially weathered and Ca and P have been removed by percolating soil water and plant uptake. Leaching of primary granite derived apatite controls rare earth element (REE) fractionation only below 50 cm depth. Rhabdophane (REE-PO4) has been observed as secondary replacement mineral of apatite. This replacement is reflected by Ca/P ratios that are low at the surface and which increase regularly with depth, implying that some of the apatite derived P has been integrated into newly formed rhabdophane and some of the P and Ca have been exported, either by soil solutions or plant uptake. During downward transport, litter derived elements are adsorbed on soil particles and/or coprecipitate with authigenic minerals such as Fe–Mn oxyhydroxide or rhabdophane. Alternatively, litter derived elements may remain in solution or, like Ca, be absorbed by vegetation. The soil solutions from the uppermost 30 cm of the soil profiles and the corresponding soil leachates show very similar REE distribution patterns and are, compared to the waters of the Strengbach stream, which drains the forested catchment, strongly enriched in light rare earth elements (LREE). However, soil solutions from 60 to 70 cm depth are, compared to litter, strongly LREE depleted and show stream water-like REE distribution patterns. It is suggested that this evolution of the REE in the soil solutions is due to the following processes taking place above 50 cm depth: (1) precipitation of LREE-rich phosphate minerals like rhabdophane, (2) diminution of the formation of organic dissolved or colloidal phases in association with Fe–Mn and Al oxyhydroxides and (3) preferential LREE uptake by vegetation. Below 50 cm the REE distribution patterns are further modified by dissolution–precipitation reactions and adsorption. The combination of all these fractionation processes finally leads to the REE patterns of the Strengbach stream at the catchment outlet.

International audience; Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi- cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Publisher Summary This chapter summarizes the recent applications of U-series nuclides to study weathering and river transport. The chapter focuses on the determination of time-scales of continental weathering and erosion, based on U-series analysis in soils, rivers, as well as in lakes. Analysis of U-series nuclides in soils and alterites can provide information on the behavior of radionuclides during weathering in the natural environment and also on the nature and time-scale of weathering processes. The chapter also indicates that determination of the time-scales of weathering processes from U-series data in soils requires a full understanding of the processes that control U–Th–Ra fractionation in regoliths. In parallel with the development of U-series disequilibrium studies in soils and weathering profiles U-series disequilibria in rivers have been used to establish weathering mass balances and/or to determine weathering time-scales at the scale of a watershed. Studies of U- and Th-series nuclides in lake waters provide information on their geochemical behavior and mixing characteristics of lakes. Some of these studies complement the investigations in river waters. Lake sediments, on the other hand, hold records of recent environmental changes. Measurements of U–Th nuclides in these sediments and associated mineral precipitates can help chronologically decipher the records in them. It has been shown that U-series disequilibria in lake water and sediment could also provide a wealth of information about the physico-chemical processes operating in lakes, such as the source and fate of elements in sediment and water, the residence time of particles or radionuclides in lakes, mixing or turnover time-scales of lakes. The chapter briefly presents some of the results obtained in these areas of research. These findings are certainly one of the major advances in U- and Th-series nuclide applications in the earth sciences and have laid the foundation for further developments in this field.

New data on the geochemistry and isotopic composition of chloride brines of the Siberian Platform are presented. The distribution of stable isotopes (2H, 18O, and 37Cl) in brines of the Tunguska, Angara-Lena, western part of the Yakutian and Olenek artesian basins and 87Sr/86Sr in brines of the western part of the Olenek artesian basin was studied in the context of the problem of genesis of highly mineralized groundwaters. Results of the study and comparative analysis of the geochemical and isotopic peculiarities of the Siberian Platform brines conform to the theory of brine formation through the interaction of connate waters with enclosing rocks.

In order to define time-scales of erosion and sedimentary transfer in the Himalaya, 238 U– 234 U– 230 Th disequilibria have been analysed in river bank sediments and in bedloads collected along the Kali Gandaki river, one of the main Nepalese rivers, and in the Ghaghara and Gandak rivers, two major plain tributaries of the Ganges. The Th activity ratios and U/Th ratios in river sediments of the two Ganges tributaries constantly decrease from upstream to downstream. This is related to the maturation of sediments by weathering during their transfer to the plain. The U-series data allow to calculate a transfer time for the sediments in the alluvial Gangetic plain from the chain front to the confluence with the Ganges of about 100 kyr for both rivers. The Kali Gandaki river sediment data highlight a decrease of both the Th isotopic and U/Th ratios which is explained by a mixing between two sources with similar U/Th ratios but having suffered a different U–Th fractionation history. Interpretation of the U-series data in the frame of this scenario gives long time-scales of weathering of several 100's kyr for the Himalayan terranes. The results imply that Himalayan bedrocks are submitted to a long in situ stage of weathering before their erosion and transfer into the rivers. In addition, occurrence of similar U–Sr signatures in dissolved (i.e.

International audience; Previous studies on waters of a streamlet in the Vosges Mountains (Eastern France) have shown that strontium and rare earth elements (REE) mainly originate from preferential dissolution of apatite during weathering. However, stream water REE patterns normalized to apatite are still depleted in the light REE (LREE, La–Sm) pointing to the presence of an additional LREE depleting process. Vegetation samples are strongly enriched in LREE compared to stream water and their Sr and Nd isotopic compositions are comparable with those of apatite and stream water. Thus, the preferential LREE uptake by vegetation might lead to an additional LREE depletion of surface runoff in the forested catchment. Mass balance calculations indicate, that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling LREE depletion in river water. This is underlined by the observation that rivers from arctic and boreal regions with sparse vegetation appear to be less depleted in LREE than rivers from tropical environments or boreal environments with a dense vegetation cover.

Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia, from the weathering of a mafic and ultramafic cumulate. The platinum/palladium (Pt/Pd) ratio evolution from the bottom to the top of the weathering profile indicates a chemical mobilization of Pd in supergene environment. The examination by scanning electron microscopy of the residual PGM collected in the weathering profile and their chemical characterization by electronic microprobe show a preservation of the structure and chemical signature (in the core of the grains) of the fresh PGM. This clearly demonstrates that the PGM studied here are residual and are affected by dissolution process. (c) 2005 Elsevier B.V. All rights reserved.

Previous studies on waters of a streamlet in the Vosges mountains (eastern France) have shown that Sr and rare earth elements (REE) principally originate from apatite dissolution during weathering. However, stream water REE patterns normalized to apatite are still depleted in light REE (LREE, La–Sm) pointing to the presence of an additional LREE depleting process. Speciation calculations indicate that complexation cannot explain this additional LREE depletion. In contrast, vegetation samples are strongly enriched in LREE compared to water and their Sr and Nd isotopic compositions are comparable with those of apatite and waters. Thus, the preferential LREE uptake by the plants at the root–water–soil (apatite) interface might lead to an additional LREE depletion of the waters in the forested catchment. Mass balance calculations indicate that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling the LREE depletion in river waters.

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.

This study uses 238 U– 234 U– 230 Th disequilibria in river sediments in order to constrain the transfer times of sediments in alluvial plains of rivers from Himalaya and the Gangetic plain. From the observed distributions we infer sediment transfer times of about 100 ka in the Gangetic plain for rivers taking their source in the Himalayan chain, and longer transfer of about 160–250 ka for foothill-fed rivers. This difference is probably related to the difference in the sediment transport dynamics of these two types of rivers.

A freshwater reservoir (area: 122 km2, volume: 2850 × 106 m3, maximum depth: 72 m) located at the outlet of a tropical Andean watershed (Rio Boconó, 1620 km2, 08°57′–09°31′N, 70°02′–70°34′W, NW Venezuela) was studied combining: (1) high resolution monitoring of the major environmental parameters using a CTD probe; (2) dissolved (major and trace) element concentration measurements; (3) water (δ[18]O, δ[2]H) isotope measurements and; (4) organic carbon (C) concentration – δ[13]C measurements on suspended matter and on sediments. Composed of two sub-reservoirs with contrasted water inputs, the Peña Larga dam provides the opportunity to better constrain C budgets and source – composition of suspended organic matter within two parts of a unique freshwater system with contrasted redox conditions. The redox status is reflected by drastic differences between the two parts of the reservoir, pictured by water residence time, water column ventilation, dissolved trace element concentration and suspended organic matter composition. Seasonal renewal of water near the main inlet supports higher surface productivity, oxic water column conditions, homogenous water chemical composition and, below the thermocline, the transport of suspended organic matter mainly derived from soil erosion on the watershed. Calculated sediment and organic C storages amount to 5.6 × 106 t a−1 and 21 ± 5 × 103 t C a−1, respectively. Despite a markedly lower suspended sediment load, outflow waters exhibit POC-δ[13]C values similar to those in inflow waters, due to a short water residence time and a rapid recycling of primary products. In the more isolated parts of the reservoir, where water discharge is thoroughly reduced, hypoxic to anoxic conditions are found immediately below the thermocline (ca. 16 m, [O2]

The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that not, vert, similar80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the α-radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates.

To evaluate the effects of tropical rainfall on the compositions of basaltic lavas, we studied 20th century lava flows from Mount Cameroon in Africa. Weathering conditions are extreme because the climate is particularly warm and humid, and vegetation grows extremely quickly on the flows. The high rainfalls and dense vegetation contribute to rapid and intense degradation of the volcanic rocks and should cause significant changes in chemical composition. Such effects need to be quantified to constrain how young a lava flow must be so that its trace element and isotopic composition remains representative of the original magma. Fresh inner parts and altered flowtops of four different lava flows were sampled and analysed for major and trace elements as well as O, U, Sr, Nd and Pb isotopic compositions. Four samples of the 1999 eruption were also analysed to constrain the composition of fresh basalts. Almost all major and trace elements display similar concentrations in inner and outer parts of the same flow. This is notably the case for elements such as K, Rb and Sr, which are highly mobile during weathering. The lack of variation suggests that the overall composition of the lava flows has not been significantly affected. However, some systematic chemical changes are observed: Loss-on-ignition (LOI) and δ18O increase slightly from inner parts of flows to near surface samples; Na and, to a lesser extent, U display significant losses in the outer samples. We interpret the Na loss in terms of hydration leading to exchange between Na+ and H+ ions. This process, associated with oxidation of Fe2+ to Fe3+, accounts for the larger loss-on-ignition in the outer parts of flows. A change in U contents is only observed in the 1922 flow, which is covered by dense vegetation. This emphasizes the role that complexation by organic ligands plays in U mobility. While U is not completely immobile, all volcanic rocks are in secular equilibrium ((234U/238U)≈1), indicating limited interaction between meteoric waters and basalts. Sr and Nd isotopic compositions remain constant and although 206Pb/204Pb ratios vary from inner to outer parts of the lavas, the changes are not systematic and cannot be attributed to weathering. The absence of significant chemical mobility and substantial isotopic exchange suggests very limited interaction between water and lavas at the centimeter scale, due, most probably, to their relatively young age. This work shows that over a period of about 100 years, no detectable geochemical changes are observed. These results are very promising for petrological and geochemical studies of ocean island basalts located in tropical areas since they provide direct evidence of very limited trace element mobility at the century time scale.

Major and trace element concentrations, as well as Sr and U isotope ratios, were measured in rainwater samples collected in three different locations in Alsace (East of France) and Luxembourg: a mid-altitude mountain site (Aubure Environment HydroGeochemical Observatory), an urban site (Strasbourg) and a peri-urban site located in an area of well developed industrial activity (Esch-sur-Alzette in Luxembourg). Results highlight the quite high spatial and temporal variability of the chemical and isotopic characteristics of rainwater at the regional scale. They also suggest a quite systematic contribution of a local component in the chemical composition of rainwater. In urban and peri-urban sites, the local component is certainly linked to human activities, as it is well illustrated in this study with the Esch-sur-Alzette samples. On the other hand, for the Aubure site, i.e. a small forested watershed of mid-altitude mountain, data presented in this study demonstrate the influence of the vegetation on the chemical composition of rainwater for alkali and calc-alkali elements, as well as for the Sr isotope ratios. Such a result questions the reliability of the method classically used to estimate the rainwater contribution on the river chemical budget. In addition, data of the present study confirm the very low content of uranium in rainwater and demonstrate, especially through the U isotope analysis of Aubure rainwater, the negligible effect of rainwater on U budget of river waters. This work thus outlines the property of U to be a geochemical tracer specific of weathering fluxes carried by rivers. To cite this article: F. Chabaux et al., C. R. Geoscience 337 (2005).

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Calcium isotope ratios relative to seawater were studied in natural waters in order to constrain the Ca isotopic flux to the oceans and to discuss the oceanic Ca budget. This study includes the analysis of (1) Ca isotopes in continental waters at the regional scale of the Upper Rhine valley and (2) the determination of the δ44Ca values of some major rivers and hydrothermal vents. The data indicate that the global δ44Ca variability of the studied waters, with a maximal range of 1‰, is limited and must be linked to mass fractionation processes. At the scale of a small watershed (Aubure, Vosges, France), the δ44Ca river flux changes due to the variation of the proportion of waters resulting from chemical alteration of rocks and biologically fractionated soil solutions. Ca isotopes could therefore be an important tool to quantify the impact of the plants and the biosphere on the river water chemistry. At the regional scale of the Rhine valley as well as at the global scale of world rivers, the δ44Ca variability of river samples ranges from 0.5 to 1‰ without any relationship with the lithology of the watershed or the climate. Similarly, the δ44Ca value of hydrothermal vents is also uniform and close to the mean value of river waters. Consequently, the δ44Ca flux to the ocean should remain rather constant through time with a mean of −1.1±0.2‰. This value confirms that the Ca budget of the present-day seawater is in steady-state. This was not necessarily the case in the past. Non-steady-state periods should have been due to intensity variations of incoming and outcoming Ca fluxes rather than to the variations in their isotopic signatures.

International audience; The activity concentrations of radionuclides within the uranium and thorium series were determined in wheat and lettuce at five sites in France, and in their respective potential sources crop soils of wheat and crop soils and irrigation waters of lettuce. These data were used to calculate concentration ratios and to enrich the database supported by the technical report series N°472 of the IAEA (2010). For wheat and lettuce, the activity concentrations were in the same range for all radionuclides studied, except for 210Pb, which had higher activity concentrations in wheat, ranging between 1.3 and 11 Bq kg-1 (fresh weight) as compared to 0.4 and 0.7 Bq kg-1 (fresh weight) for lettuce. For wheat, the range of activity concentrations (mBq kg-1; fresh weight) decreased as 210Pb andgt; 226Ra (56-1511) ≈ 228Ra (86-769) andgt; 228Th (19-176) ≈ 238U (11-169) ≈ 234U (12-150) ≈ 230Th (9.08-197.18) ≈ 232Th (8.61-121.45) andgt; 235U (0.53-7.9). For lettuce, it decreased as 228Ra (andlt;320-1221) andgt; 210Pb (409-746) andgt; 226Ra (30-599) ≈ 228Th (andlt;29-347) andgt; 238U (8-120) ≈ 234U (8-121) ≈ 230Th (5.21-134.63) ≈ 232Th (5.25-156.99) andgt; 235U (0.35-5.63). The species differences may reflect different plant physiologies. Through the study of activity ratios of wheat and lettuce in relation with those of the various radionuclide sources it has been possible to highlight the contribution of the main sources of natural radionuclides. Indeed, irrigation water when the uranium concentration is enhanced (andgt;30 mBq L-1) contributed significantly to the activity concentration of uranium in lettuces. Concerning the high activity concentrations of 210Pb, it could be explained by atmospheric particle deposition. The effect of soil particles resuspension and their adhesion to the plant surface seemed to be important in some cases. The soil-to-plant transfer factors were calculated for lettuce and wheat. The values were lower in wheat than in lettuce except for 210Pb which had similar values in the two species (0.11-0.13 respectively). For both species, 210Pb followed by 228Ra (0.015-0.10) and 226Ra (0.010-0.051) displayed the highest transfer factor, whereas 238U had intermediate values (0.0015-0.030) and 232Th exhibited the lowest (0.0014-0.013). © The Royal Society of Chemistry 2012.

Alkaline lamprophyre dykes from Taourirt (North Morocco) contain numerous xenoliths, ranging from alkaline pyroxenites, kaersutitites, gabbros and nepheline syenites to a calcite carbonatite. The silicate xenoliths and the host rocks consist of Aland Ti-rich diopside±salite, mica or kaersutitite, nepheline, plagioclase and K-feldspar, and ubiquitous apatite. Both the xenoliths and the lamprophyres are enriched in incompatible elements. The chemical composition of the lamprophyres cannot be accounted for by fractional crystallization alone. Moreover, the clinopyroxenes exhibit complex zoning, which requires repeated mixing of pulses of more or less fractionated melts. The carbonatite is a soEvite cumulate with Sr-rich calcite, pyrochlore, fluorapatite, and rare salite. The Sr±Nd isotopic compositions of the Taourirt rocks indicate a depleted mantle source, the carbonatite having the most depleted composition, and define a linear trend similar to that of the East African carbonatites. The different rocks thus represent unrelated magmas, and the trend is interpreted as mixing between two components with HIMU and EM1 mantle end-member signatures. An EM2 mantle component could also be involved for a few samples; it may correspond to hydrous metasomatized mantle of the PP± PKP (phlogopite and phlogopite K-richterite peridotite) and MARID (mica, amphibole, rutile, ilmenite and diopside) type.

The PhD thesis concerns the geochemical characterization and the determination of the residence times of the water circulating in the deep part of the substratum of the Strengbach catchment. The work is based on the comparative study of chemical, isotopic variations (87Sr/86Sr, 143Nd/144Nd and (234U/238U)) and water geochronometers (CFC-SF6, 14C, 4He) of samples from spring, piezometers (15 m deep) and deep boreholes.The results highlight two types of water, with lower cationic loads for surface waters (spring and piezometer water) than deep water, sampled in front of the main fractures. The origin of these differences has been discussed in terms of variation in residence times and from a point of view of water-rock interactions. This study has shown that an important factor explaining these differences is the residence time of waters, with recent waters in the surface system and old waters (more than 1000 years) in the deep system. The isotopic analysis has shown that the Sr-Nd signatures of the different water can, at first order, be explained by the dissolution of the phosphate-plagioclase-biotite end-members conventionally used on the catchment. The variation between the different deep waters suggests interactions with specific secondary minerals for each of the waters., Le travail de thèse porte sur la caractérisation géochimique et la détermination des temps de résidence des eaux circulant dans la partie profonde du substratum du bassin versant du Strengbach. Le travail s’appuie sur l’étude comparée des variations chimiques, isotopiques (87Sr/86Sr, 143Nd/144Nd et (234U/238U)) et des geochronomètres des eaux (CFC-SF6, 14C, 4He) d’échantillons de sources, de piézomètres (15 m de profondeur) et de forages profonds. Les résultats mettent en évidence deux types d’eaux, avec des eaux peu chargées en surface (eaux de sources et de piézomètres) et des eaux plus chargées en profondeur, au regard des principales fractures. L’origine de ces différences a été discutée en termes de variation des temps de résidence et d’un point de vue des interactions eaux-roches. Cette étude a montré qu’un facteur important expliquant ces différences est le temps de résidence des eaux, avec des eaux récentes dans le système superficiel et des eaux anciennes (sup. à 1000 ans) dans le système profond. L’analyse isotopique a montré que les signatures Sr-Nd des différentes masses d’eaux peuvent, au premier ordre, s’expliquer par la dissolution des pôles phosphates-plagioclase-biotite classiquement utilisés sur le bassin. La variation entre les différentes eaux profondes suggère des interactions avec des minéraux secondaires spécifiques pour chacune des eaux.

This work investigates the behaviour of Ra within a forest ecosystem (Montiers, Meuse). It is based on the quantification of several isotopic tracers (U- and Th- series disequilibria, isotopic ratios of 228Ra/226Ra and 87Sr/86Sr) in the different compartments of the water-soil-plant system. The research goals were : 1) to study the mobility of Ra and its radioactive ascendants in the separated mineral fractions of a soil profile, 2) to evaluate the transfer of Ra to the water-plant system, 3) to characterise the transfer of Ra and its residence time onto the vegetation, 4) to realise an account of the biogeochemical cycle of Ra. This work shows a strong redistribution of U, Th and Ra depending on their respective affinities with the different mineral fractions of the soil. Despite being predominantly concentrated in the clay fraction (

In-depth comparison of the response of various selected isotopic tools (Li, B and U) during chemical weathering in an already known profile (eg the Summit profile in the Strengbach watershed, Ackerer, 2017) to identify the driving mechanisms of isotopic fractionations of these elements to link the response of these tools compared to the chemical alteration to go back to the alteration regime.Develop the observations made on the Summit profile and apply them to five other weathering profiles distributed over the entire watershed focusing on the, Comparer de manière approfondie la réponse des différents outils isotopiques choisis (Li, B et U) durant l’altération chimique dans un profil déjà connu (e.g. le profil du Sommet sur le Bassin versant du Strengbach ; Ackerer, 2017) afin de dégager les mécanismes moteurs des fractionnements isotopiques de chacun de ces éléments pour faire le lien entre la réponse de ces outils lors de l’altération chimique et remonter ainsi au régime d’altération des roches. Développer les observations réalisées sur le profil du Sommet et les appliquer sur cinq autres profils d’altération répartis sur l’ensemble du bassin versant en se concentrant sur la fraction

In-depth comparison of the response of various selected isotopic tools (Li, B and U) during chemical weathering in an already known profile (eg the Summit profile in the Strengbach watershed, Ackerer, 2017) to identify the driving mechanisms of isotopic fractionations of these elements to link the response of these tools compared to the chemical alteration to go back to the alteration regime.Develop the observations made on the Summit profile and apply them to five other weathering profiles distributed over the entire watershed focusing on the

This work investigates the behaviour of Ra within a forest ecosystem (Montiers, Meuse). It is based on the quantification of several isotopic tracers (U- and Th- series disequilibria, isotopic ratios of 228Ra/226Ra and 87Sr/86Sr) in the different compartments of the water-soil-plant system. The research goals were : 1) to study the mobility of Ra and its radioactive ascendants in the separated mineral fractions of a soil profile, 2) to evaluate the transfer of Ra to the water-plant system, 3) to characterise the transfer of Ra and its residence time onto the vegetation, 4) to realise an account of the biogeochemical cycle of Ra. This work shows a strong redistribution of U, Th and Ra depending on their respective affinities with the different mineral fractions of the soil. Despite being predominantly concentrated in the clay fraction (, Ce travail porte sur le comportement du Ra au sein d’un écosystème forestier (Montiers, Meuse). Il s’appuie sur la mesure de plusieurs traceurs isotopiques (déséquilibres radioactifs, rapports 228Ra/226Ra et 87Sr/86Sr) dans les différents compartiments du système eau-sol-plante. Les objectifs étaient : 1) d’étudier la mobilité du Ra et ses ascendants radioactifs entre les fractions minérales séparées d’un profil de sol, 2) d’évaluer le transfert du Ra vers les compartiments eau-plante de l’écosystème, 3) de caractériser le transfert du Ra et son temps de résidence dans la végétation, 4) de réaliser le bilan du cycle biogéochimique du Ra. Ce travail montre une forte redistribution de U, Th et Ra en fonction de leurs affinités respectives avec les différentes fractions minérales du sol. Bien que le Ra soit très fortement associé à la fraction fine (< 2µm) du sol, la végétation (hêtres) semble prélever le Ra des oxydes de fer du sol. Le transfert ultérieur de ce nucléide depuis les racines vers les parties aériennes de l’arbre est moins important que celui des autres alcalino-terreux, aboutissant à un temps de résidence dans la végétation de l’ordre de quelques années (2.6 ± 1.6 ans). Enfin, le cycle biogéochimique du Ra est caractérisé par un flux de dégradation de la litière souterraine plus important que celui lié à la litière de surface, par un apport atmosphérique en Ra négligeable et par un flux d'altération très important, ce dernier ne transitant pas par les solutions de sols gravitaires. Le Ra présent dans ces dernières provient presque exclusivement de la dégradation de la litière.

The analysis of total sediments and grain-size fractions from the Gandak river (Himalaya) showed that the Uranium series variability observed for the Himalayan sediments is controlled by their mineralogical and grain-size evolution, rather than by the weathering process.This conclusion called into question the transfer timescale in Himalayan plains estimated by previous works and highlighted the need to explore radionuclide mobility at the mineral scale. For this purpose, an approach on pure mineral fractions pointed out exchange processes between the mineral phases, with a significant mobility of 230Th, strongly depleted in zircon, and 226Ra, particularly enriched in the clay fraction. The observed disequilibria are attributed to a recent weathering process of a few ky. Those observations are confirmed by the SIMS analysis of 238U-234U-230Th disequilibria on mineral grains, which show a 230Th enrichment at the grain surface in monazites and a loss of Th, 230 and 232, in zircons., L’analyse de sédiments totaux et de fractions granulométriques de la rivière Gandak (Himalaya) a montré que la variabilité des séries de l’uranium observée pour les sédiments himalayens est contrôlée par leur évolution minéralogique et granulométrique plutôt que par le mécanisme d’altération. Cette conclusion questionne les travaux antérieurs et souligne la nécessité d’étudier la mobilité des radionucléides à échelle minérale. A cette fin, une approche sur fractions minérales a mis en évidence des processus d’échange entre les minéraux, avec une mobilité significative de 230Th, fortement appauvri dans les zircons, et 226Ra, enrichis en particulier dans la fraction argileuse. Les déséquilibres observés semblent induits par un processus d’altération récent, de quelque ka.Ces résultats sont confirmés par l’analyse des déséquilibres 238U-234U-230Th dans des minéraux par SIMS, qui montre un enrichissement en 230Th à la surface des monazites et une perte de Th (232, 230) dans les zircons.

This work is focused on the study of a profile of soil and soil solutions collected on an experimental plot covered with spruce. All these samples come from the watershed of the Strengbach (environment - OHGE Hydrogeochimique Observatory), were sampled at different depths (5, 10, 30 and 60 cm) and during the period between 2009 and 2013. Characterizations of soil extracts by infrared spectroscopy allowed to highlight changes in the organic functional groups with depth and that these changes have a significant impact on the behaviour of the cations (major and trace) in the soil. Ultrafiltration experiments helped to identify flows of colloidal and dissolved organic carbon as well as those of the major and trace-element present in soil solutions. The joint use of isotope tracers (87Sr / 86Sr and δ44 / 40 Ca) and chemical (Rare Earth Elements) have highlighted processes taking place at the water-soil-plant interface, as the uptake by root or soil alteration., Ce travail est axé sur l’étude d’un profil de sol et des solutions de sol prélevés sur une parcelle expérimentale couvertes d’épicéas. Tous ces échantillons proviennent du Bassin Versant du Strengbach (Observatoire HydroGéochimique de l’Environnement – OHGE), ont été échantillonnés à différentes profondeurs (5, 10, 30 et 60 cm) et durant la période comprise entre 2009 et 2013.Les caractérisations des extraits des sols par spectroscopie Infra-Rouge ont permis de mettre en évidence les modifications des groupements fonctionnels organiques avec la profondeur et que ces modifications ont une forte incidence sur le comportement des cations (majeurs et traces) dans le sol. Des expérimentations d’ultra-filtration ont permis d’identifier les flux colloïdaux et dissous du carbone organique ainsi que ceux des éléments majeurs et traces présents dans les solutions de sol. L’utilisation conjointe des traceurs isotopiques (87Sr/86Sr et δ44/40Ca) et chimiques (Terres Rares) ont mis en évidence des processus ayant lieu aux interfaces eau-sol-plante, comme le prélèvement racinaire ou l’altération des sols.

Since the end of the 19th century, the use of copper (Cu)-based fungicides has resulted in increased Cu concentrations in vineyard soils, but also in downstream aquatic ecosystems. The aim of the thesis was to better understand the fate of this Cu in an agrosystem based on assessing Cu isotope fractionation (65Cu/63Cu). The results have shown that the surface vineyard soils have become enriched in Cu from 9 to 28 times compared to the background level during 4 to 5 decades of vine-growing and that clay minerals were the major Cu sorbing phases in the soils. During rainfall, runoff mobilized ~1% of the applied Cu during the, mainly associated with clays. The stormwater wetland collecting the runoff retained in average 68% of the dissolved and more than 92% of particulate Cu. Cu isotope ratios measured in the wetland suggested dissolved Cu sorption to the sediments and in situ reduction of Cu(II) due to biogeochemical processes., Depuis la fin du 19ème siècle, le traitement des vignes par des fongicides cupriques a engendré une augmentation de la teneur en cuivre (Cu) dans les sols viticoles, ainsi que dans les écosystèmes aquatiques en aval. Cette thèse vise à mieux comprendre le devenir de ce Cu dans un agrosystème basé sur l’étude du fractionnement isotopique du 65Cu/63Cu. Les résultats montrent que durant 4 à 5 décennies de culture de vignes, les sols en surface se sont enrichis en Cu de 9 à 28 fois par rapport au fond géochimique et que les minéraux argileux jouent un rôle important dans l’accumulation du Cu. Lors des événements pluvieux, ~1% du Cu appliqué est mobilisé, essentiellement lié à des argiles. Le bassin d’orage récoltant les lames ruisselantes retient en moyenne 68% du Cu dissous et plus de 92% du Cu particulaire. Les ratios isotopiques du Cu dans le bassin indiqueraient la sorption du Cu dissous dans les sédiments, ainsi que la réduction du Cu(II) in situ due à des processus biogéochimiques.

The main goal of the present thesis is to better constrain the parameters that control weathering processes at a small catchment scale, focusing specifically on the role of hydrology. For this purpose, temporal and spatial variations of the chemical and isotopic (U, Sr) water composition of the Ringelbach catchment are studied. Several springs of this catchment whose basement is mainly composed of granite more or less fractured located along an altitudinal profile were monthly sampled over a period of two years. The additional interest of this site is that three deep boreholes (up to 150 meters deep) allow the sampling of both deep rocks and waters. The connectivity of the different hydrological compartments is evaluated based on the geochemical interpretation of water samples. A schematic hydrological functioning is proposed based on a good knowledge of the geological context. Finally, a weathering rate and a water flux are estimated from the modeling of the uranium activity ratio along a water path. The originality of this study lays also in combination of a geochemical and modeling approaches using the software KIRMAT (Kinetic Reactions and Mass transport) that integrates geochemical reactions (dissolution/precipitation) and 1D mass transport equations. It allow to simulate the reactive transport of a fluid through a rock along a given water pathway. This modeling is based on the characterization of the mineralogical and physical properties of the rock, sampled along the boreholes and the geochemical interpretation of the waters. Thus, the modeling of the chemical composition of the spring waters and borehole waters permit to improve the understanding of weathering processes including the role of precipitated secondary phases. It also allows to better understand the interplay of parameters that that control the water chemical signature at the catchment scale., L’objectif de cette thèse est d’arriver à mieux contraindre les paramètres qui contrôlent les processus d’altération à l’échelle d’un petit bassin versant, en se focalisant plus précisément sur le rôle de l’hydrologie. Pour ce faire, les variations temporelles et spatiales des signatures chimiques et isotopiques (U, Sr) des eaux du bassin versant du Ringelbach (Vosges, France) sont étudiées. Plusieurs sources de ce bassin versant, essentiellement composé d’un granite plus ou moins fracturé, ont été échantillonnées mensuellement sur une période de deux ans, le long d’un profil altitudinal. Ce bassin versant présente également l’avantage d’être équipé de trois forages profonds (allant jusqu’à 150 mètres de profondeur) qui ont permis de prélever à la fois les roches et les eaux profondes. Sur la base de l’interprétation géochimique des eaux, la connectivité des différents compartiments hydrologiques a été évaluée et un schéma de fonctionnement hydrogéochimique est proposé s’appuyant sur une bonne connaissance du contexte géologique. Enfin, un taux d’altération et un flux d’eau sont estimés à partir d’une modélisation de l’évolution du rapport isotopique de l’uranium (234U/238U) le long d’un trajet d’eau. L’originalité de cette étude est également de coupler cette approche de traçage géochimique avec une approche modélisatrice en utilisant le logiciel KIRMAT (Kinetic Reactions and Mass transport) qui intègre les équations des réactions géochimiques (dissolution/précipitation) et les équations de transport (1D). Il est ainsi possible de simuler le transport réactif d’une eau traversant la roche le long d’un certain trajet d’eau. Cette modélisation s’appuie sur la caractérisation minéralogique, des propriétés physiques des échantillons de roches prélevés le long des forages et sur les interprétations géochimiques des eaux. Ainsi, la modélisation de la composition chimique des eaux de sources et des eaux de forages a permis d’affiner la compréhension des processus d’altération, notamment le rôle des phases secondaires précipitées. Celle-ci permet également une meilleure compréhension des phénomènes de couplage entre les différents paramètres qui contrôlent la signature chimique des eaux à l’échelle du bassin versant.

The objective of this thesis is to estimate the ability of boron and its isotopes to evaluate mineral weathering budgets and mechanisms in soils. In this goal, 3 different approaches were conducted:1) an experimental approach during which a test-mineral (biotite) undergoes the action of different weathering agents, representative of those found in soils.2) an in-situ approach in an acid soil profile led on different minerals (biotite, muscovite, K-feldspar and albite) handpicked at various depths and weathering state (Breuil-Chenue forest experimental site, France).The objective of this study is to determine the sensitivity of boron to soil formation processes.3) an second in-situ approach conducted on the same study site but this time on sub-surface horizons (horizon A and Al-oxi-hydroxides accumulation horizon), which aim is to evaluate the sensitivity of boron and its isotopes to mechanisms in close vicinity to vegetation (seasonal variations, vicinity of the roots). B and its isotopes able, thanks to its high reactivity during weathering mechanisms in comparison to major elements that constitute the crystallographic network (observed in experimental as well as in in-situ approaches), to trace with a great sensitivity primary minerals dissolution or transformation mechanisms, and opens new insights for the comprehension of soils.; L’objectif de cette thèse est d’estimer la capacité du bore et de ses isotopes à évaluer les mécanismes et bilans de l’altération des minéraux dans les sols. Pour ce faire, trois axes ont été développés : 1) une approche expérimentale, durant laquelle un minéral-test (la biotite) est soumis à l’action de différents agents altérants représentatifs de ceux trouvés dans les sols. 2) une approche in-situ dans un sol forestier acide, menée sur des minéraux séparés (biotite, muscovite,feldspath potassique et albite), au cours de leur altération à différentes profondeurs (site de Breuil-Chenue, France). L’objectif de cette étude est de déterminer la sensibilité du bore aux processus de la formation des sols. 3) une seconde approche in-situ sur le même site expérimental, menée cette fois-ci sur les horizons superficiels (horizon A et horizon d’accumulation des oxy-hydroxydes d’aluminium), dont le but est de déterminer la sensibilité du bore et de ses isotopes aux processus liés à la proximité de la végétation (variations saisonnières, proximité des racines). Le bore permet, grâce à sa très grande réactivité durant les mécanismes d’altération par rapport aux éléments majeurs constitutifs du réseau cristallin (observable aussi bien en conditions expérimentales qu’en milieu naturel), de tracer avec une grande sensibilité les mécanismes de dissolution ou de transformation des différents minéraux primaires, et ouvre de nouvelles perspectives quant à la compréhension et la modélisation des sols.

The objective of this thesis is to estimate the ability of boron and its isotopes to evaluate mineral weathering budgets and mechanisms in soils. In this goal, 3 different approaches were conducted:1) an experimental approach during which a test-mineral (biotite) undergoes the action of different weathering agents, representative of those found in soils.2) an in-situ approach in an acid soil profile led on different minerals (biotite, muscovite, K-feldspar and albite) handpicked at various depths and weathering state (Breuil-Chenue forest experimental site, France).The objective of this study is to determine the sensitivity of boron to soil formation processes.3) an second in-situ approach conducted on the same study site but this time on sub-surface horizons (horizon A and Al-oxi-hydroxides accumulation horizon), which aim is to evaluate the sensitivity of boron and its isotopes to mechanisms in close vicinity to vegetation (seasonal variations, vicinity of the roots). B and its isotopes able, thanks to its high reactivity during weathering mechanisms in comparison to major elements that constitute the crystallographic network (observed in experimental as well as in in-situ approaches), to trace with a great sensitivity primary minerals dissolution or transformation mechanisms, and opens new insights for the comprehension of soils., L’objectif de cette thèse est d’estimer la capacité du bore et de ses isotopes à évaluer les mécanismes et bilans de l’altération des minéraux dans les sols. Pour ce faire, trois axes ont été développés : 1) une approche expérimentale, durant laquelle un minéral-test (la biotite) est soumis à l’action de différents agents altérants représentatifs de ceux trouvés dans les sols. 2) une approche in-situ dans un sol forestier acide, menée sur des minéraux séparés (biotite, muscovite,feldspath potassique et albite), au cours de leur altération à différentes profondeurs (site de Breuil-Chenue, France). L’objectif de cette étude est de déterminer la sensibilité du bore aux processus de la formation des sols. 3) une seconde approche in-situ sur le même site expérimental, menée cette fois-ci sur les horizons superficiels (horizon A et horizon d’accumulation des oxy-hydroxydes d’aluminium), dont le but est de déterminer la sensibilité du bore et de ses isotopes aux processus liés à la proximité de la végétation (variations saisonnières, proximité des racines). Le bore permet, grâce à sa très grande réactivité durant les mécanismes d’altération par rapport aux éléments majeurs constitutifs du réseau cristallin (observable aussi bien en conditions expérimentales qu’en milieu naturel), de tracer avec une grande sensibilité les mécanismes de dissolution ou de transformation des différents minéraux primaires, et ouvre de nouvelles perspectives quant à la compréhension et la modélisation des sols.

Calcium is the fifth most abundant element of the silicate Earth. Its interest results from its abundance in the Earth crust and from the existence of a huge quantity of minerals containing it. A maximal fractionation of 1.3 ‰ per a.m.u was recorded up to now. The work realised during the Ph-D deals with new studies on Ca isotopes.First, a measurement protocol was developed at the Centre de Géochimie de la Surface to avoid instrumental fractionation. After chemical separation on ion exchanging resins, the Ca isotopic compositions were measured on a thermo-ionisation mass spectrometer, using a double-spike technique. The Ca isotopic ratios were expressed in a delta notation. To do this, a reference material is needed. No international standard presently exists for Ca isotopes. For that reason, we proposed to use the SRM 915a carbonaceous standard from the NIST. Seawater, the only common sample of most of previous studies has also been used: it was calibrated against the carbonaceous standard and used in present study.This protocol was then applied to study the Ca oceanic budget. The isotope data of continental waters allowed to constrain the Ca flux to the ocean. A limited variability of δ44Ca (0.5 ‰) was recorded at the outlet of main rivers or for hydrothermal vents. Moreover, no relationship was observed between δ44Ca and the lithology or the climate of the studied watersheds. This allowed to determine the mean Ca isotopic composition of the Ca flux to the ocean (-1.1 ± 0.2 ‰) and to suggest that this value remains rather constant through time. A consequence of this value is that present-day seawater is at steady-state with regard to Ca isotopes. The δ44Ca study of marine phosphates from Miocene and Pleistocene, which reflect the paleo-seawater variations through time, has shown that this was not necessarily the case in the past. At 22 Ma, a deviation in δ44Ca through more positive values suggests that the intensity of the incoming and outcoming fluxes changed, but not their isotopic composition.; Le calcium est le cinquième élément le plus abondant de la Terre silicatée. Son intérêt découle de son abondance dans la croûte terrestre et de l'existence d'une grande quantité de minéraux le contenant. Un fractionnement maximum, dont les causes précises ne sont pas connues, de 1,3 ‰ par u.m.a a été mesuré à ce jour. Le travail réalisé durant ce doctorat s'intègre à ces nouvelles études sur les isotopes du Ca.Un protocole de mesure permettant de s'affranchir du fractionnement instrumental a tout d'abord été développé au Centre de Géochimie de la Surface. Après séparation chimique sur résine échangeuse d'ions, les compositions isotopiques en Ca ont été déterminées au spectromètre de masse à thermo-ionisation par une technique employant un double spike. L'expression des rapports isotopiques du Ca sous forme de delta requiert l'emploi d'un matériel de référence. Aucun standard international n'existant pour les isotopes du Ca, le standard carbonaté SRM 915a du NIST a été proposé. L'eau de mer, seul échantillon commun à la majorité des études antérieures, a été calibré en fonction de ce standard carbonaté et utilisé pour ce travail.Ce protocole a ensuite été appliqué à l'étude du bilan océanique du Ca. L'étude d'eaux continentales a permis de contraindre le flux de Ca arrivant aux océans. Il en résulte une variabilité limitée de δ44Ca (0,5 ‰) à l'embouchure des grands fleuves et pour les eaux hydrothermales. Aucune relation n'a en outre été observée entre δ44Ca et la lithologie ou le climat des bassins versants étudiés. Ceci a permis de calculer une composition isotopique moyenne en Ca arrivant aux océans égale à -1,1 ± 0,2 ‰ et de suggérer sa constance au cours du temps. Il résulte de cette valeur que l'océan actuel est à l'état stationnaire par rapport aux isotopes du calcium. L'étude du δ44Ca de phosphates marins du Miocène et du Pléistocène, qui reflètent la paléo-variation de l'eau de mer au cours du temps, a montré que cela n'a pas toujours été le cas par le passé. Ainsi, à 22 Ma, une incursion du δ44Ca vers des valeurs plus positives suggère que l'intensité des flux entrants et sortants a varié, mais non leur signature isotopique.