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International audience; Particle shape is a major parameter for the space-filling and strength properties of granular materials. For a systematic investigation of shape effect, a numerical benchmark test was set up within a collaborative group using different numerical methods and particles of various shape characteristics such as elongation, angularity and nonconvexity. Extensive 2D shear simulations were performed in this framework and the shear strength and packing fraction were compared for different shapes.We show that the results may be analyzed in terms of a low-order shape parameter h describing the degree of distortion from a perfectly circular shape. In particular, the shear strength is an increasing function of h with nearly the same trend for all shapes, the differences being of second order compared to h. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with h from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as h is further increased. Finally, the analysis of contact forces for the same value of h leads to very similar statistics regardless of our specific particle shapes.

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$., Comment: 5 pages, 8 figures

International audience; Volcanic islands, being characterized by highly porous basaltic/andesitic lava flows and pyroclastic deposits, are subject to important chemical weathering by subsurface waters. Moreover, such subsurface weathering is impacted by hydrothermal springs in both active and non-active volcanic areas, thus increasing dissolved load concentrations. Here, we focus on the subsurface water chemistry in the volcanic islands of the Lesser Antilles and Re'union and on the origin of these subsurface flows. We are able, through the use of various isotopic tools (C, Sr, U-Th), to identify hydrothermal influences in river water. For example, Li concentrations show a positive correlation with temperature of hot and cold springs and also a relationship with d13C; from this, we can show that several sources of hydrothermal activity influence the rivers of the Lesser Antilles and that some rivers also reveal an important organic influence. As much as 20% of the subsurface hydrothermal springs go to feed the rivers. The increasing temperatures result in more dissolved elements being mobilized and an increase in chemical weathering rates. In addition, using the (230Th/238U) isochron for the well and river dissolved loads in Martinique, Guadeloupe and Re'union, we can evaluate residence times in the river water, i.e. the average residence time in the water along the circulation path to the sampling point. Alteration takes longer when the water circulates through thick soil, for example, 400-5,000 years when circulating under an ash profile and 1,200-15,000 years when circulating through a collapse zone. It would appear that waters circulation is globally three times longer for subsurface water than for surficial water. The weathering regime in tropical volcanic environments seems to be controlled mainly by such subsurface circulation with high chemical concentration from hydrothermal inputs. The origin of these compositions is varied and not controlled by a single hydrothermal spring. Consequently, it is subsurface circulation that determines the weathering regime in tropical volcanic islands with the main controlling parameters being temperature and residence time.

International audience; In this paper, we use carbon isotopes in the dissolved load of rivers from the Lesser Antilles volcanic arc (Guadeloupe, Martinique and Dominica islands) to constrain the source of the carbon dioxide (CO2) involved in the neutralization reactions during water-rock interactions. The d13C data span a large range of variations, from -19% to -5 _ 2% for DIC (dissolved inorganic carbon) concentrations ranging from 11 mM to 2000 mM. Coupled with major element concentrations, carbon isotopic ratios are interpreted as reflecting a mixture of magmatic CO2 (enriched in heavy carbon (d13C_-3 _ 5%) and biogenic CO2 produced in soils (enriched in light carbon (d13C

In "Proceedings of the Fourteenth International Symposium on Water-Rock Interaction, WRI 14"; International audience; The Allier river is one of major river in France as it is the main contributor of the Loire river, The Allier river present both natural environment upstream and a plain zone deeply impacted by mines and human activities. The original combination of d13C and d7Li data allow to decipher sources effects and in-situ phenomenon in the riverine inorganic carbon cycle such as mineral phases precipitation or equilibration with the atmosphere. These data show that the riverine system of the Allier is a mixing between DIC from organic decay produced in a natural environment (upstream) progressively enriched in DIC of anthropogenic origin (downstream) and deep carbon inputs implying thermo-water rock interactions.

International audience; Guadeloupe, Martinique and Dominica Islands alike numerous tropical environment present extreme weathering regimes. Physical denudation is mainly controlled by landside, This reflects the torrential dynamics of the rivers. For Guadeloupe, the mechanical weathering rates are 800-4000 t/km2/yr. The lithology is very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface as the chemical erosion rates are 2 to 5 time higher than the rates from surface water. We show how kinetic of chemical weathering rates depends on the age of the lava and subsurface circulation. In addition, timescale of erosion have been calculated from U-series analyses sediments from rivers. Our results show a large range: from 0 to 150 ka in Martinique and from 0 to 60 ka Guadeloupe. From analyses from the dissolved loads, we propose to evaluate residence times in the river water. It would appear that waters circulation is globally 3 times longer for subsurface water than for surficial water (Rad et al., 2011). Moreover these islands are highly impacted by agriculture. It is therefore interesting to assess the impact of such influence on the weathering rates. Our result show that human activity brings no disturbance on Critical zone processes contrary to what one might think. Indeed, we show that among the combined impact of all parameters (climate, runoff, slopes, vegetation...), the basins age seems to be the control parameter on chemical weathering and land use: the younger the basin, the higher the weathering rate. We could observe a combined effect between the higher erodibility and a higher climate erosivity of the younger reliefs.

International audience; Li isotopes has been recently used to study water-rock interactions at Earth surface. Li is a fluid-mobile element that tends to preferentially partition into the fluid phase during water-rock interaction. The relative mass difference between the two isotopes is considerable, generating large mass dependent fractionation during chemical weathering processes, even at high temperature. In the present work, the 7Li/6Li (expressed as δ7Li) was analyzed in Allier River, one of the major river basins of France. We have undertaken a systematic study of weathering products of the river. The lithology is dominant by granite rocks with current upstream, while it is mainly basaltic and Oligocene sediments in the downstream with hydrothermal manifestation. Water samples were collected during several field trips. Our results show a large variation in Li isotopes within the catchment from 4.44 ‰ to 23.52 ‰. Li signature show a geographical distribution as it decreases from upstream to downstream over 400km. It appears that the upstream portion of the river present Li signature with higher fractionation with a mean value of 18 ‰ whereas it is only 6 ‰ in the downstream. This last portion of the river is impacted by hydrothermal activity, indeed these values reflect low temperature water-rock interaction in upstream and high water-rock interactions in the downstream, which the lowest values correspond to thermal mineral water plant. Li isotopes can be used here as a tracer of hydrothermalism. Moreover it seems that Li isotopes present a negative correlation with metal elements concentrations, which means that in a natural system hydrothermal activity induce liberation of those elements in the dissolved phase of the river during chemical alteration. Those promising applications from Li isotopes to better understand chemical weathering processes will be discussed.

International audience; The Lesser Antilles have very high chemical weathering rates, with values that can reach 1290 t/km(2)/a. The tropical environment induces high precipitation rates, high temperature, dense vegetation, with sharp relief and thick soils. Because of volcanic activity, frequent pyroclastic flows produce very erodible and porous materials. In addition, agriculture induces important land use changes which replace existing native forest cover with banana and sugar cane plantations. Their surface can cover as much as 40% of the total area of a river basin. The aim of this study is to identify key parameters, either natural or anthropogenic, that control chemical weathering rates. Among the combined impact of all parameters (climate, runoff, slopes, vegetation etc.), basin age seems to be the control parameter: the younger the basin, the higher the weathering rate. A correlation between the chemical weathering rate and the basin age suggests that young volcanic rocks are more easily weathered than old ones: young fresh material is easily mobilized by erosion, while for older rocks with thick soil covers, chemical rates are much lower. A combined effect between the higher erodibility and a higher climate erosivity of the younger relief could be observed. Moreover, a correlation between banana plantations and the chemical weathering rates that can be explained by an increase of infiltration, due to stem flow processes is shown here. Banana plantations also have a correlation with the basin age, older basins being more favorable terrains for cultivation.

International audience; Water-rock interactions produced in river catchment are accompanied by fractionation or changes in stable isotopes such as H, Li, C and O during chemical weathering processes. Li is a fluid-mobile element that tends to preferentially partition into the fluid phase during water-rock interaction. The relative mass difference between the two isotopes is considerable, generating large mass dependent fractionation during chemical weathering processes. The CO2 dissolves into the water providing the main acid that attack the rock during chemical weathering. Carbon stable isotopes and concentration of Dissolved Inorganic Carbon (DIC) in the river catchment can be used to determine the origin and consumption rates of CO2. In the present work, stable isotopes were analyzed in Allier River, one of the major river basins of France. The lithology is dominated by granite rocks within current upstream, while it is mainly basaltic and Oligocene sediments in the downstream with hydrothermal manifestations. We propose a new isotopic approach by combining δ7Li and δ13CDIC analyses in river catchment waters. A first method has been applied to volcanic tropical environments with Li concentrations correlated to δ13CDIC (Rad et al., 2011). Here, we have completed this approach by lithium isotopes. Water samples were collected during several field trips. Our results show a large variation in Li isotopes and C isotopes within the catchment from 3.3 ‰ to 30.3 ‰ and from -17.9‰ to -3.5‰, respectively. Chemical weathering rates linearly increase from upstream to downstream over 400km distance, whereas Li isotope signatures decrease and global C signature increases. This is due to low water-rock interaction dominated in upstream, whereas the downstream is punctually impacted by hydrothermalism. From Li and C isotopes, our results show 4 groups reflecting different chemical weathering processes: the first group with high fractionation of Li and C, for Li, the heavy lithium partitioned into surface waters, leaving lighter lithium behind in the weathered products, the signature of C is mainly due to organic matter or partially due to biochemical interaction with assimilation of CO2 by microorganism. The second group involves atmospheric equilibrium with CO2 degassing with organic origin or "cold" CO2 degassing with important fraction of Li. The third group present high fractionation of C, reflecting presence of superficial C with organic origin, with low fractionation of Li underling the hydrothermalism impact. Finally a fourth group with low fractionation mainly due to high temperature water-rock interaction. Therefore, the combination of the two tracers, Li and C isotopes, offers a powerful tool to discriminate chemical weathering processes from sources of alteration during water-rock interactions under multi-lithology terrains.

International audience; ABSTRACT: The present study is a part of a collaborative French national research project CEGEO/shape (Change of Scale in GEOmaterials). The purpose of benchmark collaborative numerical tests performed, using CD (Contact Dynamics) or MD (Molecular Dynamics) methods, is to quantify the non-circular shape of particles, by their degree of distortion relative to the disk, and to analyse their influence on the local and the macroscopic mechanical properties of granular materials. This contribution points out the compact collection of rigid octagonal shaped particles, built in accordance with our specific shape index. It also deals with the first investigations of realistic 3D particles. The following five biaxial compressions tests (which bring difficulties such as little deformations and multiple localizations) are numerical simulations, using NSCD (Non Smooth Contact Dynamic) method and LMGC90 software. The macroscopic behavior is studied in terms of solid fraction and internal angle of friction. We show that the internal angle is influenced by the increased anisotropy of friction forces and the contact orientations with the shape indicator. The variation of the initial assembly density (area of solids over total area) and the final density in the critical state is modelled using the number of particle edges, in order to explain the higher peak shearing resistance of assemblies with smaller initial void ratio or higher coordination number. Besides unilateral contact and Coulomb friction, other parameters relevant to the shape and contact anisotropy, which influence the anisotropy of stress transmission, as the ratio of edge-to-edge or vertex-to-edge active contacts, the stick-slip phenomenon, and the friction mobility, (ratio of tangential and normal contact forces) are quantified. Finally, different approaches have been proposed for the change of scale of kinematic variables. Analysis of local kinematics, and relation between the global strain and the local relative displacements at the contacts is studied.

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected., Submited to Physical Review E