Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition ({sup 13}C and {sup 14}C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than 5 years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in mid-size to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics

Study of iodide sorption to the argillite of Tournemire in alkaline media

Argillaceous rocks are considered potential host rocks for radioactive waste repositories. The concrete matrix that could be used as a barrier could react with the groundwater of the geological site, inducing a drastic change in its chemical composition and its pH (10-13). Consequently, the physicochemical properties of the rock in contact with this alkaline solution may be modified and, in turn, may induce modification on the behaviour of radioelements. This study, applied to the argillite of Tournemire, involves characterizing I- sorption to an argillaceous rock in alkaline media in batch experiments under N2-controlled conditions. I- was added as a 125I radiotracer and measured by γ spectrometry.Preliminary experiments were conducted with different solution/solid ratios (v/m=2.5, 5 and 20 ml g-1) and contact times (1-14 days) in order to determine the optimal experimental conditions. The chosen v/m ratio was 5 ml g-1 as the best compromise between a high Kd value and a low error of the measure. The chosen experiment duration was 1 day because I- sorption was highest and to limit the effects of pyrite oxidation. One of the experiments, performed with a radio-sterilized sample to test possible effects from microorganisms, showed that they could enhance iodide retention, particularly during the first 2 contact days.The influence of pH on I- sorption was tested using solutions between values of 8.3 and 12.8. The Kd values were independent of pH and very low (0.3 ml g-1). Finally, the influence of the chemical composition of concrete fluids was also tested. Three solution compositions corresponding to different steps in the evolution of fluids in contact with altering concrete were used: fluid in contact with fresh concrete (pH 13.2), with moderately degraded concrete (pH 12.1) and with strongly degraded concrete (pH 11.5). Each solution contained variable amounts of sodium, potassium, calcium, silica and sulphate. I- sorption was also very low (Kd∼0.2 ml g-1). Additional experiments were conducted with alkaline solutions containing different amounts of SO42- ions (10-3-10-2 M) to test sulphate-iodide sorption competition. I- retention was independent of the sulphate concentration. © 2004 Elsevier B.V. All rights reserved

Comparison of the bulk geochemical features and thermal reactivity of kerogens from Mol (Boom Clay), Bure (Callovo-Oxfordian argillite) and Tournemire (Toarcian shales) underground research laboratories

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Mineralogical characterization of the Tournemire argillite after in situ interaction with concretes

This study aims to investigate, through mineralogical characterization (SEM, XRD) and mass-balance calculations, the effects of contact time, concrete types and presence of free water on the Tournemire argillite under in situ conditions. Three sampling zones from Excavation Disturbed Zone (EDZ) areas have been chosen: (1) dry contacts, collected at the tunnel masonry/argillite interface (contact time - 125 years), (2) wet contacts, taken close to drained areas below the tunnel roadbed in contact with the canal draining the Cernon fault water (contact time - 15 and 7 years) and at the tunnel masonry/argillite interface over 70 m from the Cernon fault (contact time - 125 years). This study shows that:•in the absence of water, no significant modification of argillite is observed after 125 years, except for pyrite dissolution and gypsum precipitation.•in the presence of water, precipitation of gypsum, recrystallization of mixed-layer clays, neoformation of zeolites and K-feldspars overgrowths are observed. At the concrete/argillite interface near the Cernon fault, important dolomite neoformation and leaching of chlorite and kaolinite occur. These processes are enhanced with contact time, low flow rate and the nature of the concrete (compound cement Portland CEM II 32.5). Evidence for oxidation processes linked to the EDZ (pyrite oxidation, Fe-oxy-hydroxide and gypsum precipitation) is observed. © 2006 Elsevier Ltd. All rights reserved

Interaction of Tournemire argillite (Aveyron, France) with hyperalkaline fluids: Batch experiments performed with powdered and/or compact materials

Argillaceous rocks and concrete matrix are considered as potential host rocks and engineered barrier, respectively for radioactive waste repositories. The concrete matrix could react with the groundwater of the geological site, inducing a drastic change in its chemical composition and pH. Consequently, the physico-chemical properties of the rock in contact with this hyperalkaline fluid may be modified. Therefore, an experimental programme has been developed for examining the physico-chemical changes of claystone in contact with alkaline fluids: batch experiments undertaken as a preliminary step followed by diffusion and percolation experiments in order to describe and to assess the physico-chemical phenomena observed under repository conditions. The first series of batch experiments are carried out in order to study the degradation of the Tournemire argillite properties caused by a fluid resulting from a contact with fresh concrete, considered as the most aggressive in relation to its hyper alkaline pH (≈13). Thus, the influence of several parameters is tested, such as liquid/rock ratio (L/R 3-30 g/g), contact time (1-3 months), temperature (25-70 °C) and oxygen concentration. To compare the effect of the concrete maturity on the argillite physico-chemical damage, the second series of batch experiments are performed with a fluid resulting from a contact with moderately degraded concrete. Experiments are carried out on powdered and compact samples in order to identify structural and geochemical modifications. Before and after contact with simplified concrete fluids, powdered solids were mainly characterised by XRD (with measurements of specific surface area and SEM observations for a selection of samples) whereas compact materials were only observed by SEM. The chemical composition of the recovered solutions was also determined for their major anions and cations with their carbon content and pH values. Interpretations are based on solid analyses and aqueous chemistry. The comparison of results obtained with powdered and compact solids is only feasible for short time experiments. Aqueous chemistry and solid analyses indicate the dissolution of pyrite, dolomite and organic carbon with the precipitation of calcite. Precipitations of zeolites and K-feldspars occur sometimes and are only evidenced by SEM analyses. Nevertheless, silicates dissolution or precipitation is unclear through XRD analyses. Experimental conditions influence widely the intensity/nature of processes and prevent the transposition of data obtained in given conditions to other ones. © 2006 Elsevier Ltd. All rights reserved

Evaluation of tritiated water diffusion through the Toarcian clayey formation of the Tournemire experimental site (France)

Through-diffusion experiments with tritiated water were performed on argillaceous samples from various zones of the Tournemire test site. It was intended to evaluate the homogeneity of the transport property of unfracturated samples and the influence of the orientation and the nature of the samples (presence of an opened fracture or a pre-existing tectonic fracture filled with calcite and pyrite). Homogeneous values of the tritiated water (HTO) effective diffusion coefficients were deduced from experiments carried out when diffusion occurred parallel to the stratigraphic bedding, with an apparent sensitivity to experimental conditions. Anisotropy was significant, De(HTO) perpendicular to the bedding being 1/3 lower than that parallel to the bedding. The observed fractures of the samples created by mechanical stress and partial dehydration during sawing and the presence of a pre-existing opened fracture did not affect the effective diffusion coefficients of tritiated water, which is probably due to the healing ability of the clayey medium during the re-saturation phases of the equilibrium steps performed prior to the diffusion experiments. On the contrary, a significant decrease of this transport parameter was induced by the occurrence of a pre-existing tectonic fracture, which was assigned to the dense structure of the filling phases. © 2007 Elsevier B.V. All rights reserved

Alteration of the Tournemire argillite (France) in contact with steels in natural context: experimental and modelling approaches

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