The Gomi legacy

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Significance of the Sm-Nd isotopic systematics of the Akilia Association

Samarium-Neodymium analyses were carried out on fourteen samples of basic to ultrabasic metavolcanics from several enclaves of the Amitsoq gneisses (T = to or approximately 3,700 Ma). Field observations suggest that all the analyzed rocks belong to the pre-Amitsoq Akilia Association. Consequently, a minimum age of 3,700 Ma is postulated for the emplacement of their protoliths. When all the data points are put together in a conventional isochron diagram, no clear isochron relationship can be discerned. However, the points seem to fall within a band broadly corresponding to an age of 3,600 Ma. The isotopic results are difficult to interpret satisfactorily. Two contrasting interpretations are offered and summarized: (1) data scatter as a result of open system behavior; and (2) data scatter due to a melange of data sets defining two distinct isochrons

Dosage du carbone organique dissous dans les eaux douces naturelles. Intérêt, Principe, Mise en Oeuvre et Précautions Opératoires

International audienceCe mémoire présente un travail de validation du dosage du carbone organique dissous(COD) et du carbone inorganique (CI) contenus dans les eaux douces naturelles développé surl'analyseur Shimadzu©, modèle TOC 5050A.Les limites de détection calculées pour cet analyseur avec un catalyseur de sensibilitédite normale sont de 0.31 ppm pour le COD et de 0.10 ppm pour le CI, respectivement. Leslimites de quantification sont logiquement plus élevées: 0.41 ppm (COD) et 0.13 ppm (CI).Ces résultats permettent de conclure que le catalyseur dit de "sensibilité normale" qui équipeen routine cet analyseur est adapté à l'analyse de la plupart des eaux douces naturelles.Les tests d'étalonnage effectués ont montré que l'appareil Shimadzu© TOC 5050A étaittrès stable dans le temps, les dérives constatées étant inférieures à 5%. De ce fait, uneprocédure allégée d'étalonnage comprenant l'injection d'un seul point de gamme en début dechaque série d'échantillons suffit à garantir une bonne justesse des résultats, même si celle-ciest difficile à quantifier du fait de l'absence de solutions standard certifiées.Les trois méthodes susceptibles d'être mises en oeuvre par l'analyseur Shimadzu©, TOC5050A (COD = CT – CI; NPOC, NPIW) ont été testées. Seules la méthode COD = CT – CIpermet de doser la totalité du compartiment "matière organique dissoute" des eaux doucesnaturelles. Les deux autres ne permettent pas d'appréhender les molécules les plus volatiles,les écarts entre valeurs "vraies" et valeurs "mesurées" pouvant aller jusqu'à 25%.Des expériences visant à tester les modalités de préparation et de conservation deséchantillons ont également été effectuées. Les résultats montrent que pour des eaux peuchargées en matière organique (COD 5%) entre valeurs mesurées et valeurs vraies. Dans ce cas,l'opérateur soucieux de produire des résultats justes sera conduit à filtrer les eaux directementsur le terrain et à réduire au maximum l'intervalle de temps entre le prélèvement et l'analyse

A neural compiler

AbstractThis paper describes a neural compiler. The input of the compiler is a PASCAL Program. The compiler produces a neural network that computes what is specified by the PASCAL program. The compiler generates an intermediate code called cellular code

Rare earth elements complexation with humic acid

The binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE­HA complexation experiments were performed at various pH conditions (ranging from 2 to 10.5) using a standard batch equilibration method. Results show that the amount of REE bound to HA strongly increases with increasing pH. Moreover, a Middle-REE (MREE) downward concavity is evidenced by REE distribution patterns at acidic pH. Modelling of the experimental data using Humic Ion Binding Model VI provided a set of log KMA values (i.e., the REE­HA complexation constants specific to Model VI) for the entire REE series. The log KMA pattern obtained displays a MREE downward concavity. Log KMA values range from 2.42 to 2.79. These binding constants are in good agreement with the few existing datasets quantifying the binding of REE with humic substances but quite different from a recently published study which evidence a lanthanide contraction effect (i.e., continuous increase of the constant from La to Lu). The MREE downward concavity displayed by REE­HA complexation pattern determined in this study compares well with results from REE­fulvic acid (FA) and REE­acetic acid complexation studies. This similarity in the REE complexation pattern suggests that carboxylic groups are the main binding sites of REE in HA. This conclusion is further illustrated by a detailed review of published studies for natural, organic-rich, river- and ground-waters which show no evidence of a lanthanide contraction effect in REE pattern. Finally, application of Model VI using the new, experimentally determined log KMA values to World Average River Water confirms earlier suggestions that REE occur predominantly as organic complexes (= 60%) in the pH range between 5­5.5 and 7­8.5 (i.e., in circumneutral pH waters). The only significant difference as compared to earlier model predictions made using estimated log KMA values is that the experimentally determined log KMA values predict a significantly higher amount of Light-REE bound to organic matter under alkaline pH conditions

Colloidal Control on the Distribution of Rare Earth Elements in Shallow Groundwaters

International audienceA 7-year monitoring period of rare earth element (REE) concentrations and REE pattern shapes was carried out in well water samples from a 450 m long transect setup in the Kervidy/Coët-Dan experimental catchment, France. The new dataset confirms systematic, topography-related REE signatures and REE concentrations variability but challenges the validity of a groundwater mixing hypothesis. Most likely, this is due to REE preferential adsorption upon mixing. However, the coupled mixing­adsorption mechanism still fails to explain the strong spatial variation in negative Ce anomaly amplitude. A third mechanismƒnamely, the input into the aquifer of REE-rich, Ce anomaly free, organic colloidsƒis required to account for this variation. Ultrafiltration results and speciation calculations made using Model VI agree with this interpretation. Indeed, the data reveal that Ce anomaly amplitude downslope decrease corresponds to REE speciation change, downhill groundwaters REE being mainly bound to organic colloids. Water table depth monitoring shows that the colloid source is located in the uppermost, organic-rich soil horizons, and that the colloid input occurs mainly when water table rises in response to rainfall events. It appears that the colloids amount that reaches groundwater increases downhill as the distance between soil organic-rich horizons and water table decreases. Topography is, therefore, the ultimate key factor that controls Ce anomaly spatial variability in these shallow groundwaters. Finally, the <0.2 µm REE fraction ultimately comes from two solid sources in these groundwaters: one located in the deep basement schist; another located in the upper, organic-rich soil horizon

Organo-colloidal control on major- and trace element partitioning in shallow groundwaters : confronting ultrafiltration and modelling

International audienceUltrafiltration experiments using new small ultracentifugal filter devices were performed at different pore size cut-offs to allow the study of organo-colloidal control on metal partitioning in water samples. Two shallow, circumneutral pH waters from the Mercy site wetland (western France) were sampled: one dissolved organic carbon (DOC)- and Fe-rich and a second DOC-rich and Fe-poor. Major- and trace-element cations and DOC concentrations were analysed and data treated using an ascendant hierarchical classification method. This reveals the presence of three groups: (i) a "truly" dissolved group (Na, K, Rb, Ca, Mg, Ba, Sr, Si and Ni); (ii) an inorganic colloidal group carrying Fe, Al and Th; and (iii) an organic colloidal group enriched in Cr, Mn, Co, Cu and U. However, REE and V have an ambivalent behaviour, being alternatively in the organic pool and in the inorganic pool depending on sample. Moreover, organic speciation calculation using Model VI were performed on both samples for elements for which binding constants were available (Ca, Mg, Ni, Fe, Al, Th, Cr, Cu, Dy, Eu). Calculation shows relatively the same partitioning of these elements as ultrafiltration does. However, some limitations appear such as (i) a direct use of ultrafiltration results which tends to overestimate the fraction of elements bound to humic material in the inorganic pool as regards to model calculations as well as, (ii) a direct use of speciation calculation results which tends to overestimate the fraction of elements bound to humic material in the organic pool with regard to ultrafiltration results. Beside these limitations, one can consider that both techniques, ultrafiltration and speciation calculation, give complementary information, especially for more complex samples where inorganic and organic colloids compete

Uncertainties in assessing annual nitrate loads and concentration indicators. Part 1: Impact of sampling frequency and load estimation alogorithms

International audienceThe objectives of this study are to evaluate the uncertainty in annual nitrate loads and concentrations (such as annual average and median concentrations) as induced by infrequent sampling and by the algorithms used to compute fluxes. A total of 50 watershed-years of hourly to daily flow and concentration data gathered from nine watersheds (5 to 252 km2) in Brittany, France, were analyzed. Original (high frequency) nitrate concentration and flow data were numerically sampled to simulate common sampling frequencies. Annual fluxes and concentration indicators calculated from the simulated samples were compared to the reference values calculated from the high-frequency data. The uncertainties contributed by several algorithms used to calculate annual fluxes were also quantified. In all cases, uncertainty increased as sampling intervals increased. Results showed that all the tested algorithms that do not use continuous flow data to compute nitrate fluxes introduced considerable uncertainty. The flowƒ]weighted average concentration ratio method was found to perform best across the 50 annual datasets. Analysis of the bias values suggests that the 90th and 95th percentiles and the maximum concentration values tend to be systematically underestimated in the long term, but the load estimates (using the chosen algorithm) and the average and median concentrations were relatively unbiased. Great variability in the precision of the load estimation algorithms was observed, both between watersheds of different sizes and between years for a particular watershed. This has prevented definitive uncertainty predictions for nitrate loads and concentrations in this preliminary work, but suggests that hydrologic factors, such as the watershed hydrological reactivity, could be a key factor in predicting uncertainty levels

Assessment of vanadium distribution in shallow groundwaters

International audienceShallow groundwater samples (filtered at 0.2 μm) collected from a catchment in Western France (Petit Hermitage catchment) were analyzed for their major- and trace-element concentrations (Fe, Mn, V, Th and U) as well as their dissolved organic carbon (DOC) concentrations, with the aim to investigate the controlling factors of vanadium (V) distribution. Two spatially distinct water types were previously recognized in this catchment based on variations of the rare earth element (REE) concentrations. These include: (i) DOC-poor groundwater flowing below the hillslope domains; this type has low V contents; and (ii) DOC-rich groundwater originating from wetlands, close to the river network; the latter water type displays much higher V concentrations. The temporal variation of the V concentration was also assessed in the wetland waters; the results show a marked increase in the V content at the winter-spring transition, along with variations in the redox potential, and DOC, Fe and Mn contents. In order to allow the study of organo-colloidal control on V partitioning in water samples, ultrafiltration experiments were performed at different pore size cut-offs (30 kDa, 10 kDa and 5 kDa). Two shallow, circumneutral waters were sampled: one was both DOC- and Fe-rich and the other was DOC-rich and Fe-poor. In terms of major- and trace-cations and DOC concentrations, the data were processed using an ascendant hierarchical classification method. This revealed the presence of two main groups: (i) a "truly" dissolved group (Na, K, Rb, Ca, Mg, Ba, Sr, Si, Mn, Co, Ni, Cr, Zn and Ni), and (ii) a colloidal group carrying DOC, Fe, Al, Pb, Cu, REE, U, Th and V. Vanadium has an unpredictable behavior; it can be either in the organic pool or in the inorganic pool, depending on the sample. Moreover, V speciation calculations--using Model VI and SCAMP--were performed on both samples. Speciation modeling showed approximately the same partitioning feature of these elements as compared to ultrafiltration data, namely: a slight change of the V speciation in groundwaters along the studied topographic sequence. This implies that vanadium in hillslope groundwater wells occurs as a mixing of organic and inorganic complexes, whereas V in wetland groundwater wells comprises mainly organic species. Using the dataset described above, factors such as aquifer-rock composition or anthropogenic input were demonstrated to probably play a minor role in determining the V distribution in shallow groundwaters. Although an anthropogenic impact can be ruled out at this local scale, we cannot preclude a perturbation in the global V cycle. Most likely, the two dominant factors involved are the organic matter content and the redox state either promoting competition with Fe-, Mn-oxides as V carriers in groundwater or not. In this context, it appears challenging to determine whether organic matter or redox-sensitive phases are the major V carriers involved, and a further study should be dedicated to clarify this partition, notably to address the processes affecting large-scale V transport