Nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity-related uranium deposits of the Kiggavik camp, Paleoproterozoic Thelon Basin, Nunavut, Canada

International audienceConcentrations of 7% U and 1% Cu were identified in massive, brecciated, and amorphous carbonaceous materials (CM) characterized by strongly negative values of carbon stable isotopes (δ13C = − 39.1‰ relative to PDB). The anomalies are restricted to clay alteration halos developed in Neoarchean Woodburn Lake group metagreywacke that is the predominant host of unconformity-related uranium (U) deposits in the Kiggavik exploration camp. Petrographic and microstructural analyses by SEM, X-Ray Diffraction, HR-TEM and RAMAN spectroscopy identified carbon veils, best described as graphene-like carbon, upon which nano-scale uraninite crystals are distributed. CM are common in U systems such as the classic Cretaceous roll-front deposits and the world-class Paleoproterozoic unconformity-related deposits. However, the unusual spatial and textural association of U minerals and CM described herein raises questions on mechanisms that may have been responsible for the precipitation of the CM followed by crystallization of U oxides on its surfaces. Based on the characteristics presented herein, the CMs at Kiggavik are interpreted as hydrothermal in origin. Furthermore, the nanoscale organization and properties of these graphene-like layers that host U oxide crystallites clearly localized U oxide nucleation and growth

Sources of dissolved organic matter during storm and inter-storm conditions in a lowland headwater catchment: constraints from high-frequency molecular data

International audienceThe transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of mi-cropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochem-istry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammo-nium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events

Hydrologically driven seasonal changes in the sources and production mechanisms of dissolved organic carbon in a small lowland catchment

International audienceTo obtain better constraints on the control of seasonal hydrological variations on dissolved organic carbon (DOC) dynamics in headwater catchments, we combined hydrometric monitoring with high-frequency analyses of DOC concentration and DOC chemical composition (specific UV adsorption, 13C) in soil and stream waters during one complete hydrological cycle in a small lowland catchment of western France. We observed a succession of four hydrological periods, each corresponding to specific DOC signatures. In particular, the rise of the upland water table at the end of the rewetting period yielded to a strong increase of the specific UV absorbance (from 2.5 to 4.0 L mg C 1 m 1) and of the 13C values (from 29 to 27%) of the soil DOC. Another striking feature was the release of large amounts of DOC during reduction of soil Fe-oxyhydroxides at the end of the highflow period. Comparison of hydrometric data with DOC composition metrics showed that soils from the upland domains were rapidly DOC depleted after the rise of the water table in these domains, whereas wetland soils acted as quasi-infinite DOC sources. Results from this study showed that the composition and ultimate source of the DOC exported to the stream will depend on the period within the annual hydrological cycle. However, we found that the aromatic DOC component identified during the high-flow period will likely represent the dominant DOC component in stream waters on an annual basis, because most of the annual stream DOC flux is exported during such periods

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Author Posting. © Nature Publishing Group, 2007. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 450 (2007): 407-410, doi:10.1038/nature06273.Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales

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Les transferts de matières à la surface de la Terre : Apports de la géochimie isotopique à différentes échelles de temps

La zone critique (‘Critical zone’) est un terme scientifique relativement récent, apparu en 1998(Ashley, 1998), puis utilisé internationalement à partir de 2001 (White et al., 2015). Elle désigne lacouche superficielle de la Terre à l’interface des 4 grands réservoirs terrestres : lithosphèreatmosphère‐hydrosphère‐biosphère. Actuellement cette zone concentre aussi bien la vie sous toutesses formes, que la plupart des activités humaines (et ses conséquences). Cette zone critique pourraitaussi être appelée zone essentielle, car c’est elle qui concentre les interactions complexes entre lesdifférents compartiments de la zone qui se produisent à des échelles de temps très variables(journalières à géologiques), sur des échelles spatiales variant du micromètre au bassin versant, voireau continent

Les transferts de matières à la surface de la Terre : Apports de la géochimie isotopique à différentes échelles de temps

La zone critique (‘Critical zone’) est un terme scientifique relativement récent, apparu en 1998(Ashley, 1998), puis utilisé internationalement à partir de 2001 (White et al., 2015). Elle désigne lacouche superficielle de la Terre à l’interface des 4 grands réservoirs terrestres : lithosphèreatmosphère‐hydrosphère‐biosphère. Actuellement cette zone concentre aussi bien la vie sous toutesses formes, que la plupart des activités humaines (et ses conséquences). Cette zone critique pourraitaussi être appelée zone essentielle, car c’est elle qui concentre les interactions complexes entre lesdifférents compartiments de la zone qui se produisent à des échelles de temps très variables(journalières à géologiques), sur des échelles spatiales variant du micromètre au bassin versant, voireau continent

Le cycle de l'osmium et du rhénium pendant l'érosion himalayenne

This study allowed determination of the Os isotopic compositions of the three major Himalayan formations (TSS, HHC, LH), based on both source rocks and river sediments, which represent the erosional products of Himalayan erosion. The High Himalayan formations (TSS+HHC) have 187 0s/188 0s ratios (0.9 to 1.6) similar to that of average continental crust. In contras t, the LH includes very radiogenic Os isotopic ratios, found in black shales and some carbonates. River sediments are mixtures of three components: High Himalayan silicates, LH black shales and LH carbonates. Sediments from the Brahmaputra are less radiogenic than those from the Ganga, due to either the addition of an ophiolitic fraction or the absence of detrital carbonates in, the sediments. The origin of the radiogenic Os isotopic compositions in LH carbonates is related to Himalayan metamorphism which mobilized Re and radiogenic Os from black shales and redistributed it in carbonates. Alteration at low temperature was studied in soil profiles. The concentrations of Re and Os, as well as the Os isotopic ratios, do not vary with depth. Nevertheless, an important loss of Re and Os occurs during the earlier stages of alteration, during the passage from source rock to saprolith. Sediments from the Bengal Fan show that Himalayan erosion is the major source of detritic material, especially in the active fan. In the rest of the Bay of Bengal, the Himalayan source may be masked by secondary sources, such as the erosion of the Indo-Burman range or the Sri Lanka. There is no evidence for significant loss of Os during sediment transport or after deposition in the Fan. This implies that the contribution of Himalayan erosion to seawater is limited to that carried in dissolved form, which is not nearly sufficient to explain the increase of the 187 0s/188 0s seawater ratio during the past 16 Ma.Cette étude a permis de déterminer la composition isotopique d'Os des trois principales formations himalayennes (TSS, HHC et LH) à partir de roches sources, ainsi que celle de sédiments de rivières qui représentent le produit d'érosion de la chaîne. Les formations du Haut Himalaya ont un rapport 187 0s/188 0s comparable à celui de la croûte continentale moyenne. Par contre, le LH se distingue par des rapports radio géniques rencontrés dans deux lithologies distinctes: les schistes noirs et les marbres impurs du LH. Un mélange de trois pôles (silicates du Haut Himalaya + schistes noirs du LH + carbonates impurs du LH) permet d'expliquer les signatures des sédiments de rivières à la sortie de la chaîne ou dans le Gange. Les sédiments du Brahmapoutre montrent un rapport moins radio génique que ceux du Gange, expliqué d'une part par la présence d'affleurement d'ophiolites dans le bassin versant du Brahmapoutre et d'autre part par l'absence de carbonates résiduels. L'origine des carbonates radiogéniques dans le LH est à relier au métamorphisme himalayen qui a permis la mobilisation d'Os et de Re à partir des schistes noirs radio géniques du LH. L'altération à basse température étudiée à partir de sols a montré que les compositions isotopiques et les concentrations d'Os et de Re ne varient pas avec la profondeur. Cependant une perte importante d'Os et de Re a probablement lieu dans les premiers stades de l'altération, lors du passage de la roche saine au saprolite. Les sédiments récents du Cône du Bengale montrent que l'érosion de la chaîne himalayenne est la source majeure des sédiments, surtout dans le cône actif. Cependant, dans le reste de la Baie du Bengale, cette source himalayenne peut être masquée par des sources secondaires, comme l'érosion de la chaîne indo-birmane ou l'érosion du Sri Lanka. Il n'y a aucune preuve qu'une perte significative d'Os lors du transport ou après le dépôt dans le Cône se produise. Ceci implique que la contribution de l'érosion himalayenne au budget de l'Os marin est limitée à la phase dissoute, et donc insuffisante pour expliquer l'augmentation du rapport 187 0s/188 0s marin durant les 16 derniers Ma

Le cycle de l'osmium et du rhénium pendant l'érosion himalayenne

NANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF