Low melting temperature for calcite at 1000 bars on the join CaCO3‐H2O – some geological implications

Melting experiments of calcite were performed on the join CaCO₃‐H₂O at a pressure of 1000 bars. The system evolves to the ternary CaO‐H₂O‐CO₂ system during melting experiments. Our experiments show that partial melting of calcite begins at a low temperature, below 650 °C. Such a low partial melting temperature for carbonates revives the debate about the presence of carbonate melts in the upper crust. More specifically, the conditions for carbonate partial melting are present in carbonate host rocks undergoing contact metamorphism at high temperatures in the presence of water‐rich fluid. The presence of carbonate melts influences physical parameters such as viscosity and permeability in contact aureoles, and, furthermore, decarbonation reactions release massive amounts of CO₂.     

Modelling the poroelastoplastic behaviour of soils subjected to internal erosion by suffusion

Granular soils subjected to seepage flow may suffer suffusion, ie, a selective internal erosion. Extending the classical approach of poromechanics, we deduce a new form of the Clausius-Duhem inequality accounting for dissipation due to suffusion, and we deduce restrictions on the constitutive laws of the soil. We suggest (a) a possible coupling between the seepage forces and the suffusion kinetics and (b) an extension of an existing elastoplastic model for the skeleton mechanical behaviour. Numerical integrations of the elastoplastic model are carried out under drained axisymmetric triaxial and oedometric conditions. As a result, we prove that the extended model is able to qualitatively reproduce the suffusion induced strains and the strength reduction experimentally observed. Predictions on the oedometric behaviour of suffusive soils are also provided.     

Sedimentary record of redox-sensitive elements (U, Mn, Mo) in a transitory anoxic basin (the Thau lagoon, France)

In order to further document the relation between redox conditions and the sedimentary record of Mn, U and Mo in a transitory anoxic water basin, their distribution has been studied along two profiles in the Thau lagoon (France). Sediments and pore-water have been sampled at two contrasting sites located, respectively, in the shellfish-farming area and in the centre of the lagoon. In the shellfish-farming area, the particulate organic carbon (POC) data indicate a more rapid organic matter mineralisation compared to the centre of the lagoon. This results in a sharper redox gradient characterized by the appearance of H₂S in pore-water a few millimetres below the sediment–water interface. In the centre of the lagoon, H₂S appears at a depth of 35 cm.In both cores, sedimentary Mn is relatively depleted through out the whole sedimentary column and varies with the proportion of clay minerals. After an initial release into solution at the sediment–water interface in relation to Mn-oxide reductive dissolution, authigenic U is immobilized when sulphides appear. Despite the occurrence of anoxic conditions at the sediment–water interface at the site influenced by shellfish farming, the burial of U is reduced by bioturbation, which raises reducing sediments to the surface. In the centre of the lagoon, Mo profiles reflect continuous diffusion into pore water and immobilization at 15 cm probably in anoxic microenvironments. At shellfish farms, dissolved Mo undergoes removal with sulphides but contrary to U, sedimentary Mo does not appear to be strongly affected by bioturbation. The profile indicates an increase in the frequency of anoxia crises during the second half of the 20th century.     

Sediment density flow distribution on wave-influenced deltas

Deltas are at the transition between fluvial and marine sedimentary environments where sediment density flows are often triggered during high river discharge events, forming submarine channels and sediment waves. On wave-influenced deltas, longshore currents are particularly efficient at transporting sediment alongshore, reducing the likelihood of sediment density flows from occurring at river mouths. This study describes four deltaic sedimentary systems at different stages of their evolution on a formerly glaciated continental inner shelf of eastern Canada in order to better understand the distribution of sediment density flows on wave-influenced deltas. Three types of settings are recognized as being prone to sediment density flows: (i) in the early stages of wave-influence and on large deltas, converging longshore currents can lead to offshelf sediment transport; (ii) on wave-influenced to wave-dominated deltas, a sandy spit can re-route the river mouth and sediment density flows form where the spit intersects the delta lip; (iii) in advanced stages of wave-dominated deltas and during their demise, rocky headlands are exposed and can intersect the slope, where off-shelf sediment transport occurs. These types of sediment density flows were all characterized by debris flows or surge-type turbidity currents which have limited offshore run-out. More rarely, hyperpycnal flows form at the river mouths, especially where the river incises glaciomarine clays prone to landsliding in the river, which increases fine-grained fluvial suspended sediment concentration. Overall, these results highlight the predominance of fluvial-dominated deltas during a phase of relative sea-level fall combined with high sediment supply. However, as soon as sediment supply diminishes, wave action remobilizes sediment alongshore modifying the distribution and types of sediment density flows occurring on wave-influenced deltas.     

Characterising groundwater and surface-water interconnections using hydrogeology,hydrochemistry and stable isotopes in the Ouémé Delta,southern Benin

The Mio-Pliocene aquifer of the coastal sedimentary basin of Benin is the most exploited aquifer for water supply to the urbanised region in the southern part of the country. The population explosion is putting increasing pressure on quantitative and qualitative aspects of the groundwater resources. Preventing groundwater contamination caused by surface waters requires a thorough understanding of surface-water/groundwater interactions, especially the interactions between the Mio-Pliocene aquifer and surface waters. This study aimed to investigate the interactions between groundwater and surface waters along the major rivers (Sô River and Ouémé Stream) and brooks in the Ouémé Delta. Field campaigns identified 75 springs located in the valleys which feed the rivers, and thus maintain their base flow. The piezometric results indicated, through flow direction assessment, that the Mio-Pliocene aquifer feeds Ouémé Stream and Sô River. Chemical analyses of groundwater and surface waters show similar chemical facies, and changes in the chemical composition in groundwater are also observed in the surface waters. Moreover, the isotopic signatures of surface waters are similar to those of the groundwater and springs, which led to the identification of potential groundwater discharge areas. As a result of groundwater discharge into surface waters, the fraction of groundwater in the surface water is more than 66% in the brooks, regardless of the season. In the Ouémé Stream and Sô River, the fraction of groundwater is 0–21% between June and September, while from October to March it is 47–100%.

     

Personalized sightseeing tours: a model for visits in art museums

This article describes a method to provide adapted visit tours in art museums according to the preferences expressed by the visitor and exhibits prestige. It is based on a dual approach with, on the one hand an automatic textual analysis of the official information available online (labels of exhibits) that allows to rank the exhibit attractiveness for a standard museum visitor. On the other hand, individual preferences are also taken into account to adapt the visit according to the personal cultural awareness of the visitor. We use operations research to solve a routing optimization problem, aiming at finding a visit tour with time constraints and maximization of the visitor satisfaction. Depending on the instance size and the problem scale, an integer linear programming (ILP) model and a greedy algorithm are proposed to recommend personalized visit tours and applied on two museums: ‘Musée de l’Orangerie’ in Paris and ‘National Gallery’ in London. The obtained results show that it is possible to recommend a good tour to visitors of an art museum by taking into account the common prestige of the exhibits and the individual interests, joining automatic text summarization and routing optimization in a limited geographical space.     

Cosmogenic Be dating of a sackung and its faulted rock glaciers, in the Alps of Savoy (France)

Sackungs are the largest gravitational deformation observed in mountains. They are characterized by the long-term slowness of the movements, but their mechanism is still not well understood. Nowadays cosmic ray exposure (CRE) dating methods allow dating the morphologic structures involved in sackung and can contribute at the understanding of their origin. In the Alps, the 5.3 km long Arcs sackung initiated during the activity of rock glaciers. Three samples from these faulted rock glaciers provide their first CRE ages and show that at 2000 m elevation these block accumulations moved during the Younger Dryas and stopped in early Holocene. Six ¹⁰Be ages of fault scarps show that the Arcs sackung lasted only a few thousand years and stopped at about 8462 ± 432 ¹⁰Be yr. They also reveal that deformation migrated upslope in agreement with a mechanism of flexural toppling of vertical layers. This unique and long gravitational event, characterized by migration of the deformation, does not support earthquake shaking as triggering mechanism for individual faults. It shows that, in the upper Isère valley, slope deformation was delayed of several thousand years after glacial debuttressing, and is not anymore active despite its fresh morphology.     

Structure-from-motion,multi-view stereo photogrammetry applied to line-scan sediment core images

Journal of Paleolimnology - Images of sediment cores are often acquired to preserve primary color information, before such profiles are altered by subsequent sampling and destructive analyses. In...