Global effect of irrigation and its impact on the onset of the Indian summer monsoon

In a context of increased demand for food and of climate change, the water consumptions associated with the agricultural practice of irrigation focuses attention. In order to analyze the global influence of irrigation on the water cycle, the land surface model ORCHIDEE is coupled to the GCM LMDZ to simulate the impact of irrigation on climate. A 30-year simulation which takes into account irrigation is compared with a simulation which does not. Differences are usually not significant on average over all land surfaces but hydrological variables are significantly affected by irrigation over some of the main irrigated river basins. Significant impacts over the Mississippi river basin are shown to be contrasted between eastern and western regions. An increase in summer precipitation is simulated over the arid western region in association with enhanced evapotranspiration whereas a decrease in precipitation occurs over the wet eastern part of the basin. Over the Indian peninsula where irrigation is high during winter and spring, a delay of 6?days is found for the mean monsoon onset date when irrigation is activated, leading to a significant decrease in precipitation during May to July. Moreover, the higher decrease occurs in June when the water requirements by crops are maximum, exacerbating water scarcity in this region. A significant cooling of the land surfaces occurs during the period of high irrigation leading to a decrease of the land-sea heat contrast in June, which delays the monsoon onset.     

Study of the initiation and propagation of excavation damaged zones around openings in argillaceous rock

The study of the creation and evolution of the excavation disturbed zone (EDZ) in argillaceous rocks is a major issue for the safety of nuclear wastes underground repositories. In this context, the argillaceous Tournemire site has provided a unique opportunity to study the evolution of the EDZ with time thanks to the existence of three openings of different ages. A thorough characterization of the EDZ has been conducted by different means such as visual observation, analysis of samples extracted from drilled boreholes, EDZ permeability measurements, etc. On the basis of these measurements, a conceptual model of the EDZ initiation and propagation at the Tournemire site has been proposed. In order to validate this model, numerical simulations of increasing complexity have been carried out. In a first attempt, the response of the rock mass to the excavation phase, followed by seasonal cyclic variations of temperature and relative humidity inside the opening, has been simulated by means of a purely mechanical analysis, using a simple elastic material model. The EDZ has been estimated by post-processing the calculated stress states, using a Mohr–Coulomb failure criterion. The results obtained show that no EDZ could be predicted unless adopting a low cohesion value for the rock mass. Moreover, the deferred nature of the EDZ formation in Tournemire could not be reproduced. These limitations have then been suppressed by using a coupled viscoplastic-damaging mechanical model, the parameters of which have been identified from different laboratory experiments. With this model, a time evolution of the EDZ could be predicted, but the EDZ pattern could not match the one observed in situ. Finally, in view of the importance of the hydraulic couplings, unsaturated hydro-mechanical calculations have been carried out to investigate the effect of the numerous seasonal variations cycles and the resulting shrinkage.     

Comparison of several BHT correction methods: a case study on an Australian data set

Bottom-hole temperatures (BHT) from oil exploration provide useful constraints on the subsurface thermal regime, but they need to be corrected to obtain the equilibrium temperature. In this work, we introduce several BHT correction methods and compare them using a large Australian data set of more than 650 groups of multiple BHT measurements in about 300 oil exploration boreholes. Existing and suggested corrections are classified within a coherent framework, in which methods are divided into: line/cylinder source; instantaneous/continuous heat extraction; one/two component(s). Comparisons with reservoir test temperatures show that most of the corrections lead to reliable estimates of the formation equilibrium temperature within ±10 °C, but too few data exist to perform an intercomparison of the models based on this criterion. As expected, the Horner method diverges from its parent models for small elapsed times (or equivalently large radii). The mathematical expression of line source models suffers from an unphysical delay time that also restrains their domain of applicability. The model that takes into account the difference of thermal properties between circulating mud and surrounding rocks—that is the two-component model—is delicate to use because of its high complexity. For these reasons, our preferred correction methods are the cylindrical source models. We show that mud circulation time below 10 hr has a negligible effect. The cylindrical source models rely on one parameter depending on the thermal diffusivity and the borehole radius, which are poorly constrained, but the induced uncertainty on the extrapolations remains reasonably low.     

Typologie et mode de mise en place des roches magmatiques dans les Pyrénées hercyniennes

Résumé L'évolution géodynamique hercynienne des Pyrénées se caractérise par une importante activité magmatique à l'origine de corps plutomques de taille, de composition et de niveaux de mise en place différents. A partir de ces caractéristiques nous avons pu établir une typologie qui nous a permis de distinguer deux types principaux de massifs: 1) les massifs de composition homogène, acides (massifs de type HA) ou basiques (massifs de type HB), localisés dans les grands ensembles métamorphiques régionaux et 2) les massifs de composition hétérogène, formés par l'association de roches basiques et de roches acides dont la répartition spatiale a permis de distinguer les massifs de type CA (roches acides au coeur du massif), les massifs de type CB (roches basiques au coer du massif) et les massifs de type CR (récurrences de lentilles de roches basiques dans un massif zoné de type CA ou CB), mis en place dans le Paléozoïque supérieur généralement loin des grands ensembles métamorphiques. Tous ces massifs se sont mis en place diapiriquement au cours du métamorphisme régional et de la déformation hercynienne majeure.

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One of the main characteristics of the Hercynian geodynamic evolution of the Pyrenees is an intense magmatic activity producing large plutonic bodies. They have different sizes, various compositions and distinct emplacement levels. From these characteristics, a typology for these massifs is defined. Two main types of massifs have been recognized: 1) massifs with an homogenous composition founded in large regional metamorphic areas, and 2) massifs with an heterogenous composition of acid and basic rocks, whose spatial disposition permits the recognition of three sub-types: a) the CA type where acid rocks are at the core of the massif; b) the CB type where basic rocks are at the core of the massif, and c) the CR type where numerous basic xenoliths are scattered within either a CA or a CB type massif. These heterogenous massifs generally occur within Upper Paleozoic rocks and usually far away from regional metamorphic areas. All of these massifs were emplaced diapirically during the regional metamorphism and the main Hercynian deformation.

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Zusammenfassung Ein wichtiges charakteristisches Merkmal der geodynamischen Entwicklung der Pyrenäen während der herzynischen Phase ist eine hohe magmatische Aktivität, auf die Plutone größerer Ausdehnung zurückgeführt werden. Größe, Mineralogie und Erstarrungstiefe der Plutone variieren. Anhand dieser Charakteristika können die Intrusivkörper in zwei Hauptgruppen differenziert werden. Die erste Gruppe (1) sind Plutone homogener Zusammensetzung in regionalmetamorph überprägten Gebieten. Die zweite sind heterogen zusammengesetzte Plutone (2) saurer und basischer Magmatite, deren räumliche Anordnung eine Untergliederung in drei Untergruppen ermöglicht:a) Bei dem CA-Typ befinden sich saure Gesteine im Kern; bei b), dem CB-Typ, liegen basische Gesteine im Kern vor und bei c), einem CR genannten Typ, sind zahlreiche basische Xenolithe entweder in dem CA-oder dem CB-Typ verstreut vorhanden. Diese heterogenen Plutone findet man allein in spätpaläozoischen Gesteinen und in der Regel weit entfernt von Gebieten, die metamorph beeinflußt wurden. Sämtliche Plutone haben eine Diapir-ähnliche Form und intrudierten zeitgleich mit der Regionalmetamorphose und der Hauptphase der herzynischen Deformation.

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Application of a novel cascade-routing and reinfiltration concept with a Voronoi unstructured grid in MODFLOW 6, for an assessment of surface-water/groundwater interactions in a hard-rock catchment (Sardon,Spain)

Hydrogeology Journal - Integrated hydrological modelling (IHM) can reliably characterize surface-water/groundwater interactions in complex hydrological systems such as hard-rock systems (HRS),...     

Oxygen isotopic ratios in fine quartz silt from sediments and soils of southern Africa

Fine quartz silt (1–10μm dia, important in aerosol dust) isolated from a large number of soils, as well as Phanerozoic shales, sandstones and recent sands of the southern African stratigraphie column (Carboniferous, Permian and Triassic) exhibits a mean δ¹⁸O of 12.2 ± 2.1%0. These values are similar to those reported for South Pacific pelagic sediments and adjacent land areas of similar latitude but are lower than those reported for this size fraction of 30 Mesozoic and Paleozoic shales (20.7 ± 1.8%.) and soils from mid-continental U.S.A., for aerosol quartz in the North Pacific pelagic sediments, and for eolian caps in high-elevation soils of Hawaii. The lower oxygen isotopic ratios in fine quartz silt of southern Africa reinforces the earlier indication that Southern Hemisphere detritai sedimentary reservoirs contain a higher proportion of igneous and metamorphic quartz (lower δ¹⁸O) and less lowtemperature authigenic quartz (higherδ¹⁸O) than Northern Hemisphere detritai sediments. The difference reflects climates, as continents drifted in latitude.     

Pliocene lahar deposits in the Coastal Cordillera of central Chile: Implications for uplift, avalanche deposits, and porphyry copper systems in the Main Andean Cordillera

Lahar deposits occur within a shallow marine sedimentary succession of the Pliocene La Cueva Formation in the Coastal Cordillera of central Chile (33°40′–34°15′S). Provenance studies of the abundant volcanic material in the lahar deposits suggest that they derive from denudation by mass wasting of Oligocene–Miocene volcanic rocks on the western slopes of the Main Andean Cordillera at the same latitude. Pliocene rock debris deposits preserved in the region of El Teniente (34°S) and scattered along the westernmost part of the Andes of central Chile indicate catastrophic erosive events related to the rapid uplift of the cordilleran block. This rock debris was deposited by avalanches and transformed further downslope into lahars by dilution with stream water. Lahars were channeled along the ancient drainage system that reached a shallow Pliocene sea at the site of the present Coastal Cordillera. The exceedingly rapid exhumation of active porphyry systems during the Early Pliocene in this part of the Andes may have played a role in affecting hydrothermal processes, brecciation, and diatreme formation at the porphyry systems of El Teniente and Río Blanco–Los Bronces.     

Erosion balance in the watersheds of the western Paris Basin by high-frequency monitoring of discharge and suspended sediment in surface water

An accurate quantification of erosion, based on high-frequency monitoring of river discharge and suspended sediment fluxes is proposed for two watersheds in the western Paris Basin, a sensitive area with respect to erosion phenomena. This continuous monitoring makes it possible to include flood events of short duration, but significant erosion potential. The obtained erosion rate (16 and 21 t?km^?2?yr^?1) is among the weakest of the planet (3.5 to 18?000 t?km^?2?yr^?1). However, this annual balance does not reflect the behaviour of these rivers which can be torrential in certain cases. To cite this article: B. Laignel et al., C. R. Geoscience 338 (2006).     

Generation of CO2-rich melts during basalt magma ascent and degassing

To test mechanisms of basaltic magma degassing, continuous decompressions of volatile-bearing (2.7–3.8 wt% H₂O, 600–1,300 ppm CO₂) Stromboli melts were performed from 250–200 to 50–25 MPa at 1,180–1,140 °C. Ascent rates were varied from 0.25 to ~1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubble-free to bubble-rich, the latter with bubble number densities from 10⁴ to 10⁶ cm^?3, similar to Stromboli pumices. Vesicularities range from 0 to ~20 vol%. Final melt H₂O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt CO₂ concentration. High vesicularity charges have glass CO₂ concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with CO₂ concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass CO₂ concentration gradients near the gas–melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas–melt interfaces) and the volatile diffusion distance, control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of CO₂ and H₂O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gas–melt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that CO₂-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of CO₂-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism.