67 research outputs found

    Gas emissions due to magma-sediment interactions during flood magmatism at the Siberian Traps: gas dispersion and environmental consequences.

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    International audienceWe estimate the fluxes of extremely reduced gas emissions produced during the emplacement of the Siberian Traps large igneous province, due to magma intrusion in the coaliferous sediments of the Tunguska Basin. Using the results of a companion paper (Iacono-Marziano et al. submitted to EPSL), and a recent work about low temperature interaction between magma and organic matter (Svensen et al., 2009), we calculate CO-CH4-dominated gas emission rates of 7Ă—1015-2Ă—1016 g/yr for a single magmatic/volcanic event. These fluxes are 7 to 20 times higher than those calculated for purely magmatic gas emissions, in the absence of interaction with organic matter-rich sediments. We investigate, by means of atmospheric modelling employing present geography of Siberia, the short and mid term dispersion of these gas emissions into the atmosphere. The lateral propagation of CO and CH4 leads to an important perturbation of the atmosphere chemistry, consisting in a strong reduction of the radical OH concentration. As a consequence, both CO and CH4 lifetimes in the lower atmosphere are enhanced by a factor of at least 3, at the continental scale, as a consequence of 30 days of magmatic activity. The short-term effect of the injection of carbon monoxide and methane into the atmosphere is therefore to increase the residence times of these two species and, in turn, their capacity of geographic expansion. The estimated CO and CH4 volume mixing ratios (i.e. the number of molecules of CO or CH4 per cm3, divided by the total number of molecules per cm3) in the low atmosphere are 2-5 ppmv at the continental scale and locally higher than 50 ppmv. The dimension of the area affected by these high volume mixing ratios decreases in the presence of a lava flow accompanying magma intrusion at depth. Complementary calculations for a 10-year duration of the magmatic activity suggest (i) an increase in the mean CH4 volume mixing ratio of the whole atmosphere up to values 3 to 15 times higher than the current one, and (ii) recovery times of 100 years to bring back the atmospheric volume mixing ratio of CH4 to the pre-magmatic value. Thermogenic methane emissions from the Siberian Traps has already been proposed to crucially contribute to end Permian-Early Triassic global warming and to the negative carbon isotopic shift observed globally in both marine and terrestrial sediments. Our results corroborate these hypotheses and suggest that concurrent high temperature CO emissions also played a key role by contributing to increase (i) the radiative forcing of methane and therefore in its global warming potential, and (ii) the input of isotopically light carbon into the atmosphere that generated the isotopic excursion. We also speculate a poisoning effect of high carbon monoxide concentrations on end-Permian fauna, at a local scale

    Extremely reducing conditions reached during basaltic intrusion in organic matter-bearing sediments

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    International audienceRedox conditions in magma are widely interpreted as internally buffered and closely related to that of their mantle source regions. We use thermodynamic calculations to show that high-temperature interaction between magma and organic matter can lead to a dramatic reduction of the magma redox state, and significant departure from that of the original source. Field studies provide direct evidence of the process that we describe, with reported occurrences of graphite and native iron in igneous mafic rocks, implying very reducing conditions that are almost unknown in average terrestrial magmas. We calculate that the addition of 0.6 wt% organic matter (in the form of CH or CH2) to a standard basalt triggers graphite and native iron crystallisation at depths of few hundred meters. Interaction with organic matter also profoundly affects the abundance and the redox state of the gases in equilibrium with the magma, which are CO-dominated with H2 as the second most abundant species on a molar basis, H2O and CO2 being minor constituents. The assimilation of only 0.1 wt% organic matter by a basalt causes a decrease in its oxygen fugacity of 2-orders of magnitude. The assimilation of 0.6 wt% organic matter at depths < 500 m implies minimum CO content in the magma of 1 wt%, other gas components being less than 0.1 wt%. In the light of our calculations, we suggest that the production of native iron-bearing lava flows and associated intrusions was most likely accompanied by degassing of CO-rich gases, whose fluxes depended on the magma production rates

    The chemistry of bromine in the stratosphere: Influence of a new rate constant for the reaction BrO + HO2

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    The impact of new laboratory data for the reaction BrO + HO2 yields HOBr + O2 in the depletion of global stratospheric ozone has been estimated using a one-dimensional photochemical model taking into account the heterogeneous reaction on sulphate aerosols which converts N2O5 into HNO3. Assuring an aerosol loading 2 times as large as the 'background' and a reaction probability of 0.1 for the above heterogeneous reaction, the 6 fold increase in the measured rate constant for the reaction of BrO with HO2 increases the computed depletion of global ozone produced by 20 ppt of total bromine from 2.01 percent to 2.36 percent. The use of the higher rate constant increases the HOBr mixing ratio and makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction

    Validation and data characteristics of methane and nitrous oxide profiles observed by MIPAS and processed with Version 4.61 algorithm

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    The ENVISAT validation programme for the atmospheric instruments MIPAS, SCIAMACHY and GOMOS is based on a number of balloon-borne, aircraft, satellite and ground-based correlative measurements. In particular the activities of validation scientists were coordinated by ESA within the ENVISAT Stratospheric Aircraft and Balloon Campaign or ESABC. As part of a series of similar papers on other species [this issue] and in parallel to the contribution of the individual validation teams, the present paper provides a synthesis of comparisons performed between MIPAS CH4 and N2O profiles produced by the current ESA operational software (Instrument Processing Facility version 4.61 or IPF v4.61, full resolution MIPAS data covering the period 9 July 2002 to 26 March 2004) and correlative measurements obtained from balloon and aircraft experiments as well as from satellite sensors or from ground-based instruments. In the middle stratosphere, no significant bias is observed between MIPAS and correlative measurements, and MIPAS is providing a very consistent and global picture of the distribution of CH4 and N2O in this region. In average, the MIPAS CH4 values show a small positive bias in the lower stratosphere of about 5%. A similar situation is observed for N2O with a positive bias of 4%. In the lower stratosphere/upper troposphere (UT/LS) the individual used MIPAS data version 4.61 still exhibits some unphysical oscillations in individual CH4 and N2O profiles caused by the processing algorithm (with almost no regularization). Taking these problems into account, the MIPAS CH4 and N2O profiles are behaving as expected from the internal error estimation of IPF v4.61 and the estimated errors of the correlative measurements

    Relevance of oxygen and ozone detections in the search for primitive life in extra solar planets

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    International audienceConsidering the future importance of the search for evidences of primitive life on a distant planet, we have revisited some points of the O-2 and O-3 detection criteria.The budget of free oxygen and organic carbon on Earth is studied. If one includes the organic carbon in sediments, it confirms that O-2 is a very reactive gas whose massive presence in a telluric planet atmosphere implies a continuous production. Its detection would be a strong indication for photosynthetic activity, provides the planet is not in a runaway green-house phase.In principle, the direct detection of O-2 could be possible in the visible flux of the planet at 760 nm (oxygen A-band) but it would be extremely difficult, considering the much larger flux from the star. The alternative search for the 9.7 mu m absorption of O-3 may be easier as the contrast with the star is increased by 3 orders of magnitude. A simple atmospheric model indicates that the O-3 column density is not a linear tracer of the atmospheric O-2 content. However, the detection of a substantial O-3 absorption (tau > 25%) would indicate, within the validity of this model, a O-2 ground pressure larger than 10 mbar. The question is raised of whether this pressure is sufficient to indicate a photosynthetic origin of the oxygen. If the answer was positive, it would be an even more sensitive test of photosynthetic activity than the detection of the oxygen A-band. Further studies of these points are clearly needed before determining an observation strategy

    Modélisation du transport d'espèces chimiques en période convective pour l'étude de la haute troposphère tropicale en Amérique du Sud

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    De nombreux travaux indiquent qu il est important d étudier les impacts physico-chimiques de la convection profonde tropicale. Nous avons utilisé le modèle méso-échelle 3D non-hydrostatique CATT-BRAMS pour étudier le transport de traceurs dans la troposphère tropicale au-dessus de l Amérique du Sud. J ai effectué une validation de l outil en comple ment d une étude en saison sèche et dans les basses couches menée au CPTEC (Brésil). Les résultats obtenus dans ces travaux indiquent un comportement météorologique globalement correct. Le transport en résultant montre une sur-estimation du transport d espèces chimiques dans la moyenne troposphère et une sous-estimation dans la haute troposphère. Cela vient d un déclenchement trop fréquent de la convection restreinte, de la paramétrisation de la convection profonde et de la représentation de leurs interactions. Une adaptation du modèle pour la saison humide est nécessaire. A l échelle locale des difficultés venant d une sensibilité importante de la paramétrisation au relief sont rencontrées. Le CATT-BRAMS évolue vers un modèle avec chimie, le C-CATT-BRAMS. Les premiers résultats obtenus indiquent un fort impact de l initialisation et des conditions aux limites pour les espèces NO et O3. Quelques soient l initialisation ou les conditions aux limites utilisées, on observe une augmentation du rapport de mélange de ces espèces au cours du temps. Cela peut provenir d une sur-estimation des émissions a la surface dans le modèle, en particulier pour les méga-cités. Il est important de poursuivre la validation de cet outil afin de pouvoir étudier l impact physico-chimique de la convection profonde tropicale avec ce modèle.Many works show it is important to study the phyical and chemical impacts of tropical deep convection. We used the 3D mesoscale non-hydrostatic model CATT-BRAMS to study the tracers transport in the tropical troposphere above South America. I validated the tool parallel to a study done in CPTEC (Brazil) for the dry season and in the lower troposphere. The results obtained in this work indicate a globaly correct meteorological behaviour. The associated transport show an over estimation of the chemical species transport in mid-troposphere and an under estimation in the upper troposphere.This comes from a to frequent triggering of shallow convection, from the deep convection scheme and from the representation of their interactions. An adaptation of the model for the wet season is necessary. At local scale, difficulties because of a high deep convection scheme sensitivity to the orography are encountered. The CATT-BRAMS model evolve to a model with chemistry, the C-CATT-BRAMS. The first results obtained indicate a strong impact of initialisation and boudary conditions on species NO and O3. Whatever be the initialisation or the boundary conditions, we observe an increase of the mixing ratio along time for these species. This can come from an over estimation of surface emissions in the model, especialy for megacities. It is important to continue the validation of this tool in order to be able to study the physical and chemical impacts of tropical deep convection with this model.ORLEANS-SCD-Bib. electronique (452349901) / SudocSudocFranceF

    A method of measuring the total D.C. electric field in the vicinity of a spacecraft using artificially injected charged particles

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    (ENG) A method of measuring both the perpendicular and parallel D.C. electric field in the vicinity of a spacecraft using artificially injected . charged particles is presented. This method uses a rendezvous between the spacecraft and injected particles after a number of complete gyrations around the magnetic field plus one partial gyration, to measure the field and potential structure at short distances (up to few kilometers or tenth of kilometers) from the spacecraft. The solutions of the rendezvous problem are derived in the general case, and in realistic cases related to possible potential structure due for example to anomalous resistivity and electrostatic shocks. The charac- teristics of the measurements are evaiuated,in particular the echo delay time of the particles (and hence the time resolution) and the sensitivity (i.e. the minimum electric field, the extent and the magnitude of the potential measurable). Among other, the minimum parallel electric field ( v 1 mV/m) and the minimum time resolution ( v 20 ms) are found to the very suitable to describe potential structures associated with anomalous resistivity and electrostatic shocks. Finally the requirements for guns and detectors are eval uated i.e. their orien- tations, the beam intensity and the detectors time resolution and sensitivity. (FRE) On présente une méthode de mesure des composantes perpen- diculaire et parallèle du champ électrique continu au voisinage d'un engin spatial, utilisant des particules chargées injectées artificiellement. La méthode utilise le rendez-vous entre un engin spatial et des particules injectées effec- tuant un nombre entier de rotations autour du champ magnétique plus une rotation partielle, pour mesurer le champ et la structure du potentiel à de faibles distances (jusqu'à quelques kilomètres ou des dizaines de kilomètres) de l'engin spatial. Les solutions du problème de rendez-vous sont obtenues dans le cas général puis dans des cas pratiques associés à des structures du potentiel possibles dues par exemple à la résistivité anormale ou à des chocs électro- statiques. Les caractéristiques de la mesure sont évaluées et en particulier le temps d'aller et retour des particules (par conséquent la résolution temporelle) et la sensibilité (c'est-à-dire le champ électrique minimum, l'étendue et la valeur du potentiel, mesurables). La valeur minimum de la composante parallèle du champ électrique ('" 1 mV/m) et la résolution temporelle minimum ( w20 ms) entre autres sont parfaitement bien adaptées pour décrire les structures du potentiel associées avec les phénomènes de résistivité anormale et de chocs électrostatiques. Finalement les caractéristiques nécessaires des canons et des détecteurs sont évaluées, c'est-à-dire leur orientation, l'intensité des faisceaux, la résolution temporelle et la sensibilité des détecteurs


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    International audienceA zero-dimensional model (box model) was developed for the simulation of diurnal variations of minor constituents in the atmosphere, using a new technique well adapted to the integration of stiff ordinary differential equations. The aim of developing this code is the easy coupling with any multi-dimensional dynamical model, especially 3D models using the on-line time-splitting method. The integration scheme is a semi-implicit symetric method (SIS) which is twenty times faster than Gear's scheme and has a comparable accuracy for stiff problems. In a closed system, this method strictly conserves the number of atoms, avoiding the need for any corrections due to long integration. The code accepts any photochemical system as input. In a first application, the box model is coupled with a steady-state one-dimensional model, including NOY, HOX, Cl(Y), O(X) and Br(Y) families to study the effect of increases in bromine species on stratospheric ozone. To this end, the ground-mixing ratio of the source species CH3Br is increased from the present level 20 pptv to 100 pptv. The maximum decrease of ozone abundance then occurs at 19 km. This decrease is 9% if the expected low value of HBr concentration is assumed, but it could be reduced to 4% if the concentration of HBr is higher than expected, as recently measured. A possible explanation of the high HBr concentration is that HBr and O3 are products of the reaction BrO + HO2
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