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

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 7×1015-2×1016 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

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

Evidence of convective transport in tropical West Pacific region during SHIVA experiment

Air masses in the convective outflows of four large convective systems near Borneo Island in Malaysia were sampled in the height range 11–13 km within the frame of the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere) FP7 European project in November and December 2011. Correlated enhancements of CO, CH4 and the short-lived halogen species (CH3I and CHBr3) were detected when the aircraft crossed the anvils of the four systems. These enhancements were interpreted as the fingerprint of vertical transport from the boundary layer by the convective updraft and then horizontal advection in the outflow. For the four observations, the fraction f of air from the boundary layer ranged between 15 and 67%, showing the variability in transport efficiency depending on the dynamics of the convective system

Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000–2016 period

The modeling study presented here aims to estimate how uncertainties in global hydroxyl radical (OH) distributions, variability, and trends may contribute to resolving discrepancies between simulated and observed methane (CH4) changes since 2000. A multi-model ensemble of 14 OH fields was analyzed and aggregated into 64 scenarios to force the offline atmospheric chemistry transport model LMDz (Laboratoire de Meteorologie Dynamique) with a standard CH4 emission scenario over the period 2000–2016. The multi-model simulated global volume-weighted tropospheric mean OH concentration ([OH]) averaged over 2000–2010 ranges between 8:7*10^5 and 12:8*10^5 molec cm-3. The inter-model differences in tropospheric OH burden and vertical distributions are mainly determined by the differences in the nitrogen oxide (NO) distributions, while the spatial discrepancies between OH fields are mostly due to differences in natural emissions and volatile organic compound (VOC) chemistry. From 2000 to 2010, most simulated OH fields show an increase of 0.1–0:3*10^5 molec cm-3 in the tropospheric mean [OH], with year-to-year variations much smaller than during the historical period 1960–2000. Once ingested into the LMDz model, these OH changes translated into a 5 to 15 ppbv reduction in the CH4 mixing ratio in 2010, which represents 7%–20% of the model-simulated CH4 increase due to surface emissions. Between 2010 and 2016, the ensemble of simulations showed that OH changes could lead to a CH4 mixing ratio uncertainty of > 30 ppbv. Over the full 2000–2016 time period, using a common stateof- the-art but nonoptimized emission scenario, the impact of [OH] changes tested here can explain up to 54% of the gap between model simulations and observations. This result emphasizes the importance of better representing OH abundance and variations in CH4 forward simulations and emission optimizations performed by atmospheric inversions

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

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

The Fate and Environmental Consequences of Reduced gas Mixtures Resulting from Magmatic Intrusion into Carbonaceous Rocks

International audienceRecent developments on the impacts of Large Igneous Provinces on climate changes and extinction rates emphasize the fundamental role of country rocks in gas emissions. Contact metamorphism of country rocks intruded by sills and dikes of mafic melts can be particularly important due to their long exposure to high temperatures. When the host rocks are composed of carbonates, sulphates, salts, or organic-compounds such as bituminous shales or coals, their heating can inject into the atmosphere a quantity of volatiles that greatly exceeds the amount delivered by purely magmatic degassing. We focus here on the interaction between magma and carbonaceous rocks. Recent studies have estimated the gas released by contact metamorphism of bituminous shales in the Karoo Province; we calculate the composition of the volcanic gases which results on this interaction, taking into account the magmatic contribution too. We then present an evaluation of the fate of such gases during their diffusion in the atmosphere. The modelling of the composition of the modified volcanic gases is based on gas-melt thermodynamic calculations that take into account S-H-O-C gaseous species at temperatures and pressures in equilibrium with basaltic liquids. We simulate the incorporation into the gas-melt system of organic compounds as CH or CH2, depending on the maturity of the carbonaceous rocks (coal or oil). Addition of C and H has a dramatic effect on the amount and the redox state of the gas in equilibrium with the basalt. With the incorporation of only 0.2 wt% CH, the gas composition changes from CO2-H2O dominated (typical of basaltic gases on Earth), to CO-H2 dominated (a strongly reduced mixture, which resembles Martian volcanic gases). Addition of more than 0.2 wt% CH can trigger graphite saturation, such as reported in few locations where carbonaceous rocks have been ingested by basalts. In the famous Disko Island location, for example, we calculate that an incorporation of 1 wt% CH led to saturation in metal iron. These "modified" volcanic gases are injected into the atmosphere at a rate directly proportional to the eruption rates and then disseminated. Using a 3D atmospheric algorithm coupling convective dynamics and chemistry, we model the expansion of the gas at a continental scale. Our simulations show that, at reasonable gas emission rates, no significant oxidation of CO occurs because the OH-radical concentration is strongly reduced due to the large concentration of CO and H2. This allows the gas to propagate faraway from the emission centre. The lateral propagation of the gas is significant at the scale of a continent within 2 weeks, which might reveal a new mechanism for mass extinctions

An OSSE estimation of the capacity of FCI to improve aerosols forecast, by assimilating observations of aerosols optical thickness in MOCAGE CTM: Preliminary results

International audienc

Variability of cirrus clouds in a convective outflow during the Hibiscus campaign

Light-weight microlidar and water vapour measurementswere taken on-board a stratospheric balloon duringthe HIBISCUS 2004 campaign, held in Bauru, Brazil(49W, 22 S). Cirrus clouds were observed throughout theflight between 12 and 15 km height with a high mesoscalevariability in optical and microphysical properties. It wasfound that the cirrus clouds were composed of different layerscharacterized by marked differences in height, thicknessand optical properties. Simultaneous water vapour observationsshow that the different layers are characterized bydifferent values of the saturation with respect to ice. Amesoscale simulation and a trajectory analysis clearly revealedthat the clouds had formed in the outflow of a largeand persistent convective region and that the observed variabilityof the optical properties and of the cloud structureis likely linked to the different residence times of theconvectively-processed air in the upper troposphere