Nickel Isotope Variations in Terrestrial Silicate Rocks and Geological Reference Materials Measured by MC‐ICP‐MS

Although initial studies have demonstrated the applicability of Ni isotopes for cosmochemistry and as a potential biosignature, the Ni isotope composition of terrestrial igneous and sedimentary rocks, and ore deposits remains poorly known. Our contribution is fourfold: (a) to detail an analytical procedure for Ni isotope determination, (b) to determine the Ni isotope composition of various geological reference materials, (c) to assess the isotope composition of the Bulk Silicate Earth relative to the Ni isotope reference material NIST SRM 986 and (d) to report the range of mass‐dependent Ni isotope fractionations in magmatic rocks and ore deposits. After purification through a two‐stage chromatography procedure, Ni isotope ratios were measured by MC‐ICP‐MS and were corrected for instrumental mass bias using a double‐spike correction method. Measurement precision (two standard error of the mean) was between 0.02 and 0.04‰, and intermediate measurement precision for NIST SRM 986 was 0.05‰ (2s). Igneous‐ and mantle‐derived rocks displayed a restricted range of δ^60/58Ni values between ?0.13 and +0.16‰, suggesting an average BSE composition of +0.05‰. Manganese nodules (Nod A1; P1), shale (SDO‐1), coal (CLB‐1) and a metal‐contaminated soil (NIST SRM 2711) showed positive values ranging between +0.14 and +1.06‰, whereas komatiite‐hosted Ni‐rich sulfides varied from ?0.10 to ?1.03‰.     

Thermodynamic properties of glaucophane new data from calorimetric and spectroscopic measurements

New data concerning glaucophane are presented. New high temperature drop calorimetry data from 400 to 800 K are used to constrain the heat capacity at high temperature. Unpublished low temperature calorimetric data are used to estimate entropy up to 900 K. These data, corrected for composition, are fitted for C _p and S to the polynomial expressions (J · mol^?1 · K^?2) for T> 298.15 K: $$\begin{gathered} C_p = 11.4209 * 10^2 - 40.3212 * 10^2 /T^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} - 41.00068 * 10^6 /T^2 \hfill \\ + 52.1113 * 10^8 /T^3 \hfill \\ \end{gathered} $$ $$\begin{gathered} S = 539 + 11.4209 * 10^2 * \left( {\ln T - \ln 298.15} \right) - 80.6424 * 10^2 \hfill \\ * \left( {T^{ - {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} - 1/\left( {298.15} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} } \right) + 20.50034 * 10^6 \hfill \\ * \left( {T^{ - 2} - 1/\left( {298.15} \right)^2 } \right) - 17.3704 * 10^8 * \left( {T^{ - 3} - \left( {1/298.15} \right)^3 } \right) \hfill \\ \end{gathered} $$ IR and Raman spectra from 50 to 3600 cm^?1 obtained on glaucophane crystals close to the end member composition are also presented. These spectroscopic data are used with other data (thermal expansion, acoustic velocities etc.) in vibrational modelling. This last method provides an independent way for the determination of the thermodynamic properties (C_p and entropy). The agreement between measured and calculated properties is excellent (less than 2% difference between 100 and 1000 K). It is therefore expected that vibrational modelling could be applied to other amphiboles for which spectroscopic data are available. Finally, the enthalpy of formation of glaucophane is calculated.     

Modelling flash flood propagation in urban areas using a two-dimensional numerical model

This paper reports on the numerical modelling of flash flood propagation in urban areas after an excessive rainfall event or dam/dyke break wave. A two-dimensional (2-D) depth-averaged shallow-water model is used, with a refined grid of quadrilaterals and triangles for representing the urban area topography. The 2-D shallow-water equations are solved using the explicit second-order scheme that is adapted from MUSCL approach. Four applications are described to demonstrate the potential benefits and limits of 2-D modelling: (i) laboratory experimental dam-break wave in the presence of an isolated building; (ii) flash flood over a physical model of the urbanized Toce river valley in Italy; (iii) flash flood in October 1988 at the city of Nîmes (France) and (iv) dam-break flood in October 1982 at the town of Sumacárcel (Spain). Computed flow depths and velocities compare well with recorded data, although for the experimental study on dam-break wave some discrepancies are observed around buildings, where the flow is strongly 3-D in character. The numerical simulations show that the flow depths and flood wave celerity are significantly affected by the presence of buildings in comparison with the original floodplain. Further, this study confirms the importance of topography and roughness coefficient for flood propagation simulation.     

Comparison of tropical cyclogenesis indices on seasonal to interannual timescales

This paper evaluates the performances of four cyclogenesis indices against observed tropical cyclone genesis on a global scale over the period 1979–2001. These indices are: the Genesis Potential Index; the Yearly Genesis Parameter; the Modified Yearly Convective Genesis Potential Index; and the Tippett et al. Index (J Clim, 2011), hereafter referred to as TCS. Choosing ERA40, NCEP2, NCEP or JRA25 reanalysis to calculate these indices can yield regional differences but overall does not change the main conclusions arising from this study. By contrast, differences between indices are large and vary depending on the regions and on the timescales considered. All indices except the TCS show an equatorward bias in mean cyclogenesis, especially in the northern hemisphere where this bias can reach 5°. Mean simulated genesis numbers for all indices exhibit large regional discrepancies, which can commonly reach up to ±50%. For the seasonal timescales on which the indices are historically fitted, performances also vary widely in terms of amplitude although in general they all reproduce the cyclogenesis seasonality adequately. At the seasonal scale, the TCS seems to be the best fitted index overall. The most striking feature at interannual scales is the inability of all indices to reproduce the observed cyclogenesis amplitude. The indices also lack the ability to reproduce the general interannual phase variability, but they do, however, acceptably reproduce the phase variability linked to El Ni?o/Southern Oscillation (ENSO)—a major driver of tropical cyclones interannual variations. In terms of cyclogenesis mechanisms that can be inferred from the analysis of the index terms, there are wide variations from one index to another at seasonal and interannual timescales and caution is advised when using these terms from one index only. They do, however, show a very good coherence at ENSO scale thus inspiring confidence in the mechanism interpretations that can be obtained by the use of any index. Finally, part of the gap between the observed and simulated cyclogenesis amplitudes may be attributable to stochastic processes, which cannot be inferred from environmental indices that only represent a potential for cyclogenesis.     

Geochemical modelling of fluid–rock interactions in the context of the Soultz-sous-Forêts geothermal system

The development of the Enhanced Geothermal System (EGS) at Soultz-sous-Forêts (France) has given to scientists an interesting opportunity for the application of geochemical modelling of water–rock interactions, combining theoretical studies with field and experimental data. The main results of four successive and complementary studies are summarized: geochemical modelling of fluid–rock interactions with prediction of dissolution/precipitation of minerals, feed-back effects on the mineralogy and petrography of the rock (major role of silicates in the geological past and of carbonates in the near future of the exploitation), experimental control of the dynamics of silicates under thermal gradient and relation between the evolution of the petrophysics of the rocks and the heat and mass transfers. The thermal cycle of the fluid, between 200 °C and 65 °C in the geothermal loop, may be responsible for dissolution/precipitation of minerals which modify the porosity and permeability of the granite, as it happened in the geological past, in relation with hydrothermal circulations in the Rhine Graben.     

Analysis of multidimensional aspects of agricultural droughts in Zimbabwe using the Standardized Precipitation Index (SPI)

This study focuses on a framework of methodologies used for analyzing the frequency and spatiotemporal characteristics of agricultural droughts in Zimbabwe from a vulnerability context. By employing an empirical orthogonal analysis method, the study revealed that relatively strong spatial and temporal station drought relationships prevail, making the drought spatiotemporal characteristics of the country to be considered highly homogeneous. Thus, agricultural droughts were characterized temporarily using the Standardized Precipitation Index derived from rainfall data for the longer but sparse data period from 1901 to 2004. At the same time, higher spatial density analysis was achieved from shorter but denser database for the period 1941 to 1999. The results indicated that drought is a natural climatic feature of the region and occurs from time to time in defined periods. However, severe and extreme droughts tend to concentrate near the end of the time series, suggesting that during the earlier period of the twentieth century, droughts have been smaller or less pervasive. The extreme droughts appear to inherit the coincidence of both very high values of spatial extent and intensity in a single event. This offers a possible explanation to why extreme droughts in Zimbabwe usually have dire consequences on agriculture and the national economy. By showing that the related national drought impacts on staple maize food production can be estimated, this study has demonstrated that it is possible to anticipate future drought hazard impacts and predict periods of food insecurity. As far as the forecasting of agricultural droughts is concerned, the recently discovered Indian Ocean dipole/zonal mode seems to perform better than the traditional El Niño–Southern Oscillation as a potential drought predictor during the twentieth century.     

Ethane aerosol phase evolution in Titan's atmosphere

Strong experimental evidence is presented that the northern polar cloud observed in Titan's atmosphere by the Cassini orbiter (VIMS) was indeed composed of ethane aerosol as proposed by Griffith et al. [2006. Science 313, 1620-1622]. We report on the condensation and phase behavior of ethane aerosol under atmospheric conditions of Titan (145 hPa, 40 km altitude, 70-90 K, 10-30 ppm ethane in nitrogen). The results were obtained in an in-situ collisional cooling experiment combined with Fourier-transform infrared (FTIR) spectroscopy. Apart from the liquid phase, three crystalline phases (solid I, solid II, metastable) and the transitions into each other have been observed in the ethane aerosol. The phases were found to have a significant effect on the particles' IR spectra, their growth dynamics and the final size of the aerosols which varies between 0.5 and 4 μm (compared to 1-3 μm observed on Titan). This has strong implications on the ethane vapor pressure, precipitation and optical aerosol detection.     

Mapping of prospectivity and estimation of number of undiscovered prospects for lode gold,southwestern Ashanti Belt,Ghana

In the southwestern part of the Ashanti Belt, the results of fractal and Fry analyses of the spatial pattern of 51 known mines/prospects of (mostly lode) gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales. Intersections of NNE- and NW-trending faults were likely predominantly involved in local-scale structural controls on lode gold mineralisation, whilst NNE-trending faults were likely predominantly involved in district-scale structural controls on lode gold mineralisation. The results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area. The applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines/prospects results in a one-level prediction of 37 undiscovered lode gold prospects. The applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines/prospects result in an estimate of 40 undiscovered lode gold prospects. The study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits.     

Spatial variations in chemical weathering and CO2 consumption in Nepalese High Himalayan catchments during the monsoon season

The major ion chemistry of the Marsyandi basin and six of its tributaries in the Nepalese Himalaya have been investigated during the monsoon months of 2002. Weekly water samples taken at 10 river monitoring stations in the Annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution. The river chemistry of all but one basin is heavily dominated by carbonate weathering which, compared to silicate weathering, contributes 80 to 97% of the total solute load. This prevalence is due to a combination of (a) intrinsically faster dissolution kinetics of carbonates, (b) relatively high runoff and (c) glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities. Monitoring stations with headwaters in the Tethyan Sedimentary Series (TSS) are particularly carbonate-rich and slightly supersaturated with respect to calcite through half of the monsoon season. Silicate weathering in the TSS is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric CO₂. With respect to the tributaries in the Greater Himalayan Sequence (GHS), carbonate weathering is practically as predominant as for the TSS, in spite of the largely felsic lithology of the GHS. Relative to the TSS, the primary proton source in the GHS has shifted, with at least 80% of the protons derived from carbonic acid. Averaged over the whole field area, the CO₂ fluxes, based on silicate-derived Ca and Mg, are considerably lower than the global average. Assuming that this study area is representative of the entire range, we conclude that in situ weathering of the High Himalayas does not represent a significant sink of atmospheric carbon dioxide, despite the presence of a watershed south of the GHS that is characterized by a four times higher CO₂ consumption rate than the global average. Silicate weathering rates of all basins appear to be climate controlled, displaying a tight correlation with runoff and temperature. Given the extremely low chemical weathering under transport-limited conditions in high-altitude crystalline terrains outside of the monsoon season, this would result in virtually no chemical exhumation for 2/3 of the year in such a cold and arid climate, north of the rain shadow cast by the High Himalayas.