The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy

Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO₂) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO₂ < −9) or oxidised (log fO₂ > −2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S²⁻. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV.The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO₄²⁻. Sulfite (SO₃²⁻) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S⁶⁺). S⁴⁺ does, however, occur unambiguously with S⁶⁺ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO₄ at 4-16 kbar, and when buffered with Re/ReO₂ at 10 kbar. Solubility of S⁴⁺ thus requires partial pressures of SO₂ considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO₂s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S²⁻ and S⁶⁺ were found to coexist in some samples that were not in equilibrium with the imposed fO₂.The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S²⁻, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S²⁻ + 8Fe³⁺ = S⁶⁺ + 8Fe²⁺, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S²⁻ and S⁶⁺ are frequently found together despite the very limited fO₂ range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.     

Mercury isotope fractionation during liquid-vapor evaporation experiments

Liquid-vapor mercury isotope fractionation was investigated under equilibrium and dynamic conditions. Equilibrium evaporation experiments were performed in a closed glass system under atmospheric pressure between 0 and 22 °C, where vapor above the liquid was sampled at chemical equilibrium. Dynamic evaporation experiments were conducted in a closed glass system under 10⁻⁵ bar vacuum conditions varying (1) the fraction of liquid Hg evaporated at 22 °C and (2) the temperature of evaporation (22-100 °C). Both, residual liquid and condensed vapor fractions were analyzed using stannous chloride CV-MC-ICP-MS.Equilibrium evaporation showed a constant liquid-vapor fractionation factor (α_202/198) of 1.00086 ± 0.00022 (2SD, n = 6) within the 0-22 °C range. The 22 °C dynamic evaporations experiments displayed Rayleigh distillation fractionation behavior with liquid-vapor α_202/198 = 1.0067 ± 0.0011 (2SD), calculated from both residual and condensed vapor fractions. Our results confirm historical data (1920s) from Brönsted, Mulliken and coworkers on mercury isotopes separation using evaporation experiments, for which recalculated δ²⁰²Hg′ showed a liquid-vapor α_202/198 of 1.0076 ± 0.0017 (2SD). This liquid-vapor α_202/198 is significantly different from the expected kinetic α_202/198 value ((202/198)^0.5 = 1.0101). A conceptual evaporation model of back condensation fluxes within a thin layer at the liquid-vapor interface was used to explain this discrepancy. The δ²⁰²Hg′ of condensed vapor fractions in the 22-100 °C temperature range experiments showed a negative linear relationship with 10⁶/T², explained by increasing rates of exchange within the layer with the increase in temperature.Evaporation experiments also resulted in non-mass-dependent fractionation (NMF) of odd ¹⁹⁹Hg and ²⁰¹Hg isotopes, expressed as Δ¹⁹⁹Hg′ and Δ²⁰¹Hg′, the deviation in ‰ from the mass fractionation relationship with even isotopes. Liquid-vapor equilibrium yielded Δ¹⁹⁹Hg′/Δ²⁰¹Hg′ relationship of 2.0 ± 0.6 (2SE), which is statistically not different from the one predicted for the nuclear field shift effect (Δ¹⁹⁹Hg/Δ²⁰¹Hg ≈ 2.47). On the other hand, evaporation under dynamic conditions at 22 °C led to negative anomalies in the residual liquid fractions that are balanced by positive anomalies in condensed vapors with lower Δ¹⁹⁹Hg′/Δ²⁰¹Hg′ ratios of 1.2 ± 0.4 (2SD). This suggests that either magnetic isotope effects may have occurred without radical chemistry or an unknown NMF process on odd isotopes operated during liquid mercury evaporation.     

Identification of the Bloody Creek structure,a possible impact crater in southwestern Nova Scotia,Canada

Abstract— An approximately 0.4 km diameter elliptical structure formed in Devonian granite in southwestern Nova Scotia, herein named the Bloody Creek structure (BCS), is identified as a possible impact crater. Evidence for an impact origin is based on integrated geomorphic, geophysical, and petrographic data. A near‐continuous geomorphic rim and a 10 m deep crater that is infilled with lacustrine sediments and peat define the BCS. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 1 m high buried scarp. Heterogeneous material under the floor, interpreted as deposits from collapse of the transient cavity walls, is overlain by stratified and faulted lacustrine and wetland sediments. Alteration features found only in rim rocks include common grain comminution, polymict lithic microbreccias, kink‐banded feldspar and biotite, single and multiple sets of closely spaced planar microstructures (PMs) in quartz and feldspar, and quartz mosaicism, rare reduced mineral birefringence, and chlorite showing plastic deformation and flow microtextures. Based on their form and crystallographic orientations, the quartz PMs consist of planar deformation features that document shock‐metamorphic pressures ≤25 GPa. The age of the BCS is not determined. The low depth to diameter ratio of the crater, coupled with anomalously high shock‐metamorphic pressures recorded at its exposed rim, may be a result of significant post‐impact erosion. Alternatively, impact onto glacier ice during the waning stages of Wisconsinian deglaciation (about 12 ka BP) may have resulted in dissipation of much impact energy into the ice, resulting in the present morphology of the BCS.     

Measuring Venus’ winds using the Absolute Astronomical Accelerometer: Solid super-rotation model of Venus’ clouds

We present a new method of measuring the Venus winds by Doppler velocimetry on the full visible spectrum of solar light scattered by the clouds. In January 2003, we carried out observations to measure the winds of Venus, using the EMILIE high-resolution, cross-dispersed spectrograph and its associated calibrating instrument the Absolute Astronomical Accelerometer (AAA), at Observatoire de Haute-Provence, France. The motivation of this type of measurements is that it measures the actual velocity of cloud particles, while the other method (track of cloud features) may be sensitive to the deformation of the clouds. During observations, Venus was near maximum western elongation, at a phase angle near 90°. The EMILIE-AAA system allows us to measure accurately the Doppler shift induced in the reflected solar spectrum by the radial component of the motion of the clouds of Venus. We present the measurements and compare them with a forward simulation of a solid super-rotation of the atmosphere of Venus. Taking into account the Doppler shift relative to the Sun and that relative to the Earth, the theoretical total Doppler shift induced in the solar spectra is easily computed as a function of the velocity of the reflecting target. A first forward simulation is computed, with a wind model considering a purely horizontal and zonal wind. The magnitude of the wind is assumed to depend on cos(latitude), as for a solid-body rotation. The comparison with the measurements at various points on the illuminated semi-disc allowed us to determine an equatorial velocity of 66, 75, 91 and 85 m/s on 4 consecutive mornings, consistent with previous ultraviolet cloud tracking wind measurements, showing that wave propagation is not a major factor in the apparent motion of the cloud marks. Further, we discuss the effect of the finite angular size of the Sun and its rapid equatorial rotation (that we call the Young effect). It mainly affects measurements taken near the terminator, where the largest discrepancies are found. These discrepancies are alleviated when the Young effect is taken into account in the model but then the retrieved Venus equatorial velocity is reduced to only 48±3 m/s. This is well below classical ultraviolet markings velocities, but the altitude at which the visible photons are scattered (66 km) that we use is 5 km below the UV markings, confirming the vertical gradient of the horizontal winds shown by previous in-situ measurements.     

In situ micro‐Raman and X‐ray diffraction study of diamonds and petrology of the new ureilite UAE 001 from the United Arab Emirates

Abstract— A new olivine‐pigeonite ureilite containing abundant diamonds and graphite was found in the United Arab Emirates. This is the first report of a meteorite in this country. The sample is heavily altered, of medium shock level, and has a total weight of 155 g. Bulk rock, olivine (Fo_79.8–81.8) and pyroxene (En_73.9–75.2, Fs_15.5–16.9, Wo_8.8–9.5) compositions are typical of ureilites. Olivine rims are reduced with Fo increasing up to Fo_96.1–96.8. Metal in these rims is completely altered to Fehydroxide during terrestrial weathering. We studied diamond and graphite using micro‐Raman and in situ synchrotron X‐ray diffraction. The main diamond Raman band (LO = TO mode at ?1332 cm^?1) is broadened when compared to well‐ordered diamond single crystals. Full widths at half maximum (FWHM) values scatter around 7 cm^?1. These values resemble FWHM values obtained from chemical vapor deposition (CVD) diamond. In situ XRD measurements show that diamonds have large grain sizes, up to >5 μm. Some of the graphite measured is compressed graphite. We explore the possibilities of CVD versus impact shock origin of diamonds and conclude that a shock origin is much more plausible. The broadening of the Raman bands might be explained by prolonged shock pressure resulting in a transitional Raman signal between experimentally shock‐produced and natural diamonds.     

Acceleration strategies for elastic full waveform inversion workflows in 2D and 3D

Full waveform inversion (FWI) is one of the most challenging procedures to obtain quantitative information of the subsurface. For elastic inversions, when both compressional and shear velocities have to be inverted, the algorithmic issue becomes also a computational challenge due to the high cost related to modelling elastic rather than acoustic waves. This shortcoming has been moderately mitigated by using high-performance computing to accelerate 3D elastic FWI kernels. Nevertheless, there is room in the FWI workflows for obtaining large speedups at the cost of proper grid pre-processing and data decimation techniques. In the present work, we show how by making full use of frequency-adapted grids, composite shot lists and a novel dynamic offset control strategy, we can reduce by several orders of magnitude the compute time while improving the convergence of the method in the studied cases, regardless of the forward and adjoint compute kernels used.     

Profiling of dissolved organic compounds in the oil sands region using complimentary liquid–liquid extraction and ultrahigh resolution Fourier transform mass spectrometry

Understanding and characterizing organics in aquatic environments is a great challenge for environmental monitoring, especially for the oil sands industry due to the complexity and potential toxicity of dissolved organics in water. To date, significant efforts have been made in investigating the toxicity of naphthenic acids, although other compounds may also contribute to the toxicity of oil sands process-affected water (OSPW). Here, we present a case study showing a systematic approach for profiling the organic composition of OSPW and environmental water samples by concentrating and separating dissolved organics through complementary liquid–liquid extractions followed by positive- or negative-ion mode ultrahigh resolution mass detection. Our comparative investigation shows clear differences in the composition of dissolved organics (homologues particularly) not only between OSPW samples and environmental water samples, but also differences among oil sands operators. Sulfur-containing compounds (especially the SO _n classes) appear to have great potential to be used for evaluating the impact of OSPW, while our understanding of oxygen-only containing compounds should not be limited to O₂ (i.e., classic naphthenic acids), but rather can be broadened to include many other compound classes (for instance O _n , n = 1–9). Systematic profiling of water samples should be more widely implemented for monitoring the origin and transport of organics in aquatic ecosystems of the oil sands development region, northeastern Alberta, Canada.     

Estimating greenhouse gas emissions from cattle raising in Brazil

The study estimated, for the first time, the greenhouse gas emissions associated with cattle raising in Brazil, focusing on the period from 2003 to 2008 and the three principal sources: 1) portion of deforestation resulting in pasture establishment and subsequent burning of felled vegetation; 2) pasture burning; and 3) bovine enteric fermentation. Deforestation for pasture establishment was only considered for the Amazon and Cerrado. Emissions from pasture burning and enteric fermentation were accounted for the entire country. The consolidated emissions estimate lies between approximately 813 Mt CO₂eq in 2008 (smallest value) and approximately 1,090 Mt CO₂eq in 2003 (greatest value). The total emissions associated with Amazon cattle ranching ranged from 499 to 775 Mt CO₂eq, that of the Cerrado from 229 to 231 Mt CO₂eq, and that of the rest of the country between 84 and 87 Mt CO₂eq. The full set of emissions originating from cattle raising is responsible for approximately half of all Brazilian emissions (estimated to be approximately 1,055 Mt CO₂eq in 2005), even without considering cattle related sources not explicitly estimated in this study, such as energy use for transport and refrigeration along the beef and derivatives supply chain. The potential for reduction of greenhouse gas emissions offered by the Brazilian cattle industry is very high and might constitute Brazil’s most important opportunity for emissions mitigation. The study offers a series of policy recommendations for mitigation that can be implemented by public and private administrators at a low cost relative to other greenhouse gas reduction options.     

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U-Th series and REEs

The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as thick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size.Chemical and isotopic (⁸⁷Sr/⁸⁶Sr) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the ⁸⁷Sr/⁸⁶Sr signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO₂. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations.The Sr, U and Mg contents and the (²³⁴U/²³⁸U) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (²³⁴U/²³⁸U) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 ± 0.84 kyr to 7.5 ± 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO₂ even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils.