Electrostatic solitons in multispecies electron-positron plasmas

Acoustic solitons are investigated in electron-positron plasmas containing equal hot and cool components of both species. The hot components are isothermal Boltzmann distributed, the cool constituents are modelled by adiabatic fluids. The equations are integrated exactly in terms of a Sagdeev potential. Solitons are shown to be possible, but no double layers, due to the symmetry in the model. Bearing in mind the constraints imposed by the Boltzmann assumption, small amplitude solitons only are found. Such findings are relevant for different kinds of astrophysical plasmas, as well as for other types of similar acoustic solitons.     

Sources,transport, and sinks of SO<Subscript>2</Subscript> over the equatorial Pacific during the Pacific Atmospheric Sulfur Experiment

The Pacific Atmospheric Sulfur Experiment (PASE) is the first sulfur-budget field experiment to feature simultaneous flux measurements of DMS marine emissions and SO₂ deposition to the ocean surface. We make use of these data to constrain a 1-D chemical transport model to study the production and loss pathways for DMS and SO₂ over the equatorial Pacific. Model results suggest that OH is the main sink for DMS in the boundary layer (BL), and the average DMS-to-SO₂ conversion efficiency is ~73%. In an exploratory run involving the addition of 1 pptv of BrO as a second oxidant, a 14% increase in the DMS flux is needed beyond that based on OH oxidation alone. This BrO addition also reduces the DMS-to-SO₂ conversion efficiency from 73% to 60%. The possibility of non-DMS sources of marine sulfur influencing the estimated conversion efficiency was explored and found to be unconvincing. For BL conditions, SO₂ losses consist of 48% dry deposition, while transport loss to the BuL and aerosol scavenging each account for another 19%. The conversion of SO₂ to H₂SO₄ consumes the final 14%. In the BuL, cloud scavenging removes 85% of the SO₂, thus resulting in a decreasing vertical profile for SO₂. The average SO₂ dry deposition velocity from direct measurements (i.e., 0.36 cm sec⁻¹) is approximately 50% of what is calculated from the 1-D model and the global GEOS-Chem model. This suggests that the current generation of global models may be significantly overestimating SO₂ deposition rates over some tropical marine areas. Although the specific mechanism cannot be determined, speculation here is that the dry deposition anomalous results may point to the presence of a micro-surface chemical phenomenon involving partial saturation with either S(IV) and/or S(VI) DMS oxidation products. This could also appear as a pH drop in the ocean’s surface microfilm layer in this region. Finally, we propose that the enhanced SO₂ level observed in the lower free troposphere versus that in the upper BuL during PASE is most likely the result of transported DMS/SO₂-rich free-tropospheric air parcels from the east of the PASE sampling area, rather than an inadequate representation in the model of local convection.     

Geology,geochemistry and mineralogy of the lignite-hosted Ambassador palaeochannel uranium and multi-element deposit,Gunbarrel Basin,Western Australia

The Ambassador U and multi-element deposit occurs on the SW margin of the Gunbarrel Basin, Western Australia. Low-grade, flat-lying U mineralization averaging about 2 m thick at 0.03% U occurs in lignites at the redox front at the base of the weathering profile within a laterally extensive palaeochannel network. Uranium is principally associated with organic matter within the lignitic matrix, although rare discrete U minerals, such as coffinite and uraninite, are also present. The lignite is also enriched in a suite of other elements, principally base metals and sulphur, with concentrations of 0.3 ≥ 1% Cu, Pb, Ni, Co, Zn and total rare earth elements (REE) in some samples. Other element enrichments include: Cr, Cs, Sc, Se, Ta, Ti, Th, V and Zr as detrital heavy minerals of Zr, Ti and REE (oxides and silicates) or authigenic minerals of Cu, Bi, Pb, Zn, Ni, Se, Hg, Ti, Cr, Tl, V, U and REE (sulphides, vanadates, selenides, oxides, chlorides and native metals) and diffuse lignite impregnations. The Ambassador deposit probably formed from the convergence of redox-active weathering processes to unique source/host rocks, constrained within the palaeochannel. A proximal source of U and trace elements of lamproite/carbonatite origin is probable, as constrained by U–Pb isotope and U–Th disequilibria studies. Uranium and other metals were precipitated syngenetically with organic matter as it was deposited during a humid phase in the Late Eocene. Remobilization subsequently concentrated the metals in the upper 2 m of the lignite. This may have occurred during one or more periods of weathering and associated diagenesis, with the latest episode in the last 300,000 years.     

Geological context of crustal anatexis and granitic magmatism in the northeastern Glenelg River Complex,western Victoria

The Cambro‐Ordovician Glenelg River Complex in the Harrow district, western Victoria, consists of extensive granitic rocks associated with a migmatitic metasedimentary envelope. Metasedimentary rocks comprise amphibolite facies massive‐laminated quartzo‐feldspathic schists and layered gneisses with minor sillimanite‐bearing horizons. Intercalated are stromatic and nebulitic migmatites of granitic and tonalitic character; textural evidence suggests that both varieties developed by in situ partial melting. Ranging from adamellite to leucotonalite, granitic rocks contain abundant magmatic muscovite, commonly with garnet and sillimanite, and exhibit generally unrecrystallised igneous textures. Heterogeneous structurally concordant plutons transitional to migmatites and more uniform intrusive phases are delineated with both types hosting diverse metasedimentary enclaves, micaceous selvages and schlieren; a gneissic foliation of variable intensity is defined by the latter. These petrographic attributes are consistent with derivation of plutons by anatexis of a peraluminous metasedimentary protolith. The schlieric foliation is not tectonically imposed, but rather directly inherited from the migmatitic precursor, compositional variations within which are preserved by the layered Schofield Adamellite. The most mafic granitic body (Tuloona Granodiorite) also has igneous microgranular enclaves indicating a more complex petrogenesis. Metasedimentary rocks experienced five episodes of folding, the latest involving macroscopic open warps. This is analogous to the structural history elucidated elsewhere in the Glenelg River Complex, by inference a coherent tectonic entity whose present metamorphic and stratigraphic configuration might be governed by F₅ folding. Structures within migmatites intimate that partial melting proceeded throughout the deformational history and peaked syn‐D₄ to pre‐D₅, whilst temperatures had waned to sub‐biotite grade in the southwestern Glenelg River Complex. Granitic rocks were generated during this anatectic culmination and were therefore emplaced late in the orogenic history relative to other syntectonic phases of the Glenelg River Complex.     

Misinterpreting the potential benefits of aggregation for reducing desiccation in the intertidal: a simple analogy

The potential adaptive benefits of aggregation in intertidal invertebrates, in terms of preventing desiccation stress, are logical, but have proved difficult to confirm from field measurements. Through a simple analogy we explain why this may be so, especially for those species that do not solely forage when fully immersed. Measuring water content (or osmotic potentials of body fluids) of individuals that become inactive in aggregations and comparing this with non‐aggregated individuals, which is the most common method of determining adaptive benefits of aggregation to desiccation, (i) causes any relationships between desiccation and aggregation to be, at best, weakly related due to variability in the different times that individuals enter aggregations, and (ii) is not measuring the true benefit of aggregation, which in this case should be measured as a reduction in the rate of water loss. Further, the design of field studies or experiments to determine the benefits of aggregation in terms of desiccation reduction is extremely challenging, due to the unavoidable repeated disturbances to individuals that are necessary to measure water levels at multiple occasions, in order to measure the rate of water loss. We suggest that development of biomimetic sensors, such as modification of the robolimpet sensor, are required to measure in situ rates of water loss with a high temporal resolution. Although such sensors may not be truly physiologically realistic, they are likely to provide the least confounded approach to understanding benefits of aggregation in terms of physical stress reduction.     

SiO maser emission in Miras

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infrared (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in very long baseline interferometry (VLBI) observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1–0.25, which is consistent with observed phase lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13 μm appear to be closely associated in three out of four phase samples.     

Synoptic Controls on Boundary-Layer Characteristics

We report the characteristics of the three-dimensional, time evolving, atmospheric boundary layer that develops beneath an idealised, dry, baroclinic weather system. The boundary-layer structure is forced by thermal advection associated with the weather system. Large positive heat fluxes behind the cold front drive a vigorous convective boundary layer, whereas moderate negative heat fluxes in the warm sector between the cold and warm fronts generate shallow, stably stratified or neutral boundary layers. The forcing of the boundary-layer structure is quantified by forming an Eulerian mass budget integrated over the depth of the boundary layer. The mass budget indicates that tropospheric air is entrained into the boundary layer both in the vicinity of the high-pressure centre, and behind the cold front. It is then transported horizontally within the boundary layer and converges towards the cyclone’s warm sector, whence it is ventilated out into the troposphere. This cycling of air is likely to be important for the ventilation of pollution out of the boundary layer, and for the transformation of the properties of large-scale air masses.     

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACT

In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ¹⁵N_air (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ¹⁵N values (δ¹⁵N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ¹⁵N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ¹⁵N = + 2.1 to +3.6‰).

Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.