Non-adiabatic discrete Alfvén waves in coronal loops and prominences

Discrete Alfvén waves in coronal loops and prominences are investigated in non-ideal magnetohydrodynamics. The non-ideal effects included are anisotropic, thermal conduction, and optically thin radiation. The classic ideal Alfvén continuum is not altered by these non-ideal effects, but the discrete Alfvén modes, which exist under certain conditions above or below the Alfvén continuum in ideal MHD, are shown to be influenced by non-adiabatic effects.The existence of discrete, non-adiabatic Alfvén waves, and their damping and overstability are examined for 1D cylindrical equilibrium states with twisted magnetic fields. First, analytic results are obtained for modes of high radial order by means of a WKB-analysis. The subspectrum of discrete Alfvén modes is computed with a numerical code, with particular emphasis on the modes of low radial order. The results show that discrete Alfvén waves are of potential importance for solar applications and also that the information obtained with the WKB-analysis is of limited use in this context.Research Assistant of the Belgian National for Scientific Research.     

Characterization of colloidal and particulate matter transported by the magela creek system,Northern Australia

The composition and amount of colloidal and suspended participate matter transported during a small flood event in Magela Creek in tropical northern Australia was investigated. The flood studied constituted approximately 3 % of the total annual flow, most (90%) of which occurred between mid-January and mid-February of the study year. Three fractions were separated from water samples using a sequential method involving a continuous flow centrifuge to separate suspended particulate matter (SPM; nominally > 1 μm) followed by hollow fibre filtration, first using a 0.1 μm filter to separate course colloidal matter (CCM; nominal size 1–0.1 μm) and then a 0–015 μm filter to separate fine colloidal matter (FCM; nominal size 0.1–0.015 μm). The SPM was predominantly inorganic (organic matter 21 %), whereas the colloidal fractions were dominantly organic matter (CCM 60%; FCM 83%). Analysis of individual particles using electron microprobe and automated image analysis indicated that the mineral fractions in both the SPM and CCM were dominated by iron-enriched aluminosilicates (including kaolinite) (72–82%) and quartz (9–10%), indicative of a highly weathered and extensively laterized catchment. Surprisingly there was very little difference in the composition of the SPM or CCM fractions during the flood event studied, which may indicate either that sediment availability was restricted following the major run-off events in January and February, or that all the sediment sources within the catchment are geochemically similar. Approximately the same amounts of particulate (20 tonne), colloidal (21 tonne) and dissolved material (17 tonne) were transported during the 25 hour period of the main flood peak; over 90% of the colloidal matter was 0.1–1.0 μm in size. These data suggest that previous estimates of the amounts of particulate (and colloidal) matter transported by Magela Creek, which were based on suspended solids measurements, may have underestimated the particulate matter load by as much as 50%. It is possible that the relatively high proportion of colloidal matter is unique to Magela Creek because coagulation and aggregation of colloidal matter to particulate matter is slow due to the very low concentations of calcium and magnesium in these waters. However, if the result is more widespread, there are important implications for the global estimates of fluvially transported particulate and dissolved materials as many of the previous studies may have underestimated the particulate load and overestimated the dissolved load.     

Monte Carlo simulation of temperature fluctuations in interstellar iron grains

The Monte Carlo technique is used to simulate the temperature fluctuations in interstellar iron grains exposed to a particular radiation field. It is found that iron grains attain time-averaged temperatures almost an order of magnitude greater than dielectrics and that their temperatures may fluctuate by as much as 100 K. The effect that this will have on the catalytic formation of molecular hydrogen is examined. It is also found that small metallic grains are unlikely candidates for explaining the extended red emission observed in many reflection nebulae.     

The use of magnetic field excursions in stratigraphy

The use of magnetic field excursions in stratigraphy is difficult primarily because the excursion field is complex and not dominantly dipolar. In contrast with a reversal, which is a global event, an excursion can be evidenced at one location but not another. Although this does not by itself rule out the use of excursions in stratigraphy, it does limit the geographic area over which they may be correlated. We recommend, somewhat conservatively, that excursions can be used to correlate between sedimentary cores separated by angular distances of less than 30° on Earth's surface. Correlation between cores separated by more than 45° should not be attempted.     

Pleistocene geomorphology and geochronology of eastern Grand Canyon: linkages of landscape components during climate changes

We report new mapping, soils, survey, and geochronologic (luminescence, U-series, and cosmogenic-nuclide) data from Pleistocene deposits in the arid setting of eastern Grand Canyon. The result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes, tributary streams, and the Colorado River to the last 400 kyr of glacial–interglacial climate change. The best-preserved last 80 kyr of this record indicates a stratigraphic–chronologic disconnect between both deposition and incision along the Colorado River versus along the trunks of local tributaries. For example, the Colorado River finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during Marine Isotope Stage 3, and then tributary incision followed during the millennial-scale fluctuations of the last glacial epoch, potentially concurrent with mainstem aggradation. The mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic–hydrologic changes in its mountain headwaters, with aggradation beginning during full-glacial times and continuing into subsequent interglacials. The contrasting lag time in responses of the dryland catchments within Grand Canyon may be largely a function of the weathering-limited nature of hillslope sediment supply.     

Particle Size Distributions of Clay-rich Sediments and Pure Clay Minerals: A Comparison of Grain Size Analysis with Sedimentation Field-Flow Fractionation

Two clay-rich coastal sediment samples have been characterised together with three pure clays identified as major components of the sediment samples. The mineralogy of the sediments was determined by X-ray diffraction, and their bulk properties measured (organic carbon content, leachable iron and manganese, surface area and cation exchange capacity). A simple approach is used to model the properties of the sediments from those of their pure clay components. Good agreement is found for cation exchange capacity and specific surface area indicating that the clay minerals are the most important constituents influencing the sediments' surface properties. Particle size distributions were assessed by conventional grain size analysis, and also by Sedimentation Field-Flow Fractionation (SedFFF). These two approaches yield sharply differing size distributions; in particular, conventional grainsize analysis indicates a much greater proportion of submicron particles. Similar results are obtained from analysis of pure clays. The most likely reason for this is artifacts introduced in the unavoidable 1 m prefractionation used for SedFFF.     

Geological,petrochemical and metallogenic characteristics of the Costa Rican gold belt: Contribution to new explorations

The Costa Rican Gold Belt is located in the northwestern part of Costa Rica. The geology of this region is characterized by tectonically juxtaposed volcanic-sedimentary and igneous rock units of Tertiary age.Mineralization within the Gold Belt consists of: 1) veins and fissure fillings of epithermal type which bear gold, silver and base metals; 2) disseminated mineralization of complex sulphides (Au, Ag, Pb, Zn); 3) disseminated specks of native gold; 4) eluvial Au rich lateritic soils.Hydrothermal circulation, induced by the emplacement of epizonal plutons, together with continuous and intense pervasive tectonism has been responsible for the epithermal gold-silver mineralization occurring within the Aguacate volcanic series. Contamination and assimilation of andesitic and »shoshonitic« rocks by uprising granitic magmas generated, in association with hydrothermal phenomena, composite environments for gold deposits.Calc-alkaline rocks of the Costa Rican Gold Belt appear to be enriched in incompatible elements of low ionic potential (particularly Ba). For a similar silica concentration, there is a general increase in Y, Ni, and Zr from Miocene to Pliocene rocks. K/Rb and Rb/Sr ratios increase from older to younger suites as well.On the basis of trace element distribution it is suspected that the Guacimal intrusive rocks may represent a marginal suite comparable to those of the New Guinea Mobile Belt. This would imply that the Precambrian-Paleozoic igneous and metamorphic basement, present in northern Nicaragua, might extend further south.Four different magmatic events have been recognized and correlated with regional geodynamic conditions: Eocene-Oligocene calc-alkaline and »shoshonitic« volcanism (plate convergence and subduction), Middle-Early Miocene to Late Miocene calc-alkalic magmatism (compression and uplift), Early Pliocene alkalic magmatism (E-W trending rift), Middle-Late Pliocene calc-alkaline magmatism (crustal extension followed by compression and uplift).Correlation between tectonic events, magmatism and mineralization suggests that metal enrichment may have some relation to different stress regimes associated with active subduction.

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Zusammenfassung Der goldführende Gürtel von Costa Rica befindet sich in dem nordwestlichen Teil des Landes. Die Geologie dieses Gebietes ist durch tektonisch aneinandergelagerte vulkaisch-sedimentäre und magmatische Gesteine des Tertiärs gekennzeichnet.Die Mineralisierungen in dem goldführenden Gürtel bestehen aus: 1) quarzführenden Gängen epithermaler Art, die Gold, Silber, Zink und Blei enthalten 2) Mineralisierungen mit komplexen Sulfiden (Au, Ag, Pb, Zn) 3) punktförmige Einschlüsse von gediegenem Gold 4) eluvialem, Aureichem, lateritischem Boden.Der durch das Eindringen von epizonalen Plutonen bedingte hydrothermale Umlauf, zusammen mit einer intensiven, ununterbrochenen durchgreifenden tektonischen Tätigkeit ist für die Au-Ag epithermalen Mineralisierungen in der vulkanischen Reihe von Aguacate verantwortlich. Die Kontamination und die Assimilation von andesitischen und »shoshonitischen« Gesteinen durch die aufdringenden Granitmagmen hat, in Verbindung mit hydrothermalen Erscheinungen, verschiedene Typen von Goldmineralisierungen erzeugt.Die kalkalkalischen Gesteine des goldführenden Gürtels von Costa Rica sind anscheinend an inkopatiblen Elementen mit niedrigem Ionenpotential (insbesondere Ba) angereichert. Bei vergleichbaren Kieselsäurekonzentrationen begegnet man eine allgemeine und allmähliche Zunahme an Y, Ni und Zr in den jüngsten Gesteinen, wenn man diese mit den Miozän-Gesteinen vergleicht. Au\erdem ist eine Zunahme der K/Rb und Rb/Sr Verhältnisse zu verzeichnen.Aufgrund der Verteilung der Spurenelemente vermutet man, da\ die Intrusivgesteine von Guacimal eine Gesamtheit von marginalen Plutonen darstellen, die mit denen der mobilen Kordillere (mobile belt) in Neuguinea vergleichbar sind. Dies würde bedeuten, da\ das magmatisch-metamorphe Fundament aus dem Präkambrium-Paläozoikum, das im nördlichen Teil von Nicaragua vorhanden ist, sich nach Süden ausdehen könnte.Vier unterschiedliche Magmaerzeugnisse sind ermittelt worden: kalkalkalischer und »shoshonitischer« Vulkanismus des Späteozäns-Oligozäns (Plattenkonvergenz und Subduktion); kalkalkalischer Miozän-Magmatismus (Kompression und Aufwölbung); alkalischer Magmatismus des Frühpliozäns (Entstehung des Valle Central Senkungsgrabens, mit E-W Streichen); Kalkalkali-Magmatismus des Mittel-Spätpliozäns (Krustenausdehnung und anschlie\ende Kompression und Aufwölbung).Die Beziehung zwischen den tektonischen Ereignissen, dem Magmatismus und den Mineralisierungen lä\t vermuten, da\ die Metallanreicherung der Kruste auf veränderliche Stre\bedingungen in Verbindung mit aktiver Subduktion zurückzuführen sein könnte.

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Resumen La Cintura Aurífera de Costa Rica está localizada en el sector norte-occidental del país y está caracterizada por unidades volcano-sedimentarias e igneas de edad terciaria en yuxtaposición tectónica.Las mineralizaciones dentro de la Cintura Aurífera están representadas por: 1) vetas cuarzo-auríferas y relleno de fisuras de origen epitermal asociadas con plata y metales básicos; 2) mineralizaciones diseminadas de sulfuros complejos (Au, Ag, Pb, Zn); 3) laminillas diseminadas de oro nativo; 4) suelos lateríticos eluviales ricos en oro.Se asigna como responsable de las mineralizaciones epitermales oro-plata de la Serie Volcanica de Aguacate, al tectonismo intenso y continuado, así como a la circulación hidrotermal inducida por el emplazamiento de plutones y »stocks« en condiciones epitermales. La contaminación y asimilación de rocas andesiticas y »shoshoniticas« por parte de magmas graniticos ascendentes generó, en asociación con fenomenos hidrotermales, un ambiente de campo aurífero compuesto.Las rocas calco-alcalinas de la Cintura Aurífera de Costa Rica muestran un elevado contenido de elementos incompatibles (en particular Ba). Por contenidos de SiO₂ similares, se evidencia un enrequecimiento general en Y, Ni, Zr desde las rocas del Mioceno a las rocas del Plioceno. Paralelamente se registra un aumento de las relaciones K/Rb y Rb/Sr.En particular se suspecha, por medio de la distribución de los elementos en traza, que las rocas intrusivas de Guacimal puedan representar un »grupo« de plutones marginales comparables con aquellos de la »Mobile Belt« de Nueva Guinea. Lo que indicaria la posible extensión al Sur del basamento igneo y metamórfico de edad Precambrica-Paleozoica presente en el norte de Nicaragua.Se reconocen quatro eventos magmaticos asociados con las condiciones geodinamicas regionales: vulcanismo calco-alcalino de edad Eoceno-Oligoceno (en condiciones de convergencia y subducción de placas), magmatismo calcoalcalino del Mioceno Medio y Temprano hasta Mioceno Tardio (en condiciones de compresión crostal y levantamiento), vulcanismo alcalino del Mioceno Temprano (Rift prevalente E-W), magmatismo calco-alcalino del Plioceno Medio-Tardfo (en condiciones de extensión crostal seguida por compressión y levantamiento).Las correlaciones entre condiciones geodinámicas, magmatismo y mineralizaciones, sugieren que el enriquecimiento en metales esta vinculado con los diferentes regimenes de deformación asociados con procesos de subducción activa.

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Bolide impacts, acid rain, and biospheric traumas at the Cretaceous-Tertiary boundary

Large projectiles impacting the Earth will cause severe shock heating and chemical reprocessing of the Earth's atmosphere. This occurs during atmospheric entry and, more importantly, as a result of the supersonic plume ejected on impact. In particular, very large amounts of nitric oxide would result from the impact of the putative Cretaceous-Tertiary bolide(s). We present models of the shock chemistry, the conversion of NO to NO₂ and thence to nitrous and nitric acids, and the global dispersion of the NO₂ and acids. Two plausible projectiles are considered: an ice-rich long-period comet and a much smaller rock-metal asteroid. The comet produces semi-global atmospheric NO₂ mixing ratios of 0.1% while the asteroid produces these high mixing ratios only in the immediate region of the impact. The comet produces concentrated nitrous and nitric acid rain with apH ≈ 0–1.5 globally. The asteroid produces rain with apH ≈ 0–1 near the impact and≈ 4–5 globally. Immediate environmental effects of these nitrogen species include inhibition of photosynthesis due to extinction of solar radiation by NO₂, foliage damage due to exposure to NO₂ and HNO₃, toxicosis resulting from massive mobilization of soil trace metals, and respiratory damage due to exposure to NO₂. The acid rain decreases the pH of the oceanic mixed layer affecting the stability of calcite with important implications for the survival of calcareous-shelled organisms and for exhalation of CO₂ from the mixed layer to the atmosphere. Longer-term environmental effects perhaps≈ 1000years in duration include a global warming due to the sudden CO₂ injection and the simultaneous extinction of the ocean micro-organisms which normally help remove CO₂ from the atmosphere-mixed layer system. Havens for survival include carbonate-buffered lakes and burrows. This acid rain theory therefore serves to explain the peculiar selectivity of the extinctions seen at the Cretaceous-Tertiary boundary. The first few years of acid rain will lead to massive weathering rates of continental soils characterized by large ratios of the relatively insoluble metals (e.g. Be²⁺, Al³⁺, Hg²⁺, Cu⁺, Fe²⁺, Fe³⁺, Tl³⁺, Pb²⁺, Cd²⁺, Mn²⁺, Sr²⁺) to the more soluble metals (Ca²⁺, Mg²⁺) which should have left a clear signal in the fossils of neritic organisms and in unperturbed neritic sediments near river deltas if such sediments still exist for the Cretaceous-Tertiary time frame.     

Environmental influences upon mercury, radon and helium concentrations in soil gases at a site near Denver, Colorado

Variations in soil gas Hg, Rn and He concentrations and meteorological variables were monitored daily at one site over a period of 22 months. Air and soil temperature, humidity, barometric pressure, soil moisture, wind direction and velocity, soil freeze-thaw, water table elevation, crystal strain and gas emissions were determined simultaneously in order to assess the influence of the environmental variables on gas emission.Mercury concentrations were found to be higher in the summer while Rn and He concentrations were higher in the winter. It is hypothesized that adsorption-desorption controls the migration of Hg whereas Rn and He concentrations are controlled by diffusion and mass transport. Gas emissions respond to seasonal and shorter-term changes in environmental conditions. Stepwise multiple regression using gas emissions as the dependent variables suggests that environmental parameters account for 62% of the total Hg variance, 83% of the total Rn variance, and 33% of the total He variance. Temperature, barometric pressure and soil moisture exert the most influence on gas emissions with temperature effects dominating gas emissions throughout the year. Soil gas emissions display a predictable behavior during winter and summer when more stable meteorological conditions exist. During the transitional seasons of spring and fall, soil gas emissions become erratic and exhibit increased variability. Environmental variables are interrelated and appear to control the manner of gas migration.Increased adsorption by solids during the season of falling temperature decreases soil gas Hg<0.0001 ng L⁻¹, which is desorbed as the soil warms in the spring. Transport of Rn and He is primarily by convection. During the winter months, air temperatures are less than soil temperatures promoting upward movement of Rn and He by convection. During the summer, soil temperatures are less than air temperatures and an inversion layer below the level of sampling reduces upward flux and observed concentration.