Can the ordinary chondrites have condensed from a gas phase?

The conditions under which ordinary chondrites containing iron in three different chemical states can form in thermodynamic equilibrium with a gas phase are calculated. Hydrogen depletion factors of 10²–10³ are obtained and the formation of liquid condensates from residual gases occurs at pressures (prior to hydrogen depletion) of ?1 atm.     

Interaction of organic compounds with calcium carbonate-II. Organo-carbonate association in Recent sediments

Surface area measurements as well as organic carbon, nitrogen and phosphorus analyses on various grain size fractions of carbonate mud samples confirm that in natural environments of carbonate deposition, surface sorption processes take place which are similar to those described earlier for dissolved organics and artificially suspended calcite particles in both seawater and synthetic solutions.The specific surface area of the sediment increases from 1.8m²/g for the coarse-grained fraction to 12.5 m²/g for the fine material; likewise organic carbon, nitrogen and phosphorus increase with increasing surface area so that there are 1.20 mg C, 0.175 mg N and 0.06–0.20 mg P associated with every square meter of carbonate surface irrespective of the mineralogy of the sediment particles.It appears that the organic matter in these sediments is similar in composition, structure and quantity to the organic layers produced in sorption experiments. With their apparently defined structure and ubiquitous nature, these layers could determine the mineralogy and orientation of submarine carbonate cement or could even be a prerequisite to calcification in general.     

Numerical modeling of coronal mass ejections based on various pre-event model atmospheres

We examine how the initial state (pre-event corona) affects the numerical MHD simulation for a coronal mass ejection (CME). Earlier simulations based on a pre-event corona with a homogeneous density and temperature distribution at the lower boundary (i.e., solar surface) have been used to analyze the role of streamer properties in determining the characteristics of loop-like transients. The present paper extends these studies to show how a broader class of global coronal properties leads not only to different types of CMEs, but also modifies the adjacent quiet corona and/or coronal holes.We consider four pre-event coronal cases: (1) constant boundary conditions and a polytropic gas with = 1.05; (2) non-constant (latitude dependent) boundary conditions and a polytropic gas with = 1.05; (3) constant boundary conditions with a volumetric energy source and = 1.67; (4) non-constant (latitude dependent) boundary conditions with a volumetric energy source and = 1.67. In all models, the pre-event magnetic fields separate the corona into closed field regions (streamers) and open field regions. The CME's initiation is simulated by introducing at the base of the corona, within the streamer region, a standard pressure pulse and velocity change. Boundary values are determined using MHD characteristic theory.The simulations show how different CMEs, including loop-like transients, clouds and bright rays, might occur. There are significant new features in comparison to published results. We conclude that the pre-event corona is a crucial factor in dictating CMEs properties.     

The effects of magnesium-to-calcium ratios in artificial seawater,at different ionic products,upon the induction time,and the mineralogy of calcium carbonate: a laboratory study

The effects of the Mg²⁺ ion concentration and the ionic products of carbonate upon the induction time for the onset of precipitation and the different mineralogies of calcium carbonates were studied. It was shown that Mg²⁺ ions delay the spontaneous precipitation of calcium carbonate from supersaturated solutions (e.g. seawater) with respect to calcium carbonate mineral to such an extent that only biogenic removal of skeletal calcium carbonate is possible from the open ocean. Low concentrations of magnesium ions in solution favor calcite formation while aragonite is formed at high magnesium concentrations. The mole% of MgCO₃ in magnesian calcite increases with the increase of (Mg²⁺) in solution and with the increase of (CO₃ ^2–) in the presence of (Mg²⁺) in solution.Therefore, one would expect that high Mg-calcite is formed in wormed coastal regions, where high temperature and or the increase of photosynthesis activities tend to expel CO₂ and increase supersaturation, and low-magnesian calcite is favored in meteoric-vadose environment where low concentration of magnesium ions or in burial environment where respiration and oxidation is high and decrease supersaturation.

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Zusammenfassung Die Wirkung der Mg²⁺-Ionenkonzentration und des Ionenprodukts von Karbonaten wurde in Abhängigkeit von der Induktionszeit für den Beginn der Ausfällung und der unterschiedlichen Mineralogie von Kalziumkarbonaten untersucht. Es konnte gezeigt werden, daß Mg²⁺-Ionen die spontane Ausfällung von Kalziumkarbonat aus einer übersättigten Lösung (z. B. Meerwasser), im Vergleich zu Kalziumkarbonatmineralien derart verzögert, daß nur noch der biogene Entzug für skelettbildendes Kalziumkarbonat aus dem Meerwasser möglich ist. Niedrige Konzentrationen von Magnesiumionen in der Lösung fördern die Kalzitbildung, während Aragonit bei hohen Magnesiumkonzentrationen gebildet wird. Die Mol% MgCO₃ in Magnesiumkalzit steigen mit der Zunahme von (Mg²⁺) und mit der Zunahme von (CO₃ ^2–) bei Gegenwart von (Mg²⁺) in der Lösung.Daher würde man erwarten, daß Hochmagnesiumkalzit in erwärmten Küstengebieten gebildet wird, wo hohe Temperaturen, und/oder die Zunahme der Photosyntheseaktivität dazu führen, daß CO₂ frei wird, und damit die Übersättigung ansteigen lassen. Niedrigmagnesiumkalzit wird bevorzugt in meteorische-vadosen Gebieten mit niedriger Magnesiumionenkonzentration oder in Versenkungsgebieten gebildet, wo die Respiration und die Oxidation hoch ist, bei sinkender Übersättigung.

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Résumé Une étude a été menée sur l'effet de la concentration en ions Mg²⁺ et des produits ioniques des carbonates sur le temps d'induction du début de la cristallisation et sur la nature minéralogique du carbonate de calcium formé. On a pû montrer que les ions Mg²⁺ retardent la précipitation spontanée du carbonate de Ca à partir de solutions sursaturées par rapport aux minéraux de CaCO₃ (par exemple l'eau de mer) et ce à un point tel que seule la précipitation biogénique du carbonate de Ca dans les squelettes est possible à partir de l'eau de mer. Les faibles concentrations en ions Mg²⁺ dans les solutions favorisent la formation de calcite, l'aragonite se formant pour des concentrations plus élevées. Le pourcentage molaire de MgCO₃ dans la calcite magnésienne augmente avec la teneur, dans la solution, de Mg²⁺ et de Co₃ ^2– enprésence de Mg²⁺.On présume donc que la calcite riche en Mg se forme dans des milieux côtiers chauds dans lesquels la haute température et/ou la forte activité photosynthétique provoquent la libération de CO₂, ce qui augmente la sursaturation. La calcite pauvre en Mg se forme de préférence dans des milieux météoriques vadoses à faible concentration en ions Mg²⁺ ou dans des milieux d'enfouissement où la respiration et l'oxydation importantes abaissent le degré de sursaturation.

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Relations between methane venting,geological structure and seismo-tectonics in the Okhotsk Sea

Methane investigations carried out in the Okhotsk Sea show that the methane flux from the earths interior into the water column increased during periods of seismo-tectonic activity between 1988 and 2002. In this case, methane gas hydrates found on the northeast Sakhalin slope may have decomposed due to a reactivation of fault zones. Methane emissions in the Okhotsk Sea generally can be divided into two forms. Firstly, methane vents from decomposing gas hydrates and/or free gas exist below gas hydrate saturated sediments via fault zones, venting into the water column with high bubble concentrations that were recorded by echosounding. These hydro-acoustic anomalies were named flares. Methane concentration inside these flares reached 10,000–20,000 nl/l (background methane concentrations in the Okhotsk Sea are less than 90–100 nl/l). Secondly, methane migrates as seepage into the water column from oil- and gas-bearing sedimentary source rocks on the eastern Sakhalin shelf, without showing acoustic anomalies in the water column, probably by filtration and diffusion processes. In these areas methane concentration reached 500–3,000 nl/l. In seismo-tectonically active regions, like the northwestern part of the Okhotsk Sea, many new flares were observed. Their distribution and orientation are usually controlled by fault zones (East Sakhalin Shear Zone in the Okhotsk Sea).     

Phosphate regeneration from sediments of the Peru continental margin by dissolution of fish debris

Generally, oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes. Stoichiometric modelling of such reactions takes into consideration varying proportions of ‘decomposable’ organically bound P to account for the ratios among nutrients in depth-concentration profiles of near-surface sediments. New results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off Peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P.Dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations. Phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10% of the nutrient pool in the waters of the Peru undercurrent. Partitioning of P among the two sources reveals further that fish debris phosphate is about four times more important than organically bound P in nutrient generation from sediments of the Peru continental margin. Not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area.     

Mössbauer spectroscopic studies on the iron forms of deep-sea sediments

Mössbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin. The procedure in unraveling this information includes temperature-dependent measurements from 275?K to very low temperature (300?mK) in zero–field and also at 4.2?K in an applied field (up to 6.2?T) and by mathematical procedures (least-squares fits and spectral simulations) in order to resolve individual spectral components. The depth distribution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19?cm with a subsequent steep increase (within 3?cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40?cm). The steep increase of the amount of Fe(II) defines a redox boundary which coincides with the position where the tan/green color transition of the sediment occurs. The isomer shifts and quadrupole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and silicate minerals. Goethite and traces of hematite are observed only above the redox boundary, with a linear gradient extending from about 20% of the total Fe close to the sediment surface to about zero at the redox boundary. The superparamagnetic relaxation behavior allows to estimate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. ～170?Å and ～50?Å, respectively. The composition of the sediment spectra recorded at 300?mK in zero-field, apart from the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been identified as major constituents of the sediment in the <2?μm fraction by X-ray diffraction. The specific “ferromagnetic” type of magnetic ordering in the sediment, as detected at 4.2?K in an applied field, also resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This “ferromagnetic” behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)–Fe(III) systems. A significant part of the clay-mineral iron is redox sensitive. It is proposed that the color change of the sediment at the redox boundary from tan to green is related to the increase of Fe(II)–Fe(III) pairs in the layer silicates, because of the intervalence electron transfer bands which are caused by such pairs.     

A large methane plume east of Bear Island (Barents Sea): implications for the marine methane cycle

A pockmark field extending over 35 km² at 74°54N, 27°3E, described by Solheim and Elverhøi (1993), was re-surveyed and found to be covered with more than 30 steep-sided craters between 300 and 700 m in diameter and up to 28 m deep. The craters are thought to have been formed by an explosive gas eruption. Anomalously high concentrations of methane in the shelf waters around the craters suggest that a strong methane source near this area is still active today. Methane enrichment more than 10 km away from the crater field indicates the large dimensions of a plume and the amount of gas released from sources below the seafloor of the Barents Sea shelf. From the characteristic vertical decrease of methane towards the sea surface, it is concluded that biota are extensively using this energy pool and reducing the methane concentration within the water column by about 98% between 300 m depth and the sea surface. Degassing to the atmosphere is minimal based on the shape of the methane concentration gradient. Nevertheless, the net flux of methane from this area of the Barents Sea is about 2.9 × 10⁴ g CH₄ km^–2 yr^–1 and thus in the upper range of the presently estimated global marine methane release. This flux is a minimum estimate and is likely to increase seasonally when rough weather leads to more effective vertical mixing during autumn and winter. The amount of methane consumed in the water column, however, is about 50 times greater and hence should significantly contribute to the marine carbon inventory.     

Correlated variations of planetary albedos and coincident solar-interplanetary variations

To locate two-dimensional positions of the solar decametric radio bursts a heliograph was developed on the basis of the UTR-2 radiotelescope (Khar'kov) operated in the range 10–26 MHz. The beamwidth of the heliograph rapid-scanning pencil-beam is 25 arc min at 25 MHz, and its field of view is about 3.5° (E-W) × 2.0° (N-S). The instrument yields rapidly forty records of the radio brightness of all (8 × 5) elementary parts (each 25 arc min in diameter) of the investigated sky area during every period of 1/4 s. Both coordinates of a burst center are measured with an accuracy 5 arc min. The bandwidth of the receiving system is 10 kHz. The heliograph operates in conjunction with a radiospectrograph connected to the output of a N-S arm of the UTR-2 array. The data observations with the UTR-2 correspond only to one linear polarized component.The ionospheric distortion of the test records of the radio source Cassiopeia-A that occurred sometimes is illustrated.First results of 25 MHz observations of the solar radio storm in August, 1976 with the heliograph are presented here. This storm is accompanied by the compact sunspot group travelling all over the optical disk. The type III and stria bursts were predominant during the storm. On the given day the scattering regions of their apparent centers were overlapped and the sizes of these regions were usually not more than 5 arc min. On some days there occurred additional burst sources displaced in position from the persistent storm region. It was found out that, as a rule, 25 MHz stria-bursts from the type IIIb chain coincided in position with the following type III burst at the same frequency. The difference of the daily averaged coordinates of both stria and type III bursts was considerably smaller than the mean diameter of their sources.The radial distance of the 25 MHz storm region from the solar center was calculated by using the three methods. The storm height was estimated as 1.8R from the rotation rate close to the central meridian of the storm center. Definite association of the spots with the storm near the limb allowed to determine the average value 2.1R for the height. The limb measurements give the mean height of 2.3R .The center-to-limb variation of the storm source height is a known fact in the meter-wavelength range. This is the evidence of the propagation effects in the solar corona being essential to interpret the results of the radio source location.