Physical differences between the initial phase of the formation of two types of coronal mass ejections

Physical differences in the formation of “gradual” and “impulsive” coronal mass ejections (CMEs) at heights of h < 0.2 R _⊙ just before and during the initial phase of their motion are studied using AIA/SDO ultraviolet data (h is the altitude above the solar surface and R _⊙ is the solar radius). The basic structure of a gradual CME is a magnetic rope located in the corona. During an hour or more preceding the initial phase, the magnetic rope demonstrates an increase in brightness and transverse size, first of the low, inner elements of the rope and then of elements in its outer envelope most distant from the Sun. The rope remains motionless during this time. The initial phase of a gradual CME begins from the motion of the magnetic rope’s outer envelope, which further becomes the basis for the CME frontal structure. At this stage, the inner low elements of the rope remain almost motionless. The initial phase of an impulsive CME begins with the appearance near the photosphere of a cavity moving away from the Sun; the dynamics of this cavity probably correspond to a magnetic tube filled with cool plasma rising from beneath the photosphere. This magnetic tube collides with and drags arch structures, which initially block the tube’s motion. These arch structures contribute to the CME formation, although the magnetic tube itself forms the basis of the CME.     

A new species of Cloudina from the terminal Ediacaran of Spain

The terminal Ediacaran genus Cloudina includes some of the earliest biomineralized fossils. It consists of a tube formed by stacked funnel-shaped elements, and is usually interpreted as the external skeleton of an early metazoan. Although a number of species have been described within this genus, their distinctiveness remains unclear and they may all belong to the type species, Cloudina hartmannae Germs, 1972. Here we describe the new species Cloudina carinata, from central Spain. C. carinata n. sp. has a distinct morphology characterized by external longitudinal crests, which confer on the tube an irregular polygonal cross-section. The funnels have a thickened apertural rim and a basal, slightly constricted circular opening. Successive funnels appear less deeply imbricated than in previously described material of Cloudina. The evidence of tube disarticulation in the material studied, particularly the abundance of loose funnels, indicates that funnels were secreted as independent elements, not fused to the previous ones. However, they could become fused during the life of the organism through the precipitation of inorganic cements between their walls. Several specimens of C. carinata n. sp. show evidence of asexual reproduction.     

Effect of the pulsar-tube radius on the gamma-ray curvature radiation from the polar regions of radio pulsars with non-dipolar magnetic fields

The effect of the radius of the tube of open magnetic-field lines on the gamma-ray curvature radiation from the polar regions of a radio pulsar with a non-dipolar magnetic field is analyzed. The pulsar is considered in a polar-cap model with free electron emission from the neutron-star surface. The effect of the non-dipolar magnetic field on the radius of curvature of the field lines and the field intensity is taken into account. In connection with the creation of electron-positron pairs, we take into account only the birth of pairs by curvature radiation in the magnetic field. The small non-dipolarity of the field enables the radio pulsar not to turn off, even after a considerable decrease in the pulsar-tube radius. For instance, with a 20% non-dipolarity (ν = 0.2), a pulsar with B = 10¹³ G and P = 0.5 s can still operate even for a fivefold decrease in the pulsar-tube radius. A maximum is observed in the dependence of the electrostatic potential in the diode on the non-dipolarity parameter ν at ν ～ 0.5–0.7. The pulse profile in non-thermal X-ray emission for ν ～ 0.5–0.7 may look virtually the same as for ν ～ 0.1–0.2. Decreases in the pulsar-tube radius could be due to a structure of currents in the magnetosphere that results in the pulsar diode on the neutron-star surface occupying only a small fraction of the pulsar tube, with the remainder of the tube containing an outer annular gap. The pulsar-tube size is also affected by the presence of a circum-pulsar disk. A change in the pulsar-tube radius could also be due to an external magnetic field, associated with either a magnetic white dwarf or a circum-pulsar disk.     

Horizontal magnetic fields in the solar photosphere

Two-dimensional simulations of time-dependent solar magnetogranulation are used to analyze the horizontal magnetic fields and the response of the synthesized Stokes profiles of the IR FeI λ1564.85 nm line to the magnetic fields. The 1.5-h series of MHD models used for the analyses reproduces a region of the magnetic network in the photosphere with an unsigned magnetic flux density of 192 G at the solar surface. According to the magnetic-field distribution obtained, the most probable absolute strength of the horizontal magnetic field at an optical depth of τ ₅ = 1(τ ₅ denotes τ at λ = 500 nm) is 50 G, while the mean value is 244 G. On average, the horizontal magnetic fields are stronger than the vertical fields to heights of about 400 km in the photosphere due to their higher density and the larger area they occupy. The maximum factor by which the horizontal fields are greater is 1.5. Strong horizontal magnetic flux tubes emerge at the surface as spots with field strengths of more than 500 G. These are smaller than granules in size, and have lifetimes of 3–6 min. They form in the photosphere due to the expulsion of magnetic fields by convective flows coming from deep subphotospheric layers. The data obtained qualitatively agree with observations with the Hinode space observatory.     

Jet structure of electric currents in coronal magnetic loops

We analyze the properties of the electric-current distribution over the cross sections of fairly dense coronal magnetic flux tubes in which the plasma pressure exceeds the magnetic pressure, so that the equilibrium is maintained by the ambient magnetic field. If the plasma is fully ionized, the distributions of the longitudinal and azimuthal currents over the cross section of the loop have the same spatial scale as the pressure distribution. However, even a small number of neutral atoms in the corona (with a mass fraction of the order of 10^?5, taking into account the partial ionization of helium) substantially modifies the current distribution over the tube cross section: in this case, a considerable fraction of the full current flowing along the tube is concentrated in a thin region near the axis with a radius of the order of (10^?2–10^?3)r ₀ (where r ₀ is the characteristic scale of the plasma-pressure distribution over the tube), thus forming a sort of a jet current. This comes about because the pattern of the conductivity anisotropy is substantially modified in the presence of ion-atom collisions in the magnetoactive plasma of the tube, and the Cowling conductivity dominates over the Hall and Pedersen conductivities. The high current density near the axis of the tube can ensure heating of the plasma to coronal temperatures via Joule dissipation.     

Some features of the streamer belt in the solar corona and at the Earth’s orbit

The rays of enhanced brightness making up the structure of the coronal-streamer belt can be traced to the lowest atmospheric layers in the Sun, with the angular size remaining nearly constant, d ≈ 2.5° ± 0.5°. This suggests that the physical mechanism generating the slow solar wind in the rays of the streamer belt differs from the mechanism giving rise to the fast solar wind from coronal holes. At distances of R < (4–5) R _⊙, the rays of the streamer belt are not radial in the plane of the sky and show deviations toward the corresponding pole. They then become essentially radial at R > (4–5) R _⊙. A transverse cross section of streamers in the corona and its continuation into the heliosphere—a plasma sheet—can be represented as two radially oriented, closely spaced rays (d ≈ 2.0°–2.5°) with enhanced density and an angular size of d. We also show that the ray structure of the streamer belt is involved in the development of coronal mass ejections (CMEs). The motion of a small-scale CME occurs within a magnetic flux tube (ray of enhanced brightness) and leads to an explosive increase in its angular size (rapid expansion of the tube). It seems likely that large-scale CMEs are the result of the simultaneous expansion of several magnetic tubes. We suggest that a small-scale CME corresponds to a “plasmoid” (clump of plasma of limited size with its own magnetic field) ejected into the base of a magnetic tube, which subsequently moves away from the Sun along the tube.     

Long-term variations in the asymmetry of the magnetic field of the sun and heliosphere

The asymmetry of the magnetic field of the Sun and its manifestation in the interplanetary magnetic field (IMF) are studied. The dominant magnetic polarity of the radial component of the IMF alternates from cycle to cycle, but with an overall systematic dominance of polarity directed toward the Sun. The global asymmetry is also manifest in the component of the IMF perpendicular to the plane of the solar equator. The dominance of positive values of B _z together with an appreciable linear trend in the cumulative sum of this quantity is interpreted as a manifestation of a relic solar magnetic field. The strength of this relic magnetic field near the Earth is estimated to be 0.048 ± 0.015 nT, based on the growth of the linear component of the cumulative sum of B _z . Time intervals, in which negative values of the B _z component of the IMF dominate and enhanced geomagnetic activity is observed, are identified. Our analysis of solar and heliospheric magnetic fields in an integrated representation has enabled us to compare various types of measurements and estimate their stability.     

The relative mobility of natural (Th,Pb and Po) and fallout (Pu,Am, Cs) radionuclides in the coastal marine environment: results from model ecosystems (MERL) and Narragansett Bay

The relative mobility of nuclides of Pb, Th, Pu, and Cs, which are widely used as geochemical tracers for sedimentation and bioturbation. was investigated in artificial microcosm tanks (MERL) and in Narragansett Bay. Their mobility was characterized by their removal rates from the water column, their affinity toward particles and their degree of penetration into the surface sediments. Pb, Th, and Pu transport was controlled predominantly by the affinity of these elements to particles, and the transport parameters of the particles themselves (i.e. sediment resuspension and mixing rates). Because of its low distribution coefficient, transport of Cs was controlled by molecular diffusion through pore water in the winter, while in summer, Cs transport was enhanced due to bioturbation. The increase with depth of the $\text{Cs}\text{Pu}$ ratio in core profiles of MERL and Narragansett Bay sediments is thus caused primarily by the higher mobility of Cs.     

Electron acceleration in the ionosphere of Io

A mechanism for the acceleration of electrons in the ionosphere of Io due to the moon's motion through the Jovian magnetic field and the presence of Io's ionosphere is considered. Attention is drawn to the important role of the anisotropic conductivity of the ionosphere, which results in the formation of a longitudinal (with respect to the planetary magnetic field) component of the charge-separation electric field. Owing to this anisotropy, the electric field induced by the motion of Io, E_i, produces in Io's ionosphere not only a Pedersen electrical current along E_i but also a Hall current that is approximately perpendicular to the moon's surface in the “upstream” and “downstream” parts of the ionosphere. However, this current cannot be closed through the surface, leading to the formation of a powerful charge-separation field in Io's ionosphere. This field has a component parallel to the magnetic field, with an amplitude comparable to that of the induced electric field. Electron runaway along the magnetic field is also considered, and the occurrence of “active longitudes” and preferred locations for the sources of decametric radio emission in the northern hemisphere of Jupiter are interpreted. The characteristic energies and fluxes of the accelerated electrons injected into Io's flux tube are estimated. The energy of these electron fluxes is sufficient to produce the electromagnetic radiation observed from Io's magnetic tube.     

Evolution of the first supernovae in protogalaxies: Dynamics of mixing of heavy elements

The paper considers the evolution of the supernova envelopes produced by Population III stars with masses ofM _* ?? 25?C200M _?? located in non-rotating protogalaxies with masses of M ?? 10⁷ M _?? at redshifts z = 12, with dark-matter density profiles in the form of modified isothermal spheres. The supernova explosion occurs in the ionization zone formed by a single parent star. The properties of the distribution of heavy elements (metals) produced by the parent star are investigated, as well as the efficiency with which they are mixed with the primordial gas in the supernova envelope. In supernovae with high energies (E ? 5 × 10⁵² erg), an appreciable fraction of the gas can be ejected from the protogalaxy, but nearly all the heavy elements remain in the protogalaxy. In explosions with lower energies (E ? 3 × 10⁵² erg), essentially no gas and heavy elements are lost from the protogalaxy: during the first one to threemillion years, the gas and heavy elements are actively carried from the central region of the protogalaxy (r ?? 0.1r _v , where r _v is the virial radius of the protogalaxy), but an appreciable fraction of the mass of metals subsequently returns when the hot cavity cools and the envelope collapses. Supernovae with high energies (E ? 5 × 10⁵² erg) are characterized by a very low efficiency of mixing of metals; their heavy elements are located in the small volume occupied by the disrupted envelope (in a volume comparable with that of the entire envelope), with most of the metals remaining inside the hot, rarified cavity of the envelope. At the same time, the efficiency of mixing of heavy elements in less energetic supernovae (E ? 3 × 10⁵² erg) is appreciably higher. This comes about due to the disruption of the hot cavity during the collapse of the supernova envelope. However, even in this case, a clear spatial separation of regions enriched and not enriched in metals is visible. During the collapse of the supernova envelope, the metallicity of the gas is appreciably higher in the central region ([Z] ?? ?1 to 0) than at the periphery ([Z] ?? ?2 to ?4) of the protogalaxy; most of the enriched gas has metallicities [Z] ?? ?3.5 to ?2.5. The masses of enriched fragments of the supernova envelope remain appreciably lower than the Jeans mass, except in regions at the center of the protogalaxy upon which the surrounding enriched gas is efficiently accreted. Consequently, the birth of stars with metallicities close to those characteristic of present-day Galactic stars is very probable in the central region of the protogalaxy.     

Analysis of optical spectra of V1357 Cyg≡Cyg X-1

Optical spectra and light curves of the massive X-ray binary V1357 Cyg are analyzed. The calculations were based on models of irradiated plane-parallel stellar atmospheres, taking into account reflection of the X-ray radiation, asphericity of the stellar surface, and deviations from LTE for several ions. Comparison of observed spectra obtained in 2004?C2005 at the Bohyunsan Observatory (South Korea) revealed variations of the depths of HI lines by up to 18% and of HeI and heavy elements lines by up to 10%. These variations are not related to the orbital motion of the star, and are probably due to variations of the stellar wind intensity. Perturbations of the thermal structure of the atmosphere due to irradiation in various states of Cyg X-1 (including outburst) do not lead to the formation of a hot photosphere with an electron temperature exceeding the effective temperature. As a result, variations of the profiles of optical lines of HI, HeI, and heavy elements due to the orbital motion of the star and variations of the irradiating X-ray flux do not exceed 1% of the residual intensities. Allowing for deviations from LTE enhances the HI and HeI lines by factors of two to three and the MgII lines by a factor of nine, and is therefore required for a fully adequate analysis of the observational data. Analysis of the HI, HeI, and HeII lines profiles yielded the following set of parameters for theOstar at the observing epoch: T _eff = 30 500±500 K, log g = 3.31±0.05, [He/H] = 0.42 ± 0.05. The observed HeI line profiles have emission components that are formed in the stellar wind and increase with the line intensity. The abundances of 11 elements in the atmospheres of V1357 Cyg and ?? Cam, which has a similar spectral type and luminosity class, are derived. The chemical composition of V1357 Cyg is characterized by a strong excess of helium, nitrogen, neon, and silicon, which is related to the binarity of the system.     

Emergence of magnetic flux at the solar surface and the origin of active regions

The evolution of photospheric velocities from the first minutes after the emergence of fresh magnetic flux and the formation of the first pores in active region NOAA 10488 is studied with a time resolution of 1 min and spatial resolution of 4″. The emerging magnetic flux of a major active region is initially a bundle of magnetic-flux loops. Some of these loops erupt through the system of supergranular cells with speeds of up to 1 km/s within 15–25 min and form pores and small spots. It is suggested that the development of a pore represents the emergence of a horizontal magnetic field, which is converted into elements with a strong vertical magnetic field. The region of ascending plasma initially coincides with the zero line of a bipolar magnetic pair. Downflow and upflow regions are related to and appear with the development of pores. During the first hours of their evolution, the trailing-polarity pores exhibit downflows with mean speeds of ∼500 m/s, while upflows with speeds of ∼250 m/s dominate near the leading-polarity pores. It is concluded that a matter flow from the leading to the trailing end is present in the rising loop of a magnetic flux tube, in agreement with well-known numerical-simulation results. The flow that develops in the magnetic-flux tube erupting through the convection zone persists when pores and small spots emerge in the photosphere, at least during the first hours of their evolution.     

Field and dual magnetic susceptibility proxies for heavy metal pollution assessment in the urban soil of Al-Karak City, South Jordan

A total of 115 urban soil samples collected on grid bases from Al-Karak, South Jordan, were investigated for their field and dual-frequency magnetic susceptibility (χ _field, χ _d) and heavy metal content using Bartington susceptibility meters and ICP-MS. The upper soils have higher magnetic susceptibility values than lower soils, and large particles contain more heavy metals and higher magnetic susceptibility than smaller particles. This might be attributed to the lack of pedogenesis due to arid climate influence. Within the upper soil all heavy metal showed positive significant correlation with upper soil low-frequency χ _dlf. This was evident from the distribution maps produced by Surfer 9.0 for χ _dlf and heavy metals. The results showed that higher χ _dlf is associated with traffic-dominated sites more than other areas. The frequency-dependent susceptibility (χfd %) falls between 2 and 10 %, which indicate the presence of admixture of fine supermagnetic particles. Mildly correlation exists between χfd % and χ _dlf, which implies that soils contain anthropogenic multi-domain grains. Selected samples have been analyzed for their mineral constituents; the results indicate the presence of magnetite as the main magnetic mineral. This confirms the anthropogenic source of pollution mainly from the vehicle-related materials. The results indicate the applicability of magnetic susceptibility for pollution detection.     

Stokes-meter observations of the solar mean magnetic field: Probable manifestations of strong small-scale magnetic fields

For many years, information on the solar mean magnetic field (SMMF) of the Sun—an important heliophysical and astrophysical parameter—was restricted to magnetographic measurements in only one spectral line, FeI λ525.02 nm. More informative observations of the Stokes-meter parameters of the SMMF were first initiated on a regular basis at the Sayan Solar Observatory. The availability of I and V data obtained simultaneously in several spectral lines has made it possible to study fundamentally new physical problems. In this paper, based on a comparison of SMMF observations in several spectral lines, we find high correlations in the data and important systematic differences in the magnetic-field strength B, which we interpret as a manifestation of kilogauss magnetic fields in fine-structure magnetic elements. Results of theoretical modeling of the SMMF strength ratios for the FeI λ525.02 nm-FeI λ524.70 nm and FeI λ630.15 nm-FeI λ630.25 nm lines are presented. The asymmetries of the V profiles of four lines near the FeI λ525.02 nm line are examined; these lines are important diagnostics for studies of small-scale dynamical processes. The Sayan Solar Observatory SMMF measurements are in good consistency with the Wilcox Solar Observatory data for 2003: for a comparison of N = 137 pairs of points in the two data sets, the correlation coefficient ρ is 0.92 for the linear regression between the datasets B_WSO = 0.03(±0.05) + 0.93(±0.03)B_SSO.     

Calculation of profiles of CIV,NV, OVI,and SiIV resonance lines formed in accretion shocks in T Tauri stars: A plane layer

We have calculated profiles of the CIV 1550, NV 1240, OVI 1035, and SiIV 1400 resonance doublets for a plane-parallel shock viewed at various angles. Calculations were performed for the range of preshock gas velocities V₀ and gas densities ρ₀ appropriate for classical T Tauri stars. The parameters of accretion shocks in young stars can be determined by comparing the calculated and observed profiles of the studied lines and their relative intensities. It is not possible to derive the parameters of the accreting gas from the line profiles without knowing the geometry of the accretion zone. The relation I _v (µ,V₀,ρ₀) for a plane shock, where I _v is the intensity μ=cosθ, can be used to determine the accretion parameters by either choosing a geometry for the radiating region or using a technique similar to Doppler tomography. The results obtained for DR Tau, T Tau, and RY Tau indicate that, in contrast to current concepts, the inner regions of the accretion disk are not disrupted by the magnetic field of the star, and the disk reaches the stellar surface. As a result, only a small fraction of the accreted matter passes through the shock and falls onto the star.     

The presence of a magnetic field on the pulsating subdwarf Balloon 090100001. Observations of 2012

New polarization observations of the subdwarf Bal 09 are analyzed. Bal 09 belongs to the group of hybrid sdB stars, which display both short- and long-period pulsations. Explaining certain properties of Bal 09 that were previously unknown in relation to subdwarfs (variations of the amplitude of the fundamental pulsation mode, rotational splitting of line multiplets and variation of this splitting) requires invoking information about the magnetic field of the star. According to estimates made in 2010, the longitudinal component of the magnetic field of Bal 09 is 34±63 G. This value is significantly lower than the fields found earlier for six other hot subdwarfs. New observational data for the longitudinal magnetic field of Bal 09 was obtained on July 27, 2012, using the main stellar spectrograph of the 6-m telescope of the Special Astrophysical Observatory (SAO). The longitudinal component of the magnetic field 〈B _z 〉 was found via a regression analysis. When applied to the star HD 210762 with a zero total magnetic field, this method yielded the value 〈B _z 〉 = ?12 ± 9 G. The observations also included measurements of 〈B _z 〉 for the well-studied magnetic star γ Equ, which is used at the SAO for calibration and testing of the polarimetric instruments. The estimate obtained for γ Equ is 〈B _z 〉 = ?546±16G, consistentwith the general variations of the longitudinal magnetic-field component of this star. This new study, based on data obtained on July 27, 2012, leads to a similar estimate of the longitudinal magnetic field, 〈B _z 〉 = ?23±53 G. This estimate of 〈B _z 〉 was obtained for the full analyzed spectral range (4400–4958 Å). The corresponding “limited” solution yielded ?32±63 G. The regression analysis for the individual spectral sub-bands and for bands containing characteristic spectral features, did not provide firm evidence of the presence of a magnetic field, with a strength exceeding the error in 〈B _z 〉. The data analysis leads to the conclusion that the errors of the measurements made in 2010 and 2012 are in good agreement. This testifies to the reliability of the method applied and of the resulting observational material. In addition, the estimates are in good agreement among themselves and with estimates obtained earlier, in 2010. The results unambiguously confirm the earlier conclusion that the subdwarf Bal 09 does not have magnetic field with a strength comparable to those detected earlier for six sdB and sdO stars. Estimates of 〈B _z 〉 for hot subdwarfs that have appeared in the literature since the 2010 study also provide trustworthy evidence for the absence of magnetic fields ～ 1 kG in these objects.     

The structure of solar filaments. Prominences in the corona free from external magnetic field

The new approach to the modeling of quiescent solar prominences is proposed. We solve the inverse magnetohydrostatic problem, when the pressure, density and temperature of plasma in the filament are calculated from the equilibrium equations using the given magnetic structure (magnetic flux function is proposed to be known). The new exact nonlinear solutions for dense (n ≈ (2?3) × 10¹¹ cm^?3) and cold (T ≈ (5?10) × 10³ K) filaments, embedded in the plan, vertically stratified atmosphere (hot solar corona) free of magnetic field, are derived. The filaments are stretched along the horizontal axisy(the translational symmetry is assumed: ?/?y = 0) and located parallel to and above a photospheric, magnetic polarity reversal line. The magnetic field lines have a structure of magnetic flux rope with helical field lines in three-dimensional space; the strength of magnetic field falls rapidly with distance from a rope axis. No external longitudinal magnetic field is needed to equilibrate the prominence. The net electric current along the filament is equal to zero. The model of magnetic arcade with the deflection (sag) on the top, proposed by Pikelner (1971) as a basic form of normal prominence, is calculated also using the method proposed. It is shown that such magnetic arcade, having the magnetic field strength of few gauss only, can effectively maintain the equilibrium of cool dense filament at the heights about 50–60 Mm.     

Analytical representation of spatial and temporal variations of the geomagnetic field in the Indian region

The magnetic measurements of declination (D), horizontal (H) and vertical (Z) components of earth’s magnetic field, collected from ground surveys between 1962 and 1966, are used to develop an analytical model of geomagnetic field variations over Indian region for the epoch 1965. In order to reflect spatial features with wavelengths of approximately 1000 km, sixth degree polynomial as a function of differential latitude and longitude is calculated by the method of least squares. The root mean square fit of the model to the input data is better than that accounted by the International Geomagnetic Reference Field for 1965.0. Isomagnetic charts drawn forD, H, Z and total force (F) reflect more details than that shown on world magnetic charts. Further, the values of the field at common repeat stations recorded between 1962 and 1974, after eliminating the field values for the epoch 1965.0, are used to get the secular variation as well as its spatial dependence again by means of polynomial which now includes coefficients which are functions of time and of geographical locations. The accuracy of coefficients is tested against the behaviour of secular variation at permanent magnetic observatories. The merits and limitations of the model are discussed.     

The ZONMET thermodynamic and kinetic model of metal condensation

The ZONMET model of metal condensation is a FORTRAN computer code that calculates condensation with partial isolation-type equilibrium partitioning of the 19 most abundant elements among 203 gaseous and 488 condensed phases and growth in the nebula of a zoned metal grain by condensation from the nebular gas accompanied by diffusional redistribution of Ni, Co, and Cr. Of five input parameters of the ZONMET model (chemical composition of the system expressed as the dust/gas [D/G] ratio, nebular pressure [P_tot], isolation degree [ξ], cooling rate (CR), and seed size), only two—the D/G ratio and the CR of the nebular source region of a zoned Fe,Ni grain—are important in determining the grain radius and Ni, Co, and Cr zoning profiles. We found no evidence for the supercooling during condensation of Fe,Ni metal that is predicted by the homogeneous nucleation theory. The model allows estimates to be made of physicochemical parameters in the CH chondrite nebular source regions.Modeling growth and simultaneous diffusional redistribution of Ni, Co, and Cr in the zoned metal grains of CH chondrites reveals that the condensation zoning profiles were substantially modified by diffusion while the grains were growing in the nebula. This means that previous estimates of the physicochemical conditions in the nebular source regions of CH and CB chondrites, based on measured zoning profiles of Ni, Co, Cr, and platinum group elements in Fe,Ni metal grains, need to be corrected.The two zoned metal grains in the PAT 91456 and NWA 470 CH chondrites studied so far require nebular source regions with different chemical compositions (D/G = 1 and D/G = 4, respectively) and thermal histories characterized by variable cooling rates (CR = 0.011 + 0.0022 × ΔT K/h and CR = 0.05 + 0.0035 × ΔT K/h, respectively). It appears that the metal grains of the CH chondrites were formed in multiple nebular source regions or in different events within the same source region as the CB chondrite metal grains were formed.     

Geochemistry of trace metals associated with reduced sulfur in freshwater sediments

Porewater concentration profiles were determined for Fe, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn), sulfide, SO₄ and pH in two Canadian Shield lakes (Chevreuil and Clearwater). Profiles of pyrite, sedimentary trace elements associated with pyrite and AVS were also obtained at the same sites. Thermodynamic calculations are used, for the anoxic porewaters where sulfide was measured, to characterize diagenetic processes involving sulfide and trace elements and to illustrate the importance of sulfide, and possibly polysulfides and thiols, in binding trace elements. The ion activity products (IAP) of Fe sulfide agree with the solubility products (K_s) of greigite or mackinawite. For Co, Ni and Zn, IAP values are close to the K_S values of their sulfide precipitates; for Cu and Pb, IAP/K_s indicate large oversaturations, which can be explained by the presence of other ligands (not measured) such as polysulfides (Cu) and thiols (Pb). Cobalt, Cu, Ni and Zn porewater profiles generally display a decrease in concentration with increasing ΣH₂S, as expected for transition metals, whereas Cd, Pb and Zn show an increase (mobilisation). The results suggest that removal of trace elements from anoxic porewaters occurs by coprecipitation (As and Mn) with FeS(s) and/or adsorption (As and Mn) on FeS(s), and by formation of discrete solid sulfides (Cd, Cu, Ni, Pb, Zn and Co). Reactive Fe is extensively sulfidized (51–65%) in both lakes, mostly as pyrite, but also as AVS. Similarities between As, Co, Cu and Ni to Fe ratios in pyrite and their corresponding mean diffusive flux ratios suggest that pyrite is an important sink at depth for these trace elements. High molar ratios of trace elements to Fe in pyrite from Clearwater Lake correspond chronologically to the onset of smelting activities. AVS can be an important reservoir of reactive As, Cd and Ni and, to a lesser extent, of Co, Cu and Pb. Overall, the trace elements most extensively sulfidized were Ni, Cd and As (maximum of 100%, 81% and 49% of the reactive fraction, respectively), whereas Co, Cu, Mn, Pb and Zn were only moderately sulfidized (11–16%).