The HI halo of spiral galaxies     

Filippo Fraternali  Tom Oosterloo  Renzo Sancisi 《Astrophysics and Space Science》2004,289(3-4):377-380

A deep H I survey with the VLA of the spiral galaxy NGC 2403 has revealed the existence of a thick, low density layer of neutral gas surrounding the thin ‘cold’ disk. This layer has a mean rotation velocity 25–50 km s^-1 lower than that of the disk and a 10–20 km s^-1inflow towards the centre of the galaxy. In the central parts there are velocity differences from rotation of up to 150 km s^-1.Chandra observations of NGC 2403 show a diffuse, hot X-ray emitting gas component with a temperature of a few 10⁶ K. These results point at galactic fountain type of flows between disk and halo. ‘Halo’ gas with similar characteristics has also been observed in other spiral galaxies(e.g. NGC 6946, NGC 891). Such gas is probably similar to the IVCs and to some of the HVCs of the Milky Way.  相似文献   

Surface structure effects on direct reduction of iron oxides by Shewanella oneidensis     

Andrew L. Neal  Kevin M. Rosso  Yuri A. Gorby 《Geochimica et cosmochimica acta》2003,67(23):4489-4503

The atomic and electronic structure of mineral surfaces affects many environmentally important processes such as adsorption phenomena. They are however rarely considered relevant to dissimilatory bacterial reduction of iron and manganese minerals. In this regard, surface area and thermodynamics are more commonly considered. Here we take a first step towards understanding the nature of the influence of mineral surface structure upon the rate of electron transfer from Shewanella oneidensis strain MR-1 outer membrane proteins to the mineral surface and the subsequent effect upon cell “activity.” Cell accumulation has been used as a proxy for cell activity at three iron oxide single crystal faces; hematite (001), magnetite (111) and magnetite (100). Clear differences in cell accumulation at, and release from the surfaces are observed, with significantly more cells accumulating at hematite (001) compared to either magnetite face whilst relatively more cells are released into the overlying aqueous phase from the two magnetite faces than hematite. Modeling of the electron transfer process to the different mineral surfaces from a decaheme (protoporphyrin rings containing a central hexacoordinate iron atom), outer membrane-bound cytochrome of S. oneidensis has been accomplished by employing both Marcus and ab initio density functional theories. The resultant model of electron transfer to the three oxide faces predicts that over the entire range of expected electron transfer distances the highest electron transfer rates occur at the hematite (001) surface, mirroring the observed cell accumulation data. Electron transfer rates to either of the two magnetite surfaces are slower, with magnetite (111) slower than hematite (001) by approximately two orders of magnitude. A lack of knowledge regarding the structural details of the heme-mineral interface, especially in regards to atomic distances and relative orientations of hemes and surface iron atoms and the conformation of the protein envelope, precludes a more thorough analysis. However, the results of the modeling concur with the empirical observation that mineral surface structure has a clear influence on mineral surface-associated cell activity. Thus surface structure effects must be accounted for in future studies of cell-mineral interactions.  相似文献   

Metal island growth and dynamics on molybdenite surfaces     

Udo Becker  Kevin M. Rosso  Michele Warren 《Geochimica et cosmochimica acta》2003,67(5):923-934

In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS₂) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface.The strength and the nature of the Ag-S(MoS₂) bond were further investigated by using molecular orbital calculations, ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). By applying quantum mechanical approaches using a two-dimensional periodic molybdenite slab and hexagonal MoS₂ clusters of different sizes with metal atoms adsorbed to them, it is possible to calculate the electron transfer between the mineral surface and the metal atom as well as the adsorption energy as a function of surface coverage. In addition, we used the results from the quantum mechanical runs to derive empirical potentials that model the characteristics of the forces within the crystal, within the adsorbed islands, and the metal and mineral surface. The combination of quantum mechanical calculations and empirical force field calculations explain the electronic structure and the highest stability of Ag islands that have seven atoms in diameter, which exactly agrees with the size of experimentally observed islands. UPS results also suggest that a specific new state is formed (approximately 4.5 eV into the valence band) which may describe the Ag-S bond because it does not occur in pure silver or molybdenite.This study shows how the combination of microscopic (STM), spectroscopic (STS, UPS), compositional (X-ray photoelectron spectroscopy, XPS) and molecular modeling (quantum mechanical and empirical) techniques is a useful approach to understand the nature of the metal to sulfide bond. Further insights may be gained concerning the natural association of certain metals with sulfides.  相似文献   

Radiocarbon chronology of Holocene hydrologic changes in northeastern Mali     

C. Hillaire-Marcel  J. Riser  P. Rognon  N. Petit-Maire  J.C. Rosso  I. Soulie-Marche 《Quaternary Research》1983,20(2):145-164

A detailed chronology of hydrological changes that occurred in northeastern Mali during the Holocene is based on approximately 30 ¹⁴C dates of molluscs which were collected from the lacustrine and paludal deposits of the Ine Sakane sand sea. The presence of cemented paleodunes indicates that the end of the Pleistocene was and arid period. This was followed by an early Holocene lacustrine episode (ca. 9500–6500 yr B.P.) during which numerous lakes occupied depressions formed in the Cretaceous limestones and between the sand ridges. These lakes reflect a significant rise in the water table. Between ca. 6500 and 5500 yr B.P. the lakes dried up, although the water table remained close to the ground surface in several areas. Calcareous concretions formed at the water table fringe during this time, thereby “fossilizing” some of the middle Holocene dunes. A second lacustrine episode is dated between ca. 5500 and 4500 yr B.P.; it corresponds to the establishment of numerous Neolithic settlements in the area. Arid conditions have developed since about 4500 yr B.P. to attain the conditions of the present day. The isotopic composition (¹⁸O and ¹³C) of the fauna reflects highly variable hydrological conditions. Groundwaters were recharged by precipitation which was occasionally very depleted in heavy oxygen (δ¹⁸O ? ?13%.). Evaporation induced an enrichment in ¹⁸O; high concentrations characterize some lakes. The ¹³C content of fossil shells reflect: (1) species ecological requirements, (2) isotopic composition of the total inorganic dissolved carbon (TIDC) in groundwaters, and (3) the lake metabolism.  相似文献   

The June 4th 1999 severe weather episode in San Quirino, Italy: a tornado event?     

Renzo Bechini  Dario Giaiotti  Agostino Manzato  Fulvio Stel  Stefano Micheletti 《Atmospheric Research》2001,56(1-4)

On the morning of June 4th 1999, a severe weather event took place in San Quirino, a small village of Friuli-Venezia Giulia in the northeast of Italy. This village is located near the piedmont of the Alps, 40 km west from Udine and 60 km north from Venice.Around 0900 UTC (1100 local time), a thunderstorm with an intense hail fall affected the area of San Quirino. A few minutes later (around 0920 UTC, source: a farmer), a funnel cloud from a cumulonimbus touched the ground, producing damages to houses, trees and sheds. The damaged area was quite narrow (about 300 m) and short (less than 10 km). No injuries to people were reported.In spite of the smallness of the area interested by the phenomenon, this storm is studied here starting from the synoptic scale, moving to the mesoscale and finishing with the storm scale, trying to underline its characteristics. These analyses, especially those coming from the Doppler radar images, bring us to the conclusion that the San Quirino episode was produced by a supercell storm.  相似文献   

Electron tunneling properties of outer-membrane decaheme cytochromes from Shewanella oneidensis     

Nicholas S. Wigginton  Kevin M. Rosso  Liang Shi 《Geochimica et cosmochimica acta》2007,71(3):543-555

We have characterized the outer-membrane decaheme cytochromes OmcA and MtrC from Shewanella oneidensis MR-1 at the single-molecule level using scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). These cytochrome proteins are of great interest because they are thought to mediate bacterial electron transfer reactions in anoxic waters that control the reductive dissolution of oxide minerals. In our study, to characterize the electron transfer properties of these proteins on a model surface, the purified cytochromes were chemically immobilized as molecular monolayers on Au(111) substrates via a recombinant tetra-cysteine sequence as verified by X-ray photoelectron spectroscopy. Atomic force microscopy images confirm the monolayer films were ∼5-8 nm thick which is consistent with the apparent lateral dimensions of individual cytochrome molecules obtained with STM. Current-voltage TS of single cytochrome molecules revealed that OmcA and MtrC have different abilities to mediate tunneling current despite having otherwise very similar molecular and biochemical properties. These observations suggest that, based on their electron tunneling properties, the two cytochromes could have specific roles during bacterial metal reduction. Additionally, this study establishes single-molecule STM/TS as an effective means for revealing insights into biogeochemical redox processes in the environment.  相似文献   

Magmatic History of Somma-Vesuvius on the Basis of New Geochemical and Isotopic Data from a Deep Borehole (Camaldoli della Torre)   总被引：3，自引：0，他引：3  

Di Renzo  V.; Di Vito  M. A.; Arienzo  I.; Carandente  A.; Civetta  L.; D'antonio  M.; Giordano  F.; Orsi  G.; Tonarini  S. 《Journal of Petrology》2007,48(4):753-784

A continuous-coring borehole recently drilled at Camaldoli dellaTorre on the southern slopes of Somma–Vesuvius providesconstraints on the volcanic and magmatic history of the Vesuvianvolcanic area since c. 126 ka BP. The cored sequence includesvolcanic units, defined on stratigraphical, sedimentological,petrological and geochemical grounds, emitted from both localand distal vents. Some of these units are of known age, suchas one Phlegraean pre-Campanian Ignimbrite, Campanian Ignimbrite(39 ka), Neapolitan Yellow Tuff (14· 9 ka) and VesuvianPlinian deposits, which helps to constrain the relative ageof the other units. The main rock types encountered are shoshonite,phonotephrite, latite, trachyte and phonolite. The sequenceincludes, from the base upwards: a thick succession of pyroclasticunits emplaced between 126 and 39 ka, most of them attributedto eruptions that occurred in the Phlegraean area; the CampanianIgnimbrite; the products of a local tuff cone formed between39 ka and the deposition of the products of the earliest activityof the Mt. Somma volcano; the products of the Somma–Vesuviusvolcano, which include from the base upwards a thick sequenceof lavas, pyroclastic rocks and the products of a local spattercone dated between 3· 7 ka and AD 79. The data obtainedfrom the study of the borehole show that, before the CampanianIgnimbrite eruption, low-energy explosive volcanism took placein the Vesuvian area, whereas mostly high-energy explosive eruptionscharacterized the Campi Flegrei activity. In the Vesuvian area,Campanian Ignimbrite deposition was followed by the eruptionof a local tuff cone and a long repose time, which predatedthe formation of the Mt. Somma edifice. Since 18· 3 ka(Pomici di Base eruption) the activity of Somma–Vesuviusbecame mostly explosive with rare lava effusions. The shallowestcored deposits belong to the Camaldoli della Torre cone, formedbetween the Pomici di Avellino and Pomici di Pompei eruptions(3· 7 ka–AD 79). New geochemical and Sr–Nd–Pb–B-isotopicdata on samples from the drilled core, together with those availablefrom the literature, allow us to further distinguish the volcanicrocks as a function of both their provenance (i.e. Phlegraeanor Vesuvian areas) and age, and to identify different magmaticprocesses acting through time in the Vesuvian mantle source(s)and during magma ascent towards the surface. Isotopically distinctmagmas, rising from a mantle source variably contaminated byslab-derived components, stagnated at mid-crustal depths (8–10km below sea level) where magmas differentiated and were probablycontaminated. Contamination occurred either with Hercynian continentalcrust, mostly during the oldest stages of Vesuvian activity(from 39 to 16 ka), or with Mesozoic limestone, mostly duringrecent Vesuvian activity. Energy constrained assimilation andfractional crystallization (EC-AFC) modelling results show thatcontamination with Hercynian crust probably occurred duringdifferentiation from shoshonite to latite. Contamination withlimestone, which is not well constrained with the availabledata, might have occurred only during the transition from shoshoniteto tephrite. From the ‘deep’ reservoir, magmas rosetowards a series of shallow reservoirs, in which they differentiatedfurther, mixed, and fed volcanic activity. KEY WORDS: Somma–Vesuvius; crustal contamination; source heterogeneity; radiogenic and stable isotopes; magmatic system  相似文献   

Sulphate reduction in the sediment of the Venice canals (Italy)     

Zaggia L  Rosso J  Zonta R 《Marine pollution bulletin》2007,55(10-12):415-424

The sulphur cycle in the sediment of the Venice canal network was investigated by considering the sulphate reduction rate (SRR) and the distribution of sulphur compounds, in both pore water and sediment. Sulphate reduction (SR) is the main process in the metabolism of the organic matter supplied to the network by untreated urban effluents. Although it might account for the decomposition of only a limited percentage of the total organic-C inputs, the estimated rates are among the highest observed in coastal sediments. Measured rates range from 0.26 to 0.99 micromolcm(-3)d(-1), while mean annual values, estimated by a diagenetic model, vary from 0.16 to 0.43 micromolcm(-3)d(-1). The speciation of S in the sediment reveals that pyrite-S is the most abundant component of the total reduced S pool, whereas acid volatile sulphides and elemental sulphur together account for less than 45%. A preliminary budget indicates that the rate of burial of solid-phase S is small compared to the S produced by SR (from 10 to 25%). A large amount of reduced S is then lost from the canal deposits to be re-oxidised at the sediment-water interface or in the overlying water column.  相似文献   

Pauling bond strength, bond length and electron density distribution     

G. V. Gibbs  N. L. Ross  D. F. Cox  K. M. Rosso  B. B. Iversen  M. A. Spackman 《Physics and Chemistry of Minerals》2014,41(1):17-25

The power law regression equation, <R(M–O)> = 1.46(<ρ(r _c)>/r)^?0.19, relating the average experimental bond lengths, <R(M–O)>, to the average accumulation of the electron density at the bond critical point, <ρ(r _c)>, between bonded pairs of metal and oxygen atoms (r is the row number of the M atom), determined at ambient conditions for oxide crystals, is similar to the regression equation R(M–O) = 1.41(ρ(r _c)/r)^?0.21 determined for three perovskite crystals at pressures as high as 80 GPa. The pair are also comparable with the equation <R(M–O)> = 1.43(<s>/r)^?0.21 determined for oxide crystals at ambient conditions and <R(M–O)> = 1.39(<s>/r)^?0.22 determined for geometry-optimized hydroxyacid molecules that relate the geometry-optimized bond lengths to the average Pauling bond strength, <s>, for the M–O bonded interactions. On the basis of the correspondence between the equations relating <ρ(r _c)> and <s> with bond length, it seems plausible that the Pauling bond strength might serve a rough estimate of the accumulation of the electron density between M–O bonded pairs of atoms. Similar expressions, relating bond length and bond strength hold for fluoride, nitride and sulfide molecules and crystals. The similarity of the expressions for the crystals and molecules is compelling evidence that molecular and crystalline M–O bonded interactions are intrinsically related. The value of <ρ(r _c)> = r[(1.41)/<R(M–O)>]^4.76 determined for the average bond length for a given coordination polyhedron closely matches the Pauling’s electrostatic bond strength reaching each the coordinating anions of the coordinated polyhedron. Despite the relative simplicity of the expression, it appears to be more general in its application in that it holds for the bulk of the M–O bonded pairs of atoms of the periodic table.  相似文献   

SiO bonded interactions in coesite: a comparison of crystalline, molecular and experimental electron density distributions     

K. M. Rosso  G. V. Gibbs  M. B. Boisen 《Physics and Chemistry of Minerals》1999,26(3):264-272

Bond critical point properties of electron density distributions calculated for representative Si₅O₁₆ moieties of the structure of coesite are compared with those observed and calculated for the bulk crystal. The values calculated for the moieties agree with those observed to within ∼5%, on average, whereas those calculated for the crystal agree to within ∼10%. As the SiOSi angles increase and the SiO bonds shorten, there is a progressive build-up in the calculated electron density along the bonds. This is accompanied by an increase in both the curvatures of the electron density, both perpendicular and parallel to each bond, and the Laplacian of the electron density distribution at the bond critical points. The cross sections of the bonds at the critical points become more circular as the angle approaches 180o. Also, the bonded radius of the oxide anion decreases about twice as much as that of the Si cation as the SiO bond length decreases and the fraction of s-character of the bond is indicated to increase. A knowledge of electron density distributions is central to our understanding of the forces that govern the structure, properties, solid state reactions, surface reactions and phase transformations of minerals. The software (CRYSTAL95 and TOPOND) used in this study to calculate the bond critical properties of the electron density and Laplacian distributions is bound to promote a deeper understanding of crystal chemistry and properties. Received: 23 February 1998 / Revised, accepted: 16 July 1998  相似文献