Noble-metal mineralization in the Semenov-2 hydrothermal field (13°31′N), mid-atlantic ridge

The porous fine-grained to microcrystalline copper-zinc ore of the Semenov-2 hydrothermal field, a site in the Semenov hydrothermal cluster discovered in 2007 (13°31′N, MAR), is anomalously enriched in Au (22–188 ppm) and Ag (127–1787 ppm). Chalcopyrite, isocubanite, würtzite, and opal are major minerals; sphalerite, marcasite, pyrite, and covellite are auxiliary; and galena, pyrrhotite, native gold, silver telluride, barite, and aragonite are sporadic. Gold containing 0.31 to 23.07 wt % Ag occurs as up to 9-μm-sized subhedral, dendritelike, and elongated grains mostly hosted in opal and less frequently in sphalerite and in pores within isocubanite-chalcopyrite aggregate. An elongated grain (2 × 4 μm in size) of the Ag-Te phase was found in a pore. So far only basalts have been dredged from the Semenov-2 field, but anomalous gold and silver concentrations suggest the influence of ultramafic rocks; the latter were found 1.5 km westward, in the Semenov-1 hydrothermal field. Mineral assemblage and morphology of gold particles indicate its primary origin in contrast to the hydrothermal fields hosted in basalts, where gold is a product of remobilization. Zonal gold grains, found on oceanic floor for the first time, are characterized by low Ag content in the cores and high Ag content in the outer rims, reflecting variation in formation conditions.     

Comparative analysis of marine paleogene sections and biota from West Siberia and the Arctic Region

The analysis of the main biospheric events that took place in West Siberia and the Arctic region during the Early Paleogene revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations between their separate parts. Most biotic and abiotic events of the first half of the Paleogene in the Arctic region and West Siberia were synchronous, unidirectional, and interrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages characteristic of these basins during the Paleogene are compared. The comparative analysis primarily concerned events of the Paleocene-Eocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene Azolla Beds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a common scenario related to the development of a system of estuarine-type currents in a sea basin partly isolated from the World Ocean.     

Detailed architecture of a compound incised valley system and correlation with forced regressive wedges: Example of Late Quaternary Têt and Agly rivers,western Gulf of Lions,Mediterranean Sea,France

Quaternary incised valley systems are usually characterized by the preservation of a single valley-fill attributed to the last post-glacial period. Moreover, there are very few cases of correlation between incised valley system developed on inner shelf and sedimentary units observed on the mid to outer shelf, mainly forced regressive wedges. The Roussillon shelf, in the western part of the Gulf of Lion, is a particular example of preserved Quaternary compound incised valley system also characterized by a direct correlation with the forced regressive lowstand wedges on the mid-outer shelf. High-resolution seismic data and a borehole, 60 m deep, located on the beach barrier permit an accurate study of the geometry and lithology of the system. Six imbricated and more or less preserved incised valleys and valley-fills are observed up to the inner to mid-shelf. The key surfaces associated to the incised valleys are correlated to the boundaries of the forced regressive wedges. They are assumed to be reworked surfaces. At the borehole location, only few thin layers, less than 1 m thick, of coarse grain and/or floating pebbles, are observed and should correspond to preserved fluvial lowstand deposits reworked under marine influence. The valley fills are mainly composed of estuarine muddy silts. From AMS ¹⁴C age dating it is inferred that the uppermost incised valley system is younger than 45 ky cal BP. Based on those observations, the six preserved incised valley systems are assumed to be controlled by the last six 4th order sea-level cycles — 100 ky — of the middle to late Quaternary. The paleo-topography of the underlying Plio-Quaternary deposits controls the compound incised valley system location. The deep topography of the Messinian Erosionnal Surface is a controlling factor at a lower degree. The partial preservation of the successive valley fill is attributed not only to the differential subsidence but also to the lateral migration of each incision and to the hydrodynamic regime.     

The magnetic fields and density of relativistic electrons in the nucleus of NGC 1052

We analyze observations of the compact GHZ-peaked-spectrum radio source in the nucleus of the weakly active galaxy NGC 1052, assuming that the low-frequency turnover in its spectrum is due to synchrotron self-absorption. The analysis is based on a model for an inhomogeneous source of synchrotron radiation. It is shown that the magnetic field is not uniform, but the change in the field strength from the center to the edge of the compact radio source does not exceed an order of magnitude. The maximum magnetic-field strength in the nucleus of NGC 1052 is 20 G < H _⊥ < 200 G, and the density of relativistic electrons is 0.018 cm⁻³ < n _e < 0.18 cm⁻³ on scales of 0.1 pc; everywhere in the radio source, the energy density of the magnetic field exceeds the energy density of the relativistic electrons. The physical conditions are similar to those in the nuclei of the nearby radio galaxies 3C 111 and 3C 465, and differ strongly from those in the nucleus of the radio galaxy 0108+388, which is a compact GHz-peaked-spectrum source (these three galaxies were studied by the authors earlier using the same method).     

Catagenesis of organic matter at the top and base of the Jurassic complex in the West Siberian megabasin

Thermal maturation for the top and base of the Jurassic in the West Siberian megabasin was modeled on the basis of the vitrinite reflectance (R_vt⁰) data using mathematical modeling and computer simulations. The values of thermal maturation are found to vary within substages PC₃-MC₃² for the top (or being equivalent to PC₃-MC₁² on the periphery and southern part of the basin, or to MC₂-MC₃² in the north) and PC₃-AC₃ for the base of the Jurassic (or being equivalent to PC₃-MC₂ on the periphery and southern part of the basin or MC₃¹-AC₃ in the north). Thermal maturity levels of the Jurassic in West Siberia are controlled by depths of burial and peak temperatures which the source rocks were subjected to during this period. The situation is further complicated by high heat flows superimposed on the regional background, which are observed in deep fault zones and in the proximity of numerous igneous bodies.     

Spectrophotometric observations of variable stars

We present the results of our visual and near-infrared spectrophotometric observations for 77 variable stars obtained during 1971–1991 in Chile, Armenia, and Bolivia. The quasi-monochromatic, extraatmospheric fluxes from the stars are given in absolute energy units (W m^?2 m^?1) at all wavelengths of the spectral range at 2.5-nm intervals.     

3D Coronal Density Reconstruction and Retrieving the Magnetic Field Structure during Solar Minimum

Measurement of the coronal magnetic field is a crucial ingredient in understanding the nature of solar coronal phenomena at all scales. We employed STEREO/COR1 data obtained during a deep minimum of solar activity in February 2008 (Carrington Rotation CR 2066) to retrieve and analyze the three-dimensional (3D) coronal electron density in the range of heights from 1.5 to 4 R_⊙ using a tomography method. With this, we qualitatively deduced structures of the coronal magnetic field. The 3D electron-density analysis is complemented by the 3D STEREO/EUVI emissivity in the 195 Å band obtained by tomography for the same CR. A global 3D MHD model of the solar corona was used to relate the reconstructed 3D density and emissivity to open/closed magnetic-field structures. We show that the density-maximum locations can serve as an indicator of current-sheet position, while the locations of the density-gradient maximum can be a reliable indicator of coronal-hole boundaries. We find that the magnetic-field configuration during CR 2066 has a tendency to become radially open at heliocentric distances greater than 2.5 R_⊙. We also find that the potential-field model with a fixed source surface is inconsistent with the boundaries between the regions with open and closed magnetic-field structures. This indicates that the assumption of the potential nature of the coronal global magnetic field is not satisfied even during the deep solar minimum. Results of our 3D density reconstruction will help to constrain solar coronal-field models and test the accuracy of the magnetic-field approximations for coronal modeling.