Pb–Pb Age of Carbonate Rocks of the Kamo Group,Baikit Anteclise of the Siberian Platform

Doklady Earth Sciences - The first direct Pb–Pb dating of carbonate rocks of the Kamo Group has been carried out. The Pb–Pb age of carbonates of the lower units (the Madra, Jurubchen,...     

Sr isotopic composition of Paleoproterozoic C-rich carbonate rocks: The Tulomozero Formation,SE Fennoscandian Shield

Rb–Sr systematics has been studied in ¹³C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and ¹⁸O/¹⁶O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn (<0.40) ratios in the Tulomozero carbonate rocks of Members A, B (the lower part), D, F, and E are consistent with accumulation of original carbonate sediments in evaporitic lacustrine, playa, and sabkha environments. A decrease in the Sr content with concurrent increase in the Fe/Mn ratio (>0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The ⁸⁷Sr/⁸⁶Sr values in the least altered (Mn/Sr < 2.0) marine dolostones are 0.70418–0.70442 and 0.70343–0.70409 for the earlier and late phases of the marine transgression, respectively. The decrease in the ⁸⁷Sr/⁸⁶Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.     

Paleoproterozoic Age of Carbonates and Trondhjemites of the Central Azov Group: Sr Isotope Chemostratigraphy and U–Pb Geochronology

Doklady Earth Sciences - The Sr isotope characteristic of carbonate rocks in the Central Azov Group has been reported (0.70322–0.70352), the Nd model age of silicate sediments has been...     

Behavior of parameters of the ionospheric F2 layer at the edge of the centuries: 2. Height of the layer

The trends in the height of the ionospheric F2 layer, hmF2, for the period from the end of the 1990s to the beginning of the 2010s are considered. It is shown that for a majority of the considered situations (time of the day plus season) negative trends are obtained for all stations for which series of initial data satisfying the requirements were found. They vary from ?1.2 to ?3.1 km per year. The trends in the daytime are slightly higher than in the postsunset period which apparently manifests the influence of the contraction of the upper atmosphere on changes in the hmF2 value.     

Polarization of radiation from Supernova 2005cs and possible physical mechanisms of its generation

We present R-band polarimetry and photometry for Supernova 2005cs. The polarimetric observations were performed with the AZT-8 telescope at the Crimean Astrophysical Observatory (Ukraine) using the ST-7 photometer-polarimeter of the Sobolev Astronomical Institute. A record (for supernovae) linear polarization, p = 8%, was detected; the position angle underwent a jump by ?90° during the evolution of the supernova brightness. Possible generation mechanisms of the polarized radiation at the photospheric stage of evolution of the supernova are briefly analyzed.     

Rb-Sr geochronology of Vendian shales,the Staraya Rechka Formation,Anabar Massif,northern Siberia

Fine-grained clayey subfractions (SF) with particle sizes of 1–2, 0.6–1.0, 0.3–0.6, 0.2–0.3, 0.1–0.2, and <0.1 μm were extracted from shales of the Vendian Staraya Rechka Formation in the Anabar Massif and studied by XRD and Rb-Sr methods. All the clayey subfractions are represented by illite with high crystallinity indices, which are characteristic of the low-temperature diagenesis/catagenesis zone and grow with the decrease of the particle size. The Rb-Sr systematics in clayey subfractions combined with mineralogical data provide grounds for the conclusion that illite from clayey rocks of the Staraya Rechka Formation was forming during two periods: approximately 560 and 391–413 Ma ago. The first illite generation was likely formed in the course of lithostatic subsidence of the Staraya Rechka sediments and the second one, during the Devonian lithogenesis stage. It is assumed that age of the first generation (∼560 Ma) is close to that of the Staraya Rechka Formation. This inference is consistent with biostratigraphic, chemostratigraphic, and geochronological data obtained for both rocks of the Anabar Massif and Vendian sediments from other regions of Siberia.     

Sr chemostratigraphy of carbonate sedimentary cover of the Tuva-Mongolian microcontinent

This study uses Sr isotope chemostratigraphy to place constraints on the depositional age of carbonate rocks from the Tuva-Mongolian microcontinent. The age of carbonate rocks of the Irkut Formation (⁸⁷Sr/⁸⁶Sr initial ratio equal to 0.70480–0.70485) is determined to be older than 1250 Ma, whereas carbonates of the Zabit (0.70706–0.70727 and 0.70828–0.70848) and Agarin Gol (0.70725–0.70743) formations were deposited in the interval 630–560 Ma.     

Estimating the Anthropogenic Impact on Water Bodies Using a Simulation Mathematical Model of Phosphorus System

A simulation model of the phosphorus system is used to study the ecosystems functioning in the Dautkul'skoe Reservoir and Shegekul Lake (Mezhdurech'e). Dependence of the internal flows (turnover) and normal annual concentrations of P forms in the water body on its external load onto the ecosystem is established. The budgets of P and its production–destruction flows (1993–1999), calculated using the model, are presented and analyzed. Comparison estimates are made for the anthropogenic impact on the water bodies of the southern Aral Sea region.     

Mineralogical,Mössbauer,and Isotopic–Geochronological Study of Upper Riphean Al-glauconites,Kil'din Group,Srednii Peninsula

The paper presents mineralogical, crystallochemical, and Rb–Sr age data on globular layer silicates of the glauconitic series from lower horizons of the Upper Riphean Kil'din Group, Srednii Peninsula. Chloritization significantly changed the globular Al-glauconite in sandstones of the Päräjarvinskaya Formation. However, it almost did not affect glauconite globules in sandstones of the Palvinskaya Formation and altered only clayey component of the rock. Mössbauer spectroscopy showed that Al-glauconite in the Palvinskaya Formation was affected by secondary transformations, which caused the Fe²⁺ oxidation in octahedral sheets and loss of radiogenic ⁸⁷Sr. The transformations were presumably related to exhumation of rocks of the Päräjarvinskaya and Palvinskaya formations to the circulation level of the oxygen-rich meteoric waters. Thus, Rb–Sr datings obtained for Al-glauconites of the Palvinskaya (781–786 Ma) and Päräjarvinskaya(744–751 Ma) formations correspond to the tectonic uplift of Kil'din rocks and have no stratigraphic significance.