Ocean Redox State at 2500‒500 Ma: Modern Concepts

Modern concepts on variations of the redox conditions of Proterozoic ocean and atmosphere are reviewed. Generalized data indicate a non-linear evolution of the О₂ content in atmosphere and ocean for almost 2 Ga of the geological history. Contrasting distribution of minimum and maximum values of diverse proxies of the ocean redox state was found for the Great Oxygenation Event, Early–Late Proterozoic transition stage, and the Late Precambrian–beginning of Cambrian. The last interval is marked by a wide scatter of Mo, V, and U_sed concentrations and values of Мо/TOC, δ⁹⁸Mo, δ⁵³Cr, and δ⁸²Se in different geological objects. This time interval also coincided with one more large-scale oxygenation event, Neoproterozoic. Thus, according to the modern concepts, the О₂ contents near the Earth’s surface and in ocean significantly varied during the entire Proterozoic. These concepts are consistent with the assumption by Frolov and many other researchers concerning the heterogeneous distribution of рН and Eh in ancient hydrosphere and the existence of diverse, sufficiently contrasting “gas” facies. They also agree with conclusions by Sochava that the periodical variations of О₂ content in atmosphere and hydrosphere, especially at the Precambrian–Phanerozoic boundary, are related to the mass extinctions, emergence of adaptive response of animals (appearance of exoskeletons, food search strategy change, and others) and heavy taxonomic radiation of the latters.     

The Riphean Arsha Group of the South Urals: A Problem of the Geodynamic Origin of Rock Associations

The geodynamic origin of volcanic and sedimentary rocks of the Upper Precambrian Arsha Group of the western slope of the southern Urals is considered. It is shown that both rock types are riftogenic/plume in origin. The problem of the spatial and temporal correlation of the Arsha and Karatau groups requires further study.     

Molasse of the Belskii depression of the Cis-Uralian foredeep: Modern data about provenance

The crystallochemical characteristics of Cr-spinels and tourmalines in combination with U–Pb isotope data on detrital zircons from the Upper Permian and Lower Triassic sandstones of the Belskii Depression showed that the main provenances of the molasse sequence in the southern part of the Cis-Uralian foredeep were Lower Paleozoic (Sakmara zone) and Precambrian (Uraltau zone) complexes. The absence of Late Paleozoic zircons in the Tatarian (Upper Permian) sandstones, as well as their small amount in the Lower Triassic psammites together with geochemical and petrographic data, suggest that granite massifs of the Main Granitic Axis of the Urals, as Middle–Upper Paleozoic magmatic complexes of the Magnitogorsk Megazone, were not involved in erosion. At the same time, the significant amount of Precambrian zircons is indicative of the presence of metamorphic complexes in the provenance of the Uraltau zone.     

Some Specific Features of Early Vendian Sedimentation in the Southern and Middle Urals

The existence of the pre-Early Vendian hiatus in the Bashkir Meganticlinorium is supported by large incised valleys filled with Early Vendian sediments both on the western and eastern limbs of the meganticlinorium. It is suggested that a great glacioeustatic lowering of the sealevel occurred in the sedimentation basin. Taking into account the reconstruction of similar Early Vendian events for many provinces elsewhere, we can suppose that Early Vendian sedimentation basins in the southern and middle Urals were connected with the World Ocean. The comparative study of Lower Vendian sedimentary sequences in the Urals and Norway demonstrates similarities in rock associations. The correlation of Lower Vendian sections in the Urals with modern facies models of glacial sedimentation shows that glacimarine sequences of the southern Urals fit the shelf sedimentary associations proper. The character of section at the Serebryanka level in the middle Urals and the spatial distribution of mixtites therein correspond to sedimentation on the basin shelf and slope.     

Study of dust haze in arid zone

Regularities in the effect of dust haze on the thermal regime of the air surface layer are revealed by examining air-temperature measurement data obtained at the Dushanbe, Termez, Bairam-Ali, Repetek, and Kurgan-Tyube meteorological stations. We find that the sharp decrease in daily air temperature and the slight increase in night temperature until the 1990s under strong dust storms leads to decreased temperatures in the air surface layer. Later, dust hazes of 3 to 8 days are more frequently observed with a pulsating change in the horizontal range of visibility. Depending on their power, these hazes lead to either heating or cooling of the atmospheric surface layer.     

Structure and metamorphism of the Antarctic Shield

The information on the composition, structure, P-T conditions of metamorphic facies, evolution, and time of the metamorphic events in the largest Precambrian tectonic provinces of the Antarctic Crystalline Shield gained over more than a half-century is summarized in this paper. The joining up of the ortho- and paracrystalline rocks into complexes and groups according to their geographic position, composition, age, and the character of their metamorphism allowed us to consider the main features of the structure and evolution of the provinces including (1) the near-latitudinal polycyclic Late Precambrian-Early Paleozoic Wegener-Mawson Mobile Belt, extended for more than 4000 km, which started to evolve in the Mesoproterozoic and stabilized only at the end of Cambrian; (2) the Early Precambrian relict crystalline protocratonic blocks adjoining this mobile belt; their history is traced from the Eoarchean; and (3) the near-latitudinal Late Precambrian-Early Paleozoic aulacogen in the southern protocratonic block. The P-T conditions of the metamorphism from the pyroxene-granulite subfacies in the protocratonic blocks to the greenschist facies in aulacogen, as well as the age of the magmatic and metamorphic events in all the tectonic provinces of the shield, are characterized. This made it possible to consider the metamorphic history and conditions of the continental crust’s formation in Antarctica, where the oldest crystalline rocks are dated to the Eoarchean (4060–3850 Ma) and the youngest rocks are ～500 Ma old.     

Late Vendian unfolded Molasse in the northeastern, eastern, and southwestern peripheries of the east European platform: Distinctive lithogeochemical features

The paper reports data on the lithogeochemistry of sandstones and silty mudstones from Upper Vendian sedimentary sequences in the northeastern, eastern, and southwestern peripheries of the East European Platform belonging to the so-called unfolded molasse. The sequences are dominated by wackes, arkoses, subarkoses, litharenites, and sublitharenites, i.e., chemically immature and moderately mature psammites, and can be classed with rocks produced by clastic material brought from orogens surrounding the platform. The higher TiO₂, Al₂O₃, FeO_tot, MgO, Na₂O, and K₂O concentrations of the psammites than those in the average cratonic Phanerozoic sandstone (APhSa) testify that the chemical maturing of the rocks was not completed. The silty mudstones accompanying the sandstones have a composition closer to those of the average cratonic Phanerozoic shale (APhSh), but this is likely explained by the fact that the rocks were produced of material brought from erosion territories of much greater area. The lithogeochemical data generally indicate that these territories were dominated by acid and intermediate magmatic rocks with variable fraction of sedimentary rocks when the Late Vendian sedimentary associations in question were produced. The distribution of certain indicator trace elements in the sandstones and silty mudstones show that the average composition of the eroded complexes was close to the composition of the post-Archean upper continental crust, but the erosion areas occasionally (in the Vychegorskii trough and the Shkapovsko-Shikhanskaya depression) contained relatively primitive source rocks. The data points of the great majority of the sandstones and silty mudstones plot in the SiO₂-K₂O/Na₂O and F1–F2 diagrams in the fields of sediments typical of the environments of active continental margins, which is consistent with the arrangement of the data points of these rocks in the La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 diagrams. All of these features confirm that the sedimentary rocks in question affiliate with rock associations produced at an active tectonic regime.     

Sources of Bottom Sediments in the Eastern Part of East Siberian Sea (Reconstruction from Geochemical Data)

Oceanology - Based on analytical materials obtained in 2008 an the expedition of the R/V Akademik M.A. Lavrentiev along a regional gas-geochemical profile stretching 550 km from Billings...