Pb-Pb age of Jatulian carbonate rocks: The Tulomozero Formation of southeast Karelia

The U-Pb systematics of 25 carbonate samples collected from the Upper Jatulian Tulomozero Formation in southeastern area of the Baltic shield has been studied. The U-Pb systems of Jatulian carbonates metamorphosed here under conditions of the greenschist facies likely have not been reset completely. Samples have been collected from core sections recovered by drilling 60 to 70 km apart from each other in western and eastern areas of the Onega Lake region. In majority, the rocks sampled characterize a thick upper member of the Tulomozero Formation, and a few samples have been collected in addition from its middle and lower members. The analyzed dolomitic rocks (Mg/Ca=0.60–0.68) have variable Mn (54–1450 ppm) and Sr (31–438 ppm) concentrations. Powdered dolomite samples have been treated preliminary in 1 N solution of ammonium acetate in order to get material for analysis enriched in pre-metamorphic carbonate phases in terms of U-Pb systematics. Five samples have been subjected to stepwise dissolution in 0.5 N HBr to analyze their carbonate phases L-1, L-2, L-3 and siliciclastic fraction for getting a deeper insight into the U-Pb systematics. The U-Pb characteristics of carbonate and siliciclastic fractions suggest deposition of studied carbonate sediments in two different paleobasins. In addition, they show for same samples the partial Pb redistribution between carbonate and siliciclastic components in the course of metamorphism and Pb gain from allogenic metamorphic fluids. The Pb-Pb date of 2090 ± 70 Ma (MSWD = 2.0) is estimated for the least altered dolomite samples from upper member of the Tulomozero Formation, which represent marine sediments of a paleobasin and contain a minimum of siliciclastic material, being the least-contaminated by gained Pb. The date obtained is well consistent with U-Pb and Sm-Nd ages established for the Jatulian volcanogenic rocks in northern and western areas of the Baltic shield.     

Superhumps in binary systems and their connection to precessional spiral density waves

We consider a mechanism for the formation of superhumps in the TV Col system, based on the possible existence of a precessional spiral wave in the accretion disk of the system. This mechanism can act in binaries with arbitrary component-mass ratios, and our precessional spiral wave model can be applied to explain observed superhumps of all types.     

Cyclicity of shallow-water carbonate sediments in different climatic zones

Based on the study of several different-age shallow-water carbonate sediments from different regions, it is established that cyclothems formed in basins of arid and humid climatic zones differ in their composition and structure. It is shown that the influence of climate was indirect. The climate governed salinity (particularly, in extremely shallow-water settings of initial and terminal stages of the formation of cyclothems) and, thus, stimulated changes in the biota. The biota, in turn, controlled the composition of carbonate material and mechanisms of its sedimentation.     

Sr isotope chemostratigraphy of Upper Jurassic carbonate rocks in the Demerdzhi Plateau (Crimean Mountains)

The first Sr chemostratigraphic data are obtained for the Upper Jurassic carbonate sections in the Demerdzhi Plateau of the Crimean Mountains. The oncoid, microbial, and organogenic-detrital limestone varieties representing shallow-water marine microfacies of the carbonate platform served as material for these studies. The limestone samples for reconstructing the isotopic parameters of depositional environments were selected using geochemical criteria (Mn/Sr < 0.2, Fe/Sr < 1.6, Mg/Ca < 0.024) and subjected to the preliminary treatment in a 1 N ammonium acetate solution. The ⁸⁷Sr/⁸⁶Sr value in the least altered samples increases up the section from 0.70701 to 0.70710. The obtained ⁸⁷Sr/⁸⁶Sr values are correlated with the Sr isotope properties of belemnite rostrae characterizing the Pectinatites scitulus-Pavlovia rotunda ammonite zone in the zonal scale of the Boreal realm, Hyponoticeras hybonotum-M. ponti/B. peroni Zone in the zonal scale of the Tethyan realm, and lower part of the Dorsoplanites panderi Zone in the zonal scale for the transitional domain of the East European Platform. The Sr chemostratigraphic correlation indicates the early Tithonian age of sediments developed in the eastern part of the Demerdzhi Plateau and restricts the upper boundary of the limestone section redeposited within carbonate breccia of Mount Severnaya Demerdzhi to the terminal early Tithonian. The Sr isotope data are used for calculating the sedimentation rates for limestones. They are estimated to be at least 0.23 m/1000 years, which is an anomalously high value for pre-Quaternary carbonate platforms.