A. B. Kuznetsov

The Sr isotope composition of the world ocean, marginal and inland seas: Implications for the Sr isotope stratigraphy

We studied the Sr isotope composition of shells of modern shallow-water mollusks and coral fragments. Twenty five of the studied samples were collected in beach zones of open oceans and marginal seas; twelve and eight additional samples are from saline and freshened inland seas respectively. The 87Sr/86Sr ratio in samples from the Atlantic, Indian, and Pacific oceans and their marginal seas corresponds on average to 0.709202 ± 0.000003 and coincides with the average ratio in the standard USGS EN-1 sample. The average 87Sr/86Sr ratio in inner parts of evaporite subbasins of the Mediterranean and Red seas is identical to that of the oceanic water. In shells of shallow-water mollusks from the Black Sea and Sea of Azov, where the degree of seawater dilution by riverine runoff is as high as 50 to 70%, the 87Sr/86Sr ratio is lower than that in the oceans by only a value of 0.00002 on average. As oceanic waters penetrated into these freshwater basins no earlier than in the Holocene, we conclude that the Sr isotopic equilibration with the oceanic water is realized very rapidly in the epicontinental seas even under conditions of restricted water exchange with the World Ocean. The established uniformity of the Sr isotope composition in all geographic types of currently existing sea basins open to the World Ocean proves the efficiency of the Sr isotope stratigraphy in correlation of contemporaneous chemogenic sediments.

New data on Sr-and C-isotopic chemostratigraphy of the Upper Riphean type section (Southern Urals)

New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ13C value varies in the Karatau Group succession from −2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ13C variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the “best” samples satisfying geochemical criteria of the isotopic system retentivity, the 87Sr/86Sr initial ratio increases from 0.70521–0.70532 in the lower Inzer deposits to 0.70611 in the upper Min’yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min’yar and Uk formation, this ratio grows from 0.70533 to 0.70605–0.70609 in the limestone succession of the last formation.

Lower Vendian microfossil assemblages of East Siberia: Significance for solving regional stratigraphic problems

Both published and original data are used for the analysis of the stratigraphic position and taxonomic characteristics for acanthomorphic microfossils of the Pertatataka type in Lower Vendian sediments of the central and southeastern Siberian Platform and its surrounding structures. Four taxonomically different microbiotas are distinguished in sections of the Nepa-Botuoba, Fore-Patom; Syugdzher, Anabar, Zhuya-Lena, and Berezovo lithotectonic zones: two diverse (Nepa and Ura) and two impoverished (Otradnino and Torga) each including diagnostic Early Vendian taxa. Beyond the Siberian Platform, genera and species characterizing these microbiotas occur in single assemblages and represent diagnostic taxa of the second (Tc-Sr-Vl) and third (Ti-Cg-Mp) zones of the acritarch scale proposed by Australian geologists as a biostratigraphic basis for subdividing the Ediacarian of the Standard stratigraphic scale (SSS). In the Russian General stratigraphic scale, the distribution of Pertatataka acritarchs corresponds to the Lower Vendian. The appearance and mass development of acanthomorphic microorganisms represents a global biological event, which should be taken into consideration as a biostratigraphic criterion for defining the Vendian System. For substantiating age, the following species among diagnostic Lower Vendian forms of East Siberia should be used: Appendisphaera grandis, “Appendisphaera” tabifica, A. tenuis, Ceratosphaeridium glaberosum, Dicrospinosphaera virgata, Multifronsphaeridium pelorium, “Polygonium” cratum, Tanarium conoideum, Variomargosphaeridium litoschum, and Talakania obscura. These taxa are readily recognizable, morphologically stable, and characterized by their wide lateral distribution and relatively narrow stratigraphic range. Their occurrence in the upper part of the Dal’nyaya Taiga Group of the Baikal-Patom region indicates that host sediments should be attributed to the Lower Vendian, not the Upper Riphean as is currently accepted.

The Sr Isotopic Characterization and Pb-Pb Age of Carbonate Rocks from the Satka Formation, the Lower Riphean Burzyan Group of the Southern Urals

The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formations corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the 87Sr/86Sr ratio of sedimentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leaching are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the 206Pb/204Pb-207Pb/204Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial 87Sr/86Sr ratio in the least altered limestones of this formation is within the range of 0.70460–0.70480. Generalization of the Sr isotopic data published for the Riphean carbonates from different continents showed that 1650–1350 Ma ago the 87Sr/86Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux.

The Sr isotope chemostratigraphy as a tool for solving stratigraphic problems of the Upper Proterozoic (Riphean and Vendian)

Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600–1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456–0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601–0.70611 and then quickly declined to 0.70519–0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521–0.70535 to values of 0.70874–0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561–0.70575 at approximately 760 Ma and to 0.70533–0.70538 at 670–660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812–0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806–0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.

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.

U-Pb age and Sr isotope signature of cap limestones from the Neoproterozoic Tsagaan Oloom Formation, Dzabkhan River Basin, Western Mongolia

The U-Pb and Pb-Pb methods were used for determining age of cap limestones from the Neoproterozoic Tsagaan Oloom Formation corresponding to the lower part of the sedimentary cover in the Dzabkahn microcontinent of Central Asia. The weighted average age value appeared to be equal to 632 ± 14 Ma (MSWD = 0.11, probability 0.74). This value allows the following assumptions: (1) the lower boundary of the Tsagaan Oloom Formation corresponds to the beginning of the Ediacaran; (2) Dzabkhan tillites are correlative with glacial sediments of the Marinoan Epoch. The low 238U/204Pb and 232Th/238U ratios observed in initial Pb sources of limestones from the Tsagaan Oloom Formation indicate that the Dzabkhan paleobasin received at its early development stages a bulk of material from eroded upper Riphean juvenile rocks. The 87Sr/86Sr ratio in fractions of Tsagaan Oloom limestones enriched with primary carbonate material and satisfying geochemical criteria of Rb-Sr systems retentivity (Mn/Sr < 0.20 and Fe/Sr < 1) varies from 0.70676 to 0.70691 and reflects this ratio in the World Ocean approximately 630 Ma ago.

Mössbauer characteristics, mineralogy and isotopic age (Rb-Sr, K-Ar) of Upper Riphean glauconites from the UK Formation, the southern Urals

Comprehensive mineralogical analysis, Mössbauer spectroscopy and isotopic-geochronological study have been carried out for globular phyllosilicates (GPS) of glauconite group from the Uk Formation, the second one below the top of the Upper Riphean stratotype in the southern Urals. Glauconites have been sampled in the Kurtaza and Kulmas sections remote from each other in the Alatau anticlinorium that corresponds to western facies zone of the Bashkirian meganticlinorium. As is shown, size and density monomineral fractions of globules are represented by Al-glauconite according to established structural formula. The isotopic (Rb-Sr and K-Ar) dating of glauconites from the Uk Formation is performed for the first time along with computer simulation of cation arrangements in their crystal lattice and comparison of the results obtained with data of Mössbauer spectroscopy. It has been assumed by simulation that origin and transformation of the Rb-Sr and K-Ar systems in glauconite are concurrent to stages in structural evolution of this mineral, which have been controlled by geological and geochemical events in the history of sedimentary successions. The approach has been aimed at recognition of stratigraphically meaningful isotopic dates corresponding to the glauconite formation at the stage of the early diagenesis close to sedimentation time and the “rejuvenated” dates characterizing ages of subsequent geological events. The comparison of simulated cation arrangements with data of Mössbauer spectroscopy shows that the Rb-Sr (663 ± 9 Ma) and K-Ar (669 ± 16 Ma) dates established for glauconites correspond to the time of early diagenesis in their host sediments, being suitable for age assessment of the Uk Formation. The dates obtained are of interregional and wider significance, as they must be taken into consideration when constructing the general curve of Sr isotope variations in the Late Riphean oceans.

Isotope age of boundaries between the general stratigraphic subdivisions of the Upper Proterozoic (Riphean and Vendian) in Russia: The evolution of opinions and the current estimate

The evolution of opinions about the isotope age of the general stratigraphic subdivisions of the Upper Proterozoic in Russia (Lower, Middle, Upper Riphean and Vendian) during the second half of the 20th century and the beginning of the 21st century is considered, and the current data on the age of these subdivisions are analyzed in detail. It is shown that the isotope age of the lower boundaries of the Lower, Middle, and Upper Riphean and Vendian should be estimated at 1750 ± 50, 1400, 1030, and 640 ± 5 Ma, respectively. The role of the historical–geological criteria in the substantiation of these boundaries is studied.

Stratotype of the Lower Riphean, the Burzyan Group of the Southern Urals: Lithostratigraphy, Paleontology, Geochronology, Sr- and C-Isotopic Characteristics of Its Carbonate Rocks

The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered (“best”) samples of the Burzyan carbonates, which retain the 87Sr/86Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480 and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ18O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7 to 0‰, which is consistent with the assumption that δ18O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago. C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least altered carbonates of the Early Riphean, the δ18O variation range corresponds to 4.0–4.5‰.