The article presents the results of long-term studies of Paleozoic organogenic structures in the Timan-northern Ural region. The article discusses the principles of defining reefs and other organogenic structures, which allow tracing the evolution and stages of the Upper Ordovician-Lower Permian reef formation in the region based on their temporal distribution. Three stages of reef formation have been identified in the Paleozoic of the region, differing in the geometry of organogenic structures, their growth rate, and paleogeographic position. (i) The Middle Katian-Early Emsian stage is characterized by local and barrier reefs of the outer margin of the carbonate shelf, reaching great thickness. (ii) The Middle Frasnian-Early Tournaisian stage is distinguished by the formation of large microbial mounds, both on the outer margin of the shelf and on the margins of shallow-water platforms within the dissected shelf, as well as the appearance of reefs in the coastal zone of the shelf. (iii) The Late Visean-Early Sakmarian stage is characterized by mud, microbial, and skeletal mounds arising on the slopes of depressions and uplifts in the degrading carbonate shelf. The distribution and geometry of organogenic structures were determined primarily by regional tectonic events and global geobiotic processes during this period of Earth's evolution. The uniqueness of this region allows it to be proposed as a model object for the study and understanding of Paleozoic reef formation.

The paper presents the results of a study of carbonate and sulfate–carbonate–clayey rocks of the Lower Maeotian in the sections of the bays of Cape Kazantip using a complex of analytical methods. It is established that the greatest variation of chemical, bituminological, phase, and carbon-isotope composition is characterized by carbonate–clay and clayey rocks of the section bottom. It is proved that the initial OM was accumulated mainly under reducing conditions, but has some variations in composition; it is characterized by a low degree of its catagenetic transformation, which indicates the preservation of the primary isotopic composition. The diverse phase composition of the clay fraction is revealed: dioctahedral illite, kaolinite, chlorite, glauconite, and weakly ordered mixed-layer formations of illite/smectite type with different ratios of illite and smectite components and varying degrees of ordering. Modeling of their diffraction profiles showed that the illite/smectite structure may indicate significant depths of sediment mobilization by mud volcanoes. The isotopic composition of the 13Сorg ranges widely from –33.72 to –19.27‰ the mode being –22.1…–24.93‰. The isotopic composition of 13Сorg below –25.6‰ may be related to the entry of isotope-light mass of methane-oxidizing bacteria into the OM. It was revealed that variations of isotopic composition curves 13Сcarb and 13Сorg along the section of the studied rocks are rarely characterized by unidirectional (positive or negative) variations and have different trends along the section. Direction of the 13Сcarb isotope composition curve with some variations has a pronounced upward trend toward heavier weighting, whereas the isotopic values of the 13Сorg reveals reverse tendency. The results obtained prove that the revealed variations in the composition of OM and carbon isotope composition in the Lower Maeotian sections of Cape Kazantip reflect variations in the sedimentation conditions of temperature, salinity, freshwater ingression, bioproductivity fluctuations, and the influence of local gas–fluid deposition. It is proposed to use such accessory minerals as zircon, monazite, and ilmenite as an indicator of mud paleovolcanism.

It has been established that the basement rocks of the Lower Maeotian section are characterized by the greatest variation in the chemical and isotopic composition of OM, while the carbonate rocks are more similar. It was revealed that the initial OM accumulated mainly under reducing conditions, but has some differences in its composition: for example, in the rocks of the Senkina and Sharabay bays it is an algal-cyanobacterial substance, and in the rocks of the Shirokaya bay it has a large contribution of the microbial component and the introduction of humus material. It was found that the isotopic composition of Corg has a strong variability (–19.27…–33.72 ‰), but most of the samples are in the range of –22.1…–24.93 ‰. It has been established that the 13Сcarb and 13Сorg isotopic curves in the studied rocks reflect their deviations both towards lighter and towards heavier values. The general direction of the isotopic composition, with some variations, nevertheless tends to be isotopically heavier towards the upper part of the sections. It was revealed that the litho-mineral-geochemical specificity of the sections reflects significant changes in sedimentation conditions, generally associated with fluctuations in the temperature and salt composition of waters, and bioproductivity. It is proposed to use such accessory minerals as zircon, monazite, ilmenite as an indicator of mud paleovolcanism.

The results of the study of calcite microspherulites in carbonate breccia formed at the junction of biohermal and bioclastic limestones (Lower Maeotian, Cape Kazantip, Kerch Peninsula), the nature of which is still being discussed, are presented. The following analytical methods were used: chemical carbonate, gas chromatography, Raman spectroscopy, electron microscopy with EDS, isotope. The studies resulted in the following: the presence in the fossilized biofilms of the pelitomorphic matrix and microspherulites of framboidal pyrite, halite, barite, celestine, dolomite-calcite, hematite and magnetite, which are absent in the other structural components of the breccia. Organic matter (OM) is represented by algal-cyanobacterial matter with a large contribution of the microbial component and an admixture of humic material. The carbonaceous matter in calcite microspherulites is actually characterized by primary unstructured organic matter. The accessory minerals zircon, oligoclase, rutile, neodymium-cerium monazite and vanadium silicide were identified. The formation of microspherulite calcites most likely occurred in the reducing conditions of the lagoon with high salinity in the contact zone of carbonate and microbial muds. The activation of authigenic mineral formation was facilitated by gas-fluid seeps from an active ancient mud volcano.

Research subject. Carbonate formations raised from depths up 1986 to 2973 m in the off-axis zone of the rift valley of the North Atlantic Ocean in areas of active young volcanism. The ocean floor here is composed of basaltoids and serpentinized gabbro-peridotites fragmentarily overlain by carbonate pelagic sediments.Aim. To confirm the organogenic nature of these carbonate formations and to reveal new features of deep-water carbonate structures of this type.Materials and methods. The research objects comprised 100 samples of branched and cone-shaped/crater-like carbonate formations, the primary studies of which were carried out directly on the research vessel. Analytical methods included optical microscopy, electron microscopy, X-ray fluorescence spectroscopy, X-ray diffractometry, infrared spectroscopy, inductively coupled plasma mass spectrometry, and isotope mass spectrometry.Results. Among the most important features of the studied formations were found to be a concentric-zonal structure, which forms around the axial channel, and a thin dark brown crust of carbonate-ferromanganese composition. The abundance of planktonic fauna fossils and the distribution of mineralized biofilms with bacteriomorphic structures and glycocalyx were found in the body of crusts of the studied formations. More than 50 trace elements were found, including 11 essential (vital), 18 physiogenicallly-active and 22 antibiotic elements. The ratios of group contents of essential and antibiotic elements vary from 0.67 in the upper part of the structures to 0.001 in their lower part and up to 0.0006 in the volcanogenic substrate of the carbonate buildups. The ratio of the concentrations of essential zinc to physiogenically-active copper behaves similarly. In calcite, the isotopic composition of carbon, δ13СPDB = = –0.16 ± 1.03‰, corresponds to marine sedimentary carbonates; conversely, while oxygen exhibits anomalously isotopically heavy values, δ18OSMOW = 34.44 ± 3.21‰. In ferromanganese carbonates, the corresponding values are –3…1 and 32– 35‰.Conclusions. The studied carbonate formations are solid solutions based on calcite in their body and based on siderite-rhodochrosite binary series in the composition of brown crusts. Specific features of the chemism and minal compatibility of carbonate solid solutions reflect the conditions of microbially-stimulated mineral formation. The conducted isotopic studies discovered the phenomenon of a combination of carbon and oxygen, fundamentally different in genetic nature, in the studied formations. For the explanation of this fact, a scheme for isotopic exchange of oxygen between marine bicarbonate and sulfate with the active participation of sulfate-reducing bacteria was proposed.

The lower Maeotian carbonate encrustation of bryozoan bioherms and local problematic carbonate buildups at Cape Kazantip (Kerch Peninsula) were studied to elucidate their genesis. Analytical (lithological and mineralogical, X-ray diffractometry, scanning electron microscopy, energy dispersive spectrometry, and isotopy) studies have shown that hardness of the bryozoan framework is related to the syndepositional, biologically induced cement around bryozoans and carbonate encrustation of bioherms. In addition to fossilized traces of products of the microbiotic vital activity (bacteriomorphic structures, mineralized biofilms, glycocalyx—an exopolymer substance (IPS), and framboidal pyrite), the carbonate crusts on bryozoan bioherms and mollusk–polychaete minibioherms contain abundant bitumen, strontianite, barite, celestine, Mn-rich calcite (kutnohorite), Mg-calcite, aragonite, dolomite are widespread. Mineralized biofilms include trace elements Fe, Si, Mg, Al, K, Na, Cl, Ba, S, Ni, and Co. The isotopic composition of different carbonate rock types is marked by wide variations of carbon (–2.76…7.17‰) and oxygen (24.20–33.01‰) and manifested in fluctuations of water salinity (16.67–39.74‰). The chemical composition and mineral specificity of rocks, confinement of carbonate crusts and minibioherms to saline waters, and local pattern of their formation suggest the manifestation of near-bottom cold gas-fluid seeps, probably, of a complex chloride-sodium-sulfate-magnesium composition or various modifications of these components in a shallow sea basin, whereas the bryozoan biohermal complex is most likely a near-hydrothermal oasis.

The isotopic composition of carbon and oxygen of 165 samples of carbonate rocks of genetically different organogenic structures: the Upper Ordovician Bol'shaya Kos'yu reef, the Upper Devonian Shar'yu microbial mound, the Kozhym skeletal mound, the Upper Miocene Kazantip bioherm complex and deep-water organogenic-carbonate structures of the north of the Mid-Atlantic Ridge was studied. The reef and microbial mound are characterized by δ 13СPDB values within the limits close to normal-sedimentary marine carbonates (–0.33—3.13 ‰) and (0.8—3.0 ‰), but with significant variations in δ 18ОSMOW — (22.24—30.0 ‰) and (20.4—26.3 ‰), respectively. The most isotopically-heavy carbon composition (5.1—7.3 ‰) in combination with varying δ 18ОSMOWvalues (22.4— 30.0 ‰) is characteristic of limestones and brachiopods of the skeletal mound. Fluctuations of δ 13СPDB and δ 18ОSMOW values in carbonate rocks of the Kazantip bioherm complex are the most expressive (–2.76—7.17 ‰) and (24.20—33.1 ‰), respectively. Deepwater organogenic structures from the bottom of the axial zone with active volcanism, in contrast to others, showed stable δ 13СPDB values (–0.98—0.83 ‰) within the area of normal-sedimentary marine carbonates, whereas for oxygen the values are istopic-heavy (32.27—39.75 ‰). As a result, the specificity of the development of the objects under study, established by lithological-paleoecological and chemical-physical methods, found its substantiation in isotopic values, paleosalinity and paleotemperatures caused by paleogeographic and climatic conditions.

The object of our research is the Kazantip Cape (Kerch Peninsula, Crimea). Its attraction is a ring-shaped rock massif composed of bryozoans previously considered to be a reef structure growing on the limbs of rising brachyanticline about 8 million years ago. Application of complex of investigating methods show that clay deposits underlying bryozoan structure are composed of expandable mixed-layered minerals, smectite, kaolinite, chlorite, illite with accessory minerals (zircon, monazite, ilmenite). These clays are the result of eruption of fossilized analogue of mud volcano. This process was accompanied by unloading of cold gas-fluid seepage. Specific mineralization (barite, celestine, strontianite, authigenic minerals of rare-earth elements and manganese) of bryozoan’s bioherms indicates that the seep process continued even after the waning of mud volcanism.

The complex of modern physical methods for studying authigenic carbonate crusts in the Miocene bryozoan bioherms of the Kazantip Cape (Kerch Peninsula) has made it possible to reveal fossil biofilms and glycocalyx for the first time. These finds indicate that methanotrophic carbonate-precipitated bacteria during their activity strengthened brittle bryozoan skeletons. The presence of cavities in bioherms incrusted with goethite crusts, the presence of bitumen, pyrite, strontianite, barite, kutnagorite in the composition of carbonate crusts, and traces of the vital activity of methanotrophic bacteria are associated with the significant influence of near-bottom local gas–fluid seeps. This situation was determined by the occurrence of a discharge zone of rising gas–fluid flows in the Kazantip Cape area as a result of activation of mud volcanism, typical of the Neogene–Holocene interval of the Kerch–Taman Region.

This article reports the first results of studies on microbially mediated organomineralization in carbonate ooids from the Paleozoic sections of the Timan–Northern Ural region, which were formed in different environments and had different original mineral composition (calcite, Mg-calcite, and dolomite). Using a scanning electron microscope, mineralized microbial biofilms preserved in various forms and the modified morphology of primary grains under the effect of organic acids were identified. The interrelation between microbes and organomineralization has been established, which was observed in the form of conserved nuclei of the amorphous phase of calcium carbonate on the surfaces of mineralized biofilms, including EPS relics in the composition of ooid crusts. Carbon and oxygen isotope data showed the difference in Paleozoic ooid formation from saline lagoons to open shallow sea. The Raman spectra revealed the state of structure order of carbonaceous materials (CM) ranging from amorphous to weakly ordered carbon in ooids, which enables the data on the isotope composition of ooid carbonates to be considered corresponding to their primary composition and the environmental conditions under which they were formed.

The results of mineralogical and geochemical studies of deepsea carbonate buildups dredged from areas of modern volcanism in the rift valley of the northern Atlantic Ocean were obtained for the first time. Morphology, anatomy, chemical and microelement compositions of buildups are characterized. Mineralized biofilms with bacteriomorphic structures were revealed. Using the methods of X-ray diffraction and RSMZ analysis, the main carbonate minerals of the buildups were diagnosed and studied, and the phenomenon of the anomalous nature of the isotopic composition of carbon and oxygen in them was revealed. It has been suggested that the cause of the abnormally heavy oxygen isotopic composition in carbonate buildups is the borrowing of oxygen by carbonate-forming bacteria directly from sulfates, often distinguished by isotopeheavy oxygen.

A complex of modern physical investigations of carbonate crusts covering Lower Miocene bryozoan limestone from the Kazantip Cape showed that bryozoans built a biohermal skeleton due to the synsedimentation of bioinduced cement. The mineral association (Mg-calcite + aragonite) indicates the existence of near-bottom environment typical for gas-hydrate biogenic mineral formation as a result of bacterial methane oxidation during the formation of bioherms. The presence of bitumen, pyrite, strontianite, barite, kutnohorite and traces of vital activity of carbonate-depositing methanotrophic bacteria in the composition of carbonate crusts is concerned with a significant influence of near-bottom local gas-fluid seeps.

The results of a paleoecological study of reef ecosystems in the Upper Paleozoic organic structures formed on the northeast margin of the European Platform (in modern coordinates) are presented. The main communities, microbial and metazoan associations, which are important designers at various stages of ecological continuity, are considered. Accordingly, their role in reefal as well as in microbial, skeletal and mud mound ecosystems is considered. The development of ecological successions in genetically diverse organic frameworks resulted from regional and global changes in tectonics and the biosphere.

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Complex modern micro- and spectroscopic methods for study of siderite concretions in the Lower Carboniferous terrigenous strata on the Kozhym River (Subpolar Urals) have shown that its formation was caused by destruction of clay minerals due to the activity of bacterial communities. The abundance of these bacteria was caused by gas–fluid seeps and bacterial methanogenesis processes in bottom deposits. In basins with normal marine fauna, this led to local desalination, hydrogen sulfide contamination, mass collapse of primary organisms, and the development of element-specific bacteria. The occurrence of these bacteria caused the formation of specific authigenic mineralization in the concretion of sideritic bacteriolites: the framboidal pyrite, sphalerite, galenite, barite, sulfoselenides, and tellurides.