The chemical and isotopic signatures of mud volcanic fluids were determined for 42 mud volcanoes of the Kerch–Taman region (Crimea–Caucasus district). The analysis of spatial geochemical trends of mud-volcanic waters and gases and estimates of formation temperatures using the Mg–Li geothermometer allowed us to attribute the mud volcanic fluids to sediments of the Maikop Formation, the thickness of which in the studied region varies from 1 to 5–6 km. It was shown that an increase of CO2 and $${\text{HCO}}_{3}^{ - }$$ concentrations and values of δ18O in H2O and δ13C in CH4, as well as a decrease of Cl– concentrations and δ13C values in $${\text{HCO}}_{3}^{ - }$$ and CO2 correlate with increasing thickness of the Maikop sediments in the central part of the Kerch–Taman region. At the periphery of this region, in the foothills of the Caucasus and Crimea orogens, this correlation is absent. Active tectonic processes in these areas provided contribution of additional “higher temperature” sources of water and gas to fluids in the mud volcanic systems. Using the Kerch–Taman region gases as an example, it was shown for the first time that the enrichment of mud volcanic fluids in isotopically heavy CO2 (δ13C CO2 to +22.8‰; δ13C HCO3 to +38.9‰) occurs within a narrow temperature range (40–80°C). At higher temperatures, the accumulation of isotopically heavy CO2 is ceased. These trends suggest that the high-δ13C CO2 is related to the hydrocarbon biodegradation. Using δ13C (in CH4 and CO2) and δ2H (in CH4) classification, it was shown that at least three methane sources could participate in the CH4 balance: microbial (primary), connected with “biodegradation” (secondary), and thermogenic. The content of “secondary” methane in some cases can reach almost 100%. In the studied mud volcanic gases, the isotopic composition of molecular nitrogen was determined for the first time. The δ15N values vary from –5.2 to –0.1‰ (δ15Nav = –2.3 ± 0.9‰, n = 35) and indicate a significant admixture of the non-atmospheric nitrogen. A negative correlation between values of δ15N in N2, δ13C in $${\text{HCO}}_{3}^{ - }$$ , and the calculated fluid generation temperatures (t(Mg-Li)) make it possible to attribute the origin of this nitrogen to the transformation of organic matter during the maturation of sediments.

The chemical and isotopic compositions of manganese carbonate and oxide ores and host dolomites of the Porozhinsk deposit are studied. Manganese carbonates are characterized by low values of δ13C (–19.0 to –8.4‰, PDB) and δ18O (8.8 to 27.3‰, SMOW). These data indicate the participation in their formation of the isotopically light carbon from oxidized organic matter, and the water of ore-depositing solutions was most likely meteogenic. The host dolomites (Pod”emsk Formation, Chapa Group) are characterized by a heavier isotopic composition of carbon (2.3‒2.6‰) and oxygen (22.5‒31.3‰) that are typical of marine sedimentary carbonates in the Late Proterozoic sedimentation basin. The simplest organisms played a significant role in the genesis of manganese ores. Microbial structures were preserved in manganese carbonates and oxides.

The chemical and isotopic characteristics (δ18O and δ2H) of water from 42 mud volcanoes of the Kerch–Taman region (Crimean–Caucasus district) were analyzed. The formation temperatures of mud volcanic waters were estimated using the Mg-Li hydrochemical geothermometer (t(Mg-Li) = 40–134°C). It was revealed that dehydration waters released during the smectite–to-illite transformation play significant role in the total fluid balance of the mud volcanic systems (up to 80% of the total fluid volume). These waters are characterized by low Cl ion contents, δ18O from +13 to +17‰, and δ2H from –25 to –20‰ relative to VPDB, and high concentrations of $${\text{HCO}}_{3}^{ - }$$ , B, Li, Ba and other trace elements. Correlations between the concentration of HCO3, B, δ18O, and t(Mg-Li) values were revealed.

The Imnyakh Formation (1500–1800 m thick), which is developed east of the Mama–Bodaibo Synclinorium (inner zone of the Patom Upland), comprises crystalline limestones (marbles) and carbonate shales (metamorphic marls). The carbon and strontium isotope compositions (δ13Cav = –9.0 ± 0.4‰, 87Sr /86Sr = 0.70810–0.70845) in these rocks are completely identical to those in unmetamorphosed limestones of the Zhuya Group (outer zone) and the Torgo Formation (adjacent part of the Siberian Platform), which are comparable with the global Ediacaran C isotope anomaly (Shuram–Wonoka event). Metamorphism was not accompanied by the removal of Sr whose content in marbles of the Imnyakh Formation are as high (up to 6290 μg/g) as in limestones of the Zhuya Group in the deepest parts of the Patom paleobasin. Thus, metamorphism was only manifested in the depletion of rocks with the heavy oxygen isotope by 3–4‰ (δ18Oav = 17.2 ± 1‰), relative to unmetamorphosed limestones. Carbonates of the pre-Imnyakh black carbonate-bearing shales and deep black crystalline limestones in the Khomolkha and Ugakhan formations are characterized by wide variations of δ13C (–4.0 to 7‰) and δ18O (15.2 to 23.4‰), indicating strong postsedimentary alterations. Nevertheless, their Early Vendian (Early Ediacaran) age is confirmed by such features as position in the section, abnormally high δ13C values, and the minimum value 87Sr /86Sr = 0.70753 recorded in a sample with the Sr content of 11700 μg/g. Although the isotope systems of carbonates in the Sogdiondon and Vitim formations (δ13C –14.2 to –3.7‰, δ18O 12.2 to 13.4‰, and 887Sr/86Sr 0.71120 to 0.71337) occurring at the Neoproterozoic sequence base in the Zhuya River underwent even stronger modification, it is possible that δ13C values in these formations were initially negative. In terms of the C and O isotope compositions (δ13Cav = –9.5 ± 0.7‰, δ18Oav = 17.3 ± 1.8‰), crystalline limestones and carbonate shales in the metamorphosed analogs of the Zhuya Group from the Mama River mouth (west of the synclinorium) are similar to the Imnyakh Formation. Current problems of the correlation, age, duration, and origin of the largest in geological history negative C isotope anomaly are discussed.

The behavior of stable carbon and oxygen isotopes in carbonates during the deposition and diagenesis of sediments in the bioproductive Upper Famennian Pripyat Trough (southern Belarus) is discussed. Limestones and clayey limestones (Corg 0.92 ± 0.11%) are characterized by very low δ13C values (–9.6 ± 0.3‰). Parental sediments of these rocks were deposited in the shallow-water zone during slow downwarping episodes of the seafloor. Lithification of the sediments took place in oxidative conditions of the diagenesis zone. Organic matter was actively oxidized by free oxygen. Carbon dioxide with isotopically light organic carbon formed in this process was used during the crystallization of diagenetic carbonates that are visually indiscernible from the sedimentational variety. Marls, clayey marls, and carbonate-bearing clays (Corg 6.02 ± 0.80%) are characterized by δ13C values as high as –3.5 ± 0.6‰. In combustible shales (Corg >10%), δ13C value is ‒1.2 ± 0.6‰). The clayey rocks mark the episodes of sedimentation in relatively deep-water conditions that appear during the uncompensated sagging of the basin floor. The diagenetic zone with free oxygen was significantly decreased or absent at all. Here, oxygen of marine sulfates was the main or single oxidizer of organic matter (sulfate reduction). The sulfate oxygen is a weaker oxidizing agent than free oxygen. Therefore, much more organic matter was retained and fossilized in clayey rocks than in carbonate rocks. Organic carbon released during the sulfate reduction and mobilized later for the diagenetic carbonate formation was insufficient for the significant decrease of δ13C values relative to the marine carbonate standard. Isotopic composition of carbonate oxygen in the studied rocks is invariable and does not depend on the content of clay and organic matter. In these rocks, δ18O values are at the level (approximately –5‰) shown for the Famennian in the Global Chemostratigraphic Chart. This fact is consistent with the supposition that oxygen isotope composition of atmospheric precipitates, which influenced the rock formation via continental flow, is close to that of sea water in low latitudes where the territory of Belarus was located in the Late Devonian.

The Upper Neoproterozoic–Lower Cambrian section in the Olekma River basin is divided (from bottom to top) into the Dikimdia, Seralakh, Porokhtakh, and Pestrotsvetnaya formations, which are mainly composed of dolostones with a subordinate amount of sandstone, silty shales, gypsum, anhydrite, and native sulfur. The isotope data available allow one to substantially correct the stratigraphic units of the Olekma section, which is probably the most completed section on the Siberian Platform, including the transitional strata from the Vendian (Ediacaran) to the Cambrian. The Dikimdia Formation dolostones have typical Late Ediacaran values of 87Sr/86Sr = 0.70837–0.70843 and abnormally high δ13С values (up to 5.1‰), which gives reason to correlate this formation with the Ust’-Yudoma Formation (the stratotype area), the Dengying Formation in South China (548–550 Ma), the Uluntuy Formation of the Baikal Group, and the terminal Ediacaran sections in some other areas. The 87Sr/86Sr ratio decreases upward and reaches a minimum (0.70803–0.70818) near the Porokhtakh–Pestrotsvetnaya boundary. The lowermost part of the Porokhtakh Formation contains a negative carbon isotope excursion (δ13С = –4.4‰) marking the base of the Nemakit-Daldynian Stage, whereas ~8 m below the base of the Pestrotsvetnaya Formation the positive “pre-Tommotian” excursion (δ13С = 4.1‰) is located. There is a sharp decrease in Fe and Mn contents from ~3800 and ~300 ppm to ~2000 and ~130 ppm, respectively, in dolostones at the Ediacaran–Cambrian transition boundary which corresponds to the Seralakh–Porokhtakh boundary. High δ18О values (26.0 ± 1.2‰) do not provide reasons to associate the carbon isotope excursions, the 87Sr/86Sr trend, and variations of Fe and Mn contents with diagenetic alterations.

Отношения 87Sr/86Sr в гипсах и известняках ордовикского разреза р. Мойеро уменьшаются снизу вверх по разрезу от 0.7091-0.7095 в ирбуклинской свите (няйский горизонт~тремадокский ярус нижнего ордовика) до 0.7080 в верхней части джеромской свиты (долборский горизонт~катийский ярус верхнего ордовика), что находится в хорошем соответствии с биостратиграфическим расчленением разреза и существующим представлением об эволюции изотопного состава стронция в Мировом океане. Наиболее характерной особенностью изотопно-углеродной кривой является уменьшение значений 13С в карбонатах от слабо положительных (0.5…1.1‰) в ирбуклинской свите (няйском горизонте) до резко отрицательных (-5.4…-5.8‰) в средней части кочаканской свиты (кровля кимайского горизонта~кровля дапинского - основание дарривильского яруса). Увеличение в этом интервале разреза значений 18О от 20-22‰ до 26-28‰ и отрицательная корреляция между 13С и 18О, а также уменьшение значений 34S в гипсах от 30-32‰ до 22-24‰, показывающее, что обеднение карбонатов 13С не связано с сульфат-редукцией и окислением органического вещества в диагенезе, позволяют считать отрицательный экскурс 13С первичным; наличие на близком стратиграфическом уровне отрицательных аномалий 13С в ордовикских разрезах Южной и Северной Америки [Buggish et al., 2003; Edwards, Saltzman, 2014; McLaughlin et al., 2016] указывает на глобальное или субглобальное распространение этого события, возможно, связанного с появлением древнейшей наземной растительности. На фоне общего падения величин 13С в нижней части разреза выделяются 3 положительных экскурса малой амплитуды (1-2‰), положение которых в целом подтверждает существующую схему корреляции разреза р. Мойеро с международной шкалой. Верхняя часть разреза характеризуется чередованием интервалов с пониженными значениями 13С (до -2…-3‰) и короткими положительными экскурсами с амплитудой 0.5-1.3‰. Положительный экскурс 13С, которым заканчивается ордовикский разрез р. Мойеро, сопоставляется с экскурсом 13С в середине катийского яруса, а экскурс 13С в нижней части баксанского горизонта - с экскурсом, маркирующим границу катийского и сандбийского ярусов.

The paper describes the results of study of the Silurian clayey–carbonate rocks ranging from the Telychian Stage (Llandovery) to the Gorstian Stage (Ludlow) recovered by the Borehole Davtyuny 3k in northwestern Belarus. Rocks of the Sheinwoodian Stage demonstrate a positive excursion of δ 13 C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ 18 O for the carbonate material in the studied section (25.5–29.2‰ SMOW) are close to those for Silurian rocks from the Baltic region, Scandinavia, Ukraine, Poland, and Canada. The whole section contains postsedimentary gypsum as nodules and the infilling of fissures and fenestrae. Values of δ 34 S in gypsum (21.3–26.7‰ CDT) are close to those for the Silurian rocks on the Phanerozoic isotope plot. The formation of gypsum was related to a partial development of the supralittoral environment over the sublittoral and littoral clayey–carbonate substrate. The seawater accumulated in lowlands of the supralittoral plain after storms was intensely concentrated during arid conditions and accumulated in the clayey–carbonate sediment. The subsequent underground evaporation promoted the formation of gypsum as nodules in the unlithified sediments and the infilling of fissures and fenestrae in the lithified rocks.

The 87Sr/86Sr ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091‒0.7095 in the Irbukli Formation (Nyaian Regional Stage, ~Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, ~Upper Ordovician Katian Stage), which is well consistent with biostratigraphic subdivision of the section and existing concept concerning the strontium isotope evolution of the World Ocean. The most characteristic feature of the carbon isotope curve is decrease of δ13С values in carbonates from weakly positive values (0.5…1.1‰) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (–5.4...–5.8‰) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, ~end of the Dapingian–base of the Darriwilian Stage). Increase of δ18О from 20‒22‰ to 26‒28‰, the negative correlation of δ13С and δ18О, and decrease of δ34S in gypsum from 30‒32‰ to 22‒24‰ in this interval indicate that the 13С depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative δ13С excursion was of primary nature. The presence of negative δ13С anomalies at this stratigraphic level in Ordovician sections of the South and North America (Buggish et al., 2003; Edwards and Saltzman, 2014; McLaughlin et al., 2016) indicates the global or subglobal distribution of this event, which was possibly related to the emergence of the oldest ground vegetation. Against the general decrease of δ13С, the lower part of the section reveals three low-amplitude (1‒2‰) positive excursions, the position of which in general confirms the existing correlation scheme of the Moyero River section with the international scale. The upper part of the section is characterized by the alternation of low-δ13С intervals (from–2 to–3‰) and brief positive excursions with amplitude of 0.5‒1.3‰. The positive δ13С excursion terminating the Ordovician section of the Moyero River correlates with the δ13С excursion in the middle Katian Stage, while the δ13С excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian–Sandbian boundary.

The typical for marine sediments decrease in the amount of sulfate ion in interstitial solutions with increasing depth owing to sulfate reduction and attenuation of the diffusion of near-bottom waters is manifested as the basic-to-acid zonation in the sulfur isotope composition of the diagenetic pyrite nodules (increase of δ 34 S values from the center to edge). In sandstones of the Early Carboniferous Emyaksin Formation, which is developed at middle reaches of the Vilyui River (Sakha-Yakutia), we detected a peculiar sulfide nodule. In contrast to common nodules, this nodule is marked by the δ 34 S value decreasing from 7.5…8.6‰ at the center to –30…–31.4‰ at the edge. Such acid-to-basic zonation is attributed to the origin of nodule in a freshened basin (delta) with a sulfate-depleted environment, which gave way to a sulfate-enriched environment owing to the expulsion brines from the underlying gypsiferous sequences.

Geochemical studies of pyrite crystals from the mud volcanic ejecta in Azerbaijan were studied. It is shown that all of them have cubic shape. Determination of the sulfur isotope composition revealed a wide variation range of δ 34 S values from–27.0 to +26.4‰. Signs of spatial zonation were recorded in the distribution of δ 34 S values—lower values are confined to the present-day coastline of the Caspian Sea. Appearance of pyrite with a high share of 34 S is attributed to sulfate reduction that takes place in an environment with excess organic matter. It is supposed that the isotopically heavy sulfides represent the “neck” facies that are formed at the periphery of mud volcanic conduits at the contact of the hydrocarbon-rich mud volcanic fluids with stratal waters of host sediments.

New data were obtained on δ13Сcarb and δ18O variations in the sequence of deposits of the Dal’nyaya Taiga series at the western and eastern flanks of the Ura anticline. The summary δ13С curve was plotted in view of the correlation of sequence–stratigraphic data of the basin analysis. A series of positive anomalies was found within the succession. Alternatives for global chemostratigraphic correlation of the Dal’nyaya Taiga series of the Ura uplift were considered.

The paper presents original authors' data on the O, H, C, S, and Sr isotopic composition of water and sediments from the basins into which the Aral Sea split after its catastrophic shoaling: Chernyshev Bay (CB), the basin of the Great Aral in the north, Lake Tshchebas (LT), and Minor Sea (MS). The data indicate that the δ18О, δD, δ13C, and δ34S of the water correlate with the mineralization (S) of the basins (as of 2014): for CB, S = 135.6‰, δ18О = 4.8 ± 0.1‰, δD = 5 ± 2‰, δ13C (dissolved inorganic carbon, DIC) = 3.5 ± 0.1‰, δ34S = 14.5‰; for LT, S = 83.8‰, δ18О = 2.0 ± 0.1‰, δD =–13.5 ± 1.5‰, δ13C = 2.0 ± 0.1‰, δ34S = 14.2‰; and for MS, S = 9.2‰, δ18О =–2.0 ± 0.1‰, δD =–29 ± 1‰, δ13C =–0.5 ± 0.5‰, δ34S = 13.1‰. The oxygen and hydrogen isotopic composition of the groundwaters are similar to those in MS and principally different from the artesian waters fed by atmospheric precipitation. The mineralization, δ13С, and δ34S of the groundwaters broadly vary, reflecting interaction with the host rocks. The average δ13С values of the shell and detrital carbonates sampled at the modern dried off zones of the basins are similar: 0.8 ± 0.8‰ for CB, 0.8 ± 1.4‰ for LT, and –0.4 ± 0.3‰ for MS. The oxygen isotopic composition of the carbonates varies much more broadly, and the average values are as follows: 34.2 ± 0.2‰ for CB, 32.0 ± 2.2‰ for LT, and 28.2 ± 0.9‰ for MS. These values correlate with the δ18O of the water of the corresponding basins. The carbonate cement of the Late Eocene sandstone of the Chengan Formation, which makes up the wave-cut terrace at CB, has anomalously low δ13С up to –38.5‰, suggesting origin near a submarine methane seep. The δ34S of the mirabilite and gypsum (11.0 to 16.6‰) from the bottom sediments and young dried off zone also decrease from CB to MS in response to increasing content of sulfates brought by the Syr-Darya River (δ34S = 9.1 to 9.9‰) and weakening sulfate reduction. The 87Sr/86Sr ratio in the water and carbonates of the Aral basins do not differ, within the analytical error, and is 0.70914 ± 0.00003 on average. This value indicate that the dominant Sr source of the Aral Sea is Mesozoic–Cenozoic carbonate rocks. The Rb–Sr systems of the silicate component of the bottom silt (which is likely dominated by eolian sediments) of MS and LT plot on the Т = 160 ± 5 Ma, I0 = 0.7091 ± 0.0001, pseudochron. The Rb–Sr systems of CB are less ordered, and the silt is likely a mixture of eolian and alluvial sediments.