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The Aptian deposits on Spitsbergen Island are poorly studied. Moreover, there were no published data on fossil distribution in the Aptian and Albian in the vicinity of the town of Longyearbyen. This article provides new data on ammonite-based biostratigraphy of the Carolinefjellet Formation, indicating the presence of Beds with Tropaeum arcticum (middle Aptian) and Beds with Grantziceras (lower Albian). The age of the formation was verified and the presence of lower Albian strata was justified. The results of microscopic and isotope studies of five samples of glendonites collected from the middle Aptian of the Carolinefjellet Formation section are presented. Glendonites from the Carolinefjellet Formation are composed of three calcite phases: ikaite-derived calcite and two successive types of cement, which fill cavities and develop partially after the first-phase calcite. The O and C isotope compositions of glendonites were measured in five bulk samples from the middle Aptian interval of the section. The δ18O isotope composition of glendonites is significantly different from that of seawater, indicating the input of diagenetic fluids; the δ13C values are characteristic of sedimentary organic matter and methanotrophy. For the first time, findings of ammonites allowed us to clarify the stratigraphic intervals of glendonite occurrence and associated cold-climate episodes and prove the early Albian age of glendonites from the top of the Carolinefjellet Formation.

A number of ecological and geochemical transformations occurred during late Ediacaran and early Cambrian time, the effects of which are difficult to overestimate. However, the strong linkage of biostratigraphic and chemostratigraphic methods with lithofacies makes the localization of the Precambrian–Cambrian boundary and its correlation with lithologically contrasting sections highly debatable. We analyse the taxonomy and stratigraphic distribution of small skeletal fossils and trace fossils, the carbonate carbon and oxygen isotope composition, and U–Pb detrital zircon age in the Ediacaran–Cambrian transitional interval of the Irkutsk Cis–Sayans Uplift (southwestern Siberian Platform). This interval (Moty Group) comprises a transgressive succession with red-coloured alluvial to deltaic siliciclastic deposits (Shaman Formation) and overlying shallow-marine carbonates (Irkut Formation). The lower Irkut Formation hosts sporadic and poorly preserved tubularCambrotubulusfossils, which are known from both the terminal Ediacaran Period (c. 550–541 Ma) and the Terreneuvian Epoch (541–521 Ma), and typical Fortunian trace fossils, including an index ichnotaxon of the Cambrian boundaryTreptichnus pedum. The biostratigraphic and carbonate carbon isotope data and U–Pb concordia ages of 531.1 ± 5.2 Ma (mean weighted, 530.6 ± 5.3 Ma) of the five youngest zircon grains from the lower Irkut Formation indicate that at least the shallow-marine carbonates of the upper Moty Group correspond to the Cambrian Stage 2 (c. 529–521 Ma). In the Irkutsk Cis–Sayans Uplift, the Cambrian Period tentatively began before or during the accumulation of the alluvial to deltaic siliciclastic Khuzhir and Shaman formations, and this crucial divide remained unmarked in the palaeontological and isotopic records.

The paper presents the first data on the nitrogen isotope composition of the associated gases of mineral waters in the North Caucasus and newly acquired data on the carbon isotope composition of the CO2 and CH4. Most of the samples were taken from the high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters of the Elbrus and Kazbek volcanic areas in the Greater Caucasus. It is shown that δ15N changes in the range of –3.9 to +5.6‰ and synchronously increases in gases of the Caucasus Spa Area (CSA) with an increase in the N2 and CH4 concentrations. This correlation points to a genetic association of non-atmogenic nitrogen with sedimentary methane-generating processes. The values of δ13С in carbon dioxide in the Elbrus area vary from –11.8 to –3.0‰. The mean value of δ13С (СО2) tends to increase north of Elbrus volcano, and this trend may be a consequence of the increasing content of СО2 produced by the thermal decomposition of sedimentary carbonates in the composition of gases, as well as a result of low-temperature interaction of the high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters with the Jurassic and Cretaceous carbonate rocks. It is shown that high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters associated with Elbrus volcano often contain up to 12.4% CH4. This methane is characterized by high δ13С of –32.0 to –17.2‰. Analysis of the distribution of concentrations and δ13С values of СН4 around Elbrus and the presence of N2 with positive δ15N values in the gases suggest a crustal genesis of the methane. The role of magmatic activity in this situation is reduced to producing a temperature anomaly that activates carbon isotope exchange in the CO2–CH4 system.

We studied the composition and isotopic characteristics (δ13C and δD) of hydrocarbon gases from the Kotelnikovsky, Zmeiny, and Goryachnisky hot springs located in the coastal zone of Lake Baikal. Of greatest interest is the Zmeiny Spring located on the eastern coast, where the methane has abnormally high values of δ13C (up to +11‰) and δD (up to +267‰). These values are related to the fractionation of carbon and hydrogen isotopes during the oxidation of microbial methane. The δ13C-C1 value (‒40.2‰) in the Goryachinsky Spring can formally indicate the thermogenic origin of methane. However, the modification of carbon isotope composition in this spring due to methane oxidation is more likely, because gas from the Goryachinsky Spring contains the microbial methane as admixture. Isotopic characteristics typical of microbial gases undistorted by secondary oxidation (δ13C-C1 = ‒61.5‰, δD-C1 = ‒230.2‰, δ13C-C2 = ‒56.6‰) are recorded in the Kotelnikovsky Spring northwest of Lake Baikal.

In 13 out of 14 samples of carbonate xenoliths from mud volcanic breccias of Azerbaijan, the 87Sr/86Sr values (0.70675–0.70823, 87Sr/86Srmd = 0.7073 ± 0.0005) are within the same limits as in the volcanic waters (Bujakaite et al., 2019). Genetically, they can be associated both with Mesozoic carbonate and saline deposits or with Cenozoic volcanosedimentary strata. Similarity of the Sr isotopic composition with isotopic values in carbonate extracts from terrigenous rocks of the Maikop Formation was noted only in the calcite sample from the Cheildag Volcano (87Sr/86Sr = 0.71047). The most interesting results with the lowest Sr ratios were obtained for carbonate fragments from breccias of the Byandovan Volcano. The combination of low δ13C values (down to –49.2‰), high Sr contents (up to 3108 μg/g) and 87Sr/86Sr (0.70675‒0.70690) values typical of the Late Jurassic brings them closer to caprocks of salt domes of Jurassic oil-bearing formations in the western United States.

The results of lithological, petrographic, and C–O isotope studies of various types of the diagenetic and postdiagenetic carbonate cement in silty mudstones, as well as coeval limestones, in postglacial deposits of the Dal’nyaya Taiga Group (Patom Basin, Siberia) are presented. Evidence of methane generation but not its anaerobic oxidation has been obtained. High δ13C (up to 14.9‰) values of the earliest carbonate cement generations in mudstones from the Barakun and Ura formations are related to extreme isotopic fractionation during the biogenic methanogenesis. The absence of isotopic signatures of the anaerobic oxidation of methane indicates its isolation during the subsequent reactions, suggesting the accumulation of gas hydrate compounds inside the sedimentary layers. Analysis of the distribution and correlation of δ180 and δ13C in the authigenic and sedimentary carbonates provides new arguments in favor of link of the Ediacaran global δ13C trend with epochs of the predominant accumulation of gas hydrate hydrocarbon compounds inside the deep-sea sedimentary strata and their subsequent anaerobic oxidation.

Carbon and oxygen isotope compositions were determined in the pedogenic, sedimentary, and organogenic carbonates from Upper Permian (Vyatkian Stage) and Lower Triassic (Induan Stage) continental rocks in the northeastern Moscow Syneclise. Variations of δ18O (18.0 to 29.1‰ SMOW) and δ13C (‒8.8 to 0.7‰ PDB) in them could be provoked both by regional climatic fluctuations and by perturbation of the global carbon cycle at the Permian‒Triassic boundary. Decrease of δ18O in pedogenic carbonates at the top of the Vyatkian Stage was likely caused by cooling on the northeastern side of Pangea. The negative excursion of δ13C values at this stratigraphic level correlates with the analogous excursion recorded in marine carbonates of the Paleo- and Neotethys and corresponds to the end-Permian biotic event.

The results of integrated study of the Cretaceous/Paleogene transition in the northeastern Caucasus are reported. The lithological, geochemical and micropaleontological characteristics of the sediments from two sections (Okhli and Gubden, Mountainous Dagestan) accumulated in different parts of paleobasin are displayed. An analysis of nannofossil distribution at the Cretaceous/Paleogene transition allowed to define the stratigraphic position of the stage boundary and reveal the interval of sediments (1–2 m thick transitional zone) accumulated under stress conditions. The large-scale sea-level fall occurred at the onset of the Cretaceous/Paleogene boundary event is evidenced. It led to the erosion and redeposition of the Maastrichtian calcareous sediments. In Okhli section, the occurrence of palygorskite is detected in the clay mineral assemblage of the transitional zone. The local concentrations of coprolites (Decapoda?) and spherical nodules of different composition (more likely, concretions) are found in the clay layer onlapping the erosional surface at the top of Maastrichtian. There is no reliable evidence of the impact nature of these nodules. The lithological record at the Cretaceous/Paleogene transition of the studied sections is similar to those exposed in other areas of the NE Peri-Tethys (e.g., southern Turkmenistan and Mangyshlak Peninsula).

We present the results of an optical microscopy, cathodoluminoscopy and isotopic study on nine glendonite concretions (calcite pseudomorphs replacing metastable hexahydrate ikaite) from Low...

The Sr isotope composition was studied in waters from 22 mud volcanoes in Azerbaijan, where the O and H isotope compositions, as well as concentrations of macro- and microelements, were determined earlier in (Lavrushin et al., 2015). Values of the 87Sr/86Sr ratio in the Azerbaijanian mud volcanoes range from 0.7067 to 0.7083. Its minimum value was recorded in water of Neftechala Yuzhnaya Volcano located in the southern area (Kura region), where volcanoes are generally characterized by low average values of 87Sr/86Sr (0.7070 ± 0.0003) and δ18O (2.0 ± 1.8‰), in contrast to volcanoes located in the northern area (Shemakha‒Gobustan, Apsheron, and Caspian regions) with 87Sr/86Sr = 0.7076 ± 0.0005 and δ18O = 4.8 ± 2.5‰. Ratios of the Sr and O isotope compositions of mud volcanic waters in Azerbaijan suggest that isotope characteristics of these waters are related to two major sources: the first source is related mainly to Cretaceous‒Paleogene sedimentary carbonates, whereas the second source dominating in the Kura region bears the signs of isotope exchange with volcanic rocks. The role of Sr dissolved in sea water and of Sr, which could be extracted from the recent terrigenous sediments in the Caspian Sea and the Maikopian terrigenous clayey rocks, is negligible in the Azerbaijanian mud volcanic waters.

The role of O2 in the evolution of early animals, as represented by some members of the Ediacara biota, has been heavily debated because current geochemical evidence paints a conflicting picture regarding global marine O2 levels during key intervals of the rise and fall of the Ediacara biota. Fossil evidence indicates that the diversification the Ediacara biota occurred during or shortly after the Ediacaran Shuram negative C-isotope Excursion (SE), which is often interpreted to reflect ocean oxygenation. However, there is conflicting evidence regarding ocean oxygen levels during the SE and the middle Ediacaran Period. To help resolve this debate, we examined U isotope variations (δ238U) in three carbonate sections from South China, Siberia, and USA that record the SE. The δ238U data from all three sections are in excellent agreement and reveal the largest positive shift in δ238U ever reported in the geologic record (from ~ −0.74‰ to ~ −0.26‰). Quantitative modeling of these data suggests that the global ocean switched from a largely anoxic state (26%–100% of the seafloor overlain by anoxic waters) to near-modern levels of ocean oxygenation during the SE. This episode of ocean oxygenation is broadly coincident with the rise of the Ediacara biota. Following this initial radiation, the Ediacara biota persisted until the terminal Ediacaran period, when recently published U isotope data indicate a return to more widespread ocean anoxia. Taken together, it appears that global marine redox changes drove the rise and fall of the Ediacara biota. Published version

This paper is devoted to the vein and dispersed carbonates from ejecta of mud volcanoes of Azerbaijan. The vein calcites are morphologically diverse and related to the healing of fracture systems. Most of them have δ13C (from –4 to +2‰) and δ18O (from +20 to +26‰) typical of marine carbonates. Some volcanoes contain fragments of the vein calcites enriched in light carbon (δ13C from –49.2 to –6‰), which could be formed in methane seep discharge sites during the Caspian Sea highstand period. In the dispersed carbonates from the clay pulp, the values of δ13C and δ18O change from –6.1 to +11.9‰ and from +23.8 to +32.7‰, respectively. It is shown that the studied carbonate matter has no genetic relation with the modern mud volcanic waters, which are characterized by high concentrations of $${\text{HCO}}_{3}^{-}$$ (up to 8 g/L) and values of δ13C(TDIC) (δ13Cav= +20.0‰) and δ18O(H2O) (δ18Oav= +4.0‰). Based on isotope characteristics and REE patterns, the vein calcites can be ascribed to the products of seawater-assisted postsedimentation transformation of sedimentary carbonates. The obtained data revealed a complete hydrodynamic isolation of mud volcanic channels from aquiferous complexes of host rocks.

Significant influence of anoxic events on faunal turnovers in marine communities is well-established. However, many studies are focused on the impact of anoxic conditions on benthic organisms, while coeval changes in pelagic cephalopod assemblages remain relatively poorly understood. In the present paper we discuss the trends in cephalopod assemblages coinciding with the onset of the early Aptian black shale deposition in European Russia coeval with OAE1a. In few sections of the Saratov Volga area (central part of the Russian Platform), representing both offshore and more proximal nearshore lithofacies of the epicontinental Middle Russian Sea, we have recognized simultaneous changes in ammonite and belemnite successions. Belemnites, represented by the late members of the family Oxyteuthididae, are common in the interval directly preceding anoxic event, but totally disappear with the onset of the black shale deposition. For the Deshayesites ammonites shell size reduction across the mudstone – black shale boundary (maximum shell diameter of adults reduces from ∼20 cm to 7–8 cm) can be observed. Some other ammonites become numerous (Sinzovia) within the black shale interval or show a first occurrence in it (Koeneniceras and Volgoceratoides). In our opinion diminishing of Deshayesites shell size during the early Aptian OAE could be caused by coupling of palaeoenvironmental factors such as progressive warming and regional input of brackish water. Preliminary results of carbon isotope studies of aragonite deriving from the ammonite nacreous layer are also provided.

The paper reports lithological and geochemical data on the stratigraphic interval corresponding to the Paleocene/Eocene Thermal Maximum (PETM) event in the Paleogene section of Eastern Crimea (Nasypnoe section). The section is located on the western continuation of the sublatitudinal profile consisting of 15 PETM sections spanning the area from Crimea to Tajikistan (over 2500 km). It is shown that PETM sediments have the negative δ13C and δ18O isotope excursions and are enriched in kaolinite, which is typical of most sections of this interval around the world. At the same time, the extremely low content of organic matter (OM) in sediments of the Nasypnoe section makes them sharply different from the highly carbonaceous rocks of the PETM interval in Central and Eastern Caucasus, and Central Asian regions. This is correlated with the low contents of many chemical elements in the studied rocks, and their extremely high contents in the highly carbonaceous rocks of the easterly sections. Thus, Eastern Crimea in the end of the Paleocene‒beginning of the Eocene was occupied by the low-bioproductive marine paleobasin, whereas the coeval paleobasin in Central Asia was characterized by extremely high bioproductivity owing to the presence of significant amount of phosphorus.

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 origin of high-Mg melts remains one of the most highly debated questions in igneous petrology. There are two contrasting points of view, namely, (1) melt initiation in a rising high-temperature plume, and (2) mantle melting due to fluxing by water. To address this question we determined H2O, CO2, F, Cl and S concentrations in olivine-hosted melt inclusions of high-Mg volcanic rocks of the Siberian Traps Large Igneous Province that bracket the main pulse of volcanism at about 252–250 Ma, and can be classified as melanephelinites and meimechites. Both rock types belong to the high-Ti rock series. Correcting measured H2O, CO2, F, Cl and S concentrations in homogenized primary meimechite melt inclusions to primary meimechite melt composition using experimental melt compositions resulted in corrected melt-inclusion, volatile compositions of ˜3.88 wt% H2O, ˜1477 ppm CO2, ˜4214 ppm F, ˜2.08 wt% Cl and ˜2490 ppm S. These values are viewed as minimum estimates for the original volatile concentrations in the melt because of the high probability for degassing during melt crystallization and/or during experiment homogenization. Olivine-hosted homogenized melt inclusions from melanephelinites yielded lower corrected concentrations of ˜1.06 wt% H2O, ˜998 ppm CO2, ˜3242 ppm F, ˜607 ppm Cl and ˜2131 ppm S. We also measured water concentrations in clinopyroxenes of melanephelinites by FTIR, obtaining values as high as 133 ppm H2O, which corresponds to 0.91 wt% in the melt, in general agreement with data obtained by SIMS on the olivine-hosted melt inclusions. Olivine grains from melanephelinites are characterized by evolved compositions (Fo 0.80–0.86). Extrapolation to a primitive melanephelinite melt by simple fractional crystallization suggests that it could also contain high H2O concentrations (up to ˜ 3 wt%). Analyzed meimechite and melanephelinite whole-rock samples are characterized by trace-element patterns that are typical of mantle-derived melts and by Sr-Nd isotope ratios that exclude crustal contamination or derivation from ancient lithospheric mantle. Thus, high volatile concentrations can be attributed to sublithospheric mantle source regions. This supports the notion that high-Mg melts form by volatile fluxing of the asthenospheric mantle rather than by decompression melting under relatively dry conditions of a rising abnormally high-temperature mantle plume.

It is shown that the gas and water phases of the thermal nitrogen–methane waters in the Talysh fold zone of the Lesser Caucasus mountain system contain helium and strontium with mantle isotope signatures (3Не/4Не from 200 × 10–8 to 401 × 10–8 and 87Sr/86Sr from 0.70490 to 0.70562). At the same time, clear signs of the mantle component in other gases (nitrogen, methane, and carbon dioxide) are absent. The δ15N value in nitrogen varies from +0.3 to +1.7‰, methane is mainly characterized by δ13C from–57.4 to–38.0‰, while δ13C(CО2) varies from–24.4 to–11.3‰. An increase of the CО2 content is accompanied by the decrease of δ13C in CО2, against the background of increasing SO4 content in the salt composition of waters. This indicates a microbial nature of CO2 in the studied gases. Thus, the presence of mantle helium and strontium in the thermal waters is likely related to their leaching from the Pleogene–Neogene host volcanic rocks. The studies of the oxygen and hydrogen isotope composition in water revealed quite different mechanisms for the formation of cold and thermal waters of the region. The cold waters are mainly fed by local infiltration, whereas the feeding of thermal nitrogen–methane waters is strongly provided by transit atmogenic waters (>50%), which are formed in the mountain ranges at altitudes no less than 1600 m and spaced at 20–40 km or more from the thermal discharge sites.

The summary of the study of invertebrates used for decorative items from Khvalynsk Eneolithic Cemeteries is presented. AMS radiocarbon dating confirmed that freshwater Unio mother-of-pearl shells and nacre discs, manufactured from them, are coeval with the site and most likely local in origin. Tubes of the sea worms and marine shells are fossil and some may originate from local deposits. Other are probably related to the ancient Paratethys basin sediments and can have diverse origin spanning from Caucasus and Caspian shores to Carpathian and Mediterranean area. Glycymeris L. clams are most likely originated in the latter area. The discussion of the obtained AMS 14 C results agrees with a possibility of a noticeable ingress of younger carbon, irremovable by standard methods and affecting the determinations, probably related to a bacterial activity in deposits.

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 δ13C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ18O 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 δ34S 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 paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The 87Sr/86Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ18O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in 18O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ13C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ13C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.

The studies of past climatic changes form the basis for predicting our future anthropogenic world and are among the most prominent topics in current Earth sciences. Although the Cretaceous is generally considered as a greenhouse period in Earth's history, a number of significant cooling events based on an array of climatic proxies have been identified. Here we present the first data on Berriasian (Ryazanian) glendonite findings from the paleontologically well dated Lower Cretaceous succession of northeastern Siberia. Based on well calibrated Buchia and ammonite biostratigraphy, the stratigraphic interval across which the glendonites occur is restricted to the late Berriasian. Stable carbon isotope (δ13C) values of the studied glendonites clearly suggest the precipitation of ikaite from marine water without any significant contamination from biogenic methane. Our results, when integrated with other available paleoclimatic proxies from elsewhere in the high latitudes, suggest a revision of the initiation of Early Cretaceous cooling in the high latitudes from the Valanginian to the late Berriasian. All known occurrences of Lower Cretaceous glendonites in both the northern and southern hemispheres are reviewed.

The sedimentary record of the Paleocene–Eocene Thermal Maximum (PETM) in the wide epeiric sea of the NE Peri-Tethys contains a sapropelitic bed (SBD) characterized by a specific microfossil assemblage and negative oxygen and carbon isotope excursions (CIE). New results obtained from a high-resolution sampling of this interval in the Kheu section, central Caucasus, allows us to explore the succession, interrelationships and consequences of global and regional palaeoenvironmental events during the PETM. The CIE apparently lasted twice as long as the SBD accumulation period; thus, the SBD represents a response to the early and most dramatic phase of the PETM related to rapid transgression and greatly enhanced eutrophication of the basin. The lithological architecture of the SBD represents four bands, each of which consists of a dark, low-calcareous TOC-rich clay and relatively paler calcareous clays. The highest TOC concentrations are found in the lower and upper bands of the SBD; however, the lower band mostly consists of basinal organic matter, while the upper band represents mixed, basinal and terrestrial, organic matter. Pre-PETM events include the initial evolution of the Rhomboaster nannofossil lineage with the early first appearance of a short-arm species, relatively reduced calcareous plankton productivity, changes in the ratios of dinocyst ecological groups, and minor fluctuations in δ13C and δ18O. The onset of the CIE corresponds to the base of the SBD, which exhibits a dramatic decrease in total nannofossil abundance and an increase in total dinoflagellate abundance, the occurrence of “excursion taxa” of nannofossils (rhomboasters, asymmetric discoasters) and dinocysts (Axiodinium augustum, Epelidinium pechoricum), and significant variations in the species ratios of both nannoplankton and dinoflagellate communities, with widespread dominance of warm-water and eutrophic species. Above the SBD but during the late phase of CIE, the nannofossil abundance and species composition show a slight recovery, but most Paleocene taxa become extinct during the CIE recovery. Apectodinium spp. significantly decrease in abundance above the SBD, but rare specimens of Ax. augustum persist higher in the section after the end of the CIE. Thus, the major extinction of the Paleocene nannofossil taxa occurs after the termination of the most critical conditions during the CIE recovery phase, while the dinocyst “excursion taxa” survive even later.

Stellate and plate-like carbonate bodies, traditionally called anthraconites, are found throughout the Baltic-Ladoga Klint in bituminous shale of the Koporye Formation (Tremadocian, Lower Ordovician). Although this time interval is usually considered as a greenhouse, there is some evidence for the existence of at least temporary cold conditions during the Cambrian&ndash, Ordovician. However, the origin of anthraconites is still strongly debated. We studied the mineralogical, petrographic, cathodoluminescence, geochemical, and isotopic characteristics of anthraconites from five sections of the Russian part of the Baltic paleobasin. A close similarity between the morphological, petrographic, cathodoluminescence, and isotopic characteristics of the studied anthraconites with those of glendonites allow us to suggest that these bodies formed in a similar paleo-environment and should be considered as pseudomorphs of the mineral ikaite. The oxygen and carbon isotope ratios reveal that ikaite precipitation occurred in low-temperature conditions on the seafloor. The carbon isotopic values reveal influence of inorganic seawater carbon along with organic matter decomposition and/or methane oxidation during ikaite-glendonite transformations. The oxygen isotopic composition significantly changed after deposition due to meteoric diagenesis. We propose that the studied Tremadocian anthraconites formed under a region of upwelling, where cold phosphate-rich deep waters rose to the relatively shallow part of the Baltic paleobasin, providing favorable conditions for ikaite precipitation. Based on our cathodoluminescence study, we suggest that ikaite was transformed to calcite over several stages during diagenesis. Mineralogical studies also reveal that primary calcite was transformed to sulfate (gypsum) or dolomite during late superimposed processes.