An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23070395

The article provides new data on the structure of the Laptev Sea flank of the Gakkel Ridge. The intensive supply of clastic material from the Laptev Sea shelf leads to the development of a thick alluvial fan at the continental rise, which determines the structure of the bottom topography. In the northwestern direction, the influence of the fan decreases and tectonics becomes the main relief-forming factor. The bathymetric survey traced the asymmetrical rift valley of the Gakkel Ridge, the western flank of which is complicated by terraces. The presence of fault structures, bottom subsidence, extensive sediment supply, and the widespread development of subaqueous slump processes indicate the high neotectonic activity of the Laptev Sea flank of the Gakkel Ridge. For the first time in this region, numerous carbonate rocks have been discovered, the authigenic cement of which is represented by magnesian calcite or aragonite with an admixture of terrigenous material. The palynological and micropaleontological analysis of the carbonate rocks indicates the Quaternary formation of authigenic carbonate cement. An important role in the formation of authigenic carbonates was played by diagenetic solutions coming from the sedimentary cover together with methane and oxidation products of gases and organic matter. The authigenic carbonates were precipitated mainly in an isotopic equilibrium with bottom water at a temperature of about 0°C. The negative correlation between 87Sr/86Sr and δ13C indicates the presence of at least two different sources of carbonate-forming solutions.

The article describes the first find of authigenic carbonates on the southern flank of the Gakkel Ridge in the zone of its junction with the Laptev Sea continental margin of the Russian Federation. The samples are represented by dense magnesian calcites and aragonites, including rounded and angular fragments of terrigenous material, as well as microphytoplankton of different ages, spores and pollen of terrestrial and aquatic plants. Elemental and organochemical characteristics indicate the predominance of oxidizing or intermediate between oxidizing and reducing conditions of carbonate crystallization, which may be a consequence of their formation near the bottom surface. The isotopic composition of O, C, and Sr allows us to conclude that the diagenetic carbonates of the Gakkel Ridge were deposited mainly in isotopic equilibrium with bottom water at a temperature of about 0°C, which corresponds to measurements from the ship. A wide range of δ13С (–23.5 до –37.3) indicates that methane was an important, but not the only source of carbon in carbonates. The wide variations in the 87Sr/86Sr (0.70906–0.70933), which correlate with the δ13С values, show that the carbonate-forming fluid was not only modern sea water, but also diagenetic solutions coming from the sedimentary cover together with methane and the products of methane and organic matter oxidation. Intense discharge of heterogeneous methane-bearing fluids may be related to the high modern tectonic activity of the studied region.

The paper presents the results of studies of authigenic carbonates sampled in the near-surface sediments of the gas-hydrate-bearing CHAOS structure located on the western slope of the Deryugin Basin in the Sea of Okhotsk. Carbonates were at different stages of formation, the most common morphological forms of which were concretions. Microscopic studies have shown that the concretions are formed by pelitomorphic carbonate cementing the terrigenous clay-clastic matrix and organic detritus. According to X-ray phase analysis, the dominant mineral composing nodules is cryptocrystalline high-Mg calcite (14.2-16.9 mol% Mg). Based on the geochemical modeling the intervals of the sediments with favorable conditions for the formation of basic carbonate minerals (aragonite, calcite, and dolomite) were identified. Almost over the entire area of the structure, the formation of carbonates occurs at more than 1m and up to 5m sub-bottom depths. In the central part of the structure (station LV31-27GC) the formation of carbonates is also possible in the upper part of the sedimentary section - from 0 to 1,5 m. On the basis of balance calculations of the δ13C isotopes content in nodules, the contribution of the main carbon sources involved in their formation was estimated. These sources turned out to be organic matter and microbial methane in a ratio of 67.5: 32.5. It was found that the conversion of carbon dioxide into bicarbonate ion occurs under conditions of early diagenesis due to the interaction of the fluid with aluminosilicates. Calculations of the isotopic composition of pore waters (the main source of oxygen in carbonates) and paleotemperatures indicate crystallization of carbonates at low (close to 0 ° C) temperatures. Based on 230Th/U-dating of carbonates, the age of the studied unloading source was determined as ~ 3.5 thousand years. It was found that the CHAOS structure is one of the “youngest” on the northeastern slope of the Sakhalin Island.

Authigenic carbonates are crystallized in subaquatic and subaerial settings at various stages of sedimentary rock formation, and are often found in both ancient and modern deposits. The explanation of their genesis may be problematic. However, in most cases the study of stable isotopes makes it possible to reconstruct the authigenic carbonates crystallization mechanism. The fact of the frequent genetic association of authigenic carbonates with hydrocarbons determines the importance of the development of methods for the reliable identification of their genesis.

The current stage of strategic development of Russia’s economic and scientific interests is characterized by the study and subsequent development of hydrocarbon resources of the continental shelf and the bottom of the World Ocean. At the same time, special attention is paid to the waters of the Arctic seas, the ecological and climatic changes of which are interconnected with the degradation of underwater permafrost and associated gas hydrates. To study the latter, specialized equipment is needed to take unmodified samples of natural gas hydrates. The paper analyzes the technical characteristics and analytical capabilities of modern samplers and analyzers and evaluates the possibilities of their use for studying hydrate-containing sediments in the Arctic seas.

Seismostratigraphical studies of the 11.8‐km2‐large and ~140‐m‐deep Lake Bolshoye Shchuchye, Polar Ural Mountains, reveal up to 160‐m‐thick acoustically laminated sediments in the lake basin. Using a dense grid of seismic lines, the spatial and temporal distributions of the sedimentary history have been reconstructed. Three regional seismic horizons have been identified and correlated with the well‐dated 24‐m‐long sediment core retrieved from the lake. Isopach maps constructed from the seismic data show four phases of sedimentation. A contour map of the deepest regional seismic reflector represents the earliest hemipelagic sedimentation in the lake. Three contour maps represent time intervals covering the last 23 cal. ka based on the well‐dated core stratigraphy from the lake. The detailed time constraints on the upper stratigraphical units in the lake allow calculation of the lake's development in terms of sediment fluxes and the denudation rates from the Last Glacial Maximum (LGM) to the present. The sedimentation in Lake Bolshoye Shchuchye has been dominated by hemipelagic processes during at least the last 24 cal. ka BP only locally interrupted by delta progradation and slope processes. A major shift in the sediment accumulation at c. 18.7 cal. ka BP is interpreted to mark the end of the local glacial maximum, greatly reduced denudation and the onset of the deglaciation period; this also demonstrates how fast the glaciers melted and possibly disappeared at the end of the LGM. The denudation rate during the Holocene is only a fifth of the LGM rate. The age of the oldest stratified sediments in Lake Bolshoye Shchuchye is not well constrained, but estimated as c. 50–60 ka.

Study of the microstructure and isotopic composition of authigenic tubule-shaped carbonate concretions from sediment core PS51/154-11 on the western Laptev Sea continental slope (present water depth 270 m) has allowed for reconstruction of the conditions prevailing during their formation and identification of the mechanisms controlling their genesis. Concretions were collected from the basal sediment unit with an extrapolated age estimate of 16.3–17.6 cal.ka. Crystallization of carbonate tubules occurred at the beginning of the last deglaciation when the site was located in the proximity to the former coastline and the mouths of the Olenek and Anabar-Khatanga rivers in water depths of about 150–170 m. Microprobe analysis showed that the studied carbonate tubules consist of the minerals belonging to the siderite–rhodochrosite isomorphic series. The measured isotopic composition of δ13С and δ18O in the carbonates varies between − 21.0 and − 17.0‰ and between − 9.86 and 1.72‰ VPDB, respectively. The δ18O values in the authigenic carbonates give evidence for the gradual transition from a freshwater affected to modern-like marine sedimentation environment during carbonate crystallization. Water freshening is confirmed by the co-occurrence of authigenic Fe–Mn carbonates and Fe-phosphate vivianite that is a typical mineral of freshwater environments. The dominant source of dissolved inorganic carbon in the pore water was the isotopically light carbon derived from the diagenetic decomposition of organic matter. Two possible scenarios of authigenic carbonates formation are proposed: penetration of freshened ground waters and/or enhanced freshwater influence during short seasonal floods in combination with geochemical processes in a narrow marginal filter zone that was located extremely close to the Laptev Sea continental slope and the studied core site.

The purpose of this study is to forecast the scale and distribution character of gas hydrate stability zone in the Chukchi Sea under simulated natural conditions and basing on these results to estimate resource potential of gas hydrates within this area. Three types of stability zone have been identified. A forecast map of gas hydrate environment and potentially gas hydrate-bearing water areas in the Chukchi Sea has been plotted to a scale of 1:5 000 000. Mapping of gas hydrate stability zone allowed to give a justified forecast based on currently available data on geologic, fluid dynamic, cryogenic, geothermal and pressure-temperature conditions of gas hydrate formation in the Chukchi Sea. It is the first forecast of such kind that focuses on formation conditions for hydrates of various types and compositions in the Arctic seas offshore Russia. Potential amount of gas, stored beneath the Chukchi Sea in the form of hydrates, is estimated based on mapping of their stability zone and falls into the interval of 7·1011-11.8·1013 m3.

This paper presents an estimation of the intensity of lithodynamic processes, and calculation results of sediment accumulation for the seaway navigation canal of the Sabetta port (the Yamal Peninsula), on the basis of data of complex engineering surveys carried out in the field seasons in 2011–2013. The sediment accumulation in the canal is assessed using data of the methods of seismic stratigraphy, sedimentation traps, and using data of the newly proposed method of artificial canals. The authors calculate the volume of sediments carried into the canal due to exaration. The intensity of lithodynamic processes is estimated as high or very high in the north of the canal, and as medium or high in its central and southern parts.

Studied samples of ikaite have been collected in the southern part of the Laptevs’ sea in 2008. XRD analysis data have shown that by destruction ikaite is transformed into calcite and waterite pha- ses. Isotopic composition of carbon (d13C) and oxygen (d18O) in the sample composed of the ikaite, calcite and vaterite mixture was –31.0 and –0.39 ‰, respectively, and after the complete destruction of ikaite it has become –29.0 and 0.67 ‰ VPDB, respectively. Calculated theoretical values of d18O in porous water, from which ikaite has formed in equilibrium conditions, vary from 4.0 to –2.8 ‰ VSMOW. It evidences the weak influence of the Lena river water upon formation of the studied sam- ple. The result of the balance calculations points that organic matter was the principal source of car- bon in ikaite (88—92 %), with supplementary supply from methane (8—12 %).

It is essential to study methane in the Arctic environment in order to understand the potential for large-scale greenhouse gas emissions that may result from melting of relict seafloor permafrost due to ocean warming. Very few data on the sources of methane in the Chukchi Sea were available prior to initiation of the Russian-American Long-term Census of the Arctic (RUSALCA) program in 2004. This article documents for the first time the spatial variation of methane concentrations in the sediment and water column in a significant region of the Pacific Arctic and the influence of methane turnover and net transport from organic-rich environments within the western Chukchi Sea. The study combines historical observations, new data obtained during the RUSALCA collaborative program, and modeling results to provide insights into the contemporary methane dynamics of the western Chukchi Sea. We compare methane evolution at two sites with distinct geological settings, depositional patterns, and methane sources: (1) the deeper, fault-bounded Herald Canyon (northern site) where methane flux is controlled by both northward CH4 transport via ocean currents and diffusive influx of thermogenic methane (formed under high-temperature conditions) from source rocks at depth in the canyon's seafloor, and (2) the shallow Chukchi shelf (southern site), where sulfate reduction and anaerobic methane oxidation play a significant role in biogenic methane production and its flux within and from the sediments into the water column. Diffusive methane fluxes at the sediment-water interface within the southern and northern sites were estimated to be 14.5 mu mol dm(-2) day(-1) and 0.7 nmol dm(-2) day(-1), respectively. In addition, we suggest that biogenic methane emanating from the organic-rich southern region is transported northward by the Anadyr Current, leading to a mix of both biogenic and thermogenic methane in Herald Canyon surface waters. Study results indicate that the South Chukchi Basin is an important source of atmospheric CH4. Further work is required to accurately quantify this flux.

Abstract An area of focused fluid venting off NE Sakhalin, Sea of Okhotsk, was investigated in 2003 during the 31st and 32nd international expeditions of R/V Akademik M. A. Lavrentyev within the framework of the CHAOS Project. More than 40 structures related to seafloor gas venting were discovered and gas hydrates were sampled from three of these: CHAOS, Hieroglyph and Kitami. Geochemical analyses were used to define the mechanisms of gas hydrate accumulation and the sources of fluids involved. Chemical and isotopic analyses of the interstitial and hydrate waters suggest that hydrates were formed from seawater (or in-situ pore water) and an ascending fluid enriched in salts. Hydrate formation occurs at locations of the most intensive saline water upflow, and this is probably a function of the gas solubility in water in equilibrium with hydrate. The water involved in gas hydrate formation consists of about 70% pore water derived from the host sediment and 30% from the ascending fluid. The overall isotopic composition of the ‘fluid’ taking part in hydrate formation was calculated as δ2H≈−11‰ and δ18O≈−1.5‰.