Four cores of 1–3 m length recovered by gravity corers from the Laptev Sea and the East Siberian Sea (Russian East Arctic shelf) were subjected to paleomagnetic studies. The latter together with radiocarbon dating and magnetic susceptibility allowed to make a stratigraphic reference to several other sections of marine sediments obtained in the shelf. The distribution of the Late Pleistocene-Holocene sediments along the shelf of the East Arctic seas of Russia appears to be controlled by the hypsometric position and association with certain geomorphological elements of the relief. All cores revealed a common trend of gradual decrease of inclination downcore compared to the current dipole inclination at the coring sites. Several, well-defined shallow positive and negative inclinations have been identified which could reflect geomagnetic excursions in the Holocene or at the Holocene-Pleistocene boundary.

A 3D model of the crustal structure of the region including the Chukchi Borderland with the adjacent ocean and Chukchi‒Alaska Shelf has been compiled and calculated. A specific feature of the crustal structure consists of three-sided isolation of the Chukchi Borderland and a transitional zone from the south connecting the Chukchi Borderland with the Chukchi‒Alaska Shelf. The connection between the Chukchi Borderland and Wrangel Rise of the Chukchi Shelf is traced through the North Chukchi Rise located between the North Chukchi Trough and the Hanna Trough. Clockwise rotation of the Chukchi Borderland began in the Early Cretaceous, because basalts at the bottom of sedimentary strata sections along the eastern and southern boundaries of the Chukchi Basin have reversed magnetization: their outpouring occurred before the beginning of the Cretaceous superchron, earlier than 121 Ma. Near the upper boundary of the Neocomian, there was a large-scale shear displacement of crustal blocks along the eastern boundary of the Chukchi Rise, which had a thrust pattern before that. The 3D model demonstrates that the Chukchi Borderland and Chukchi Basin are natural components of the continental margin, since they have a strong geological connection with the continental masses of the Chukchi Shelf.

In 2011‒2020 a significant number of seismic lines were carried out in the Eurasian Basin of the Arctic Ocean, which made it possible to study the structure of the junction zones of the Gakkel Ridge with the Nansen and Amundsen basins on a number of profiles. During 2019‒2020 15 sections of the Gakkel Ridge and its rift valley were studied using a sub-bottom profiler and seismo-acoustic profiling. New data on the relief of the basement, as well as the use of databases of bathymetry, gravity, and magnetic anomalies updated at VNIIOkeangeologia, made it possible to calculate the magnetization of the rocks of the Gakkel Ridge along a number of profiles crossing the ridge and to perform model calculations of the structure of the Earth’s crust using a complex of geological and geophysical data in the area of the southeastern termination of the ridge. The Gakkel Ridge is a structure that was isolated in the Early Oligocene (34 Ma)–Early Miocene (23 Ma) in the process of radical restructuring of the spreading kinematics in the existing ocean basins in the regions of the North Atlantic and the Arctic. The values of the calculated magnetization of the magnetic layer of the Earth’s crust show that this layer is partly composed of oceanic basalts, but mainly of deep-originated rocks, gabbro, and peridotites that were brought to the surface during detachment accompanying spreading. The Laptev Sea continuation of the rift valley of the Gakkel Ridge to the south of the caldera passes above many kilometers of sediments, at the base of which sedimentary rocks of Cretaceous and Late Jurassic age occur.

— The age assignment to the Arctic Basin sediments is complicated by the insufficient microfauna in them. Under these conditions, the paleomagnetic method is the major method to determine age boundaries. This method was used to construct the first successful age model of Arctic sediments in the 1970s. In recent decades, a number of works made it possible to describe changes in natural remanent magnetization direction as a consequence of secondary geochemical processes, in fact, discrediting the possibility of applying paleomagnetism to the Arctic Basin sediments. Based on our research of natural remanent magnetization in sediment cores from the Central Arctic submarine elevations, the reference paleomagnetic horizons were determined reliably. The sedimentation rates at the Alpha and Mendeleev ridges was calculated to be low (<2 mm/kyr). Mean sedimentation rates increase toward the Lomonosov Ridge due to the influence of the Transpolar Drift and toward the shelf. Based on the comprehensive analysis of the paleomagnetic and seismoacoustic data, low sedimentation rates have been characteristic of the Mendeleev Ridge and Podvodnikov Basin since the Early Miocene.

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23050057

The Gebel EL-Zeit area in the southwestern Gulf of Suez, Egypt, is an area with a significant hydrocarbon potential in sedimentary basins, so that the three-stage inversion method was proposed for the Bouguer anomalies observed therein. Salt diapirs obscured the deep structure of the main central El-Zeit basin; hence, this method was implemented to overcome challenges in 3D seismic modeling. Our study included direct and inverse parameterization sequences that involved analyzing the inputs and outputs within trial-and-error initiations and inverse estimations to assess whether and how much the constraining parameters used in the calculations could achieve the intended aim. Data reduction, filtering, optimization, and constraint assumptions were used to determine the minimal set of density model parameters needed to set limits on the acceptable range of density contrasts that are required to study the basement depths, swells, troughs, faulting/folding and intra-sedimentary structures, and for direct modeling aimed at creating a simple model to save time. The thirteen constrained wells with a total depth ranging from shallow to deep were not involved in direct modeling but provided quality control over the graphical display of the inverse results for the entire study area. Moreover, many parameter constraints were inverted to regulate the way the calculated data are related to the model’s solution that allowed us to determine which inversion trial provided the best parameterization sequence and, therefore, yielded the most appropriate solution for the depth-density model which is approximating reality with a minimal computation error in the study area.

The bottom sediments of the Russian Arctic seas have been studied to varying degrees. The least attention has been paid to the East Siberian Sea, the Quaternary geology of which remains largely overlooked. This article summarizes the results of a comprehensive research on the East Siberian Sea, including the first paleomagnetic analysis of nine sediment cores collected during three cruise expeditions as part of the program “State Geological Mapping of the Territory and Continental Shelf of the Russian Federation at the Scale of 1:1 000 000”. The results obtained show that the processes and conditions of sedimentation vary in different parts of the East Siberian Sea.

A 3D model of the Earth’s crust for the continental margin of the Laptev Sea and the adjacent part of the Eurasian Basin was developed using the latest seismic and gravity data. The thickness of the consolidated part of the Earth’s crust in the study area is estimated at 7–11 km, which corresponds to a highly extended continental or oceanic crust. The formation of the basement and sedimentation in this area most likely began in the Late Jurassic. The southeastern part of the Eurasian Basin is separated from the rest of the basin by a dextral shear zone, the displacement along which during the Paleogene was more than 100 km.

— The Chukchi Borderland is a tectonic unit of the eastern part of the Arctic continental margin of Eurasia that is part of the complex of the Central Arctic rises, along with the Lomonosov Ridge, the Alpha‒Mendeleev Rise, the Podvodnikov, Chukchi and Mendeleev basins. The study provides data on the structure of the Chukchi Borderland and the surrounding geological structures, morphology and geology, uses bathymetric materials, seismic materials from CDP and deep seismic survey, sampling and drilling data. A review of materials on the study region was carried out. The latest results of geomorphological analysis of bathymetric data and 3D modeling of the Earth’s crust of the study region using geophysical data are presented. To explain the identified features of the morphology and deep structure of the Chukchi Borderland, a tectonic model is proposed that explains the deep mechanisms of its formation and adjacent structures, as well as a structural-tectonic scheme of the Arctic Alaska–Chukotka microplate, which presents the morphological and geological connection of the Chukchi Borderland with the continental shelf.

Summary The new airborne geophysical survey allowed to obtain more detailed data on gravity and magnetic anomalies in the northwestern area of the East Siberian Sea. The basement relief was modelled due to correlation of inversion of magnetic and gravity anomalies with the basement depth according to the seismic data along the seismic profiles. The position of main tectonic boundaries – Vilkitsky Basin in the north, North Chukchi Basin in the east, and Zhokhov Basin in the south – was outlined in accordance with the features of the basement relief. Ten graben-like depressions were detected in the eastern part of De Long Rise. Combine interpretation of the magnetic, gravity, and seismic data allowed to clarify the structure of the studied area and to select the areas prospective for hydrocarbons.

This paper presents the results of high‐resolution sedimentological analyses of sediment cores from the eastern Gulf of Finland (Baltic Sea). Sampling sites in the periphery of sedimentary basins were selected on the basis of acoustic profiling analyses. The research allowed tracing of the transition from the freshwater Ancylus Lake to the Littorina Sea. A specific transitional layer of ‘blue clays’, indicating the first stage of brackish water inflow into the Gulf of Finland, was dated to 9.1 ka BP. The date of first appearance of Littorina silty clay sedimentation was as follows: from 8.0 ka BP near Gogland Island, from 7.0 ka BP near Moshchny Island and from 5.9 ka BP near the Berezovye Islands. Holocene cycles of hypoxia, associated with periods of warming, were identified and cycles of ‘warming – transgression – anoxic conditions’ and ‘cooling – regression – oxygen‐rich conditions’ were revealed. During the first stage of Littorina transgression (8.0–7.0 ka BP), the near‐bottom environment in the deepest sedimentary basin of the eastern Gulf of Finland was characterized by oxygen deficiency. In contrast, 7.0–6.0 ka BP was dominated by oxygen‐rich conditions and active processes of bioturbation. Anoxic conditions occurred again from 6.0–4.8 ka BP (Holocene Climatic Optimum), resulting in the accumulation of undisturbed silty clays with subhorizontal lamination. The interval from 4.8–2.0 ka was then characterized by oxygen‐rich near‐bottom conditions favourable for benthic organisms. The grain‐size distributions throughout the sediment cores from the easternmost sedimentary basins suggest a relative lowering of the sea level from 3.5–1.8 ka and a rise after 1.8 ka BP.

Accurate dating of marine sediments from the Arctic Ocean remains a subject of great debate over the last decades. Due to the lack of adequate materials for biostratigraphy and stable isotope analyses, paleomagnetic reconstructions came into play here but though yielded ambiguous interpretations. Moreover, sedimentation rates in the Quaternary, determined for isolated morphological features in the Arctic Ocean, are often applied to the entire Arctic Ocean realm resulting in an inappropriate oversimplification of probably diverging regional depositional regimes. Paleomagnetic studies on four long sediment cores, collected from the Mendeleev Ridge and the Lomonosov Ridge, complemented by the results from one core from the Podvodnikov Basin, have provided an opportunity to compare the sedimentation history of these profound structures in the Arctic Ocean. Cores PS72/396-5 and PS72/410-3 (Mendeleev Ridge), PS87/023-1, PS87/030-1 (Lomonosov Ridge) and PS87/074-3 (Podvodnikov Basin) were retrieved during expeditions of RV Polarstern in 2008, and 2014. Paleomagnetic, rock magnetic and physical properties measurements were carried out at the Center for Geo-Environmental Research and Modeling (GEOMODEL) of the Research Park in St. Petersburg State University, at the University of Bremen, and the Alfred Wegener Institute. According to the results on the Mendeleev Ridge’s cores, complemented with 230Th excess study on core PS72/396-5, the Brunhes Matuyama boundary (0.78 Ma) is observed at the first meters below the seafloor. That, together with the Matuyama Gauss transition (2.58 Ma) recorded in both cores, implies the mean sedimentation rate in this area to be in the order of mm/kyr. In contrast to the Mendeleev Ridge, the cores from the Lomonosov Ridge and the Podvodnikov Basin have shown a more complex paleomagnetic record with a relevant shift to negative inclinations significantly deeper downcore. This could signify a relevant difference in the sedimentation regimes between both ridges during the Quaternary.

The chronology of investigations and geomorphological analysis of the underwater terrain in the abyssal parts of the Arctic Ocean is presented. History of airborne and ship-borne gravity and magnetic surveys is accompanied by the detail technical, quantitative and qualitative analysis of existing datasets and regional potential fields anomalies maps. The up-to date location maps show the MCS seismic coverage grid inside the Arctic Basin. The important geological structures are highlighted by detail fragments of both TWT and depth converted interpreted seismic sections. The composite velocity models calculated from DSS data along regional geotraverses are also present. The progress of the seismological observations over several decades presents a telling picture of distribution of hypocenters and focal mechanisms of the modern seismicity related to the mid-ocean spreading zone and its continuation onto the Laptev Sea shelf. The results of the Arctic Basin deep water seafloor sampling and drilling provide additional input to our knowledge base.

Summary Seismic and drilling exploration maturity of the Russian Arctic shelf is tens and hundreds of times lower than the exploration maturity of the water areas in the United States of America, Norway, and the United Kingdom. The best studied shelf areas are the southern parts of Barents and Kara seas. The northern Barents and Kara seas and the entire East Arctic shelf (Laptev, East Siberian and Chukchi seas) aren't covered by the parametric drilling. With a limited seismic data, data of gravity and magnetic surveys allowed to outline the main regional structures including sedimentary basins. According to the modern level of geological and geophysical investigation three areas have to be considered as most prospective for the giant oil field discoveries: eastern slope of the North Barents Basin, shelf part of the West Laptev Basin, and southern edge of the North Chukchi Basin.

Summary The gravity and magnetic anomalies, seismic and seismological data were analyzed to determine the nature and timing of tectonic deformations in the Laptev Sea and the western part of the East Siberian Sea. Formation of narrow elongated extensional structures (grabens) on the Laptev and East Siberian shelves may be attributed to different stages of evolution. This can occur due to westward shift of spreading activity in the Eurasian Basin which tentatively took place at Oligocene-Eocene interface (chron LMA 13) and in Pliocene. Direct geological observations in the marginal parts of one of the currently passive grabens on the shores of the Blagoveschensky Strait dividing the islands of Novaya Sibir’ and Faddeevsky make possible to determine the time line of deformation cessation in this area. Direct geological observations in the marginal parts of one of the currently passive grabens on the shores of the Blagoveschensky Strait dividing the islands of Novaya Sibir’ and Faddeevsky make possible to determine the time line of deformation cessation in this area. A detailed combine analysis of different data made it possible to derive information on the tectonic events in the Laptev Sea – East Siberia Sea area. The main result is a tectonic subdivision of active deformation in present time the eastern Laptev Sea area and the western East Siberian Sea, where seismic and deformation activity took place from Cretaceous time and has finished in Pliocene.