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.

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 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.

The paleoclimatic events of the Late Pleistocene and Holocene have been clarified based on the results of detailed lithological, geochemical, pollen, and paleomagnetic studies of sediment cores collected in the eastern Gulf of Finland. The time frame of the change from lacustrine to marine sedimentation conditions has been refined based on the radiocarbon dating. For deposits of the marine phase of the Baltic Sea in the eastern Gulf of Finland, hypoxia cycles associated with periods of warming during the Holocene have been revealed.

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.

Based on obtained data, the paper considers the structure of the sedimentary cover and basement in the continent–ocean transition zone. We analyze the structure of modern tectonic activity zones in the Laptev Sea and structurally similar zones in the Novosibirsk Trough and the De Long Massif. Three sedimentary Anisin–Laptev, Amundsen, and West Laptev basins separated by basement uplifts are distinguished in sedimentary cover. The Anisin–Laptev Basin is separated from the West Laptev Basin by the North Laptev Horst and from the Amundsen Basin by an uplift stretching from the Lomonosov Ridge and covered by the Neogene–Quaternary deposits. The modern tectonic activity zone, marked by a rift valley and earthquakes, stretches across the continental slope from the Gakkel Ridge above a sedimentary rock sequence possessing many-kilometers thickness. The zone reached its present-day position in the Pliocene. Near the shelf boundary, the zone bifurcates, with one branch departing into the West Laptev Basin, and the other branch departing into grabens that developed to the west of New Siberian Islands forming the Laptev microplate.

A giant caldera located in the eastern segment of the Gakkel Ridge could be firstly seen on the bathymetric map of the Arctic Ocean published in 1999. In 2014, seismic and multibeam echosounding data were acquired at the location. The caldera is 80 km long, 40 km wide and 1.2 km deep. The total volume of ejected volcanic material is estimated as no less than 3000 km 3 placing it into the same category with the largest Quaternary calderas (Yellowstone and Toba). Time of the eruption is estimated as ~1.1 Ma. Thin layers of the volcanic material related to the eruption had been identified in sedimentary cores located about 1000 km away from the Gakkel Ridge. The Gakkel Ridge Caldera is the single example of a supervolcano in the rift zone of the Mid-Oceanic Ridge System.

ЗАКОНОМЕРНОСТИ РАСПРЕДЕЛЕНИЯ ОСАДКОВ В АРКТИЧЕСКОМ БАССЕЙНЕПо данным последних сейсмических исследований, проведенных на шельфе Восточно-Сибирского и Чукотского морей, а также на примыкающем глубоководье Амеразийского бассейна, уточнена карта мощности осадочного чехла Арктического бассейна, включающего глубоководье Северного Ледовитого океана и примыкающие шельфы Евразии.Впервые выделен кольцевой мегапрогиб, представленный системой сверхглубоких впадин, окружающей глубоководье Северного Ледовитого океана и примыкающий Баренцево-Северокарский шельф.Этот прогиб является основной областью аккумуляции осадков всей акватории Арктики, поступающих с разных сторон к оси прогиба.На основании рассмотрения каналов стока (транзита) осадков подтверждается континентальная природа Амеразийского бассейна, естественное продолжение его поднятий через прогиб на шельф, а также рассматривается структурная последовательность накопления осадков в Арктике в связи с фазами континентального рифтогенеза

Nowadays the Arctic Ocean is a region of high scientific interest and it’s more and more considered like an essential key to understanding of the Earth paleoclimate. With the lack of biological information and age limitations of radiocarbon method paleomagnetic studies become a crucial tool for marine sediments dating here. A 9-meter length core was retrieved from the Mendeleev Ridge and subjected to paleomagnetic analysis. Magnetic susceptibilities measurements were performed by different types of equipment. The 6 meters of the core were continuously sub-sampled and measured for natural remanent magnetization (NRM) using a spinner magnetometer JR-6A. Patterns of NRM inclination, NRM intensity and magnetic susceptibility were constructed against depth. Specimens of interest was chosen for following demagnetization procedure to remove viscous overprints based on the revealed NRM inclination reversals. Stepwise thermal demagnetization was performed at 130°C and further in 50°C steps in the 200-500°C temperature range. Characteristic remanent magnetization (ChRM) with maximum anglular deviations (MAD) were calculated by using the principal component analysis method [Kirschvink, 1980]. Alternating field demagnetization was performed in the 5 mT – 100 mT field peak range and NRM results in 20, 30 mT were used as an addition to the ChRM inclination pattern. The distinct drop from positive to a negative inclination at 123.5 cmbsf is interpreted as a polarity reversal from the Brunhes chron to the Matuyama chron. Prevalence of reverse polarity is supported by demagnetization data up to at 394-397 cmbsf and short positive intervals within zone are probably reflect subchrons of normal polarity. Change from reverse to normal polarity at 394-397 cmbsf is considered as the Matuyama – Gauss boundary. This zone with prevailed normal polarity is traced from 394-397 cmbsf up to 490 cmbsf and its lower bound isn’t clearly definable downward due to lack of demagnetization data and frequent interchanges of polarity. The decrease of the NRM intensity values is observed in the reverse polarity zone in comparison to normal polarity zones, which is derived from the presence of viscous remanent magnetization (VRM) and its superimposing with the initial one. Spikes of NRM intensity and magnetic susceptibility are discovered near the determined chron boundaries, and it may act as an independent factor for determination of polarity boundaries. The secondary processes occurring in sediments may play some role and affect alterations in the paleomagnetic record. Calculated mean sedimentation rates during Brunhes and Matuyama chrons are 1.58 and 1.5 mm kyr-1 respectively. These values represent sedimentation rate on the Mendeleev Ridge as low and approach to other studies with the similar estimation [Piskarev et al., 2013, Gusev et al., 2013] Thus the obtained sedimentation rates don’t exceed 1.58 mm kyr-1 for last 2.5-2.6 Myr.