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In this study, we consider in detail the July 13, 2023, earthquake that occurred on the shelf of the eastern Laptev Sea (Belkov–Svyatoi Nos rift). On the one hand, our interest in this event is due to the location of its epicenter, to the east of which there is a sharp decrease in seismic activity. Conversely, detailed common depth point (CDP) data on the structure of the upper crust are available for its epicentral zone, making it possible to analyze the seismotectonic position of the earthquake source. Focal parameters in the instantaneous point source approximation are calculated from surface waves recorded at teleseismic distances. As a result, we have obtained a scalar seismic moment (M0 = 9.8 × 1016 N m), corresponding moment magnitude (Mw = 5.3), source depth (h = 8 km), and focal mechanism (a normal fault along a gently dipping nodal plane with a NW–SE strike). Our results are compared with data from seismological agencies. It has been shown that differences between them are most likely caused by various initial data, including their different frequency ranges. Our estimates agree better with the available geological and geophysical information on the tectonics of the study area. Taking into account the data on strike, dip, and penetration depth of faults and our source parameter values, we have concluded that the July 13, 2023, earthquake could have been associated with a major listric normal fault on the western slope of the Belkov–Svyatoi Nos rift. [ABSTRACT FROM AUTHOR]

The near-Laptev part of the Eurasian basin is an area of transition from modern spreading to intraplate rifting. We propose an approach to the construction of a 3D model of the basement of the near-Laptev part of the Eurasian basin based on all currently available geological and geophysical data. The structure of the acoustic basement of the Eurasian basin consists of alternating troughs and highs in cross-section. The sparse seismic data do not allow us to directly trace the strike of these structures, however, they can be correlated with linearity established by gravity and magnetic data and related to the sequential opening of the basin. For the near-Laptev part of the Eurasian Basin, where the linearity is no longer traceable from magnetic data, we propose to determine the strike of the basement structures on the basis of a seismic stratigraphic analysis. The new 3D model of the acoustic basement in the studied area provided the basis for a tectonic scheme of the entire Eurasian Basin. The model reflects the main stages of the basin development: continental rifting for up to 56 Ma, normal spreading for 56–45 Ma, ultraslow spreading for 45–34 Ma, and ultra-ultraslow spreading for 34–20 Ma. The southern part of the study area is overlain by sedimentary cover with an age of 20 Ma and younger, which is associated with the cessation of spreading here no later than 20 Ma ago. [ABSTRACT FROM AUTHOR]

Thermospores, the dormant resting stages of thermophilic bacteria, have been shown to be frequent but enigmatic components of cold marine sediments around the world. Multiple hypotheses have been proposed to explain their distribution, emphasizing their potential as model organisms for studying microbial dispersal via ocean currents. In the Arctic Ocean, the abundance and diversity of thermospores have previously been assumed to be low. However, this assessment has been based on data mainly from the western fjords of Svalbard, thus leaving most of the Arctic unexplored. Here, we expand the knowledge about the distribution of thermospores in the Arctic Ocean by investigating the abundance and diversity of thermospores in heated shelf sediments from three sites in the outer Laptev Sea. Two of the sites are located in an area with methane‐emitting cold seeps with a thermogenic source signature suggestive of an origin in a deep hydrocarbon reservoir, while the third site is a reference site not known to be impacted by seepage. We found that activity of viable thermospore populations was more prominent at one of the investigated seep sites. This finding is supported by both radiotracer growth experiments showing thermophilic, sulfate‐reducing activity triggered by heating, as well as 16S gene sequence analyses showing significantly enriched ASVs affiliated to the phylum Firmicutes following high‐temperature incubations. An enrichment of the sulfate‐reducing, endospore‐forming class Desulfotomaculia in heated samples compared to unheated samples was also observed. Furthermore, several ASVs identified at the seep site are closely related to thermospore‐producing bacteria associated with the deep biosphere, including hydrocarbon and hydrothermal systems. Based on the combined information from induced activity, estimated abundance, and phylogenetic composition using 16S rRNA gene sequencing, we propose likely source environments and dispersal vectors for thermospores in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Continental margins receive, process and sequester most of the terrestrial organic carbon (terrOC) released into the ocean. In the Arctic, increasing fluvial discharge and collapsing permafrost are expected to enhance terrOC release and degradation, leading to ocean acidification and translocated CO2 release to the atmosphere. However, the processes controlling terrOC transport beyond the continental shelf, and the amount of terrOC that reaches the slope and the rise are poorly described. Here we study terrOC transport to the Laptev Sea continental slope and rise by probing surface sediments with dual‐isotope (δ13C/Δ14C) source apportionment, degradation‐diagnostic terrestrial biomarkers (n‐alkanes, n‐alkanoic acids, lignin phenols) and 210Pbxs‐based mass accumulation rates (MAR). The MAR‐terrOC (g m−2 yr−1) decrease from 14.7 ± 12.2 on the shelf, to 7.0 ± 5.8 over the slope, to 2.3 ± 0.3 for the rise. Scaling this to the respective regimes yields that 80% of the terrOC accumulates on the shelf, while 11% and 9% of the accumulation occurs in slope and rise sediments, respectively. TerrOC remineralization is evidenced by biomarker degradation proxies (CPI of n‐alkanes and 3,5Bd/V) indicating 40% and 60% more terrOC degradation from slope to rise, consistent with a decline in terrOC concentrations by 57%. TerrOC degradation only partially explains this decline. An updated Laptev Sea terrOC budget suggests that sediment transport dynamics such as turbidity currents may drive terrOC shelf‐basin export, contributing to the observed accumulation pattern. This study quantitatively demonstrates that Arctic shelf seas are key receptor systems for remobilized terrOC, emphasizing their importance in the carbon cycle of the rapidly changing Arctic. Key Points: Terrestrial carbon export from the Laptev Sea shelf to the slope and rise is studied using δ13C/Δ14C, biomarkers and 210Pb mass accumulationThe accumulation of terrestrial carbon declines by 52% at the shelf edge and by 68% from slope to rise due to transport and degradation dynamicsA terrestrial carbon budget for the Laptev Sea suggests 80%–90% of the input is retained on the shelf via accumulation and re‐mineralization [ABSTRACT FROM AUTHOR]

Introduction: Methane seeps influence surrounding benthic communities in different ways from slight changes in benthic abundance and biomass to major altering the species composition. Materials and Methods: We studied benthic communities of 14 methane seep flares in Laptev and East Siberian seas with comparative analysis of species composition and abiotic parameters at the nearby areas not affected by methane seeps. The species diversity was comparable at seep and non-seep sites varying from 3.9 to 39.6 taxa per 100 individuals and from 4.5 to 34.8 taxa per 100 individuals, correspondingly. Results: The Laptev Slope community corresponds with the Polychaeta-community, encircling the upper continental slope area of the entire Siberian Arctic. The Lower Shelf community described in this study apparently corresponds with the Ophiocten sericeum community identified in the shelf areas of the Kara, Laptev and East Siberian seas. The Upper Shelf community is mostly inhabited by the bivalves species such as Portlandia arctica, Ennucula tenuis and Astarte montagui communities. The Estuarine community, which is the poorest by diversity but has high values of abundance and biomass is directly influenced by the Lena River runoff in terms of lower salinities and higher sedimentation rates. Discussion: Throughout the study area, the differences between the Estuarine, Upper Shelf Lower Shelf and Laptev Slope communities exceeded the differences between the seep and background non-seep areas. Several taxa demonstrated correlations with different environmental factors, including the latitude, depth, temperature, salinity, pH and methane content, not depending on the revealed benthic community. Eight taxa demonstrated correlations with the methane content measured at different sediment depths. Two siboglinids taxa demonstrated high abundances at stations with highest methane content deep in the sediment. At the Siberian shelf, our geochemical data for siboglinid habitats are the first to be published so far. [ABSTRACT FROM AUTHOR]

This is the first report on abundance and diversity of cultured filamentous fungi and on abundance of yeasts in the sediments of the Laptev and East Siberian seas. A total of 63 isolates of filamentous fungi belonging to 32 morphotypes were obtained from 35 sediment samples. Most morphotypes were represented by single colonies in individual samples; the most frequently occurring species were Pseudeurotiumzonatum, Pseudogymnoascuspannorum, and Cordycepsfarinosа. In both seas, the abundance of filamentous fungi (up to 12 colonies per 1 g wet sediment) was lower than that of yeasts (up to 68 colonies per 1 g wet sediment). In the Laptev Sea, yeast abundance was low at all stations, while in the East Siberian Sea higher values were found at near-estuarine stations. [ABSTRACT FROM AUTHOR]

The duration of ice-free periods in different parts of the Arctic Ocean plays a great role in processes in the climate system and defines the most comfortable sea ice conditions for economic activity. Based on satellite-derived sea ice concentration data acquired by passive microwave instruments, we identified the spatial distribution of the dates of sea ice retreat (DOR), dates of sea ice advance (DOA), and the resulting ice-free period duration (IFP) between these days for the Kara and Laptev seas during 1979–2022. The monthly decline in sea ice extent was detected from June to October in both seas, i.e., during the whole ice-free period. The annual mean sea ice extent during 2011–2021 decreased by 19.0% and 12.8% relative to the long-term average during 1981–2010 in the Kara and Laptev seas, respectively. The statistically significant (95% confidence level) positive IFP trends were detected for the majority of areas of the Kara and Laptev seas. Averaged IFP trends were estimated equal to +20.2 day/decade and +16.2 day/decade, respectively. The observed DOR tendency to earlier sea ice melting plays a greater role in the total IFP extension, as compared to later sea ice formation related to the DOA tendency. We reveal that regions of inflow of warm Atlantic waters to the Kara Sea demonstrate the largest long-term trends in DOA, DOR, and IFP associated with the decrease in ice coverage, that highlights the process of atlantification. Also, the Great Siberian Polynya in the Laptev Sea is the area of the largest long-term decreasing trend in DOR. [ABSTRACT FROM AUTHOR]

The sources of supply and transformation of organic matter (OM) as it transitions from dissolved form to particulate matter, fluffy layer, and bottom sediments were investigated along a meridional transect from the Lena River delta to the continental slope (in the depth range of 10 to 2390 meters, cruise 63 of the R/V Akademik Mstislav Keldysh, September 2015). The study of OM molecular markers showed that dissolved and particulate organic matter of marine and terrigenous origin biodegrades in the water column and accumulates insignificantly in bottom sediments. Thus, OM from Lena River runoff does not contribute to the formation of OM in bottom sediments in summer. An area of influence of under-ice blooms on the sedimentation process was identified on the outer shelf. [ABSTRACT FROM AUTHOR]

Microbial communities of the Arctic Ocean are poorly described in comparison to aquatic environments of other regions regarding their patterns of distribution and change. The present work aims to investigate free-living bacterial communities (size fraction 0.22–1.5 μm) of different Arctic seas regions, from the river discharge area to the continental slope. Illumina MiSeq metabarcoding analysis using V3–V4 region of 16S rRNA gene was employed to study the microbial diversity of 11 Arctic seawater samples, collected from the surface and near-bottom layers in the Kara and Laptev Seas in August–September 2018. Additionally, we determined environmental parameters, bacterial abundance, biomass, and respiratory activity. Redundancy analysis, Spearman's rank correlation, and multiple linear regression analysis were used to reveal environmental factors that modulate the bacterial community structure. The differences in the free-living bacterial community composition were associated with environmental characteristics of water layers (salinity and temperature) rather than with geographical area. Although the communities from all examined sites were dominated by Gammaproteobacteria and Alphaproteobacteria followed by Flavobacteriia and Actinobacteria, they showed distinct variations in the distribution at all taxonomical levels. No archaeal taxa were observed. The distribution patterns of the quantitative parameters of total bacterial community were not associated with defined environmental characteristics. [ABSTRACT FROM AUTHOR]

The features of the spatial distribution of the long-term average ice-free period (IFP) characteristics and their trends in the Laptev Sea are calculated and described quantitatively based on the microwave remote sensing data for 1979–2021. The spatial distribution of the long-term average duration of the IFP mainly follows the spatial distribution of the long-term average IFP start dates. The highest value of the long-term average duration of the IFP is observed in the areas where the Anabar–Lena and Western New Siberian stationary flaw polynyas are formed. It is shown that south of 78° N, the duration of the IFP increased statistically significantly by +17 ± 7 days/10 year on average. The areas of the Laptev Sea north of 80° N are characterized by statistically insignificant changes in IFP characteristics. The duration of the IFP extends due to both the shift of the IFP start dates to the earlier time (an average rate of changes is –10 ± 4 days/10 year) and the shift of the IFP end dates to a later time (an average rate of changes is +8 ± 4 days/10 year). [ABSTRACT FROM AUTHOR]

Within the program "Marine Ecosystems of the Siberian Arctic," based on materials collected during cruises 69 and 72 of the R/V Akademik Mstislav Keldysh, the distribution of chemical elements (As, Ba, Bi, Co, Cr, Cr, Cd, Co, Cu, Cr, Mn, Ni, Pb, Sc, Ti, Tl, Th, V, U, and Zn) and organic carbon in the benthic mass taxa of methane seepage fields in the Laptev Sea and outside of them was examined. For the first time, the bioaccumulation potential (BP) was quantified, taking into account, in addition to the concentrations of elements in organisms, their biomass. On the C-15 methane field, the ophiuroid population accumulates each element per square meter of seafloor up to 40 times greater than at the background station. Ophiuroids and bivalves exhibited elevated BP for Ni, As, Ba, Cu, Ti, V, Mn, and Zn compared to other elements. At the same time, the BP of ophiuroids significantly exceeds that of bivalves. It is suggested that the increased bioaccumulation of some heavy metals and metalloids is related to greater bioavailability of organic matter in the bottom sediments of methane seeps. Organic carbon revealed the highest BP, which was especially pronounced in the C-15 methane field. At the background station, taxa accumulate a significantly smaller (by a factor of 10) amount of carbon in their biomass. As a result of the detritus feeders' activity, bottom sediments are enriched in organic carbon, which indicates an important contribution of benthic organisms to the carbon cycle in the Arctic seas. [ABSTRACT FROM AUTHOR]

This paper reports the obtained analysis data on the focal mechanisms and general distribution of earthquake epicenters in the Laptev Sea region. The principal stress axes directions were calculated by the formal stress inversion method for four groups of event clusters with known focal mechanisms. The distributions of the earthquake epicenters and crust thickness were compared. According to the seismological data, the extension axis of the Gakkel Ridge on the Laptev Sea shelf currently continues in the vicinity of the group of extension detachments located along the eastern boundary of the Anisin, Zarya, and Belkovskii–Svyatonosskii rift chains. The more ancient extension axis located along the group of detachments marking the eastern boundary of the Ust'-Lena and Omoloi rift systems and continuing the Gakkel Ridge axis is currently much less active. It carries residual stresses near its intersection with the Khatanga–Lomonosov fault zone in the northwestern parts of the shelf area and with the Lena–Taimyr zone of boundary uplifts, in the southwestern part. The axes oriented along the Olenyok and Bykovskii channels and the Siberian Platform boundary form extension conditions in the eastern part and strike–slip conditions in the western part of the Lena delta area. [ABSTRACT FROM AUTHOR]

Radium isotopes, which are sourced from sediments, are useful tools for studying potential climate‐driven changes in the transfer of shelf‐derived elements to the open Arctic Ocean. Here we present observations of radium‐228 and radium‐226 from the Siberian Arctic, focusing on the shelf‐basin boundary north of the Laptev and East Siberian Seas. Water isotopes and nutrients are used to deconvolve the contributions from different water masses in the study region, and modeled currents and water parcel back‐trajectories provide insights on water pathways and residence times. High radium levels and fractions of meteoric water, along with modeled water parcel back‐trajectories, indicate that shelf‐ and river‐influenced water left the East Siberian Shelf around 170°E in 2021; this is likely where the Transpolar Drift was entering the central Arctic. A transect extending from the East Siberian Slope into the basin is used to estimate a radium‐228 flux of 2.67 × 107 atoms m−2 d−1 (possible range of 1.23 × 107–1.04 × 108 atoms m−2 d−1) from slope sediments, which is comparable to slope fluxes in other regions of the world. A box model is used to determine that the flux of radium‐228 from the Laptev and East Siberian Shelves is 9.03 × 107 atoms m−2 d−1 (possible range of 3.87 × 107–1.56 × 108 atoms m−2 d−1), similar to previously estimated fluxes from the Chukchi Shelf. These three shelves contribute a disproportionately high amount of radium to the Arctic, highlighting their importance in regulating the chemistry of Arctic surface waters. Plain Language Summary: Half of the Arctic Ocean is composed of shallow seas that extend over the continental shelf, so understanding what controls the chemical composition of the seawater in these regions is imperative to predicting how the Arctic Ocean as a whole may respond to climate change. Radium isotopes are naturally occurring radioactive elements that are produced in seafloor sediments, and they can be used to study the transfer of elements from the continental shelf into the overlying water. Here we combine measurements of radium isotopes with other chemical tracers and physical oceanographic models to study the transport and chemical signature of the Laptev and East Siberian Seas into the open Arctic Ocean. We find that the transport of elements from shelf sediments into the ocean is particularly strong in these seas and in the neighboring Chukchi Sea. Our data indicates that this region has an impact on the chemistry of the broader Arctic Ocean and emphasizes the need to continue to monitor this area for potential climate‐driven changes. Key Points: In 2021, shelf‐ and river‐influenced waters crossed the shelf‐basin boundary near 170°E, indicating the likely position of the Transpolar Drift originModels suggest a residence time of ∼13 months in the Laptev and East Siberian Seas before surface water enters the central Arctic OceanThe Laptev, East Siberian, and Chukchi Shelves contribute a disproportionately high amount of radium to Arctic surface waters [ABSTRACT FROM AUTHOR]

The features of the spatial distribution of the long-term average ice-free period (IFP) characteristics and their trends in the Laptev Sea are calculated and described quantitatively based on the microwave remote sensing data for 1979–2021. The spatial distribution of the long-term average duration of the IFP mainly follows the spatial distribution of the long-term average IFP start dates. The highest value of the long-term average duration of the IFP is observed in the areas where the Anabar–Lena and Western New Siberian stationary flaw polynyas are formed. It is shown that south of 78° N, the duration of the IFP increased statistically significantly by +17 ± 7 days/10 year on average. The areas of the Laptev Sea north of 80° N are characterized by statistically insignificant changes in IFP characteristics. The duration of the IFP extends due to both the shift of the IFP start dates to the earlier time (an average rate of changes is –10 ± 4 days/10 year) and the shift of the IFP end dates to a later time (an average rate of changes is +8 ± 4 days/10 year). [ABSTRACT FROM AUTHOR]

Large areas of the seafloor in the Laptev Sea consist of submarine permafrost, which has experienced intense degradation over the last decades and centuries. Thermal abrasion of the submarine permafrost results in upward advection of suspended matter, which could reach the surface layer in shallow areas. This process is visually manifested through increased turbidity of the sea surface layer, which is regularly detected in optical satellite imagery of the study areas. In this study, satellite data, wind and wave reanalysis, as well as in situ measurements are analyzed in order to reveal the main mechanisms of seafloor erosion in shallow areas of the Laptev Sea. We describe the synoptic variability in erosion at the Vasilyevskaya and Semenovskaya shoals in response to wind and wave conditions. Finally, using reanalysis data, daily suspended matter flux from this area was evaluated during ice-free periods in 1979–2021, and its seasonal and inter-annual variabilities were described. The obtained results contribute to our understanding of subsea permafrost degradation, the sediment budget, and carbon and nutrient cycles in the Laptev Sea. [ABSTRACT FROM AUTHOR]

Single-cell cyanobacteria, being an integral part of picoplankton in marine ecosystems, have been suggested to be important contributors to primary production and carbon cycles in the global ocean. The spatial distribution, abundance and diversity of natural communities of picocyanobacteria (PC) in estuaries of Khatanga, Indigirka and Kolyma rivers and adjacent shelves of the Laptev and East Siberian seas were studied in September 2017. The PC concentrations were higher in the estuaries than in the shelf stations of the seas. The abundance of PC was 1.25 × 106 cells/L, 0.42 × 106 cells/L and 1.58 × 106 cells/L in the surface layer of Khatanga, Indigirka and Kolyma estuaries, respectively. The contribution of PC to total autumn picophytoplankton abundance averaged 6% and 3% in the Khatanga and Indigirka estuaries and reached 5% in the Kolyma estuary. Phylogenetic analysis of the 16S rRNA gene and ITS region clone libraries revealed picocyanobacterial sequences related to marine Synechococcus subclusters 5.1-I, 5.2 and 5.3. Of the phylotypes from Synechococcus S5.1-I and S5.2 that were found, only several were discovered earlier, while the remaining clones were unique. Two groups of phylotypes (clades A and E) were found that were not closely similar to those previously described in both marine and freshwater habitats. It can be expected that a more detailed study of the phytoplankton of the Arctic seas will further expand our understanding of the diversity of these key components of the food chains of oceanic biocenoses. [ABSTRACT FROM AUTHOR]

Within the program "Ecosystems of the Siberian Arctic Seas," carried out by Shirshov Institute of Oceanology, Russian Academy of Sciences since 2007, studies of the water structure and spatial variability of the parameters of the carbonate system have been performed, and the intensity and direction of the carbon dioxide flux over the continental slope of the Laptev Sea and in the Vilkitsky Strait in September 2018 have been calculated. The presence of several main water masses that govern the water structure in the study area is shown. A strong spatial variability of the parameters of the carbonate system of seawater, determined by complexes of physical and chemical–biological processes, has been revealed. The intensity and direction of the carbon dioxide flux at the water–atmosphere boundary were calculated, which range from –12 to 4 mmol m–2 day–1. It was revealed that the investigated area of the outer shelf and continental slope of the Laptev Sea is an emitter of carbon dioxide into the atmosphere as of September 2018. Conversely, the area of the Vilkitsky Strait, is a CO2 sink zone. [ABSTRACT FROM AUTHOR]

Ongoing global warming accelerates release of relict terrigenous organic carbon from permafrost onto the Arctic shelf waters. When transported in the land-sea system, it can further be accumulated in bottom sediments in the shelf or deep-sea zone and undergo degradation and remineralization, which leads to critical environmental consequences. This study aims at assessing the sources and degradation degree of terrigenous organic matter in the surface sediments of the Eastern Arctic seas. Within this study, marine bottom sediments taken from the surface horizon (0–10 cm) were investigated. Sampling was carried out during the 2011–2019 marine research expeditions. Lateral consistency of hydrogen index values in modern marine sediments on the Eastern Arctic shelf (mainly in the Laptev Sea) is associated with the great contribution of heterogeneous biolabile terrestrial organic matter, in contrast to other Arctic waters, where growing hydrogen index values are associated with the consistently growing contribution of autochthonous organic matter with increasing distance from the coast. While considering the δ13C and HI/OI correlation, there are also significant deviations from the linear dependence which usually indicates a conservative marine geochemical regime. Sediments of the Buor-Khaya Bay are characterized by an increased HI/OI values in contrast to the deep-water sediments of the continental slope which shows lower hydrogen content and a higher proportion of oxygen-containing compounds, indicating a strong transformation of organic matter. These findings confirm a key role of terrigenous supply in specific biogeochemical conditions in the studied area and reveal that geochemical indicators of immature organic matter sources in the Eastern Arctic seas should be interpreted differently from other Arctic continental margins.

Relevance. The paper presents the results of studying the morphology and size distribution of authigenic pyrite, both present in sediments and in carbonate nodules. From the point of view of studying the conditions of early diagenesis, authigenic pyrite is considered the most important iron sulfide mineral due to its greatest diagenetic stability relative to other iron sulfides. Numerous previous studies of the morphology and size of authigenic pyrite show the possibility of using this information to assess the redox conditions of sedimentation and early diagenesis in both modern and ancient sedimentary basins. The aim of the research was to study the morphology and size distribution of authigenic pyrite to assess the effect of sulfate-controlled anaerobic oxidation of methane on the conditions of early diagenesis. Methods: field hydroacoustic researches (Kongsberg EA600), X-ray diffraction (Bruker D2 Phaser), scanning electron microscopy with local energy dispersive analysis (TESCAN VEGA 3 SBU). Results. Morphologically, pyrite is represented by idiomorphic and hypidiomorphic crystals, framboids and their clusters, radial outgrowths, and also rod like aggregates. The mean diameter of framboids, as well as the high value of standard deviation, indicate the diagenetic origin of pyrite. Sulfate-driven anaerobic oxidation of methane is the dominant process that controls the formation of iron sulfides, which is confirmed by the presence of fairly large framboids up to 49 µm in diameter. The observed diversity of pyrite morphology may reflect the change in the diagenetic environment over time, due to the variability of the flow of methane-bearing fluids.

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. [ABSTRACT FROM AUTHOR]

Understanding of the primary production of phytoplankton in the Kara Sea (KS), the Laptev Sea (LS), and the East Siberian Sea (ESS) remains limited, despite the recognized importance of phytoplankton in the Arctic Ocean. To address this knowledge gap, we conducted three NABOS (Nansen and Amundsen Basins Observational System) expeditions in 2013, 2015, and 2018 to measure in situ primary production rates using a 13C-15N dual-tracer method and examine their major controlling factors. The main goals in this study were to investigate regional heterogeneity in primary production and derive its contemporary ranges in the KS, LS, and ESS. The daily primary production rates in this study (99 ± 62, 100 ± 77, and 56 ± 35 mg C m−2 d−1 in the KS, LS, and ESS, respectively) are rather different from the values previously reported in each sea mainly because of spatial and regional differences. Among the three seas, a significantly lower primary production rate was observed in the ESS in comparison to those in the KS and LS. This is likely mainly because of regional differences in freshwater content based on the noticeable relationship (Spearman, rs = −0.714, p < 0.05) between the freshwater content and the primary production rates observed in this study. The contemporary ranges of the annual primary production based on this and previous studies are 0.96–2.64, 0.72–50.52, and 1.68–16.68 g C m−2 in the KS, LS, and ESS, respectively. Further intensive field measurements are warranted to enhance our understanding of marine microorganisms and their community-level responses to the currently changing environmental conditions in these poorly studied regions of the Arctic Ocean. [ABSTRACT FROM AUTHOR]

The structure and productivity of planktonic microbial communities in the ecosystems of the Siberian Arctic seas are significantly dependent on freshwater input. During the study, we determined the spatial distribution of the abundance, biomass, and production of heterotrophic bacterioplankton in the Western Laptev Sea on the transect from the Khatanga River estuary to the continental slope and assessed the impact of river freshwater discharge. The influence of fresh water on bacterioplankton was restricted mainly to Khatanga Bay (KHAB) and the transitional zone (TZ) and was poorly recognized in the Western shelf (WS) and continental slope (CS) areas. The total bacterial abundance decreased from KHAB to the CS. Particle-attached bacteria constituted on average 63.0% of the total abundance of bacterioplankton in KHAB and 1.0% at the CS. Average bacterial production in the water column was highest in KHAB (10.3 mg C m−3 d−1), decreasing towards the CS (0.7 mg C m−3 d−1). In KHAB and TZ, bacteria were the main component of the planktonic community (44−55%). These results show that at the end of the growing season, bacterial processes prevailed over autotrophic ones and contributed largely to the total biological carbon flux in the coastal ecosystem of the Western Laptev Sea. [ABSTRACT FROM AUTHOR]

It is shown that the area of the winter Siberian polynya in the Laptev Sea has a negative correlation with the Arctic Oscillation index (AO) for the winter period. Using this, a transfer function has been developed that connects the variations in the winter polynya (the time series of the winter AO) and the geochemical time series of bottom sediments accumulated during the instrumental period (1898–2018) in the zone of distribution of the Lena polynya. It was established that the influence of the winter polynya on the chemical composition of the accumulated bottom sediments is realized through a change in the redox conditions of near-bottom waters. Reconstructions using the developed transfer function of the polynya over the past three centuries showed that its active development increases the average annual air temperature in the adjacent areas, but affects the duration of the ice-free period multidirectionally. [ABSTRACT FROM AUTHOR]

Macrofauna can contribute substantially to the organic matter cycling on the seafloor, yet the role of terrestrial and chemosynthetic organic matter in the diets of microphagous (deposit and suspension) feeders is poorly understood. In the present study, we used stable isotopes of carbon and nitrogen to test the hypothesis that the terrestrial organic matter supplied with river runoff and local chemosynthetic production at methane seeps might be important organic matter sources for macrofaunal consumers on the Laptev Sea shelf. We sampled locations from three habitats with the presumed differences in organic matter supply: "Delta" with terrestrial inputs from the Lena River, "Background" on the northern part of the shelf with pelagic production as the main organic matter source, and "Seep" in the areas with detected methane seepage, where chemosynthetic production might be available. Macrobenthic communities inhabiting each of the habitats were characterized by a distinct isotopic niche, mostly in terms of δ13C values, directly reflecting differences in the origin of organic matter supply, while δ15N values mostly reflected the feeding group (surface deposit/suspension feeders, subsurface deposit feeders, and carnivores). We conclude that both terrestrial and chemosynthetic organic matter sources might be substitutes for pelagic primary production in the benthic food webs on the largely oligotrophic Laptev Sea shelf. Furthermore, species-specific differences in the isotopic niches of species belonging to the same feeding group are discussed, as well as the isotopic niches of the symbiotrophic tubeworm Oligobrachia sp. and the rissoid gastropod Frigidoalvania sp., which are exclusively associated with methane seeps. [ABSTRACT FROM AUTHOR]

Macrofauna can contribute substantially to the organic matter cycling on the seafloor, yet the role of terrestrial and chemosynthetic organic matter in the diets of microphagous (deposit and suspension) feeders is poorly understood. In the present study, we used stable isotopes of carbon and nitrogen to test the hypothesis that the terrestrial organic matter supplied with river runoff and local chemosynthetic production at methane seeps might be important organic matter sources for macrofaunal consumers on the Laptev Sea shelf. We sampled locations from three habitats with the presumed differences in organic matter supply: “Delta” with terrestrial inputs from the Lena River, “Background” on the northern part of the shelf with pelagic production as the main organic matter source, and “Seep” in the areas with detected methane seepage, where chemosynthetic production might be available. Macrobenthic communities inhabiting each of the habitats were characterized by a distinct isotopic niche, mostly in terms of δ13C values, directly reflecting differences in the origin of organic matter supply, while δ15N values mostly reflected the feeding group (surface deposit/suspension feeders, subsurface deposit feeders, and carnivores). We conclude that both terrestrial and chemosynthetic organic matter sources might be substitutes for pelagic primary production in the benthic food webs on the largely oligotrophic Laptev Sea shelf. Furthermore, species-specific differences in the isotopic niches of species belonging to the same feeding group are discussed, as well as the isotopic niches of the symbiotrophic tubeworm Oligobrachia sp. and the rissoid gastropod Frigidoalvania sp., which are exclusively associated with methane seeps.

The Arctic sedimentary basins are still poorly studied in comparison with other regions. The lack of deep wells across the eastern Russian Arctic has resulted in numerous contrasting geodynamic models for the geological evolution and age of sedimentary successions within this frontier region, where a modern mid-ocean ridge breaks through the continental crust in the Laptev Sea. The only onshore evidence of rifting processes is a number of small graben-like depressions exposed on the New Siberian Islands and along the Laptev Sea coast. We present U-Pb detrital zircon provenance and palynology study results of the Cenozoic sedimentary rocks filling graben-like depressions across western Kotel'nyi Island. Palynological data indicate that these sedimentary rocks are Early Eocene to Pleistocene in age. Based on U-Pb detrital zircon dating, Early Eocene and Late Oligocene clastic sediments were sourced from underlying deformed Palaeozoic rocks as well as by reworking of Upper Mesozoic rocks outcropping elsewhere on Kotel'nyi Island, which bear Siberian signature. Plio-Pleistocene clastic sediments were not derived from the erosion of deformed Palaeozoic rocks, suggesting the cessation of active uplift by this time and the development of a regional peneplain. Therefore, by extrapolating our onshore observations to the neighbouring offshore, we propose that graben structures imaged by seismic profiles along the eastern flank of the Laptev Rift System are likely to host Eocene and Oligocene sediments. Thus, it implies the Cenozoic extension led to formation of grabens on- and offshore in the eastern portion of Laptev Sea as early as Eocene. Further studies are necessary to evaluate the age of graben-related basins in the central and western part of the Laptev Sea. [ABSTRACT FROM AUTHOR]

A single specimen of the northern wolffish Anarhichas denticulatus Krøyer, 1845, 393 mm in length, was documented for the first time in the Siberian Arctic (Laptev Sea, Russia). Species identification was confirmed by an integrative taxonomic approach that included examination of external morphology and DNA barcoding using the COI mtDNA gene. This species is widely distributed in the North Atlantic, but records in the Arctic Ocean are limited to the Canadian and US coasts. This record might represent a significant range extension of about 7500 km for the species and may be associated with the eastward transport of a pelagic juvenile specimen from the Northeastern Atlantic to the Laptev Sea by the North Atlantic current, consistent with the hypothesis of Atlantification of the Arctic Ocean. X-ray images of the Laptev Sea specimen and photographs showing ontogenetic variations of species' coloration are provided for reference. The Laptev Sea specimen had a more elongated shape, longer preorbital distance, and longer pectoral, dorsal, and anal fins, as well as a larger eye and wider caudal fin compared to North Atlantic samples. The size differences are likely associated with conditions experienced as a juvenile during the pelagic stage of the lifecycle. [ABSTRACT FROM AUTHOR]

Biodiversity in the Laptev Sea was assessed for gutless marine worms of the family Siboglinidae (Annelida), whose metabolism is provided by symbiotic bacteria that oxidize hydrogen sulfide and methane. Seven siboglinid species were found within the geographical boundaries of the Laptev Sea, and another species was found in an adjacent sector of the Arctic Basin. The largest number of finds and the greatest biological diversity of siboglinids were observed in the eastern part of the Laptev Sea in a field of numerous methane flares. One find was made in the estuary area of the Lena River at a depth of 25 m. A possible association of siboglinids with methane seepage areas is discussed. [ABSTRACT FROM AUTHOR]

The new results of the frequency-resonance technologies application for the study of the sources and processes of seeps and pockmarks formation in the Arctic and the North Sea are considered. The study of the methane seeps and pockmarks fields shows that the intensity and dynamics of their formation depend on the inflow of gas fluids from deep sources because of a result of active degassing processes. The use of FR-sounding technologies allows us to remotely determine the spatial regularities of intense gas emission zone formation and the probable depth of geological sources of migration. The seep and pockmark field study results confirm the crust-mantle gas fluids' influence on the nature and degassing processes features in the structures of the continental margins. Modern degassing centers' studies have confirmed the presence of zones of active migration of gases into the atmosphere, which is an important factor in the dynamics of global climate change processes. The experience of FR- technologies used in the fields of methane seeps and pockmarks study in the structures of the Arctic polar seas showed their versatility and the possibility of their use in the study of gas emissions in structures of different regions. [ABSTRACT FROM AUTHOR]

Macrobenthos of soft bottoms on the shelf of two Arctic seas was investigated in the survey conducted aboard RV TINRO in August-September 2015. The area of 32,800 km2 in the depth range of 15–34 m was researched in the East Siberian Sea where 13 taxonomic groups of bottom fauna were found, mainly Bivalvia (75.9 % by biomass). The area of 22,500 km2 in the depth range of 45–96 m was surveyed in the Laptev Sea, where 20 groups of macrobenthos were identified, with predominance of Polychaeta (34.1 %), Bivalvia (27.7 %), and Sipuncula (21.2 %), by biomass. Significant changes in structure of benthic communities are revealed on the external shelf of the Laptev Sea (below of 70 m), in comparison with the data of previous years, possibly caused by warming of the environments: the previously dominated group of Ophiuroidea was replaced by Sipuncula. Although the dominant groups were widely occurred, their quantitative distribution was neither uniform nor similar. The dominant species had different trophic habits and occupied disjointed zones with domination of sestonophages or detritophages, in dependence on the habitat conditions. The average abundance and biomass of macrobenthos were assessed as 881 ± 174 ind./m2 and 149.2 ± 25.5 g/m2 in the East Siberian Sea, and 251 ± 32 ind./m2 and 71.0 ± 13.0 g/m2 in the Laptev Sea, respectively.

The discharge of rivers and the subsequent dispersion of their plumes play a pivotal role in the biogeochemical cycling of the Arctic Ocean. Based on the data collected during annual transects conducted in the autumn period (September-October) from 2015-2020, this study explores the effect of River Lena plume dispersion on the seasonal and interannual changes in the hydrophysical and biogeochemical structure of the southeastern Laptev Sea. The temperature-salinity relationship (T-S), Redfield ratio and multiparameter cluster analysis were used to investigate variations in the water mass structure along the transect. The results revealed that the plume’s interannual and seasonal spreading patterns play a crucial role in regulating the local physical, biogeochemical, and biological processes in the southern Laptev Sea. During September-October, the hydrochemical water mass structure along the transects shifted from highly stratified to unstratified as the plume’s mixing intensity increased. Anomalous hydrochemical distributions were observed due to coastal upwelling, which was primarily characterized by high total alkalinity and nitrate levels, and low organic phosphorus, nitrite, and ammonia levels in the seawater. Wind and cold weather conditions drive deep vertical mixing of seawater, causing the resuspension of bottom sediment and the subsequent enrichment of bottom water by nutrients. Multi-parameter cluster analysis is used to describe the details of water mass structures in the highly dynamic southern Laptev Sea, with water mass structures typically undergoing significant changes within two weeks between September and October. The migration and transformation of water masses throughout the seasons are influenced by the volume of river discharge, fall-winter cooling, and atmospheric circulation patterns. Furthermore, the general atmospheric circulation is confirmed to be the primary cause of the interannual variation in the spread of the Lena River plume over the southeast Laptev Sea.

The distribution of the concentrations of CH4 and CO2 and other gases in the sediments of the Kara, Laptev and East Siberian seas was studied. A correlation was found between CH4 and CO2 in most sediment cores. The correlation coefficient R for the sediments of the Kara Sea ranges from 0.58 to 0.97 and does not depend on the gas distribution with depth. Methanogenesis in marine sediments is presumably associated with sulfate reduction, and hydrogenotrophic methanogenesis leads to an increase in CH4 concentration against the background of high CO2 concentration. The high concentration of dimethyl sulfide in the sediments indicates the simultaneous formation of CH4 and CO2 through methylotrophic methanogenesis with the participation of CH3SCH3. In the upper layers of the sediment, opposite trends in the CH4 and CO2 distribution can be observed. The fact that the correlation coefficients between CH4 and CO2 gases in the sediments of the Laptev Sea acquire both negative and positive values is probably related to the fact that gas formation processes in the upper layers of the sediment captured by short columns differ from those of deep layers. [ABSTRACT FROM AUTHOR]

Bottom sediments at methane discharge sites of the Laptev Sea shelf were investigated. The rates of microbial methanogenesis and methane oxidation were measured, and the communities responsible for these processes were analyzed. Methane content in the sediments varied from 0.9 to 37 µmol CH4 dm−3. Methane carbon isotopic composition (δ13C-CH4) varied from −98.9 to −77.6‰, indicating its biogenic origin. The rates of hydrogenotrophic methanogenesis were low (0.4–5.0 nmol dm−3 day−1). Methane oxidation rates varied from 0.4 to 1.2 µmol dm−3 day−1 at the seep stations. Four lineages of anaerobic methanotrophic archaea (ANME) (1, 2a–2b, 2c, and 3) were found in the deeper sediments at the seep stations along with sulfate-reducing Desulfobacteriota. The ANME-2a-2b clade was predominant among ANME. Aerobic ammonium-oxidizing Crenarchaeota (family Nitrosopumilaceae) predominated in the upper sediments along with heterotrophic Actinobacteriota and Bacteroidota, and mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae). Members of the genera Sulfurovum and Sulfurimonas occurred in the sediments of the seep stations. Mehtanotrophs of the classes Alphaproteobacteria (Methyloceanibacter) and Gammaproteobacteria (families Methylophilaceae and Methylomonadaceae) occurred in the sediments of all stations. The microbial community composition was similar to that of methane seep sediments from geographically remote areas of the global ocean. [ABSTRACT FROM AUTHOR]

Sea ice conditions over the Arctic can be studied remotely using various satellites. In the current paper, two seas of the Arctic, Beaufort and Laptev are considered. The difference in the sea ice conditions of Beaufort and Laptev is separately observed using the parameters; Sea Ice Area, Concentration and Thickness for the span of 2010—2021. In order to understand the peripheral and lateral condition of sea ice in a given region for a given time, we have attempted to understand each derivable product of the sea ice spatially and temporally throughout its life cycle. It has been found that the Laptev which is a major warehouse of arctic sea ice with an average outflow of 483,000 km2 per year is found to have a low range of Sea Ice Area and Thickness. Laptev Sea Ice Thickness maxima shift from 1 to 1.2—1.3 to 1.4 m from January to February to March. Whereas over Beaufort, the peak of Sea Ice Thickness shifts from 1.2 to 1.5 to 1.7 m from January to February to March. This means the variability in Beaufort is higher than that in Laptev. The method discussed in this paper presents a unique idea of using the Sea Ice Thickness variable over Sea Ice Area and Concentration, especially during winter months, for a better understanding of the sea ice growth. The information obtained using these sea ice parameters would be further utilized by modelers, navigators, etc., which in turn helps improve, forecast studies. [ABSTRACT FROM AUTHOR]

Our research was motivated by significant warming in the Arctic in recent decades and the influence of this warming on diatoms, which are the main producers in the seas of the Eastern Arctic. For this purpose, we studied the qualitative concentrations and quantitative ratio of diatoms from the surface sediments of the Laptev Sea (LS), the East Siberian (ESS) and Chukchi seas (ChS), and the Arctic Ocean (AO), obtained by box corers in 2016 and 2018. The ecological structure of the diatom assemblages of these sediments, reflecting the current environmental conditions, was also analyzed. Compared with the end of the last century, there were significant changes in species composition and quantitative ratio of the diatom assemblages in the sediments from the AO and the LS and ESS. In contrast, the diatom assemblages in the sediments from the ChS had not significantly changed in terms of species composition or quantitative ratio over the same three decades. The observed transformations in the diatom assemblages of the surface sediments were associated with substantial changes in water temperature, current flow, salinity, ice melting, and prolonged ice-free periods associated with global warming in the Northern Hemisphere in recent decades. The foregoing processes have clearly had strong impacts on the environment and the biota of the Arctic region. [ABSTRACT FROM AUTHOR]

The distribution of fresh water in the East Siberian Sea (ESS) and Laptev Sea (LS) in the period of the least ice coverage (August–October) has been analyzed using satellite data on sea level and CTD data from ship observations. It is shown that there is a good agreement between the salinity/density distribution in the surface water layer and the sea level in the study area. An increase (decrease) in the runoff of the Lena River during the summer flood leads to an increase (decrease) in the sea level in the eastern part of the LS and in the western and southwestern parts of the ESS in September and October (r = 0.52–0.59 in 1993–2019). The intensification of the western winds is accompanied by a decrease in salinity and a rise in sea level in the coastal areas of the LS and ESS. Eastern winds cause a decrease in sea level in the southern regions of the LS and ESS and the flow of waters with low salinity from the coastal to the open part of the sea. Western/eastern winds are accompanied by eastern/western directed geostrophic currents in the region of the Sannikov Strait and in the southwestern part of the ESS. [ABSTRACT FROM AUTHOR]

The features of the distribution of the 226Ra, 232Th, 238U concentrations in the surface layer of bottom sediments in the northern part of the Laptev Sea are considered. Data are obtained for the bottom sediments of the shelf zone of the sea, the continental slope, and abyssal depths. The concentrations of 226Ra and 232Th varied in the range 17–36.3 and 28.7–38.1 Bq/kg, respectively, and the 238U concentration, within 12.5–38.7 Bq/kg. It is assumed that the enrichment of sediments in 226Ra and 238U with depth is due to the reduction of part of the carbonates containing 226Ra and 238U, followed by the sorption of 226Ra from sea water and the sedimentation of hydrolysis forms of 238U to the bottom. An increase in the 226Ra and 238U concentrations in the sediment with depth in the eastern section, as the concentration of inorganic carbon decreases, is consistent with this assumption. In this case, the correlation of the 226Ra and 238U concentrations with the concentration of inorganic carbon is negative (R = –0.98 and –0.94, respectively). The 232Th concentration correlates with the concentration of organic carbon (R = 0.85). In the area of the outer shelf and in the Vilkitsky Strait, the 226Ra, 232Th, and 238U concentrations in the surface layer of sediments depend on the fractional composition of the sediment and change with a change in the fractional ratio. The concentrations of 238U and 232Th correlate with each other (R = 0.84) and with the concentration of inorganic carbon (R = 0.75 and 0.87, respectively). [ABSTRACT FROM AUTHOR]

Relevance. A specific feature of the Laptev Sea shelf is the sites of discharge of methane-containing fluids from the surface of the seabed into the water column (methane cold seeps). The key biogeochemical processes occurring during methane migration through the sedimentary environment are anaerobic oxidation of methane and bacterial sulfate reduction. The activity of these processes encourages a change in the redox conditions of sedimentation, which affects the biogeochemical cycles of some redox-sensitive elements. The aim of the research is to study the influence of methane-containing fluids on the geochemical cycles of iron, carbon and some redox-sensitive elements. Objects. The data of the concentrations of carbon, iron, and some of redox-sensitive elements (Mn, Co, Ni, Cu, Zn, Cr, Ba, Mo, U) in three bottom sediment cores sampled on the outer shelf of the Laptev Sea were analyzed. Two of the three cores were obtained at methane cold seep sites and were considered as sediments subject to anaerobic methane oxidation. Methods: pyrolysis (Rock-Eval 6 Turbo, Vinci Technologies), X-Ray analysis (HORIBA X-Ray Analytical Microscope XGT 7200), Inductively coupled plasma mass spectrometry (ICP-MS, ELAN DRC-e). Results. TOC and Fe contents in sediments do not reflect the impact of anaerobic oxidation on their geochemical cycles and controlled by the specifics of the spatial distribution of sedimentary material. In all the studied cores, there are elevated Mn concentrations confined to the surface layer of sediments. At methane cold seep sites, the surface layer of bottom sediments is characterized by enrichment in Mo, Ni and Cr. The selective enrichment of the surface layer of sediments with some redox-sensitive elements can be caused by the migration of methane-containing fluids, which facilitate the transport of elements in dissolved form from deeper horizons. The sorption of these elements by organic matter and Fe-Mn oxihydroxides appears to be the key mechanism controlling the deposition of Mo, Ni, and Cr.

The central Arctic Ocean is rapidly changing due to amplified warming and sea ice retreat. Nonetheless, it remains challenging to document and decipher impacts on key ecosystem processes such as primary production and pelagic-benthic coupling, due to limited observations in this remote area. Here we investigated environmental changes at the Laptev Sea continental slope (60-3400 m water depth) from the surface to the seafloor, by replicating sample transects two decades apart. Mean break-up of sea ice occurred earlier and mean freeze-up occurred later in 2012 compared to 1993, extending the ice-free period by more than 30 days. On average, observations and model results showed an annual increase in primary production of 30% and more in the study area in 2012. In contrast, calculated and modelled fluxes of particulate organic carbon (POC) to the seafloor were only slightly higher in 2012 and did not extend as far into the deep Laptev Sea as the increase in primary production, possibly due to a more developed retention system. Nevertheless, benthic surveys revealed a substantial increase in phytodetritus availability at the seafloor along the entire transect from the shelf edge to the deep sea. This calls for carbon input by lateral advection from the shelves, additional input from sea ice, and/or a late summer bloom. We also investigated the composition and activity of bacterial communities at the seafloor and potential linkages to the observed environmental changes. While bacterial abundance, biomass and overall community structure showed no systematic differences between the two contrasting years at all depths, extracellular enzymatic activities had increased as a result of higher food availability. This was partly reflected in higher benthic oxygen uptake, indicating a moderate impact on benthic remineralization rates at the time of sampling. Our results show considerable effects of ocean warming and sea ice loss on the ecosystem from the surface ocean to the seafloor in the Laptev Sea, which are likely to continue in the coming decades.

The results of new local seismic observations in the Laptev Sea obtained using ocean bottom seismographs were compared with actual data from global and regional earthquake catalogs. Most earthquake epicenters on the Laptev Sea shelf are grouped into several clusters located within the East Laptev province of horsts and grabens and enclosed between two extension detachments. The first one marks the eastern boundary of the Ust–Lena and Omoloi rift systems, and the second one is the eastern boundary of the Anisin, Zarya, and Belkovskii-Svyatoi Nos rift chains. At the same time, the linear zone of epicenters confined to the Gakkel Ridge, when moving to the shelf, shifts along the Khatanga–Lomonosov fault zone to the northeast in the direction of the second extension detachment, which probably remains the only active one. The spatial distribution of earthquakes in the Laptev Sea, taking into account the epicenters of micro- and weak seismic events on the shelf, does not have sufficiently clear boundaries that determine the contours of the previously assumed lithospheric microplate. [ABSTRACT FROM AUTHOR]

Information on the distribution and occurrence of three species of the genus Liparis, Liparis bathyarcticus Parr, 1931, L. cf. fabricii Krøyer, 1847 and L. tunicatus Reinhardt, 1836 on the shelf of the Laptev and East Siberian seas, depending on the temperature, salinity and sediment types is presented according to the data of trawl surveys in 2014 and 2017. L. cf. fabricii is the most common species in the surveyed water area with the frequency of occurrence of 22.4% in the Laptev Sea and 24.7% in the East Siberian Sea. The total relative abundance and biomass of all species of the genus Liparis in the East Siberian Sea (377.7 ind./km2 and 9.14 kg/km2) were slightly higher than in the Laptev Sea (326 ind./km2 and 8.32 kg/km2). The maximum length of L. tunicatus in the East Siberian Sea (172 mm) exceeds the previously known one (160 mm). The average length and weight of fish of all three species of Liparis in the East Siberian Sea are greater than in the Laptev Sea. [ABSTRACT FROM AUTHOR]

Realistic prediction of the near‐future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization, and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift (TPD), we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), silicic acid (DSi), and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013 and 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom‐dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River and was supplied during the spring freshet, while riverine DSi in the southeastern Laptev Sea was continuously supplied during the summer. Primary productivity fueled by river‐borne nutrients was enhanced by admixture of DIN‐ and DIP‐rich Atlantic‐sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm‐induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240%) and DIP (+90%) beneath the Lena River plume were caused by sea ice‐driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev Shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom‐dominated primary productivity in the TPD. Plain Language Summary: Ongoing warming and sea ice loss in the Arctic Ocean may significantly impact biological productivity, which is mainly controlled by light and nutrient availability. To investigate nutrient inputs from land, biological utilization, and the influence of water mass mixing and winter processes on the nutrient distributions, we measured nutrient concentrations and silicon isotopes in the Laptev Sea. We found high concentrations in the southeastern Laptev Sea in agreement with nutrient inputs from the Lena River. Toward the northern Laptev Sea, nutrient concentrations decreased in the surface layer and the silicon isotope signatures shifted to heavier values, consistent with nutrient utilization by phytoplankton. In contrast to the depleted surface layer, the bottom layer beneath the Lena River plume was strongly enriched in some nutrients, which we attribute to different physical and biogeochemical processes. These observations are important for our understanding of nutrient bioavailability in the Laptev Sea and the Transpolar Drift (TPD), which is a surface current that connects the Laptev Sea with the central Arctic Ocean and the Fram Strait. The changing hydrography of the Laptev Sea will likely cause a decrease in silicic acid concentrations and thus a reduction in nutrient export and diatom‐dominated primary productivity in the TPD. Key Points: Surface dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), silicic acid (DSi), and Si isotope dynamics are controlled by marine and riverine inputs and uptake by phytoplanktonStrong DIP and DSi enrichments beneath the Lena River plume are due to sea ice‐driven nutrient redistribution and remineralizationEnhanced DSi utilization in the Laptev Sea will lead to a reduced diatom‐dominated primary productivity in the Transpolar Drift [ABSTRACT FROM AUTHOR]

The structure and productivity of planktonic microbial communities in the ecosystems of the Siberian Arctic seas are significantly dependent on freshwater input. During the study, we determined the spatial distribution of the abundance, biomass, and production of heterotrophic bacterioplankton in the Western Laptev Sea on the transect from the Khatanga River estuary to the continental slope and assessed the impact of river freshwater discharge. The influence of fresh water on bacterioplankton was restricted mainly to Khatanga Bay (KHAB) and the transitional zone (TZ) and was poorly recognized in the Western shelf (WS) and continental slope (CS) areas. The total bacterial abundance decreased from KHAB to the CS. Particle-attached bacteria constituted on average 63.0% of the total abundance of bacterioplankton in KHAB and 1.0% at the CS. Average bacterial production in the water column was highest in KHAB (10.3 mg C m−3 d−1), decreasing towards the CS (0.7 mg C m−3 d−1). In KHAB and TZ, bacteria were the main component of the planktonic community (44−55%). These results show that at the end of the growing season, bacterial processes prevailed over autotrophic ones and contributed largely to the total biological carbon flux in the coastal ecosystem of the Western Laptev Sea.