Cold-seep carbonates of the Laptev Sea continental slope: Constraints from fluid sources and environment of formation

This study presents results of the petrographic, mineralogical, stable isotopes of oxygen and carbon, and trace element investigation of authigenic carbonates collected at newly discovered active cold seeps on the Laptev Sea continental slope at ∼300 m water depth. These carbonates are mainly represented by Mg-calcite with MgCO3 content from 9.1 mol% to 14.0 mol%. The low δ13C values of carbonates ranging from −50.6 ‰ to −32.4 ‰ (V-PDB) indicate that they were formed from anaerobic oxidation of biogenic methane and minor participation of other carbon sources. The difference between measured (from 4.7 ‰ to 5.5 ‰) and calculated (4.0 ‰) δ18Ocarb values might be inherited from fluids enriched in 18O due to dissociation of gas hydrates, which could be the source of methane. The carbonates exhibit weak enrichment in Co, moderate and strong enrichments in As, Mo, and Sb, and strong enrichment in U. Interestingly, As, Sb, and Co correlate with the pyrite content. This indicates that authigenic iron sulfides promote the immobilization of these redox-sensitive elements in seep sediments. The (Mo/U)EF values and anomalies of concentration of Mo and U probably indicate variations in the redox conditions during carbonate formation due to episodically seepage activity changes. Ascending methane-bearing fluids were the main contributor to the enrichment of cold-seep carbonates in As, Mo, Sb, and U at the Laptev Sea continental slope. However additional input from the particulate shuttle process can not be ruled out

Sonar Estimation of Methane Bubble Flux from Thawing Subsea Permafrost: A Case Study from the Laptev Sea Shelf

Seeps found offshore in the East Siberian Arctic Shelf may mark zones of degrading subsea permafrost and related destabilization of gas hydrates. Sonar surveys provide an effective tool for mapping seabed methane fluxes and monitoring subsea Arctic permafrost seepage. The paper presents an overview of existing approaches to sonar estimation of methane bubble flux from the sea floor to the water column and a new method for quantifying CH4 ebullition. In the suggested method, the flux of methane bubbles is estimated from its response to insonification using the backscattering cross section. The method has demonstrated its efficiency in the case study of single- and multi-beam acoustic surveys of a large seep field on the Laptev Sea shelf

Active high-frequency acoustic thermometry of frozen water-saturated media

The article discusses promising method of active high-frequency acoustic thermometry, which measures the duration of the backscattering signal of ultrasonic pulses Delta t in the upper layer of water-saturated rocks or seabed sediments. An acoustic model is proposed to explain the strong temperature dependence of the duration of this signal in the region of zero and negative temperatures. It is shown that Delta t increases with decreasing temperature due to an increase in the reflection coefficient of ultrasonic waves from a layered structure and to a decrease in the absorption coefficient in a water-saturated medium. Field studies carried out on a vast area of the Arctic shelf confirmed the existence of such a dependence. The proposed approach can be effective for remote monitoring of temperature changes in frozen water-saturated rocks on land and in the upper layer of seabed sediments on the shallow Arctic shelf, which is necessary to estimate various environmental risks associated with global warming

ORGANIC CARBON IN SURFACE SEDIMENTS OF LAPTEV SEA AND EAST SIBERIAN SEA: OBSERVATION OF PYROLYSIS DATA

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

METHANE SEEPAGE IMPACT ON AUTHIGENIC PYRITE MORPHOLOGY IN SEDIMENTS OF THE LAPTEV SEA CONTINENTAL SLOPE

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

East Siberian Sea: Interannual heterogeneity of the suspended particulate matter and its biogeochemical signature

The East Siberian Sea (ESS) is the largest, shallowest and most icebound Arctic marginal sea. It receives substantial input of terrigenous material and climate-vulnerable old organic carbon from both coastal erosion and rivers draining the extensive permafrost-covered watersheds. This study focuses on the interannual variability and spatial distribution of suspended particulate matter (SPM) in the surface and bottom waters of the ESS during the ice-free period in 2000, 2003, 2004, 2005 and 2008. We report on the composition and variability of particulate organic carbon (POC), total nitrogen (TN), POC/TN ratios, carbon and nitrogen isotopes (δ13C, δ15N) and provide estimates of the contribution of terrestrial organic carbon (terrOC) based on the δ13C isotopic values. The results show that interannual SPM distribution and elemental-isotopic characteristics of POC differ significantly between the western biogeochemical province (WBP; West of 165oE) and the eastern biogeochemical province (EBP; East of 165oE) of the ESS. The SPM mean concentration in the WBP is almost an order of magnitude higher than in the EBP. From west-to-east of the ESS, SPM tends to become more depleted in δ15N, while the δ13C becomes isotopically heavier. This trend can be explained by a shift in organic matter sources from terrigenous origin (erosion of the coastal ice complex and riverine POC) to becoming dominantly from marine plankton. The maximum contribution of terrOC to POC reached 99% in parts of the WBP, but accounts for as low as 1% in parts of the EBP. At the same time, the type of atmospheric circulation and its associated regime of both water circulation and ice transport control a displacement of the semi-stable biogeochemical border between WBP and EBP to the east or to the west if compared to its long-term average position near 165oE. Our multi-year investigation provides a robust observational basis for better understanding of the transport and fate of terrigenous material upon entering the ESS shelf waters. Our results also provide deeper insights into the interaction in the land-shelf sea system of the largest shelf sea system of the World Ocean, the East Siberian Arctic Shelf system

A Complex of Marine Geophysical Methods for Studying Gas Emission Process on the Arctic Shelf

The Russian sector of the arctic shelf is the longest in the world. Quite a lot of places of massive discharge of bubble methane from the seabed into the water column and further into the atmosphere were found there. This natural phenomenon requires an extensive complex of geological, biological, geophysical, and chemical studies. This article is devoted to aspects of the use of a complex of marine geophysical equipment applied in the Russian sector of the arctic shelf for the detection and study of areas of the water and sedimentary strata with increased saturation with natural gases, as well as a description of some of the results obtained. This complex contains a single-beam scientific high-frequency echo sounder and multibeam system, a sub-bottom profiler, ocean-bottom seismographs, and equipment for continuous seismoacoustic profiling and electrical exploration. The experience of using the above equipment and the examples of the results obtained in the Laptev Sea have shown that these marine geophysical methods are effective and of particular importance for solving most problems related to the detection, mapping, quantification, and monitoring of underwater gas release from the bottom sediments of the shelf zone of the arctic seas, as well as the study of upper and deeper geological roots of gas emission and their relationship with tectonic processes. Geophysical surveys have a significant performance advantage compared to any contact methods. The large-scale application of a wide range of marine geophysical methods is essential for a comprehensive study of the geohazards of vast shelf zones, which have significant potential for economic use