Understanding the Permafrost–Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf

This paper summarizes current understanding of the processes that determine the dynamics of the subsea permafrost–hydrate system existing in the largest, shallowest shelf in the Arctic Ocean; the East Siberian Arctic Shelf (ESAS). We review key environmental factors and mechanisms that determine formation, current dynamics, and thermal state of subsea permafrost, mechanisms of its destabilization, and rates of its thawing; a full section of this paper is devoted to this topic. Another important question regards the possible existence of permafrost-related hydrates at shallow ground depth and in the shallow shelf environment. We review the history of and earlier insights about the topic followed by an extensive review of experimental work to establish the physics of shallow Arctic hydrates. We also provide a principal (simplified) scheme explaining the normal and altered dynamics of the permafrost–hydrate system as glacial–interglacial climate epochs alternate. We also review specific features of methane releases determined by the current state of the subsea-permafrost system and possible future dynamics. This review presents methane results obtained in the ESAS during two periods: 1994–2000 and 2003–2017. A final section is devoted to discussing future work that is required to achieve an improved understanding of the subject

Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

Sonar surveys provide an effective mechanism for mapping seabed methane flux emissions, with Arctic submerged permafrost seepage having great potential to significantly affect climate. We created in situ engineered bubble plumes from 40 m depth with fluxes spanning 0.019 to 1.1 L s−1 to derive the in situ calibration curve (Q([sigma])). These nonlinear curves related flux (Q) to sonar return ([sigma]) for a multibeam echosounder (MBES) and a single-beam echosounder (SBES) for a range of depths. The analysis demonstrated significant multiple bubble acoustic scattering - precluding the use of a theoretical approach to derive Q([sigma]) from the product of the bubble [sigma] (r) and the bubble size distribution where r is bubble radius. The bubble plume σ occurrence probability distribution function ([PSI]([sigma])) with respect to Q found [PSI] ([sigma]) for weak σ well described by a power law that likely correlated with small-bubble dispersion and was strongly depth dependent. [PSI] ([sigma]) for strong σ was largely depth independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core. [PSI] ([sigma]) was bimodal for all but the weakest plumes

Role of Warming in Destabilization of Intrapermafrost Gas Hydrates in the Arctic Shelf: Experimental Modeling

Destabilization of intrapermafrost gas hydrates is one of the possible mechanisms responsible for methane emission in the Arctic shelf. Intrapermafrost gas hydrates may be coeval to permafrost: they originated during regression and subsequent cooling and freezing of sediments, which created favorable conditions for hydrate stability. Local pressure increase in freezing gas-saturated sediments maintained gas hydrate stability from depths of 200–250 m or shallower. The gas hydrates that formed within shallow permafrost have survived till present in the metastable (relict) state. The metastable gas hydrates located above the present stability zone may dissociate in the case of permafrost degradation as it becomes warmer and more saline. The effect of temperature increase on frozen sand and silt containing metastable pore methane hydrate is studied experimentally to reconstruct the conditions for intrapermafrost gas hydrate dissociation. The experiments show that the dissociation process in hydrate-bearing frozen sediments exposed to warming begins and ends before the onset of pore ice melting. The critical temperature sufficient for gas hydrate dissociation varies from ?3.0 °C to ?0.3 °C and depends on lithology (particle size) and salinity of the host frozen sediments. Taking into account an almost gradientless temperature distribution during degradation of subsea permafrost, even minor temperature increases can be expected to trigger large-scale dissociation of intrapermafrost hydrates. The ensuing active methane emission from the Arctic shelf sediments poses risks of geohazard and negative environmental impacts

Role of Salt Migration in Destabilization of Intra Permafrost Hydrates in the Arctic Shelf: Experimental Modeling

Destabilization of intrapermafrost gas hydrate is one possible reason for methane emission on the Arctic shelf. The formation of these intrapermafrost gas hydrates could occur almost simultaneously with the permafrost sediments due to the occurrence of a hydrate stability zone after sea regression and the subsequent deep cooling and freezing of sediments. The top of the gas hydrate stability zone could exist not only at depths of 200–250 m, but also higher due to local pressure increase in gas-saturated horizons during freezing. Formed at a shallow depth, intrapermafrost gas hydrates could later be preserved and transform into a metastable (relict) state. Under the conditions of submarine permafrost degradation, exactly relict hydrates located above the modern gas hydrate stability zone will, first of all, be involved in the decomposition process caused by negative temperature rising, permafrost thawing, and sediment salinity increasing. That’s why special experiments were conducted on the interaction of frozen sandy sediments containing relict methane hydrates with salt solutions of different concentrations at negative temperatures to assess the conditions of intrapermafrost gas hydrates dissociation. Experiments showed that the migration of salts into frozen hydrate-containing sediments activates the decomposition of pore gas hydrates and increase the methane emission. These results allowed for an understanding of the mechanism of massive methane release from bottom sediments of the East Siberian Arctic shelf

Signatures of Molecular Unification and Progressive Oxidation Unfold in Dissolved Organic Matter of the Ob-Irtysh River System along Its Path to the Arctic Ocean

The Ob-Irtysh River system is the seventh-longest one in the world. Unlike the other Great Siberian rivers, it is only slightly impacted by the continuous permafrost in its low flow. Instead, it drains the Great Vasyugan mire, which is the world largest swamp, and receives huge load of the Irtysh waters which drain the populated lowlands of the East Siberian Plain. The central challenge of this paper is to understand the processes responsible for molecular transformations of natural organic matter (NOM) in the Ob-Irtysh river system along the South-North transect. For solving this task, the NOM was isolated from the water samples collected along the 3,000?km transect using solid-phase extraction. The NOM samples were further analyzed using high resolution mass spectrometry and optical spectroscopy. The obtained results have shown a distinct trend both in molecular composition and diversity of the NOM along the South-North transect: the largest diversity was observed in the Southern “swamp-wetland” stations. The samples were dominated with humic and lignin-like components, and enriched with aminosugars. After the Irtysh confluence, the molecular nature of NOM has changed drastically: it became much more oxidized and enriched with heterocyclic N-containing compounds. These molecular features are very different from the aliphatics-rich permafrost NOM. They witnesses much more conservative nature of the NOM discharged into the Arctic by the Ob-Irtysh river system. In general, drastic reduction in molecular diversity was observed in the northern stations located in the lower Ob flow

Composition of Sedimentary Organic Matter across the Laptev Sea Shelf: Evidences from Rock-Eval Parameters and Molecular Indicators

Global warming in high latitudes causes destabilization of vulnerable permafrost deposits followed by massive thaw-release of organic carbon. Permafrost-derived carbon may be buried in the nearshore sediments, transported towards the deeper basins or degraded into the greenhouse gases, potentially initiating a positive feedback to climate change. In the present study, we aim to identify the sources, distribution and degradation state of organic matter (OM) stored in the surface sediments of the Laptev Sea (LS), which receives a large input of terrestrial carbon from both Lena River discharge and intense coastal erosion. We applied a suite of geochemical indicators including the Rock Eval parameters, traditionally used for the matured OM characterization, and terrestrial lipid biomarkers. In addition, we analyzed a comprehensive grain size data in order to assess hydrodynamic sedimentation regime across the LS shelf. Rock-Eval (RE) data characterize LS sedimentary OM with generally low hydrogen index (100–200 mg HC/g TOC) and oxygen index (200 and 300 CO2/g TOC) both increasing off to the continental slope. According to Tpeak values, there is a clear regional distinction between two groups (369–401 °C for the inner and mid shelf; 451–464 °C for the outer shelf). We suggest that permafrost-derived OM is traced across the shallow and mid depths with high Tpeak and slightly elevated HI values if compared to other Arctic continental margins. Molecular-based degradation indicators show a trend to more degraded terrestrial OC with increasing distance from the coast corroborating with RE results. However, we observed much less variation of the degradation markers down to the deeper sampling horizons, which supports the notion that the most active OM degradation in LS land-shelf system takes part during the cross-shelf transport, not while getting buried deeper

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

Carbon depth cycle and formation of abiogenic hydrocarbons

The relevance. Identification of mechanisms of carbon metamorphic transformation in convergent and divergent regions of the Earth, assessment of the scale of deep transport and the transfer on the generation of abiogenic hydrocarbons in tectonic discharge zones are some of the most urgent problems of modern geology. The aim of the research is to describe multi-stage and polycyclic carbon transformation and transfer in the crust and mantle. Sedimentary rocks covered in subductions zones are destroyed and transformed by metamorphic processes. Some of the newly formed carbon compounds are transferred by convective flows of the mantle to the rift zones of mid-ocean ridges, brought to the surface, decomposed in the presence of water and form a wide range of hydrocarbons and carbon dioxide. There, they are again deposited on the sea floor in the form of sediments forming carbonate and carbon/containing structural/material complexes. Result. It is determined that the manifestation of a multi-stage mechanism of physicochemical transformations in crust-mantle areas of the Earth leads to occurrence of features of abiogenic origin in biogenic hydrocarbons. The identified crust-mantle carbon cycle is a part of a global process of carbon cyclic transport from the atmosphere into the mantle and back. The scale of its manifestation, most likely, is not so large. Numerous small (mm and fractions of mm) particles of exogenous matter and dispersed carbon pulled in plate subduction zones form a stable geochemical train of the crustal trend in the mantle spreading along the surface of convection flows motion. It is possible to judge indirectly the scale of this process manifestation by degassing amount of hydrocarbon and carbon dioxide gases and hydrogen in Earth's crust rift systems. In this case the amount of generated depth/origin hydrocarbon gases cannot form large gas and oil and gas fields as their significant part is released in the atmosphere. Only a small amount of compounds may be deposited in oceanic sediments and form gas hydrate accumulations in them

Carbon depth cycle and formation of abiogenic hydrocarbons

Актуальность. Выявление механизмов метаморфической трансформации углерода в конвергентных и дивергентных областях Земли, оценка масштабов глубинного переноса и влияние на процессы генерации абиогенных углеводородов в зонах тектонической разгрузки являются одними из наиболее актуальных задач современной геологии. Цель исследования заключается в описании процессов многостадийного и полициклического преобразования и переноса углерода в коре и мантии. Затянутые в зонах поддвига плит осадки разрушаются, трансформируются и преобразуются метаморфическими процессами. Часть вновь сформированных углеродистых соединений переносится конвективными течениями мантии в рифтовые зоны срединно-океанических хребтов, выносятся на поверхность, разлагаются в присутствии воды и образуют широкий спектр углеводородов и углекислого газа. Там они снова отлагаются на морском дне в виде осадков, образуя карбонатные и углеродсодержащие структурно-вещественные комплексы. Результаты. Определено, что проявление многоступенчатого механизма физико-химических преобразований в коромантийных областях Земли приводит к тому, что биогенные углеводородные соединения приобретают черты абиогенного происхождения. Выявленный коромантийный цикл углерода является частью глобального процесса циклического переноса углерода из атмосферы в мантию и обратно. Масштабы его проявления, скорее всего, не столь широки, а многочисленные мелкие (мм и доли мм) частицы экзогенного вещества и рассеянного углерода, затянутые в зоны поддвига плит, образуют устойчивый геохимический шлейф коровой направленности в мантии, распространяющийся в плоскости перемещения конвективных потоков. Косвенно о масштабе проявления данного процесса можно судить по объемам дегазации углеводородных и углекислого газов, а также водорода в рифтовых системах земной коры. При этом количество генерируемых углеводородных газов глубинного происхождения не могут формировать крупных газовых и нефтегазовых месторождений, т. к. значительная их часть переносится в атмосферу. Лишь некоторое количество соединений может отлагаться в океанических осадках и формировать в них залежи газогидратов.The relevance. Identification of mechanisms of carbon metamorphic transformation in convergent and divergent regions of the Earth, assessment of the scale of deep transport and the transfer on the generation of abiogenic hydrocarbons in tectonic discharge zones are some of the most urgent problems of modern geology. The aim of the research is to describe multi-stage and polycyclic carbon transformation and transfer in the crust and mantle. Sedimentary rocks covered in subductions zones are destroyed and transformed by metamorphic processes. Some of the newly formed carbon compounds are transferred by convective flows of the mantle to the rift zones of mid-ocean ridges, brought to the surface, decomposed in the presence of water and form a wide range of hydrocarbons and carbon dioxide. There, they are again deposited on the sea floor in the form of sediments forming carbonate and carbon/containing structural/material complexes. Result. It is determined that the manifestation of a multi-stage mechanism of physicochemical transformations in crust-mantle areas of the Earth leads to occurrence of features of abiogenic origin in biogenic hydrocarbons. The identified crust-mantle carbon cycle is a part of a global process of carbon cyclic transport from the atmosphere into the mantle and back. The scale of its manifestation, most likely, is not so large. Numerous small (mm and fractions of mm) particles of exogenous matter and dispersed carbon pulled in plate subduction zones form a stable geochemical train of the crustal trend in the mantle spreading along the surface of convection flows motion. It is possible to judge indirectly the scale of this process manifestation by degassing amount of hydrocarbon and carbon dioxide gases and hydrogen in Earth's crust rift systems. In this case the amount of generated depth/origin hydrocarbon gases cannot form large gas and oil and gas fields as their significant part is released in the atmosphere. Only a small amount of compounds may be deposited in oceanic sediments and form gas hydrate accumulations in them

Organic carbon in surface sediments of Laptev Sea and East Siberian Sea: observation of pyrolysis data

Актуальность исследования обусловлена необходимостью изучения механизмов трансформации и накопления терригенного органического углерода, высвобождаемого из мерзлотных толщ, на шельфе арктических морей. При переносе в системе «суша-море» он может в дальнейшем накапливаться в донных осадках в шельфовой или глубоководной зоне и подвергаться деградации и реминерализации, что приводит к критическим экологическим последствиям. Цель: установление источников и степени диагенетической преобразованности терригенного органического вещества в поверхностных осадках морей Восточной Арктики. Объектом исследования послужили пробы донных осадков, взятые с поверхностного горизонта (0-10 см). Отбор проб проводился в морских исследовательских экспедициях 2011-2019 гг. Результаты. Латеральная выдержанность значений водородного индекса в современных осадках в море Лаптевых связана с вкладом гетерогенного наземного органического вещества, характеризующегося относительной биогеохимической доступностью: водородонасыщенное наземное органическое вещество сменяется морским с сохранением величины водородного индекса. Это отличает море Лаптевых от других арктических акваторий, где по мере удаления от берега отмечался устойчивый рост значения водородного индекса в связи с усилением вклада автохтонного органического вещества. В соотношении параметров δ13C и HI/OI наблюдаются значительные отклонения от линейной зависимости, характерной для консервативного геохимического режима морских акваторий: терригенный материал в осадках губы Буор-Хая характеризуется легким изотопным составом δ13C и повышенными отношением HI/OI, нетипичным для наземного генезиса. Для органического вещества, накопленного в глубоководных осадках континентального склона, напротив, отмечается низкое содержание водорода и высокая доля кислородсодержащих соединений, свидетельствующих о значительной степени диагенетической преобразованности органического вещества.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 oxygencontaining 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