Subsea permafrost organic carbon stocks are large and of dominantly low reactivity

Subsea permafrost carbon pools below the Arctic shelf seas are a major unknown in the global carbon cycle. We combine a numerical model of sedimentation and permafrost evolution with simplified carbon turnover to estimate accumulation and microbial decomposition of organic matter on the pan-Arctic shelf over the past four glacial cycles. We find that Arctic shelf permafrost is a globally important long-term carbon sink storing 2822 (1518-4982) Pg OC, double the amount stored in lowland permafrost. Although currently thawing, prior microbial decomposition and organic matter aging limit decomposition rates to less than 48 Tg OC/yr (25-85) constraining emissions due to thaw and suggesting that the large permafrost shelf carbon pool is largely insensitive to thaw. We identify an urgent need to reduce uncertainty in rates of microbial decomposition of organic matter in cold and saline subaquatic environments. Large emissions of methane more likely derive from older and deeper sources than from organic matter in thawing permafrost.

Numerical Simulation of Coastal Sub-Permafrost Gas Hydrate Formation in the Mackenzie Delta, Canadian Arctic

The Mackenzie Delta (MD) is a permafrost-bearing region along the coasts of the Canadian Arctic which exhibits high sub-permafrost gas hydrate (GH) reserves. The GH occurring at the Mallik site in the MD is dominated by thermogenic methane (CH4), which migrated from deep conventional hydrocarbon reservoirs, very likely through the present fault systems. Therefore, it is assumed that fluid flow transports dissolved CH4 upward and out of the deeper overpressurized reservoirs via the existing polygonal fault system and then forms the GH accumulations in the Kugmallit–Mackenzie Bay Sequences. We investigate the feasibility of this mechanism with a thermo–hydraulic–chemical numerical model, representing a cross section of the Mallik site. We present the first simulations that consider permafrost formation and thawing, as well as the formation of GH accumulations sourced from the upward migrating CH4-rich formation fluid. The simulation results show that temperature distribution, as well as the thickness and base of the ice-bearing permafrost are consistent with corresponding field observations. The primary driver for the spatial GH distribution is the permeability of the host sediments. Thus, the hypothesis on GH formation by dissolved CH4 originating from deeper geological reservoirs is successfully validated. Furthermore, our results demonstrate that the permafrost has been substantially heated to 0.8–1.3 °C, triggered by the global temperature increase of about 0.44 °C and further enhanced by the Arctic Amplification effect at the Mallik site from the early 1970s to the mid-2000s.

Permafrost and Gas Hydrate Stability Zone of the Glacial Part of the East-Siberian Shelf

By using thermal mathematical modeling for the time range of 200,000 years ago, the authors have been studying the role the glaciation, covered the De Long Islands and partly the Anjou Islands at the end of Middle Neopleistocene, played in the formation of permafrost and gas hydrates stability zone. For the modeling purpose, we used actual geological borehole cross-sections from the New Siberia Island. The modeling was conducted at geothermal flux densities of 50, 60, and 75 mW/m2 for glacial and extraglacial conditions. Based on the modeling results, the glaciated area is characterized by permafrost thickness of 150–200 m lower than under extraglacial conditions. The lower boundary of the gas hydrate stability zone in the glacial area at 50–60 mW/m2 is located 300 m higher than the same under extraglacial conditions. At 75 mW/m2 in the area of 20–40 m isobaths, open taliks are formed, and the gas hydrate stability zone was destroyed in the middle of the Holocene. The specified conditions and events were being formed in the course of the historical development of the glacial area with a predominance of the marine conditions peculiar to it from the middle of the Middle Neopleistocene.

Evidence of Gas Emissions from Permafrost in the Russian Arctic

The active emission of gas (mainly methane) from terrestrial and subsea permafrost in the Russian Arctic has been confirmed by ample evidence. In this paper, a generalization and some systematization of gas manifestations recorded in the Russian Arctic is carried out. The published data on most typical gas emission cases have been summarized in a table and illustrated by a map. The tabulated data include location, signatures, and possible sources of each gas show, with respective references. All events of onshore and shelf gas release are divided into natural and man-caused. and the natural ones are further classified as venting from lakes or explosive emissions in dryland conditions that produce craters on the surface. Among natural gas shows on land, special attention is paid to the emission of natural gas from Arctic lakes, as well as gas emissions with craters formation. In addition, a description of the observed man-caused gas manifestations associated with the drilling of geotechnical and production wells in the Arctic region is given. The reported evidence demonstrates the effect of permafrost degradation on gas release, especially in oil and gas fields.

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.

Formation and Accumulation of Pore Methane Hydrates in Permafrost: Experimental Modeling

Favorable thermobaric conditions of hydrate formation and the significant accumulation of methane, ice, and actual data on the presence of gas hydrates in permafrost suggest the possibility of their formation in the pore space of frozen soils at negative temperatures. In addition, today there are several geological models that involve the formation of gas hydrate accumulations in permafrost. To confirm the literature data, the formation of gas hydrates in permafrost saturated with methane has been studied experimentally using natural artificially frozen in the laboratory sand and silt samples, on a specially designed system at temperatures from 0 to −8 °C. The experimental results confirm that pore methane hydrates can form in gas-bearing frozen soils. The kinetics of gas hydrate accumulation in frozen soils was investigated in terms of dependence on the temperature, excess pressure, initial ice content, salinity, and type of soil. The process of hydrate formation in soil samples in time with falling temperature from +2 °C to −8 °C slows down. The fraction of pore ice converted to hydrate increased as the gas pressure exceeded the equilibrium. The optimal ice saturation values (45−65%) at which hydrate accumulation in the porous media is highest were found. The hydrate accumulation is slower in finer-grained sediments and saline soils. The several geological models are presented to substantiate the processes of natural hydrate formation in permafrost at negative temperatures.

Shallow carbon storage in ancient buried thermokarst in the South Kara Sea

Geophysical data from the South Kara Sea reveal U-shaped erosional structures buried beneath the 50–250 m deep seafloor of the continental shelf across an area of ~32 000 km². These structures are interpreted as thermokarst, formed in ancient yedoma terrains during Quaternary interglacial periods. Based on comparison to modern yedoma terrains, we suggest that these thermokarst features could have stored approximately 0.5 to 8 Gt carbon during past climate warmings. In the deeper parts of the South Kara Sea (>220 m water depth) the paleo thermokarst structures lie within the present day gas hydrate stability zone, with low bottom water temperatures −1.8 ^oC) keeping the gas hydrate system in equilibrium. These thermokarst structures and their carbon reservoirs remain stable beneath a Quaternary sediment blanket, yet are potentially sensitive to future Arctic climate changes.

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records

Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ13CH4 and δD(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.

Spatial and seasonal variability of CH4 in the eastern Gulf of Cadiz (SW Iberian Peninsula)

Methane (CH₄) concentrations were measured along three sections of the eastern Gulf of Cadiz (designated "Guadalquivir", "Sancti Petri" and "Trafalgar") during eight cruises in 2014 and 2015. The concentration of CH₄ varied from 3.6 to 19.7nmolkg^-1 (CH₄ saturation percent of 122 and 916%), showing seasonal variation. The highest values were found in December 2014 and November 2015. In most of the sampling area the highest concentration of CH₄ was found in subsurface waters at depths close to the thermocline, and in the bottom waters near the coast. The seawater-air flux of CH₄ ranged between 0.8 and 59.7μmolm^-2d^-1, showing seasonal variation in function of the temperature of the surface water. In the "Guadalquivir" and "Sancti Petri" sections, the CH₄ fluxes increased with proximity to the coast; this may be a result of continental inputs and CH₄ emissions from sediments. The whole study area behaves as a source of CH₄ to the atmosphere with mean values of 0.5 and 0.6GgCH₄yr^-1 in 2014 and 2015, respectively.

A synthesis of the arctic terrestrial and marine carbon cycles under pressure from a dwindling cryosphere

The current downturn of the arctic cryosphere, such as the strong loss of sea ice, melting of ice sheets and glaciers, and permafrost thaw, affects the marine and terrestrial carbon cycles in numerous interconnected ways. Nonetheless, processes in the ocean and on land have been too often considered in isolation while it has become increasingly clear that the two environments are strongly connected: Sea ice decline is one of the main causes of the rapid warming of the Arctic, and the flow of carbon from rivers into the Arctic Ocean affects marine processes and the air-sea exchange of CO₂. This review, therefore, provides an overview of the current state of knowledge of the arctic terrestrial and marine carbon cycle, connections in between, and how this complex system is affected by climate change and a declining cryosphere. Ultimately, better knowledge of biogeochemical processes combined with improved model representations of ocean-land interactions are essential to accurately predict the development of arctic ecosystems and associated climate feedbacks.

Massive marine methane emissions from near-shore shallow coastal areas

Methane is the second most important greenhouse gas contributing to climate warming. The open ocean is a minor source of methane to the atmosphere. We report intense methane emissions from the near-shore southern region of the North Sea characterized by the presence of extensive areas with gassy sediments. The average flux intensities (~130 μmol m⁻² d⁻¹) are one order of magnitude higher than values characteristic of continental shelves (~30 μmol m⁻² d⁻¹) and three orders of magnitude higher than values characteristic of the open ocean (~0.4 μmol m⁻² d⁻¹). The high methane concentrations (up to 1,128 nmol L⁻¹) that sustain these fluxes are related to the shallow and well-mixed water column that allows an efficient transfer of methane from the seafloor to surface waters. This differs from deeper and stratified seep areas where there is a large decrease of methane between bottom and surface by microbial oxidation or physical transport. Shallow well-mixed continental shelves represent about 33% of the total continental shelf area, so that marine coastal methane emissions are probably under-estimated. Near-shore and shallow seep areas are hot spots of methane emission, and our data also suggest that emissions could increase in response to warming of surface waters.

Review of methane mitigation technologies with application to rapid release of methane from the Arctic

Methane is the most important greenhouse gas after carbon dioxide, with particular influence on near-term climate change. It poses increasing risk in the future from both direct anthropogenic sources and potential rapid release from the Arctic. A range of mitigation (emissions control) technologies have been developed for anthropogenic sources that can be developed for further application, including to Arctic sources. Significant gaps in understanding remain of the mechanisms, magnitude, and likelihood of rapid methane release from the Arctic. Methane may be released by several pathways, including lakes, wetlands, and oceans, and may be either uniform over large areas or concentrated in patches. Across Arctic sources, bubbles originating in the sediment are the most important mechanism for methane to reach the atmosphere. Most known technologies operate on confined gas streams of 0.1% methane or more, and may be applicable to limited Arctic sources where methane is concentrated in pockets. However, some mitigation strategies developed for rice paddies and agricultural soils are promising for Arctic wetlands and thawing permafrost. Other mitigation strategies specific to the Arctic have been proposed but have yet to be studied. Overall, we identify four avenues of research and development that can serve the dual purposes of addressing current methane sources and potential Arctic sources: (1) methane release detection and quantification, (2) mitigation units for small and remote methane streams, (3) mitigation methods for dilute (<1000 ppm) methane streams, and (4) understanding methanotroph and methanogen ecology.