Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure. The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects. In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.

Fundamental principles and applications of natural gas hydrates

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain. © 2013 Macmillan Publishers Limited.

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Three decades of global methane sources and sinks

Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.

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Abrupt climate change

Large amounts of methane are produced in marine sediments but are then consumed before contacting aerobic waters or the atmosphere1. Although no organism that can consume methane anaerobically has ever been isolated, biogeochemical evidence indicates that the overall process involves a transfer of electrons from methane to sulphate and is probably mediated by several organisms, including a methanogen (operating in reverse) and a sulphate-reducer (using an unknown intermediate substrate)2. Here we describe studies of sediments related to a decomposing methane hydrate. These provide strong evidence that methane is being consumed by archaebacteria that are phylogenetically distinct from known methanogens. Specifically, lipid biomarkers that are commonly characteristic of archaea are so strongly depleted in carbon-13 that methane must be the carbon source, rather than the metabolic product, for the organisms that have produced them. Parallel gene surveys of small-subunit ribosomal RNA (16S rRNA) indicate the predominance of a new archael group which is peripherally related to the methanogenic orders Methanomicrobiales and Methanosarcinales.

Methane-consuming archaebacteria in marine sediments

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).

For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters.

Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning.

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The global methane budget 2000–2017

Previous studies of deep-sea sediment cores have found evidence for Milankovitch cycles, climatic cyclicity due to the periodicity of the Earth's orbital parameters. Many of the cores recovered on Leg 114 of the Ocean Drilling Program showed outward signs of cyclicity, especially at Site 704. We have analyzed the GRAPE density, carbonate content, and magnetic susceptibility using both standard and nonstandard spectral analysis techniques. One of the nonstandard techniques used was the Lomb-Scargle spectral estimation method, which is designed for unequally spaced data and which yields as part of the process the statistical significance of any observed spectral peaks. Pairs of spectra were compared for statistical similarity using the Kolmogorov-Smirnov method. All of the data sets contain some spectral peaks, including both the expected Milankovitch cycles as well as other peaks. Upon further investigation, we have found that the other peaks could be explained as the nonlinear climate system response to the Milankovitch orbital forcing functions, because the extra peaks appear to be simple linear combinations, harmonics and subharmonics of the Milankovitch peaks. At Site 704, there is a marked change in the response at the Brunhes/Matuyama boundary (0.73 Ma B.P.) from strong long-period cyclicity in the Brunhes to more prevalent shorter period cyclicity in the Matuyama. INTRODUCTION The hydrosphere of the Earth is complex and inherently nonlinear. The Navier-Stokes equations and the associated thermodynamic equations form a coupled highly nonlinear set of governing equations for atmospheric and oceanic flow. The climate of the Earth represents a long-term average of the parameters of the nonlinear system and thus should also exhibit nonlinearity. Nonetheless, numerous researchers have sought simple (linear) mechanisms to explain major climatic variations. The most prominent such mechanism recently proposed for driving climatic variations has been the effect of the periodicity in the Earth's orbital parameters, Milankovitch cycles (see, for example, Berger et al., 1984). The analysis of deep-sea drill core and logging data has naturally expanded to include spectral analysis for the purposes of identifying Milankovitch cycle peaks, and numerous papers have found evidence for such peaks (see Berger et al. 1984; Ruddiman et al., 1986). In many cases extraneous peaks have been found, for example, by Pestiaux and Berger (1984) and Ruddiman et al., (1986). Figure 1 (from Borehole Research Group, 1986) shows results from Ocean Drilling Program (ODP) Site 646; Milankovitch peaks are identified at 19,000-23,000, 41,000, 95,000, and 410,000 yr. The presence of other peaks, however, can be explained as linear combinations of the main peaks. We 1 Ciesielski, P. F., Kristoffersen, Y., et al., 1991. Proc. ODP, Sci. Results, 114: College Station, TX (Ocean Drilling Program). 2 Department of Earth Sciences, Department of Physics, and Quaternary Sciences Institute, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. 3 Department of Physics and Quaternary Sciences Institute, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1. GEOMAR, Forschungszentrum der Christian-Albrechts-Universitàt zu Kiel D-2300 Kiel, FRG. Alfred Wegener Institut für Polar und Meeresforschung, Postfach 120161, D-2850 Bremerhaven (Present address: Université de Nantes, Laboratoire de Sédimentologie Sciences de la Terre, 2 Rue de la Houssiniere, 44072 Nantes Cedex, France). believe that these extraneous peaks are evidence for the nonlinear nature, of the climatic system (Bloomer and Nobes, 1989; Bloomer, 1989). The prominent spectral power at 100,000 yr found in most ocean core and logging data spectra is another compelling piece of evidence for the nonlinear nature of climatic response. Because there is no primary power at the eccentricity (100,000 yr) cycle in the expansions of theoretical solar insolation (the eccentricity term acts only to modulate the precessional cycles), the high power in paleoclimatic spectra associated with the 100,000-yr cycle has been attributed to the effect of ice sheets (Pollard, 1982; Le Treut and Ghü, 1983) or some other nonlinear mechanism. To demonstrate how 100,000-yr power can arise in paleoclimatic spectra, Wigley (1976) considered an amplitudemodulated periodic signal of the form F(t) = ßsin{2f2i))sin{2fλt), (1) where fλ is the frequency of the basic signal and f2 is the modulator frequency. This is analogous to the insolation signal, where the 100,000-yr cycle acts as a modulator of the precessional cycles at 23,000 and 19,000 yr. In the plot of F(i) in Figure 2A,/i = 1/20,000 cycles/yr,/2 = 1/100,000 cycles/yr, and ß = 0.4. Spectral analysis of this signal yields a major peak at/i, minor peaks/j ± f2 (16,000 and 25,000 yr), and no peak at the modulator frequency, f2 (Fig. 2B). If the output is related to the input by the simple nonlinear response model Z{t) = (F(t)), (2) the spectrum of Z(i) has a dominant peak at the modulator frequency, with a series of peaks centered at 2/t (10,000 yr) and a peak at 2/2 (50,000 yr) (Fig. 2C). This demonstrates how primary output power can arise from relatively minor input power and how combination tones can arise. We have analyzed data from sediment cores of Quaternary age recovered on ODP Leg 114. The Leg 114 sites form a transect across the South Atlantic (Figs. 3 and 4; Shipboard

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Milankovitch Cycles and Nonlinear Response in the Quaternary Record in the Atlantic Sector of the Southern Oceans

Extensional rise and fall of a salt diapir in the Sørvestsnaget Basin, SW Barents Sea

Two-dimensional seismic data from the Mid-Norwegian margin provide evidence for sediment liquefaction and fluid mobilisation within the sediments that were located at the base of the hydrate stability zone before the Storegga Slide occurred. The disturbed subsurface sediments are overlain by a prominent roll-over structure and sea-floor collapse. This indicates fluid escape from the formerly hydrated sediment and suggests that the landslide caused a pressure drop strong enough to dissociate, the gas hydrates. We calculate that this fluid escape must have taken place within less than 250 years after the slide, as the effect of pressure decrease on hydrate stability was later compensated by a temperature decrease, related to the slumping process. The volume of expelled fluids from the collapse structure exceeds the volume of the gas hydrate dissociation products, implying that gas hydrate dissociation significantly affected the surrounding sediments.

Gas hydrate dissociation and seafloor collapse in the wake of the Storegga Slide, Norway

A number of reefs are found along the coast of northern Norway, and a cluster of particularly high reefs off Troms County at 70°N are known collectively as the Fugloy Reef. The reefs, up to 40 m high and more than 200 m wide, consist mainly of the reef-building Lophelia pertusa. Most of the reefs identified in the study area are located on moraine ridges in water depths of 140–190 m, and in water masses dominated by the relatively warm and saline Norwegian Current. Several of the reefs are located on the flanks of channels incising the moraine highs, where currents are tidally dominated and periodically reach velocities of 30 cm/s. Gravity cores were acquired from the reefs and their surroundings, and thorough analyses of the sampled sediments provide valuable information about three (paleo-) sedimentary environments surrounding the reefs. The immediate vicinity of the reefs consists of coarse and unsorted deposits that are interpreted to be moraine material deposited by the retreating inland ice. Elevated current velocities have prevented fine sediments from settling since the ice retreated. The second province is a pockmarked basin at water depths down to ∼300 m. Gravity cores from the basin reveal silty sand deposits of more than 4 m thickness representing postglacial sedimentation in the area. Gas analyses reveal that the hydrocarbons found in the sediments clearly are of biogenic origin, although it is somewhat enigmatic whether biogenic gas is the sole driving force behind the pockmarks in the area. No direct link between the reefs and the pockmarks is found. The third sedimentary province is characterised by resedimentation of coral debris, clearly illustrated by sorted deposits and U/Th-datings from the allochthonous deposits. Remobilisation of coral debris is modest in areal extent, but an important mechanism linked to the occurrence of the coral reefs.

Sedimentological and geochemical environment of the Fugløy Reef off northern Norway

The southwestern Barents Sea, part of the epicontinental Barents Sea located between Norway and the Svalbard archipelago, hosts several mature petroleum systems. Large gas chimneys, leaking faults and seabed pockmarks are proof that hydrocarbon leakage has taken place previously in this part of the Barents Sea. Glacial erosion, being one of the major episodes of erosion and uplift, has probably caused fluid leakage out of proven hydrocarbon reservoirs and fluid migration along permeable carrier beds. We use 3D seismic data from the Snohvit gas field to focus on the nature of the fluid transport systems and the origins and potential mechanisms for how fluids migrated from deep-seated reservoirs. Mapping of the trap of the reservoirs at Snohvit has allowed us to determine potential spill point locations, coinciding with major gas chimneys and several faults that extend into shallow strata. We explain how uplift and erosion caused gas expansion in the reservoirs allowing the gas to reach below the spill point located at the gas–water contact level at the bottom of the closure. Fluids, such as hydrocarbons, may have escaped through these spills points, leading to the emplacement and development of the aforementioned gas chimneys. Similarly, migration of fluids may have taken place along faults, reactivated by glaciotectonic processes and connected with spill points at reservoir depth. Both gas chimneys and faults have allowed gas to migrate into and be trapped in shallow gas accumulations within the Torsk Formation and below the upper regional unconformity.

https://link.springer.com/content/pdf/10.1007/s41063-016-0026-z.pdf

Jürgen Mienert

Papers

Milankovitch Cycles and Nonlinear Response in the Quaternary Record in the Atlantic Sector of the Southern Oceans

Extensional rise and fall of a salt diapir in the Sørvestsnaget Basin, SW Barents Sea

Gas hydrate dissociation and seafloor collapse in the wake of the Storegga Slide, Norway

Sedimentological and geochemical environment of the Fugløy Reef off northern Norway

Mechanisms initiating fluid migration at Snøhvit and Albatross fields, Barents Sea