Methane Transport Research Articles

Meteorological insights from planetary heat flow measurements

Planetary heat flow measurements are made with a series of high-precision temperature sensors deployed in a column of regolith to determine the geothermal gradient. Such sensors may, however, be susceptible to other influences, especially on worlds with atmospheres. First, pressure fluctuations at the surface may pump air in and out of pore space leading to observable, and otherwise unexpected, temperature fluctuations at depth. Such pumping is important in subsurface radon and methane transport on Earth: evidence of such pumping may inform understanding of methane or water vapor transport on Mars. Second, the subsurface profile contains a muted record of surface temperature history, and such measurements on other worlds may help constrain the extent to which Earth’s Little Ice Age was directly solar-forced, versus volcanic-driven and/or amplified by climate feedbacks.

Gross photosynthesis explains the ‘artificial bias’ of methane fluxes by static chamber (opaque versus transparent) at the hummocks in a boreal peatland

The closed chamber technique has been widely employed to detect methane emissions, despite little being known about whether the absence or presence of light will impact the flux estimation. Here, we employed a laser greenhouse gas analyzer with an opaque—transparent chamber pair to measure the methane emission rate in a boreal peatland complex. Microtopography (i.e., hummocks and hollows) in natural and drained peatlands, and plant communities (i.e., grasses and shrubs) in a pasture converted from natural peatlands, were considered to cover the local heterogeneity. Our results indicated that opaque chambers (0.58–0.78 g CH4 m−2 during the growing season) measured a significantly higher (∼2–3 times) methane emission at the hummocks than transparent chambers (∼0.24 g CH4 m−2); however, a similar phenomenon was not found at the hollows or at other measurement plots. Gross photosynthesis explained 44%–47% of the temporal variation of the ‘artificial bias’ (the difference in methane flux obtained by the opaque versus transparent chambers) at the hummocks. Additionally, both water table depth and surface soil moisture significantly explained spatial variations of methane emissions. Our study suggests that microtopography has a significant influence on the artificial bias in methane emission estimation and the artificial properties of a chamber (transparency/opacity) method can be vitally important in some cases (i.e., hummocks), and negligible in others (i.e., hollows). The observed connection between the photosynthesis process and the ‘artificial bias’ of closed chambers (opaque versus transparent) can be used to improve methane flux modeling. Separate parameterization schemes are needed for methane transportation under the presence or absence of light.

Carbon isotope effects of methane transport through Anahuac Shale — A core gas study

Compositional data and stable isotope ratios are critical datasets for the study of hydrocarbon generation, expulsion, and migration. These geochemical parameters are also important for studies of leakage detection for geological carbon sequestration. Diffusion, gas–liquid partitioning and adsorption, the three most crucial processes to affect gas transport through shale and other low-permeability formations, are known to cause stable isotope fractionation. A considerable number of studies have been conducted on stable isotope fractionation associated with diffusion and gas–liquid partitioning. However, significantly fewer data are reported in the literature that specifically addresses adsorption carbon isotope effects. Moreover, adsorption isotope effects are rarely verified or seldom demonstrated in geologic systems. In this study, a shale core taken from the Anahuac Formation in Texas Gulf Coast, which overlies several Frio reservoirs, was analyzed for composition and stable isotope ratios of the residual gas. These geochemical parameters are used to identify gas migration and associate transport mechanisms in the caprock formation. Gas samples extracted from the core contain alkane gas components (C1–C5) and show a systematic variation in carbon isotope ratios of methane along depth. An analytical model is proposed to interpret the observed carbon isotope trend on the basis of coupled processes of diffusion and adsorption. At a ratio of retardation of 0.99 between 12CH4 and 13CH4 (with 12CH4 having a preferential bonding with the clay mineral) and a diffusion time of 1.5Ma, the modeled curve fits well with observed upward increasing δ13CCH4 values. This study demonstrates the isotope effect of adsorption in a natural system and its importance in quantifying gas-migration rate and distance in low-permeability systems.

Can Guest Occupancy in Binary Clathrate Hydrates Be Tuned through Control of the Growth Temperature?

Clathrate hydrates are nonstoichiometric compounds comprised of a hydrogen-bonded water network that forms polyhedral cages that can be occupied by small guest molecules. Clathrates are candidate materials for storage and transportation of methane and H2. Promoter molecules, such as THF, reduce the pressure or temperature needed to form clathrates of these gases, resulting in the formation of binary clathrates with the promoter molecule hosted in the large cages of the hydrate. In this work, we study the growth and occupancy of binary clathrates as a function of supercooling of the solution using molecular dynamics simulations with the mW water model and small and large guest molecules of sizes similar to those of H2 and THF, respectively, but that are both highly soluble in water and produce single hydrates with identical melting point. The large guest molecules only fit into the 51264 cages, while the small guest molecules can fit into both types of cages. We find that the large guest act as a kinetic promoter for growth, increasing the rate of uptake of small guests into the clathrate. Our results also indicate that the growth of binary clathrates is limited by the arrangement of guest molecules in the large 51264 cages at the clathrate/solution interface. The occupancy of large cages of binary clathrates can be tuned by varying the growth temperature. The simulations indicate that with increasing supercooling there is an increase in the percentage of 51264 cages occupied by the small guest molecules at the expense of the large guest molecules, while the occupancy of 512 cages remains relatively constant. The results of this work show that the composition of clathrates grown at high driving force does not necessarily reflect the composition of the most stable phase.

STRUCTURE AND DYNAMICS OF COLD WATER SUPER-EARTHS: THE CASE OF OCCLUDED CH4AND ITS OUTGASSING

We study the transport of methane in the external water envelopes surrounding water-rich super-Earths and estimate its outgassing into the atmosphere. We investigate the influence of methane on the thermodynamics and mechanics of the water mantle. We find that including methane in the water matrix introduces a new phase (filled ice) resulting in hotter planetary interiors. This effect renders the super-ionic and reticulating phases accessible to relatively low mass planets lacking a H/He atmosphere. We model the thermal and structural profile of the planetary crust and discuss five possible crustal regimes. The formation of methane clathrate in the subsurface is shown to inhibit the formation of a subterranean ocean. This effect results in increased stresses on the lithosphere making modes of ice plate tectonics possible. The dynamics of the tectonic plates are analysed. We derive overturn and resurfacing time scales as well as the melt fraction underneath spreading centers. Ice mantle dynamics is found to be important for assessing the composition of the atmosphere. We formulate the relation between the outgassing flux of methane and the tectonic mode dynamics. We give numerical estimates for the global outgassing rate of methane into the atmosphere.

Seasonal and spatial methane dynamics in the water column of the central Baltic Sea (Gotland Sea)

The influence of hydrodynamic events on the distribution of methane and its microbial turnover was investigated during the period from August 2011 to August 2013 along a transect from the eastern (EGB) to the western Gotland Basin (WGB), central Baltic Sea. The water column was characterized by a pronounced methane concentration gradient between the methane-rich deep anoxic and the methane-poor upper oxic water layer. In both basins, enhanced vertical turbulent diffusivities in fall (November 2011) and winter (February 2012) lead to an enhanced flux of methane from the deep anoxic water towards the oxic–anoxic transition zone (redox zone). In both basins, the increased vertical transport of methane in fall/winter was mirrored by reduced methane turnover times measured within the redox zone. Moreover, specific biomarkers indicative for aerobic methanotrophic bacteria implied an increase in the microbial population size from August 2011 till February 2012, indicating a methanotrophic community adapting to the variable methane fluxes. The deep water methane inventory of the EGB showed a seasonal pattern, with concentrations increasing during spring (May) and summer (August) and decreasing during fall (November) and winter (February) as a direct result of the seasonality of the vertical turbulent diffusivity. In contrast, the WGB showed no clear correlation between the seasons and the observed deep water methane variability. Here, the impact of lateral weak intrusions penetrating the deep water layer was identified as the main factor controlling the variability of the deep water methane concentration.Moreover, methane concentration and carbon stable isotopic data (δ13C CH4) demonstrate that the previously reported production of methane in the oxic water column below the thermocline occurs in the entire central Baltic Sea from May through November, and despite the large methane pool in the underlying anoxic deep water, might govern the moderate methane flux to the atmosphere in this area in summer.

First evidence of widespread active methane seepage in the Southern Ocean, off the sub-Antarctic island of South Georgia

An extensive submarine cold-seep area was discovered on the northern shelf of South Georgia during R/V Polarstern cruise ANT-XXIX/4 in spring 2013. Hydroacoustic surveys documented the presence of 133 gas bubble emissions, which were restricted to glacially-formed fjords and troughs. Video-based sea floor observations confirmed the sea floor origin of the gas emissions and spatially related microbial mats. Effective methane transport from these emissions into the hydrosphere was proven by relative enrichments of dissolved methane in near-bottom waters. Stable carbon isotopic signatures pointed to a predominant microbial methane formation, presumably based on high organic matter sedimentation in this region. Although known from many continental margins in the world's oceans, this is the first report of an active area of methane seepage in the Southern Ocean. Our finding of substantial methane emission related to a trough and fjord system, a topographical setting that exists commonly in glacially-affected areas, opens up the possibility that methane seepage is a more widespread phenomenon in polar and sub-polar regions than previously thought.

Lattice Boltzmann simulation of shale gas transport in organic nano-pores.

Permeability is a key parameter for investigating the flow ability of sedimentary rocks. The conventional model for calculating permeability is derived from Darcy's law, which is valid only for continuum flow in porous rocks. We discussed the feasibility of simulating methane transport characteristics in the organic nano-pores of shale through the Lattice Boltzmann method (LBM). As a first attempt, the effects of high Knudsen number and the associated slip flow are considered, whereas the effect of adsorption in the capillary tube is left for future work. Simulation results show that at small Knudsen number, LBM results agree well with Poiseuille's law, and flow rate (flow capacity) is proportional to the square of the pore scale. At higher Knudsen numbers, the relaxation time needs to be corrected. In addition, velocity increases as the slip effect causes non negligible velocities on the pore wall, thereby enhancing the flow rate inside the pore, i.e., the permeability. Therefore, the LBM simulation of gas flow characteristics in organic nano-pores provides an effective way of evaluating the permeability of gas-bearing shale.

Analytical modeling of pressure transient behavior for coalbed methane transport in anisotropic media

Resulting from the nature of anisotropy of coal media, it is a meaningful work to evaluate pressure transient behavior and flow characteristics within coals. In this article, a complete analytical model called the elliptical flow model is established by combining the theory of elliptical flow in anisotropic media and Fick's laws about the diffusion of coalbed methane. To investigate pressure transient behavior, analytical solutions were first obtained through introducing a series of special functions (Mathieu functions), which are extremely complex and are hard to calculate. Thus, a computer program was developed to establish type curves, on which the effects of the parameters, including anisotropy coefficient, storage coefficient, transfer coefficient and rate constant, were analyzed in detail. Calculative results show that the existence of anisotropy would cause great pressure depletion. To validate new analytical solutions, previous results were used to compare with the new results. It is found that a better agreement between the solutions obtained in this work and the literature was achieved. Finally, a case study is used to explain the effects of the parameters, including rock total compressibility coefficient, coal medium porosity and anisotropic permeability, sorption time constant, Langmuir volume and fluid viscosity, on bottom-hole pressure behavior. It is necessary to coordinate these parameters so as to reduce the pressure depletion.

The role of vegetation in methane flux to the atmosphere: should vegetation be included as a distinct category in the global methane budget?

Currently, the global annual flux of methane (CH4) to the atmosphere is fairly well constrained at ca. 645 Tg CH4 year−1. However, the relative magnitudes of the fluxes generated from different natural (e.g. wetlands, deep seepage, hydrates, ocean sediments) and anthropogenic sources remain poorly resolved. Of the identified natural sources, the contribution of vegetation to the global methane budget is arguably the least well understood. Historically, reviews of the contribution of vegetation to the global methane flux have focused on the role of plants as conduits for soil-borne methane emissions from wetlands, or the aerobic production of methane within plant tissues. Many recent global budgets only include the latter pathway (aerobic methane production) in estimating the importance of terrestrial vegetation to atmospheric CH4 flux. However, recent experimental evidence suggests several novel pathways through which vegetation can contribute to the flux of this globally important, trace greenhouse gas (GHG), such as plant cisterns that act as cryptic wetlands, heartwood rot in trees, the degradation of coarse woody debris and litter, or methane transport through herbaceous and woody plants. Herein, we synthesize the existing literature to provide a comprehensive estimate of the role of modern vegetation in the global methane budget. This first, albeit uncertain, estimate indicates that vegetation may represent up to 22 % of the annual flux of methane to the atmosphere, contributing ca. 32–143 Tg CH4 year−1 to the global flux of this important trace GHG. Overall, our findings emphasize the need to better resolve the role of vegetation in the biogeochemical cycling of methane as an important component of closing the gap in the global methane budget.

Modeling Methane Emissions from Amazon Floodplain Ecosystems

A new process-based simulation model to estimate methane emissions from Amazon floodplain ecosystems is described and evaluated in comparison to independent measurements of methane fluxes. The model’s three major components are 1) types of wetland vegetation and the changes in water level, temperature and dissolved oxygen of flooded areas, 2) plant production, biomass accumulation, and litterfall decay in soils and sediments, and 3) methane production and transport pathways through the water column and into the atmosphere. Ecological and limnological data from Lake Calado, a well-studied site in the central Amazon basin, were used to develop the model. One set of model simulations were generated for floating macrophytes. Predicted rates of CH4 emission to the atmosphere by all simulated transfer pathways were typically in the range of 0.25 to 0.33 g C m−2 day−1. Simulated CH4 emissions from flooded forests were predicted to be around 0.25 g m−2 day−1, nearly all by ebullition. These rates compare favorably to rates measured in Amazon floodplain habitats.

Transformation of methane in peatland environments

Abstract Wetlands and particularly peatlands are the main natural source of methane. Data indicate that 10-45% of methane emission comes from these sources. Methane emission from wetlands is the result of the balance between methanogenesis and methanotrophic processes and is actively affected by the wetland plant community composition. There are many factors affecting the balance of CH4: for instance, vegetation has a strong effect on CH4 emissions from wetland ecosystems by influencing methane production, consumption and transport in the soil. The effects of plants on methane fluxes may be mediated by: molecular diffusion, internal transport through plant aerenchyma tissues and ebullition. Methane is formed in the process of methanogenesis under anaerobic conditions. It may then be emitted into the atmosphere directly from the soil or by internal transport through the plant. Alternatively, it may undergo methane oxidation by methanotrophic bacteria, both free-living in the root zone and associated with the host plant in symbiosis. Sphagnum moss is of particular importance for this processes as it contains methanotrophic bacteria in its endophytic system. Methanotrophic bacteria live inside the dead hyaline cells or on the surface of Sphagnum leaves and are able to oxidise methane produced in the soil during methanogenesis, making peatlands a natural biofilter for methane, one of the main greenhouse gases

Effects of Thermal Treatment and Physical Aging on the Gas Transport Properties in Matrimid®

Carbon dioxide and methane transport in a commercial polyimide, Matrimid 5218<sup>®<sup/>, has been characterized in order to evaluate the effect of membrane thermal treatment and physical aging on its potentialities for CO<sub>2<sub/>/CH<sub>4<sub/> separation. In particular, CO<sub>2<sub/> and CH<sub>4<sub/> permeabilities and diffusion coefficients were measured at three different temperatures (35, 45 and 55°C) in films pretreated at 50, 100, 150 and 200°C, respectively. The performances of each sample were examined for a period of more than 3 000 hours. Permeability and diffusivity values for both penetrants showed a marked decrease with increasing the pretreatment temperature up to 150°C and remained, then, substantially unchanged for specimens pretreated at the highest temperature. Interestingly, the samples characterized by the higher flux after film formation also showed a faster aging phenomenon, leading to a 25 % decrease of CO<sub>2<sub/> permeability in the period inspected. Conversely, the samples pretreated at temperatures of 150°C, or higher, displayed very stable transport properties for the entire duration of the monitoring campaign. At the end of the aging period considered, the differences among samples were definitely reduced, suggesting that the initial behaviors are related to different polymer structures induced by pretreatment, which slowly evolve in time towards more similar and more stable configurations. Such aging rearrangements affect both CO<sub>2<sub/> and CH<sub>4<sub/> permeability in a similar way, so that no significant changes were observed for selectivity, which showed only a slight increment by increasing the temperature of the thermal treatment or the duration of the aging period.

Plant-mediated transport and isotopic composition of methane from shallow tropical wetlands

The methane (CH4) content of gas bubbles in sediments and CH4 emission from plants in the Pantanal wetland of Brazil was examined across a depth gradient to ascertain the relative importance of plant-mediated CH4 emission versus ebullition. Sediment bubbles contained >50% CH4 at sites with water depths >1 m but often contained <50% CH4 in shallower waters. Previous work has shown that bubble % CH4 is related to ebullition rates, and therefore these patterns are suggestive of greater CH4 emission by ebullition from deeper waters compared to waters <1 m in depth. Stable isotope ratios of carbon and hydrogen in the CH4 showed evidence for significant CH4 oxidation but no apparent relation with water depth. Shallower waters typically contain rooted emergent plants with erect, vertical stems or petioles, whereas in waters deeper than ~1 m, emergent aquatic plants are limited to rooted forms with long stems that float at the water surface. Rooted emergent plants typical of deeper waters had low rates of CH4 emission (0.0–0.2 mg d−1 CH4 for individual plants) compared to species typical of inundation depths <1 m (1.6–7.5 mg d−1 CH4). These observations suggest that tropical savanna floodplains like the Pantanal with inundation depths >1 m have low rates of CH4 emission via rooted emergent and floating-leaf plants, in contrast to results from shallower rice fields and temperate wetlands in which plant-mediated transport dominates CH4 emission from vegetated waters.

Comparative studies of pelagic microbial methane oxidation within the redox zones of the Gotland Deep and Landsort Deep (central Baltic Sea)

Abstract. Pelagic methane oxidation was investigated in dependence on differing hydrographic conditions within the redox zone of the Gotland Deep (GD) and Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which indicates the transition between oxic and anoxic conditions, was characterized by a pronounced methane concentration gradient between the deep water (GD: 1233 nM, 223 m; LD: 2935 nM, 422 m) and the surface water (GD and LD &lt; 10 nM). This gradient together with a 13C CH4 enrichment (δ13C CH4 deep water: GD −84‰, LD −71‰; redox zone: GD −60‰, LD −20‰; surface water: GD −47‰, LD −50‰; δ13C CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial methane consumption within the redox zone. Expression analysis of the methane monooxygenase identified one active type I methanotrophic bacterium in both redox zones. In contrast, the turnover of methane within the redox zones showed strong differences between the two basins (GD: max. 0.12 nM d−1, LD: max. 0.61 nM d−1), with a nearly four-times-lower turnover time of methane in the LD (GD: 455 d, LD: 127 d). Vertical mixing rates for both deeps were calculated on the base of the methane concentration profile and the consumption of methane in the redox zone (GD: 2.5 × 10–6 m2 s−1, LD: 1.6 × 10–5 m2 s−1). Our study identified vertical transport of methane from the deep-water body towards the redox zone as well as differing hydrographic conditions (lateral intrusions and vertical mixing) within the redox zone of these deeps as major factors that determine the pelagic methane oxidation.

Modeling fluid flow in sedimentary basins with sill intrusions: Implications for hydrothermal venting and climate change

Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH4. Basin‐scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative δ13C excursions observed in the fossil record over short time scales (&lt;10,000 years).

Variability of internal frontal bore breaking above Opouawe Bank methane seep area (New Zealand)

Large internal wave breaking is observed exceeding a vertical array of 61 high‐resolution temperature sensors at 1 m intervals between 7 and 67 m above the bottom. The array was moored for 5 days at 969 m of Opouawe Bank, New Zealand, a known methane seep area. As breaking internal waves dominate sediment resuspension above sloping topography in other ocean areas, they are expected to also influence methane transport. Despite being visible in single beam echosounder data, indications for turbulence due to rising gas bubbles are not found in the present 1 Hz sampled temperature records. Likely, the mooring was too far away from the very localized bubble release spot. Instead, the temperature sensors show detailed internal wave‐turbulence transitions. Every tidal cycle, a solibore (a frontal turbulent bore with a train of trailing solitary waves) changes shape and intensity. These solibores are highly turbulent and they restratify the bottom boundary layer, thereby maintaining efficient mixing. Details of different turbulent bore developments are discussed. Averaged over a few tidal cycles and over the sensors range, mean vertical eddy diffusivity amounts to 3 ± 1 × 10−3 m2 s−1 and mean turbulent kinetic energy dissipation to 1.6 ± 0.7 × 10−7 W kg−1, with variations over 4 orders of magnitude. Such turbulence will affect the distribution of dissolved methane and other geochemical species in the lower 100–150 m above the bottom and their release from the bottom. The above mean values are remarkably similar to those found at various other sites in the NE Atlantic Ocean.

Gas origin and migration in the Ulleung Basin, East Sea: Results from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2)

We have investigated the gas chemistry of marine sediments (up to 250 m below seafloor, m bsf) recovered during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) to identify and compare the gas origin and its migration at seismically non-chimney, truncated chimney and chimney sites in this basin. The molecular and isotopic data (C1/C2+ ratios > 300, δ13CCH4 < −65‰, and δDCH4 < −181‰) of the gases indicate that methane predominantly originates via microbial carbon dioxide reduction. The carbon isotopic fractionation (ɛc) between methane and carbon dioxide below 30 m bsf has a relatively constant value (ɛc = 66–68) at all sites, consistent with the microbial pathway. In contrary, samples shallower than 30 m bsf display significant differences with the seismic characteristics of each site. ɛc ranges from 65 to 81 in non-chimney sites but shows values as low as 45 in chimney sites, which is agreement with previously reported piston core data (Kim et al., 2012), and thought to reflect two-phase methane transport processes. In chimney sites, deep-sourced methane migrates independently of the carbon dioxide, which remains dissolved in the aqueous phase leading to the extremely low ɛc values observed in the upper 30 m bsf. Upward methane diffusion at non-chimney sites may also add a transport-induced fractionation as evidenced in mixing diagrams, but this process does not significantly affect the ɛc values.δ13CCO2 shows usually minimum values at the Sulfate–Methane Transition Zone (SMTZ) at all sites. However, in chimney sites, these values have a much more depleted signature, indicating faster rates of methane consumption by Anaerobic Oxidation of Methane (AOM) supported by the higher methane flux. Moreover, here the massive hydrate that forms in shallow sediments incorporates gases with a lower C1/C2 ratio (=higher C2/C1) than the surrounding sediment, as documented in hydrate-bound gas (BG) and void gas (VG) samples. Gas hydrate dissociation during the core recovery is released enriched ethane (C2H6) trapped in gas cage, which leads to have high C2H6 in VG samples.

Spatiotemporal distribution patterns of dissolved methane in lakes: How accurate are the current estimations of the diffusive flux path?

AbstractLakes have been identified as an important source of atmospheric methane. Here the spatiotemporal distribution of dissolved methane was measured in a medium‐sized freshwater lake. The data reveal that littoral zones (nearshore, shallow) are the predominant source of methane. Offshore‐directed gradients of dissolved methane suggest the transport of methane from the nearshore zone to the pelagic epilimnion. The distribution patterns of epilimnetic methane were highly heterogeneous, independent of the mean lake‐wide methane concentration. Consequently, the diffusive flux of methane to the atmosphere strongly varies with location in the lake. A comparison of the diffusive methane flux from different offshore sampling stations indicates that single‐point measurements are not necessarily sufficient to estimate lake‐wide emissions to the atmosphere accurately. Thus, spatially resolved measurements of methane emissions are needed to improve the reliability of estimates of the methane that lakes contribute to the global methane budget.

Natural hydrocarbon seepage on the continental slope to the east of Mississippi Canyon in the northern Gulf of Mexico

AbstractFrom 5 June to 15 September 2010, a multidisciplinary marine survey was undertaken onboard the M/V Ryan Chouest in the region of the BP Deepwater Horizon incident site in the Gulf of Mexico. The primary objective of the survey was the continuous monitoring of hydrocarbon abundance from sea surface down to a maximum depth of 120 m. Compound abundances were inferred using a hydrocarbon sensor array with associated vertical cast system. In order to better understand the potential inputs from natural seepage in the vicinity of the spill, a Simrad EK60 high‐resolution split beam echo sounder, operated at 38 kHz, was included in the survey between 7 July and 15 September 2010. During this period, three fields of natural seeps characterized by hydroacoustic flares were studied in detail. These seep fields are at water depths of approximately 430 m, 880 m, and 1370 m. They are associated with extensive cold seep systems. In particular, the area around Seep Field 1 (the vicinity of Deepwater Horizon) seems to present a vast area of active natural seepages in the Gulf of Mexico. The repeat surveys at two of the fields suggested that the cold seep systems here were active, with expulsions of hydrocarbons into the water column, at least during the periods of our acoustic surveys. Multiple lines of evidence gathered during the survey indicated that the observed hydroacoustic flares at the three fields identified consisted of oily bubble streams of gases of thermogenic origin. However, direct observation and sampling are required to reveal the precise nature of the flares. In the deep water Gulf of Mexico, the formation of a hydrate rim around bubbles seems to be a very important mechanism for the long transport of methane and oil in the water column.