Enhanced lifetime of methane bubble streams within the deep ocean. Geophys Res Lett

Geophysical Research Letters (Impact Factor: 4.2). 08/2002; 29(15). DOI: 10.1029/2001GL013966


1] We have made direct comparisons of the dissolution and rise rates of methane and argon bubbles experimentally released in the ocean at depths from 440 to 830 m. The bubbles were injected from the ROV Ventana into a box open at the top and the bottom, and imaged by HDTV while in free motion. The vehicle was piloted upwards at the rise rate of the bubbles. Methane and argon show closely similar behavior at depths above the methane hydrate stability field. Below that boundary ($520 m) markedly enhanced methane bubble lifetimes are observed, and are attribute to the formation of a hydrate skin. This effect greatly increases the ease with which methane gas released at depth, either by natural or industrial events, can penetrate the shallow ocean layers.

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Available from: Gregor Rehder, Jun 23, 2015
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    • "The rapidity of this process strongly depends on the bubble size, the rise velocity, as well as the composition and conditions of the surrounding medium and the presence of upwelling flows (Leifer and Judd, 2002). Several studies have demonstrated that methane escapes the bubbles well before final bubble dissolution (Leifer and Patro, 2002; McGinnis et al., 2006; Rehder et al., 2002). Our suggestion that most of the methane discharged from the South Georgia northern shelf does not reach the upper water column is additionally strengthened by the relatively low concentrations of dissolved methane (about 5 nmol/l) in the intermediate to uppermost water masses at two hydrocast stations, deliberately acquired close to recorded flares in the Cumberland Bay area (Figs. "
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    ABSTRACT: 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.
    Earth and Planetary Science Letters 10/2014; 403:166–177. DOI:10.1016/j.epsl.2014.06.036 · 4.73 Impact Factor
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    • "At the same time, gas exchange between the gas bubble and the ambient water might take place resulting in a rapid decrease of the methane portion inside the bubble. This process is hampered within the GHSZ in which gas hydrate rims form around methane bubbles (Maini and Bishnoi, 1981), hence, preventing them from rapid dissolution until they pass the top of the GHSZ (Rehder et al., 2002). Basin-wide modeling suggests that less than 1.5% of the methane escaping the Black Sea seafloor reaches the atmosphere with the rest remaining in the hydrosphere (Reeburgh et al., 1991). "
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    ABSTRACT: We investigated gas bubble emissions at the Don–Kuban paleo-fan in the northeastern Black Sea regarding their geological setting, quantities as well as spatial and temporal variabilities during three ship expeditions between 2007 and 2011. About 600 bubble-induced hydroacoustic anomalies in the water column (flares) originating from the seafloor above the gas hydrate stability zone (GHSZ) at ~ 700 m water depth were found. At about 890 m water depth a hydrocarbon seep area named "Kerch seep area" was newly discovered within the GHSZ. We propose locally domed sediments ("mounds") discovered during ultra-high resolution bathymetric mapping with an autonomous underwater vehicle (AUV) to result from gas hydrate accumulation at shallow depths. In situ measurements indicated spatially limited temperature elevations in the shallow sediment likely induced by upward fluid flow which may confine the local GHSZ to a few meters below the seafloor. As a result, gas bubbles are suspected to migrate into near-surface sediments and to escape the seafloor through small-scale faults. Hydroacoustic surveys revealed that several flares originated from a seafloor area of about 1 km 2 in size. The highest flare disappeared in about 350 m water depth, suggesting that the released methane remains in the water column. A methane flux estimate, combining data from visual quantifications during dives with a remotely operated vehicle (ROV) with results from ship-based hydroacoustic surveys and gas analysis re-vealed that between 2 and 87×10 6 mol CH 4 yr −1 escaped into the water column above the Kerch seep area. Our results show that the finding of the Kerch seep area represents a so far underestimated type of hydrocarbon seep, which has to be considered in methane budget calculations.
    Marine Geology 08/2012; 319-322:57–74. DOI:10.1016/j.margeo.2012.07.005 · 2.71 Impact Factor
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    • "NOMENCLATURE c speed of sound f frequency G shear modulus K bulk modulus L resonator length n mode number P hydrostatic pressure S salinity sI structure I gas hydrate sII structure II gas hydrate T temperature VSA vector signal analyzer λ acoustic wavelength ρ density χ volume fraction INTRODUCTION Gas hydrates are often formed and found in ocean sediments along continental margins. The compounds are also known to form as a skin on rising methane bubbles, as reported by Rehder et al. [1], Heeschen et al. [2], and Sauter et al. [3]. To improve seismic detection and characterization of hydrates, it is necessary to gain a better understanding of their acoustic behavior in the variety of media in which they are found. "
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    ABSTRACT: The unique nature of the molecular structures of gas hydrates results in curious acoustic properties which have yet to be adequately characterized. Understanding the acoustic behavior of hydrates in liquids, in bubbly liquids, and in sediments containing liquids and/or gas is vital for surveying their location using seismic or echosounding techniques and may become a key tool for monitoring hydrate dissociation and its possible link to climate change. Acoustic properties of gassy substances are known to have a strong dependence on excitation frequency; however, tabulated values of hydrate sound speeds are most often measured at high frequencies (>200 kHz) despite modern location methods which use frequencies below 100 kHz. This presentation details a laboratory experiment in which the dissociation pressures of natural structure I and structure II methane hydrate samples were determined by measuring their low-frequency acoustic velocity in a liquid as a function of hydrostatic pressure.
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