Enhanced lifetime of methane bubble streams within the deep ocean

Geophysical Research Letters (Impact Factor: 3.98). 01/2002; 29(15). DOI: 10.1029/2001GL013966

ABSTRACT 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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    ABSTRACT: Objective To explore which baseline physiological and psychosocial variables predict change in body mass index (BMI) z-score in obese youth after 12 weeks of a dietary weight management study. Methods Participants were obese young people participating in a dietary intervention trial in Brisbane Australia. The outcome variable was change in BMI z-score. Potential predictors considered included demographic, physiological and psychosocial parameters of the young person, and demographic characteristics of their parents. A multivariable regression model was constructed to examine the effect of potential predictive variables. Results Participants (n = 88) were predominantly female (69.3%), and had a mean(standard deviation) age of 13.1(1.9) years and BMI z-score of 2.2(0.4) on presentation. Lower BMI z-score (p < 0.001) and insulin resistance (p = 0.04) at baseline, referral from a paediatrician (p = 0.02) and being more socially advantaged (p = 0.046) were significantly associated with weight loss. Macronutrient distribution of diet and physical activity level did not contribute. Conclusions Early intervention in obesity treatment in young people improves likelihood of success. Other factors such as degree of insulin resistance, social advantage and referral source also appear to play a role. Assessing presenting characteristics and factors associated with treatment outcome may allow practicing clinicians to individualise a weight management program or determine the ‘best-fit’ treatment for an obese adolescent.
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    ABSTRACT: 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.
    Geochemistry Geophysics Geosystems 06/2013; 14(6):1940-1956. · 2.94 Impact Factor
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    ABSTRACT: While bubble plumes have been acoustically imaged in the water column above marine gas hydrate deposits in many studies, little is known about the temporal variation in plume intensity. In July 2008, we conducted surveys using 3.5 and 12 kHz echosounders and a 75 kHz acoustic Doppler current profiler (ADCP) over the northern and southern summits of Hydrate Ridge, on the Cascadia continental margin. Our study included multiple surveys at both sites, including a survey of the northern summit that was repeated 16 times in 19 h. Seafloor depth at the northern summit is ∼600 m, well within the hydrate stability zone (HSZ), which was below ∼510 m during our survey based on CTD data. Three distinct flares (a term used to denote the acoustic signature of bubble plumes) were detected at Northern Hydrate Ridge (NHR) and one was detected at Southern Hydrate Ridge (SHR), coincident with where flares were observed a decade ago, indicating that the supply of gas is stable on this time scale. High-resolution bathymetric surveys of NHR and SHR acquired with an Autonomous Underwater Vehicle (AUV) flown ∼50 m above the seafloor indicate that flare locations are correlated with a distinctive pattern of short-wavelength seafloor roughness, supporting the inference of long-term stability in the location of bubble expulsion sites. As in previous studies at Hydrate Ridge, flares were not detected with the 3.5 kHz echosounder but were clearly imaged at 12 kHz. By reprocessing routine shipboard ADCP data, we show that they are also observed in at 75 kHz, indicating that a wide range of bubble sizes is present. The intensity of the flares varied strongly with time. Two primary sources for flares were observed. One, located on the regional topographic high, showed continuous activity, with two times periods of particularly strong flares that are not correlated with tidal height. The other, located on a local topographic high, shows a pulse of increased backscatter that occurred on a falling tide. While the time period of observation is not enough to constrain the effect of tidal changes in seafloor pressure on venting, the data suggest that tides are not the dominant factor controlling release of bubbles from the seafloor. The data support previously suggested models in which temporary sealing of vents by gas hydrate formation and breaking of these barriers as gas pressure builds up is responsible for “burp-like” pulses of gas expulsion. We also report the first observations of flares originating within the HSZ that extend well above the HSZ with little loss of backscatter intensity. These represent a new source of methane injection into the upper ocean and possibly the atmosphere. Flare extension above the HSZ may be due to coating of some bubbles by oil or biofilms or to inclusion of particulate matter (possibly including floating pieces of hydrate) in the plumes. Although this study provides tantalizing new information on both short-term variability in gas expulsion rate and long-term stability of vent sites, longer, well-calibrated observations that integrate bubble flux over entire vent fields as a function of time are needed to develop accurate models for the flux of methane into the ocean and atmosphere from seafloor methane vents.
    Earth and Planetary Science Letters 04/2013; 368:33–42. · 4.72 Impact Factor


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