20-12. Seafloor methane bubble emissions -- Implications for gas hydrates and the environment

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Background: Natural methane hydrates are widespread in the sediments of marine continental margins and sequester an estimated one-sixth of global methane. Because gas hydrate is stable only within a specific range of pressures and temperatures, ocean warming is likely contributing to the breakdown of gas hydrate deposits in some settings, particularly on upper continental slopes at the landward edge of gas hydrate stability (Ruppel and Kessler, 2017). In the past decade, hydroacoustic techniques have been used to discover more than 1500 methane seeps on the northern U.S. Atlantic margin and offshore the Pacific Northwest at seafloor depths that bracket the landward limit of gas hydrate stability or, more rarely, are far within the gas hydrate stability zone (e.g., Johnson et al., 2015; Merle et al., 2021; Riedel et al., 2018; Skarke et al., 2014). Methane bubbles emitted from these seeps represent a substantial mobilization of once-buried carbon to ocean waters and feed water column microbial oxidation that in turn produces CO₂ that can locally acidify ocean waters (e.g., Garcia-Tigreros et al., 2021). At some seeps, bubbles grow a gas hydrate shell, which slows dissolution of the methane during bubble ascent and allows the bubble-encased methane to rise closer to the sea-air interface.

Bubble dynamics at seafloor seeps has primarily been studied using seafloor instrumentation and visualization or water column hydroacoustics (e.g., Greinert et al., 2010; Skarke et al., 2014; Wang and Socolofsky, 2015; Weber et al., 2014). However, laboratory and theoretical studies are also needed to advance understanding of the numbers and nature of methane bubbles released from the seafloor to the ocean-atmosphere system and to explore the links between bubble processes and gas hydrate degradation, ocean chemistry, and climate synergies. Laboratory studies could constrain the size, shape, and volume of bubbles that make up bubble streams emitted at seafloor seeps, ultimately addressing key knowledge gaps related to local- and regional-scale seafloor methane fluxes.  Laboratory studies could also determine which subset of bubbles acquires hydrate shells and quantify how these shells impede dissolution of a bubble’s methane into the surrounding waters. Such laboratory experiments are an important test of existing theoretical and observational studies (e.g., Fu et al., 2021; Leifer and Patro, 2002; McGinnis et al., 2006; Wang et al., 2020). Laboratory acoustic studies of bubbles with and without hydrate shells could also provide calibration for hydroacoustic data obtained at sea, allowing researchers to estimate the fraction of remotely-detected bubble streams that becomes hydrate coated (McGinnis et al., 2006).

Description of the Research Opportunity: The Mendenhall Fellow will use laboratory and theoretical studies to investigate processes related to the fate of methane bubbles once released from the seafloor. The Fellow, who will ideally have a background in ocean engineering, marine acoustics, and/or physical oceanography and an understanding of geophysical detection methods, will have access to a clear-walled, pressurized, bubble flow loop (Waite et al., 2017) at the USGS Woods Hole Coastal and Marine Science Center. The flow loop can be used to track the evolution, release, and water column ascent of gas bubbles, to visualize possible hydrate formation on bubble surfaces, and to quantify the dissolution of the bubbles’ gas into the surrounding water bath. The flow loop is equipped with high-speed cameras for visualization of bubble processes and can be outfitted with ultrasonic acoustic sensors for calibrating hydroacoustic properties of bubbles with and without gas hydrates. Analyses of the changing chemistry of water in the flow loop allow measurement of bubble dissolution rates, a critical parameter for refining the most widely used bubble dissolution models (e.g., Gros et al., 2020; McGinnis et al., 2006).  The Fellow will have wide latitude to refine the research focus; to develop new instrumentation, methodology, and analytical techniques to advance the broader goals of this work; and to innovate in extrapolating laboratory results to existing USGS hydroacoustic datasets on seafloor methane emissions or to shipborne bubble stream data that are newly acquired with the Fellow’s involvement and direction.  The Fellow could also collaborate with USGS Gas Hydrate Project scientists to explore the broader implications of the research for past, contemporary, and future methane flux across the sediment-ocean and sea-air interfaces and to draw conclusions relevant for ocean chemistry, microbial ecology, and/or carbon cycling.

Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.

References:

Fu, X., W. F. Waite, and C. D. Ruppel (2021), What controls how bubbles escape the seafloor in deep cold seeps? Lessons learned from seafloor videos on North American marine margins (abstract), Amer. Geophys. Union, Fall Conference, Dec. 13-17, 2021, New Orleans, LA.

Garcia-Tigreros, F. et al. (2021), Estimating the impact of seep methane oxidation on ocean pH and dissolved inorganic radiocarbon along the U.S. Mid-Atlantic Bight, J. Geophys. Res. Biogeo., 126(1) doi: 10.1029/2019JG005621.

Greinert, J. et al. (2010), Methane seepage along the Hikurangi Margin, New Zealand: Overview of studies in 2006 and 2007 and new evidence from visual, bathymetric and hydroacoustic investigations, Marine Geology, 272(1-4), 6-25, doi: 10.1016/j.margeo.2010.01.017.

Gros, J. et al. (2020), Dynamics of live oil droplets and natural gas bubbles in deep water, Environ. Sci. Technol., 54(19), 11865-11875, doi: 10.1021/acs.est.9b06242.

Johnson, H. P. et al. (2015), Analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope, Geochem. Geophys., Geosys., 16(11), 3825-3839, doi: https://doi.org/10.1002/2015GC005955.

Leifer, I., and R. K. Patro (2002), The bubble mechanism for methane transport from the shallow sea bed to the surface: A review and sensitivity study, Continental Shelf Res., 22(16), 2409-2428, doi: 10.1016/S0278-4343(02)00065-1.

McGinnis, D. F. et al. (2006), Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere?, J. Geophys. Res. Oceans, 111(C9), doi: 10.1029/2005jc003183.

Merle, S. et al. (2021), Distribution of methane plumes on Cascadia Margin and implications for the landward limit of methane hydrate stability, Frontiers in Earth Science, 9, doi: 10.3389/feart.2021.531714.

Riedel, M. et al. (2018), Distributed natural gas venting offshore along the Cascadia margin, Nature Comm., 9(1), 3264, doi: 10.1038/s41467-018-05736-x.

Ruppel, C. D., and J. D. Kessler (2017), The interaction of climate change and methane hydrates, Rev. Geophys., 55, doi: 10.1002/2016RG000534.

Skarke, A. et al. (2014), Widespread methane leakage from the sea floor on the northern US Atlantic margin, Nat Geosci, 7(9), 657-661, doi: 10.1038/Ngeo2232.

Waite, W. F. et al. (2017), Laboratory observations of the evolution and rise rate of bubbles with and without hydrate shells, in Proc. 9th Int. Conf. on Gas Hydrates, Denver, CO, 14 pp.

Wang, B., I. Jun, S. A. Socolofsky, S. F. DiMarco, and J. D. Kessler (2020), Dynamics of gas bubbles from a submarine hydrocarbon seep within the hydrate stability zone, Geophys. Res. Lett., 47(18), e2020GL089256, doi: 10.1029/2020GL089256.

Wang, B. B., and S. A. Socolofsky (2015), A deep-sea, high-speed, stereoscopic imaging system for in situ measurement of natural seep bubble and droplet characteristics, Deep-Sea Res. Part I-Oceanogr. Res. Pap., 104, 134-148, doi: 10.1016/j.dsr.2015.08.001.

Weber, T. C., L. Mayer, K. Jerram, J. Beaudoin, Y. Rzhanov, and D. Lovalvo (2014), Acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the Northern Gulf of Mexico, Geochem. Geophys. 15(5), 1911-1925, doi: 10.1002/2014GC005271.

Proposed Duty Station: Woods Hole, Massachusetts

Areas of PhD: Ocean engineering, marine acoustics, physical oceanography, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet the qualifications for:  Research Geophysicist

(This type of research is performed by those who have backgrounds for the occupation stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Human Resources Office Contact:  Audrey Tsujita, 916-278-9395, atsujita@usgs.gov