Time‐series measurements of bubble plume variability and water column methane distribution above Southern Hydrate Ridge, Oregon

Abstract

An estimated 500–2500 gigatons of methane carbon is sequestered in gas hydrate at continental margins and some of these deposits are associated with overlying methane seeps. To constrain the impact that seeps have on methane concentrations in overlying ocean waters and to characterize the bubble plumes that transport methane vertically into the ocean, water samples and time‐series acoustic images were collected above Southern Hydrate Ridge (SHR), a well‐studied hydrate‐bearing seep site ∼90 km west of Newport, Oregon. These data were coregistered with robotic vehicle observations to determine the origin of the seeps, the plume rise heights above the seafloor, and the temporal variability in bubble emissions. Results show that the locations of seep activity and bubble release remained unchanged over the 3 year time‐series investigation, however, the magnitude of gas release was highly variable on hourly time scales. Bubble plumes were detected to depths of 320–620 m below sea level (mbsl), in several cases exceeding the upper limit of hydrate stability by ∼190 m. For the first time, sustained gas release was imaged at the Pinnacle site and in‐between the Pinnacle and the Summit area of venting, indicating that the subseafloor transport of fluid and gas is not restricted to the Summit at SHR, requiring a revision of fluid‐flow models. Dissolved methane concentrations above background levels from 100 to 300 mbsl are consistent with long‐term seep gas transport into the upper water column, which may lead to the build‐up of seep‐derived carbon in regional subsurface waters and to increases in associated biological activity.

Multiple gas reservoirs are responsible for the gas emissions along the Marmara fault network

“…Intensive gas emissions are common and widespread processes in oceanic and continental marine basins. Amongst the areas on Earth where they have been densely observed, one can cite offshore Siberia (Shakhova et al, 2014), the Norwegian continental margin including the well-studied Hakon Mosby Mud Volcano (Gentz et al, 2014;Sauer et al, 2015;Westbrook et al, 2009), the North Sea (Borges et al, 2016;McGinnis et al, 2011;von Deimling et al, 2011), the Black Sea (Klaucke et al, 2006;Roemer et al, 2012a), the Sea of Marmara (Dupré et al, 2012;Dupré et al, 2010a), the Aquitaine Shelf (Dupré et al, 2014;Ruffine et al, 2017), the Central Nile Deep-Sea Fan (Dupré et al, 2010b;Roemer et al, 2014a), the US Atlantic Margin (Skarke et al, 2014;Weinstein et al, 2016), the Gulf of Mexico (Bernard et al, 1976;Hu et al, 2012), the Santa Barbara Basin (Clark et al, 2010), the Hydrate Ridge (Haeckel et al, 2004;Milkov et al, 2005;Philip et al, 2016), the Makran continental margin (Roemer et al, 2012b), the South China Sea (Di et al, 2014;Huang et al, 2009), the Japan Sea (Aoyama et al, 2007), as well as offshore New Zealand (Greinert et al, 2010) and the Southern Ocean (Roemer et al, 2014b). Such phenomena occur either as dissolved or free gases, and they lead to the formation of specific sites called cold seeps (Hovland and Judd, 1988;Suess, 2014;Talukder, 2012).…”

Using Time-Series Videos to Quantify Methane Bubbles Flux from Natural Cold Seeps in the South China Sea

“…Methane bubbles released from cold seeps may reach the surface and enter the atmosphere directly or dissolve in the mixed layer and become released to the atmosphere through seawater-air exchange [10]. According to the CTD data and the observed height of the bubble plumes [25,68], areas of methane enrichment may be located at the depth of bubble plume dissipation or higher than the plume height [49,69]. Although methane continuously outgasses from rising bubbles within the hydrate stability zone, the maxima in methane concentrations may not extend into the mixed layer (<50 m) in the SCS [70,71].…”

“…In addition, seep methane deposited below the mixed layer depth may be transported away from seep vents due to complex local currents and eventually become oxidized to carbon dioxide through methanotrophic microbial processes that can promote the phytoplankton and chlorophyll growth [72]. Furthermore, the deposition of methane at the depth of bubble plume dissipation may sustain methanotrophy within the water column [69].…”

Better Regional Ocean Observing Through Cross-National Cooperation: A Case Study From the Northeast Pacific

“…A second OOI cable leg loops south from Pacific City and hosts infrastructure on the Southern Hydrate Ridge (SHR) site to provide insight into actively venting methane hydrate systems (Kelley et al, 2016). Processes of interest include the temporal evolution of methane hydrate systems in response to seismic events, determining chemical fluxes from the seafloor and impacts on overlying ocean chemistry, and understanding biogeochemical coupling associated with gas-hydrate formation and dissolution (Philip et al, 2016). Since 2016, significant instrument expansion of the Axial and SHR sites has occurred through funding by NSF, United States Office of Naval Research, and Germany.…”