Hydrate Formation on Marine Seep Bubbles and the Implications for Water Column Methane Dissolution

Abstract

Methane released from seafloor seeps contributes to a number of benthic, water column, and atmospheric processes. At seafloor seeps within the methane hydrate stability zone, crystalline gas hydrate shells can form on methane bubbles while the bubbles are still in contact with the seafloor or as the bubbles begin ascending through the water column. These shells reduce methane dissolution rates, allowing hydrate‐coated bubbles to deliver methane to shallower depths in the water column than hydrate‐free bubbles. Here, we analyze seafloor videos from six deepwater seep sites associated with a diverse range of bubble‐release processes involving hydrate formation. Bubbles that grow rapidly are often hydrate‐free when released from the seafloor. As bubble growth slows and seafloor residence time increases, a hydrate coating can form on the bubble's gas‐water interface, fully coating most bubbles within ∼10 s of the onset of hydrate formation at the seafloor. This finding agrees with water‐column observations that most bubbles become hydrate‐coated after their initial ∼150 cm of rise, which takes about 10 s. Whether a bubble is coated or not at the seafloor affects how much methane a bubble contains and how quickly that methane dissolves during the bubble's rise through the water column. A simplified model shows that, after rising 150 cm above the seafloor, a bubble that grew a hydrate shell before releasing from the seafloor will have ∼5% more methane than a bubble of initial equal volume that did not grow a hydrate shell after it traveled to the same height.

Estimates of Methane Release From Gas Seeps at the Southern Hikurangi Margin, New Zealand

“…where A is the spatial extent of the acoustic anomaly in the proximity of the seafloor as interpreted from the processed MBES data, ρ is the mean density of bubbles in the water in m −3 , v(a) is the mean rising velocity of the gas bubbles, BSD is the bubble size distribution and a is the bubble radius. The bubble rise velocity valuesare based on the work by Leifer and Patro (2002) and consider the two endmembers clean and coated bubbles, where the latter model represents gas bubbles which are coated with oil or hydrate (Fu et al, 2020).…”

“…On the seafloor, gas seeps are the most common manifestations of ongoing subsurface fluid flow (Judd and Hovland, 2009). The gases that are expelled from gas seeps on continental margins are primarily composed of methane, leaving major questions open on: 1) the amount of methane reaching the ocean surface (McGinnis et al, 2006;Shakhova et al, 2010;Fu et al, 2020), 2) the connectivity of seeps to deeper hydrocarbon systems (Crutchley et al, 2021), 3) the role of gas hydrate dissociation (Reagan et al, 2011), 4) how gas flux rates change over time and the potential influence of seismicity on subsurface fluid flow (Bassett et al, 2014;Bonini, 2019;Legrand et al, 2021). The southern Hikurangi Margin, off the North Island of Aotearoa/ New Zealand, reveals evidence of widespread methane seepage (Greinert et al, 2001;Barnes et al, 2010;Watson et al, 2020).…”

Fate of Methane Released From a Destroyed Oil Platform in the Gulf of Mexico

“…Methane seepage on outer continental margins supports microbial consortia and symbioses that are the basis of chemosynthetic food webs. Seeps can be identified visually by the presence of pockmarks (Marcon et al, 2014;Mason et al, 2019), authigenic carbonates (Feng et al, 2010), gas hydrate outcroppings-when within the hydrate stability zone- (MacDonald et al, 2003), lush biological communities (Roy et al, 2007), or bubble plumes (Fu et al, 2021). Seeps can be detected acoustically because bubbles are strong reflectors in scanning or swath-mapping sonar (Römer et al, 2012).…”

Assessing the Benthic Response to Climate-Driven Methane Hydrate Destabilisation: State of the Art and Future Modelling Perspectives

“…That is, the impact is approached by equating the totality or a fraction of the hydrate-sourced methane released to the ocean to the ocean-atmosphere flux. Yet, this approach is based on weak estimates of hydrate-sourced benthic methane emissions (see Section 3.2) and overlooks potential effects of methane turnover on greenhouse gas (GHG) emissions, ocean chemistry, ocean ecosystems, and biogeochemical cycles at different temporal and spatial scales [29,49,55,179,180].…”