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

• Main Authors: Fu, X., Waite, W. F., Ruppel, C. D.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: American Geophysical Union 2021
• Subjects: gas and hydrate systems; oceanography: biological and chemical; carbon cycling; biogeochemical cycles; processes; and modeling; Methane hydrate
• Online Access: https://doi.org/10.1029/2021jc017363

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. © 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Issue Online: 15 September 2021; Version of Record online: 15 September 2021; Accepted manuscript online: 01 September 2021; Manuscript accepted: 11 August 2021; Manuscript revised: 14 July 2021; Manuscript received: 15 March 2021. X. ...