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

• Published in: Journal of geophysical research. Oceans 2021-09, Vol.126 (9), p.n/a
• Main Authors: Fu, X., Waite, W. F., Ruppel, C. D.
• Format: Article
• Language: English
• Subjects: Acidification; Atmospheric processes; Benthos; biogeochemical cycles, processes, and modeling; Bubbles; carbon cycling; Coatings; Crystal structure; Crystallinity; Deep water; Dissolution; Dissolving; ENVIRONMENTAL SCIENCES; gas and hydrate systems; Gas exchange; Gas hydrates; Geophysics; Greenhouse effect; Hydrates; Methane; Methane hydrates; Natural gas; Ocean acidification; Ocean floor; Ocean warming; Oceanography; oceanography: biological and chemical; Residence time; Seepages; Shells; Stability; Water chemistry; Water circulation; Water column; Water depth
• ISSN: 2169-9275; 2169-9291
• DOI: 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. Plain Language Summary Methane is the primary component of natural gas. Processes that affect the formation, consumption, and redistribution of methane in natural settings have significant environmental consequences, such as ocean acidification and greenhouse warming. In water deeper than 500–600 m, methane bubbles can acquire shells of gas hydrate, a crystalline solid made of methane and water molecules. Here, we study how the process of hydrate shell formation alters the fate of methane that naturally bubbles up from the seafloor. We find that bubbles taking longer than ∼10 s to release at seafloor that is within the local methane hydrate stability often grow hydrate shells before rising into the water column. As bubbles ascend through the water column, the hydrate shells slow down bubble dissolution and gas exchange, allowing more methane to be delivered to shallower water depths. Key Points In deepwater