Abstract
AbstractMethane 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.
Highlights
Methane is an environment-altering molecule, function as a fuel for human activities, a carbon source for benthic, water-column, and wetland ecosystems (Levin, 2005; Valentine et al, 2001), and a greenhouse gas in the atmosphere (Sundquist & Visser, 2003)
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
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
Summary
Methane is an environment-altering molecule, function as a fuel for human activities, a carbon source for benthic, water-column, and wetland ecosystems (Levin, 2005; Valentine et al, 2001), and a greenhouse gas in the atmosphere (Sundquist & Visser, 2003). It has long been postulated that such bubbles may form enclosing shells of gas hydrate that could reduce the diffusion of methane into the surrounding water during the bubble's ascent, thereby allowing the methane to reach shallower depths in the water column before dissolution occurs. This hypothesis was further supported by field experiments (Rehder et al, 2002) and later theorized into models (McGinnis et al, 2006), which suggest that a hydrate shell can slow the rate of transport of methane out of a bubble by 80%. We combine theoretical constraints with the quantitative analyses of seafloor videos collected at deepwater seeps on North American continental margins to determine the characteristics of bubbles that are most likely to acquire hydrate coatings and to assess the impact of hydrate shells on the preservation of methane within bubbles
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