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

Abstract:

Methane released from marine seeps can contribute to a number of benthic, water column and atmospheric processes. Constraining seep methane contributions to these processes requires accurately tracking methane migration into and through the water column. Methane from deep-water seeps (500-600 meters) can form methane hydrate, a crystalline solid that can coat gas bubbles and complicate efforts to predict methane dissolution rates. We have analyzed seafloor videos from five deep-water seep sites in the Gulf of Mexico and offshore British Columbia that record a diverse range of bubble-release processes involving hydrate formation. Though these processes produce a variety of bubble sizes, shapes and hydrate coatings, certain patterns of bubble release dynamics emerge, and we assess their implications for bubble dissolution in the water column. Bubbles growing and releasing rapidly from the seafloor are often hydrate-free. As bubble growth slows and a bubble’s seafloor residence time increases, a hydrate coating can form on the gas-water interface. Most seafloor bubbles become fully coated within ~10 seconds, which agrees with water-column observations that the majority of bubbles become hydrate-coated after the initial ~150 centimeters (cm) of rise, even if they are completely clean when they release from the seafloor. Whether a bubble is coated or not when it releases from the seafloor impacts the bubble’s initial amount of methane, and how quickly that methane dissolves as the bubble rises. We derived a simplified model considering hydrate formation and hydrate shielding effects on methane dissolution. At 150 cm above the seafloor, initially hydrate-coated bubbles contain ~5% more methane than bubbles that were initially hydrate-free at seafloor (but become fully hydrate-coated at 150 cm). Over the initial 150 cm rise, our model also confirms a published observation that dissolution rates of hydrate-coated bubbles are only ~20% of the dissolution rate of hydrate-free bubbles.