(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2019-09)
Johnson, Andrew S.
Methane (CH4) is a potent greenhouse gas that is often found in a solid, hydrate clathrate form in marine sediments along continental margins and will often escape from the seafloor and rise through the water column as bubbles. The estimated marine methane hydrate inventory is over 600 times greater than the current atmospheric concentration so the fate of this ebullitive methane flux is of great interest. Traditional methods of measuring this flux such as acoustic imaging, optical sensors, and modeling suffer from limited information regarding the bubbles’ composition. Studies that attempt to constrain CH4 bubble composition suffer from low spatiotemporal resolution and adaptability. The current study presents the design, development and deployment of a novel, in situ bubble sampling system, the Bubble Delivery System (BDS), to quantify gas chemical composition in the water column. The BDS was deployed at the Cascadia Margin – a region well known for its active CH4 bubble seeps – where 95 samples were collected from McArthur Ridge, Hydrate Ridge, Heceta Deep and Heceta Shallow over the course of seven remotely operated vehicle dives. By combining this approach with the use of an underwater mass spectrometer, in situ analysis of these samples indicated that the bubbles contained between 84.6 to 100% CH4 and exhibited a high level of variability both spatially and temporally. Bubbles emitted from Heceta Deep exhibited anomalously elevated levels of carbon dioxide compared to the other sites.
