From the Seafloor to the Surface: In situ Chemical Analysis of Rising Bubbles along the Cascadia Margin

Anna Michel1, Andrew Stafford Johnson2, Kristen Fauria3, Victoria Preston1, David P Nicholson4, Daniel Hoer5, Peter R Girguis6 and Scott D Wankel7, (1)Woods Hole Oceanographic Institution, Applied Ocean Physics and Engineering, Woods Hole, MA, United States, (2)Woods Hole Oceanographic Institution, Woods Hole, United States, (3)Woods Hole Oceanographic Institution, Woods Hole, CA, United States, (4)Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (5)Harvard University, United States, (6)Harvard University, Organismic and Evolutionary Biology, Cambridge, MA, United States, (7)Harvard University, Woods Hole, United States
The fate of methane contained within bubbles emitted from seafloor hydrocarbon seeps is not fully understood. Here we present results from a 2018 R/V Falkor cruise to Cascadia Margin to investigate the chemical fate of bubbles emanating from methane seeps using a suite of advanced in situtechnologies. To characterize the composition of bubble emissions from deep sites (>100m), a novel bubble sampling system was designed and deployed along with an in situmass spectrometer for gas compositions and an in situlaser spectrometer for carbon isotopes of methane. Dives were made to seep sites located at McArthur Ridge, Hydrate Ridge and Heceta Bank. 22 bubble samples from 7 sites were collected and analyzed in situand found to contain between 85 to 100% methane. Bubbles from one deep site at Heceta Bank exhibited anomalously high levels of carbon dioxide compared to other sites. To investigate transport of methane from shallow seep sites (<75 m), a remote controlled surface platform, the ChemYak, was used to survey methane concentrations in the surface ocean. From these measurements, regions of elevated methane in near surface shallow waters were identified. We will highlight results from all of these deployments, illustrating how these advanced technologies offer new insight on the fate of the methane at both deep and shallow sites.