Monitoring of Methane Emissions at Southern Hydrate Ridge using Deep-Sea Cabled Observatory

Natural methane gas release from the seafloor is a widespread phenomenon that occurs at cold seeps along most continental margins. Since their discovery in the early 1980s, seeps have been the focus of intensive research, partly aimed at refining the global carbon budget. However, deep-sea research is challenging and expensive and, to date, few programs have successfully monitored the variability of methane gas release over several weeks or more. Long-term monitoring is necessary to study the mechanisms that control seabed gas release. Located at 800 m depth on the Cascadia accretionary prism offshore Oregon, Southern Hydrate Ridge is one of the most studied seep sites where persistent, but variable gas release has been observed for more than 20 years. Using a series of instruments connected to the Ocean Observatories Initiative's (OOI) Regional Cabled Array observatory, we monitored the venting activity at Southern Hydrate Ridge over several months. We will present results from the systematic monitoring, which include in particular acoustic sensing of bubble plumes and time-lapse photography of selected vents at the seafloor. The data reveal a very dynamic system characterized by frequent and significant changes in seabed morphology and highly variable gas emissions. Acoustic data show how bubble plume variability is linked to the local tidal cycles. Photo and video imagery reveal how intense gas ebullition contributes to rapidly shaping the seabed morphology. This work is funded by the German Ministry of Education and Research (Bundesministerium für Bildung und Forschung). The OOI is funded by the National Science Foundation.