Spatial mapping of dissolved methane using an in situ sensor in Puget Sound

Release of methane, as gas bubbles or in the dissolved phase, from the seafloor has been observed in coastal waters (<200 m) and deep ocean basins (>1000 m). Methane dissolution within the water column affects the geochemistry of the surrounding water, leading to localized oxygen loss and ocean acidification. Traditional methods for detecting and quantifying dissolved methane rely on collecting discrete water samples for ship- or land-based ex-situ analysis and post processing. Here, we report on the use of a real-time, in-situ methane sensor, Sensor for Aqueous Gases in the Environment (SAGE), for detecting and quantifying dissolved methane concentrations in a wide range of seafloor environments. During a Fall 2022 research cruise on the R/V Thomas G. Thompson in Puget Sound, SAGE was integrated to a towed conductivity/temperature/depth (CTD) rosette and deep-sea camera system with real-time 1Hz telemetry and used to spatially map the concentration of methane approximately one meter above the seafloor. The site had been previously identified as an active methane plume field characterized by gas bubbles, fluid venting, and a faulted seabed. The widespread background dissolved concentration of methane measured by SAGE was 83 nM, and a range of 78–670 nM was observed throughout the survey. The results highlight the capacity of SAGE to capture the spatial and temporal variability of dissolved methane concentrations in-situ and to identify and localize sites of variable methane emissions from the seafloor.