Variability of Natural Methane Bubble Release at Southern Hydrate Ridge. (5th October 2021)

• Record Type: Journal Article
• Authors: Marcon, Yann; Kelley, Deborah; Thornton, Blair; Manalang, Dana; Bohrmann, Gerhard
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 10(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-05
• Subjects: methane -- bubbles -- plume -- variability -- tides -- multibeam; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009894
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Current estimations of seabed methane release into the ocean (0.4–48 Tg yr −1 ) are based on short‐term observations and implicitly assume that fluxes are constant over time. However, the intensity of gas seepage varies significantly throughout a seep lifetime. We used instruments operated by the Ocean Observatories Initiative's Regional Cabled Array to monitor variations of gas emissions over the entire Southern Hydrate Ridge summit. We show that bubble plumes emanate from distinct and persistent vents. Multiple plumes can occur within each vent and the location of their outlets may shift progressively. Active bubble plumes vary temporally in number and intensity, even within single vents. Gas emission fluctuations are partly periodic and linked to the local tide. However, short‐term variability and high ebullition events unrelated to tidal cycles are also commonly observed. Our data indicate that small‐scale processes beneath or at the sediment surface are responsible for the short‐term variability of the venting activity that is otherwise modulated by tides. Furthermore, a decrease of venting at one vent may coincide with an increase in plume activity at other vents. Our results depict a spatially and temporally dynamic seep environment, the variability of which cannot be fully characterized without systematic and comprehensive monitoring of the entire area. These results indicate that flux estimations may be largely overestimated or underestimated depending on the time, duration, and place of observation. Although sudden ebullition bursts are hardly predictable, we argue that tidal cycles must be taken into consideration when estimating gas fluxes. Plain Language Summary: Methane emission from the seabed into the ocean occurs naturally along continental margins. Methane release in the form of bubbles commonly escapes the seabed and rises through the water column forming bubble plumes. Since methane is a potent greenhouse gas, understanding which factors influence the methane release rate from submarine sources is important. This study focuses on one submarine source, Southern Hydrate Ridge, located in the Northeast Pacific 85 km offshore Oregon at a 780 m water depth. We used instruments installed at the seafloor and operated through an underwater cabled observatory to monitor bubble plumes and to study why their intensity varies over time. We confirmed that pressure variations caused by tides affect methane release rates and that bubble plumes are more intense during decreasing tides than rising tides. However, we found that not all fluctuations could be accounted for by tides and that distinct bubble plumes could have decoupled behaviors. The data suggest local and temporary permeability changes near the sediment‐water interface as the most likely cause of the short‐term gas emission variability. These findings are significant because they show that methane flux estimations from submarine sources may be largely inaccurate if based on short‐term or small‐scale measurements. Key Points: Fluctuations of methane emissions from the Southern Hydrate Ridge summit are modulated by the barotropic tide Permeability changes in shallow hydrate‐bearing sediments cause local pressure buildups that produce strong short‐term venting variability Distinct vents have different ebullition behaviors and monitoring a single vent would give an incomplete picture of the venting dynamics