Characterization of Sea-Air Methane Fluxes Around a Seafloor Gas Seep in the Central Laptev Sea

Abstract:

The fate of CH₄ released from thawing subsea permafrost on the East Siberian Arctic Shelf (ESAS) is unclear. In recent years, interest has focused on the possibility of large emissions of CH₄ directly to the atmosphere from this remote area. It is uncertain how high those emissions are and whether they are primarily of biogenic or thermogenic nature, or some combination of sources.

The SWERUS-C3 expedition onboard the Swedish icebreaker Oden during July-August 2014 sought to document possible CH₄ release from subsea permafrost, and to understand mechanisms and magnitudes of such CH₄ being released to the atmosphere. During the first leg of the expedition continuous high-resolution measurements were made to determine the in situ concentrations of CH₄ in both the atmosphere and surface water.

During SWERUS-C3, several underwater gas flares were found within the ESAS region showing elevated CH₄ concentrations collocated in the surface waters. Here we focus on one seep area, a so-called “mega-flare” site, in the central Laptev Sea. Over individual gas flares of this site the surface water concentration of CH₄ reached as high as 200ppm. The atmospheric concentrations of CH₄ briefly (< 1 s) reached a maximum of ~3.2 ppm. More typical atmospheric values around the seeps were between 1.9-2.0 ppm (background values were approximately 1.88 ppm). However, such peak concentrations in both air and water were highly localized, returning to background levels within a few hundred meters of the source seeps.

Together with continuous high-precision eddy-covariance measurements that were made during the SWERUS-C3 expedition, the combined dataset allows an intensive analysis these highly inhomogeneous gas flares. This gives the opportunity to calculate accurate high-resolution CH₄ fluxes and thus give a better insight into the current rates of subsea CH₄ outgassing reaching the atmosphere.