New Evidence of the Existence of Gas Migration Pathways Through Sub-Sea Permafrost in the East Siberian Arctic Shelf

To assess whether sudden, large-scale releases are taking place in the East Siberian Arctic Shelf (ESAS) or are likely to occur in the future we investigated the migration pathway characteristics and identified controlling factors of methane (CH4) flux from the seabed, through the water column, and into the atmosphere. Evidence of the existence of migration pathways through permafrost is provided by seismic data, specifically as low-amplitude anomalies sometimes referred to as washed-out or semi-blanked zones. In the marine environment, widespread washed-out zones often have been attributed to gas hydrates. In permafrost, low seismic amplitude may also result from variations of physical properties or property changes associated with development of deep taliks. The most prominent features of the seafloor morphology associated with the bubble releases observed in the mid-outer shelf (water depth >30 m) were morphological features that could be attributed to gas release from permeable/unconsolidated sediments: pockmarks (PMs), PM-induced erosion channels, collapse depression, and features related to mass wasting and sub-marine sediment slides. Acoustic anomalies observed in high-resolution seismic images obtained in mid-outer shelf reflect large volumes of free gas ascending within highly permeable sediments, so-called "gas curtains" and "gas blankets". Where such acoustic anomalies were observed, we detected very high concentrations of bubble seeps, which appeared not as single bubble streams (i.e. individual bubbles released continuously) that were mostly observed in the inner part of the ESAS, but as columns of bubbles (termed flares) rising to the sea surface. One of the most prominent morphological features observed in the ESAS (water depth <90 m) was associated with temporal gas releases was caused by ice scouring. The effect of this scouring is the creation of a long linear furrow that follows a relatively straight line and extends from only a few tens of meters to many tens of kilometers in length. In the ESAS, ice scouring penetrated up to 8 m deep into the sediments, and where surface sediments are underlain with gas fronts, gas releases have occurred. Since shallow gas fronts appeared to be a ubiquitous feature observed over the entire ESAS, ice scouring provides an important mechanism in the inner and mid shelf, allowing CH4 to escape from the sediments to the atmosphere by avoiding slow diffusion and aerobic oxidation in the sulfate-reduction zones. Additional pathways for CH4 release from the inner-mid shelf could be provided by completely submerged thaw lakes, underlain by taliks, which formed on the Siberian coastal plain prior to inundation. A number of such lakes have been transformed into sea lagoons or left seabed depressions in the ESAS interpreted as a typical thermokarst terrain landscape similar to the terrain of the Siberian Lowland. In such areas, we observed so-called "gas columns," which are characteristic of locally permeable sediments (within taliks), or gas movement within low-permeability sediments observed as "gas plumes".