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Box 3.1 Methane bubble plumes from the sea floor off West Svalbard

Over 200 methane gas plumes have been observed coming from the seabed offshore West Svalbard (Fig. TB-3.1.1). There are plumes on the continental shelf in relatively shallow water near the shelf edge, on the continental slope in water shallower than the ~400 m water depth required for methane hydrates to be stable, and a few plumes have even been observed in deeper water (Westbrook et al. 2009; Sarkar et al. 2012; Rajan et al. 2012). From a climate-change perspective, it is of interest to know if 1) methane in the plumes reaches the atmosphere to directly increase atmospheric greenhouse gas levels, and 2) if recent climate change is responsible for creating the methane plumes. It does not appear methane in these plumes is reaching the atmosphere (Fisher et al. 2011), but there is ongoing debate about whether the plumes are caused by gas hydrates that are dissociating in response to bottom-water warming over the past ~30 years (Thatcher et al. 2013; Sarkar et al. 2012; Rajan et al. 2012; Berndt et al. 2014).

Much of the methane feeding into this region appears to be migrating toward the sea floor from below the gas hydrate stability zone, further down the continental slope than where most of the methane flares are found (Hustoft et al. 2009, Thatcher et al. 2013; Sarkar et al. 2012; Rajan et al. 2012). As methane migrates upward through sediment, it can occasionally find flow conduits through which it can escape vertically through the gas hydrate stability zone (GHSZ) (Liu and Flemings 2006), but in this region that type of flare is quite rare (Sarkar et al. 2012). More generally, methane either becomes incorporated into the gas hydrate or migrates up the continental slope through permeable sediment layers and conduits, some of which reach all the way to the shelf (Thatcher et al. 2013; Sarkar et al. 2012; Rajan et al. 2012) (Fig. TB-3.1.2). Of particular interest, however, are the plumes located just upslope from the current limit of gas hydrate stability (Fig. TB-

Figure TB-3.1.1: Methane-rich plumes in the water column on the West Svalbard continental margin. A: Location of survey area west of Svalbard; bathymetry (Jakobsson et al. 2008). B: Positionsof acoustically imaged plumes are depicted by “pins” superimposed on a perspective view of the bathymetry of part of the area of plume occurrence. The 396-metre isobath contour is the expected landward limit of the GHSZ. C: Part of the record from an acoustic survey showing examples of observed plumes. All plumes show a deflection towards the north caused by the West Svalbard Current. The sea floor, at around 240-metre depth, is shown by the strong (red) response (adapted from Westbrook et al. 2009).

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