Thermogenic methane injection via bubble transport into the upper Arctic Ocean from the hydrate‐charged Vestnesa Ridge, Svalbard

Abstract

We use new gas-hydrate geochemistry analyses, echosounder data, and three-dimensional PCable seismic data to study a gas-hydrate and free-gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of 3.3 C result in a shallow top of the hydrate stability zone (THSZ) at 340 m below sea level (mbsl). Therefore, hydrate-skinned gas bubbles, which inhibit gas-dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to 150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas-hydrate and free-gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice-covered Arctic environments.

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

“…The system is restricted to the upper stratigraphic sequence (YP3) and has a series of gas chimneys and pockmarks associated along the full extent of the Vestnesa Ridge. However, only pockmarks located toward the easternmost part of the ridge (where our 3D seismic survey is located; Figure 1) are actively seeping gas presently (Bünz et al, 2012;Smith et al, 2014). Gas chimneys toward the westernmost part of the ridge seem inactive presently, but foraminiferal records indicated past activity approximately 8000 my ago (Consolaro et al, 2014).…”

“…An inline has been selected from seismic data in which a BSR is clearly identified by high seismic amplitudes at approximately 1.9 s two-way traveltime in the seismic section (Figure 4a) (Bünz et al, 2012;Smith et al, 2014). The BSR separates hydrate-bearing sediments from an approximately 100-m-thick free-gas zone (Hustoft et al, 2009).…”

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

“…Potentially, part of the CH 4 oversaturation in AIW and ADW in the Nansen Basin comes from cold seeps in the subsurface layers of the continental margin of Svalbard (Westbrook et al, 2009;Steinle et al, 2015). There is evidence for most of this gas coming from thermogenic sources in the Vestnesa Ridge (Fram Strait) in the form of gas-hydrates (Smith et al, 2014). This ecosystem in known to contains large amounts of CH 4 in the form of solid gas hydrates, gaseous reservoirs or dissolved gas in pore waters (Wallmann et al, 2012), which may ascend from the sea floor and be available to anaerobic and aerobic methanotrophic microbes (Knittel and Boetius, 2009;Boetius and Wenzhöfer, 2013).…”

“…More recently, CH 4 formation was reported to proceed the metabolism of organic methyl-compounds of methylotrophs, such as methylphosphonate (MPn) (Karl et al, 2008), dimethylsulfoniopropionate (DMSP) (Damm et al, 2010), and dimethylsulphide (DMS) (Florez-Leiva et al, 2013). Other geological/thermogenic production processes, such as CH 4 hydrates in continental margin sediments, mud volcanoes, and cold seeps, could also be responsible for CH 4 release, as is the case in the Arctic Ocean, which has been the focus of scientific interest in previous decades (Westbrook et al, 2009;Shakhova et al, 2010;Smith et al, 2014).…”

Submarine vortices derived from natural gas hydrate conversion: a mechanism for ocean mixing

“…Undirected hydrostatic pressures decrease during bubble ascent resulting in gas exsolution and bubble expansion and fragmentation, which in turn results in increased buoyancy, possible expansion of the bubble plume, and changes in the interfacial tension of hydrate-shelled bubbles. Vortex tube generation could explain the consistent relatively slender morphologies associated with plumes seen in multibeam data (Solomon et al, 2009;Colbo et al, 2014;Skarke et al, 2014;Smith et al, 2014;Tudino et al, 2014;Weber et al, 2014;Garcia-Pineda et al, 2015;Wilson et al, 2015) and recent observations indicate that vertical rise of buoyant plumes may be augmented by spiral flow geometries (von Deimling et al, 2015). Because the reflection and scattering from a bubble plume is a bulk effect, rotation has not been observed in seismic reflection data.…”