Escape of methane gas from the seabed along the West Spitsbergen continental margin

Abstract

[1] More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150 -400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean. Citation: Westbrook, G. K., et al.(2009), Escape of methane gas from the seabed along the West Spitsbergen continental margin, Geophys. Res. Lett., 36, L15608,

Enhanced methane emissions from oil and gas exploration areas to the atmosphere – The central Bohai Sea

“…Since the average depth of the Bohai Sea is only 18 m, the water column provides a short conduit for the seafloor-released CH 4 to rise to the surface layer. Although microbial consumption has been realized as a significant sink for spilled CH 4 in deep oceans (Westbrook et al, 2009;Kessler et al, 2011;Schmale et al, 2011), the strength of this process remains to be assessed in shallow sea areas such as the Bohai Sea.…”

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

“…Seawater sampling sites are shown in Figure 1 and listed in supporting information Tables S1 and S2, and air sampling sites are shown in supporting information Figure S1. Bubble plumes were identified by shipboard sonar: a Simrad EK60 ''fishfinder '' sonar (38,120,and 200 kHz) and a 12 kHz Simrad EM122 sonar were used on the RRS James Clark Ross [Westbrook et al, 2009], and a PARASOUND sonar (18 kHz parametric echo sounder) was used on the RV Maria S. Marian [Berndt et al, 2014]. In 2012, an equilibrator system [G€ ulzow et al, 2011] was used to continuously measure surface water methane concentrations along the cruise track of RV Heincke during cruise He-387 (20 August 2012to 6 September 2012; the area covered by the cruise track is shown in Figure 1a [Sahling et al, 2014]).…”

“…However, new sites of seafloor methane seepage continue to be discovered [e.g., R€ omer et al, 2014;Skarke et al, 2014], and recent studies suggest that sea to air methane fluxes at some locations may be far higher than previously thought [e.g., Shakhova et al, 2010a]. Seepage of methane from seafloor sediments in the Arctic Ocean has been linked to release from temperature-sensitive shallow marine sediment reservoirs, including permafrost and methane hydrate [Berndt et al, 2014;Ferre et al, 2012;Sahling et al, 2014;Shakhova et al, 2014;Westbrook et al, 2009]. As Earth's climate continues to warm [Intergovernmental Panel on Climate Change, 2013], there is a need to constrain the potential for feedback between seafloor and atmospheric methane systems.…”

“…The presence of hydrate reduces sediment permeability such that subsurface gas flow can be directed along the base of the hydrate layer, with seepage at the seafloor in waters shallower than the landward limit of the hydrate stability zone [Naudts et al, 2006;Schmale et al, 2011]. Destabilization of methane hydrate in seafloor sediments is proposed to have contributed to previous episodes of major climate change, including the Paleocene-Eocene thermal maximum [e.g., Dickens, 2011], and recently discovered methane emissions from the seafloor offshore western Svalbard may, at least in part, be related to hydrate dissociation linked to warming of bottom waters [Berndt et al, 2014;Biastoch et al, 2011;Reagan and Moridis, 2009;Thatcher et al, 2013;Westbrook et al, 2009].…”

“…As seafloor methane seepage may increase in response to climate warming, particularly in the Arctic, observations of methane fluxes and oxidation rates and investigation of the processes that regulate them are critical. The discovery of more than 250 plumes of methane gas seeping from the seafloor at the landward limit of the gas hydrate stability zone (GHSZ) on the upper continental margin offshore western Svalbard [Westbrook et al, 2009] provides an ideal setting to address these issues. To this end, we have determined the distribution of methane in the water column in the vicinity of the landward limit of the GHSZ for three separate sampling campaigns.…”