Widespread methane seepage along the continental margin off Svalbard - from Bjørnøya to Kongsfjorden

Abstract

Numerous articles have recently reported on gas seepage offshore Svalbard, because the gas emission from these Arctic sediments was thought to result from gas hydrate dissociation, possibly triggered by anthropogenic ocean warming. We report on findings of a much broader seepage area, extending from 74° to 79°, where more than a thousand gas discharge sites were imaged as acoustic flares. The gas discharge occurs in water depths at and shallower than the upper edge of the gas hydrate stability zone and generates a dissolved methane plume that is hundreds of kilometer in length. Data collected in the summer of 2015 revealed that 0.02-7.7% of the dissolved methane was aerobically oxidized by microbes and a minor fraction (0.07%) was transferred to the atmosphere during periods of low wind speeds. Most flares were detected in the vicinity of the Hornsund Fracture Zone, leading us to postulate that the gas ascends along this fracture zone. The methane discharges on bathymetric highs characterized by sonic hard grounds, whereas glaciomarine and Holocene sediments in the troughs apparently limit seepage. The large scale seepage reported here is not caused by anthropogenic warming.Methane is, after water vapor and CO 2 , the most abundant greenhouse gas on Earth. When averaged over a 100 yr timescale, the warming effect of methane per unit mass is 28 times higher than that of CO 2 1 . Methane is produced in oceanic sediments either by methanogens at temperatures typically below ~80 °C, or through the breakdown of organic molecules at higher temperatures 2,3 . Buoyancy and pressure gradients can drive gas advection to shallower sediments where methane can be consumed via anaerobic oxidation of methane (AOM) 4 at the sulfate-methane transition zone and aerobic methane oxidation at the sediment surface 5 . Methane can also be sequestered within a cage of water molecules, in a gas hydrate structure, stable under the low temperature and high pressure conditions that define the gas hydrate stability zone 6 . If the upward methane flux is not fully exhausted by these processes, methane is emitted to the ocean either dissolved in the venting fluids or, in case of over-saturation, as gas bubbles 7 . As the bubbles ascend through the water column, a fraction of the methane gas dissolves 8 , generating patches of high methane concentration 9 . When the gas discharge is persistent and vigorous, it leads to the formation of large dissolved methane plumes. The dissolved methane is diluted by mixing with the surrounding ocean water and it is further oxidized by aerobic methanotrophs. Only in cases where dissolved methane reaches the surface-mixed layer in concentrations above saturation, can it be transferred to the atmosphere via sea-air gas exchange 10 . At present, the oceanic methane source to the atmosphere is very small (2-10%) 11 , as it is limited to emissions from vigorous and shallow seeps (<100 m) 1,7,8 . There is, however, an ongoing controversy regarding the methane discharge from sediments during warming events througho...

Methane distribution and oxidation around the Lena Delta in summer 2013

“…A comprehensive study by Myhre et al (2016) calculated a median methane flux of only 3 µmol m 2 d −1 , which is supported by a median methane flux of 2 µmol m 2 d −1 for the coastal waters of Svalbard (Mau et al, 2017), and this value lies within the previously reported range of 4 to 20 µmol m 2 d −1 ; Table 5). For the North American Arctic Ocean and its shelf seas, rather low methane fluxes of 1.3 µmol m 2 d −1 have been reported (Fenwick et al, 2017).…”

“…When the total methane inventory was set to 100 %, a median of 1 % (range 0.3-3.8 %) was consumed within 1 day by bacteria within the system, while a median of 8 % (1-47 %) left the system and entered the atmosphere. A similar estimation has been made for the coastal waters of Svalbard (Mau et al, 2017), where a much higher fraction of the dissolved methane (0.02-7.7 %) was oxidised, and only a minor fraction (0.07 %) was transferred into the atmosphere. However, the water in this region was much deeper; thus, the ratio of water volume (including the methane oxidation activity) to the surface area (including the diffusive methane flux) was much larger.…”

“…In polar waters, off the coast of Svalbard and unaffected by ebullition sites, values of 0.26 to 0.68 nmol L −1 d −1 (Mau et al, 2017) and 0.5 ± 1 nmol L −1 d −1 have been reported. Thus, our values are well within the reported ranges for coastal and polar MOX.…”

“…It has a deep central basin and is characterised by extensive shallow shelf areas, including the Barents, Kara, Laptev, East Siberian, Chukchi, and Beaufort seas. Methane sources in these Arctic areas can include the thawing methane hydrates off the coast of Svalbard (Westbrook et al, 2009) and ebullition of methane from diverse geologic sources (Mau et al, 2017;Shakhova et al, 2014). In addition, the extensive shallow-water areas of the Arctic continental shelf are underlain by permafrost, which was formed under terrestrial conditions and subsequently submerged by post-glacial rises in sea level.…”

Methane-oxidizing seawater microbial communities from an Arctic shelf

“…During the last few years, the first studies have determined methane oxidation rates from seawater in these regions to cover a range from 10 −4 up to 3.2 nmol L −1 d −1 (Gentz et al, 2014;Lorenson et al, 2016;Mau et al, 2013Mau et al, , 2017Steinle et al, 2015). In only two of these studies, both performed off Svalbard, oxidation rate measurements were combined with analysis of the microbial community.…”