Leakage on the continental margin

Off the coast of Spitsbergen, on the upper continental margin between Bear Island and Kongsfjord, methane gas is emitted from the seafloor at more than a thousand sites. Past expeditions have reported methane gas seeps off the coast of Prins Karls Forland, presumed by some scientists to be a result of the dissociation of methane hydrates in the sediments caused by warming in recent years.

The results of two cruises of the Research Vessel HEINCKE in 2015 have revealed, however, that methane emissions are not limited to this site, but extend across five degrees of latitude along the continental margin, and is very likely associated with the Hornsund Fracture Zone. Susan Mau has now published the results together with her colleagues at MARUM – Center for Marine Environmental Sciences at the University of Bremen, and others from the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung in Bremerhaven and Oregon State University (USA).

The investigations of Susan Mau and her colleagues are based on data from two research cruises in the summer of 2015. The gas discharge sites were identified hydroacoustically as patterns called “flares.” “We already knew about the gas seeps off Prins Karls Forland because that site has been thoroughly studied,” explains Dr. Mau.

Their data, however, reveal the presence of numerous emission sites along the entire coast. The sites follow a fracture zone on the upper continental margin that could be the conduit for the methane rising from depths along this zone. Like air from a perforated bicycle inner tube submerged in water, the rising gas can escape along the fractures and ascend through the sea water. According to Mau, the scientists measured consistently high methane concentrations along the entire slope off the coast of Svalbard. “But they were especially high at the sites where multiple flares were seen.”

In addition, the gas seepage conspicuously occurred at bathymetrically elevated areas rather than in the troughs between them. Susan Mau presumes the reason for this is fine-grained deposits sealing the pathways that gas could otherwise escape from.

Why have so many emission sites been discovered off Prins Karls Forland? Are there even more sites off the coast of Svalbard? These were the initial questions to be addressed by the cruises led by Susan Mau and Gerhard Bohrmann. Furthermore, rock samples and seismic studies have shown that the entire coast is characterized by similar tectonic conditions and glacial history. The gas seeps verified off the coast of Svalbard are interesting primarily because scientists have thought that the methane escaping here is released from methane hydrates. Methane hydrates have a solid ice-like structure that is only stable under specific pressure conditions at rather well defined depths and relatively low temperatures. When the water becomes warmer methane hydrate is no longer stable and methane is released.

Should the water warm up – due to climate change, for example – methane hydrates can only occur in deeper sediments. The boundary zone in which gas hydrates become stable is effectively shifted downward. The gas bubble emissions observed by Mau and her colleagues, however, also occur above this boundary, and thus are not anthropogenically triggered gas released from methane hydrates.

These methane seepage sites are the result, rather, of large amounts of gas from great depths escaping along the Hornsund Fracture Zone, an extensive fault zone in the Earth’s crust, a natural geologic process. This produces high gas concentrations that the team has confirmed over a range of hundreds of kilometers along the coast. The data from the summer of 2015 also indicate that the dissolved methane is oxidized by microbes within the water column and only a small proportion escapes into the atmosphere. The microbes thus prevent an increase in greenhouse gas concentrations in the atmosphere.

From their current results, numerous new questions arise for the geologists Susan Mau and Gerhard Bohrmann: What is the precise course of the actual fracture zone? What is the character of the substratum? Where are the gas reservoirs located? And: What is the age of the escaping gas? In any case, a connection between the large number of gas seeps and human-produced warming of the oceans has not been confirmed.

Because the expeditions off the coast by Mau and her colleagues were carried out in the summer, it is not certain what happens during other colder and stormier seasons. “Our results cry out for long-term studies of the seeps,” Mau emphasizes. “We have to strive to learn the reason for high methane gas concentrations that have occurred repeatedly throughout the Earth’s history. The goal is to observe the seepage sites to find out what happens over the course of the year. Only then will it be possible to draw accurate conclusions – including, for example, whether gas emissions at these depths and at this temperate zone are climate relevant.”

Contact:
Dr. Susan Mau
Telephone: +49 (0) 421-21865059
Email: smau@marum.de

Original publication:
Susan Mau, Miriam Römer, Martha E. Torres, Ingeburg Bussmann, Thomas Pape, Ellen Damm, Patrizia Geprägs, Paul Wintersteller, Chieh-Wei Hsu, Markus Loher und Gerhard Bohrmann: Widespread methane seepage along the continental margin off Svalbard – from Bjørnøya to Kongsfjorden. Sci. Rep. 7, 42997; doi: 10.1038/srep42997 (2017)

Further information / Photo material:
Ulrike Prange
MARUM-Öffentlichkeitsarbeit
Telephone: 0421 218 65540
Email: medien@marum.de

http://www.marum.de/en/Leakage_on_the_continental_margin.page