Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.
Large amounts of the greenhouse gas methane are released from the seabed to the water column 1 where it may be consumed by aerobic methanotrophic bacteria 2. This microbial filter is consequently the last marine sink for methane before its liberation to the atmosphere. The size and activity of methanotrophic communities, which determine the capacity of the water column methane filter, are thought to be mainly controlled by nutrient and redox dynamics 3-7 , but little is known about the effects of ocean currents. Here we show that cold bottom water at methane seeps west off Svalbard, containing a large number of aerobic methanotrophs, was rapidly displaced by warmer water with a considerably smaller methanotrophic community. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, constitutes an oceanographic switch severely reducing methanotrophic activity in the water column. Strong and fluctuating currents are widespread oceanographic features common at many methane seep systems and are thus likely to globally affect methane oxidation in the ocean water column. Large amounts of methane are stored in the subsurface of continental margins as solid gas hydrates, gaseous reservoirs or dissolved in pore water 8. At cold seeps, various physical, chemical, and geological processes force subsurface methane to ascend along pathways of structural weakness to the sea floor where a portion of this methane is utilised by anaerobic and aerobic methanotrophic microbes 1,9. On a global scale, about 0.02 Gt yr-1 (3-3.5% of the atmospheric budget 10) of methane bypasses the benthic filter system and is liberated to the ocean water column 1 where some of it is oxidised aerobically (aerobic oxidation of methane-MOx) (ref 2), or less commonly where the water column is anoxic, anaerobically (anaerobic oxidation of methane-AOM) (refs 2, 11). MOx is performed by methanotrophic bacteria (MOB) typically belonging to the Gamma-(type I) or Alphaproteobacteria (type II) (refs 12, 13): CH 4 + 2 O 2 → CO 2 + 2 H 2 O Water column MOx is consequently the final sink for methane before its release to the atmosphere, where it acts as a potent greenhouse gas. The water column MOx filter could become more
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