Abstract:Mud volcanism is an important natural source of the greenhouse gas methane to hydrosphere and atmosphere 1,2 . Recent investigations show that the number of active submarine mud volcanoes may be much higher than anticipated (eg. 3-5), and that gas emitted from deep-sea seeps may reach the upper mixed ocean [6][7][8] . Unfortunately, global methane emission from active submarine mud volcanoes cannot be quantified because their number and gas release is unknown 9 . Another uncertainty is the efficiency of methane oxidizing microorganisms in methane removal. Here we investigated the methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea, 72°N, 14°44'E; 1250 m water depth), to provide quantitative estimates of in situ composition, distribution and activity of methanotrophs in relation to gas emission. The HMMV hosts three key communities; aerobic methanotrophic bacteria (Methylococcales), anaerobic methanotrophic archaea (ANME-2) thriving below siboglinid tubeworms, and a novel clade of archaea (ANME-3) associated with bacterial mats. We found that upward flow of sulphate-and oxygen-free mud volcano fluids restricts the availability of these electron acceptors for methane oxidation, and hence the habitat range of methanotrophs. This mechanism limits the capacity of the microbial methane filter at active marine mud volcanoes to <40% of the total flux.The HMMV (Fig. 1), a circular structure of 1 km diameter and <10 m elevation above the adjacent seafloor, has been studied since the 1990s as a typical example of an active mud volcano 9 . Its formation may have coincided with a submarine landslide during the late Pleistocene, 330-200 ka before present 10 . Today, fluids, gas and muds rise from 2-3 km depth through a conduit below the HMMV 11,10 . The emitted gas is of a mixed microbial/thermogenic origin and consists of >99% CH4 with a δ 13 C-isotope signature of -60‰ 12,13 . The rising fluids are depleted in sulphate, chloride and magnesium caused by subsurface clay dewatering 11 . Investigation of the HMMV with RV POLARSTERN and ROV VICTOR 6000 in 2003 showed extensive outcroppings 1 2006-01-01028b_Boetius_MS 3 of fresh subsurface muds associated with steep thermal gradients 14 , gas and fluid vents, and a large gas plume reaching the mixed upper water column above the HMMV 12,8 .Seafloor videography in combination with geochemical measurements provided in situ estimates of gas flux 8 , fluid flow 15 and habitat distribution 16 . We focused on the three main concentric habitats above the gassy muds (Fig. 2): the centre of HMMV, which was devoid of epifauna; thiotrophic bacterial mats dominated by a Beggiatoa species; and surrounding fields of siboglinid tubeworms. Gas concentrations in sediments and bottom water were elevated in all three habitats (Tab. (Fig. 3a). Only minor amounts of methanotroph lipids (<0.1 µg gdw -1 ) and very low cell numbers (~10 7 cells cm -3 ) were found below 5 cm sediment depth ( Fig. 3a3-3a4). ANME cells were not microscopically detectable in the centre cores using all know...

The assessment of climate change factors includes a constraint of methane sources and sinks. Although marine geological sources are recognized as significant, unfortunately, most submarine sources remain poorly quantified. Beside cold vents and coastal anoxic sediments, the large number of submarine mud volcanoes (SMV) may contribute significantly to the oceanic methane pool. Recent research suggests that methane primarily released diffusively from deep-sea SMVs is immediately oxidized and, thus, has little climatic impact. New hydro-acoustic, visual, and geochemical observations performed at the deep-sea mud volcano Håkon Mosby reveal the discharge of gas hydrate-coated methane bubbles and gas hydrate flakes forming huge methane plumes extending from the seabed in 1250 m depth up to 750 m high into the water column. This depth coincides with the upper limit of the temperature-pressure field of gas hydrate stability. Hydrographic evidence suggests bubble-induced upwelling within the plume and extending above the hydrate stability zone. Thus, we propose that a significant portion of the methane from discharged methane bubbles can reach the upper water column, which may be explained due to the formation of hydrate skins. As the water mass of the plume rises to shallow water depths, methane dissolved from hydrated bubbles may be transported towards the surface and released to the atmosphere. Repeated acoustic surveys performed in 2002 and 2003 suggest continuous methane emission to the ocean. From seafloor visual observations we estimated a gas flux of 0.2 (0.08-0.36) mol s−1 which translates to several hundred tons yr−1 under the assumption of a steady discharge. Besides, methane was observed to be released by diffusion from sediments as well as by focused outflow of methane-rich water. In contrast to the bubble discharge, emission rates of these two pathways are estimated to be in the range of several tons yr−1 and, thus, to be of minor importance. Very low water column methane oxidation rates derived from incubation experiments with tritiated methane suggest that methane is distributed by currents rather than oxidized rapidly.

Depth distribution data were compared for 172 European and 157 Antarctic benthic invertebrate species occurring in the respective shelf areas. Antarctic species showed significantly wider depth ranges in selected families of the groups Bivalvia, Gastropoda, Amphipoda and Decapoda. No differences were found in Polychaeta, Asteroidea and Ophiuroidea, where European species also showed comparatively wide bathymetric ranges. These extended levels of eurybathy in the Antarctic benthos may be interpreted either as an evolutionary adaptation or pre-adaptation to the oscillation of shelf ice extension during the Antarctic glacial-interglacial cycle.

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