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 Okinawa Trough, lying to the east of China, is a back arc basin formed by extension within continental lithosphere behind the Ryukyu trench‐arc system. Middle to late Miocene uplift, associated with normal faulting of the initially adjacent Ryukyu nonvolcanic arc and the Taiwan‐Sinzi folded belt, corresponds to the first rifting phase. The timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern Okinawa Trough. The rifting occurred after a major early Miocene change in the motion of the Philippine plate with respect to Eurasia and ceased during the Pliocene. A second rifting phase started about 2 m.y. ago, at the Plio‐Pleistocene boundary and has continued until the present time. It has proceeded to a more advanced stage in the middle and southern Okinawa Trough than it has farther north. Detailed bathymetric (Sea Beam), seismic reflection, and magnetics data collected during the POP 1 cruise of the R/V Jean Charcot reveal the principal features of the extensional processes. The back arc spreading phase started very recently in the southern and middle Okinawa Trough, as exemplified by several en échelon and, in some cases, overlapping active, central graben oriented N70°E–N80°E. Some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies. Transform faults between these en échelon active rifts are not obvious. We suggest that the major part of the southern Okinawa Trough is underlain by a thinned continental crust and that except for the system of en échelon rifts of the southern Okinawa Trough, the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough. The system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of Okinawa Island whereas active volcanoes in the Ryukyu arc occur only north of Okinawa Island. We refer to this transition between active arc and back arc volcanism as the volcanic arc‐rift migration phenomenon (VAMP). Globally, back arc volcanism propagated from the southern Okinawa Trough to the Okinawa VAMP area. Rifting continues to occur in the northern Okinawa Trough but is not yet accompanied by associated volcanism. The Okinawa VAMP area is characterized by a series of parallel basaltic ridges oriented N75°E with associated linear magnetic anomalies characteristic of dyke intrusions. We suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending N75°E and that this zone presently ends at the southern extremity of the active volcanic chain. The initial phase of formation of back arc basin oceanic crust is non‐steady state and is characterized by the lack of a developed fracture zone pattern. The termination of the VAMP area in the direction of the volcanic zone of the arc is consistent with the suggestion of Molnar and Atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc...

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Detailed patrems of near surface tectonics and heat flow were investigated at two locations at the toe of the Barbados Ridge accretionary prism. At the two locations, in the thickly sedimented southern portion of the prism near 12ø202q and in the northern area drilled during Ocean Drilling Program (ODP) leg 110 near 15ø322q, relative highs of heat flow were associated with zones of thrusting. We attribute them to upward advection of water rising from the d6collement along faults through the sediments. In the ODP area, a significant heating plate effect from warm fluids flowing along the d6collement and thrusts is required to account for the high regional heat flow. This effect implies high rates of fluid flow at the d6collement, which can only be non-steady state or would require massive lateral transport of fluids along strike. Paper number 90IB00828.0148-0227/90/90JB00828505.00 decrease in heat flow to the background value over a lateral distance of 300 m. Although the heat flow profile does not extend all the way across the second thrust-outcrop wedge farther upslope, the two westernmost measurements suggest that a similar heat flow anomaly is associated with this upper wedge.The only reasonable explanation that we find for this anomaly, in particular because of its short wavelength, is upward fluid 8859 J.P. Foucher, D6partement de G6osciences Marines